# Presentation: 116045

## The Big Bang

***Koji Mukai***

- Elements 2002 Workshop

## What is the Big Bang?	

**‘****Big Bang’ Cosmology is **
**The theory that the expansion of the universe began at a finite time in the past, in a state of enormous density and pressure (Weinberg)**
**Tightly constrained by observations**
**Highly successful family of theories with no obvious competitor**

**Notes:**
“The First Three Minutes” by Steven Weinberg is somewhat old, but still a great introduction to Big Bang cosmology.

## Unanswerable Questions

**This subject can generate countless questions: some are based on misconceptions, some are about the unobservable.**
**Where did the Big Bang occur?**
**What is the Universe expanding into?**
**What happened before the Big Bang?**
**Are there many universes?**

**Notes:**
This subject can produce so many questions that are, to put it somewhat unfairly, are the equivalent of “when have you stopped beating your wife?”
The best outcome for a class may be for the students to be able to ask superficially very similar questions but with a better understanding of the current state of the art.

## Key Concepts

**The Universe as a whole is expanding.**
**Hubble’s Law: recession velocity is proportional to distance**
**Expansion implies early Universe was dense and hot, sufficient for fusion**
**Further back in time you go, the more uncertain the theory becomes**
**Predicts a Microwave Background**

## A Cosmic Census

**What objects are there in the Universe?**
**A galaxy is made up of billions of stars**
**Many galaxies are found in groups and clusters**

## Cosmological Principle

**The axiom that the universe is isotropic and homogeneous.**
**Does not apply to stars within Milky Way Galaxy, or to bright, nearby galaxies**
**Applies to the average distribution of galaxies on the largest scales**
**Implies that we are not a privileged observer.**

**Notes:**
Nature of an axiom?

## Isotropic Distribution: an Example

## Hubble’s Discovery	

## Hubble’s Law	

**Hubble’s Law **
**Recession velocity is proportional to distance**
**Hubble constant: ~65 km/s per Mpc**
**I.e., the Universe is expanding!**

## How Do We Know Their Distances?

**Astronomers use a series of steps, or “the cosmic distance ladder” **
**Parallax for nearby stars**
**Cepheid and other “standard candles”**

**Notes:**
Parallax diagram also taken from Ned Wright’s site.
Standard Candles are objects of known intrinsic brightness --- by measuring its apparent brightness, you can estimate the distance.  Cepheids are somewhat more complicated: it’s a class of pulsating star whose period of pulsation is related to its intrinsic brightness.  Cepheids of known periods are standard candles.  Type Ia supernovae are similar, in that their intrinsic brightness is strongly correlated with the rate of decline of the brightness.
We are skipping many many steps of the Cosmic Distance Ladder --- see, for example, the Cosmic Distance Scale site.

## How Do We Know Their Velocities?

**We use Doppler Shifts. **
**Redshifts when object is receding**

## Fingerprints of Atoms	

**Atoms emit and absorb light at specific wavelengths;  these can be used to identify composition and velocity.**
**Examples (hydrogen, helium, carbon)**

## Back to Hubble’s Law

**Space itself is expanding, not just a few galaxies! **
**Same expansion in every direction**
**Same seen from another galaxy**

**Notes:**
Cartoon from Ned Wright’s site again.
Again, emphasize Cosmological Principle, and use balloon and/or raisin cake analogy, but be careful these are only analogies!

## Looking Back in Time	

**Hubble’s Law describes the current expansion of the Universe. **
**General Relativity is our current best theory of gravity and motion.**
**Inputting the current expansion speed and density to GR equations, we can “run the film backward”**
**Universe began as a “singularity” of infinite density (there is no “before”)**

**Notes:**
Imagine  a fly ball in a game of baseball.  If you see the motion of the ball, you can guess where it’s going (that’s how outfielders catch fly balls).  But you can also trace back the arc and figure out where the fly ball came from (not necessary in normal game situation).  But observations of the ball cannot tell you what caused it to fly in the first place (was it thrown, or hit with a bat?) This is similar to what scientists can do with the Big Bang.  Thinking about what caused the Big Bang, what “happened before” --- these are speculations, outside the realm of observational science.

