How could universe inflate itself out of the very dense and curved early spacetime? Could it happen in a black hole too? Wasn't spacetime as much curved as a black hole directly after Big Bang, because mass was so densly packed? Wasn't everything like an event horizon and how could things expand across it? Could inflation happen in a black hole too so that its content does become accessible to the outside universe?

Great question! Sorry for this huge response, and it might not be a satisfying answer, but it'll address your questions.

Sadly, as with most of astronomy, the Big Bang is surrounded in mystery. It is one of the biggest uncertainties out there, and you'll come to this realization if you've researched it enough.

Most of what we can get out of the Big Bang is through extrapolation. If we observe the expansion of the Universe, we can presume that at one point, there was far less space between all the matter in the Universe. This is where things begin to break down, though.

Through General Relativity — which explains our modern understanding of gravity — and our extrapolation, we would reach the phenomena you're describing; the entire Universe would have originated as an infinitely small point (called a singularity), from which matter was incredibly hot.

However, General Relativity begins to break down at those levels, since its equations cannot explain the conditions of such a universe accurately. For one, as you hint at, an infinite density and potentially infinite mass would likely have an infinitely strong gravity.

There is no place for such infinities in a proper mathematical description of the universe, and at that point, Einstein's equations — the basis for predicting the evolution of the cosmos — collapse. These equations provide us with unreliable results and we run into some of the problems you are describing.

Quantum mechanics is more fit for such small scales, but it fails to explain how gravity acts and what its role was at the time. Thus, this tells us that QM and GR are necessarily incomplete. Their equations cannot describe the Big Bang singularity in its entirety, especially since GR (which deals with large, high-mass phenomena) and QM (which deals with small, low-mass phenomena) are incompatible and cannot describe small, high-mass phenomena.

Thus, we're left with a lot of unanswered questions: did the Big Bang create the Universe, or was it simply an event in the Universe's history? Why would the Universe be compressed to such a small scale? Would the idea of an infinitely dense universe go against the presumption of an infinite universe?

There has long been hope that these questions can be explained within a theory of quantum gravity. Quantum gravity attempts to unify general relativity and the concepts of quantum theory (QM and QFT). These hopes have recently taken on a more concrete shape, with the help of what is called loop quantum gravity, and its applications to cosmology.

Quantum gravity is still unproven and hypothetical. String theory also tries to unify GM and QFT, but is unfalsifiable. Thus, the Big Bang is a point in time that astronomers really know little about. It's unfortunate and surprising, but it's something physicist are trying to overcome.

The Earth's gravity (call it the curvature of space) at the center of the Earth is approximately zero. Similarly, we assume a dense early universe was still infinite and homogeneous, so no worries, the gravity canceled out to approximately zero. There is no evidence that it was a lump of matter surrounded by an empty space, like a black hole usually is.

Remember, the Big Bang happened right here where you are now, and all the matter became more and more distant from the point where you are. Simply all the distances increased all the time without matter accelerating or moving into some imagined "outer space".


Asteroid locations at the time of the crucifixion of Jesus I am interested in learning the locations and velocities of the near earth asteroids at the time of the crucifixion of Jesus Christ.
 I would like to back-calculate the paths of all known near earth asteroids to find out if any was between the earth and the sun at the time of Jesus' crucifixion on Good Friday.  The Horizons system of NASA JPL only permits back calculations to about 1400AD.

I want to see if there was a clustering of asteroids on Good Friday between the earth and sun.  If so they could be the remnants of a gigantic collision that would have been capable of making a dust cloud in interplanetary space that could have produced a shadow on the entire earth which would account for the observations reported by the gospels and the Chinese astronomers and St. Dionysius and others.

In other words if any asteroids are calculated to have been between the earth and sun on Good Friday they may be left-over debris from the possible collision that produced the dust cloud.

My question is this.  Do you know of any available computer program like the JPL  Horizons program that will work to give asteroid positions back in the time period between 29AD and 35AD?  (I give a range of years because biblical scholars give a range of possible years for the Crucifixion of Jesus.  The determination of a debris asteroid would help historians know the timing of the Crucifixion better, also.)
Thank you for your attention and assistance.
Best Regards,
Dr. Francis Kelly
President, The Catholic Association of Scientists and Engineers

The computer program you are asking for is literally impossible:  Even ignoring the affect of the greater universe and transient visitors such as comets on hyperbolic orbits, the Solar system is a chaotic complex system.  We have a pretty good handle on the motion of the bodies we are, and have been aware of, during the time modern astronomy has been studying the skies, i.e., over the past few hundred years.  (Remember that in the earlier years of telescopic astronomy, we were unaware of most of the currently known members of the Solar system, including all of the asteroids, and every massive object beyond Saturn.)  As you deviate further from the present (either past or future), the chaotic nature of the system comes more into play, especially on smaller bodies such as asteroids.  As a result, stating orbital positions either far in the past or far in the future requires a larger margin of error as the time differential grows.  Eventually you get to a point where the error is so great that we cannot say reliably where an object was or will be.  Perhaps if someone had come up with the laws of planetary motion sooner than Kepler, we would have more reliable data further into the past, and therefore more accurate orbital elements over a longer range than we do, but that didn't happen.

The asteroid belt (white) and Jupiter's trojan asteroids (green)

From Wikipedia, linked from the Asteroid page

In addition to the chaotic nature of the Solar system, there's also the fact that the vast majority of the asteroids revolve around the Sun outside of the Earth's orbit, so the chances of any asteroids we know of having "a gigantic collision ... that could have produced a shadow on the entire Earth" (i.e., between the Earth and the Sun) are between slim and none.  (Note also that the amount of material present in said "dust cloud" would have to be immense in order to have a noticeable affect on the amount of sunlight falling on the Earth.)  The only way such a collision could have occurred would be if two objects traveling through the Solar system happened to collide in such a manner that the "dust cloud" the collision produced happened to fall between the Earth and Sun - and also left the Solar system without a trace afterwards.  This would have required the objects being in similar trajectories crossing the plane of the ecliptic at a high angle and speed - a chaotic event if there ever was one!

So, basically you're looking for a way to prove that a chaotic event happened sufficiently far in the past that chaotic errors make our knowledge of most of the Solar system (asteroids and such) and outside influnences questionable - no, that's not going to happen.  Math doesn't work that way.

First off, Jesus death was on a Wednesday and not a Friday. Why, Creation of the Sun, moon, and stars was on the 4th day. Also, anything to do with the life of Yeshua, has to line up with Leviticus 23 which is the word Season's in Genesis 1:14. Years is the Shemitah and Jubilee Years.
    The Menorah has 7 candles. The fourth candle is called the Christ candle or servant Candle and also you have to line up His death to Daniel 9:27 in the MIDST of/Middle of so the year would have been on Passover (not a Sabbath) on Wednesday, 4000 years from Creation (Aleph-Tav in Genesis 1:1) in 30 CE (Wednesday) between 27(Sunday) to 33 CE (Saturday).

2). NASA (NOT a straight aswer) is trying to hide Trappist, Nemesis, El Shaddai, the Destroyer who brings in the Trumpet and Bowl Judgments a of Revelation in 2019 between Passover (April 20th) and Day of Atonement on October 10th. The orbital path of Nibiru( the Crossing) is an elliptical orbit and comes around every 300-350 years. Gill Broussard's work will help you out on this one or Jardalkalataol.blogspot.com.

Yes, there was an asteroid that did occur at Yeshua's death just like the one's at  His second Coming during this week.
Asteroid 2016 JP
21.1000 H
7,292,673 km
19 April 2019

Asteroid 2017 UT2
24.6000 H
19,652,952 km
21 April 2019

Asteroid 2016 WQ3
28.8000 H
10,862,251 km
21 April 2019

Asteroid 2019 FY
22.9030 H
9,793,095 km
26 April 2019

Asteroid 2013 KJ6
19.9000 H
17,478,956 km
26 April 2019

Asteroid 137805 1999 YK5
16.6000 H
7,923,219 km
4 May 2019

https://www.asteroidsnear.com/year?year=2019
As you can tell we are in the baseball park and parking lot for an exciting two weeks of hitting balls (asteroids)  as they come by the earth. At the time of this reporting, there will not be any home runs until May to October time frame with 2 hits (Asteroid by Sea and asteroid by land (wormwood) of Revelation chapter 8.


Why is the discovery of merging neutron stars important? I'm fairly certain people here will have heard about it, already, but apparently, two supernova leftovers clashed some 130 million years ago and some billion billion kilometres away ...

What I haven't heard yet, however, is why we should care.

I mean sure, it's an interesting phenomenon and measuring it can't have been easy.

But now that we've heard it ... what changes?

I'll admit it, I don't know particularly much about astronomy, but I'm curious:

What's the significance of having achieved this? Why does it matter whether or not we know?

Reasons why this is important:

It is the first simultaneous detection of a gravitational wave and electromagnetic signal, and the strongest GW signal yet in terms of signal to noise (Abbott et al. 2017a). It spectacularly corroborates the reality of the GW detection technology and analysis. The progenitor has been unambiguously located in a (relatively) nearby galaxy (Soares-Santos et al. 2017), allowing a host of other telescopes to obtain detailed measurements.

It shows that GWs travel at the speed of light, a further verification of Einstein's General Relativity (Abbott et al. 2017b).

It shows that most of the very heavy elements such as gold, platinum, osmium etc. are plausibly produced by merging neutron stars and constrains the rate of such mergers in the local universe (e.g. Chornock et al. 2017; Tanvir et al. 2017).

It shows that short gamma ray bursts — some of the most energetic explosions in the universe — can be caused by neutron star mergers (e.g. Savchenko et al. 2017; Goldstein et al. 2017).

