Do narrow lines in the spectra of O- and B- type stars always indicate magnetic fields? I was reading a paper on the differential emission measures of a set of hot O- and B- type stars. As the authors discuss in Section 3 (page 959), two stars, $\tau$ Sco and $\theta^1$ Ori C, have narrow emission lines compared to other stars of the same spectral type. There is a good amount of evidence that both stars are young and have magnetic fields; the stellar winds travel along the field lines and collide at the magnetic equator, producing shocks and, indirectly, X-ray emission. This is in contrast to the mechanism behind shock formation in other O- and B- type stars, where shocks come from the line instability transition.

Now, the plasma at the magnetic equator is roughly stationary because of the collision; this means - if I'm interpreting things correctly - that there is little to no broadening of the lines, and so they are comparatively narrow. Other spectral characteristics and observations support this model.

I'm wondering two things:

Has the presence of such comparatively narrow lines in the spectra of O- and B- type stars been observed in cases where there is no magnetic field present? Very few O-type stars have significant magnetic fields, and theory predicts that none should, so these stars are the few exceptions to the rule.
If so, are there other mechanisms that could be responsible for this in cases without magnetic fields? I'm grasping at straws for ideas, like low collisional broadening.

I talked with my advisor, and we briefly discussed colliding wind shocks (CWS), which form at the interface between the stellar winds in a binary system. It's thought that these shocks contribute only a small amount to the total X-ray production of the system (see Gagné et al. (2011)), though, so any narrow lines produced in that plasma - if any exist - wouldn't have much influence over the measured spectra. So that seems to be ruled out.

Do narrow lines in the spectra of O- and B- type stars always indicate magnetic fields?

The narrow lines don't necessarily require a magnetic field, but any star that large will have one.

I'm wondering two things:

Has the presence of such comparatively narrow lines in the spectra of O- and B- type stars been observed in cases where there is no magnetic field present?

O-type main-sequence stars have high metallicity, they will have a magnetic field. Tau Scorpii and Theta$^1$ Orionis C info at their links. The Phys.Org article: "Strong magnetic fields discovered in majority of stars" says:
"An international group of astronomers led by the University of Sydney has discovered strong magnetic fields are common in stars, not rare as previously thought, which will dramatically impact our understanding of how stars evolve.".
This Wikipedia article: "Solar Dynamo" could use some citations, but here's what it says:
"The solar dynamo is the physical process that generates the Sun's magnetic field. A dynamo, essentially a naturally occurring electric generator in the Sun's interior, produces electric currents and a magnetic field, following the laws of Ampère, Faraday and Ohm, as well as the laws of hydrodynamics, which together form the laws of magnetohydrodynamics. The detailed mechanism of the solar dynamo is not known and is the subject of current research.
Mechanism
A dynamo converts kinetic energy into electric-magnetic energy. An electrically conducting fluid with shear or more complicated motion, such as turbulence, can temporarily amplify a magnetic field through Lenz's law: fluid motion relative to a magnetic field induces electric currents in the fluid that distort the initial field. If the fluid motion is sufficiently complicated, it can sustain its own magnetic field, with advective fluid amplification essentially balancing diffusive or ohmic decay. Such systems are called self-sustaining dynamos. The Sun is a self-sustaining dynamo that converts convective motion and differential rotation within the Sun to electric-magnetic energy.
...".

If so, are there other mechanisms that could be responsible for this in cases without magnetic fields? I'm grasping at straws for ideas, like low collisional broadening.

Our Q&A: "Different width of spectral lines for different groups of stars" has Rob Jefferies answer.
This seems well explained in section 3.4 of "Magnetically Confined Wind Shocks in X-rays - a Review" (22 Sept 2015), by Asif ud-Doula and Yael Nazé, on page 10:
"3.4. Structure of confined winds, as revealed by high-resolution spectra
High-resolution spectra can yield a wealth of information. With current instrumentation, line widths and shifts can be evaluated with precisions down to a few tens km s$^{−1}$ in the most favorable cases (a few hundreds km s$^{−1}$ more typically). Furthermore, the comparison of lines from H-like and He-like ions and of components of $f ir$ triplets of He-like ions constrain the temperature and location of the emitting plasma. However, such measurements are currently only possible for the brightest X-ray sources, so that few magnetic massive stars have been investigated
in this respect (τ Sco - Mewe et al. 2003; Cohen et al. 2003, θ$^1\!$ Ori C - Schulz et al. 2000; Gagné et al. 2005a, b, HD 191612 - Nazé et al. 2007, HD 148937 - Nazé et al. 2008, 2012, β Cep - Favata et al. 2009, IQ Aur - Robrade and Schmitt 2011).
Within noise limitations, the X-ray lines of magnetic massive stars were found to be symmetric, and globally unshifted. This agrees well with MHD models. In the case of θ$^1\!$ Ori C, global fitting however suggests small variations in velocity (Gagné et al., 2005a): from −75 km s$^{−1}$ when the star is seen pole-on to about 100 km s$^{−1}$ when seen edge-on. This change needs to be confirmed as the errors are large but also because one cannot exclude a stochastic variation when only a single observation per phase is available. If further observations provide evidence that velocity varies with phase, then refinement of models will be needed, as no such changes are currently predicted (Gagné et al., 2005a).
Reported widths of X-ray lines largely depend on object and ion considered. The narrowest widths, so far, were found for β Cep, whose lines are dominated by instrumental resolution, yielding only upper limit on intrinsic widths (<600 km s$^{−1}$, Favata et al., 2009). Larger widths, full width at half-maximum (FW HM) ∼ 600 − 800 km s$^{−1}$, were reported for ions with high ionization potential (Mg, Si, S) in τ Sco, θ$^1\!$ Ori C, and HD 148937, three stars with more rapid winds than β Cep. Such widths are much smaller than observed for “normal” O-type stars (FW HM ∼ v∞), where lines arise in embedded wind shocks distributed all over the wind, hence cover a larger velocity range. They indicate formation in slowly moving plasma, in agreement with the confined winds scenario. However, most MHD models predict even narrower lines (Gagné et al., 2005a).
Furthermore, lines from ions with lower ionization potential, notably oxygen, appear broader (FW HM ∼ 1800−2000 km s$^{−1}$ Gagné et al., 2005a; Nazé et al., 2007, 2008). These lines are associated with cooler plasma, which could have a different origin than hotter plasma. For example,
the dominant hot plasma in θ$^1\!$ Ori C is thought to arise in confined winds while the cooler one could arise in embedded wind shocks as in normal O-stars (Gagné et al., 2005a). This dual origin could be supported by the different temperatures derived from the different ions (Schulz et al., 2000). In Of?p stars, however, the spectra are dominated by the cooler component, i.e. confined winds emit soft X-rays in these objects (see above), but it cannot be excluded that current errors, which are large, are somewhat blurring the picture.
In He-like triplets, the forbidden line is suppressed when the density is high or the UV radiation is intense. In the case of massive stars, the latter effect is the most important one and, thanks to dilution with distance, enables us to locate the emitting region. In τ Sco, θ$^1\!$ Ori C, HD 148937, and β Cep, the start of the emitting region is found to be close to the photosphere, at radii r ∼ 1.5 − 3 R$_∗$ for the first three stars and r ∼ 4 − 6 R$_∗$ for the latter case. These values are only slightly lower than the corresponding Alfvén radii of these stars, they thus appear qualitatively compatible with MHD simulations. In IQ Aur, the forbidden line was found to be normal, suggesting a formation radius larger than 7R$_∗$ (Robrade and Schmitt, 2011) - despite a large value, this also appears compatible with the supposed location of confined winds in this star.".
Other reading:
"The surprising magnetic topology of $\tau$ Sco: fossil remnant or dynamo output?" (7 Jun 2006), by JF Donati, ID Howarth, MM Jardine, P Petit, C Catala, JD Landstreet, JC Bouret, E Alecian, JR Barnes, T Forveille, F Paletou, and N Manset
"The magnetic field and confined wind of the O star θ$^{1\!}$ Orionis C" (26 Jan 2006), by G.A. Wade, A.W. Fullerton, J.-F. Donati, J.D. Landstreet, P. Petit, S. Strasser
"X-ray hardness ratios for stars of different spectral types" (2006), by Meurs, E. J. A., Casey, P., & Norci, L.
Wikipedia - Zeeman Effect
"Magnetic Fields of the A-Type Stars" (30 Apr 1958), by H. W. Babcock


Question about space-time In the Big Bang model of the Universe, every observable thing is thought to have expanded very abruptly from a point of infinite density and zero volume. However, the problem with this assertion is that for something to expand it first needs a locality in which to do such, which brings us to my question. How can scientists believe that "everything" was condensed and then suddenly expanded, since such is automatically an oxymoron?

Expansion is a physical property of the universe and yet if "everything" had no where to expand, how could it have expanded? There was "nothing," was there not? Even if we drop the big bang model and talk about "everything's" being in a singularity point, the question still stands.

Where was the singularity point? Asked differently, into what is everything expanding (which is a rhetorical question I ask to implicate space-time as necessarily existing irrespective of any theoretical expansion and likely so vastly that one might as well assume that space-time is infinitely large).

As well, it seems counter-intuitive, as little impressive a word as that is given its context here, that something like... well, "everything" could be condensed in the first place, let alone infinitely condensed and with zero volume. When thinking about the big-bang model, I can't help but conclude that a mere analogy was originally used for stellar birth, evolution and finally explosion. The issue though is still extension.

As far as I understood it, the big bang model states that space itself was also condensed along with everything else in the observable universe to a point of singularity. So, if that's the case, then [where] was the singularity point?

It ends up becoming an infinite regress, of course. Or, if you don't like the issue of "time" presupposed by my question, then look at the question after the universe started expanding. All the matter had to expand somewhere and "somewhere" is not "nothing." - It just seems that the model is somehow overlooking said feature of the actual universe and trying to account for it by mere assertion.

If you want to call it space-time, extension, space, where everything supposedly expanded to and is and is still expanding to, then that's my question. How does the theory allege where everything is yet at the same time assert that prior to the singularity point, there was literally nothing.

