Probing the Dark Side of the Universe

Analysis by Jennifer Ouellette, for Discovery News

On July 13, 1998, the New Yorker featured one of its classic cartoons (by Jack Ziegler), depicting a TV newscaster somberly announcing, "Scientists revealed today that everything we thought we knew about the universe is wrongedy-wrong-wrong." That's pretty much sums up how the physics and astronomy crowd felt in 1998, when news broke that the expansion of our universe is actually accelerating, due to the effects of a mysterious thing called dark energy.

Ziegler's TV newscaster had a point: dark energy changed everything we thought we knew about the composition (and ultimate fate) of our universe. Scientists currently estimate it accounts for a whopping 72% of everything the universe is made of, followed by 24% dark matter, and 4% "normal" matter -- the latter being every star, galaxy, planet, and so forth contained in the cosmos. (My spousal unit, Caltech cosmologist Sean Carroll, is fond of telling how folks are always giving him grief that we only know about 4% of all the matter in the universe. "The miracle is that we understand 4%!" he exclaims.)

It's tough to probe dark energy to better understand its nature; it's pretty intangible stuff. At least with dark matter, we can study its gravitational influence on visible matter. But a paper appeared in Science last week outlining a new method for measuring dark energy, using data from NASA's Hubble Space Telescope.

The focus of the study, conducted by an international team of scientists, focused on a massive galaxy cluster known as Abell 1689 (pictured top). It's so massive that its gravitational influence creates a magnifying lens of sorts, such that the 34 or so galaxies behind it are split into multiple distorted shapes. This funhouse mirror effect is known as gravitational lensing, one of the more intriguing predictions Einstein made with his theory of general relativity.

Gravitational lensing is a useful tool for astronomers, because the way the light is bent by massive objects like Abell 1689 can shed light on things like dark matter and dark energy. For the latter, though, the scientists had to combine their Hubble data with other techniques, including careful building of precise mathematical models and maps of both the dark and "normal" matter that makes up the cluster, as well as ground-based measurements of the changing distance and speed at which all those background galaxies are hurtling away from us.

The result: The team of scientists were able to narrow the current range of scientific estimates about the effect of dark energy on our universe (usually denoted by the symbol w) by 30%. That result also takes into account data collected using other techniques: supernovae, X-ray imaging of galaxy clusters, and data from the Wilkinson Microwave Anisotropy Probe (WMAP). That's what JPL astronomer Eric Julio calls tackling the issue of dark energy from all sides.

Who cares how fast the universe is expanding, and why is that 30% significant? Well, if you know the rate of expansion, you can deduce the shape (geometry) of the universe, and that shape offers vital clues to our cosmic date. Or, as lead author Priyamvada Natarajan of Yale University put it in the official press release: "The content, geometry, and fate of the universe are linked, so if you can constrain two of those things, you learn something about the third."

See, matter curves space and time around it and gives rise to what we recognize as gravity. The more matter there is, the stronger the pull of gravity, and the more space will curve – making it more likely that expansion will halt and the universe will collapse back in on itself in the antithesis to the Big Bang, dubbed the “Big Crunch.”

If there’s not enough matter, the pull of gravity will gradually weaken as galaxies and other celestial objects move farther apart, and the universe will expand forever with essentially no end. Dark energy joins other evidence indicating a flat universe: not only will the universe expand for ever, the rate of that expansion will continue to accelerate.

These latest findings confirm previous results indicating that the nature of dark energy corresponds to a flat universe, and a universe that will expand faster and faster, with galaxies and stars getting further and further away from each other, even black holes evaporating into trace radiation, until all that's left is a cold, vast, nearly empty expanse in near-thermal equilibrium. That should keep Alvie in Annie Hall up at nights -- even if Brooklyn, per se, is not yet expanding at any perceptible rate.