It is the closest detected short gamma ray burst (with a known distance). That the progenitor has also been characterised allows a closer investigation of the interesting physics underlying the ejection and jet mechanisms thought to be responsible for the gamma rays and later X-ray and radio emission (e.g. Margutti et al. 2017; Alexander et al. 2017).

It provides observational constraints on how matter behaves at extremely high densities, testing our understanding of fundamental physics to its limits — for example, the details of the gravitational wave signal moments before merger are diagnostic of the interior conditions of neutron stars at densities of $\sim 10^{18}$ kg/m$^3$ (Hinderer et al. 2010; Postnikov et al. 2010).

It provides an independent way of measuring the expansion of the universe. Merging binary gravitational wave sources are known as "standard sirens", because the distance to the GW source pops straight out of the analysis and can be compared with the redshift of the identified host galaxy (Abbott et al. 2017c). The result agrees with measurements made using the cosmic microwave background and the distance-redshift relation calibrated by other means, verifying our estimation of distances, at least in the local universe.

Finally, this event will turn out to be important because it was lucky; in the sense that the source was detected well-inside the sensitivity horizon of LIGO (Abbott et al. 2017a). The detection itself, was not unexpected given the rates predicted based on studying the neutron star binary systems in our own Galaxy (e.g. Kim et al. 2015), but the fact that it was so close —
 within the closest 5% of the sensitive survey volume where it could have been detected — is fortunate.

In the end, if someone thinks none of the above is interesting or important, then nothing I can write will convince them otherwise. The vast majority of people I speak to are curious and fascinated to find out about our cosmic origins and how the universe works.

Because its awesome (SMBC)

So this guy called Copernicus suggested that the Earth orbits the Sun (not the other way round) - What changes?

This guy Newton had a theory for how a mass responds to force, and how gravity works - So what?

Another guy called Maxwell had this idea of how light could actually be waves of electromagnetic fields - does this matter?

A guy called Monet decided to paint some pictures of some waterlilies. Who cares?

Last February, some guys from Denver carried a ball over a line more often than some guys from Carolina carried the ball over another line. So what?

It is worth finding things out because it means finding things out. It is worth understanding our world because it is there to be understood. Discovery is its own reward. It is not measured in by £ or $.

The observations of GW170817 show that heavy elements are created by the merger of neutron stars. Heavy elements like gold, platinum on Earth were likely created in a neutron star merger in The Milky Way, billions of years ago, that enriched the interstellar dust.

If it doesn't matter to you that's fine. If Monet's Waterlilies or the Superbowl leave you cold, that's no problem either. But not everything of value is of use.


Camera reaching the event horizon This question came to my mind after reading this.

I think it is safe to assume that the supposedly thrown camera would not reach the event horizon in one piece. So my question is :

If I dropped a TV camera into a black hole, will it be destroyed before it reaches the event horizon, and if so, why?

If the black hole has no surrounding matter, so that there is no violent radiation generated by accretion or the like, then it still depends on the mass of the black hole. If it is in the thousands of solar masses or more, it is possible for a realistic camera to survive free-fall to the horizon. This is essentially a non-issue for supermassive black holes. On the other hand, smaller black holes are quite a bit more punishing.

For Newtonian gravity with potential $\Phi$, in a free-falling frame, a particle at $x^k$ near the origin of the frame will be accelerated at
$$\frac{\mathrm{d}^2x^j}{\mathrm{d}t^2} = -\frac{\partial\Phi}{\partial x^j} = -\frac{\partial^2\Phi}{\partial x^j\partial x^k}x^k\text{,}$$
where the the second derivatives of the potential, $\Phi_{,jk}$, form the so-called tidal gravitational field. Since $\Phi = -GM/r$ for a point-source, you should expect the tidal forces on a free-falling object to be proportional to $GM/r^3$ times the size of the object. Thus, at the Schwarzschild radius, this is on the order of $c^6/(GM)^2$.

Of course, black holes are not Newtonian. However, it turns out that for the non-rotating, uncharged (Schwarzschild) black hole, radial free-fall of a test particle has the same form as in Newtonian theory, except in Schwarzschild radial coordinate (not radial distance) and proper time of the particle (not universal time), so the above is essentially correct even for Schwarzschild black holes.

To be relativistically correct, the tidal forces on an free-falling object are described by the equation of geodesic deviation, in which the gravitoelectric part of the Riemann curvature provides forms the tidal tensor:
$$\frac{\mathrm{D}^2x^\alpha}{\mathrm{d}\tau^2} = -R^\alpha{}_{\mu\beta\nu} u^\mu u^\nu x^\beta\text{.}$$
In Schwarzschild spacetime, this turns out to be $+2GM/r^3$ in the radial direction, stretching the free-falling object, and $-GM/r^3$ in the orthogonal directions, squeezing it. This stretching-and-squeezing due to gravitational tidal forces is sometimes called spaghettification.

Some example numbers: say the camera size is on the order of $0.1\,\mathrm{m}$. The the following are the approximate tidal accelerations near the horizon for black holes of different multiples of solar masses:

$M\sim 10\,\mathrm{M_\odot}$: $\sim 10^6$ Earth gravities;
$M\sim 10^4\,\mathrm{M_\odot}$: $\sim 1$ Earth gravity;
$M\sim 10^6\,\mathrm{M_\odot}$: $\sim 10^{-4}$ Earth gravities.

The curvature around a rotating black hole is more complicated, but the moral of the story is basically the same.

If I dropped a TV camera into a black hole, will it be destroyed before it reaches the event horizon?

I think so. I also think it will be destroyed before tidal forces have any effect.

  If so, why?

Because it would otherwise end up falling faster than the speed of light.

Sounds strange I know, but take a look at the Shapiro delay: "the speed of a light wave depends on the strength of the gravitational potential along its path". Or see Professor Ned Wright's Deflection and Delay of Light:  "in a very real sense, the delay experienced by light passing a massive object is responsible for the deflection of the light":

Alternatively see this PhysicsFAQ article by Don Koks: "this difference in speeds is precisely that referred to above by ceiling and floor observers". He's referring to the way Einstein said light curves because the speed of light varies with position. See the Einstein digital papers for examples of that. Here's one from 1920, see the second paragraph:

Light doesn't curve because spacetime is curved. Einstein never actually said that. It curves because the speed of light is lower at a lower altitude, rather like the way sonar waves curve:

As to why this isn't common knowledge, I don't know. There's this myth that Einstein gave up on a varying speed of light in 1911, but he didn't, see this Wikipedia article and this example from 1914. I don't know why the reason matter falls down isn't common knowledge either. You know about pair production and electron diffraction and the wave nature of matter, just think of an electron as a wave going round a closed path, then simplify it to a square path, like this:

The horizontals bend downwards, so the electron falls down. Could it be any simpler? Anyway, your TV camera falls down because the speed of light is reducing with altitude. We tend to call this the "coordinate" speed of light nowadays, even though Einstein just called it the speed of light. But regardless of what we call it, you don't have to be the Brain of Britain, or the Brain of France, to work out that half down, there's some kind of problem. At this point the camera will be falling as fast as the "coordinate" speed of light at that location. And it isn't going to slow down. Things always fall faster, not slower. But matter can't go faster than light, because of the wave nature of matter. When matter is made of waves, there is no way that matter can move faster than the speed of those waves.

So what's going to happen? I can see no other option: that wave has to break. Again it sounds strange, but take a look at an old version of the firewall article on Wikipedia. Follow the reference 7 link to Friedwardt Winterberg's Gamma-Ray Bursters and Lorentzian Relativity: "If the balance of forces holding together elementary particles is destroyed near the event horizon, all matter would be converted into zero rest mass particles which could explain the large energy release of gamma ray bursters". I think this is what happens to your TV camera. Flash! It turns into a gamma-ray burst. It would be like an atom bomb, but a lot more efficient. So make sure you drop it from a safe distance.


How is dark matter different from ordinary (visible) matter? How is dark matter different from ordinary (visible) matter?

1) Does dark matter consists of invisible astronomical objects like white dwarfs, black holes?

2) Has dark matter not been directly observed?

Is there a better explanation for dark matter. I could not understand completely the difference between dark matter and ordinary matter.

There is certainly one and possibly two types of dark matter to solve two dark matter problems.

The first dark matter problem is that most gravitating matter is dark. Most of the inferred mass that appears to be responsible for the observed gravitational dynamics of large scale structures like galaxies and clusters of galaxies cannot be seen (i.e. is dark). That is, we can't detect it through visible light or any other electromagnetic waves.

Now it is possible that you could make up this mass out of very low mass stars,   cold white dwarfs, planets, black holes, lost golf balls or indeed any other form of cold, non-luminous matter. Such objects (bar the golf balls perhaps) certainly do exist, are known as baryonic dark matter (because they are made of "normal" protons and neutrons - baryons), but there is unlikely to be enough of this stuff to explain the observations.

The second problem is that most of the dark matter must be non-baryonic. This is deduced from the lack of sufficient plausible baryonic dark matter to explain the dynamics of galaxies and clusters, but also from estimates of the primordial abundances of helium, deuterium and lithium. Big bang nucleosynthesis depends on the density of baryonic matter compared to the universe as a whole and it looks like baryonic matter can only be responsible for a fifth of the dark matter.

Thus the bulk of the dark matter is supposed to be some non-baryonic stuff, probably in the form of particles that do not interact via the electromagnetic interaction and therefore do not emit or absorb light.

Read Before the Big Bang (1997) by Ernest Sternglass.  The short answer is that a big piece of "dark matter" is the source of a quasar.

