The Expanding Universe

The history of the universe in a nutshell*On to the next question: "Yes, I have many burning universal questions. This one is a two-parter. Is there evidence that the universe's rate of expansion is slowing down? And if so, does it allow us to infer what would happen if expansion stopped, i.e. would it put the universe in stasis or would it contract? This keeps me up at night, so I would appreciate any insights."

Einstein had the same question, and it kept him up at night also. In fact, he got this one wrong. Einstein had mistakenly assumed that the universe had to be static, looking exactly today as it will look tomorrow. When he formulated his improved version of the theory of gravity, known as general relativity, he found that his equations predicted otherwise.

The theory of general relativity not only describes how gravity effects planets and stars, but also how matter and energy affect space-time itself. The universe can actually expand or contract, depending on its composition. Einstein became very concerned when he saw this prediction from his own equations, and added a fudge factor known as the cosmological constant to "fix" the universe.

In 1929, Edwin Hubble, namesake of what may be the astronomy's greatest instrument, the Hubble Space Telescope, observed that almost all of the nearby galaxies were moving away from us. This observation was the first evidence that the universe is expanding AND the first evidence that Einstein made mistakes. Here is a site with further explanation and a less than exciting animation of the expanding universe.

So what's gonna happen next? Well, if there is enough matter out there, the universe may slow down, begin to contract, and eventually collapse on itself in a Big Crunch. Is there enough matter out there? We don't think so, we think that there is too much dark energy in the universe. This dark energy counteracts the affects of matter and pushes space apart.

In the dark energy scenario, the universe doesn't ever slow down to the point where it stops expanding. Instead, it actually begins to accelerate. Observations indicate that this acceleration began a few billion years ago and will continue for the rest of time. If this interpretation is correct, than we will experience not a Big Crunch, but just the opposite: A Big Chill.

If dark energy causes the universe to accelerate forever, then all of those receding galaxies we observe will eventually speed out of sight. In this future universe, astronomers will look out with their fancy telescopes, and instead of observing a universe rich with galaxies, they'll see only empty space. It's a bleak future for our breed; if we thought research funding was difficult now, imagine how bad it will be then. Fortunately we should have plenty of time over the next 50 billion or so years to figure it out. I'll just have to remind everyone to take notes.

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and
development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.

*The composition of the universe determines its ultimate fate. If we live in a universe with an excess of matter, as shown in the lowest dotted curve, the universe will eventually collapse on itself in a "Big Crunch". However, it appears that we may live in a universe dominated by a so-called "dark energy." This universe will not only continue to expand, but will actually accelerate in its growth as demonstrated by the top dashed curve.