Full sky map of the cosmic microwave background. The color red indicates a cool spot while the color blue indicates a hot spot. Image credit: NASA.

Astronomers Map Dark Matter Throughout the Entire Universe

Article Updated: 23 Dec , 2015

Warped visions of the cosmic microwave background – the earliest detectable light – allow astronomers to map the total amount of visible and invisible matter throughout the universe.

Roughly 85 percent of all matter in the universe is dark matter, invisible to even the most powerful telescopes, but detectable by its gravitational pull.

In order to find dark matter, astronomers look for an effect called gravitational lensing: when the gravitational pull of dark matter bends and amplifies light from a more distant object. In its most eccentric form it results in multiple arc-shaped images of distant cosmic objects.

The Hubble Space Telescope shows the effect of gravitational lensing as background galaxies are warped by the galaxy cluster MACS J1206. Image Credit: NASA

The Hubble Space Telescope shows the effect of gravitational lensing as background galaxies are warped by the galaxy cluster MACS J1206. Image Credit: NASA

But there’s one caveat here: in order to detect dark matter there must be an object directly behind it. The ‘stars’ have to be aligned.

In a recent study led by Dr. James Geach of the University of Hertfordshire in the United Kingdom, astronomers have set their eyes on the cosmic microwave background (CMB) instead.

“The CMB is the most distant/oldest light we can see,” Dr. Geach told Universe Today. “It can be thought of as a surface, backlighting the entire universe.”

The photons from the CMB have been hurling toward the Earth since the universe was only 380,000 years old. A single photon has had the chance to run into plenty of matter, having effectively probed all the matter in the universe along its line of sight.

“So our view of the CMB is a bit distorted from what it intrinsically looks like – a bit like looking at the pattern on the bottom of a swimming pool,” Dr. Geach said.

By noting the small distortions in the CMB, we can probe all of the dark matter throughout the entire universe. But doing just this is extremely challenging.

The team observed the southern sky with the South Pole Telescope, a 10 meter telescope designed for observations in the microwave. This large, groundbreaking survey produced a CMB map of the southern sky, which was consistent with previous CMB data from the Planck satellite.

The characteristic signatures of gravitational lensing by intervening matter can not be extracted by eye. Astronomers relied on the use of a well-developed mathematical procedure. We wont go into the nasty details.

This produced a “map of the total projected mass density between us and the CMB. That’s quite incredible if you think about it – it’s an observational technique to map all of the mass in the universe, right back to the CMB,” Dr. Geach explained.

But the team didn’t finish their analysis there. Instead, they continued to measure the CMB lensing at the positions of quasars – powerful supermassive black holes in the centers of the earliest galaxies.

“We found that regions of the sky with a large density of quasars have a clearly stronger CMB lensing signal, implying that quasars are indeed located in large-scale matter structures,” Dr. Ryan Hickox of Dartmouth College – second author on the study – told Universe Today.

Finally, the CMB map was used to determine the mass of these dark matter halos. These results matched those determined in older studies, which looked at how the quasars clustered together in space, with no reference to the CMB at all.

Consistent results between two independent measurements is a powerful scientific tool. According to Dr. Hickox, it shows that “we have a strong understanding of how supermassive black holes reside in large-scale structures, and that (once again) Einstein was right.”

The paper has been accepted for publication in the Astrophysical Journal Letters and is available for download here.


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