The "non-standard" and therefore controversial theory presented in this book says that most "dark matter" might be
“fragments of the original primeval atom [of Georges Lemaitre’s model --- please google it if you need to]  ejected to large distances in the explosive ‘mini-Bang’ that had to accompany the formation of every cosmological structure [ie, every moon, planet, star, galaxy,  etc.] in a Lemaitre-type model … the existence of quasars and active cores of galaxies over a wide range of distances indicated that there were apparently delayed mini-Bangs in which new galaxies were created, as Maarten Schmidt had conjectured, together with vast amounts of dust and gas ejected into space” [p.211, BTBB].

Plus:  “All these results strongly suggested that some of the original fragments … from the Lemaitre [“primeval”] atom had managed to survive in the massive centers of large galaxies for very long periods … these massive electron[-positron] pair fragments were apparently ejected long after the Big Bang, as the Russian astrophysicist Novokov and the Israeli physicist Ne’eman had in fact suggested independently in the mid-1960s … their nuclei would be so massive that they would be invisible black holes, yet they could account for a dominant fraction of the total mass of the universe even today” [p.212, BTBB].

According to Sternglass’s model, these fragments of the primeval atom are “seeds of galaxies and stars”, and are spread throughout our universe, ever since the Big Bang.  He says that a seed remains dormant for millions or billions of years,lurking in space, and then, after a long “count-down” process, during which the system divides in half, again + again + again,  it suddenly explodes, violently.  He calls this explosion a “mini-Bang”, and says that the “gamma-ray bursters” —(also called “quasars”)— which astronomers have observed since the 1970s, are in fact the “delayed mini-Bangs” which his model predicts.


Can Jupiter's nightside be classified as spectral type Y? As far as I am aware, the latest spectral types that have been assigned are around Y2, for objects like WISE 0855-0714 that have temperatures around 250 K or so. I've also seen several directly-imaged exoplanets have had their spectra classified as L- or T-type (e.g. Bonnefoy et al. 2016 who find that the giant planets HR 8799 d and e are good matches for L6–L8 dwarfs).
Jupiter is colder and less massive than the observed Y dwarfs and differs from isolated brown dwarfs because it is being illuminated by the Sun, but there is thermal emission which could be observed from the nightside of the planet. Has Jupiter's nightside spectrum (i.e. the spectrum of Jupiter excluding contributions from reflected sunlight) been observed in sufficient detail to compare it to the spectra of Y dwarfs, and if so does it fit the trends observed in spectral type Y or is it so different that it cannot be spectrally classified as a Y dwarf?
For clarity: I am asking purely in terms of spectral classification, i.e. the classification of the spectrum. I am not asking about whether Jupiter is a brown dwarf making the Sun+Jupiter a binary system. That has been asked elsewhere.

This Wikipedia page lists Jupiter as a disputed sub-brown dwarf because its mass reportedly is sufficient to qualify for one. Distance to another star like the Sun doesn't matter, nor does the visibility in the infrared spectrum either since every planet is emitting some infrared light. The fact that Jupiter emanates more energy than it gets from the Sun can't be accounted for either, since that's also a matter of distance from the Sun, and Saturn and Neptune do so too. But, as the article states, Jupiter obviously has enough mass to possibly count as a sub-brown dwarf. Sub-brown dwarfs are still planets because they don't undergo nuclear fusion unlike brown dwarfs and hydrogen-fusing stars and never did.
Edit: There is no reason to only classify the night side just because the day side is illuminated by a(nother) star. Spectral classifications are given to the entire object. Since sub-brown dwarfs reportedly have a mass of at least one Jupiter mass, you may want to classify Jupiter a Y9V sub-brown dwarf.


Why don't we build an interferometer between Earth and the Moon? The Event Horizon Telescope (EHT) does interferometry from an array of telescopes spread all across Earth. The data is locally stored on a hard drive and shipped to a central location, so the telescopes in the array don't need to be connected to each other in real time. Would it be possible to make an EHT-like interferometer with telescopes on the Earth and on the Moon? It seems like such a telescope would have a much higher resolution than the EHT, and would make it possible to observe in wavelengths shorter than radio.

Of course, money is likely an issue here, but is there any technical reason why this wouldn't be feasible?

Put simply: controlling the phasing would be impossible (not just difficult).  Not only do you have to deal with a constantly changing physical distance but also with the dynamics of the atmosphere.  It's difficult enough to maintain transverse phasing across an earth-based receiver (think adaptive optics);  I can't imagine a reliable way to track the radial variation.  Without that, unless you are sticking with multi-meter wavelength radio systems, you cannot hope to phase up the receivers.


Is blue light not traveling faster then red light in space? I just can't understand why light and other massless particles are not traveling at different speeds?

Gravity effects anything with a mass but light has no mass but a black hole, and weak lensing changes the trajectory of light to create more red shifted light more than blue shifted light. That tells me light has mass. Can someone clarify this to me in Laymons terminology?
Pictures would be helpful.

Photons are massless. This doesn't depend on their energy, so doesn't depend on their frequency or wavelength.

Massless particles travel at the speed of light.

Even if we abandon particles and look at classical electrodynamics, we find that the speed of an electromagnetic wave (in vacuum) has a fixed value. It doesn't depend on wavelength.

Gravitational lensing affects light of all wavelengths equally. Lensing is caused by light travelling on null geodesics - basically the "straightest line" it can follow in space-time that has been curved by the presence of mass.

EDIT (Not required for any understanding of my answer, which stands as is, but to address some comments below).

The gravitational bending of light must take into account the curvature of space-time. If that is not done, then one gets the "Newtonian" formula which is half the deflection of a correct GR treatment. For particles with mass (which isn't what this question is about), the appropriate formula for their deflection will depend on their speed. At low speeds, the Newtonian formula will work just fine. At high speeds (approaching $c$) the deflection asymptotically approaches that for a light beam (i.e. twice the Newtonian value and the same as light). A mathematical proof of the latter in the Schwarzschild and Kerr metrics is provided by Barrabes & Hogan 2004 (see their eqn 4.5). The reason, as discussed by Misner, Thorne & Wheeler (in "Gravitation", 2017 Princeton University Press, chapter 1, section 1.6 - freely available from the linked site) is that gravitation is not just due to the curvature of space, but the curvature of space-time. The tracks of light and (non-relativistic) particles follow distinctly different paths, with different curvatures, through space; but their tracks through space-time have the same curvature.

Is blue light not travelling faster then red light in space?

No it isn't. Oddly enough, it isn't travelling faster than red light in a prism either. Here's an analogy: A little old lady scurries along the pavement at 4mph. A tall young man strides along the pavement at 4mph. When the pavement is clear, that's like space. When the pavement is crowded, that's like a prism. Then the little old lady and the tall young man are both moving at 4mph, but the tall young man has big strides so he spends longer getting round the people in his way. So the little old lady makes better progress. So it looks like she's moving faster.

  I just can't understand why light and other massless particles are not traveling at different speeds?

As far as we can tell photons and neutrinos travel at the same speed in space regardless of their energy.

  Gravity effects anything with a mass but light has no mass but a black hole, and weak lensing changes the trajectory of light to create more red shifted light more than blue shifted light. That tells me light has mass. Can someone clarify this to me in Laymans terminology? Pictures would be helpful.

Gravity doesn't just affect anything with mass. Light is massless, but gravity affects it. As to why, see what Einstein said in 1920: “Second, this consequence shows that the law of the constancy of the speed of light no longer holds, according to the general theory of relativity, in spaces that have gravitational fields. As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable”.

Did you see what he said about light curving because the speed of light is spatially variable? It's rather like the way sonar waves curve in the ocean:

Image from FAS and the US Navy, see course ES310 chapter 20

Also see Professor Ned Wright’s Deflection and Delay of Light article. He says this: “In a very real sense, the delay experienced by light passing a massive object is responsible for the deflection of the light. The figure below shows a bundle of rays passing the Sun at various distances”:

Gif from Ned Wright’s Deflection and Delay of Light

That delay ties in what Einstein said. Which ties in with the Shapiro delay. See the Wikipedia article on that, and note this: "according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path". It doesn't matter what the wavelength is, the light curves the same regardless of wavelength.

PS: Ned Wright points out that the deflection of light is twice the Newtonian deflection of light. The latter is based on the deflection of matter, see for example https://arxiv.org/abs/physics/0508030. That ought to tell you that what you usually hear about light and matter  "following the curvature of spacetime" is just lies to children.


Can we observe changes in the CMB (surface of last scattering) over time? It is hard to determine the age of the Sun, because it won't change much over 100 million years or so. But what about the cosmic microwave background radiation, that is being mapped with ever better precision. It dates to only about 380 000 years after Big Bang. Is it possible to observe changes in it over a period of 10 years or 100 years, as it ages and cools, as the Hubble Bubble expands?

And how is the landscape of the CMB-map expected to change? Are relatively cool and hot areas permanent over time scale much larger than 380 000 years, or would our CMB-map be unrecognizable if we had lived, say, a billion years earlier or later?

It isn't all that hard to determine the age of the sun, provided you are content with an accuracy of 100 million years or so either way. The radiometric dating of zircon crystals, which are only a little younger than the sun, gives a date of about 4.6 billion years, and it is generally agreed that the sun is about 5 billion years old. This is compatible with its position in the main sequence of a Hertzsprung-Russell diagram. You certainly wouldn't notice any change on the CMB map in a mere ten years, but a billion years hence it will have a noticeably greater red shift, except for the fact that there will be nobody here to notice or record it. Our descendants, every last one of them, will all have gone to meet their maker in far less than a billion years.


How to determine the eccentricity, knowing the orbital speed and distance How to determine the orbital eccentricity of a planet, knowing the instantaneous orbital speed of it and the distance from the Sun to the planet at that moment?

How to determine the orbital eccentricity of a planet, knowing the instantaneous orbital speed of it and the distance from the Sun to the planet at that moment?