In the big bang model of the Universe, every observable thing is thought to have expanded very abruptly from a point of infinite density and zero volume.

Thats not quite true. Have a look at Wikipedia where you can read this:

"Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.[18] This singularity signals the breakdown of general relativity and thus, all the laws of physics. How closely this can be extrapolated toward the singularity is debated—certainly no closer than the end of the Planck epoch. This singularity is sometimes called "the Big Bang",[19] but the term can also refer to the early hot, dense phase itself,[20][notes 1] which can be considered the "birth" of our universe."

You can't necessarily extrapolate back all the way to a point singularity.

  However, the problem with this assertion is that for something to expand it first needs a locality in which to do such, which brings us to my question.

There's no problem with space expanding. Don't think of space as nothing, Einstein didn't, and nor do modern physicists such as Nobel prizewinner Robert B Laughlin.

  How can scientists believe that "everything" was condensed and then suddenly expanded, since such is automatically an oxymoron?

Because there's good scientific evidence that the universe is expanding. And when you "wind back the clock" you find that all the galaxies were in the same place 13.8 billion years ago.

  Expansion is a physical property of the universe and yet if "everything" had no where to expand, how could it have expanded? There was "nothing," was there not?

No. Space expanded, and space isn't nothing. As to what caused the big bang to occur, I don't know. Nor do I know what the universe was like 13.8 billion years ago. But I know of no infinities in nature, and I would challenge the alleged point singularity at the centre of a black hole and refer instead to the frozen star interpretation. In similar vein I would challenge the alleged point-singularity 13.8 billion years ago.

  Even if we drop the big bang model and talk about "everything's" being in a singularity point, the question still stands. Where was the singularity point?

I think you shouldn't drop the big bang model, but instead you should drop the singularity.

  Asked differently, into what is everything expanding?

Nothing. I know this is hard to imagine, but try to. Space is expanding, and there is no space beyond it. So there is no beyond it. As to whether this means you end up coming back round full circle or the universe is some "hall of mirrors" or something else, I don't know.

  (which is a rhetorical question I ask to implicate space-time as necessarily existing irrespective of any theoretical expansion and likely so vastly that one might as well assume that space-time is infinitely large).

No, one might not. Because the universe doesn't expand for nothing. It expands because space has a kind of innate "pressure", as per the energy-pressure diagonal in the stress-energy-momentum tensor. And if the universe was infinite, the pressure would be counterbalanced at all locations. An infinite universe could not expand.

  When thinking about the big-bang model, I can't help but conclude that a mere analogy was originally used

For an analogy, get a stress ball and squeeze it down in your fist. Then let go.

  The issue though is still extension. As far as I understood it, the big bang model states that space itself was also condensed along with everything else in the observable universe to a point of singularity.

See what I said above. Big bang cosmology doesn't actually say everything was condensed to point singularity.

  All the matter had to expand somewhere

There wasn't any matter initially. Remember the wave nature of matter, and pair production. We can quite literally make matter out of electromagnetic waves. These wave propagate through space, and are effectively wave sin space. So we make matter out of waves in space. So start with space, and do somehting to it such that it rings like a bell and is full of waves and starts expanding. As to what, I don't know.

  How does the theory allege where everything is yet at the same time assert that prior to the singularity point, there was literally nothing?

It doesn't. It goes back as far as a very dense universe. The stuff you hear about a point singularity and creation ex-nihilo is popscience, not big bang cosmology.

The Big Bang did not happen at a single point, nor did the Universe begin at a single point. The Universe is, and has always been, infinite. The Big Bang was just when the Universe's expansion really began — that is, when objects started drifting away from each other. The Universe was still infinite, but there was less space between the matter.

This density caused the Universe to get extremely hot and expand. So here, space itself was distorted and began to expand. More and more space was created between the matter, and still is now (although now it is mainly due to dark energy instead of heat).


Can two heavy objects circling around their C.M. be separated because of the speed of gravity? Imagine two massive objects, with the same mass (M) circling around their center of mass (C.M.). Let's assume that the distance between them is 1 light hour. Don´t the two bodies get accelerated and move away from each other because they feel the gravity of each other as it was 1 hour ago, because of which a force tangent to the direction of the speed develops?
Can it be that gravity acts instantaneously? I've read about experiments proving the speed of gravity to be equal to the speed of light, but these were disputed.
Isn't there both in Newtonian mechanics and GR there a time delay? The difference being though that space and time in Newtonian mechanics are separate and absolute (implying instantaneity?), while in GR the both are a whole and absolute.

Can two heavy objects circling around their C.M. be separated because of the speed of gravity?

No. Newtonian mechanics does quite well assuming instantaneous gravity. You can get relativistically correct orbits either by doing a complete GR calculation, or using a simple approximation as already eluded alluded to in @antispinward's answer.
This excellent answer to Besides retarded gravitation, anything else to worry about when calculating MU69's orbit from scratch? in Space Exploration SE explains that we get the wrong answer if purely Newtonian mechanics is used except for slowing down the speed of gravity, but that's because one is neither treating gravity correctly nor using Newtonian mechanics correctly.
Many answers to How to calculate the planets and moons beyond Newtons's gravitational force? include a way to treat this problem using a well-accepted approximate treatment of general relativity.
From this answer:

The acceleration of a body in the gravitation field of another body of standard gravitational parameter $GM$ can be written:
$$\mathbf{a_{Newton}} = -GM \frac{\mathbf{r}}{|r|^3},$$
where $r$ is the vector from the body $M$ to the body who's acceleration is being calculated. Remember that in Newtonian mechanics the acceleration of each body depends only on the mass of the other body, even though the force depends on both masses, because the first mass cancels out by $a=F/m$.

and later:

The following approximation should be added to the Newtonian term:
$$\mathbf{a_{GR}} = GM \frac{1}{c^2 |r|^3}\left(4 GM \frac{\mathbf{r}}{|r|} - (\mathbf{v} \cdot \mathbf{v}) \mathbf{r} + 4 (\mathbf{r} \cdot \mathbf{v}) \mathbf{v} \right),$$

It is not only possible, we have almost this situation in our neighborhood. Alpha Centauri only 4 lightyears away with two Stars. Alpha Centauri A and B. A has 1.1 times the mass of the sunn and B 0.9. This is very similar. And the distance is is 11 AU. 1.5 ~ 90 Lightminutes.
Alpha Centauri Wikipedia


How can we calculate velocity of an object given the distance from an object in parsecs? If you had the distance from a celestial object, in parsecs, how would you calculate the velocity of the said object?

Edit: I'm sorry I don't have more context, the specific questions is: "A galaxy that is 10 million parsecs away will be moving at what velocity?"

You can't, not very well anyway. That's simply because the velocity of an object isn't calculable from the Hubble constant until the so-called Hubble flow starts to dominate (which in turn only happens at cosmological distances, i.e. very far away).

That said, if you are in the regime where Hubble flow dominates, you can do this easily. The Hubble constant is approximately $70 km/s/Mpc$. Just convert your distance into megaparsecs, and multiply. You end up with a velocity, which is the one you're looking for (this neglects peculiar velocity, which will affect everything in the sky).

If the object is moving directly towards or away from us & we can get a spectrum,the Doppler shift will give us its velocity. Blue shift means it's approaching us,reshift means it's receding. If the object is orbiting some larger body with a known mass,the period of its orbit will indicate velocity (if the orbit is elliptical,its velocity will vary,speeding up as it gets nearer to the larger body). If its distance is neither increasing nor decreasing,we can track its motion within a certain timescale to calculate its velocity. Sometimes more than one method may need to be used.


Black hole darkness a result of gravity or temporal distortion? Please correct me if I am wrong as I may have made some incorrect assumptions.

Okay so we know that at some stage of "nearness to a black hole", light is no longer reflected back at us from the black hole itself. Some scientists have proposed the speed of light and it's relationship to relativity is natures way of adjusting the rate at which time passes seemingly to disallow "weirdness that could result" .. for example the fast moving object would become more massive.

We also know that the mass of the black hole causes a nearby observer to behave as if he/she were moving at incredible speeds and subsequently slows time for the observer "relatively speaking of course".

I realize that from the perspective of the observer light is traveling to and away from him/her at it's normal rate even though effectively the observer is traveling very fast. Imagine however what would happen to the frequency of the light. The red/blue shift would undoubtedly be obscene.

Since we here on earth are "relatively speaking" experiencing time passing at our normal "earth rate". I can't help but imagine that immense gravity near a black hole is distorting time and its darkness is a result of either extreme red/blue shift or the light is somehow phased or altered by the temporal distortion. Yes gravity is the origin of the darkness but is the direct cause or an indirect relationship where the rate of time passing is the culprit?

As an object falls towards a black hole, light emitted by the object will become red-shifted by the gravity. As photons rise out of the gravity well of the black hole they lose energy, they can't slow down but their wavelength is stretched. The object will go dark as it approaches the event horizon due to gravitational redshifting.

Photons starting from the event horizon would be redshifted to zero energy, and no photons can travel from inside the event horizon.

We also will observe gravitational time dilation, the last second before the object reaches the event horizon will be stretched out indefinitely. The two effects are all part of the same relativistic theory of gravity. You can't unpick the two effects.

Gravity is in fact a side-effect or result of spacetime distortion.

Consider this simple diagram:

The blue grid representing timespace, and the orange sphere being a high-mass object.

Moving through space means going from one tile to another, so when light passes into the distorted dip in spacetime it not only has to simply pass from square to square, but do so whilst those squares are being pulled in towards the massive object.
For a planet or average star, this distortion is negligible as far as light is concerned, but the dip a Black Hole would make would be many, many times deeper than the above diagram - space would be flowing in to the hole as fast, or faster than light can travel along said space to pass through it.

Like trying to climb up a downwards escalator that always moves faster than you can climb it.

But spacetime flows, it should eventually be dragged out, like pulling a table cloth out from under a load of dishes, but the mass of a black hole is so great, timespace essentially becomes pinned, like jamming your fork into the tablecloth as your friend tries to pull it away; time comes to a standstill at the epicentre of the hole.

