An Astronomer Puts the COSMOS Survey In Perspective

Cosmic Evolution Survey - Dark Matter Credit: NASA, ESA, and R. Massey (California Institute of Technology)

After decades of attempts, astronomers have unveiled the dark side of the Universe.

Dark matter, the ubiquitous yet ethereal stuff filling the cosmos, has been mapped three-dimensionally for the first time by a team of astronomers using a fleet of orbiting and ground-based telescopes. Implementing techniques not even dreamed of when dark matter was first postulated, they have created a map two degrees on a side (roughly 15 times the area of the full Moon) and 6 billion light years deep. The Cosmic Evolution Survey, or COSMOS, reveals the spatial distribution of dark matter stretching back to a time when the Universe was half its present age.

The first hint of dark matter came in the 1930s, when astronomer Fritz Zwicky investigated clusters of galaxies. He found that the galaxies in the clusters were moving so rapidly that the clusters should fly apart, yet they were clearly holding themselves together just fine. Zwicky concluded that there must be a lot more mass to the cluster than what he could see with his telescope—in fact, 90 percent of the mass of the clusters was “missing.”

As time went on, more and more data suggested that the bulk of the Universe is invisible. Astronomers weren’t thrilled with this (who wants to be told you can’t see the majority of what you’re trying to study?), but the evidence kept mounting. Galaxies rotated too quickly, implying that they had extra matter surrounding them in halos that were totally invisible. X-ray images revealed galaxies that were submerged in vast pools of million-degree gas, which should quickly dissipate unless the gravity from some unseen matter held it in place. And so on.

Astronomers knew dark matter was out there, but it was frustratingly, well, dark. What could it be? There was no lack of theories—maybe dark matter was made up of black holes, or planets, or faint old dead stars, or cold gas—but all these ideas fell short. Each of these objects would have betrayed its presence in one way or another. In fact, recent observations suggest that dark matter is some exotic form of matter that only interacts with normal matter through gravity.

So how do you find what you can’t see and see what you can’t find?

Astronomers got clever. The COSMOS team made their map of dark matter by exploiting a peculiar characteristic of gravity: its ability to warp space. Einstein postulated that gravity bends space like a bowling ball placed in the middle of a bed distorts the mattress. Light moving through empty space travels in a straight line, but if it passes by a mass, the gravity will bend the light’s path. How much the path bends depends on how much mass there is and how it’s distributed.

That is how scientists can detect dark matter. Mass (both visible and invisible) twists, bends, and warps the light from distant galaxies on its way from there to here. The visible mass can be measured in several ways, and by subtracting the visible component from the total mass, researchers are able to find the location and quantity of dark matter.

Of course, actually doing this is a bit trickier. It takes the combined might of telescopes, such as the orbiting Hubble, Spitzer, and XMM-Newton observatories together with ground-based instruments including the Very Large Telescope and the Subaru Observatory, to be able to make these high-precision observations.

The COSMOS team—over 100 astronomers in a dozen countries—used this formidable array of telescopes to map the positions, distances, and shapes of over 2 million galaxies. The most distant objects they were able to measure were about 6 billion light years away, halfway to the observable edge of the Universe. The team carefully applied statistical methods to the shapes of the galaxies to deduce the amount and location of dark matter between them and us.

The dark matter detected by the COSMOS survey can be mapped in three dimensions, with distance running from left (near) to right (far). Credit: NASA, ESA, and R. Massey (California Institute of Technology)

The result is nothing less than profound: a three-dimensional map millions of light years across and billions deep, showing the location of trillions of solar masses of invisible ethereal stuff that only decades ago was a complete mystery.

Even glancing at the map reveals insights into the Universe. The left hand side represents matter that is close to us, and the right side is farther away. We see more distant matter as it was farther in the past, so in a sense we have a time machine that lets us understand the Universe as it was 6 billion years ago. In the past, dark matter formed huge structures spanning hundreds of millions of light years across. But in more recent history, these enormous blobs have broken into smaller, scattered clumps. This shows that over time, the gravity of the big structures made them collapse into an array of smaller ones—just as modern theories of cosmology have predicted.

COSMOS verifies theory’s next prediction, too: Once dark matter condensed into smaller blobs, its gravity would increase, drawing in more dark matter and normal matter. Eventually, the normal matter would gather near clumps of dark matter, so wherever we see large amounts of dark matter today, we should also see normal matter. The survey confirms this; the visible matter detected lies roughly along the same positions as the dark matter.

As incredible as it is, COSMOS represents only the first tentative steps into understanding dark matter. While the map is a vast repository of data, it only covers about 1/10000th of the sky. Larger scale surveys that will map more ambitious areas are already in the planning stages.

Nor does this survey answer the biggest mystery of all: Just what is dark matter? No one knows for sure. But scientists love a mystery, especially a big one. And they don’t get any bigger than figuring out what the matter is with the Universe.

Phil Plait is an astronomer and writer. He runs the Bad Astronomy website and writes the Bad Astronomy Blog, where he reports on all manners of cosmic news, both good and bad.

Originally published February 2, 2007


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