We are given three things:

$v$ "instantaneous orbital speed" (a scalar)
$r$ "distance from the Sun to the planet at that moment" (a scalar)
$GM$ since the OP specifies "the Sun"

Let's see how far we can get with the vis-viva equation and look at the problem purely from an energy standpoint.

$$v^2 = GM(2/r-1/a)$$

becomes

$$a = 1/(2/r-v^2/GM).$$

Let's look at a numerical example. Say $r = 1AU \approx 1.5 \times 10^{11}$ meters, and $v = 25,000$ m/s.

We know then that the semimajor axis of the orbit must be 0.773 AU. If the orbit reaches 1AU, the velocity will be 25,000 m/s. If it reaches 1.1 AU (which it might or might not, depending on eccentricity) it's velocity will be 21,500 m/s.

We're almost there but not quite. We need one more piece of information.

The periapsis and apoapsis $r_p$ and $r_a$ are related to the semimajor axis and eccentricity by

$$r_p = (1-e)a$$
$$r_a = (1+e)a$$

So for example if you had mentioned that the apoapsis was 1.1 AU, then we could calculate eccentricity from

$$e = r_a/a -1$$

and get $e = 0.4228$ AU and a periapsis $r_p = 0.387$ AU.

You can't do that. The object has 6 degrees of freedom and hence needs 6 variables to describe it. A common method is to use the orbital elements given here.
A description is given by parameters such as eccentricity, semi-major axis and inclination.


About how many revolutions has the Earth made around the sun? The Earth is about 4.543 billion years old.
However, the length of a year can be changed by anything from an impact to a natural cycle of orbit changes to a slow drift toward or away from the sun to the tug of a nearby body.

So while the Earth is about 4.543 billion modern years old, it may have revolved around the sun a significantly different number of times.

What is a reasonable estimate for how many revolutions - factoring in known geologic events - that the Earth has made around the Sun?

This question touches on the topic of orbital changes but never really answers what I'm asking here.
The second answer on this Quora question seems to answer my question but I don't know if that source is reputable, and the actual calculations and reasoning are not given
This reddit discussion touches on the subject but has differing / inconclusive answers

Since the very early solar system, there have not been large-scale movements of the planets.

In the early solar system, while the planets were still embedded in a protoplanetary disk, there were large movements. (notably the hypothesised "Grand Tack" of Jupiter) However once Jupiter reached it's current position of 5.2AU it remained there, and the Earth formed then too.

There have not been any significant changes to the Earth's orbit since the Earth formed, otherwise there would have been significant heating or cooling. So the average year length was and is about 30 000 000 seconds, and Earth has made about 4.5 billion orbits of the sun since then.

Back of the envelope time :

Taking a rough estimate of 35 days per half billion years as the change in orbital period over time (from that Reddit page), we get the orbital period of about 365+4.5*2*35 days 4.53 billion years ago ( about 680 days ).

The average period over that time is about 522 days, so over 4.5 billion years (with those years being our current definition of year ) that's about 3.1 billion orbits.

But it's a pretty useless number, I think.

It's worth noting that we could not accurately simulate such a large number of orbits with any accuracy.


Cosmological redshift vs doppler redshift I'm reading Harrison's "Cosmology: Science of the universe" because Harrison focuses on the distinction between cosmological redshift (he calls it expansion redshift) and the Doppler redshift.

He states that "they [Doppler redshifts] are produced by peculiar and not by recession velocities"
and
"[expansion redshifts] are produced by recession and not
peculiar velocities".

I understand the concepts of both kinds of redshifts but have a hard time understanding this strict separation. Please correct me, where/if I am wrong:

Suppose a galaxy has no peculiar movement. This means that its position will stay (approximately) the same in comoving coordinates. In fact though ( proper distance), it IS moving away from Earth with recession velocity V, caused by the expansion of the universe. So it should have a Doppler effect based on this recession velocity AND a cosmological (expansion) redshift should also take effect because the light gets stretched with the expansion of the universe. Even though the recession velocity is not due to a peculiar movement, it means that the source of light is moving away from the observer and hence the light should be redshifted and on top the light gets redshifted on its way through expanding space.

Please correct me or tell me if I am right or wrong, I have spent a lot of time reading but still don't fully understand this. Thanks.

After considering @benrg's comments, I realize that my first answer contained too strong statements about the relation between the two redshifts. I try here to moderate my answer, but you might want to accept their answer instead.

It is common to think of the two redshifts as having nothing to do with each other. Doppler shifts arise when the observer and/or the emitter moves through space, whereas the cosmological redshift can be derived considering stationary emitters and stationary observers in an expanding space.
Because the cosmological redshift doesn't involve movement through space, it is often considered completely different from the Doppler. However, it is also possible to derive the cosmological redshift by considering it as infinitely many infinitesimally small Doppler shift (e.g. Lewis 2016). I admit that I'm not well enough versed in general relativity to be certain about my statements, but just because an infinitesimally small patch of spacetime is flat doesn't necessarily mean that infinitely many such patches add up to be flat. However, as @benrg says, in GR there is only one redshift.
Different or the same?
The reason I think it makes sense to view the Doppler shift and the cosmological redshift as two separate mechanisms is the following:
In principle you could have a universe (non-capitalized, since it's not our universe, the Universe) that were static when a distant galaxy emitted a photon, then at some point expanded quickly by a factor of 2, and then again is static. In this hypothetical case, the observer would still measure the photon to have been redshifted by a factor of 2 (i.e. $z=1$).
That this is true can bee seen from considering the mathematical derivation of the cosmological redshift (see e.g. here) which involves an integral, the result of which only depends on the initial and the final state, not on the expansion history.
In contrast, if you and your friend stand still with respect to each other while your friend shines her flashlight at you, then run away from each other with a relative velocity of $0.6c$, then stand still before you receive the light (i.e. analogously to the hypothetical universe above), then you would measure no redshift; you wouldn't measure the special relativistic Doppler shift of $z+1 = \sqrt{\frac{1+v/c}{1-v/c}} = 2$ that you would if you were receding from each other while either emitting or observing.
In the real Universe, galaxies move through space (i.e. they change their comoving coordinate $\chi$), and space expands (i.e. the scale factor $a$ evolves). If the physical distance to a galaxy is
$$
d = a \chi,
$$
then the change in this distance gives their total velocity wrt. us, and is obtained through differentiation:

$$
\begin{array}{rcl}
v_\mathrm{tot} & =      & \dot{a}\chi + a\dot{\chi} \\
               & \equiv & v_\mathrm{rec} + v_\mathrm{pec},
\end{array}
$$
where dots denote differentiation with respect to time, and the two terms have been identified as the cosmological recession velocity, and the peculiar, "normal" velocity. Each of these terms give rise to a redshift, but through two very different mechanisms. Only the latter term is called a Doppler shift.

There are several sorts of redshifts and they are not always easy to separate from one another. First you have cosmological redshift. These redshifts tend to be so high they could not be caused by anything else. Then you have gravitational redshifts, which when they originate with distant quasars having a mass equivalent to trillions of suns might become entangled with their cosmological redshifts. We also have redshift (sometimes blueshift) which arises from the proper motion of the galaxy in question. These red or blue shifts tend to mask any cosmological redshift for nearby galaxies, which redshift is in any case trivial. The Andromeda galaxy, for example, is slightly blue shifted, as it is coming towards us and is 'only' 2.5 million light years away. The distant quasars I referred to are supermassive black holes many billions of light years away.


Using the Sun as a Gravitational Lens Can the Sun be used as a gravitational lens to achieve better telescopic viewing? Can this effect be practically used to view celestial objects?

Yes, it would be possible. There are two roads here:

Visible light
In case of detecting light in the range of visible wavelength, perhaps you would consider that rare occasion when there is a solar eclipse.
And it may also be possible at times of 'early' dawn and 'late' dusk.

Invisible light (outside visible range, beyond the IR and the UV)
Now, other than that, we have telescopes for radio, infra-red, UV rays and X-rays which wouldn't require us to look into the 'eyepiece'. Astronomers have detectors for that and light received is stored in 'pixels'.

You might also want to look at a similar question in Physics-SE here.
As pointed out by Jeremy in a comment below and also pointed out here, it is actually a far fetched idea. You need to be really far away from the sun to use it as a G-lens. The telescope has to be around 50 billion miles away and to cover that distance within one generation, probably we haven't achieved that technologically advanced level yet.

In a word, no. The light path for a gravitational lens around the sun would be too close to the surface to see the star behind the sun. Usually, a gravitational lens is from a galaxy far away so that the light can converge after passing the galaxy.

Aside from the sun, do you see any stars during the day?


Mountains are higher than the atmosphere? Is it theoretically possible that planets exist with mountains so high that their peaks overtop the planets atmosphere? And which physical laws are relevant for this question? I'm just curious.
The question came while I imagined the atmosphere like a second ocean above our ocean of water. And I thought it would be nice if such a gaseous ocean could have islands as well.

This is a bit of a gray area, as an atmosphere doesn't have a clear boundary. That being said, Olympus Mons on Mars is so tall, the atmospheric pressure on top of it is only 12% the average pressure on the surface of Mars. That's near vacuum by terrestrial standards.

https://en.wikipedia.org/wiki/Olympus_Mons#Description

In general, for this to happen you need:

a pretty thin atmosphere to begin with
some exceptional geology that ends up producing very tall anomalies like Olympus

It's not a very likely combination, but it can happen, as seen on Mars.

I think the planet's size and it's geological activity will dictate the mountains height. Mars is smaller than Earth and no active core, So has high probability to have mountain like Olympus Mons.


