What the heck is dark energy? We still don't know. We're pretty sure it exists; several independent observations indicate that the expansion of the Universe is accelerating. We've known for almost a century that the Universe is getting bigger, but in 1998 it was discovered that the rate at which it expands is itself getting bigger every day. Whatever is behind this is acting almost like anti-gravity, or more accurately a pressure that is making the Universe inflate faster every second of every day.
But we don't know what the heck this stuff is. We've hung the name "dark energy" on it, but we know very little about it. For example, is it a constant across space, with a strength that never changes with distance or time? Or is it a function of space itself? If it's the latter, that means it gets stronger as space itself expands. That is, if there is some amount of dark energy in every cubic centimeter of space, and there are more cubic centimeters of space as the Universe expands, then dark energy will get stronger as time goes on. That means the acceleration will accelerate, growing ever-more until the Universe tears itself apart! We're talking here about determining the eventual fate of the entire Universe. Obviously, there's some interest in this topic. And now astronomers have found a new way to measure dark energy that may be able to differentiate between the two contenders. When the Universe was young, matter started to coalesce by gravity, forming huge structures millions of light years across. These collapsed to form galaxies and clusters of galaxies, like cities composed of thousands of smaller towns. If the Universe were not expanding, forming clusters would be easy. As time went on, more matter could fall in to the cluster, forming more galaxies and making the cluster bigger. But since the Universe was expanding, there was a limit to how big the clusters could get; the outermost fringes would be moving away from the central regions, and that limited the amount of raw material available to make galaxies. It's like going to the grocery store and trying to fill your cart with cans of spaghetti sauce, only to find workers removing the cans from the shelf at the same time. The number of cans you wind up with depends on how quickly the grocery store clerks are unshelving them. So the sizes of clusters today depends on how quickly the Universe expands, and this cosmic expansion depends on the amount and flavor of dark energy infusing it. Aha! If we carefully study the sizes of clusters in the near and distant Universe, we can get a handle on dark energy! That's just what Alexey Vikhlinin of the Smithsonian Astrophysical Observatory and his team did. They examined the hot gas in more than a dozen clusters with the Chandra X-ray Observatory. The gas is the raw building material of clusters, and can be used as a tracer for how the clusters formed and how big and massive they are now. What they found is, to me, something of a relief. First, they confirmed that the growth of clusters is consistent with the presence of dark energy in the first place (some people still doubt that dark energy exists, but this is another nail in the coffin of any alternative hypotheses). Second, they found that their results indicate that dark energy is a constant throughout the Universe. That is, it is not growing in strength with time, and the Universe won't rip itself to shreds in the dim future. Phew! I'm impressed with this work. One good thing about it is that it's independent of any other measurements of dark energy that have gone on before. We've been using things like distant supernovae to measure how fast the Universe is expanding; that's how dark energy was discovered in the first place. Other studies have looked at the pervasive microwave glow of the Universe, and other indicators have been used as well. This is the first to use the hot gas in clusters, and it doesn't rely on these other methods. That makes scientists more confident the result is correct. Not that we really understand dark energy yet. We can measure its effects, but we don't know what the heck it is. The most recent work is also bothersome: it indicates that the amount of dark energy in the cosmos we measure is 10^120 times smaller than it should be. That's a bit off! Obviously, there is something we're missing, and many people suspect that there are extra dimensions to the Universe that we cannot see, and this may be the problem (it may also explain why gravity is so much weaker than the other three fundamental forces, but that's another story...). The good news is that this new work with clusters may shed some light on dark energy, and on the very nature of the Universe itself. But to me, there is more good news... What all of this is telling us is that the Universe is even more complex than we thought. That makes it more interesting, more cool, and more fun! There's so much more left to learn, and the path to that knowledge is where the adventure lies.
The galaxy cluster Abell 85 (optical image on left) is infused with million degree gas that gives off X-rays (right). By measuring the amount of this gas, astronomers have determined that dark energy exists and that it's a constant throughout the cosmos. Not bad from a pink blob. Credit: NASA/CXC/SAO