Discussions of the evidence supporting the so-called Big Bang theory (an unfortunate name for a number of reasons, but that's another story) are provided here and here (after a summary of previously suggested models).

Still more information about cosmology and the Big Bang theory may be found at my list of links (caution: some, though not all, are quite technical). Here is a site that has links to common questions from laypersons with answers an informed layperson can understand. Note that much of this stuff is rather terse - meant to keep the non-specialist's eyes from glazing over - and detailed explanations of the observational evidence exist elsewhere. A longer discussion of the Big Bang theory and observational evidences can be found here.

Astronomers didn't make this stuff up. We could spend a whole semester discussing the current evidence for an evolving universe, one that did not always have stars and galaxies, one that was much denser and hotter than it is today. Obviously, I am not about to do this here. So instead let us discuss one of the more sublime pieces of evidence that we live in an evolving universe....

 Why the Night Sky is Dark...Or is it?
by Kirk Korista
The following (slightly modified) article appeared in the Kalamazoo Astronomical Society newsletter Prime Focus in December 2001.

The next time you are outside on a clear night, far away from the blasted city lights, look up, and then ask yourself, "why is the sky so dark?" That may seem like a question with an obvious answer, but you'd be wrong. Thomas Digges in 1576, followed by Johannes Kepler in 1610, Edmund Halley (of Halley's Comet fame) in 1720, Isaac Newton (of falling apples fame), and many others thought long and hard over this question. This is because the implications of the answer to this question have to do with the size and age of the universe. Is it infinite? or not? Is it infinitely old? or not? Surprised? Ponder for a while. What does it mean that our night sky is dark? This suprisingly complex question was again addressed by the astronomer Heinrich Olbers in 1823 and has since been known as Olbers' Paradox.

Stated briefly, here is the paradox. It can be demonstrated that a universe of infinite extent and age, populated everywhere with stars1 will produce a sky that is as bright as the surface of the Sun hovering just over your head! No kidding (would I joke?). This is NOT the universe we live in, is it?

Here is an analogy. Imagine standing in the middle of a large, mature oak forest that itself sits in a vast grassland plain. Let's assume it is late autumn and the trees are leafless. For a given average number of standing trees per acre, the further the forest stretches away from you, the fewer gaps you can see between the trunks of the trees to the plains and horizon outside the forest. For a sufficiently large forest your view to the outside becomes fully blocked, and everywhere you look, as far as you can see, your sight line runs into a tree. You'll agree, too, that denser forests need not be as large in extent before your view to the outside is blocked. Let's call this distance at which your view to the outside just becomes filled with tree trunks the lookout limit. Interchange tree trunks with stars and you have yourself an awfully bright sky.

Obviously, we don't live in a blast furnace universe (and if we did, stars could not exist and nor would we - another key point overlooked, but let's leave that issue aside). So what's gone wrong? For starters, one or more of our assumptions we (and many others) made is wrong. There is also one more important concept related to this paradox: as you gaze up at the sky, you look out into space and back into time, simply because the speed of light is a finite quantity.

Now, what if our imaginary forest is a mature one only in the vicinity of where we are standing, and that at greater distances the forest becomes younger? As you look out past the fully mature part, the trunks become ever narrower, then you see shorter saplings further separated from each other and mixed in with the grasses of the plains, then just seedlings, and finally just acorns scattered upon the prairie on out to the horizon. So then as long as the mature forest is neither too dense nor too extensive, you will see through the forest out to the vast grassy plains, all the way out to the horizon. That is, the lookout limit is much larger than the size of the forest. Now we add the final ingredient to our analogy: interchange the distance from your position in the forest with time going back into the past - because in our universe these are completely equivalent. Hmmm....I'll bet light bulbs are going off in your head.

Ok, I'll stop torturing you with analogies - here is the solution. The experimental evidence of the dark sky, which you can confirm with your two eyes, implies that the phenomenon we call stars was born at a finite time in the past. Simply put, stars and galaxies have not been around forever - but came into existence sometime in the distant past when the physical conditions were proper for them to do so. Like our imaginary forest, our universe at present contains both old and young stars, but if we look far enough out, we look back to a time when all stars and galaxies are young - the "saplings" of the forest analogy. Astronomers are building bigger telescopes to better study the "saplings" and to find the equivalents of the "seedlings" of stars and galaxies - we are just now gaining the ability to observe hints of a transition to the "age of darkness", an era before there were stars (the grassy plains). The lookout limit for stars in our universe is far too large in comparison to the distance light can travel over the time span that stars and galaxies have been around. In fact it's a factor of roughly 10 trillion too large! When averaged over the sky, starlight from all the stars in all the galaxies in the observable universe amounts to a feeble intensity of 1-ten trillionth the intensity of light from the Sun's surface. The sky is dark.