How do I plot galactic coordinates using matplotlib and astropy in python? I want to make a projection of millisecond pulsars on the galactic plane, much like this one from Sala et al. 2004:

I have tried several methods, and have not gotten anywhere. This is my current code, along with what it is producing:

import math as m
import numpy as np
import csv
import matplotlib.pyplot as plt
import pandas as pd
import astropy.coordinates as coord
import astropy.units as u
from astropy.io import ascii
from astropy.coordinates import SkyCoord

data = pd.read_csv('galacticwperiod.csv')

xarr = np.array(data.iloc[:,0])
yarr = np.array(data.iloc[:,1])
eq = SkyCoord(xarr[:], yarr[:], unit=u.deg)
gal = eq.galactic

#print(xarr)
#xarr = np.array(df.iloc[:,0])
#yarr = np.array(df.iloc[:,1])
#zarr = np.array(df.iloc[:,2])

#ra = coord.Angle(xarr[:], unit=u.hour)
#ra.degree
#ra = ra.wrap_at(180*u.degree)
#dec = coord.Angle(yarr[:], unit=u.deg)
#print(ra)

plt.figure(figsize=(6,5))
fig = plt.figure()
ax = fig.add_subplot(111, projection="aitoff")
plt.plot(gal.l.wrap_at(180*u.deg), gal.b.wrap_at(180*u.deg), linestyle='None')
ax.scatter(gal.l, gal.b, linestyle='None')

#ax.set_facecolor('xkcd:battleship grey')
#fig.patch.set_facecolor('white')
#ax.tick_params(axis='both', which='major', labelsize=10)
#ax.grid(color='b', linestyle='solid')
fig.show()
#plt.savefig('millisecondcoloraitoff.png', dpi=600)

Here are a few lines of the input file 'galacticwperiod.csv':

Gl,Gb
111.383,-54.849
305.898,-44.888
305.913,-44.877

This is the image it produces:

I am almost certain this is wrong because they are not distributed along the galactic plane, which they should be. The data I am using is from the ATNF Pulsar Catalog.

These are the sites I have already looked at, for reference:

https://stackoverflow.com/questions/33105898/astropy-matplotlib-and-plot-galactic-coordinates
https://astropy4scipy2014.readthedocs.io/_static/notebooks/06_Celestial_Coordinates_solutions.html

Any help with this would be greatly appreciated.

This code reads coordinates as equatorial (ra, dec) and transforms them to galactic (l, b):

eq = SkyCoord(xarr[:], yarr[:], unit=u.deg)
gal = eq.galactic

The contents of 'galacticwperiod.csv' are already in galactic coordinates and should not be transformed.
Something like this may give better results:

gal = SkyCoord(xarr[:], yarr[:], frame='galactic', unit=u.deg)

The other issue is that pyplot's geographic projections seem to expect angles in radians.
This code:

plt.subplot(111, projection='aitoff')
plt.grid(True)
plt.scatter(gal.l.wrap_at('180d').radian, gal.b.radian)

produces this plot:

from astropy.io import fits
from astropy import wcs
import matplotlib.pyplot as plt

x = fits.open('f160w_noopt_drz.fits')['SCI']
y = wcs.WCS(x.header)

plt.subplots(figsize=(8, 8), subplot_kw={'projection':y})
plt.imshow(x.data, cmap='viridis', origin='lower', vmin=0, vmax=10)
plt.show()


Does the mass of the observable universe ever change? First do we have anyway to even estimate the mass of the entire observable universe?  And then is there any data that shows mass being gained or lost?  Would we ever know if someone was playing with the til.

Also I want to be clear that I am not talking about small masses on the outskirts of the "universe" or small discrepancies in measurement or anything of the sort.

Note:  I would like to add that maybe we should define the observable universe as NOW (x-date) so that we aren't calculating a moving target.

Even if you're only referring the "ordinary" matter (such as stars, gas, and bicycles) and dark matter, the mass of the observable Universe does increase, not because mass is being created, but because the size of the observable Universe increases. In a billion years from now, we can see stuff that today is too far away for the light to have reached us, so its radius has increased. Since the mass $M$ equals density $\rho_\mathrm{M}$ times volume $V$, $M$ increases.

As called2voyage mentions, we have several ways of measuring the density, and we know it's close to $3\times10^{-30}\,\mathrm{g}\,\mathrm{cm}^{-3}$. The radius is $R = 4.6\times10^{28}\,\mathrm{cm}$, so the mass is
$$M = \rho_\mathrm{M} \frac{4\pi}{3}R^3 \simeq 10^{57}\,\mathrm{g},$$
or $5\times10^{23}M_\odot$ (Solar masses).

However, another factor contributes to the mass increase, namely the so-called dark energy, which is a form of energy attributed to empty space. Since the Universe expands, dark energy is being created all the time.

The apparent density of the universe is inconsistent with available observations.  Because of the relatively slow speed of light, observations do not reflect actual distributions.

Galaxies are now being observed in positions and size billions of years ago.  While our own Milky Way has a computational time dispersion over millions of years.

Add to this that galaxies are observably moving apart from each other at greater than the speed of light, means that there are galaxies moving away from us at speeds greater than the speed of light and are likely not detected because of their weakly dispersed signature.

The observable universe is likely skewed with sufficient variability in trustworthy evidence, that makes measuring universal density impractical, and untrustworthy.


Slow moving star like objects in the night sky So yesterday at about 2am I was watching the sky when I saw a small star like object moving very slowly across the sky. My initial thought was that it was a satellite due to the fact it was moving so slowly and wasn’t flashing. However  then I saw another one moving in the same direction about 10 minutes later. I had to keep letting my eyes adjust to the darkness to see it as it was so small but in total, I saw 4 of them moving in very similar directions, almost like they were following a path. There was about a 5-10min gap between each one and I could see it the entire time until it dipped over the tree line. I live in a very rural area so there was no light pollution or anything to interfere with the view. Now I know that around about now we are able to observe the Perseid meteor shower from the comet Swift-Tuttle but these object were very slow moving and I don’t really know how slow a meteor can move, but isn’t it strange that all of them were moving in the same direction? I also did see some normal meteors but as you’d expect, they only lasted for a second or under. Anyone have a clue what’s going on? Thanks.

Unlike meteors, which burn out in a second or two, satellites can be visible for a few minutes at a time.
In twilight they may be visible all the way across the sky; at darker times of night the Earth's shadow cuts their visibility short unless they're in high orbits.

One good resource for such things is Heavens Above.
From Hereford, UK, their Daily predictions for brighter satellites list a few matching your description on the morning of 2019-08-07.
For example, the polar-orbiting Russian intelligence satellite Cosmos 1975 passed from WSW to N between 02:00 and 02:04.

Forget about meteors. They are only the size of a sand grain or small pea. They hit the atmosphere at anything between 15km per sec and 25km per sec and burn up almost instantly. Anything large enough to last a lot longer would probably be a meteorite, and would be much brighter (I have seen a couple which lit up the countryside}. What you saw was probably a chunk  of satellite debris in the early stages of re-entry. This often causes bits to break off and trail behind, which almost never happens with a meteorite. Meteorites are meteors large enough to reach the ground.


What would you find within a void? I'm aware that voids are relatively empty regions of the universe, but just how empty can they be? Wikipedia states that voids have 'few or no' galaxies, but I can't find much else.

To make my question specific, suppose I were in the centre of a large void, way outside of the few galaxies that may still exist in this void. Would I occasionally encounter asteroids (if I may call them such), rogue planets, or even some occasional intergalactic star?

Remark: I'm aware that this question is a bit of a one-up of What is there in the intergalactic space?. If this is too much of a duplicate, let me know and I remove the question.

The Wikipedia article on voids is pretty good (though IMO unusually awkwardly written.) The key thing is that voids are not empty, they are just large volumes which have a lower density (typically around 10% of average) compared with the rest of the universe.

These low density areas still contain stars and galaxies, just fewer of them and the galaxies they contain tend to be smaller. The Lambda-Cold-Dark-Matter (LCDM) model, which is currently favored as our best description of the universe, predicts that the voids should not have many large galaxies in them, but should have a fair scattering of small, diffuse galaxies (SDGs).  (A small diffuse galaxy is, well, small compared with the Milky Way --
sized more like the Magellanic Clouds or one of the small galaxies which are being devoured by the Milky Way today such as the Sagittarius dwarf. They are also more spread out than the Magellanic Clouds.)

Here's an interesting senior thesis reporting on a search for small diffuse galaxies in voids. (They think they found some, but need confirmation.)

You ask what the night sky would look like from within a void. First, we'll assume you're in one of the SDGs, though this wouldn't make a huge difference.  What you'd see mostly is whatever planets share your solar system -- they'd be comparable in brightness to our own planets from Earth. You'd also see a few dim stars.  SDGs tend not to have much star formation, so there would be very few young stars and thus very few intrinsically bright stars. The visible stars would all be ones which happened to be nearby. And because SDGs are diffuse, there'd be many fewer nearby stars. At a guess there would be 20-200 naked eye stars, with only a handful of magnitude 3 or brighter. (Compare that with 6000 naked eye stars visible from Earth.)

There would be no naked eye galaxies. The SDG itself would not appear as a milky way, but more like the Zodiacal Light -- a vague glow dimly visible under exceptional conditions.

This would be a world whose astronomy would be heavily focused on the planets.

(Planetary formation might be quite different in SDGs, but I don't know even to try to speculate on how it would be different. So I assume that the planetary system would be similar to ours.  YMMV.)

Apart from what you read in Wikipedia, nobody knows. All the things you mention could be in there, but these voids are just too far away to detect them. Commonsense dictates that like the rest of space, they must be seething with neutrinos, photons & other particles. QM mechanics will also tell you that space,including the voids,is seething with virtual particles.

