Particle Physics

Gravitational wave signals provide a unique insight into the dynamics of the early universe. Notably, secondary gravitational waves sourced by curvature perturbations from inflation are a generic feature. I will outline the details of this mechanism and discuss several scenarios where a significant signal may arise.

Inflaton couplings during warm inflation result in the production of a thermal bath. Thermal fluctuations can dominate the standard de Sitter analogues, resulting in a modified slow-roll scenario with a new source of density fluctuations. It is generally difficult to reliably calculate the predictions of warm inflation, and therefore assess its viability as a model of the early universe. We will present a fully calculable perturbative model of warm inflation, and discuss its main features. We will see that the thermal bath develops a chemical potential. The chemical potential has a rather large impact, and in extreme cases the chemical potential can stop warm inflation from happening all together.

I discuss particle physics models for dark matter that can leave a non-trivial imprint in the dark matter density perturbations and serve as targets of precise cosmological measurements. Scenarios include sterile neutrino dark matter enabled by novel neutrino self-interaction, thermal relic via an entropy dilution mechanism, and dark matter mass generated by a first-order phase transition in the early universe. Interplay with other probes will also be discussed when applicable.

Horizon-scale images from the Event Horizon Telescope (EHT) now reveal two stable features of M87⋆: a dark inner shadow and a bright, polarized ring. These two features provide clean and robust standing points for new-physics searches. In the shadow region, any model that adds microwave emission would partially illuminate it, so the observed darkness becomes a background-free intensity test that yields strong bounds, which we showcased with dark matter annihilation. On the ring, the differential linear polarization angle between observing days, predicted and observed to be nearly zero, cancels accretion flow systematics; its stability leads to leading limits on the axion–photon coupling in the ultralight mass range, assuming a superradiant origin. I will also highlight recent progress on controlling observational uncertainties, strengthening which parts of these conclusions can be regarded as robust.

Gravitational waves (GW) offer a unique window into primordial density perturbations that may differ significantly from the adiabatic fluctuations observed in the CMB and large-scale structure. This raises an exciting prospect of seeing fluctuations of a completely new field from inflation in the anisotropy patterns of GW maps. However, CMB constraints impose an apparent tradeoff between distinctive anisotropy and sizable isotropic signal of such a GW background. I will show that rotating axion-like fields can naturally circumvent this limitation, producing both a strong GW background and large anisotropies within it. I will present the key characteristics of this signal and explore its detection prospects with future GW experiments.

The past decade has seen rapid advancement in electron excitation-based direct detection experiments, which target light, MeV-GeV mass dark matter. Recently, the DAMIC-M collaboration began to probe the benchmark light dark matter models, renewing questions about the broader range of dark matter models these experiments can detect, and the necessity of new experiments to search beyond. In this talk we'll discuss a novel mechanism to cosmologically produce light, keV-GeV mass, dark matter coupled to electrons via a magnetic dipole moment. We consider a minimally extended dark sector in which only heavier states are charged under the kinetically mixed dark photon. These states are produced via freeze-in and subsequently thermalize the rest of the dark sector, efficiently populating the lighter, neutral, magnetic dipole-coupled dark matter. Finally, we'll discuss how magnons, collective excitations of spin waves, can be used to search for keV-MeV mass magnetic dipole-coupled dark matter, outside the reach of any semiconductor based direct detection technology.