It's important to note that at no point does light slow down travelling through the vacuum of space here, it's just space is moving in the opposing direction preventing the light from escaping in any meaningful amount beyond the expected level of Hawking Radiation.


Why don't black holes quickly eat up the galaxy/universe? It's my understanding that the radius of the event horizon of a black hole is proportional to it's mass.
Which means the surface area of a black hole is proportional to it's mass squared.
Lets just assume that black holes eat stuff that by chance "runs into them".
Using this (perhaps naive) assumption, this would mean black holes gain mass at a rate proportional to their surface area, which is the same as their radius squared, which is their mass squared.
So then we get something like:
$\frac{dm}{dt} \propto m^2$
$m = 1 / (C - t)$
Which means that all black holes should grow to an infinite size in a finite length of time.
Indeed they will eventually get to a point where even small amounts of mass will grow the volume of the black hole so much (because volume increase is proportional to the current mass squared) and even if the density of the universe is very low this will eventually mean any growth will set off runaway growth because each atom the black hole swallows increases it's volume such that on average that new volume contains a new atom.
Is it just that the constant factors here are very high, or are one of my assumption quite incorrect? I say quite incorrect because even if they're slightly incorrect (the growth rate is proportional to mass not mass squared) many of these issues still occur on long enough timescales.

Your rough analysis is basically correct; everything may well end up in black holes eventually. But the timescale for that is very long indeed.
The cross-section for direct interaction as you describe is vanishingly small. The diameter of a ten solar mass black hole is 60 km. The rate at which it scoops up interstellar material from a cylinder of diameter 60 km is totally negligible. e.g. A speed of 10 km/s with respect to interstellar gas and a typical gas density of $10^6$ H atoms per cubic metre, would result in an accretion rate of about 1.5 kg/year.
The chances of direct star to star collisions in a galaxy is exceedingly rare, so the chances of a wandering star crashing directly into a black hole are even smaller.
With supermassive black holes, the issue with your analysis is that they sit in the centres of galaxies and stars do not encounter them by chance. The stars and gas in a galaxy have angular momentum that must be conserved. This takes them on orbits which do not go near the black hole unless perturbed in some way. These supermassive black holes feed while they can, but when all the easily available matter has been consumed, they become dormant. This is one of the reasons that the peak of quasar activity was about 10-12 billion years ago.

Because they're gravitational force isn't strong enough. They need to take some time to eat, like us if we are eating a Gigantic Burger. But for black holes its like us eating a continent.


Escaping a black hole I often hear that nothing can escape a black hole because its "escape velocity" is greater than c. If that is accurate, what about the following? I know that the following has a lot of most likely impossible assumptions, but I'm trying to understand whether the escape velocity explanation makes sense.
Suppose you've got some sort of space ship that's inside the event horizon, and it hasn't been destroyed by the gravity or tidal forces. Also suppose that you've got a whole lot of reaction mass and a fantastic power supply.
Could this space ship, through applying enough constant force escape the event horizon?

One of the clearest way to see what's going on is to look at a Penrose diagram of a Schwarzschild black hole. A Penrose diagram is like a map of the spacetime drawn in such a way as to preserve angles and put the light rays diagonally at $45^\circ$ angles, forming the light cones.

Since we're mapping all of an infinite space(time) into a finite drawing, distances are necessarily distorted, but that's a small price to pay.

Time goes up on the diagram, and a typical infalling trajectory is in blue.

Because every massive object must be locally slower than light, it must stay within those light cones. Hence, no matter how you accelerate, at every point of your trajectory you must go in a direction that stays within those $45^\circ$ diagonals at that point. But once you're inside the horizon, every direction that stays within the light cones leads to the singularity.

Accelerating this way or that just means that you get to choose where you hit the singularity--a little on the left on the diagram or a little to the right. Trying to escape using high acceleration brings you closer to the light's $45^\circ$ lines, which because of time dilation will actually shorten your lifespan. You'll actually get to the singularity sooner if you struggle in that manner.

This particular image came from Prof. Andrew Hamilton. Note that it pictures an eternal Schwarzschild black hole, i.e. one that has always and will exist. An actual black hole is formed by stellar collapse and will eventually evaporate, so its diagram will be slightly different (in particular, there will be no "antihorizon"). However, in all respects relevant here, it's the same situation.

Although I partially agree with @Alexander here are a few more things.

According to Einstein's special theory of relativity, particles cannot travel at the speed of light (c). It would take infinite energy as stated above. However, the special theory of relativity states that an object can travel slower or faster than the speed of light but not at the speed of c. We may refer to the objects that travel faster than the speed of light as tachyons. There are celestial objects which are accelerated faster than the speed of light.

A common thing that everyone says is that nothing can escape a blackhole. But it is totally wrong. Blackholes do emit particles and radiation called as Hawking radiation. Even stephen hawking has changed  his mind about blackholes as the general concept that blackholes absorb everything can be disproved by quantum mechanics theories.

So, If there exists a possibility that we can indeed escape a blackhole.

At CERN, reports says that ghostly subatomic  particles called neutrinos were observed to travel faster than light. Researches are still trying to confirm it.

There is also another possible theory existing around the scientific community is the concept of wormholes. If a ship were trying to escape a blackhole, opening a wormhole to escape the blackhole and reaching another spot is a proposed theory.

There are also researches that say that objects are emanated from blackhole.


If Venus and Mars changed places, would we then have 3 habitable planets? This is the supposition:
Venus is too hot, Mars is too cold. If they switched places then Solar energy would change to make both more Earth like. We might as well have had three instead of just one "Earth" in the Solar system, if the roulette of early planet formation had played out just a little bit different.

That's of course too simple. But wouldn't it help alot if it were the case from the beginning that Venus had had Mars' mass, and Mars had had Venus' mass? Or do other factors dominate mass and distance from the Sun?

Would Venus still have had a thick, but not too thick, atmosphere if it were at 1.5 AU, because it has mass enough to keep one, and wouldn't that have kept it warmer than Mars is today? Including flowing water on its surface under its atmospheric pressure?

Would a Mars at 0.7 AU have been warmer and maybe have had a passing atmosphere created from melting volatiles during a longer era in its history?

This is a very interesting question. Of course, as you noted, you have simplified things quite a bit; there are other factors besides temperature that affect habitability.

Regarding Venus, you probably know that Venus is extremely hot at its surface not just because it is closer to the Sun, but because it has a thick CO2 atmosphere and is warmed by the greenhouse effect. There are, in fact, two things about Venus that would prevent it from being habitable wherever you put it. One is the lack of a magnetosphere, which is necessary to prevent ionizing radiation (particularly from the solar wind) from reaching the planet's surface. The lack of a magnetosphere appears to be due to the lack of a geodynamo on Venus, which has to do with the structure of its core. Second, Venus appears to lack tectonic plates, which you may know are responsible for earthquakes here on Earth. Interestingly, tectonic plates play a major role in controlling the buildup of CO2 in the atmosphere (see here for details). Lacking tectonic plates, Venus is doomed to have a large CO2 atmosphere wherever you put it, which would not make it a nice place to live.

Mars, on the other hand, is a very different matter. It has both a magnetosphere (albeit it is very weak) and it likely has tectonic plates (although last I heard it is thought to only have two). One of the reasons that NASA has sent so many probes to Mars is that it was thought to be habitable at one point. It is thought that Mars's growth was stunted because of gravitation effects from Jupiter and Saturn. So, in another universe, Mars could have ended up very much like another Earth.

If Mars was placed near the Sun and Venus relocated to Mars' current orbit, neither one would be habitable. To make both Mars and Venus habitable(in Theory), a large lunar moon is required because it would cause a tidal crust movement that would help to ignite the planet's magnetic field. For Venus, Mercury would become its new moon as it could not only ignite a strong magnetic field, but it could correct the planet's rotation by making it rotate in a normal 24hr Earth rotation, and allow it CO2 atmosphere to cool down as it will be able to recycle it into the ground. As for Mars, Jupiter's moon Io would be ideal as it can increase the planet's surface pressure and magnetic field. Both Venus and Mars are capable of terraforming by themselves. All that's needed is a large lunar moon like Io and Mercury respectively.


How could a neutron star collapse into a black hole? White dwarfs usually do not collapse, as they have electron degeneracy pressure due to the Pauli exclusion principle. However, if one accretes mass beyond the Chandrasekhar limit, it is energetically favorable for the electrons to combine with protons and form neutrons. This gives us a neutron star.

However, neutron stars usually do not collapse into black holes due to neutron degeneracy pressure. How is it possible that beyond the LOV limit, the Pauli exclusion principle no longer prevents the collapse? Shouldn't it still prevent neutrons, which are fermions, from being compressed together any further?

I've seen answers involving quark stars, but those are purely hypothetical. What is the most accepted explanation for this?

The scenario you describe may occur. On the other hand it may actually be that neutronisation in a white dwarf is the trigger for a thermonuclear type Ia supernova.

You may be misunderstanding the Pauli Exclusion Principle (PEP).The PEP states that no two fermions can occupy the same quantum state, not that they cannot occupy the same space or be compressed to whatever density you like. The quantum states here consist of two spin states for every possible momentum state. In a degenerate gas, all these states are filled up to the Fermi energy. All that happens when the neutron star gets smaller (or collapses), is that the Fermi energy just keeps increasing as the neutron density climbs, and the neutron degeneracy pressure just keeps increasing as a consequence.

However, in General Relativity, pressure (like mass/energy) is a source of gravitational curvature and actually increases the required pressure gradient needed to support the star. At a certain threshold radius - a small factor larger than the Schwarzschild radius, a point of instability is reached where increasing the pressure is actually counter-productive.  Beyond this, you can make the pressure as large as you like and it will not prevent the formation of a black hole.

Even inside the BH there is not necessarily a problem with the PEP. You can compress fermions to infinite density so long as they can have infinite momentum.

