Spectra from quasars suggest that intergalactic gas was heated by a type of dark matter known as dark photons.
C. Stark/UC Berkeley and K. G. Lee/Max Planck Institute for Astronomy
The forecast is cloudy. Light from distant quasars travels through the Universe, imprinted with the absorption signatures of hydrogen gas encountered along the way. These absorption lines point to unusual heating, which could be explained by dark matter. The forecast is cloudy. Light from distant quasars travels through the Universe, imprinted with the absorption signatures of hydrogen gas encountered along the way. These absorption lines point to anomalous heating, which could be explained... Display more
The forecast is cloudy. Light from distant quasars travels through the Universe, imprinted with the absorption signatures of hydrogen gas encountered along the way. These absorption lines point to unusual heating, which could be explained by dark matter. ×
Throughout the Universe, dense gas clouds absorb light from distant quasars, resulting in absorption lines in quasar spectra. A new study suggests that the wider-than-expected widths of these lines from nearby gas clouds could be caused by a type of dark matter known as dark photons . These particles may heat the clouds, causing the absorption lines to widen. Other explanations for the broadening have been proposed, based on more conventional heating sources, but if the dark-photon mechanism is at work, it may also cause heating in low-density clouds from earlier epochs of the Universe. This prediction is already being tested by researchers.
When looking at the spectrum of a distant quasar, astronomers frequently notice absorption lines from the intervening clouds of gas. The Lyman-alpha line of hydrogen is the most visible absorption line. Indeed, some quasar spectra contain a "forest" of Lyman-alpha lines, each of which originates in a different cloud from our Galaxy (or different epochs). Researchers can learn about the density, temperature, and other characteristics of clouds by examining the widths, depths, and other details of the line shapes. This data can be compared to the outcomes of cosmological simulations that attempt to replicate the clumping of matter into galaxies and other large-scale structures.
Comparisons of forest data and simulations have generally revealed good agreement, but there is a discrepancy for relatively nearby gas clouds. Observations show that these so-called low redshift clouds have broader absorption lines than simulations predict. "This could be an indication of a specific candidate of dark matter known as a dark photon," says Andrea Caputo of CERN in Switzerland. "This dark photon can inject some energy and heat up the gas, [making] the lines a little wider and more in line with the data."
Cambridge's P. Gaikwad/Kavli Institute for Cosmology
Observing the trees The light from a distant quasar travels through dense gas (purple) regions in the intergalactic medium. Because the gas absorbs light at specific frequencies, the quasar spectrum contains a "forest" of absorption lines (green).
Observing the trees The light from a distant quasar travels through dense gas (purple) regions in the intergalactic medium. Because the gas absorbs light at specific frequencies, the quasar spectra (green) show a "forest" of absorption lines.
Caputo and his colleagues used dark photons in cosmic simulations to investigate how this energy injection might work. The theory of dark photons assumes that the particles can spontaneously convert to normal photons with a small probability, but that this conversion can be accelerated when dark photons enter an ionized gas that meets a resonance condition. The condition results in a gas with a specific density determined by the mass of the dark photon. If the density of an intergalactic cloud is this, then the ordinary photons produced by the resonance conversion will heat the gas.
Caputo emphasizes that because the density of a cloud changes over time, the resonance condition will only be met for a limited time. Other proposed types of heat-producing dark matter, such as those that decay or annihilate, are expected to be "switched on" all the time, whereas dark photons are expected to be "switched off" all the time. Other cosmological observations, such as the cosmic microwave background, do not show signs of unexplained heating, so models of continuous heating are constrained.
Caputo and colleagues' simulations suggest that dark photons with a mass of around 1014 eV/ c2 (roughly 1019 times smaller than the electron mass) would resonantly convert to photons in low-redshift Lyman-alpha clouds. This conversion would add between 5 and 7 eV of energy per hydrogen atom to the gas, which would explain the observations.
Furthermore, the researchers believe that dark-photon heating may have occurred at higher redshifts, but only in so-called under-dense clouds, which had higher densities in the past—potentially high enough to meet the resonance condition. The researchers are currently running simulations to see if the predicted heating matches observations of high-redshift clouds.
According to astrophysicist Blakesley Burkhart of Rutgers University in New Jersey, exotic dark matter physics models may not be required to explain the Lyman-alpha data. She believes that while dark photons are an exciting possibility, researchers have not ruled out more traditional heating sources, such as supermassive black hole jets at galaxies' centers, known as active galactic nuclei.
The dark photon explanation is more speculative, according to Sam Witte, a cosmologist at the University of Amsterdam, but he believes the researchers have made a compelling case with testable predictions. "Should future studies rule out conventional astrophysical explanations," he says, "it is compelling to consider the possibility that we are witnessing the first nongravitational imprint of dark matter."
Michael Schirber is a Physics Magazine Corresponding Editor based in Lyon, France.
"Comparison of low-redshift Lyman- forest observations to hydrodynamical simulations with dark photon dark matter," J. S. Bolton et al., Phys. Rev. Lett. 129, 211102 (2022).