From Ethan Siegel: “Scientists Discover Space’s Largest Intergalactic Bridge, Solving A Huge Dark Matter Puzzle”

From Ethan Siegel
Jun 13, 2019

This image shows a composite of optical, X-ray, Microwave and radio data of the regions between the colliding galaxy clusters Abell 399 and Abell 401. The X-rays are concentrated near where the clusters are, but there’s a clear radio bridge between them (in blue). (M. MURGIA / INAF, BASED ON F. GOVONI ET AL., 2019, SCIENCE)

Dark matter’s naysayers latched onto one tiny puzzle. This new find may have solved it completely.

Imagine the largest cosmic smashup you can. Take the largest gravitationally bound structures we know of — enormous galaxy clusters that might contain thousands of Milky Way-sized galaxies — and allow them to attract and merge. With individual galaxies, stars, gas, dust, black holes, dark matter and more inside, there are bound to not only be fireworks, but novel astrophysical phenomena that might not show up elsewhere in the Universe.

The gas within these clusters can heat up, interact, and develop shocks, causing the emission of spectacularly energetic radiation. Dark matter can pass through everything else, separating its gravitational effects from the majority of the normal matter. And, in theory, charged particles can accelerate tremendously, creating coherent magnetic fields that could span millions of light-years. For the first time, such an intergalactic bridge between two colliding clusters has been discovered, with tremendous implications for our Universe.

This Chandra image shows a large-scale view of the galaxy cluster MACSJ0717, where the white box shows the field-of-view of an available Chandra/HST composite image. The green line shows the approximate position of the large-scale filament leading into the cluster, suggesting a connection between the great cosmic web and the galaxy clusters that populate our Universe. (NASA/CXC/IFA/C. MA ET AL.)

In our cosmos, astronomical structures aren’t all created equal. Planets are dwarfed by stars, which themselves are far smaller in scale than Solar Systems. Collections of many hundreds of billions of these systems are required to make up a large galaxy like the Milky Way, while galactic groups and clusters might contain thousands of Milky Way-sized galaxies. On the largest scales of all, these enormous galaxy clusters can collide and merge.

Back in 2004, two sets of observations came in concerning a pair of galaxy clusters in close proximity: 1E 0657–558, more commonly known as the Bullet Cluster. From an optical image alone, two dense collections of galaxies — the two independent clusters — can clearly be identified.

The Bullet cluster, the first classic example of two colliding galaxy clusters where the key effect was observed. In the optical, the presence of two nearby clusters (left and right) can be clearly discerned.(NASA/STSCI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.)

There are then two additional things you can do to tease out additional information about what’s going on in this system. One physically interesting measurement you can make is to look at the light from all the galaxies you can see in the image, and identify which ones are behind (background galaxies) the clusters versus which ones are in front (foreground galaxies) of them.

When you look at the foreground galaxies, their orientations should be random: they should be circular or elliptical or disk-like with no average distortion skewed to favor any particular direction. But if there’s a large mass in front of the light, there should be gravitational lensing effects that distort the background images. The statistical differences in shape between the background and foreground galaxies can tell you information about how much mass is located at various positions in space, at least from our point of view.

Gravitational Lensing NASA/ESA


Any configuration of background points of light, whether they be stars, galaxies or galaxy clusters, will be distorted due to the effects of foreground mass via weak gravitational lensing.

Weak gravitational lensing NASA/ESA Hubble

Even with random shape noise, the signature is unmistakable. By examining the difference between foreground (undistorted) and background (distorted) galaxies, we can reconstruct the mass distribution of massive extended objects, like galaxy clusters, in our Universe. (WIKIMEDIA COMMONS USER TALLJIMBO)

The second thing you can do is to observe the exact same region of the sky in X-rays, using an advanced X-ray observatory in space. Observations that were conducted with NASA’s Chandra X-ray observatory were sufficient to do exactly that. What Chandra discovered was fascinating: two enormous clumps of gas were spotted, each one moving along with its home galaxy cluster. As expected, there’s an enormous amount of gas not only associated with each galaxy, but with the overall cluster as a whole.

But what was unexpected was the finding that the gas, making up about 13–15% the overall cluster’s mass, was actually separated from the gravitational effects! Somehow, the normal matter and the gravitational effects were separated, as though the overall mass had simply passed straight through. This result was taken as overwhelming astrophysical evidence for the existence of dark matter.

The gravitational lensing map (blue), overlayed over the optical and X-ray (pink) data of the Bullet cluster. The mismatch of the locations of the X-rays and the inferred mass is undeniable. (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.)

Since that time, more than a dozen other galaxy groups and clusters have been spotted colliding with one another, with each one demonstrating the same effect. Before a collision, if a cluster emits X-rays, those X-rays are associated with the cluster itself, and any gravitational distortion is found coincident with the location of the galaxies and the gas.

But after a collision, the X-ray emitting gas is offset from the matter, implying that the same physics is at play. When the clusters collide:

the galaxies take up only a small volume inside each cluster, and pass straight through,
the intracluster gas interacts and heats up, emitting X-rays and slowing down,
while the dark matter, expected to occupy an enormous halo surrounding each cluster, passes through as well, affected only by gravitation.

In every colliding group and cluster we’ve observed, the same separation of X-ray gas and overall matter is seen.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. Although some of the simulations we perform indicate that a few clusters may be moving faster than expected, the simulations include gravitation alone, and other effects may also be important for the gas.(X-RAY: NASA/CXC/ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND/D.HARVEY NASA/CXC/DURHAM UNIV/R.MASSEY; OPTICAL/LENSING MAP: NASA, ESA, D. HARVEY (ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND) AND R. MASSEY (DURHAM UNIVERSITY, UK))

You might think that this empirical proof of dark matter, seen in so many independent systems, would sway any reasonable skeptic. Alternative theories of gravity were concocted to try to explain the misalignment between the gravitational lensing signal and the presence of matter, postulating a non-local effect that resulted in a gravitational force that was offset from the matter. But any theory that worked for one particular alignment of colliding clusters failed to explain clusters in a pre-collisional state. 15 years later, alternatives still fail to explain both configurations.

But a Universe with dark matter has a very high burden of proof: it has to explain every single observed property of these clusters. While many of these colliding groups and clusters have speeds that are predicted by a dark matter-rich Universe, the Bullet cluster — the original example — moves extremely quickly.

The formation of cosmic structure, on both large scales and small scales, is highly dependent on how dark matter and normal matter interact. Despite the indirect evidence for dark matter, we’d love to be able to detect it directly, which is something that can only happen if there’s a non-zero cross-section between normal matter and dark matter. The structures that arise, however, including galaxy clusters and larger-scale filaments, are undisputed. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

When you know the ingredients of your Universe and the laws of physics that govern what’s in it, you can run simulations to predict what types of large-scale structure emerge. When we include simulations with gravitation alone, the fastest colliding clusters we predict move slower than the Bullet cluster does; the likelihood of having a single example like it in our Universe is less than 1-in-a-million.

When we buck the cosmic odds like this, we demand an explanation. While it’s always possible that our Universe is simply a lottery-winner in terms of what’s present within it, this observation poses a legitimate problem. Either the observations were wrong, or something else — some physical mechanism — is causing this normal matter to accelerate beyond what the gravitational effects alone would indicate.

See the full article here .


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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan