Note: I am in no way a trained cosmologist. I play with radio telescopes, galaxies, and the ionosphere. I’m a bit out of my depth, so I do apologize in advance for inaccuracies and welcome corrections!
Thursday was the second day of the URSI conference in Boulder, CO. The plenary session took up the entire morning and included two very interesting talks on particle physics and cosmology. These are some pretty brain-bending subjects, as the universe turns out to work in ways that are completely alien to our brains which adapted to life on a particular planet with particular size scales. The very small and the very large scales have only been probed very recently in our history. I did some live-tweeting of these talks, which can be found by searching the #ursi hashtag. I promised to blog later about how I came to intuit the acceleration of the universe’s expansion do to mysterious dark energy.
For decades, cosmologists weren’t sure if the universe would keep expanding forever or if it would have enough mass to collapse back on itself. So, several teams of astronomers looked for very distant, bright, supernovae, specifically, Type Ia SNe. These are pretty much all the same brightness, so determining the distance is possible. What they found in 1998 was shocking. The expansion of the universe is actually accelerating. How the heck can that happen?
We know from relativity that a “cosmological constant” could be added to the equations to make this acceleration. In fact, Einstein had hypothesized it himself to make the universe come out stable, but retracted it when it was discovered that the universe was indeed expanding. Scientists started to attribute the acceleration to “dark energy,” which means, “we don’t know what the heck it is.”
After the supernova results, the Wilkinson Microwave Anisotropy Probe looked at the “echo” of the Big Bang, and determined that this dark energy actually makes up a significant chunk of the universe’s energy budget. In fact, we have many lines of evidence that converge on this story, now called the “concordance model” or “Lambda-CDM model” of cosmology.
I know, I know. You are probably looking at that figure and saying, WTH? Well, the x-axis indicates the density of matter in the universe and the y-axis the density of dark energy. The color contours represent likelihood of each factor having those values, where the darkest colors are 95% confidence in the values. Each color represents a different method of measurement, green from supernovae, blue from the large scale structure of galaxies, and orange from WMAP. Where they all overlap is the most likely reality! What ever dark energy is, it looks as if we are stuck with it.
You know how they always write headlines saying “scientists are baffled” when they really are not? I would dare to say that this is a case where astronomers and physicists are legitimately baffled. There are several hypotheses for what dark energy could be, and more experiments and observations need to be done to distinguish among them. One of the favorite candidates (well, my favorite) is vacuum energy.
Vacuum energy is the idea that empty space isn’t all that empty. This has been demonstrated by the Casimir effect, in which adjacent metal plates in a vacuum seem to have some force acting on them. It may be that the vacuum of empty space itself has some kind of repulsive force that keeps pushing the universe’s expansion. Interestingly enough, as the expansion continues, which makes more space, which makes more energy, which makes space expand faster, which makes MORE of this energy… etc. So in that way, it seems natural that the universe is just “falling out.”
Usually, the forces we deal with get stronger as the things creating them get closer together. For dark energy, however, the repulsive force gets stronger the further away things get. Weird, until thought about in context of vacuum energy.
Of course, we can’t settle on the vacuum energy hypothesis just yet. Theoretical physics predicts an energy density that is hugely different from the measured energy density in astronomical observations. Like 10 to the 123rd power times wrong. (That’s 10 with 123 zero’s following. Ouch.) However, it has not been ruled out, and indeed evidence shows that it is still quite likely…. if only we could understand it better.
So don’t let your head explode over our accelerating universe just yet. As physics gets weirder and weirder, we may find ways to wrap our teeny brains around this after all.
Many thanks to Mark Whittle who first couched it in these terms for me. His lecture notes are available!