| Summary: |
The nature of dark matter is one of the most important open problems in modern cosmology and particle physics. In the past decades, both theoretical and experimental efforts in this field have focused mostly on Weakly Interacting Massive Particles (WIMPs), which have so far evaded direct detection in the laboratory. Also given the absence of any sign of new particles at the Large Hadron Collider, the scientific community's attention is gradually shifting towards more feebly interacting dark matter candidates. In this proposal, we will explore some of the "darkest" corners of the theoretical dark matter landscape, motivated by (i) the lack of empirical evidence for dark matter other than through its gravitational effects and (ii) the puzzling comparable cosmic abundances of dark and ordinary matter. We have two main goals. Firstly, we aim to show that the large dark matter densities in some astrophysical systems may compensate for the feebleness of its interactions in the scenarios that we will consider, yielding novel and unique observational signatures that take advantage of the advent of gravitational wave/multi-messenger astronomy. Secondly, we plan to relate the necessarily non-thermal dark matter production mechanisms with other open problems in particle cosmology, namely inflation and baryogenesis (which is hard, if at all possible, for thermal WIMPs), and propose new cosmological signatures to test such unified scenarios. To achieve these goals, we have elected two promising research arenas that define the two tasks of this research project: Task 1: Primordial Black Holes Black holes formed in the early Universe are natural dark matter candidates, and their properties may reveal crucial information about the primordial conditions, even if they only account for a small fraction of dark matter. Primordial black holes (PBHs) could, in fact, explain the low spins of black hole binaries recently detected with gravitational waves [1] or even ac  |
Summary
The nature of dark matter is one of the most important open problems in modern cosmology and particle physics. In the past decades, both theoretical and experimental efforts in this field have focused mostly on Weakly Interacting Massive Particles (WIMPs), which have so far evaded direct detection in the laboratory. Also given the absence of any sign of new particles at the Large Hadron Collider, the scientific community's attention is gradually shifting towards more feebly interacting dark matter candidates. In this proposal, we will explore some of the "darkest" corners of the theoretical dark matter landscape, motivated by (i) the lack of empirical evidence for dark matter other than through its gravitational effects and (ii) the puzzling comparable cosmic abundances of dark and ordinary matter. We have two main goals. Firstly, we aim to show that the large dark matter densities in some astrophysical systems may compensate for the feebleness of its interactions in the scenarios that we will consider, yielding novel and unique observational signatures that take advantage of the advent of gravitational wave/multi-messenger astronomy. Secondly, we plan to relate the necessarily non-thermal dark matter production mechanisms with other open problems in particle cosmology, namely inflation and baryogenesis (which is hard, if at all possible, for thermal WIMPs), and propose new cosmological signatures to test such unified scenarios. To achieve these goals, we have elected two promising research arenas that define the two tasks of this research project: Task 1: Primordial Black Holes Black holes formed in the early Universe are natural dark matter candidates, and their properties may reveal crucial information about the primordial conditions, even if they only account for a small fraction of dark matter. Primordial black holes (PBHs) could, in fact, explain the low spins of black hole binaries recently detected with gravitational waves [1] or even account for some unexplained microlensing events [2, 3]. We plan to (a) explore novel mechanisms for PBH production in the context of alternative inflationary models (e.g. warm inflation [4, 5] and thermal inflation [6]); (b) study their dynamical evolution (focusing on evaporation and superradiance); and (c) search for novel electromagnetic and gravitational wave signatures [7]. A largely unexplored possibility that we plan to investigate is (d) whether small black holes can act as dark matter progenitors, e.g. through Hawking evaporation [8]. We will look for potential cosmological signatures of such processes and explore whether they can explain the similarity between the present dark and ordinary matter abundances. Task 2: Axions and other cosmological scalar fields Massive scalar fields are ubiquitous in several extensions of the Standard Model of particle physics and behave naturally as cold dark matter while oscillating about their potential minimum, typically interacting feebly with known particles. The QCD axion, in particular, is amongst the most promising dark matter candidates, being predicted by the Peccei-Quinn solution to the strong CP-problem. We plan to, on the one hand, (a) develop new models that may overcome the long-standing problems of large axion isocurvature density perturbations, domain-walls and the "axion quality problem" [9,10]. On the other hand, (b) we will investigate novel astrophysical signatures of both axions and axion-like particles (ALPs), focusing on black hole superradiant instabilities (alongside Task 1), photon propagation in axion backgrounds and cosmolog |