New Way to Explore Alternative Scenarios of Standard Cosmological Model

SISSA scientists have been searching for solutions to questions related to the origin of the Cosmos, moving through spider webs and cosmic forests in deep space.

Lyman-alpha Forest simulations (Projection of the neutral hydrogen fraction at redshift z = 2 and z = 4.0) (Image credit: The Sherwood Simulation Suite)

We have tested a scenario in which dark matter is composed by non-stellar black holes, formed in the primordial Universe.

Riccardo Murgia, Study Lead Author, SISSA

The research was recently reported in Physical Review Letters. The study was performed in collaboration with his coworkers 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.

Primordial black holes (or PBH as cosmologists refer to them) are objects that were formed within fractions of a second following the Big Bang, regarded by various scientists as one of the main candidates in elucidating the nature of dark matter. Most importantly, they were regarded so after direct observations of gravitational waves by the LIGO and VIRGO detectors in 2016.

Primordial black holes remain hypothetical objects for the moment, but they are envisaged in some models of the primordial universe. Initially proposed by Stephen Hawking in 1971, they have come back to the fore in recent years as possible candidates for explaining dark matter.

Alvise Raccanelli, of CERN

Raccanelli continued, “It is believed that this accounts for approximately 80% of all matter present in the Universe, so to explain even just a small part of it would be a major achievement. Not only, but looking for evidence of the existence of PBHs, or excluding their existence, provides us with information of considerable relevance on the physics of the primordial universe.”

Cosmic Forests and Spider Webs

In this study, the focus of the researchers was on the plethora of PBHs, which are 50 times larger compared to the solar mass. In other words, the scientists have attempted to better explain various parameters related to their presence (particularly mass and abundance) by studying the interaction of the light emitted from very distant quasars with the cosmic web.

The cosmic web is a network of filaments formed of gas and dark matter that exists throughout the entire Universe. In this dense weave, the researchers have focused on the “Lyman-alpha forest,” that is, the interactions between the photons and the hydrogen of cosmic filaments. These interactions present properties closely associated with the basic nature of dark matter.

Between Supercomputers and Telescopes

Simulations performed using the Ulysses supercomputer of ICTP and SISSA were able to replicate the interactions between hydrogen and photons. The interactions have been compared with “real” interactions, observed by the Keck telescope (in Hawaii). Then, the scientists could trace various properties of primordial black holes to gain insights into the effects of their occurrence.

We used a computer to simulate the distribution of neutral hydrogen on subgalactic scales, which manifests itself in the form of absorption lines in the spectra of distant sources. 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 can constitute dark matter.

Riccardo Murgia, Study Lead Author, SISSA

The outcomes of the research appear to negate the case that all dark matter is formed of a specific type of primordial black holes (those that have 50 times greater mass than that of the sun). However, the outcomes do not completely rule out that they could make up 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 particles called WIMPs (Weakly Interacting Massive Particles).”

These outcomes are vital to building novel theoretical models and to develop new hypotheses related to the nature of dark matter. They also offer ever more accurate indications to follow the difficult path to gain insights into one of the largest puzzles of the cosmos.


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