Will we ever look back to the Big Bang?
We're getting closer and closer to understanding what was going on right at the Big Bang. We have laws of physics that get us pretty close to the beginning, but not all the way to the beginning, and certainly not before the beginning. These are frontiers right now that offer tantalizing clues that perhaps we live in just one cosmic bubble of an infinite number of bubbles. We're coming close to being able to ask and answer those questions theoretically and observationally, and I see that over the next 20 maybe to 50 years, as the string theorists come forth with more clever statements about what went on right at the first singularity, right at that first moment of the big bang.
When we find them, what will gravity waves tell us?
The Big Bang is currently our most verified and supported theory for the origin of the cosmos, but there's several barriers to looking all the way back to the start of the clock. One of them is an optical barrier, and that's the famous cosmic microwave background. Right now, it's the wall of microwaves that emanates from everywhere in the cosmos that's a leftover light signal from the original explosion. But that light hails from 300,000 years after the moment of the initial explosion, so using ordinary telescopes we have no hope of seeing, of penetrating that wall anymore than you have any hope of looking through a smoked glass and describing details happening on the other side of that. That's an optical boundary, but all hope is not lost.
There are other kinds of telescopes for example, one that detects neutrinos. Neutrinos don't have that wall problem. They emanate from an early time in the cosmos, so if we perfect neutrino detectors, we in principle will be able to see much farther back in time than this optical wall created so soon after the Big Bang (a mere 300,000 years after).
But there's something else that comes from the early universe and these are gravity waves, ripples in the fabric of space and time, predicted by Einstein but still not observed. We've got good people working on the problem, they're in the process of building sensitive enough telescopes to detect them right now. I have no doubt that we'll detect them, and when we do, then we'll be able to see even farther back -- back to the earliest moments where the actual fabric of space and time sprung from what might have been this cosmic meta-soup, which we think gave birth to universes left and right!
Now a lot of that is fantasy at this point, but it's not complete fantasy, it's consistent with modern thinking of what the behavior of quantum mechanics would be under those circumstances. Quantum mechanics is the study of matter on its smallest scale, whereas Einstein's general relativity is the behavior of matter on its largest scale. Once you have the Big Bang and you have all that matter compressed down into a tiny volume, these two theories have to come together somehow . But they don't, or we don't yet see how they marry each other.
We think you need a third thing there, and therein is the limit of our theoretical understanding of the cosmos. Once that's formulated, then we may be able to describe what kind of gravitational signature we think was made from the actual explosion itself, and tune our gravity wave telescopes to see that.
Talk about the advances in your field.
Cosmology has made tremendous progress in the past 15 years. I think people don't have the opportunity to appreciate the level of detail that cosmologists think they understand the universe and the ways we have of testing that.
For example, we have the Big Bang theory, which says the universe started as a hot, expanding ball. That means much more to theoretical physicists who can actually calculate how fast the universe was expanding at any time, what the temperature would have been at any given time, what the density of matter would have been at any time. It's a really quantitative theory. And we physicists know enough about nuclear physics to actually calculate what the rates of the different reactions would have been under the conditions we've calculated for the early universe. With all those put together we can accurately predict the abundances of about four or five of the lightest chemical elements around today, and our prediction agrees with what the astronomers predict, even though the calculations we do are based on nuclear physics experiments and theirs are based on astronomical measurements. But the theory and the measurements nonetheless agree! And I find that astounding and I think it really means that we two groups are on to something here.
What's the current thinking about gamma ray bursts?
The story about gamma ray bursts is that they were thought to be very powerful explosions that created lots of intense gamma rays from distant sources. What now seems to be the case is that they're not quite as powerful as we once thought. We're not absolutely sure, but it looks like they're only as powerful as a typical supernova explosion. Not as powerful as a 100 or 1,000 supernova explosions, as we were thinking.
And the way that we have come to this conclusion is that there is now fairly good evidence that the bursts themselves are not spherical explosions, but in fact, almost one-dimensional explosions, like jets. And if that's true, we only see the gamma ray bursts when the jets come in our direction. And so if the energy is only focused in this beam, then we can say that it's only about one percent or so of what we originally thought it might be. So that's one of the latest discoveries about gamma ray bursts.
The other one is that there is pretty good evidence now that although we aren't sure what the cause of the gamma ray burst is, it does appear to be associated with young and massive stars and that points the finger at supernovae. And again it's not absolutely sure, but that's certainly looking like a stronger and stronger possibility.
What's the cosmological constant?
The cosmological constant is essentially a mathematical term that Albert Einstein introduced into his general theory of relativity . It's the possibility that there was something in addition to the regular gravity that holds the solar system together. Dark energy in some sense is a more modern version of the cosmological constant. It's basically the same idea, and there is now observational evidence -- which of course there wasn't for Einstein -- that it really is present in the universe.