New Dark Matter Map Confirms Current Theories

NOTE: This post originally appeared on Scientific American's blog network on April 13, 2015. [URL]

The American Physical Society is holding its annual April Meeting at the moment in Baltimore, Maryland, and one of the highlights, research-wise, comes to us courtesy of the Dark Energy Survey (DES) collaboration. This afternoon, the researchers released the first in a series of maps of the dark matter that makes up some 23% of all the "stuff" (matter and energy) in our universe. The map was constructed based on data collected by the Dark Energy Camera, the primary instrument of the DES.

The camera is perched high on a mountaintop, mounted on a telescope at the Cerro Tololo Inter-American Observatory in Chile, the better to get high-resolution images with minimal interference. Now in its second year, the DES began taking data on August 31, 2013, with an eye toward better understanding dark matter's role in the formation of galaxies. The resulting map unveiled today is, as one might expect, spectacular — the first to trace in fine detail how dark matter is distributed across a huge swathe of sky, although it's a mere 3% of the area the DES will cover by the time it finishes its five-year scheduled run. It's not the first dark matter map ever, but it's the largest and highest resolution so far. Check it out:

The analysis — carried out by a team led by Argonne National Laboratory's Vinu Vikram and Chihway Change of the Swiss Federal Institute of Technology (ETH) in Zurich — looked at very subtle distortions in the shapes of two million galaxies to construct the map, thanks to a technique called gravitational lensing, whereby the invisible gravitational effects of the dark matter bend light around said galaxies in predictable ways. And so far, the researchers have found that the distribution of dark matter is pretty well in line with current theories — namely, that because there is significantly more dark matter than visible matter (a mere 4%) in the cosmos, galaxies were formed in those places where there are large concentrations of dark matter, and thus stronger gravity.

Think of it as a delicate interplay between mass and light. You can see that clustering in the color-coded image above, where the blue areas are where the density is about average, and the red and yellow areas depict regions of far greater density — places where there is more dark matter. The circles represent galaxies and galaxy clusters, which do indeed show up more in the higher-density areas. "Zooming into the maps, we have measured how dark matter envelops galaxies of different types and how together they evolve over cosmic time," Chang said in an official press release. "We are eager to use the new data coming in to make much stricter tests of theoretical models."

As more data becomes available over the next few years, the DES will further improve the scope and resolution of its dark matter maps. But the ultimate goal is to find out more about the accelerating universe, and more specifically, to suss out the nature of the mysterious dark energy that physicists believe is driving that acceleration. Dark energy accounts for a whopping 73% of the "stuff" in the universe, so, yanno, it's pretty important. Like, Nobel Prize worthy important. In fact, it's among the top research questions in 21st century physics.

The tools and techniques the survey will use to do so aren't limited to gravitational lensing. DES researchers will also study data from certain kinds of supernovae — the most common "standard candles" used to estimate cosmological distances. (Side note: a new paper in the Astrophysical Journal questions whether those standard candle Type 1a supernovae are as uniform as astronomers have assumed, which means the universe might be expanding at a slower rate than previously inferred.)

They will also keep track of how many galaxy clusters are detectable by the Dark Energy Camera. Monitoring how that changes over time should shed more light on the ongoing tug-of-war between gravity and dark energy (essentially an anti-gravity). Finally, the collaboration will study sound waves (a.k.a. baryonic oscillations) to map how the universe is expanding. Sound waves were created hundreds of thousands of years after the big bang that left an imprint in that galaxy distribution. Measure the positions of some 300 million galaxies, and physicists should be able to detect the pattern of that imprint, and use it to make inferences about the history of how the universe has been expanding.

As it happens, the Dark Energy Camera just won Symmetry's Physics Madness contest for favorite big physics machine, beating out the heavily favored Large Hadron Collider. Serendipity! Given this lovely new map, and the promise of even better ones to come, I'd say it's an honor well deserved.


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