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HomeSciencePhysicsDark Matter as an Intergalactic Heat Source

Dark Matter as an Intergalactic Heat Source

• Physics 15, 180

Spectra from quasars recommend that intergalactic gasoline might have been heated by a type of darkish matter referred to as darkish photons.

Ok. G. Lee/Max Planck Institute for Astronomy and C. Stark/UC Berkeley

Cloudy forecast. Light from distant quasars travels through the Universe toward Earth and is imprinted with the absorption signatures from hydrogen gas it encounters along the way. These absorption lines suggest anomalous heating that could be explained by dark matter.Cloudy forecast. Light from distant quasars travels through the Universe toward Earth and is imprinted with the absorption signatures from hydrogen gas it encounters along the way. These absorption lines suggest anomalous heating that could be explai… Show more

Dense gasoline clouds throughout the Universe take up mild from distant quasars, producing absorption strains within the quasar spectra. A brand new examine reveals that the larger-than-predicted widths of those strains from close by gasoline clouds might end result from a type of darkish matter referred to as darkish photons [1]. These particles might warmth the clouds, resulting in a widening of the absorption strains. Other explanations of the broadening—primarily based on extra standard heating sources—have been proposed, but when the dark-photon mechanism is at work, it may also trigger heating in low-density clouds from earlier epochs of the Universe. Researchers are already planning to check this prediction.

When viewing the spectrum of a distant quasar, astronomers typically observe absorption strains coming from the intervening clouds of gasoline. The most outstanding absorption line is the Lyman-alpha line of hydrogen. Indeed, some quasar spectra have a “forest” of Lyman-alpha strains, with every coming from a cloud at a special distance from our Galaxy (or completely different epochs). By inspecting the widths, depths, and different particulars of the road shapes, researchers can extract details about the density, the temperature, and different options of the clouds. This data could be in contrast with the outcomes of cosmological simulations that attempt to reproduce the clumping of matter into galaxies and different large-scale buildings.

Comparisons between forest knowledge and simulations have typically proven good settlement, however a discrepancy seems for comparatively close by gasoline clouds. Observations present that these so-called low redshift clouds produce broader absorption strains than predicted in simulations. “This may be an indication of a particular candidate of dark matter, which is called a dark photon,” says Andrea Caputo from CERN in Switzerland. “This dark photon can inject some energy and heat up the gas, [which makes] the lines a bit broader, in better agreement with the data.”

P. Gaikwad/Kavli Institute for Cosmology, Cambridge

Seeing the trees. The light from a distant quasar passes through regions of dense gas (purple) in the intergalactic medium. The gas absorbs light at specific frequencies, leading to a “forest” of absorption lines in the quasar spectra (green).

To discover how this vitality injection would possibly work, Caputo and his colleagues ran cosmic simulations with darkish photons. The principle of darkish photons assumes that the particles can spontaneously flip into regular photons with some small chance, however this conversion could be enhanced when darkish photons enter an ionized gasoline satisfying a resonance situation. The situation quantities to the gasoline having a sure density, which is decided by the darkish photon’s mass. If an intergalactic cloud has this density, then the odd photons generated by the resonance conversion will warmth the gasoline.

Caputo stresses {that a} cloud’s density modifications over time, so the resonance situation can be met for under a sure time frame. This time-dependent heating is exclusive to darkish photons, as different proposed varieties of heat-producing darkish matter, reminiscent of those who decay or annihilate, are anticipated to be “switched on” on a regular basis. However, fashions of steady heating are constrained by different cosmological observations, such because the cosmic microwave background, which don’t present indicators of unexplained heating.

The simulations of Caputo and colleagues recommend that darkish photons with a particularly small mass of round 10−14 eV/c2 (roughly 1019 occasions smaller than the electron mass) would resonantly convert to photons in low-redshift Lyman-alpha clouds. This conversion would inject between 5 and seven eV of vitality per hydrogen atom into the gasoline, sufficient to account for the observations.

In addition, the group predicts that dark-photon heating may need occurred at increased redshift, however solely in so-called under-dense clouds, which prior to now had increased densities—doubtlessly excessive sufficient to satisfy the resonance situation. The group is at the moment operating simulations to see if this predicted heating would agree with observations of high-redshift clouds.

However, unique darkish matter physics fashions will not be required to elucidate the Lyman-alpha knowledge, says astrophysicist Blakesley Burkhart from Rutgers University in New Jersey. She says darkish photons are an thrilling risk, however researchers haven’t but dominated out extra standard heating sources, reminiscent of supermassive black gap jets on the facilities of galaxies, often known as energetic galactic nuclei.

Sam Witte—a cosmologist from the University of Amsterdam—agrees that the darkish photon clarification is extra speculative than different eventualities, however he thinks the researchers have made a convincing case with testable predictions. “Should future studies exclude conventional astrophysical explanations, it is compelling to consider the possibility that we might be observing the first nongravitational imprint of dark matter,” he says.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics Magazine primarily based in Lyon, France.


  1. J. S. Bolton et al., “Comparison of low-redshift Lyman-

    forest observations to hydrodynamical simulations with dark photon dark matter,” Phys. Rev. Lett. 129, 211102 (2022).

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