The Big Bang!

Einstein's equations describing the large scale structure of the universe imply expansion and it is only through very contrived means that he was able to make the cosmos static. Expansion implies origin and this was soon recognized. Implicit in the equations is the idea that at a point in time the universe expanded outward from a point. Hubble's discovery of the recessional motion of galaxies is cited as evidence of this. The first version of the Big Bang was developed by Gamow, Alpher and Herman in the 1940's. They were interested in explaining the origin of the elements and reasoned that the elements were formed from the bare nuclear raw materials (neutrons, protons and electrons) during a singular event that also produced the observed expanding universe. This, in crudest form is the Big Bang Cosmology (BBC). Note, it begins with creation ex nihilo . The cosmology must posit something - it must reach beyond a scientific account to begin. We cannot claim that our account of creation is solely scientific but neither is it just conjecture - it will speak to a considerable amount of scientific data and evidence.

An Uncertain Start!

The Big Bang - in a more modern form - posits that the universe emerged from the enormous energy well of a cosmic singularity (ie., Black Hole). Key Ideas flow out of the following pair of simple equations developed by Heisenberg in the mid 1920's:
This law "occurs" because of the wave-particle nature of matter in the universe. Two related ideas that come out of this are:

the Planck length and the Planck time: the minimum size of spatial or temporal measures for the universe.

Modern quantum theory tells us that the Planck length and time present fundamental limits to our laws of physics. We cannot meaningfully speak of either times or lengths smaller than these. One consequence of this idea is that at the smallest scale the structure of space is not uniformly smooth - rather it is "frothy" (an analogy often used is the foamy surface of a glass of beer!) with Planck length sized bubbles. The actual cause of universal expansion (and the coming into being of the universe itself) is beyond the scope of science. Our known laws of physics cannot be expected to work on scales smaller than the Planck length or time. At this stage we would do well to consider the words of Mark Twain:
.

There is something fascinating about science. One gets such a wholesale return of conjecture out of such a trifling investment of fact

(Mark Twain, Life on the Mississippi ,1883)

 

This raises an interesting problem.

The Very Early Universe - The Radiation Era

The early universe - up to an age of about 1000 years was dominated by radiation and hence is called the radiation era. The universe in the BBC starts as a, presumably, infinitely dense "nut" that suddenly expands, producing space and time. The universe begins at an enormous temperature. Matter as we know it would not exist at these temperatures. Physicists expect that in the early universe - only a few billionths to a few 10's of seconds old - would be made up of strange, exotic particles continually colliding and transmuting into other particles. After 10 s or so the universe would have cooled enough to produce he "normal" nuclear matter that we are familiar with. Now comes a critical time in the universe - the formation of elements.
Accounting to the Hot - BBC all the original elements of the cosmos would be made in the next 35 minutes!

The Problem of the Heavy Elements

According to the BBC the elements formed as follows: Ooops! Something is wrong! The BBC can't get past Lithium - great for making long lasting batteries but not so good for the formation of semi-sentient carbon based bipedal lifeforms (ie: politicians). Lithium is just too fragile an element to continue this process. By the time of Lithium formation the universe had cooled sufficiently so that element production ceased and the primordial ratios of He and its isotopes to H and its isotopes were produced. We know think that the problem of heavy elements has been solved by the process of nucleosynthesis occuring in the cores of supernovae.

The Matter Era

For the first few thousands of years after the BB the universe was still too hot for ordinary atoms to exist. After about 380 000 years the universe had cooled to about 3000 K and hydrogen atoms were able to form. This means that the entire universe was something like the surface of an M-type star. Cosmologists call this the beginning of the matter dominated era of the universe and it marks the beginning of the universe in the form we know it. Prior to this era the universe was dominated by radiation and prior to that by leptons. It is the hot 3000 K universe that some think we are seeing when we detect the 2.7 K background glow. So, we have in a sense resolved a question - can we look back to the very beginning of the universe? No - we can look back to the boundary between the radiation dominated and matter dominated eras.

Formation of the Galaxies and Stuff

Now that we have entered the matter dominated universe, we have an expanding universe that has condensed into atoms of H and He with a light dusting of Be and Li. Eventually (how?) after about 100 million years any random variations in density began to grow and triggered the collapse of large structures. This leads to a puzzle. How do we get collapse to occur in an expanding universe? As we shall soon see this is a nifty problem and requires - it would seem - a very good balance between the rate of expansion and the density of the universe. An interesting side light is the theoretical implication that objects like globular clusters may have in fact collapsed prior to the collapse of their parent galaxies.

Summary of the History of the Cosmos

 
Time (s) Radius (m) Temp (K) What's Happening....
10-43
10-35
1032
 Planck sclae - we don't have good enough physics to go beyond this (or maybe we do!!)
10-38
10-29
1029
 Grand Unification of All Forces, Rapid Inflation
10-12
10-3
1016
 Electroweak Forces Unified
10-6
10-3
1013
 Heavy subatomic particles form
1
108
1010
 Neutrinos
102
1011
109
 Formation of light nuclei (H, He, Li)
1013
1022
3 x 103
 Hydrogen atoms form, end of radiation dominated universe, start of matter dominated universe
5 x 1017
1026
3
 Galaxies and stars and US!

So Why Should YOU Believe the Big Bang Model?

It works! The BB cosmology is a testable scientific model - it makes predictions and provides an explanation for a number of things. You should believe the BB model only to the extent that you believe other scientific models and theories - does it make testable predictions and lead to new insights? You should also remember that this does not exclude the possibility that other models or theories could be constructed that could do just the same or better! As of 2004 there aren't any clearly better physical cosmologies but the recent discoveries of dark energy, the accelerating universe and string theory have spawned a new class of cosmologies.

Let's summarize the success of the Big Bang Cosmology:

  1. Expansion of the universe
    Hubble and Humanson's key discovery of 1929 provides a natural way to talk about cosmic expansion and - "playing the movie backwards" provides a natural way to talk about a point or epoch of creation.
  1. 2.7 K background radiation
    Penzias and Wilson's discovery of the 2.7 K background radiation was consider by many in the 1960's as the "smoking gun". The existense of this background "glow" was predicted by George Gamow as early as 1948 in his first development of the "hot big bang".
  1. Element abundance ratios
    Why does the universe contain approximately 73% H and 24% He and smaller amounts of the heavier elements? We think that the BB cosmology provides us with a very good answer and prediction of these ratios.

Sometimes Things are Just Too Good to be True!

As we shall see in the next lecture, the Standard Big Bang Cosmology has a number of serious problems. If it is correct then, it would seem, our present universe is just too good to be true. By the mid 1970's astronomers were deeply troubled by the unbelievable "good fortune" that would be needed to produce a universe like the one we live in. Something was missing ...
Seeds: Chp18