# Sources of cosmological origin

AbstractNo Abstract Found

# The primordial soup as a source of energy

The investigation of the fundamental laws of the universe, in particular the physics of the microcosm, always requires the use of an energy reservoir. Contemporary researches in particle physics make use of powerful particle accelerators; this approach led to great successes with the building of the Standard Model of particle physics. The latest achievement in that field is the discovery of the Higgs boson that witnesses the breaking of the electroweak symmetry and most likely describes how elementary particles acquire inertial mass. As we shall see, the Standard Model, although it perfectly reproduces hundreds of high-precision measurements, suffers from internal inconsistencies and lacks a microscopic description of phenomena observed on very large scales: dark matter, dark energy, inflation. Another flaw of the model is its inability to offer a quantum description of gravitation. Challenging the Standard Model, with the idea of discovering what lies beyond it, is a task that requires either very high-precision measurements or to have access to an even more powerful energy reservoir, or both.

A possible alternative to particle colliders is to use natural environments to conduct particle physics experiments. Among other possibilities one can to use of stars or neutron stars to search for axions –those light particles that could explain the conservation of CP symmetry in strong interactions–, or the environment of supermassive black holes. The later are used in gamma-ray observations of blazars to search for axions or breaking of Lorentz invariance (see the *article/chapter/part* of Dieter and Agnieszka). In the present *article/chapter/part*, we review some investigations that make use of the tremendous energy and density that the first phases of the big bang can offer. The study of the early universe’s thermal history has shown great successes. For instance at energies of the order of atomic bindings, the description of the physical phenomena that took place then allows a very precise description of the recombination era that is used as a tool for cosmology with the success that we know. At higher energies, the knowledge of the temperature and density evolution permits to compute quite accurately the rate of the nucleosynthesis of He and Li. Here we go back in time event further and consider a hypothetical dark matter particle that would have been in thermal equilibrium at some point in the early universe. This scenario is very well motivated by both cosmological measurements and particle physics models, and it triggered a lot of experimental researches. Other interesting phenomena could have happen in the very first phases of the evolution of the universe are relevant to gamma-ray astronomy. For example very little is known about the far end of the fluctuation spectrum of the matter, at very small scales. It could be that small black holes have been created then. The evolution of those would be a very slow mass loss though the Hawking-Bekenstein evaporation mechanism ending in a explosive phase leading to the emission of bursts of gamma rays. In some initial-mass range, the final explosion could happen to end up in the local universe, making these photons observable with gamma-ray telescopes.

In this article we focus mainly on the role that gamma-ray observation play in the search for new phenomena related to the physics of the early universe. It is organized as follows; the first parts deal with primordial black holes, where the status of primordial black hole searches with gamma rays is reviewed. Then different sections are devoted to particle dark matter searches, in Sec. \ref{relic} a description of the establishment of the relic density is presented, then some examples of particle physics models with dark matter candidates are given. In Sec. \ref{struct} we give some details about the dark matter distribution and density in the objects that will be targeted by our experiments. In Sec. \ref{targets} we review the searches towards known targets with both satellite-borne telescopes and ground telescopes, and Sec. \ref{other} presents blind searches and searches in the diffuse emissions. Sec. \ref{future} and \ref{complementarity} give a short update on future searches and other ways to search for particle dark matter. This article aims at providing a pedagogical introduction to the field, relevant references are given for the reader who would like to go into further details.

# Primordial black holes

\label{pbh}

A primordial black hole (PBH) is a type of black hole that is not formed by the gravitational collapse of a star, but by the extreme density of matter present during the Universe’s early expansion, so that their initial mass scales as $$M(t)\approx 10^{15}(t/10^{-23}\mathrm{s})\mathrm{g}$$ with their creation time $$t$$ after the Big Bang. Due to Hawking radiation, PBHs evaporate in approximately $$\tau\approx 400(M/10^{10}\mathrm{g})^3\mathrm{s}$$, so that PBHs in the last stages of their lifetime at the current epoch have been created at a time close to $$10^{-23}\mathrm{s}$$ after the Big Bang, and thus with an initial mass of order $$10^{15}\mathrm{g}$$. The corresponding black body temperature of the PBH reaches $$T\approx20\mathrm{MeV}$$, but more importantly the integrated spectral energy distribution at the current epoch peaks at 100 MeV (MacGibbon et al., 1991), and occurs in a final burst of order 0.1 second. In the context of the present review, this points to the fact that PBHs could shine in gamma rays, either during the last stages of their evaporation at the current epoch, or via the cumulative gamma-ray emissivity along the universe’s history. Furthermore, PBHs constitute a class of baryonic Dark Matter candidates that may constitute a significant fraction of Dark Matter in the Galaxy.

## Constraints from gamma-ray diffuse emission

As the photon spectrum of evaporating black holes peaks at about 100 MeV, the cumulative emissivity of PBHs, up to the mass of the PBHs that would complete their evaporation at the present epoch, sums