Heavy Dark Matter, Models and Methods of Detection beyond the Standard Paradigm

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Weakly Interacting Massive Particles (WIMPs) around the TeV scale have since long been among the best-motivated and most studied Dark Matter (DM) candidates. However, the absence of experimental evidence for such particles either in colliders, at telescopes or in underground laboratories, has stimulated model-building and studies of new methods of detection beyond the WIMP paradigm. This thesis performs several steps in this direction.We are in particular interested in the case where DM is much heavier than a TeV. A well-known obstacle for such a realization is the unitarity bound on the annihilation cross-section which constrains the mass of thermal DM to be smaller than ∼100 TeV. However, the unitarity bound can be evaded in presence of entropy injection which dilutes the DM abundance. In this thesis, we investigate two possible sources of entropy injection.First, we study the entropy injection following reheating after an early matter era, when a heavy spectator field, which we choose to be the DM mediator, dominating the energy density of the universe, decays into radiation. We study in detail the corresponding constraints from indirect detection, Cosmic Microwave Background (CMB) and 21-cm, and we show that experimentalists have interests to extend the thermal DM constraints to DM masses beyond the 10/100 TeV range.Second, we study the entropy injection following reheating after an early stage of vacuum domination generated by a supercooled confining first-order phase transition. Considering the well-motivated scenario where DM is a composite state of a new confining force, we found that a variety of new effects, e.g. string fragmentation and deep-inelastic- scattering in the early universe, play an important role for setting the final DM abundance. In both cases, we show that we can increase the DM mass up to the EeV scale, 4 orders of magnitude larger than the unitarity bound.Such scenarios involve non standard cosmologies (either matter era or inflationary era inside the radiation era) and we show that these can be probed using the would-be imprints on the Gravitational-Waves (GW) spectrum from Cosmic Strings (CS) if observed with future GW detectors. In this thesis, we study in detail the computation of the GW spectrum from CS in the presence of non-standard cosmology and the associated constraints on DM models responsible for such a change of cosmology.