Testing the Sensitivity of the Galactic Center Excess to the Point Source Mask.

The Fermi Large Area Telescope (Fermi-LAT) Collaboration has an updated point source catalog, referred to as 4FGL. We perform the first template fit using a mask based on this new catalog and find that the excess in gamma rays detected at the Galactic Center in Fermi-LAT data persists. On the other hand, we find that a search for point sources is highly sensitive to the use of the 4FGL catalog: no sizable excess of bright pixels is apparent in the inner Galaxy when we mask out 4FGL point sources. Combining these observations restricts the ability of point sources to contribute to the Galactic Center excess. After identifying which bright sources have no known counterpart, we place strong constraints on any point source luminosity function capable of explaining the smooth emission identified in the template fit.

Beyond the Standard Model Explanations of GW190521.

The LIGO-Virgo Collaboration has recently announced the detection of a heavy binary black hole merger, with component masses that are difficult to account for in standard stellar structure theory. In this Letter, we propose several explanations based on models of new physics, including new light particle losses, modified gravity, large extra dimensions, and a small magnetic moment of the neutrino. Each of these affect the physics of the pair instability differently, leading to novel mechanisms for forming black holes inside the mass gap.

Constraining the Self-Interacting Neutrino Interpretation of the Hubble Tension.

Large, nonstandard neutrino self-interactions have been shown to resolve the ∼4σ tension in Hubble constant measurements and a milder tension in the amplitude of matter fluctuations. We demonstrate that interactions of the necessary size imply the existence of a force carrier with a large neutrino coupling (>10^{-4}) and mass in the keV-100 MeV range. This mediator is subject to stringent cosmological and laboratory bounds, and we find that nearly all realizations of such a particle are excluded by existing data unless it carries spin 0 and couples almost exclusively to τ-flavored neutrinos. Furthermore, we find that the light neutrinos must be Majorana particles, and that a UV-complete model requires a nonminimal mechanism to simultaneously generate neutrino masses and appreciable self-interactions.

Constraining Dissipative Dark Matter Self-Interactions.

We study the gravothermal evolution of dark matter halos in the presence of dissipative dark matter self-interactions. Dissipative interactions are present in many particle-physics realizations of the dark-sector paradigm and can significantly accelerate the gravothermal collapse of halos compared to purely elastic dark matter self-interactions. This is the case even when the dissipative interaction timescale is longer than the free-fall time of the halo. Using a semianalytical fluid model calibrated with isolated and cosmological N-body simulations, we calculate the evolution of the halo properties-including its density profile and velocity dispersion profile-as well as the core-collapse time as a function of the particle model parameters that describe the interactions. A key property is that the inner density profile at late times becomes cuspy again. Using 18 dwarf galaxies that exhibit a corelike dark matter density profile, we derive constraints on the strength of the dissipative interactions and the energy loss per collision.

Can the Inflaton Also Be a Weakly Interacting Massive Particle?

We propose a class of models in which a stable inflaton is produced as a thermal relic in the early Universe and constitutes the dark matter. We show that inflaton annihilations can efficiently reheat the Universe, and identify several examples of inflationary potentials that can accommodate all cosmic microwave background observables and in which the inflaton dark matter candidate has a weak scale mass. As a simple example, we consider annihilations that take place through a Higgs portal interaction, leading to encouraging prospects for future direct detection experiments.

Severely Constraining Dark-Matter Interpretations of the 21-cm Anomaly.

The EDGES Collaboration has recently reported the detection of a stronger-than-expected absorption feature in the global 21-cm spectrum, centered at a frequency corresponding to a redshift of z≃17. This observation has been interpreted as evidence that the gas was cooled during this era as a result of scattering with dark matter. In this Letter, we explore this possibility, applying constraints from the cosmic microwave background, light element abundances, Supernova 1987A, and a variety of laboratory experiments. After taking these constraints into account, we find that the vast majority of the parameter space capable of generating the observed 21-cm signal is ruled out. The only viable models are those in which a small fraction, ∼0.3%-2%, of the dark matter consists of particles with a mass of ∼10-80 MeV and which couple to the photon through a small electric charge, roughly 10^{-6}-10^{-4} as large as the electron charge. Furthermore, in order to avoid being overproduced in the early Universe, such models must be supplemented with an additional depletion mechanism, such as annihilations through a L_{µ}-L_{τ} gauge boson or annihilations to a pair of rapidly decaying hidden sector scalars.

Is Self-Interacting Dark Matter Undergoing Dark Fusion?

We suggest that two-to-two dark matter fusion may be the relaxation process that resolves the small-scale structure problems of the cold collisionless dark matter paradigm. In order for the fusion cross section to scale correctly across many decades of astrophysical masses from dwarf galaxies to galaxy clusters, we require the fractional binding energy released to be greater than v^{n}∼(10^{-(2-3)})^{n}, where n=1, 2 depends on local dark sector chemistry. The size of the dark-sector interaction cross sections must be σ∼0.1-1 barn, moderately larger than for standard model deuteron fusion, indicating a dark nuclear scale Λ∼O(100 MeV). Dark fusion firmly predicts constant σv below the characteristic velocities of galaxy clusters. Observations of the inner structure of galaxy groups with velocity dispersion of several hundred kilometers per second, of which a handful have been identified, could differentiate dark fusion from a dark photon model.

Inflatable Dark Matter.

We describe a general scenario, dubbed "inflatable dark matter," in which the density of dark matter particles can be reduced through a short period of late-time inflation in the early Universe. The overproduction of dark matter that is predicted within many, otherwise, well-motivated models of new physics can be elegantly remedied within this context. Thermal relics that would, otherwise, be disfavored can easily be accommodated within this class of scenarios, including dark matter candidates that are very heavy or very light. Furthermore, the nonthermal abundance of grand unified theory or Planck scale axions can be brought to acceptable levels without invoking anthropic tuning of initial conditions. A period of late-time inflation could have occurred over a wide range of scales from ∼MeV to the weak scale or above, and could have been triggered by physics within a hidden sector, with small but not necessarily negligible couplings to the standard model.