Astronomers Might See Dark Matter by Staring into the Void

We could be likened to drunks looking for lost keys under lampposts, where the light shines the best. There are regions of space teeming with galaxies and galaxy clusters which are thought to be embedded in dense clouds. What if we trained our sights on the vast emptiness of the universe? While dark matter's overall signal from these parts of the universe would be weaker, it would also be less contaminated by astrophysical sources, which could make it easier to spot.

Hamaus, who was not part of the study, said it was a new idea. The idea is not the only one. There are some calculations that make sense.

Most of the material in the universe is made up of dark matter. The estimate is based on the idea that this mysterious substance exerts influence on the gas, dust, stars and galaxies that make up normal matter. Without the gravity of dark matter to hold them together, the galaxies would have fallen apart.

Dark matter is thought to be made of weakly interacting massive particles. Despite decades of looking for them in particle accelerators and exquisitely sensitive detectors buried deep underground, direct evidence for them has not been found. According to study co-authorNicolao Fornengo of the University of Torino in Italy, WIMPs are still the favored candidate.

If these particles are heavy, they should be between a few giga-electron-volts to a few Tera-electron-volts. The signal should be in there if dark matter producesgamma rays.

NASA has a large area telescope called the LAT, which is used to detect a diffuse all-sky "background" of gamma rays. Once the contributions from all known astrophysical sources are subtracted, the background is still unexplained. It isn't evenly distributed across the sky, which is consistent with what astrophysicists expect from dark matter emission and small astrophysical sources. The glow of decaying and annihilating WIMPs should correspond with large-scale Cosmic structure, shining brighter from matter-packed regions and fainter from the voids. The correlation may exist, but so far such studies have mostly avoided the voids and focused on the brighter parts of the universe.

The team modeled how the signal should come from both types of structures in order to see if it could be teased out better from voids. The relative lack of normal matter means fewer astrophysical sources that would otherwise obscure dark matter. It is a trade-off between having a stronger but more polluted signal and a weaker but cleaner signal. The study was submitted to the journal.

The team found that most of the dark matter in these voids should decay rather than be killed. The chances of WIMPs finding each other in a void are low. The particles should decay even if their density is low. Fornengo says "Decay just probes the whole mass within a volume of space." The mass of a void is much larger than that. The object is still large. Less dense is what it is.

Hamaus says that the technique might offer novel insights into dark matter's properties that would be hard to find using other methods. Less decay should occur over a given region of space and time if the average lifetime of a dark-matter particle is higher. It should not be possible for a faint signal in a void to be seen. He says that because your signal-to-background noise is higher, you can explore the space further.

Anthony Pullen is an astronomer at New York University who is not affiliated with the study. The European Space Agency, NASA, and Vera C. Rubin Observatory are some of the facilities that will be used to conduct large-scale surveys of the universe. You will have large data sets when those surveys come online. Pullen says that the better we can detect the bigger the voids are. This type of study would benefit from that. You could see a proof of concept in the next few years.

The data collected by the LAT isn't up to the job, according to Fornengo and his colleagues. The ability to tell apart sources in the sky would need a new generation of instruments with five times the angular resolution. According to Fornengo, it would be a great addition to have a new detector. Fermissimo is the nickname the team has given it.