An age of 9 billion years (for a critical universe) or even an age of 13 billion years (for an open universe) creates a problem, since the globular cluster stars are at least 13 billion years old. Where could we have gone wrong? Well, our estimates for the ages of stars may be slightly wrong. Our theories do very well predicting the properties of the Sun, the existence of main sequence stars, giant stars, supernovae, etc., but there might be some error.

On the other hand, maybe the distance ladder is wrong. After all, there are many steps to estimating the distance to a galaxy, and if any one is wrong, the whole scale of the universe will be in error. Perhaps the age of the universe can be increased by a billion or so just due to the various uncertainties at each rung of the ladder. This is possible. So if the ages of the globular clusters have been slightly overestimated and the distances to galaxies have been slightly underestimated and the gravity in the universe is not important (i.e., the universe is very open), then our model of the universe can work. But if the universe is near the critical density (which, after all, is a very elegent model and one which many people like), then there is no way there can be agreement.

Another possibility is that our entire idea of a Big Bang Universe is in error. About 40 years ago, some astronomers proposed a universe based on the Perfect Cosmological Principle. Recall that the cosmological principle says, in effect, that the universe must look the same to everyone, no matter where they are inside it. The Perfect Cosmological Principle states that not only must the universe look the same at every place, but also at every time. The Big Bang theory states that at one time the universe was much denser than it is now. That violates the Perfect Cosmological Principle. But suppose the Big Bang Theory is wrong. Perhaps there is a little magician that is running around the universe creating matter to replace the stuff that has moved too far away. This is sometimes called the Steady State Theory.

Can we prove that the galaxies were once all together, and that they were somehow ejected into space via a Big Bang? Well, if all the matter in the universe was at one time squeezed together, then galaxies in the past would have been closer together, and there would have been many more interacting galaxies. If we had a time machine, we could see this.

Well, we do have a time machine -- it's called a telescope. Light travels at a finite speed, so, for some galaxies, the light has taken literally billions of years to get here. We are seeing those objects as they were billions of years ago. We are seeing back in time. When we take the deepest pictures of the universe, we see many irregular galaxies that are obviously tidally distorted. Far away galaxies do seem to be closer together.

In addition, in the 1940's, a prediction was made. If the entire universe were squeezed into a small primordial soup, then the matter would have been under tremendous pressure. High pressure means high temperature, and high temperature means that the universe would have emitted blackbody radiation. With a suitable telescope, it might it be possible to pear back in time to the very beginning of the universe.

Of course, the Hubble law says that the further we see back in time, the greater the redshift of the light. So if the beginning of the universe did emit (optical) light like a star, that light would today be redshifted to much longer wavelengths -- the microwave region, to be exact. In the early 1960's, a group of Princeton astronomers attempted to build a microwave receiver to detect this light. While they were debugging their instrument, two scientists working for Bell Labs (about 30 miles away) discovered that the microwave receivers they built for the phone company had excess noise in them. Tests showed that the excess microwaves weren't coming from anything specific, such as the Sun, or stars, or the center of the Milky Way. It was as if microwaves were coming from all over the universe. To them, the universe itself seemed glowing with blackbody radiation, just like it had a temperature of 3 degrees above absolute zero. Arno Penzias and Robert Wilson won the Nobel prize for this discovery --- light from the original Big Bang of the universe.

The presence of the microwave background is exceedingly strong evidence for the Big Bang. But even more evidence comes from the composition of the universe. If all the matter in the universe was at one time squeezed together, then the pressure in that soup would have been tremendous. For a few instants, the universe would have been like the interior of a star. In those moments, it's reasonable to assume that, like in the center of a star, some nuclear fusion would occur. Specifically, the Big Bang theory predicts that if the universe began with only hydrogen, then in the explosion, some helium would have been made, and that about 1 out of every 10 atoms in the present day universe should be helium. This is exactly what is observed.