## Smaller Means Hotter

**The early Universe was not only dense, but was also hot. **
**You can heat gas by compressing (bicycle pump experiment) or cool it by letting it expand (fridge)**
**Temperature is the measure of average energy per particle**
**This applies to photons as well as to protons, electrons, etc.**

**Notes:**
At what grades do students first learn about thermodynamics?

## The History of the Universe

**Notes:**
By looking at previous slide, audience should determine that stars have different colors, and deduce that this means different temperatures..
They won’t be able to tell from the image that stars are of different sizes and masses, but they may be able to deduce that from the different temps. 
With different masses and sizes, make analogy that some people are tall and others are short.

## The First Second

**We  can reconstruct the earliest history of the Universe, based on GR and quantum physics.**
**More uncertain earlier we go - a family of theories for this stage of Big Bang**
**T=10,000,000,000 K soup of particles at t=1 s (antiparticles mostly gone)**
**Electrons and positrons decreasing but numerous, photons, protons, ...**

**Notes:**
Involves many concepts too advanced for this purpose (quarks, grand unification, spontaneously broken symmetries, inflation, ...)
Slight imbalance (initially 1 in 10,000,000,000 or some such) between particles and antiparticles results in all the antiparticles going away.  The universe is made predominantly of matter, with very little antimatter.

## Electrons and Positrons	

**Electron and its anti-particle, positron, are among the least massive particles.**
**At high temperatures, they can be created and destroyed ~equally**
**At T<~10,000,000,000K, annihilation starts to win (not enough energy)**

## Protons and Neutrons

**Isolated neutrons decay into protons with a life time of ~10 min. **
**When the universe is hot, reactions go both ways so there are equal numbers**
**Imbalance develops as Universe cools**

**Notes:**
Balance can be calculated from temperature by knowing the mass difference (apparently the free neutron lifetime is a red herring).  3 percent neutron at 3e10K (0.11s) and 24% at 1e10K (1.09s) according to Weinberg.
Figure again from the Smoot group.

## The First Three Minutes	

**The first three minutes is the era of Big Bang nucleosynthesis. **
**Too hot (T~10,000,000,000K), and protons and neutrons won’t stick (left)**
**Still needs high T and high density for fusion to occur (right)**

**Notes:**
Diagrams are again from the Smoot group site.

## Deuterium Bottleneck	

**Deuterium (heavy hydrogen) nuclei are the first step in creating heavier elements, but they don’t stick easily. **
**Almost all deuterium nuclei will immediately end up as helium**
**Deuterium formation requires T=1,000,000,000K or less, depending on density of protons+neutrons**
**Universe cools sufficiently at t=3 min.**

**Notes:**
Will defer detailed nuclear reaction net work to “small stars”?

## Elements Created	

**The ratio of neutrons to protons when deuterium begins to form determines the ratio of helium to hydrogen. **
**~10% He4**
**Also He3 etc.**
**Neutrons disappear**

**Notes:**
This diagram from Ned Wright’s site.
History of neutron-proton balance is first determined by thermal equilibrium (first ~ sec), and by free neutron decay after the first 3 min; in between, you need to calculate the details.

## Origin of Hydrogen and Helium	

**The Big Bang created most of the hydrogen and helium in the Universe. **
**Protons are stable, and more numerous than neutrons; they survive to become hydrogen nuclei**
**Most neutrons end up in the very stable alpha particles (2 protons, 2 neutrons), or He4 nuclei**
**Observed ratios agree with theory**

**Notes:**
Here is the conclusion of this talk, as far as ‘origin of elements’ goes.

## Recombination

**There are a lot of photons in the Universe. **
**Spectrum is determined by the temperature (blackbody)**
**When atoms form (recombination of nuclei and electrons) at ~3,000K, Universe becomes transparent**
**The 3,000K blackbody has been redshifting ever since**

**Notes:**
By looking at previous slide, audience should determine that stars have different colors, and deduce that this means different temperatures..
They won’t be able to tell from the image that stars are of different sizes and masses, but they may be able to deduce that from the different temps. 
With different masses and sizes, make analogy that some people are tall and others are short.

## Cosmic Microwave Background	

**Notes:**
Pictures from the MAP site.

## CMB as Evidence of Big Bang

**Cosmic Microwave Background and its approximate temperature was predicted before discovery in 1964: a triumph of Big Bang theory!**
**Current temperature is 2.725K, and uniform to better than 1 part in 1,000.**
**No alternative theory can explain the uniformity.**
**Small “wiggles” are seeds of galaxies.**

**Notes:**
I don’t know much about CMB --- better refer to the MAP site.

## Elements from the Big Bang