Two Kinds Of Gravitational Lenses Both Reveal Dark Matter

The Universe is dark, but the distorted light reveals its presence.

When we look at the objects in the Universe, the mass just doesn’t add up.

A galaxy that was governed by normal matter alone (L) would display much lower rotational speeds in the outskirts than towards the center, similar to how planets in the Solar System move. However, observations indicate that rotational speeds are largely independent of radius (R) from the galactic center, leading to the inference that a large amount of invisible, or dark, matter must be present. (WIKIMEDIA COMMONS USER INGO BERG/FORBES/E. SIEGEL)

All the normal matter in the Universe — atoms, plasma, stars, black holes, etc. — can’t explain what we see.

According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. The effects of gas, feedback, star formation, supernovae, and radiation all complicate this environment, making it extremely difficult to extract universal dark matter predictions, but the biggest problem may be that the cuspy centers predicted by simulations are nothing more than numerical artifacts. (NASA, ESA, AND T. BROWN AND J. TUMLINSON (STSCI))

There needs to be more mass than normal matter alone to explain what we see.

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 in their support for dark matter. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

Some novel, exotic, invisible “dark matter” is the leading theoretical idea.

In modern cosmology, a large-scale web of dark matter and normal matter permeates the Universe. On the scales of individual galaxies and smaller, the structures formed by matter are highly non-linear, with densities that depart from the average density by enormous amounts. On very large scales, however, the density of any region of space is very close to the average density: to about 99.99% accuracy. (WESTERN WASHINGTON UNIVERSITY)

It’s a radical proposition to presume dark matter’s existence, but gravitational lenses can reveal it.

This illustration shows how the presence of a foreground mass, such as a massive galaxy cluster, can magnify and distort the light coming from a background galaxy or quasar. This phenomenon has many different manifestations, but is always known as some form of gravitational lensing. (NASA/ESA)

In Einstein’s General Relativity, the presence of mass curves the fabric of spacetime.

In the centre of this image, taken with the NASA/ESA Hubble Space Telescope, is the galaxy cluster SDSS J1038+4849 and it seems to be smiling. You can make out its two orange eyes and white button nose. In the case of this happy face, the two eyes are very bright galaxies and the misleading smile lines are actually arcs caused by an effect known as strong gravitational lensing. (NASA & ESA ACKNOWLEDGEMENT: JUDY SCHMIDT)

A large collection of foreground mass distorts the background light through the process of gravitational lensing.

When light magnifies and creates multiple images of distant objects, that’s strong gravitational lensing.

In this image, six examples of the rich diversity of 67 strong gravitational lenses found in the COSMOS survey. The lenses were discovered in a recently completed, large set of observations as part of a project to survey a single 1.6-square-degree field of sky (nine times the area of the full Moon) with several space-based and Earth-based observatories. Gravitational lenses occur when light travelling towards us from a distant galaxy is magnified and distorted as it encounters a massive object between the galaxy and us. These gravitational lenses often allow astronomers to peer much further back into the early Universe than they would normally be able to. The COSMOS project, led by Nick Scoville at the California Institute of Technology, used observations from several observatories including the Hubble Space Telescope, the Spitzer Space Telescope, the XMM-Newton spacecraft, the Chandra X-ray Observatory, the Very Large Telescope (VLT), and the Subaru Telescope. In total 67 gravitational lenses were found. (NASA, ESA, C. FAURE (ZENTRUM FÜR ASTRONOMIE, UNIVERSITY OF HEIDELBERG) AND J.P. KNEIB (LABORATOIRE D’ASTROPHYSIQUE DE MARSEILLE))

These strong lenses indicate six times the total mass as normal matter alone.

By leveraging a total of eight quadruply lensed systems (six are shown here), astrophysicists were able to use gravitational lensing to place constraints on dark matter substructure in the Universe, and hence on the mass/temperature of dark matter particles as a result. (NASA, ESA, A. NIERENBERG (JPL), AND T. TREU AND D. GILMAN (UCLA))

They also reveal dark matter substructures: small sub-halos embedded within larger structures.

Any configuration of background points of light — stars, galaxies or clusters — will be distorted due to the effects of foreground mass via weak gravitational lensing. Even with random shape noise, the signature is unmistakeable. (WIKIMEDIA COMMONS USER TALLJIMBO)

Additionally, weak gravitational lenses reveal dark matter as well.

The overlay in the lower left hand corner represents the distortion of background images due to gravitational lensing expected from the dark matter “haloes” of the foreground galaxies, indicated by red ellipses. The blue polarization “sticks” indicate the distortion. These effects are consistent with the observations. (MIKE HUDSON/WATERLOO; HUBBLE & NASA)

These less ideal configurations still distort the shapes and orientations of background galaxies.

Clumps and clusters of galaxies exhibit gravitational effects on the light-and-matter behind them due to the effects of weak gravitational lensing. This enables us to reconstruct their mass distributions, which should line up with the observed matter. (ESA, NASA, K. SHARON (TEL AVIV UNIVERSITY) AND E. OFEK (CALTECH))

By observing this light, we reconstruct the foreground masses of these weak lenses.

A galaxy cluster can have its mass reconstructed from the gravitational lensing data available. Most of the mass is found not inside the individual galaxies, shown as peaks here, but from the intergalactic medium within the cluster, where dark matter appears to reside. The time-delay observations of the Refsdal supernova, for example, cannot be explained without the presence of dark matter. (A. E. EVRARD. NATURE 394, 122–123 (09 JULY 1998))

In both cases, dark matter is absolutely necessary.

The galaxy cluster MACS 0416 from the Hubble Frontier Fields, with the mass shown in cyan and the magnification from lensing shown in magenta. That magenta-colored area is where the lensing magnification will be maximized. Mapping out the cluster mass allows us to identify which locations should be probed for the greatest magnifications and ultra-distant candidates of all. (STSCI/NASA/CATS TEAM/R. LIVERMORE (UT AUSTIN))

The same ratio — 5 times the abundance of normal matter — explains everything we’ve observed.

This image showcases the massive, distant galaxy cluster Abell S1063. As part of the Hubble Frontier Fields program, this is one of six galaxy clusters to be imaged for a long time in many wavelengths at high resolution. The diffuse, bluish-white light shown here is actual intracluster starlight, captured for the first time. It traces out the location and density of dark matter more precisely than any other visual observation to date. (NASA, ESA, AND M. MONTES (UNIVERSITY OF NEW SOUTH WALES))

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.