To create a black hole a body must occupy a radius that is smaller than its Schwarzschild limit. If a body has a mass greater than 3.2 solar masses then the theoretical behaviour of the neutron is that the force acting upon the neutrons due to the pressure of the collective mass of the star would cause the neutron to collapse. Then the way that a neutron star would form a black hole is by having a companion star which looses matter to the neutron star. If the mass of the neutron star is close to the mass of 3.2 solar masses then the star will theoretically collapse into a black hole when enough matter is accreted to exceed the critical limit.

The caveat is that there is no force in the universe that can accelerate  energy faster than the speed of light. When neutron collapses it’s surface must accelerate at a rate that exceeds the speed of light, so this in fact is not allowed. An alternate theory is that the matter does not collapse into a black hole nut rather exists at an energy density that is just above its Schwartzschild limit. In fact recent observations made by Ligo of colliding black holes and by the recent observations of Sagittarius A*'s G2 gas cloud collision in fact fits much more closely with this model than these object being in fact true black holes. When the G2 gas cloud interacted with Sagittarius A* it did not create a large output of energy, but rather there was a just small release of energy consistent with a black star that exists at an energy state just above its true event horizon. These objects do not emit light and so are virtually indistinguishable from black holes, except when they interact with other objects.

When LIGO first observed a gravitational wave there was a small gamma ray burst 0.04 seconds after the gravitational wave struck. There should be no Gamma Ray Bursts when black holes merge. There was a paper written in June of 2016 that used statistical models to suggest that the GRB did not occur. (Someone's job was at stake.) When you look at the method they used it virtually eliminates any possible observation of a GRB that would be associated with a dark star merger due to the weak GRB it would produce. Thus, no black hole merger could be observed in the future associated with a gravitational wave observation, unless it as a neutron star merger, which produces a much larger GRB. Recently LIGO announced just such an event where two neutron stars merged and this coincided with a GRB. When asked why this has not been observed before the officials at LIGO said it was because the merging of black holes do not produce GRBs.

The problem with creating black holes is that the matter at the event horizon, or simply the energy, must accelerate faster than the speed of light, which is not allowed by relativity. The comeback is that spacetime itself is collapsing faster than the speed of light. One must then ask how this could happen if the relativity does not allow super luminous speeds. To accelerate spacetime to super luminous speeds something must have dragged spacetime to this acceleration. But, that is not allowed. In fact what the observations are really showing is that it is most probable that black holes do not exist in nature. To back this up the search for extraterrestrial planets has found zero black holes orbiting other stars. Since 1968 only about 20 stellar mass black hole candidates have been found anywhere. If they exist where are they? Every observation made is consistent with black stars and only mathematical models are consistent with black holes.

WE must also consider that the present theory predicts that black holes will explode at the end of their lifetimes as they evaporate. No spikes of energy of this nature has been observed and yet there should some if the theory is correct. We must also consider that CERN should have produced black holes at the energy levels they have employed and no black holes have been created. This was not expected.

Black stars paper.
https://arxiv.org/pdf/1611.03853.pdf

A better solution to the runaway endpoint problem
https://journals.aps.org/prd/abstract/10.1103/PhysRevD.77.044032

The statistical trick and logic used to eliminate future black star merger observations

http://iopscience.iop.org/article/10.3847/2041-8205/827/2/L38/meta;jsessionid=5902D1B053AA93E11FDDF2A02A79BFBF.ip-10-40-2-120

The non-event of the Sagittarius A* G2 collision
http://earthsky.org/space/milky-ways-black-hole-more-active


How to find the source of the light? There are millions of galaxies out there in the entire universe. When a telescope is set to a direction to to identify the stellar objects, it can capture all lights from all the objects. How do we identity that the particular light is emitted from one star/galaxy and not the other one?

Even if we precisely point to one star, there are chances that other luminous objects interfere with the light ray.

The positions of many galaxies are recorded, so astronomers can point their telescopes to the exact point in the sky that they wish to observe. In addition,the relatively nearby galaxies can be recognised by their appearance,& these are the galaxies most intensively studied. Professional astronomers have memorised most of the stars you will see in the night sky, and the constellations (Sagittarius, Ursa Major, Orion etc) guide them to their target. Those galaxies near the limit of observation are of lttle interest except when something unusual & very luminous happens within them,such as a supernova or gamma ray burst.


Meaningful theories of the shape of the universe Normally, when people say "the universe is infinite" they generally mean something like "the observable universe is locally flat, it is easy to assume that it is flat way beyond the observable universe, I don't know what lies beyond that, I won't talk about that, that is enough for practical purposes" (or any other proposition implying infinity in any direction and assumed for simplicity's sake while actually focussing on a small region of the space, the observable universe). So, "the universe is infinite" does not seriously mean that physical space goes on forever because this simply makes no sense at all, "infinite" is not a number, it is not a magnitude, it is not a property of real things (if you disagree with this premise please explain how the notion of infinity can be seriously applied to any direction of physical space, i.e. how you can seriously think to extend, sic et simpliciter, such a property of your mathematical model to the real world). (Personally I don't think this should be counted as a theory of the shape of the universe at all).

On the contrary, "the universe is finite and unbounded, compact, closed in all directions" is indeed a meaningful and in principle acceptable theory because it can be thought, described, discussed for what it actually implies. Of course, we don't know what lies beyond the observable universe but the idea of a compact universe makes sense and it seems to me this is the only meaningful explanation currently available of what "could" be (it is not necessarily the right, true explanation, of course).

Is there any other meaningful alternative (which can be thought, described, discussed)? Is there any alternative to the dichotomy "finite universe" vs "infinite universe"? Is there anybody out there working on models that overcome this dichotomy?

I disagree with your premise. First, the universe being infinite does not imply anything at all about its spatial curvature without additional assumptions. Second, even with the usual cosmological assumptions of large-scale homogeneity and isotropy, being infinite would not imply spatial flatness, as negative curvature would still be possible.

  On the contrary, "the universe is finite and unbounded, compact, closed in all directions" is indeed a meaningful and acceptable theory.

This this alternative still makes grand assumptions about the structure of the universe beyond what we can observe, I don't see why you attribute more 'meaning' or 'acceptability' to it. At the end of the day, all we really know is that the bit of the universe around us is very close to being flat. Any statement whatsoever about the larger structure of the universe is going to introduce some assumptions not backed by observations, and you haven't provided much guidance to what kind are 'meaningful and acceptable'.

  Is there any other meaningful alternative?

You seem to be using that word in an idiosyncratic sense I'm not familiar with. In any case, you're mistaken about what "the universe is infinite" implies.

When something explodes in a concentric manner it would require two opposing planes of like polarity to "sandwich" the exploding matter into the shape of an expanding flat plane. So what is your theory on what these sandwiching forces could be?


Does the observed period of a pulsar change with the time of year? Over in the Physics SE a question was posted asking about the difference in the time dilation of the Earth between perihelion and aphelion:

Does Earth experience any significant, measurable time dilation at perihelion?

Rather to my surprise it turns out that because of the changes in the Earth-Sun distance and the Earth's orbital velocity there is a difference of about $60\mu$s per day between the two extremes.

A commentator pointed out that pulsars can be measured accurately enough to detect this difference. However I have never heard of a pulsar measurement having to be corrected for the time of year, and Googling has found me nothing related. I would be interested to know if this is something that needs to be considered.

The difference is slightly over one part in $10^9$, so presumably it depends on whether pulsars can be timed this accurately.

Yes. In terms of pulsar timing measurements, this is a massive effect! A +/- 30 km/s doppler shift changes the pulsar frequency by +/- 1 part in 10000. This sounds small, but the accumulated phase shift over many periods is readily apparent. In addition, the light travel time across the solar system has to be taken into account, as well as the Earth's rotation and some other smaller effects - such as Shapiro delay.

If the question refers to the specific annually varying difference in clock rates caused by the different gravitational potential experienced by an Earthbound telescope on an elliptical (as opposed to circular) orbit -the answer is still yes.

This is item 4 in the list of applied corrections given on p.52 of "Pulsar Astronomy" by Lyne et al. The maximum effect is a rate change of $3\times 10^{-9}$ leading to a maximum lead or lag of 1.7 ms.

It depends on the quantity of matter surrounding the pulsar. If the surrounding matter gets too close, via accretion, it will increase its rotational rate, making the pulse change.*


Can gravity cause a Big Crunch, if the cosmological principle holds? Say we have a closed universe that follows the cosmological principle. As a result, mass is distributed equally and, if as far as I know, gravity should have no net effect on the movements of galaxies (as in, there are no peculiar velocities).

However, in the Big Crunch, the Universe stops expanding and gravity causes the scale factor to decrease. How is this possible? Shouldn't gravity have no effect on a homogenous and isotropic universe?

On that note, if gravity actually caused a collapse, would gravity cause the comoving coordinates of the galaxies to change?

You are thinking about gravity and how it affects the universe as a whole in the wrong way.

The dynamics of the universe are governed by the Friedmann equations, which can be solved to say how the scale factor of a homogeneous, isotropic universe behaves with time.

If there is enough gravitating matter/energy density in the universe then the scale factor may eventually decrease to zero in a finite time. The simplest case is a closed universe with no cosmological constant and a mass density greater than the critical density (which is not the universe we currently inhabit: our universe appears to be (a) flat and (b) expanding at an accelerating rate).

The interpretation of this is that the space between the galaxies is contracting. It is the reversal of what we currently observe in the expanding universe.

In the same way that you do not interpret the universal expansion of a homogeneous, isotropic universe as some sort of repulsion between galaxies; neither would the contraction be interpreted as some sort of attractive force between galaxies. As you rightly point out, in a strictly Newtonian sense, there would be no net gravitational force on any one galaxy and so its co-moving coordinates would be unchanged during the contraction.

Can gravity cause a Big Crunch, if the cosmological principle holds?

