|
|
|
Fire and Ice Some say the world will end in fire,
|
|
Robert Frost (1916) |
I have summarized the standard thinking about the universe today. But I don't want to leave you with the impression that I am confident that this story is all told, or even that it will hold up. If we end our discussion with questions rather than answers, you will leave this course with a more realistic impression of the actual state of cosmology today. Here are some of the big ones:
The Fate of the Universe
The Friedmann models say that if the closure parameter W0 > 1, the universe will collapse again: it will end in fire. If this happens, it will be a long time from now -- perhaps 50 - 100 billion years. Our descendants, if they exist then, will first see the nearby clusters of galaxies stop expanding from us and begin to fall back. The microwave background will begin to warm up. Later, they will see the more distant galaxies turn around. When the final collapse occurs, all the galaxies will crash together and the microwave background will heat up to billions of degrees again: the big crunch.
Shiva, the creator and destroyer: will his dance go on forever? Will the universe rebound into another cycle of expansion? If so, will its next reincarnation be similar to this one, or perhaps something entirely different? Or will it just collapse out of existence: no space, no time, no nothing? We can't answer these questions. As with the very early universe, we don't know how space, time, matter, and energy behave at such high densities.
But today, we think that the closure parameter W0 = 0.3 - 0.4, which implies that the universe will expand forever -- if the Friedmann models are correct. If so, we can describe the fate of the universe with some confidence: it will end in ice. As it continues to expand, the clusters of galaxies will get further and further apart. The universe is much less active today than it was several billion years ago, and this fading will continue. The galaxies will slowly deplete their interstellar gas and will collide less often. Star formation will slow down and eventually cease. After hundreds of billions of years, most galaxies will contain only faint red dwarf stars.
In fact, if the recent discovery of cosmic acceleration holds up, the universe will have an even more lonely fate. As the universe speeds up, the redshifts of galaxies will increase. The most distant ones will vanish across the cosmic horizon. If we wait long enough, all the galaxies that participate in the cosmic expansion will be gone. We'll only have the local group to look at. But don't panic; this won't happen for hundreds of billions of years.
But do we really know the dynamics of the universe? Today, most cosmologists think that it's accelerating. But that news is just a few years old. When things are changing this fast, perhaps it's a good idea to regard today's most popular idea with a bit of skepticism.
In another years or two, the MAP satellite should measure the value of W0 and the cosmological constant very accurately -- if you can believe the current models for the fluctuations of the CMB.
|
|
What is the Universe Made of?
The diagram above (source) summarizes our best understanding today of the composition of matter and energy in the universe. The gravity and dynamics of the universe are dominated by two constituents that we don't understand. The "dark energy," which appears to be causing the universe to accelerate, comprises about 65% of the energy of the universe and its repulsive force dominates the attractive force of the matter. The "dark matter" dominates the gravity of the Milky Way and other galaxies. Normal atomic matter -- mostly hydrogen and helium, comprises only about 4% of the matter and energy in the universe, and most of that is in the form of interstellar and intergalactic gas. Only about 0.5% of the matter and energy is in the form of luminous matter, i.e., the stars that we see in the Milky Way and other galaxies.
It should also give you pause that the fate of the universe depends on stuff we have never seen. The only reason we know it's there is that we can see the effects of its gravity. People have spent a lot of money and effort to build ultra-sensitive detectors that might detect the dark matter, but no luck so far. Better review dark matter. Here's a good place: Dark matter.
I don't know. What's worse, I don't know of any experiments or observations that can be done to test the theory. It's an interesting idea. Perhaps someday somebody will find a way.
Now we think that, in addition to the dark matter, the universe is pervaded by a kind of "dark energy" that is causing the universe to accelerate, not decelerate. What is this energy? And, even stranger, why is it comparable with the energy density locked up in the dark matter? According to Einstein's cosmological equations, the attractive force due to the dark matter will diminish as the universe expands, while the repulsive force due to the dark energy will remain, so that the universe will continue to accelerate into the infinite future. It seems that we are living in a special epoch of the universe, not long after it has changed from a decelerating phase dominated by the dark matter to an accelerating phase dominated by the dark energy.
According to the theory of inflation, the initial acceleration of the universe, which occurred during the first 10-34 seconds, was dominated by the inflaton matter, which in many respects resembles the the kind of dark energy (alias cosmological constant) that causes the acceleration of the universe today. But then gravity due to the dark matter caused the universe slowed down for several billion years, and the universe is only now beginning to accelerate again. It's as if the universe were a two-stage rocket, with the initial boost stage occurring during the epoch of inflation, and the second boost stage occurring now. Perhaps, the dark energy of today is simply a remnant of that inflaton matter that drove the inflation? We don't know.
You can believe anything you like! If you remember only one lesson from this course, I hope it is this: science is not about belief, it's about exploration. We guess where we are, and then we make observations to confirm our guesses. If all measurements are consistent, we proceed further into the unknown. If not, we try harder to figure out where we are. One thing that scientists do believe is that we will eventually arrive at some reasonable approximation to the truth about nature if we simply keep trying. But once we understand something, it will be a path to a new and deeper puzzle.
I'll end this course by giving you my own perspective on cosmology today: I think the evidence that the universe started from a hot big bang is pretty convincing. I am also impressed by the chain of logic that takes us from the observed fluctuations of the microwave background to the large-scale distribution of galaxies and clusters today. There's still a lot of uncertainty in these observations, though. Mainly, I'm impressed by what we stand to learn within the next few years. The MAP satellite will produce much more precise measurements of the fluctuations of the CMB than we have today; and ground-based programs such as the Sloan Digital Sky Survey will provide much more detailed and extensive maps of the distribution of galaxies in the universe. The Hubble Space Telescope and several new 8- to 10-meter telescopes will give us a better look at the most distant galaxies; and powerful new X-ray telescopes such as Chandra and XMM will give us a much better picture of the distribution of the hot gas between the galaxies. And in about a decade, we'll be able to see the first light of the universe with the Next Generation Space Telescope and the Atacama Large Millimeter Array. Taken together, these new observations will provide much more rigorous tests of our present picture of the universe. Will the present picture hold up under such scrutiny? I don't know. If history is any guide, there will be some surprises. We'll have a bit of a mess on our hands. That's what makes science fun!
(Return
to course home page)
Last modified April 19, 2002
Copyright by Richard McCray
