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Starts With A Bang

Dark Matter Proved Real By Colliding Galaxy Clusters

It’s the 10th anniversary of dark matter’s most compelling proof.

“It may be that ultimately the search for dark matter will turn out to be the most expensive and largest null result experiment since the Michelson-Morley experiment, which failed to detect the ether.” –John Moffat

If you look at rotating galaxies or the motions of galaxies within clusters, there’s a mismatch between the matter we see and the gravitational effects we observe.

Traceable stars, neutral gas, and (even farther out) globular clusters all point to the existence of dark matter, which has mass but exists in a large, diffuse halo well beyond the normal matter’s location. Image credit: Wikimedia Commons user Stefania.deluca.

Even on the largest scales, the way galaxies clump and cluster together cannot be explained without some new physics.

Numerical simulation of the density of matter when the universe was 4.7 billion years old. Galaxy formation follows the gravitational wells produced by dark matter, where hydrogen gas coalesces, and the first stars ignite. This pattern in the Universe requires dark matter to match what’s observed. Image credit: V. Springel et al. 2005, Nature, 435, 629.

Observations show that it can’t be gas, dust, plasma or black holes; there’s truly something unaccounted for.

The Coma cluster of galaxies, whose galaxies move far too quickly to be accounted for by gravitation given the mass observed alone. Image credit: KuriousG of Wikimedia Commons, under a c.c.a.-s.a.-4.0 license.

Attempts to modify gravity can solve some of these problems, but the leading explanation is a new type of matter: dark matter.

From simulations and inferred maps, dark matter (blue) may form some clumps, but overall exists in a massive, diffuse halo around the luminous, disk-like part of galaxies we’re familiar with. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).

For a long time, these two possible explanations fought for dominance, but in 2006, dark matter emerged victorious.

The Bullet cluster, the first classic example of two colliding galaxy clusters where the key effect was observed. Image credit NASA/STScI; Magellan/U.Arizona/D.Clowe et al., of the Bullet Cluster.

Two colliding galaxy clusters were caught in the act by multiple telescopes, including Hubble and Chandra.

The X-ray observations of the Bullet Cluster. Image credit: NASA/CXC/CfA/M.Markevitch et al., from Maxim Markevitch (SAO).

The X-ray observatory showed that the colliding gas and plasma, made of normal matter, heated up and slowed down.

The gravitational lensing map (blue), overlayed over the optical and X-ray (pink) data of the Bullet cluster. The mismatch is undeniable. Image credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al., of the Bullet Cluster.

Yet mass reconstructions from gravitational lensing show that most of the matter passed right through.

Four colliding galaxy clusters, showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. Images credit: X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A. Mahdavi et al. (top left); X-ray: NASA/CXC/UCDavis/W.Dawson et al.; Optical: NASA/ STScI/UCDavis/ W.Dawson et al. (top right); ESA/XMM-Newton/F. Gastaldello (INAF/ IASF, Milano, Italy)/CFHTLS (bottom left); X-ray: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University) (bottom right).

This mismatch between where the matter is located and where the gravitational effects are has now shown up in many colliding clusters, independently.

The colliding galaxy cluster “El Gordo,” the largest one known in the observable Universe, showing the same evidence of dark matter. Image credit: NASA, ESA, J. Jee (Univ. of California, Davis), J. Hughes (Rutgers Univ.), F. Menanteau (Rutgers Univ. & Univ. of Illinois, Urbana-Champaign), C. Sifon (Leiden Obs.), R. Mandelbum (Carnegie Mellon Univ.), L. Barrientos (Univ. Catolica de Chile), and K. Ng (Univ. of California, Davis).

This empirical proof of dark matter cannot be explained by any theories of modified gravity.

Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images and video in no more than 200 words.

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