No. That's a myth that grew out of the Friedmann equations. Alexander Friedmann was a ballistics instructor on the Austrian front. He modelled the expanding universe as something like a cannonball fired aloft. See John Peacocks’s cosmological physics for an example: “the dynamics of the entire universe are the same as those of a cannonball fired vertically against the Earth’s gravity. Just as the Earth’s gravity defines an escape velocity for projectiles, so a universe that expands sufficiently fast will continue to expand forever. Conversely, for a given rate of expansion there is a critical density that will bring the expansion asymptotically to a halt”.

                                                                                Image from Norbert Rumiano’s article a cosmological model

It’s totally wrong I'm afraid. Gravity doesn’t stop the universe expanding. A gravitational field is akin to a pressure-gradient in space. It makes matter fall down, but it doesn’t make space fall down. You can tell it’s wrong because Big Bang cosmology is said to be balanced on a knife edge, such that the density of the universe is incredibly close to the critical density. That’s just a re-heat of Einstein’s greatest blunder, wherein a slight increase in density would trigger contraction, and a slight decrease would trigger expansion. It’s also wrong because the 1998 supernova observations didn’t match any of the cannonball trajectories. They didn’t measure the expansion slowing down, they measured it speeding up. Like the ascending light beam speeds up, because the speed of light is spatially variable. Unfortunately people like John Moffat and João Magueijo and other VSL guys said the speed of light in the universe is slowing down, not speeding up. Unfortunately the Friedmann equations feature not just a fixed c, but a curvature k. Even though a gravitational field is a place where the speed of light varies because space is neither homogeneous nor isotropic. Not a place where space is curved.

  Say we have a closed universe that follows the cosmological principle.

We have no evidence that the universe is closed, or that the cosmological principle is scientific. I think it's rather like saying I live in a forest, so everybody must see trees.

  As a result, mass is distributed equally and, if as far as I know, gravity should have no net effect on the movements of galaxies (as in, there are no peculiar velocities).

The issue isn't with galaxies, it's with space. Space is expanding like the raisin cake analogy, and gravity won't stop that.

  However, in the Big Crunch, the Universe stops expanding and gravity causes the scale factor to decrease. How is this possible? Shouldn't gravity have no effect on a homogenous and isotropic universe?

I don't think it is possible. Note that Einstein described a gravitational field as a place where space was neither homogeneous nor isotropic. So a homogeneous isotropic universe is one where there is no gravity.

  On that note, if gravity actually caused a collapse, would gravity cause the comoving coordinates of the galaxies to change?

I would say no. I'd also say gravity doesn't cause a collapse.


"Up" and "down" in Space From my understanding, "up" and "down" in space is going towards (down) an object's gravitational pull or (up) which is going away from it. I get confused with this explanation, and I believe simply because of how we perceive up and down here on earth.

My question(s) is this: Can we go in all directions in space with the "Earth" interpretation of up, down, left, right, north west, south east, etc if we can get past gravitational pulls of objects in space?

Looking at my Gimp version of the universe, I have the Earth and stars. So going up would essentially be go towards any of the stars in the picture that has the strongest gravitational pull, although each object are on different planes?

In relation to my question above, I have imagined Earth in space as to like the 10 floor on an infinite elevator that can go in all directions. Say we can somehow go through star A and escape it's gravitational pull, we would be essentially "going down" right?

Now my other question: Are there objects in every single direction is space? How about in the Bootes void? Since it's such a massive void and there isn't much objects around to pull you in "down" or go away from "up", essentially can't we go in any direction?

If it may seems that my question about this is kind of confusing, I apologize because I'm confused about this myself!

I think that one thing confusing you, is that up/down is a 2 dimensions concept (just like your image by the way), but the Universe is in 3 physical dimensions (let's not talk about time here).

So when you say « going up from Earth is extracting from it's gravity », it can be in many direction. If two persons are in North Pole and South Pole of Earth, and if they are both going up. They are in fact going in opposite directions.

The second point is that you need a frame of reference. You go up/down « to » or « from » something.
For example, you go up (going away) from the North Pole, but from Sun's or Moon's point of view, it's not going up at all.

The up/down/left/right concepts always need a reference. See this very simple example: If we are facing each other, what is on my left, is on your right.
Your question here is exactly like asking : « What is on my left in the universe ? ». There is no real answer to that.

Hope I could help.

It is not only about up and down, there are deeper dimensions to this. Like infront and behind.

The Moon seems to be "behind" us, as in expressions like: "Going BACK to the Moon". Although it does go all around and never gets anywhere, much like ones own bottom, actually. Even Ptolemy didn't argue with that fact. Might this be a clue to this geometric puzzle?

Your grandparents might argue against this and say that they definitely remember that the Moon was the horizon in front of them. How little they knew about what would become our behind!


Is it possible to use a laser to propel a spaceship? Reference: http://adsabs.harvard.edu/full/1968BAN....19..421H
It would be necessary to aim a laser to the target star. How would the density of the laser compare with the interstellar radiation?

Yes, it might be possible to send laser-propelled spaceships to the stars.
There are a whole series of technical challenges in the laser technology and the materials that such a spacecraft might be made of.
The basic idea is to accelerate mini-spaceships with very low mass, but a large, reflective surface area, up to relativistic speeds. The project that I know of is called "Breakthrough Starshot".
Contrary to what you say in your question, the laser would not be aimed at the target star, it would be aimed at the spaceships! These would have to be set off on a carefully calculated trajectory, because during their decades-long journey, the relative positions of the Sun and target star would change significantly. The impulse phase would take place within the Solar System and the craft would cruise most of the way.
The intensity of the lasers required would be immense compared to the light from a star. The baseline plans involve something like $10^9$ W/m$^2$ delivered to the light sail for 10 minutes. This is to be compared with about $10^3$ W/m$^2$ from the Sun at the orbit of the Earth.

No. There are at least two crucial problems:

Even the best lasers diverge after some thousands km. We could not focus the beam to the ship.

With laser, you can give energy to the ship. Most likely, as heat (maybe as electrical energy). You can not give impulse. The only way to give impulse to the ship by lasers, using the impulse of the light, i.e. by light pressure. This is extremely small and it would require unimaginable power to have any measurable effect.

"Lesser" hard problem is that the atmosphere scatters light a little bit, what could make the aiming of the beam impossible (it would also "help" it to diverge). Making the lasers space based would hugely increase the costs.

Extension: a quick calculation. The Parker Solar Probe is 8 solar radii away from the Sun in the nearest point of its orbit. The Earth is 200 solar radii away, and gets 1400 $\frac{W}{m^2}$ light intensity from it. Thus, the PSP gets $1400 \frac{W}{m^2} \cdot \big(\frac{200}{25}\big)^2 \approx 900 \frac{kW}{m^2}$ energy.
See the best case, if the receiving side is a perfect mirror, which radiates back the whole beam. Using the relevant formulas here, we get the pressure of $\approx 0.003 Pa$. If the "laser sail" surface area is $1m^2$, this results 0.003N force, also $\approx$ the weight of 0.3g on the Earth.
But the heat shield of the PSP is 11cm thick reinforced carbon-carbon composite (this was the heat shield also of the Space Shuttle).


Mass of the universe How do we calculate the mass of the universe taking into consideration dark matter?
Mass of visible matter can be computed by stellar method, but how do we calculate mass of dark matter which we don't see? Why do we assume that dark matter is more abundant than visible matter?

Check out the Wikipedia page on the missing baryon problem. It's how we compute the mass of ordinary matter. You can see there are methods of getting the mass of ordinary matter without adding them up. In other words, you can find ordinary matter even if they're invisible.

This is similar to how we find dark matter. Again, look at Wikipedia's article on dark matter, in particular the observational evidence section. Ignore the more technical methods if you're new to the field and just look at the first few:

With galaxy rotation curves, you know you need more matter to recover Kepler's laws. How much matter? You can tweak that number and therefore get an estimate for the dark matter needed. You can't prove that this dark matter isn't just invisible ordinary matter using this method, however.
The same goes for velocity dispersions.
Gravitational lensing can measure the total mass of an intervening cluster without knowing anything about the kinematics of the cluster.

If you're wondering how we know dark matter can't just be invisible ordinary matter, again look at the Wikipedia article, in particular this section.

Big bang nucleosynthesis constrains the amount of ordinary matter in the universe. If dark matter were just invisible ordinary matter, we also expect more helium, lithium, etc in the universe than we observe.
Gravitational microlensing constrains the amount of MACHOs (massive compact halo objects, such as black holes and invisible planets) we can have in a large mass range, covering most of the plausible candidates.
Data from the cosmic microwave background indicates a large fraction of matter does not react with photons.


Triple stars with retrograde orbits Where can I find the observed frequency of triple star systems where the outer star is in a retrograde orbit?

The usual recipe used in the population synthesis literature is that triple stars comprise ~10% of all stellar systems and the mutual inclination is uniform in the cosine.  Most of these systems have a large enough semi-major axis ratio that the inner binary is dynamically decoupled from the outer binary, so retrograde orbits will be just as likely as prograde orbits.  So, to an order of magnitude, ~5% of stellar systems should be triples with retrograde orbits.

Things get more complicated if you are interested in more details.  More massive stars have higher multiplicities, so the fraction is probably somewhat higher for them.  For more compact systems, dynamical effects like the Kozai-Lidov mechanism will become important and can lead to prograde orbits becoming retrograde and vice versa.  Moreover, in compact systems, retrograde orbits are slightly more stable than prograde orbits, so they are likely to be a little more overrepresented, though I'm not aware of any observational evidence that there is any great difference between the two.

If you want more details on the observations of multiple stellar systems you should read Raghavan et al. (2010).  However, I don't think that they measured mutual inclinations.  In general those measurements are very difficult to do.  Andrei Tokovinin has done a lot of great observational work on triple stars as well, so you might want to look at Tokovinin (2014) and Tokovinin (2008) among other papers.  There's also a good review by Duchene & Kraus (2013).

I would say that this sort of system would be near impossible. Both the mechanics of triple star systems being very complicated and the fact that have the outermost star would have to be captured, not from the original local stellar material, no easy feat.

Basically, the system would be virtually non existent, especially in the long term due to large gravitational wave energy decay (thanks LIGO) and general instability.


