In our search for cosmic signs of dark matter, we might compare ourselves to that drunk looking for his lost keys under a streetlight simply because it looked better there. In this case, the “streetlights” are the regions of space teeming with galaxies and galaxy clusters, which are thought to be embedded in dense clouds or “halos” of dark matter.
But what if we turned our gaze instead to cosmic voids, vast expanses of nearly empty space? In a recent paper published in the arXiv repository, a trio of researchers argue that while the dark matter signal from those parts of the cosmos would be weaker, it would also be less contaminated by astrophysical sources and thus could be more easy to spot.
“This is an innovative idea,” says Nico Hamaus, a cosmologist at the University of Munich, who was not involved in the study. “And it’s not just the idea. It’s also backed up by some reasonable calculations.”
Dark matter is thought to make up more than 80 percent of the matter in the universe. This estimate is based primarily on the gravitational effects that this mysterious substance appears to exert on the gas, dust, stars, and galaxies that make up ordinary matter. For example, based on their rotational speeds, galaxies should have disintegrated long ago if the gravity of dark matter didn’t hold them together.
Physicists’ best guess is that dark matter is made up of “weakly interacting massive particles” (WIMPs). But direct evidence of the existence of such particles has yet to be found, despite searching for decades in particle accelerators and ultrasensitive detectors buried deep underground to minimize spurious signals from cosmic rays and other sources. Even so, WIMPs remain the leading candidates for making up dark matter, according to Nicolao Fornengo, a researcher at the University of Turin and co-author of the study.
According to almost all WIMP-based models, if these particles are as heavy as scientists expect—between a few gigaelectronvolts (GeV) and a few teraelectronvolts (TeV), where one GeV roughly corresponds to the mass of a proton—they should end up to disintegrate or to collide with each other and annihilate. And both possibilities would produce gamma rays. “If dark matter produces gamma rays, the signal should be there,” says Fornengo.
Current gamma-ray observatories, especially NASA’s Fermi mission and its Large Area Telescope (LAT), detect a diffuse “background” of gamma rays across the sky. That background is the excess that remains after contributions from all known astrophysical sources, such as pulsars and matter-devouring supermassive black holes, are subtracted. And it’s not evenly distributed across the sky, which fits astrophysicists’ expectations for emission from dark matter and from astrophysical sources so small they can’t be resolved even with the best-in-class LAT telescope.
As far as dark matter is concerned, the brightness of gamma rays from WIMP decay and annihilation should be related to the cosmic large-scale structure: it would be strongest in regions packed with matter and faintest in dark matter. cosmic voids. Preliminary studies suggest that such a correlation exists, but such work has tended to avoid gaps and focus on the brightest regions, filled with galaxies and clusters.
To test whether it is easier to extract the signal from voids than from denser regions, the team modeled how it should emanate from both types of cosmic structures. Their results suggest that although the total gamma-ray signal from dark matter and ordinary matter contained in a vacuum would be much weaker than that from a denser region, that weakness actually has an advantage: the relative The lack of ordinary matter ensures that there will be fewer astrophysical sources to mask the gamma-ray emission from dark matter. “It’s about a compromise between having a stronger signal, but more polluted, or a weaker signal, but cleaner,” explains Fornengo. He and his collaborators intend to publish his study in the journal Journal of Cosmology and Astroparticle Physics.
The team also found, as expected, that most of the gamma rays from the dark matter contained in these voids should come from particle decay, not annihilation. For two particles to annihilate, they must first collide, and the chances of WIMPs meeting in cosmic voids are low. Instead, the particles should disintegrate no matter what their concentration. “Disintegration gives information about all the mass contained in a volume of space,” says Fornengo. “And the mass of a vacuum is not small: it is still a large object, just less dense.”
Hamaus says that the technique, thanks to its better signal-to-noise ratio and its tendency to detect gamma rays from decays, could offer new insights into the properties of dark matter that would not be available if we just studied the gamma rays from denser areas. For example, the longer the half-life of a dark matter particle, the fewer decays should occur in a given region of space and time. Although such a weak signal would usually be undetectable, that shouldn’t happen in voids. ‘Because the signal-to-background ratio is better, it is possible to study other regions of the parameter space,’ he says.
Anthony Pullen, an astrophysicist at New York University not involved in the study, is cautiously optimistic about the possibility of testing such ideas in the near future. Several exhaustive studies of the cosmic structure with new-generation instruments, such as the European Space Agency’s Euclid Space Telescope, NASA’s Nancy Grace Roman Space Telescope (previously called WFIRST) or the Vera C. Rubin Observatory, located on the surface. “As these studies get under way, we will have large data sets. The more galaxies we can detect, the better we can determine where the gaps really are,” says Pullen. “And that would help with these kinds of studies. In the next few years, maybe a proof of concept will take place.”
At present, such a proof of concept would have to rely on gamma-ray data collected by the Fermi mission’s LAT telescope, which is not good enough, according to Fornengo and his colleagues. To make unequivocal detections, they calculate, a new generation of gamma-ray instruments with twice the detection volume and five times more angular resolution (the ability to distinguish sources in the sky) than LAT would be needed. “A “new Fermi” would be a great signing,” Fornengo stresses, although he acknowledges that, for now, such a detector only exists in his dreams. However, that has not prevented the team from baptizing it with a name, which (how could it be otherwise) is Italian: Fermissimo.
Reference: “Got plenty of nothing: cosmic voids as a probe of particle dark matter.” Stefano Arcari, Elena Pinetti and Nicolao Fornengo at arXiv:2205.03360 [astro-ph.CO]May 6, 2022.
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