One of the neatest things about the blogosphere is that you can talk directly to experts in any field. Professor Sean Carroll is such an expert. From the fourth grade, Dr. Carroll wanted to be a physicist and a cosmologist. Fortunately for us, he happened to become one at a time when anyone can head over to his group blog,
Cosmic Variance, and talk to cosmologists and physicists about the deepest mysteries in the Universe. They've even put together a great online pictorial
about Dark Matter and Dark Energy in the Cosmos. Sean studies it all, from the infinitesimally small to the grandest edge of infinity.
DarkSyde (DS): How big is the universe?
Sean Carroll (SC): We've learned amazing things about the universe in the last hundred years. Our Milky Way galaxy has about a hundred billion stars, the observable universe has about a hundred billion such galaxies, and it all started about fourteen billion years ago. So it's a big universe, and getting bigger.
DS: When I was in school we were taught the standard inflationary Big Bang model, has that changed in the last twenty-years?
SC: The overall "Big Bang model" is healthier than it's ever been. That's just the basic idea that the universe is uniform on large scales and expanding from a hot, dense state. We don't know much about the Bang itself, but we still know an awful lot about early times, all the way back to a few seconds after the Bang. At that point the universe was a nuclear reactor, and we can look at the leftover elements to check whether their abundances match what we predict -- and they do! The fact that we can extrapolate our theories fourteen billion years into the past to a time just a few seconds after the Big Bang, and check that they give the right answers, is one of the most impressive feats in all of modern science.
Even though the Big Bang model works really well, it raises questions. Why is the universe so smooth, and what made it "Bang" in the first place? The best current answer is inflation, proposed by Alan Guth in 1981. Inflation imagines that a tiny region of space can be dominated by "false vacuum energy" for a brief time, stretching out space and making it smoother as it expands. This model both explains why the universe is smooth on large scales but not perfectly so, since tiny quantum jiggles lead to fluctuations in the density of matter. These fluctuations show up in the Cosmic Microwave Background, leftover radiation from the Big Bang itself, and recent observations from NASA's WMAP satellite confirm that they look exactly like inflation predicts. A few billion years later, gravity has magnified these small perturbations into the galaxies that we observe today.
Left: The WMAP, an image of the very early universe. The individual colors and patterns represent the earliest precursors to galaxies/clusters. Center: NASA-Computer generated image of what the universe may have looked like a few hundred million years after the big bang. The glowing streamers are hundreds of millions of light years long and represent proto-superclusters of galaxies. Right: The Hubble Ultra-Deep Field View (Enlarge) shows the most distant galaxies we can observe as they appeared a few billion years after the Big Bang. Each point of light is an entire galaxy containing billions of individual stars.
DS: What is Dark Matter?
SC: "Dark" is a euphemism -- it means not only that the stuff is completely invisible, but that it isn't anything ever seen in a laboratory here on Earth. Clearly, we'd rather not have to invoke such stuff. Nevertheless, the data have forced us to believe that ordinary matter is only about 5% of the universe; another 25% is "dark matter," and the remaining 70% is "dark energy."
Dark matter is some kind of particle that doesn't interact with light, so that we can't see it directly. We know it's there because it creates a gravitational field, and we can detect that gravitational field. In fact, we detect it over and over again -- the single idea of dark matter allows us to account for the behavior of our Milky Way, of other galaxies, of large-scale groups of galaxies, of the expansion of the universe, and of the patterns we observe in the microwave background. The most popular dark-matter candidates are "weakly-interacting massive particles" (or WIMPs), which are predicted by models such as "supersymmetry." Supersymmetry is an ambitious idea that proposes a new kind of fermion (matter-like particle) for every existing boson (force-like particle), and vice-versa. The Large Hadron Collider in Geneva, scheduled to turn on next year, will be looking for supersymmetry, among other things.
DS: OK, then what is Dark Energy?
SC: Dark energy is completely different -- it's a certain amount of energy density inherent in space itself. Every cubic centimeter of space contains a fixed and immutable amount of dark energy, even if it's completely empty! The dark energy doesn't cluster into galaxies like dark matter, nor does it dilute away as the universe expands. Again, we have multiple independent lines of evidence in favor of this surprising idea -- from precision measurements of the expansion of the universe, to the evolution of large-scale structure, to the overall curvature of space itself. What the dark energy really is, and why it comes in the amount it does, remain a mystery; but there is plenty of evidence that it really is there.
Enlarge Illustration--Recent observations indicate the presence of mysterious Dark Energy which is accelerating the rate at which the universe expands. if this process continues unabated, it could result in The Big Rip: Time and Space, down to the scale of individual atoms, will literally explode.
DS: What is string theory?
SC: String theory is the very simple idea that, if we looked with a good enough microscope, we would be able to resolve elementary particles into tiny loops of vibrating string. Sounds simple (and goofy) enough, but it has powerful consequences, including the most interesting one of all - it includes gravity! Since gravity does exist, and since it's been extremely difficult to reconcile gravity with particle physics by any other means, string theory has become a very popular subject of study among theoretical physicists.
The problem is, at the moment we don't understand the theory very well, and in particular we don't know how to connect it directly to the world of experiment. It does predict supersymmetry -- but doesn't say whether it will be accessible at particle accelerators. It also predicts dark energy -- but at a much larger value, naively, than what we actually observe. At this point, string theory is a promising idea, but one that isn't understood nearly well enough to know how it fits into the rest of physics, if at all.
DS: Are there any indications from experimental physics or observations that strings even exist?
SC: No - not yet, anyway. But gravity exists, and we have to explain it somehow. There was one piece of data that dramatically affected work in the field: the discovery of dark energy. Before that, string theorists were considering solutions that had zero dark energy, and were hoping that such a solution would be unique. Now that they have started looking for solutions with non-zero dark energy, it has begun to look like such solutions are extremely non-unique! There could be something like 10 500 possibilities. Some of them will look like the world we see, most will look dramatically different. It's possible that there are many different parts of the universe, and that every possible solution to string theory is represented somewhere or another.
Maybe, maybe not. Ultimately, the important thing is the data that will be coming in from new experiments. Not only new telescopes and new particle accelerators, but schemes to detect dark matter, to measure gravitational waves from distant black holes, and look for new forces in laboratory tests. Over the last hundred years we went from knowing almost nothing to knowing quite a bit about our universe, and there's every reason to believe that the next hundred years will be just as exciting.
Prof. Sean Carroll is the author of the graduate level textbook Spacetime Geometry: An Introduction to General Relativity. Currently at the University of Chicago, he will be moving to Caltech in the fall. He blogs along with several other physicists and cosmologists at Cosmic Variance.