Can a pulsar hit us with both beams? Can a pulsar have an axial tilt close enough to $90^\circ$ to hit us with both beams for two pulses per rotation?

Yes. One of the features that helps this scenario is the gravitational bending of light close to the neutron star, which allows for a larger fraction of the surface to be visible at a given time.

Many pulsars have double-peaked light curves, for example here's the light curve of the Crab Pulsar. In the pencil beam model these peaks correspond to the beams from the two poles.

According to Annala & Poutanen (2010) "Constraining compactness and magnetic field geometry of X-ray pulsars from the statistics of their pulse profiles", the ratio of single-peaked to double-peaked pulsar light curves puts a most likely value for the maximum inclination of the magnetic axis relative to the rotational axis of 40°±4°.

It depends on where the beams are facing. If the magnetic poles are facing you and the spinning axis is facing the right position, then both can. So it depends on two things:

Direction of magnetic poles
Direction of spinning axis


The laws of physics through out the universe How do we know laws of physics are the same throughout the universe? Intuitively I would say they would vary in two natural ways: the constants in the equations may vary or the math in the equations may vary. As a guess they could change over a long time. What is the furthest radius we can prove from earth, with absolute certainty, that the laws of physics do not vary? I am aware this may not be a radius but a more complex shape that cannot be simply described by a radius.

The nearest answer I can think of for a radius is a guess. And that guess is based on the furthest physics experiment we have done from earth. Which I think is an experiment with mirrors on the moon. Therefore if we assume (I don't know if this assumption is totally 100% reasonable) all physics laws hold because this experiment works. Then the radius is to the moon. This doesn't give an concrete answer for the radius, merely an educated guess.

Let's start in the middle:

  What is the furthest radius we can prove from earth, with absolute certaintity, that the laws of physics do not vary?

Zero. Proofs are found in mathematics and court rooms, and are impossible in natural science. The best we can do is have falsifiable theories. This holds for every description of reality - there's no "proof" even for the Laws of Gravity.

So, what could we observe that would tell us that physical constants or relationships between physical quantities are different in other parts of the universe, or at other times during its existence?

Gravity: For galaxy clusters, we have independent mass measurements from several different sources that agree within their (admittedly large) error bars. Gravitational lensing, velocity dispersion of the member galaxies and X-ray temperatures are all in agreement. So the laws of gravity seem to work even at redshifts up to 0.5 or even higher.
Atomic physics: We observe highly redshifted objects. The wavelength of the light emitted by these objects is made longer by the expansion of the universe. Observing redshifted spectral lines of different chemical elements (or molecules) tells us that atomic physics worked the same when and where this light was emitted. If the transition levels between electron orbits had changed over time, we would get different redshifts for the same objects depending on what element's spectral line we observe.
Nucleosythesis: Shortly after the big bang, the temperature lowered such that protons and neutrons were no longer created and destroyed constantly. A free neutron has a half live of about 8.5 minutes before it decays into a proton and an electron. Our theories predict that we'd get a helium (2x proton, 2x neutron) content in the universe of about 25%. (The rest of the "normal" matter being essentially all hydrogen), and that is indeed what we observe. Now, the helium content is dependent both on the matter density at the time this took place and the half live of the neutron. From other observations (BAO come to mind) we are fairly certain that we got the matter density about right. Which leaves only a small wiggle room for the half live of the neutron, and hence for changes in the weak force.
We've covered gravity, electromagnetism, and the weak force. I don't know any good test for the strong force.

For a change of natural laws over time, we can look at the isotope distribution in rocks here on earth. We should be able to tell whether the decay rate of various elements was different at earlier times by looking at how many of each of their decay products are around.

To summarize, we cannot say with "absolute certainty", but what we observe seems to indicate that natural laws are the same throughout the universe.

i am by no means a scientist, thus nor an astrophysicist. i have a background in electrical engineering and a curiosity for cosmology. i ended up here essentially because i'm looking for answers to the question asked above.

it seems to me that the following information is revelant to the question : a fairly recent (2017.09.20) article published on the NASA website mentions a study which reveals that the two methods used to compute Hubble's constant (one is based on observations of type 1a supernovae, the other on the CMB) disagree (although the Standard Model of Cosmology predicts their agreement) :

« A recent study using the first method yielded an 8% greater expansion rate than the second method’s result. » - https://science.nasa.gov/science-news/news-articles/hubbles-contentious-constant-news

the article doesn't mention a clear explanation for this discrepency. for instance, perhaps there are holes in one or both computation methods.

if i understand correctly : since it is believed that the CMB informs us about the early universe, but this is not so for type 1a supernovae, then another possible explanation is that both measurements are valid, and the discrepency means that something has changed over time. for instance, the article asks the question « Or are dark energy’s or dark matter’s properties changing over time? ». given the importance of Hubble's constant, perhaps this points to the fact that physics have changed over time.


What is left of a universe where no matter and energy exist and is there a term for this? In the final energy state of the universe, there exists a hypothesis that all matter will have fallen into black holes and neutron stars. If, through Hawking radiation, after all matter is consumed and black holes eventually do dissipate, what is left of our Universe?

I may be describing Heat Death, but I'm not 100% sure. However, due to the Law of Conservation of Energy, I suspect that energy will be all that is left, only in equilibrium meaning no ability for energy to form matter.

In the event I'm wrong and there exists no matter and no energy, would the term Universe still be applicable here?

However, due to the Law of Conservation of Energy, I suspect that energy will be all that is left, only in equilibrium meaning no ability for energy to form matter.

The law of conservation of energy states that energy is conserved in a closed system. This means the Universe, which is likely an open system, does not necessarily have to obey that conservation law. Not to mention, scientific laws are not absolute, despite their names; they are entirely empirical, and only apply as far as we've observed. As Edwin Hubble put it, "Beyond the limit of the observations, the form of [a] function, or relation, is speculative. For this reason, [a] law is empirical and it must remain empirical until it is explained by an accepted theory."

Anyway, let's get on with the answer. You've just described the Black Hole Era in the heat death scenario. If Hawking radiation occurs, then the black hole will continue to emit particles. During the last stages of its evaporation, a black hole will emit not only massless particles, but also heavier particles, such as electrons, positrons, protons, and antiprotons.

Once all black holes have evaporated, the Universe will be nearly empty, sans some particles that will almost never interract. Eventually, even the protons and antiprotons emitted by the black holes will decay into photons and leptons. Thus begins the Dark Era, when the Universe will be dominated by dark matter, electrons and positrons.

Electrons and positrons will rarely come together and form positronium atoms (in which the electron and positron are electromagnetically bound). Positronium decays very within nanoseconds, since the two particles annihilate. Thus, over time, the energy levels in the Universe will become lower and lower.

What happens next is a matter of speculation. According to Carroll & Chen (2004), quantum tunnelling and quantum fluctuations could hypothetically create another Big Bang, approximately $10^{10^{10^{56}}}$ years from now.

Adams & Laughlin (1997) speculates that quantum effects will dominate once all macroscopic matter is gone. Essentially, the laws of "macro-physics" will break down, while quantum mechanical phenomena prevail. As a result, the smallest perturbations would make the biggest difference in this era, so there is no telling what may happen to space or time.

Furthermore, they speculate that if the Universe is a false vacuum, the vacuum may decay into a lower-energy state, creating a new "child universe":

  It is also possible for the universe to spontaneously create “child
  universes” through a quantum tunneling process roughly analogous to
  that considered above (e.g., Sato et al., 1982; Hawking, 1987; Blau,
  Guendelman, & Guth, 1987). In this situation, a bubble of false vacuum
  energy nucleates in an otherwise empty space-time. If this bubble is
  sufficiently large, it will grow exponentially and will eventually
  become causally disconnected from the original space-time. In this
  sense, the newly created bubble becomes a separate “child universe”.

So yes, something as small as quantum tunnelling will potentially have an enormous effect on the future of the Universe.

In the end, we really have no idea what will happen after enough time, so there's no definitive answer to your question in the long run.

There couldn't be a universe where no matter & energy exists;the universe IS matter & energy,so without it,no universe.


Why doesn't the sun pull the moon away from earth? If the suns gravitational pull is strong enough to hold much larger masses in place (all the planets) and at much greater distances (all planets further away from the sun then earth) why does it not pull the moon away from earth?

Why doesn't the sun pull the moon away from earth?

Short answer: Because the Moon is much closer to the Earth than it is to the Sun. This means the gravitational acceleration of the Earth toward the Sun is almost the same as is the gravitational acceleration of the Moon toward the Sun.

The Moon's acceleration toward the Sun, $-GM_\odot\frac{\boldsymbol R+\boldsymbol r}{||\boldsymbol R+\boldsymbol r||^3}$ is indeed about twice that of the Moon toward the Earth, $-GM_\oplus\frac{\boldsymbol r}{||\boldsymbol r||^3}$. This is irrelevant. What is relevant is the Moon's earthward acceleration due to gravitation compared to the difference between the Moon's and Earth's sunward gravitational acceleration,
$$\boldsymbol a_{\odot,\text{rel}} = -GM_{\odot}\left(\frac{\boldsymbol R + \boldsymbol r}{||\boldsymbol R + \boldsymbol r||^3} - \frac{\boldsymbol R}{||\boldsymbol R||^3}\right)$$
This relative acceleration toward the Sun is a small perturbation (less than 1/87th in magnitude) on the Moon's gravitational acceleration toward the Earth. Given the current circumstances, the Sun can't pull the Moon away from the Earth.

Longer answer:

The gravitational force exerted by the Sun on the Moon is more twice that exerted by the Earth on the Moon. So why do we say the Moon orbits the Earth? This has two answers. One is that "orbit" is not a mutually exclusive term. Just because Moon orbits the Earth (and it does) does not mean that it doesn't also orbit the Sun (or the Milky Way, for that matter). It does.

