Dark Energy

by Justin

As I am utterly unequipped to articulate such grand awesomeness any better than Lisa Grossman over at the Wired blog, I’ll stick to her version of last week’s news:

Our view of dark energy, the mysterious force that is shoving the universe apart, just got a little clearer. By observing the way large clumps of mass distort their local space-time into enormous cosmological lenses, astronomers have zoomed in on a quantity that describes how dark energy works.

The universe’s composition breaks down roughly as follows: traditional atomic matter = 4%; dark matter = 24%; dark energy = 72%. Dark matter is observable because of its gravitational effects on the 4% with which we’re well acquainted. The majority of the universe, however, is composed of a mysterious energy that astronomers and physicists believe must be responsible for our ever-expanding (at an ever-accelerating rate) cosmos.

Forgive the sheen of my ignorance on this subject – I’ll do my best to keep things accurate. The catch with dark energy is that it seems to drive the universe outward, but without the observable particles of other fields (i.e. electromagnetic and photons). And it’s doing it at a faster and faster rate.

Before I touch on the Wired article and the new study, here’s a slice of weirdness that is worth exploring if you’ve got the time and inclination to have your brain rocked:

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence ormodulidynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant is physically equivalent tovacuum energy. Scalar fields which do change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.

Dive on in. What’s especially slayerly to me is that the cosmological constant was contributed by Einstein as his work on relativity began to predict a contracting universe – and his initial model required a static universe. Then Hubble exploded physics with the redshift photographic evidence that the universe was expanding. Einstein called the cosmological constant a mistake, but it’s since been adopted as useful again. The gross oversimplification is this: if the equilibrium component of the equation is itself unstable, then each expansion releases vacuum energy which drives further expansion. So the original Einstein model doesn’t lead to a static vision of the universe. I wish I had a better understanding of Einstein’s ability to think outside the then existing parameters of physics and offer grand, beautiful theories that even now have components that are newly valuable. What a guy.

Back to the present (future). Astrophysicists at Yale used the Hubble Telescope’s images of a cluster of galaxies to view the geometric manipulation of space-time:

This galaxy cluster contains so much matter — both dark matter and the regular type — that light passing through it is distorted into long, stringy arcs. The cluster acts as a gigantic magnifying glass called a gravitational lens, and produces multiple, distorted images of the galaxies behind it.

They then compare the data from the warped images with the actual distance of the perceived galaxies from earth and can thereby map dark energy:

“Knowing exactly where the object is, and knowing about the big lump that is causing the bumps in space-time, allows us to accurately calculate the light path,” Natarajan said. “The light path depends on geometry of space-time, and dark energy manifests itself there. That’s how we get at it.”

Get it? They use a galaxy cluster as a giant lens through which the Hubble focuses. Gravitational lenses!

The study, full text of the published article right here, is worth reading for all nerds if you’re willing to skip through the denser equation bits. There’s a lot of that, sure, but there’s also a lot of stuff that just sounds awesome. Really, really cool sections about Pseudo-Isothermal Elliptical Mass Distribution as a qualifier for a potential lens.