“Primordial black holes (PBHs) remain hypothetical objects for the moment,” says Alvise Raccanelli of CERN. “Initially proposed by Stephen Hawking in 1971, they have come back to the fore in recent years as possible candidates for explaining dark matter. It is believed that dark matter accounts for approximately 80 percent of all matter present in the universe, so to explain even just a small part of it would be a major achievement. Looking for evidence of the existence of PBHs, or excluding their existence, provides us with information on the physics of the primordial universe.”
Primordial Black Holes and the Lyman-Alpha Forest
Within the dense weave of the cosmic web, the scientists concentrated on the so-called Lyman-alpha forest, the interactions of photons with the hydrogen of cosmic filaments, which reveal characteristics closely linked to the fundamental nature of dark matter. Cold neutral hydrogen atoms contain a single electron sitting in the ground n = 1 state, but the electron can be excited to the n = 2 orbital if the hydrogen atom absorbs a photon with a wavelength of 121.6 nanometers, the Lyman-alpha line.
Due to the expansion of the Universe, distant pockets of hydrogen gas that absorb Lyman-alpha photons are redshifted to longer wavelengths. Multiple pockets of hydrogen gas along the same line of sight but at different distances and thus different redshifts will appear as a “forest” of Lyman-alpha absorption lines. Various cosmological models predict different spacings and depths of Lyman-alpha absorption lines. In particular, the new research detailed below suggests that a population of primordial black holes can produce a distinct recognizable signature in the Lyman-alpha forest.
Never Seen Before Structure in Early Universe
“There is all this structure in the distant Universe that has never been seen before. Sometimes I feel like an adventuring cartographer from the Middle Ages!” says Anže Slosar, a physicist at the Brookhaven National Laboratory who works on data and theory related to surveys of the large scale structure of the universe. He was not part of the research about the Lyman-alpha forest.
The image below from the EAGLE simulation, a simulation of the universe, shows the context for a single galaxy forming within the large-scale cosmic web. A recent study suggests that early galaxies may form from cool gas that accretes from the cosmic web. (The Virgo Consortium, Schaye et al.)
Quasars (PKS 2349 shown at top of the page is 1.5 billion light years from Earth) are the brightest objects in the universe, which provide back lights to illuminate the intervening hydrogen gas that fills the universe between us and them. We can see their shadows, and the details in their shadows –- specifically, the absorption features in their spectra known as the Lyman-alpha forest. What is amazing, says Riccardo Murgia, lead author of the study published in Physical Review Letters, is that this allows us to see the universe so very far away.
The Dark-Matter Scenario
“We have tested a scenario in which dark matter is composed of non-stellar black holes, formed in the primordial universe,” says Murgia. The research was carried out together with his colleagues Giulio Scelfo and Matteo Viel of SISSA—International School for Advanced Studies and INFN—Istituto Nazionale di Fisica Nucleare (Trieste division) and Alvise Raccanelli of CERN.
Simulations carried out using the Ulysses supercomputer of SISSA and ICTP have been able to reproduce the interactions between photons and hydrogen. The models have been compared with real interactions detected by the Keck telescope in Hawaii. The researchers were then able to trace several properties of primordial black holes to understand the effects of their presence.
“We used a computer to simulate the distribution of neutral hydrogen on sub-galactic scales, which manifests itself in the form of absorption lines in the spectra of distant sources,” says Murgia. “Comparing the results of our simulations with the data observed, it is possible to establish limits on the mass and abundance of primordial black holes and determine whether and to what extent such candidates constitute dark matter.”
The results of the study seem to constrain the case that all dark matter is composed of a certain type of primordial black hole (those with a mass greater than 50 times that of the sun) but they do not totally exclude that they could constitute a fraction of it.
“We have developed a new way to easily and efficiently explore alternative scenarios of the standard cosmological model, according to which dark matter would instead be composed of weakly interacting massive particles (WIMPs).”
These results, important for the construction of new theoretical models and for the development of new hypotheses about the nature of dark matter, offer much more precise indications for tracing the intricate path to understanding one of the enduring mysteries of the cosmos.