In a roundabout way this was actually first suggested by the author and poet Edgar Allen Poe (!) in 1848 in his essay Eureka. The definitive solution was derived by Lord Kelvin in 1901, but was somehow ignored. That's 15 years before Einstein's Theory of General Relativity, 20 years before we understood the nature of galaxies, and 28 years before Hubble's observations established the expanding universe. You don't need an expanding universe or one that organizes stars into galaxies to explain why starlight doesn't fill our night sky and roast us to a crispy crunch. Expansion and stars concentrated within galaxies only make the paradox easier to solve. Unfortunately, the process of "ignoration" resulted in a confusion concerning the resolution of the paradox that remained in astronomy textbooks until the 1990s!

But let us now go further. What do we see when we gaze out past the first and youngest stars and galaxies...past the last saplings in our forest analogy? We see the cosmic background radiation- represented by the horizon in our forest analogy. This radiation comes from all directions in the sky and has a spectrum that differs by no more than 1 part in 10 thousand from a perfect, single temperature blackbody (thermal) radiation source (more typically 1-3 parts in 100 thousand). Such a spectrum can have its origin in a universe homogeneously filled with relatively dense gas, opaque to light, with almost no temperature variations - a situation akin to local conditions found within the interior of a star!

"Now, wait a minute.  That's not the universe we live in. What are you, nuts?" Yes, you're right - that isn't the universe we live in today. This light comes to us from a time in the far distant past when a very smooth distribution of gas filled the universe; at that time the universe was much denser, much hotter, and just becoming transparent to the light within it. Conditions everywhere were then similar to standing on the inside of a star - nearly perfect thermal equilibrium2. As the universe expanded and cooled to a temperature of about 3000 K (about the temperature of a hot lightbulb filament), the matter (primarily hydrogen and helium gas) became transparent to light. That primordial light, decoupled from matter, has since been freely traveling through expanding space-time, for nearly as long as the age of the universe.  You might reply, "But, how could the universe have been so different?" Good question; time for another thought experiment.

If you were to take the universe we have today, and run the observed expansion of space-time backwards, the universe would become denser, and we can use the laws of nature to follow what happens as it does so. Initially, clusters of galaxies would begin crashing into each other, then galaxies would crash into galaxies, followed by individual gas clouds, then stars. Gravity and tidal forces would do their work until nothing with any structure could exist - no galaxies, no stars, no planets - just a smooth distribution of matter gas filled with light, one that becomes hotter as the density increases (compress a gas - what happens?). To get to this point would take about 14 billion years given what we know about the expansion rate of space-time3. As the universe became much denser and hotter, electric forces would tear electrons from their atoms as they collided with greater energy. The typical photon energy will have also increased so that a fraction of them are able to ionize the hydrogen and helium atoms. At about this point, the matter in the universe would become opaque to light (light and matter in close contact)...resulting in the production of a nearly perfect blackbody radiation spectrum.

We see this same light today completely covering our sky, but mercifully redshifted by the expansion of space-time to a blackbody with a temperature of 2.725 K (-455 F), and so is emitted at very low intensity at microwave (millimeter) wavelengths. It's called the cosmic background radiation. The paradox of Olbers' paradox is that it isn't paradox after all. The sky is bright, not with starlight - but with light echoing the creation of the cosmos. And remember the extremely small temperature variations in the blackbody radiation across the sky? Those are due to minute variations in the density of matter - the seeds or acorns (I couldn't resist) of the formation of structure. These are just the variations in density that gravity needed to begin pulling matter together to eventually produce galaxy clusters, galaxies, stars and planets...

1 Strictly speaking, the universe needn't be infinite in extent or age to produce the same result, but these are just details.

2 In the present universe, the condition of nearly perfect thermal equilibrium is found locally within the interior of a star, and not much else.

3 Conditions would differ significantly in detail in this "universe run backwards" from those that were present in our actual universe at some particular point in time. For example, this hypothetical gas in the universe run backwards would contain traces of heavy elements, absent in our universe before the births of stars. It is also true that the matter in the present universe is highly clumped (as planets, stars, and galaxies with plenty of empty space) due to the force of gravity acting on matter over the past 14 billion years, and other issues pertaining to entropy. The above description is meant only as simple thought experiment.