At the risk of being condemned for heresy, I have to admit that so far as virtual particles are concerned, I'm an atheist. I also have a suspicion that in some of these voids; the Boötes void for example, there is something mysterious in there or has passed through there to create the void, but that is speculation.


Plant on large Asteroid? I'm just curious if it is possible for a plant to survive on a large asteroid. Some say no, some say it depends.

With the scenario of an asteroid at about 1 AU from the Sun, receiving a similar amount of sunlight to Earth, and with wet soil or something similar.

It might depend on what qualifies as a plant.   A thin film of rugged mold for example . . . just maybe.

The tricky part is that asteroids are too small to retain an atmosphere and you need an atmosphere to have liquid water which plants and all life (so far as we know) needs.   I don't want to dismiss the possibility that there's underground water and some kind of very sturdy life that can eek out a living on the airless surface of an asteroid, but I wouldn't bet money on it.  I think it's unlikely, maybe unlikely multiplied by about 5 orders of magnitude.    It's very close to impossible.

That said, Ceres is an asteroid and Ceres may have a liquid underwater ocean.   Life in an underwater ocean is thought to be possible.

It is certainly not possible for a plant to survive on any of our asteroids, even on Ceres the conditions are completely unsuitable (moulds, by the way, are fungi, not plants, and couldn't survive unless there were plants present). The other obstacle to plants emerging on a Ceres-sized asteroid is that Ceres never had suitable conditions for life to emerge.

I think Pluto or Eris is about as large as an asteroid could get and still qualify as an asteroid, and there probably are solar systems with asteroids that size within the habitable zone, in which case it is conceivable that life might arise on them. Unfortunately they would be likely to end up like Mars, which once had conditions suitable for life, but lost most of its atmosphere over the course of billions of years. Micro-organisms might survive on such asteroids, but nothing more advanced.


What's the name of outside the cosmic horizon? The word we use to refer to what's inside the cosmic horizon is the 'universe', so what would you call the empty space outside of the horizon?

The outside is also the universe, the inside is just the observable universe.

This relates to what I was saying about the universe was once the size of a grapefruit morphing into the observable universe was once the size of a grapefruit. Along with the universe is infinite. IMHO there is no evidence for the latter.

To be pedantic, I guess the answer to your question is "the non-observable universe". That’s because the word universe is derived from “uni” as in unicycle and “verse” as in vice versa. It means turned into one. It means everything. Hence the word "multiverse" doesn't float my boat.


Why does a planet rotate and revolve? Why do planets rotate and revolve in the universe?

As the planets evolve during their protoplanetary stage and accrete materials from the protoplanetary disks, which are gravitationally collapsing interstellar dust and gases, these accreted particles retain some of the angular momentum from the materials they form from and being in constant motion.

      Generated image (virtual fly-by) from a simulation of the accretion period of the protoplanetary disk, showing preservation of      angular momentum in the orbit around a Jupiter-size planet, as it clears its neighborhood. (Source: Frédéric Masset)

One nice description for this angular momentum preservation, and why the planets appear to rotate faster than their surrounding protoplanetary disk goes like this:

  Conservation of angular momentum explains why an ice skater spins more
  rapidly as she pulls her arms in. As her arms come closer to her axis
  of rotation, her [rotation] speed increases and her angular momentum remains the
  same. Similarly, her rotation slows when she extends her arms at the
  conclusion of the spin.

  Source: Scientific American article on Why and how do planets rotate? (George Spagna)

So it could be described as this axial rotation of planets resulting in conservation of the angular momentum of the materials in the protoplanetary disk, forming during the accretion period of the planetary system as the protoplanets gain in weight, and preserve this angular momentum due to inertia of their radial velocity.

Newton’s Law of universal gravitation and Kepler’s Law of planetary motion describes the movement of planets around the sun. Equations are derived from these laws and hence the birth of Celestial Mechanics. But nowhere in the literature can we find an accepted law of Planetary Rotation because everyone is convinced that there is nothing special about the rotation of the planets. Except for the “overused” explanation…

“A long time ago in a galaxy far, far away...spinning gas and dust flattened into a protoplanetary disk and due conservation of angular momentum the planets are now rotating with RANDOM velocities”

It’s the same as saying we don’t really know how it works. We have a concept but not enough to express it in numbers.

This is a quote from Lord Kelvin (William Thomson):“I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.”

Follow the below link for the Planetary Spin Equations.

https://www.quora.com/What-determines-the-rotation-period-of-planets/answer/Randy-Evangelista-1


Milky way: How do we know its appearance? How can we know how does the Milky Way look like if we are in it?
Sorry if the answer is evident, I am not an expert.

There are lots of reasons. First of all because we can see that a typical galaxy is a spiral galaxy and we seem to live somewhere pretty typical.

Secondly because we can see the arms of the spiral through close examination of the Milky Way (ie the band in the sky) - through stellar distances and measurements of hydrogen cloud red shifts and so on.

But you your question is a perfectly good one because, ultimately, we are guessing a bit as we cannot see the galaxy from anywhere other than one vantage point.

Milky Way is a young galaxy, filled with neutral hydrogen along its arms.
This hydrogen can undergo state transition, a rare event, which is compensated by the huge amount of hydrogen itself, i.e., even if this transition is rare it is still strong because a large amount of atoms can exhibit it.
The basics of this transition is explained by quantum physics through state-change in the energy levels of the hydrogen atoms, and can be summarized in the following picture:

The electron in the atom transits from a level with parallel spin (with respect to the nuclear proton), to the lower state, where the electron spin is antiparallel.
When this happens, the atom emits a photon, known as the 21 cm radiation, or hydrogen line.
This line falls into the radio domain, which can be observed by radio telescopes.

Since this radiation comes from rotating arms, doppler shift of the line can trace the distance of the emitter region.

And voilà, you have your map.

EDIT: ops, I missed the previous links where the answer was already given.


The size of Copernicus’ and Ptolemy’s corrections I understand that both Copernicus and Ptolemy needed epicycles, but one might assume that a heliocentric system would need much smaller corrections to account for the non-circular motion of the planets.  This is however not mentioned as an important reason to favor Copernicus’ system. Is this not the case?

I gather the worse problem for Copernicus was Mercury.  Not only is Mercury's orbit quite elliptical, it has relatively high precession due to the gravitational influence of other planets.

I found a quote attributes to Copernicus in this document (sorry, only direct link available) where he says :

  It is interesting to note that Copernicus had difficulty explaining Mercury’s motion and once commented that ”this planet has .  ..  influenced many perplexities and labours on us in our investigation of its wanderings".

This is apparently originally from this source :

Baum, Richard and Sheehan, William.In Search of Planet Vulcan:  The Ghost in Newton’s Clockwork Universe. Plenum Trade, New York. 1997.

The gravitational perturbation of the planets, particularly by Jupiter, and the concept of barycenter for even ideal elliptical orbits required the work of Newton (following Kepler's concept of elliptical orbits) to explain.  Up until Newton there was no solid theory for the motion at all and people were "just" (poor choice of words) trying to find the simplest model.  Copernicus provided a simple model, but also a less accurate one.

To the authors of both systems, Mars occasionally seemed to move backward in its orbit, so epicycles were needed to account for this. It was not until Newton discovered the laws of gravity that it was realised that Mars was compelled to move slower than the Earth and therefore was sometimes overtaken by the Earth in the journey around the sun. The impression that Mars was moving backwards was of course an illusion, but still had to be accounted for. Copernicus was unaware of Newton's laws, but at least he realised that the orbits of the planets were heliocentric rather than geocentric. He also realised that Mars had further to travel than the Earth.


Why aren't neutron stars full of dark matter? Dark matter interacts with the gravitational force right? Well, unlike black holes, neutron stars are actually visible, and they're an enormous gravitational sink, so dark matter should collect to them.

But if all that is true, which it seems to be, why haven't astronomers detected or used neutron stars to detect dark matter?

Yes, neutron stars might actually accumulate weakly interacting dark matter and this allows some observational constraints on its nature. Basically, the temperature and continued existence of neutron stars places bounds on the density and interaction cross-section of dark matter.

A dark matter particle that does not interact with matter will just have its trajectory bent by the gravity field of a heavy object, so most unbound particles will just swoop past on a hyperbolic trajectory. But as discussed in (Adams & Laughlin 1997), if there is some matter-dark matter interaction then the particle may scatter from a matter particle and now have less than escape velocity. This way white dwarfs and neutron stars would indeed accumulate dark matter in their cores. The rate of accumulation is proportional to $\rho v \sigma$ where $\rho$ is the dark matter density, $v$ the average relative velocity and $\sigma$ the cross section. Adams & Laughlin estimate that a white dwarf star would accumulate its own mass in $10^{25}$ years, but this is going to be depend on the cross section (if it is too small the dark matter will pass through) which is at present unknown.

Were this accumulation the only thing happening it would eventually make white dwarfs and later neutron stars implode. However, dark matter is plausibly a mix of particles and antiparticles that annihilate each other at a rate $\sim \rho^2$; in an enriched environment like a white dwarf core this would produce energy from emitted photons heating things up. Adams & Laughlin estimate the luminosity as to about $10^{-12}L_\odot$, which is imperceptible in the current era but would eventually keep white dwarfs at 63 K in the far future (until the dark matter halo runs out).

Other, more elaborate, calculations lead to accretion estimates that are higher. If the rate were high enough, then we would not see any cool dense objects - so white dwarf and neutron star cooling gives some bounds on the possible density and cross sections, albeit not very strict ones. For example, one model suggests that neutron stars would level out at 10,000 K. Cool star observations can also already rule out some dark matter models.

There are even some arguments that super-earth planets in dense dark matter halos might be heated significantly, although this may require unrealistically dense halos and big cross sections. The current heath flow of Earth does give some constraints on how strongly it can interact.