But if the Big Bang happened, and the universe started out as a huge ball of gas, how did we get the galaxies and clusters of galaxies that we observe today? To answer that questions, let's return to looking at the 3 degree microwave background. At first glance, it looks to be the same over the whole sky.

However, the microwave background is not 3 degrees everywhere, and it is not perfectly uniform. The earth is moving around the Sun; the Sun is moving around the center of the Galaxy, the Galaxy is moving towards the center of our Local Group of galaxies. In the direction of the earth's movement, the microwave background has a slightly bluer wavelength (due to the Doppler shift). In other words, it appears to be slightly hotter (by a few thousandths of a degree). Conversely, the microwave background in the direction opposite the earth's movement has a slightly redder color.

When the effects of the earth's motion is removed, the microwave background is still not quite perfectly smooth. That's not surprising, either. There is alot of very cold gas and dust in the plane of the Milky Way, and it emits like a blackbody that is a few degrees above absolute zero. But when we remove that component, we still see that the microwave background is not perfectly uniform -- some spots are brighter than others (by a couple of parts in 10,000.) The areas of over-density in the Big Bang caused the excess gravity in regions to overcome the Hubble Flow and form structures --- galaxies, clusters of galaxies, and clusters of clusters of galaxies. The fact that we see lumps in the universe today (galaxies), comes from the fact that the Big Bang was not absolutely, perfectly smooth.

The fluctuations in the microwave background also tells us about the shape of the universe. Astronomers can predict how far apart the high points and low points of the microwave background should be from each other. (This distance has to do with how fast matter will flow into a low density region.) But how far apart with observe the high points and low points to be will depend on geometry. If the universe is flat, then angles are Euclidean (like one learns in a geometry class), but if the universe is saddle-shaped or shaped like a ball, the angles will be different. Astronomers have recently measured those angles. It is almost certain now that the universe is flat.

In order to determine the age and fate of the universe, we not only need to know the Hubble Constant, but the amount of gravity in the universe as well. The Hubble Law states that the farther away galaxies are from us, the faster they are moving away. Galaxies at the other end of the universe, are therefore moving away from us at a very large fraction of the speed of light. Will gravity be able to stop this expansion and pull the universe together? Asking this question is equivalent to asking, ``will a beam of light keep going on forever, or will be gravitationally forced to return to its starting point?'' In other words, is the universe a black hole?

One way of measuring how much gravity there is in the universe is to look for deceleration over time. If there's alot of matter in the universe, the galaxies would have been moving apart faster in the past. They would therefore be slightly closer ( i.e., slightly brighter) than if they were always moving away at the same speed. Very recently, this experiment was done using Type Ia supernovae as standard candles.

The result of the experiment is that the universe is not slowing down. In fact, it's speed up. The universe used to be expanding slower than it is today! The universe is accelerating, as if something is pushing it apart. It appears that there is a Cosmological Constant!

This is most peculiar. We don't know what it is, but apparently the more empty space there is in the universe, the more something is causing the universe to expand. Right now, there's alot of empty space in the universe, so this Dark Energy dominantes the universe. (It's 3 times more important than Dark Matter!) The Dark Energy does, however, explain the problem with the globular cluster ages. Because the universe was expanding more slowly in the past, it is older than we think. It's age is 13.7 billion years.

What's causing the universe to expand? No one knows! There are many, many theories, none of which is satisfactory. (And many are not testable.)

If were to give a brief history of the universe (going backward in time), it would be like this. The Sun was born when the universe was 9 billion years old. Most galaxies were formed at time = 2 billion years. Our Milky Way just began to form at t = 400 million years. The microwave background that we see came from a time when the universe was only 100,000 years old. The helium in the universe was formed in the universe's first 3 minutes. Protons and neutrons formed in the first 0.00001 sec. Before that? Well, there is a theory that the universe underwent a BIG expansion, called inflation when it was only 10^(-34) seconds old. (In much less than a nanosecond, the universe expanded 10^30 times!) If that happened, then our universe is just part of a much bigger bubble of a universe. And that bubble might be just one of many other bubbles.