The other answer is that gravitational force as-is is not a good metric. The gravitational force from the Sun and Earth are equal at a distance of about 260000 km from the Earth. The short-term and long-term behaviors of an object orbiting the Earth at 270000 km are essentially the same as those of an object orbiting the Earth at 250000 km. That 260000 km where the gravitational forces from the Sun and Earth are equal in magnitude is effectively meaningless.

A better metric is the distance at which an orbit remain stable for a long, long, long time. In the two body problem, orbits at any distance are stable so long as the total mechanical energy is negative. This is no longer the case in the multi-body problem. The Hill sphere is a somewhat reasonable metric in the three body problem.

The Hill sphere is an approximation of a much more complex shape, and this complex shape doesn't capture long-term dynamics. An object that is orbiting circularly at (for example) 2/3 of the Hill sphere radius won't remain in a circular orbit for long. Its orbit will instead become rather convoluted, sometimes dipping as close to 1/3 of the Hill sphere radius from the planet, other times moving slightly outside the Hill sphere. The object escapes the gravitational clutches of the planet if one of those excursions beyond the Hill sphere occurs near the L1 or L2 Lagrange point.

In the N-body problem (for example, the Sun plus the Earth plus Venus, Jupiter, and all of the other planets), the Hill sphere remains a reasonably good metric, but it needs to be scaled down a bit. For an object in a prograde orbit such as the Moon, the object's orbit remains stable for a very long period of time so long as the orbital radius is less than 1/2 (and maybe 1/3) of the Hill sphere radius.

The Moon's orbit about the Earth is currently about 1/4 of the Earth's Hill sphere radius. That's well within even the most conservative bound. The Moon has been orbiting the Earth for 4.5 billion years, and will continue to do so for a few more billions of years into the future.

Newton law: https://en.m.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation

F=G*(m1*m2)/d² is the gravitationnal force between 2 things of mass m1 and m2, separated by a distance d. G is the gravitational constant (I don't remember the value).
--> F_earth/moon=F_moon/earth=G*(m_moon*m_earth)/d²
Same thing for F_sun/moon

You'll notice that F_earth/moon is greater than the other force, so the Moon is more attracted by Earth than Sun.


Calculating star radius from temperature and absolute magnitude - what am I missing? In an attempt to visualise nearby stars, I've tried to get the star radius (measured in solar radii) using the equation provided on the following site:

http://cas.sdss.org/dr4/en/proj/advanced/hr/radius1.asp

Now though the equation seems to work fine for a star like Sirius, when I plug in the values for absolute magnitude and temperature for Barnard's star (according to wikipedia, 13.21 and 3134 K respectively) I get a radius of 0.0722. The radius provided by wikipedia is more than twice that value, at 0.196.

There are many potential culprits. Could it be floating point errors in the php code? The equation looks like this by the way:

$$\text{radius = pow(5800/3134, 2)} \times \text{pow(pow(2.512, (4.83 - 13.21)), 0.5})$$

Another possibility is that the equation is too simplistic, and there are other variables I need to take into account.

Any insights would be greatly appreciated!

Michael

The page you quote is guilty of a gross simplification (or error). The luminosity ratio cannot be estimated from the difference in visual magnitudes for stars of different spectral type. You need to use the difference in bolometric magnitude, which indicates the luminosity integrated over all wavelengths. This will involve making bolometric corrections to the visual absolute magnitudes.

A star like Proxima Cen radiates much of its light outside the visible band and has a much different and larger bolometric correction than the Sun or Sirius. This accounts for your problem.

Example:

The correct calculation uses a table of bolometric corrections vs temperature. This rapidly changes for M-dwarfs in the visual band (you would be more precise using K magnitude), but is about 2.6 mag for a star at 3100 K (see Zombeck 1992). That is, the bolometric magnitude is smaller than $M_V$ by 2.6 mag. The equivalent correction for the Sun is only 0.1 mag.
$$ M_{\rm bol}= M_V - BC$$

Thus there is a 2.5 mag less difference in the bolometric magnitudes in your final equation than the difference in absolute visual magnitudes, and I get a radius for Barnard's star of $0.23\ R_{\odot}$.

The probable reason for the discrepancy between this and the radius quoted in wikipedia (actually it is the radius determined by an interferometric angular diameter measurement plus the parallax by Lane et al. 2001; a more recent measurement by Boyajian et al. 2012 gives $0.187\pm 0.001\ R_{\odot}$) is that (a) the assumed temperature is too low or (b) that the bolometric correction scale for V-band magnitudes is not very well defined for very cool stars. Both (a) and (b) are possible, especially as Barnard' star probably has a significantly subsolar metallicity.

In order to calculate this, you can use the Stefan-Boltzmann law to calculate the star's surface flux and its absolute magnitude to get the luminosity.  Once you know the surface flux and luminosity, you can find the radius of the star.

Stefan-Boltzmann Law:
$$
F=\sigma T^4
$$

Absolute Magnitude - Luminosity calculation:
$$
\frac{L}{L_\odot} = 10^{0.4(M_\odot - M)}
$$
where $L_\odot$ is solar luminosity and $M_\odot$ is the absolute magnitude of the sun.

Next, you use the formula for intensity and solve for distance:
$$
r = \sqrt{\frac{L}{4\pi F}}
$$
Compiling all this into one equation,

$$
r = \sqrt{\frac{L_\odot * 10^{0.4(M\odot - M)}}{4\pi \sigma T^4}}
$$

As long as you use SI units for $L_\odot$, $\sigma$, and $T$, you'll get $r$ in meters.


Would dark energy save the earth for a while as the sun heats up? Would the expansion of space (e.g. dark energy) move the earth further away from the sun over time, possibly saving us for perhaps a few billion years as the sun heats up?

Scientists now tend to think that the sun is heating up very gradually (e.g. over billions of years):
http://tvblogs.nationalgeographic.com/2010/07/21/hotter-and-hotter/

Dark energy is the unexplained accelerating expansion of the universe, as described here:
http://en.wikipedia.org/wiki/Dark_energy

Put another way, my question is, is dark energy working at such a rate that it would move earth (and other planets) further from the sun and prolong life on earth as the sun heats up?

I don't think this is a bad question, and I think Pela is essentially correct, the effect is very small.

Dark energy doesn't expand objects, for example, cause those are bound together, but dark energy should expand space between the sun and the earth though the effect is tiny.

If we take this number, 68 KM per second per 1 million light years (link below), that's 245,000 KM per hour per million light years or about 1 KM every 4 hours per lightyear.   With 63,000 Astronomical units per light year, between the sun and the earth, we can expect the earth (assuming it's in a perfectly stable orbit around the sun - which it's not, but lets say it is), a velocity of 1KM/30 years.   And that's over the age of the universe.   The earth/sun is only 1/3rd the age of the universe, so dark energy has accelerated the earth maybe 1 KM/90 years velocity since the formation of the solar system.   That's a pretty small push.  A velocity change of a bit a meter per year over the age of the solar-system.   The effect of the solar wind and the Jupiter and Venus orbital tugging are probably greater than that.

Source:  http://www.space.com/26279-universe-expansion-measurement-quasars-boss.html

Now, over a few billion years, a small velocity change of 1 meter per year would add up - so I think dark energy is part of the solar system's long term orbital calculation.  I might be wrong on that, but I think it has a measurable effect over billions of years even in distances as small as planetary orbits.

But would it save the earth - I doubt it.  The sun's heating up accelerates over time.   It will at some point in the future, move too fast and get too hot for the earth, so, we'll have to save ourselves, either by moving the earth or terraforming mars, or building space colonies or some combination of all 3.   The good news, we have tens of millions, maybe hundreds of millions of years to figure that stuff out.

Dark energy has been useful though.   It's probably kept us from crashing into Andromeda and/or the great attractor - however far away that is.   Certainly delayed those collisions (We're never going to crash into the great attractor, I gather, thanks to dark energy), and crashing into Andromeda, when it happens, might harmless anyway, but I'm just kind of thinking out-loud.

I do think it's a good question as there probably is some small expansion due to dark energy between the earth and the sun.   If my numbers are wrong, I welcome correction.

I welcome further discussion, and evidence supporting, or refuting my answer.

If my idea is correct that antimatter is distributed fairly evenly through the voids between galaxies, and that antimatter is the cause of dark energy, then no, dark energy will not move earth out of the way of a sun that is heating up (e.g. it would have no effect, or very little effect).  This is because antimatter would not exist within the solar system.  Solar wind, and other particles would annihilate any antimatter that found its way into the solar system.  A "galactic wind" would also likely prevent antimatter from even entering the galaxy.

It would be interesting to see if we can observe earth's past position in relation to the sun (e.g. perhaps by examining the rocks or fossils) and see if earth is migrating out further from the sun or not.  This analysis may show us if dark energy is indeed "everywhere" (true if the earth used to be closer, and is migrating away from the sun) or just in the "voids" (true if the earth is not migrating away from the sun).

I very much welcome further discussion and answers.  I especially welcome someone to perform the research as discussed in the prior paragraph.


Can you create a black hole using kinetic energy? Imagine you have a particle accelerator that you can crank up to arbitrarily high energies.  Because of General Relativity, the particles get heavier and heavier as you dump more energy into them.  Will these particles ever become a black hole from this relativistic mass?  If so, what will an observer particle moving parallel to the black hole, just below that speed, see?  What I'm getting at is, kinetic energy is relative.  So therefore relativistic mass is as well.  So would these particles be black holes to some observers but not others?

You can't make a black hole simply by accelerating a particle, but it has been theorized that smashing some together with ridiculously high energies could produce micro black holes. This, as far as I know, has never been known to happen at particle accelerators, simply because they are not powerful enough.

Relativistic mass increase is inertial, not gravitational, so no, you couldn't make a black hole by accelerating a particle. What happens is that the more energy you use to accelerate the particle, the more its inertial mass increases, but the more its inertial mass increases, the harder it is to accelerate. There comes a point near the speed of light when virtually all the energy you pump into the system is manifested as mass increase rather than velocity, and you can't make the particle go any faster. The best idea, if you want a really fast particle with huge amounts of energy, much more than could be produced by an accelerator, is to use a cosmic ray. Cosmic rays are fast protons or atomic nuclei which have been accelerated by natural events such as supernova explosions to much higher energies than you could get in an accelerator.