So neutron stars are not directly giving us dark matter detection, but they (and planets and white dwarfs) are giving us some information.

Why aren't neutron stars full of dark matter?

Because dark matter doesn't consist of particles. There's something of a myth  that it does, which I think comes from particle physicists who have never actually read Einstein's original material. I also think science is something of a competitive business, and there's a tendency for advocates to promote their own theory (eg WIMPs) and claim that a competitor theory (eg MOND) is flawed.

  Dark matter interacts with the gravitational force right? Well, unlike black holes, neutron stars are actually visible, and they're an enormous gravitational sink, so dark matter should collect to them.

Remember that we have good scientific evidence for flat galactic rotation curves and other phenomena. These suggest that either a) there's some unseen "dark matter" around somewhere, or b) that gravity doesn't work quite the way that people think. However the evidence does not actually say dark matter is made out of particles and falls down.

  But if all that is true, which it seems to be, why haven't astronomers detected or used neutron stars to detect dark matter?

Because we do not live in some Chicken Little world where the sky is falling in. I'm referring to Gullstrand-Painlevé coordinates which model a gravitational field as a place where space is falling down. Einstein rejected the idea, but some contemporary physicists take it seriously, see this for example.

Image credit Andrew Hamilton

Why is this relevant? Because in his 1916 Foundation of General Relativity Einstein said “the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy”. This is spatial energy, and it isn't made of particles. The energy density of space near the Earth is greater than the energy density of space further away from the Earth. Because of this, there's a gravitational effect. This is why "gravity gravitates". Einstein also described a gravitational field as a place where space is "neither homogeneous nor isotropic". So dark matter might simply be inhomogeneous space. Don't forget that as per the raisin-cake analogy, the space between the galaxies expands whilst the space between the galaxies does not. Conservation of energy tells me that this will surely lead to an inhomogeneous spatial energy density. And that an older galaxy will be surrounded by a bigger/steeper halo of inhomogeneous space than a younger galaxy, so it will look like there's more dark matter present.

What Einstein said means that there's "dark matter" of sorts in the room you're in, right in front of your face. Only it isn't made of particles, and it isn't falling down. Instead it's made of space. Don't forget that space is dark, and there's a lot of about.


Can there be an infinity of humans in the Universe? I was reading this question about if the number of stars can or cannot be infinite.
The answer to this question is:

"if the universe is considered infinite, the number of stars can be infinite, otherwise not."

So, I came across a mental problem: If the universe is considered infinite, and therefore we can consider that the number of stars is too, then, we can consider that the number of humans (example) is also infinite, or the number of anything is also infinite?

If the answer is No, please describe why u think this is not a real affirmation.

IMO, this sounds more like a philosophical question rather than an astronomical question, but since I like both subjects, I'll give it a shot.

  Note: This is hypothetical question, so this answer is purely subjective (and hypothetical).

Infinity

If the number of stars is infinite, so would the numbers of humans.

I believe that in every system where an entity of infinity is introduced, all other entities will (eventually) be infinite. But, (and this is the fun "part") you cannot measure infinity. It would give you infinite results over a time span of infinity.

Take your example for instance. If there were infinite numbers of stars. Picture youself flying around in a spaceship with the goal to find out if this question is true or not. What do you see? "Worse case" scenario; you see only stars. Wherever you go. Forever. The opposite;  only humans.

One might think that it's the "in between" that is the most likely scenario. But given the probability theory, we should flatten out at approx. 50%.

Finite

If there is a finite numbers of stars then I think it's "safe" to conclude that there ain't an infinite number of humans as we're all made of starstuff.

Conclusion

  Yes: If there's an infinite numbers of stars.

  No: If there's a finite numbers of stars.

There are different infinities.

You could say that there are an infinite number of planets, so probability says that there could be humans on a small percentage of them. Say for arguments sake humans exist on 0.001% of those planets.0.001% of infinity is infinity, right? Well, sort of, but it's obviously a smaller infinity than the number of planets.

Conversely there are 99.999% of planets without humans, which is a much bigger infinity.

So to all intents and purposes the answer is no, for this mental exercise.

And in reality it's a no, as any humans on other planets would pretty much have to not be humans any more, as they are so far away that unless we have instantaneous or FTL travel, evolution would have changed them/us.


Number of stars & planets in Milky Way Galaxy? Is there any specific numbers on stars & planets count on Milky Way Galaxy ?

A consensus number is that there roughly $10^{11}$ stars in our Galaxy (though this number is certainly uncertain by a factor of at least two, because it is based on extrapolating what we know about stellar populations in our vicinity). Most of these stars are of lower mass and are much less luminous than the Sun.

The number of planets is even more uncertain. It now seems probable that most stars like the Sun have at least one planet, but we really don't know that much about the diversity of planetary systems, how typical something like the solar system is, or how planetary systems might change as a function of position in the Galaxy. We also don't know that much about planetary systems around the dominant (by number) very low-mass stars. They certainly can have planetary systems, but the fraction that do is still a work in progress.

The number of stars and planets in the milky way can only be given in approximate values.There are about 100-400 billion stars and as of 2016 there are about 2097 planets discovered from 1337 planetary systems.


Naming of the planets of the solar system Planets of the solar system have been named with Roman mythology gods names. I have a few questions on my mind concerning that subject for a while:

Who decided to name them like this?
When did the planets got named?

Finally, the ultimate question is: if the planets got named by the Romans, implying they had not technology to observe the solar system whatsoever, why some of the names match the color of the planets?

Mars is red - Mars is the god of war
Neptune is blue - Neptune is the god of the oceans

Or maybe I am wrong (most likely) and the planets have been named more recently.

Despite of the explanation of your question, it is a valid question to ask why planets all have Roman names. First of all, the Romans could, like the Greeks and Sumerians, could only see Mercury, Venus, Mars, Jupiter and Saturn. These planets can be seen with the naked eye. However, the fact that the Romans could see these planets, didn't give all the planets latin names.

If the Names of the planets were given by the ones that discovered them first, the planets would have Sumerian names. The answer to the question why the planets have latin names is because they don't. The first western scientist took over the Roman names, because latin was they scientific language of the middle ages and the renaissance. After the discovery of Uranus and Neptune these planets were not given these names by their discoverers. Only after longer controversy western astronomers have standardized the names of planets and moons according to Greek and Roman mytholohy, but this was only around the second half of the 19th century.

Since the beginning of the 20th century the IAU is setting the standards for naming celestial bodies including planets. However, the IAU doesn't set the names, it only sets the standards. Newly discovered objects are now named according to these rules by their discoverers. Older planets are still living with their old names and for western astronomers those are still the names from Greek and Roman mythological figures. However, the IAU allows you to consistently name the planets by their Arabic or Chinese names. So, the question is valid, the text around your question is unfortunately not true, since the planets don't necessarily have Roman names. If you are An Asian person and correctly think the planets should have their Chinese names, because they were earlier in discovering these objects than the Romans (although not earlier than the Sumerians), you are allowed by the IAU (sic) to call them by their Chinese names.

Cavemen were able to see the first six planets, up to Saturn. All these planets were named after the Roman and Greek mythological characters.

The rule they followed was something like this: The planet will be named after the Roman god and all its satellites will be named after the corresponding Greek gods. For example, Jupiter was named after the Roman King of Gods, and one of its satellites, Ganymede , was named after the man-servant of Zeus. Now Zeus is the Greek counterpart of Jupiter.

Now, Uranus is the lone exception for this scheme. All of its satellites, Miranda, Ariel, Umbriel, Titania, and Oberon were named after Shakespearean characters. This was to please the British because, they were very powerful in that period of time and its discovery was made by William Herschel in the 1780's.

P.S. Uranus was first named 'George' by Herschel because the King of England, King George, was his main source of funding and he didn't want to annoy him.


What differentiates a "group of rogue stars" from a galaxy? According to Wikipedia, a rogue star or intergalactic star is

  not gravitationally bound to a galaxy.

But according to a citation on the same Wikipedia page, the first intergalactic stars were discovered in the Virgo cluster of galaxies.

  These stars form a massive group  aproximately 300,000 light years away from the nearest galaxy. (Emphasis mine)

Is there any clear, stated definition which differentiates galaxies from "massive groups of rogue stars"?

It is just a poor choice of words. The comment refers to a discovery paper by Ferguson et al. (1998), where they used a very deep image in the Virgo cluster to establish that there were an excess of distant stars compared with a control field outside of the cluster.

This excess of about 630 stars were all in one tiny HST field of view, so form a "group" in some sense, but there is no suggestion that these stars are not found throughout the Virgo cluster. Indeed subsequent work has found evidence for intracluster stars in other areas of the cluster (e.g. Durrell et al. 2002).

According to theoretical work by Dr. Ernest Sternglass, there are expected to be many such groups of "rogue stars".  In Sternglass's model [the "electron-positron pair model of matter"] there are expected to be many stars and clusters of stars which are not near any galaxy because of the process by which stars and star clusters form.

Briefly, the model says that each kind of astronomical object, such as a single star, or a star cluster, or a small galaxy, or a large galaxy, or a galaxy cluster, etc., is formed by a single massive explosion, (known as a "quasar").  A not-very-energetic quasar might form a star cluster, while a very-energetic quasar might form a large galaxy.

In other words, the model says that a quasar is a galaxy or star cluster, or other astronomical object, in the process of being born.


How is the size of a distant planet determined? How is an object's size determined from other solar systems than our own? How are new large bodies being found in our solar system currently?

One method of finding planets orbiting stars is to analyze the transit effect. This is when a planet crosses the face of a star between the star and observer.  This will block some of the light reaching the observer. By measuring this decrease in light from the star we can determine how much of the surface area of star is occluded.

