Finding all the matter in the cosmos — even the invisible stuff

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“Weak Lensing” Helps Astronomers Map the Mass of the Universe

By making galaxies a little bit brighter, it points the way to elusive galaxies and lets us detect that most mysterious of substances: dark matter

For Smithsonian Magazine:

In ordinary visible light, this cluster of galaxies doesn’t look like much. There are bigger clusters with larger and more dramatic-looking galaxies in them. But there’s more to this image than galaxies, even in visible light. The gravity from the cluster magnifies and distorts light passing near it, and mapping that distortion reveals something about a substance ordinarily hidden from us: dark matter.

This collection of galaxies is famously called the “Bullet Cluster,” and the dark matter inside it was detected through a method called “weak gravitational lensing.” By tracking distortions in light as it passes through the cluster, astronomers can create a sort of topographical map of the mass in the cluster, where the “hills” are places of strong gravity and “valleys” are places of weak gravity. The reason dark matter—the mysterious substance that makes up most of the mass in the universe—is so hard to study is because it doesn’t emit or absorb light. But it does have gravity, and thus it shows up in a topographical map of this kind.

The Bullet Cluster is one of the best places to see the effects of dark matter, but it’s only one object. Much of the real power of weak gravitational lensing involves looking at thousands or millions of galaxies covering large patches of the sky. [Read the rest at Smithsonian Magazine]


Astronomers measured the rotation of a black hole from halfway across the Universe.

What, I need to say more?

Astronomers have now used gravitational magnification to measure the rotation rate of a supermassive black hole in a very distant galaxy. From four separate images of the same black hole, R.C. Reis, M.T. Reynolds, J.M. Miller, and D.J. Walton found it was spinning nearly as fast as possible. That likely means it was spun up by a small number of mergers with other black holes rather than a gradual increase from eating smaller amounts of mass.

This marks the first measurement of black hole rotation outside the local Universe…. [Read more]

Measuring black hole rotation halfway across the Universe