Cosmic rays have been used for particle experiments, but the snag is that they are random events, so you don't know exactly where to put your particle detectors to intercept the best ones. To give you an idea of how great their energies can sometimes be, the most powerful cosmic ray particles have the kinetic energy of a rifle bullet, yet the atomic nucleus which carries this energy is so minute that it cannot be seen in even the most powerful electron microscope!


Why do we see galaxies with their actual spiral shapes? Since the actual position of stars is relative, and we see the light they emitted long time ago, and one galaxy has thousands of millions of stars, and those stars can be separated by hundred of thousands of light years, why don't we see galaxies as abstract forms or lines in the sky  as the light of every galaxy star arrives to us at different times instead of well formed spiral shapes? It's like if those millions stars were forming the galaxy as a single light spot, instead of millions of spots separated by light years.

This has been in my head for decades, but I didn't find the question (not even the answer, but the question) anywhere, so either there must be an really easy answer to it or a really complex one or it's a glitch in the Matrix.

Basically, because the relative speeds of stars within galaxies are much less than the speed of light.

Imagine a structure with dimension 100,000 light years (about the size of a galaxy). Now suppose the components of that structure move with speeds relative to each other of around 100 km/s (or 0.03% of the speed of light). In the time it takes light to cross the structure (100,000 years of course), the components within it will have moved by just 30 light years (0.03% of the size of the structure).

There would therefore be no significant "blurring" of our snapshot of a galaxy caused by the differing light travel times from its structural components.

The other thing you may have not understood, is that the lives of stars are generally much longer than 100,000 years. So the stars that emit the light on the far side of a galaxy are still in  existence 100,000 years later, when their light passes stars that are present at the near side of a galaxy.

I am not rewriting what nicely said in another answer. I just point out, as it is just implicit there, that even the closest stars in our galaxy are really far away!  Not too say other galaxies. If, as I suppose, you aren't surprised by the fact that the stars look as fixed over millennia, then considering distance should fix your question.


How probably is it that galaxies will extinguish? In the nebulaes of galaxies are stars born, but the gasses and dust they are made of comes from exploded stars (fe supernovas). So it looks like these processes can go on continuesly. But is that really the case, or does according to entropy(?) galaxies extinguish because fe the galaxie went too big so gravity has no opportunity to form other stars any more. And before colliding with other galaxies it is passed away. So is there any sign that galaxies are getting smaller or less active?

Galaxies are gradually being extinguished. Most star formation activity occurs near the start of a galaxy's life, or in response to merger activity with other galaxies.

The star formation rate of the universe peaked at redshifts of around 3, corresponding to a look-back time of around 9 billion years. Since then the star formation rate has declined as the universe expands; mergers are less frequent, gas is driven out of galaxies by supernovae and active galactic nuclei.

However, most of the stars that have been formed are of lower mass (K- and M-dwarfs) than the Sun and will live on for tens or hundreds of billions of years. They are however much fainter than the Sun. So although high-mass, luminous O- and B stars live their short lives and are not replaced at the same rate, the low-mass stars continue to shine. This means that galaxies will get fainter on average as the remaining stellar populations increase in average age and decrease in average mass.

It is a slow process though. High mass stars are still being formed in our galaxy after 12 billion years, and most spiral galaxies have ongoing star formation. However, star formation has more-or-less ceased in gas-poor elliptical galaxies.

Far more stars die than stars are born, The number isn't exact but I would say 90 to 10.Then what occurs next is a large percentage of those dead stars form black holes from supernovas and the occasional neutron star collision will form black holes, Giving the new stars less of a chance to reach maturity.The Universe is being consumed from the point it was created.The reason why the Universe can not repair itself is because it was almost entirely created all at once so everything will decay at once leaving no chance for restructure.


What's the largest non-spherical astronomical object in the universe? Some asteroids and comets are non-spherical. But is the nature of big things and gravity so that large things in the universe are always spherical? What is the biggest astronomical object in terms of volume out there that's not spherical?

Note: Astronomical object is a defined term. See Astronomical object - Wikipedia.

The largest sub-galactic astronomical object (in volume) that we know of is the Carina Nebula, which is a non-spherical diffuse nebula. The Carina Nebula has a radius of about 100 parsecs.

Image credit: ESA

If you consider astronomical objects of the galactic scale, then the galaxy IC 1101 is the largest astronomical object (in volume) that we know of. From Wikipedia:

  The galaxy has a diameter of approximately 6 million light years, which makes it currently (as of 2013) the largest known galaxy in terms of breadth. It is the central galaxy of a massive cluster containing a mass (mostly dark matter) of roughly 100 trillion stars. Being more than 50 times the size of the Milky Way and 2000 times as massive

IC 1101 is on the left in the image below.

Here is an image from the Sloan Digital Sky Survey (SDSS) of IC 1101:

I'll leave it to you to decide if an elliptical galaxy counts as "spherical" or not.

If you don't count elliptical galaxies, the largest spiral galaxy is NGC 262, pictured below. It has a diameter of 1.3 million light years.

Well, this is my relatively inaccurate size comparison of galaxies. IC 1101 is 6 million LY at this scale, NGC 263 is 2.3 million LY, and there are some other galaxies.


M87 Black hole. Why can we see the blackness? So, as the title states, why are we able to actually see the 'blackness' of the black hole? I get that what we are actually seeing is the event horizon, or accretion disk. But should this not extend all the way round? Surely the black hole is not a 2D thing so we can "look into it from above" (I use that term loosely as obviously there are no directions in space!), so why are we able to actually see the blackness?

The only thing I can think of is that a black hole, much like our solar system has some sort of ecliptic, that the vast majority of matter is orbiting, and everywhere else just doesn't have enough matter for the light to be visible, kind of like why we are unable to see the Oort cloud.

I hope that made sense, and I could be way off, but it is the only thing I could think of to explain it. If it is the case, then would we be able to get a similar image of Sagittarius A, seeing as we could be in this "ecliptic" of it, so surely we would only be able to see the heated matter around the event horizon and not the blackness?

Rob Jeffries' answer is excellent, I just wanted to add this picture trying to explain the geometry. Here, I assume a non-rotating black hole (BH); for a rotating BH the exact numbers are slightly different.

The photon sphere

Photons move on straight lines, but in the heavily curved space around a BH these straight lines appear curved. Although the event horizon (EH) at a distance of $r = 2GM/c^2 \equiv r_\mathrm{S}$ (the Schwarzschild radius) from the BH marks the region from which no photons may escape if emitted radially, photons on partially tangential orbit will fall back out to a distance of $r = 1.5r_\mathrm{S}$, where photons traveling fully tangentially will stay on the photon sphere (although this is an unstable orbit).

The innermost stable orbit and the accretion disk

Ordinary matter will spiral inward out to twice this distance; hence, inside the innermost stable circular orbit (ISCO) at $r=3r_\mathrm{S}$, matter is pretty much bound to be absorbed. Outside this region matter may orbit, forming the accretion disk, but since friction between the particles will cause them to lose energy, they will slowly approach the ISCO, after which they will rapidly fall into the BH. Note that the M87 BH doesn't have a thin accretion disk like the one depicted in the movie Interstellar; rather a thick "cloud" surrounding most of the BH.

Photons emitted tangentially just outside the photon sphere will spiral around the BH many times, slowly increasing their distance, until eventually they escape at a projected distance of $\sqrt{27/4}r_\mathrm{S} \simeq 2.6r_\mathrm{S}$ from the BH (e.g. Frolov & Novikov 1998).

The shadow

Just as the path of light rays are curved around the BH, so are the sightlines from you toward the BH (you can think of sightlines as reversed photons). That means that all sightlines that are closer than (a projected distance of) $2.6r_\mathrm{S}$ to the BH will, eventually, end up on the EH, even if taking several orbits around the BH. These sightlines comprise the so-called shadow (Falcke et al.(2000); Event Horizon Telescope Collaboration et al.(2019a), ). On the other hand, along sightlines that a farther away, you see the radiation emitted from the matter falling into the BH, both in front of and behind the BH. And since the first sightlines that don't terminate at the EH circle the photon sphere many times, those sightlines are actually very long paths through matter shining its last light before being engulfed, and hence they look exceptionally bright (e.g. Event Horizon Telescope Collaboration et al.(2019b)). This bright ring just outside the shadow is called the photon ring, or the emission ring.

The drawing

The drawing below may help understand. All the red lines are sightlines toward the BH. Only the uppermost one just grazes the photon sphere (and the luminous matter behind). The rest terminate at the EH, and hence look black (except for luminous matter in front). Close to the center, you see the front of the EH; farther out you actually see the back of the EH; even farther out you again see the front of the EH, and so on ad infinitum until you reach the photon ring.

The observation

Despite the observational resolution being an astonishing $\sim25$ micro-arcseconds, the photon ring is smeared out over a larger region, resulting in the doughnut shape you've seen. That is, what you see in that image is not "the EH in front of an accretion disk", but rather "the EH seen from all sides at the same time and enlarged, with light emitted from the photons ring".

Unless the accretion disk is viewed exactly face-on, half of the accretion disk$^\dagger$ has a velocity component toward you, making it brighter than the other half through a special relativistic effect called beaming. This is seen in the southern part of the M87 BH.

The figure below (from Event Horizon Telescope Collaboration et al.(2019b)) shows, from left to right, the actual observation, a model where you see the rather sharp photon ring, and this model blurred to match the resolution of the observation.

$^\dagger$At least the material just before it plunges into the BH, which follows the rotation of the BH. Farther out, the rest of the accretion disk may in principle rotate the other way.

This black hole has an accretion disk, which is a disk of matter that rotates around the black hole at extreme speeds, causing it to heat up. The orange colour you see in the picture is that matter. The matter seems "thicker" on one side because the bottom of the disk is tilted slightly towards us. The "blackness" you see is simply the event-horizon stopping the light from that region from escaping.