The speed of a planet across the face of a star will let us calculate the orbital radius considering a circular orbit. The temperature of the star can be determined from its brightness and location on HR diagram and spectroscopic data. Combining this information lets us determine the size and of the planet.

Using space-based or ground-based telescopes and automated data collection and analysis methods large regions of the sky can be viewed for these transits. This method is highly accurate and will probably surpass the radial velocity method as most common method to discover new planets.

Another method for finding planets is known as the radial velocity method. When a planet orbits a star it is really the planet and star rotating about the center-of-mass of the system. Hence the star wobbles due to the bodies orbiting the star. It wobbles because usually the center-of-mass is inside the star and this results in the star rotating about an axis running thru the star but offset from the center. By doing a doppler analysis it can be determined whether the star is revolving or not and the period of revolution. The radial velocity method is the most effective method. The transit method is very limited by the viewing angle, the planet must pass between observer and star.

A good source to better learn about these methods is here.

The size of a planet is determined by

The amount of gas & dust available to build it.
The type of star it orbits.
Its distance from the star.
Whether or not the sort of resonances we see in Bode's Law come into play. We don't know for sure whether Bode's Law operates for all stars, some stars, or only our star.


Up to 384 minor planets (including Pluto) qualify as planets? In 2006, the International Astronomical Union redefined the definition of planet in order to exclude Pluto, Eris, and several other objects whose category was disputed. This new definition of a planet featured 3 criteria:

A planet must be massive enough to achieve hydrostatic equilibrum and
become ellipsoidal.
A planet must orbit a star or brown dwarf (with the exception of planemo).
A planet must be massive enough to clear any nearby objects.

Pluto and the other objects apparently failed to meet the 3rd criteria, as they had not cleared the nearby Kuiper belt objects, so they were reclassified under the term "dwarf planets".

But what if Pluto actually has met the third criteria? It's probably well-known by now that Pluto has 5 moons. The largest moon Charon is over half the size of  Pluto, and is frequently classed as a dwarf planet in its own right. The other 4 minor moons Nix, Styx, Hydra, and Kerberos, are much smaller then Charon, and are similar to the many asteroid-like moons found around gas planets.

The leading theory for how these smaller moons formed is that they are leftover debris from a collision between Pluto and another object, but it's just as likely that they were Kuiper belt objects that were captured by Pluto (just like the moons of Jupiter and Saturn). If they were captured, this means that Pluto has technically cleared 4 objects from its neighborhood, and so it meets the 3rd criteria.

The same logic can be applied to Eris, 2007 OR10, and the 400 other dwarf planets known to host moons. So should we have 392 planets, keep the current definition, or scrap or redefine the term "planet" entirely?

That actual IAU Resolution B5 adopted at the IAU General Assembly in 2006 states:

(1) A planet is a celestial body that:
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it   assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.

The last part being the neighbourhood around its (whole) orbit is the important part; it's not enough for Pluto to dominate its family of moons (and this last part is debatable as Pluto and Charon orbit a common barycenter (center of mass) which is between Pluto and Charon since Charon is so massive, compared to Pluto).
Since Pluto orbits in the Trans-Neptunian region with a similar orbit to other bodies, some of which are similar or larger in size and mass (Eris), and cross the orbit of Neptune, it can't really be said have to "cleared the neighbourhood around its orbit". Indeed Pluto is the prototype of the Plutinos; object which have a 3:2 mean motion resonance with Neptune (orbiting twice for every three orbits Neptune makes).

No, Pluto is not a planet by that definition. However, that definition has a serious issue: Earth is also not a planet. There are objects that have not been cleared from the Earth's orbit. (see https://www.nationalgeographic.com/news/2011/7/1107128-trojan-asteroid-earth-planet-orbit-nasa-space-science/)


Is the eventual heat death of the universe due to the expansion of the universe? I am reading about the accelerating expansion of the universe and something struck me. The observation that things further away were accelerating away faster and that early observations made it look as though we were situated at the center of the universe.

Specifically, it seems as though the universe itself is expanding and not that the things within the universe were moving away from each other. Instead it seems as though the expansion itself is creating more space between the contents.

Recently I also saw an amazing video on the eventual death of everything in the universe. This seems weird to me because as I understand it energy and matter don't go away they just change.

And it got me thinking, what if this eventual heat death of the universe is simply this expansion eventually ripping everything apart until every particle simply is to far apart to interact.

Basically, as things progress the expansion will affect the contents of the universe even down to the particle level.

Is this the case? Or am I misunderstanding this subject?

As far as I understand it, the heat death of the universe is a consequence of entropy, not expansion. All processes result in the shifting of some energy to higher entropy. Though the observable universe is an open system, the entire universe is an isolated system, so as more and more energy gets shifted to higher entropy over time the universe will eventually reach a state where effectively no work can be done.

Take a balloon and inflate it until it is firm. Then take a soft pen which won't burst the balloon and mark lots of dots all over it, fairly evenly spaced. The balloon is the universe and the dots are the galaxies. Now  put an ant on one of the galaxies and begin to inflate the balloon further. From the point of view of the ant, his galaxy is stationary and at the centre, with all the other galaxies moving away and the more distant galaxies moving away fastest of all.

Then put an ant on one of the other galaxies and try to visualise the situation as he sees it. This second ant will also see himself as at the centre of the universe with all the other galaxies rushing away, with the furthest moving away fastest of all. It doesn't matter which dot or galaxy you put the ant on, the picture will always look the same. There is an elastic force in the balloon which is always trying to collapse it, but the expansion resists this force. Its counterpart on a galactic scale is gravity, which is always trying to resist the expansion and gradually slowing it down. Until recently it was thought that eventually gravity would win, and the universe would be brought to a halt and collapse on itself to become a contracting, blue shifted universe, gradually speeding up and hurtling toward destruction in what was called the Big Crunch. But then dark energy threw a spanner in the works.

Dark energy is a mysterious force which astronomers think is speed ing up the expansion too much for gravity to slow it down and halt it. Observations and measurements of the redshift of distant type 1a supernovae are what brought them to this conclusion. However, the measurements are difficult to make and difficult to interpret, so there is always the possibility of error. It also introduces new difficulties, such as a one-off universe with no cause and which will go on expanding forever into the black nothingness of space, making the Big Crunch and the Big Bounce which follows it impossible if current interpretations of the data turn out to be correct. The Big Crunch model has not been entirely abandoned, though dark energy is the favoured theory at present. Dark energy has the disadvantage that no one can explain where it comes from or where it has been hiding, and it also contravenes the 1st Law of Thermodynamics. Some people say that the 1st Law doesn't apply on a cosmic scale, but that sounds like special pleading.

The heat death of the universe, as called2voyage says, is an entropy effect due to the 2nd Law of Thermodynamics, which for some reason can't be abandoned on a cosmic scale, though the 1st Law can. Unlike the 1st Law, the 2nd Law has over a dozen different formulations. From a dark energy, accelerated expansion perspective, it is the cold death of the universe we are talking about, for the black nothingness into which it is reportedly heading is also bitterly cold.


How far could be an object from the Sun and still be under the influence of its gravitational field? I'm trying to see how far can our star reaches with its gravity. I'm asking if anyone could give info as to what's our star's limit or the furthest object found in our solar system.

The Sun's gravity extends infinitely, but eventually solar objects would be unstable due to the influence of other stars. The minor planet "Sedna" has an orbit which takes it nearly 1000 AU (0.016 light years) from the sun at its furthest point (but now it is a lot closer)

It is also thought that billions of comets must orbit in the outer part of the solar system, out to 50000AU, or 0.8 light years, (or possibly further) forming the Oort Cloud. However, at such distances, they could not be directly observed. This marks the greatest distance at which orbiting solar system bodies can be found.

Take a simple case where we know the mass of two solar systems (M_1 and M_2) and the distance between their centers of gravity (x). We want to find the location between them where the two forces of gravity from each system cancel out. Where an object placed on one or the other side of that point would eventually fall into one or the other the star system.
To find the point of equilibrium between 2 systems, first we need the formula for the force of gravity:
F = GmM/R^2
(this is Newtonian gravity, so it is ultimately wrong but a fair approximation anyhow).
I am using 'M_1' for the mass of the first system, 'R_1' for the distance from the center of the first system to the placed object, and 'm' for the mass of the placed object.
F_1 is the force from the first system acting on the placed object:
F_1 = GmM_1/R_1^2
We do the same thing for the second system:
F_2 = GmM_2/R_2^2
And then we set the forces equal eachother to find the point where they will cancel out:
F_1 = F_2
At this exact point an object is pulled equally by the forces from both systems and will stay motionless. We can see that several variables cancel out, the mass of the object (m) and the gravitational constant (G), and we are left with:
M_1/R_1^2 = M_2/R_2^2
Since we know the distance between the systems (x) we can make a substitution using the formula:
R_1 + R_2 = x

What we are left with (after some algebra) is the infamous quadratic equation solution, where:
a = 1 - M_1/M_2
b = - 2*x
c = x^2
Lastly, we need the quadratic equation:
R_2 = [-b +/- sqrt(b^2 - 4ac)]/[2a]

Skipping the algebra, simply plug your 3 knowns into a, b, and c and apply these to the quadratic solution.
You can then find R_1 using the formula:
R_1 = x - R_2
The concept is similar to rain basins, these are space basins. In the case of a basin in space we merely create a point and draw a line perpendicular to an imaginary line connecting the 2 systems. In a more complex set of multiple systems we find the equilibrium points between all neighboring systems and extend the perpendicular basin lines to where they first touch one another.

Sample problem:
M_1 = 1 solar masses
M_2 = 2 solar masses
x = 100 au
Solution:
R_2 = 58.5786437626905 au