When I first got interested in theoretical physics, it must have been about 1991. Stephen Hawkings first popular book "A Brief History of Time" had recently come out. It became a best seller and helped generate a lot of interest in cosmology and physics. Thinking back to what was in the book, it amazes me how much has been discovered in physics, astronomy, and cosmology since then.
If you've read the book one of the debates that used to go on among cosmologists was whether or not the universe was going to recollapse on itself or die in a runaway expansion. Neither alternative sounded very pleasant. If the universe recollapses on itself it brings to mind all kinds of bizarre scenarios. Just consider that in that case, which Hawking seemed to favor at the time, the universe would have always existed. A big bang happens, the universe expands, then it reaches a maximum size and shrinks down again, starting over with a new big bang. Since its always been here there have been an infinity of universes, and probably an infinity of life forms. Even though life may be unlikely since the universe has recycled an infinite number of times then countless civilizations must have sprung up through the eons. But the forces of nature are huge, so no matter what the recollapse (the so-called "big crunch") of the universe would leave even the most advanced of beings doomed.
That kind of made life seem like a bad, eerie dream for me. Well no matter. A few years later astronomers discovered that the universe was not only expanding, its expanding at faster and faster rates! This is something undreamed of back in 1991 as far as I know. It was a complete shock and it led to the notion of dark energy. Now we may find out that dark energy, something scientists only recently became aware of, may be a dominant force in the physics of the universe.
As Michio Kaku once said on television, his favorite venue, physicists now believe the universe would not die a fiery death (the big crunch again) but would instead die in ICE! (his emphasis). Basically the mysterious dark energy would push all the galaxies far apart, and everything would eventually cool and decay. In the distant future there won't be anything around except swarms of decaying fundamental particles. A sad end to a universe that gave birth to Brent Favre, Paris Hilton, and Oprah. Little wonder we haven't discovered any alien civilizations-they probably self-destruct in a frenzy of narcissism.
Well I digress. What got me thinking about this was an article in the New York Times about cosmology that appeared today. Astronomers have once again measured the Hubble constant, and it appears that the evidence looks better that the universe is about 13.7 billion years old. If you aren't an NY Times subscriber I recommend going through the pain of becoming one, its free and they have lots of great articles about cosmology and physics.