Top Story: January 10, 2012

Dark Matter Map

“Dark” Doesn’t Mean “Doesn’t Exist”

By Ryan Wyatt

Reporting from first day of the American Astronomical Society (AAS) meeting in Austin, Texas…

This morning’s major announcement related to a new large-scale map of dark matter released by the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS).

Before I go any further, I should note that in chatting with people about dark matter (you’d be surprised how much it comes up), I find that the topic causes some confusion. Folks seem to think that just because we can’t see dark matter and we don’t know what it is, it must not exist. By that argument, oxygen didn’t exist until Lavoisier pinned down its nature in the late 18th Century… So how did everyone breathe before then?

Anyway, it’s absolutely true that we have not yet determined the nature of dark matter. But we can observe its effect on the visible universe. Indeed, the evidence takes many forms: dark causes stars to move differently in their host galaxies, and as one NASA press release described observations of a cluster of galaxies, “without dark matter,… the fast-moving galaxies and the hot gas would quickly fly apart.” Furthermore, out measurements of the cosmic microwave background reveal that nearly a quarter of our universe is made up of dark matter.

So, stepping off my soapbox… What’s so important about this new map?

First off, the CFHTLenS map covers a remarkably large portion of the sky! Analysis of about 10 million galaxies in four different regions of the sky led to four separate maps that cover a total area hundreds of times bigger than the full moon. (For comparison, an image shows all four regions together with a picture of the Moon and a postage-stamp version of an earlier survey, the COSMOS dark matter map.) Astronomers studied the distortion of the light emitted from these galaxies, bent by dark matter’s gravity as it makes its way to Earth.

We need to understand the distribution of dark matter in order to compare observations with predictions from supercomputer models of the evolution of the early Universe (for example, the Bolshoi simulation run at NASA Ames). Taken together, the COSMOS and CFHTLenS maps allow cosmologists to study the distribution of dark matter at different scales. New maps should improve on these results.

So… We may not be able to see dark matter, but we can map it! And as our maps get better with time, so will our understanding of the evolution of the Universe.

More to come from tomorrow’s sessions at the AAS meeting.

Ryan Wyatt is the director of the Morrison Planetarium and Science Visualization at the California Academy of Sciences.


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