AAS Post #6: The cosmological not-so-constant

Contributed by
Jan 11, 2006
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What's powering the Universal acceleration?

This may be the biggest question is modern cosmology, the study of the structure and evolution the Universe. We've known for nearly a century that the Universe is expanding. Distant galaxies appear to be moving away from us, indicating the Universe gets bigger every day. This was a hugely shocking result in the early 20th Century, but bigger shocks were in the works.

In the 1990s, two independent teams of astronomers determined that not only is the Universe expanding, but that expansion is accelerating, growing. Not only is the Universe bigger every day, it's getting bigger faster. This was completely unexpected; everyone assumed that the gravity of all the combined matter in the Universe acted as a cosmic brake, slowing the expansion. Some people even theorized the gravity would grind the expansion to a halt, and the Universe would recollapse on itself in the dim distant future.

But that was wrong. Something, some mysterious something, was pumping energy into the Universe, acting -- to stretch the earlier analogy a bit-- like a cosmic gas pedal. This is so bizarre it's hard to wrap your head around it. But the data all point that way.

But there's still a problem. To be able to measure this acceleration, you need to look at really distant objects. The effects nearby are way too small to measure, so you need to look at objects as far away as possible. To astronomers, this meant supernovae, exploding stars-- they're bright enough to see even from billions of light years away. It was tough-- you need to be able to examine a lot of supernovae, and each one is a bit different. This difference can screw up your results! So astronomers had to figure out a way to correct for all these little individualities. In fact, it is possible to do this pretty well, and it was the results from these studies which uncovered the acceleration of the expansion.

After studying this for some time, astronomers think that the acceleration is caused by some form of previously unknown energy permeating the cosmos. It acts like a pressure, forcing the Universe to expand ever-faster. One key thing is that astronomers assumed that the force by this "dark energy" is constant in time. In other words, it's always been the same, and always will be. Einstein was the first person to propose this, and he called it the Cosmological Constant. But to test its constancy, you need to see really distant objects, even farther away than we can spot supernovae. Seeing something 10 billion light years away is good, but 12 would be a whole lot better. The effects of expansion are a lot more obvious that far away. But what can be bright to be seen at that numbing distance?

There may be something. Gamma-ray bursts (called GRBs for short) are titanic explosions that dwarf even supernovae. Theoretically, they can be seen at distances of up to 13 billion light years! So astronomers turned to them to use as milestones in the distant Universe.

The problem is, GRBs are stubbornly individualistic. Every one appears to be really different from the other. A half-joking expression among astronomers is, "When you've seen one gamma-ray burst, you've seen one gamma-ray burst". It seemed impossible to be able to use them to measure the acceleration.

But now an astronomer thinks he's found the key. Brad Schaefer, from Louisiana State University, has been studying the problem. He thinks that he's found five separate characteristics of GRBs that, when viewed as a whole, herds this unruly lot into a usable yardstick. His numbers are still not absolutely rock solid, but what he's found is very intriguing: the GRB data seems to show that the amount of dark energy in the Universe is not constant with time. It appears to be greater now than it was in the distant past.

In the plot above, Brad shows the distance for a GRB plotted against its redshift, basically how fast the expansion of the Universe is carrying that burst away from us. What you need to see in that graph is that the GRBs all appear to fall below the prediction of their distance predicted by having a cosmological constant. The error bars on the most distant (high redshift) objects are big, but there is enough data there to cast doubt on the cosmological constant being quite so solidly constant.

Let me be clear: if this is true, it is a fundamental change in the way we view the Universe. It's weird enough that the Universe is accelerating, but if that acceleration is itself accelerating, that makes things a whole lot weirder. It's a huge monkey wrench in the works of cosmology, and if it's true it'll have theorists scrambling to figure out what's behind it. Mind you, this doesn't cast doubt on the Big Bang theory itself, it just changes one aspect of it.

But is it true? Right now it's hard to say. Brad has studied 52 GRBs, which is a nice sample, but needs more. At current rates of detection, we'll probably double this number in a year or two.Statistically, the confidence in this new model is at the 97% level, which that there is a 3% chance that this is just due to randomness in the data (like seeing three heads in a row when you flip a coin 5 times-- that may be because the coin is weighted, or it may just be a coincidence). 97% sounds pretty good, but we're talking about a major change in physics, so we'd rather have that at the 99.99% level!

At the press conference held this morning on this topic, Brad did an excellent job showing that this is a preliminary result, that's it's not as confident as it needs to be to overturn current thinking in cosmology, and that as time goes on we'll get more data we can plug into it. I was proud to be a scientist, to hear another scientist carefully explaining how results are tentative. As cosmologist Michael Turner said after the press conference, "It's an exciting time to be an astronomer. The mysteries run -- literally -- from our own back yard to the edge of the Universe."