BSM PANDEMIC Delta Series: Ying Ying Li (Fermilab) and Michael Fedderke (Johns Hopkins)


Ying Ying Li:

Cosmologically Degenerate Fermions

Even in the total absence of thermal kinetic energy, fermionic dark matter must have nonzero momentum due to the Pauli degeneracy pressure. As the fermions were inevitably denser at higher redshifts, a typical fermion may gain a fermi momentum that can exceed its mass. I will talk about the impacts of the transition between nonrelativistic and relativistic behaviour, as revealed by measurements of DNeff and the matter power spectrum. Minimal fermion mass bound will be presented, for a given fraction of the dark matter energy density the fermionic dark matter is occupying. I will also remark on implications for direct detection and suggest models of dark sectors that may give rise to cosmologically degenerate fermions.


Michael Fedderke:

Asteroids as a microhertz gravitational wave detector

The science case for a broad program of gravitational wave (GW) detection across all frequency bands is exceptionally strong. At present, there is a dearth of coverage by existing and proposed searches in the GW frequency band lying between the peak sensitivities of PTAs and LISA, roughly 0.1-100 microhertz. In this talk, I will outline a conceptual mission proposal to access this band. I will demonstrate that a few carefully chosen asteroids which orbit in the inner Solar System can act as excellent naturally occurring gravitational test masses despite the environmental noise sources. As such, a GW detector can be constructed by ranging between these asteroids using optical or radio links. At low frequencies, I will discuss how gravity gradient noise arising from the combined motion of the other ~ million asteroids in the inner Solar System sharply cuts off the sensitivity of this proposal. Sensitivity in the middle of this band is mostly limited by various solar perturbations to the asteroid test masses, while the high-frequency sensitivity is limited by noise in the ranging link. The projected strain-sensitivity curve that I will present indicates significant potential reach in this frequency band for a mission of this type.