arXiv subjects

Sarah R. Geller

Publications and source records attributed to Sarah R. Geller.

6 recordsLinked to original sources

arXiv2025

Primordial Black Holes from Inflation with a Spectator Field

How is the production of primordial black holes (PBHs) in single-field models of inflation impacted by the presence of additional scalar fields? We consider the effect of a spectator field - a free scalar field with sub-Hubble mass, no direct coupling to the inflaton, and which makes a subdominant contribution to the total energy density - in the context of single-field models of inflation featuring a transient phase of ultra-slow roll (USR) evolution. Despite the modest title, a spectator field can have a dramatic impact: the slow-roll evolution of the spectator prevents the combined inflaton-and-spectator system from entering into USR, which naively might be expected to preclude the production of PBHs. However, we demonstrate that the growth of perturbations is maintained or enhanced by the spectator, through the rich interplay of curvature and isocurvature perturbations. We show in a model-independent way that the single-field phase of ultra-slow-roll is replaced by two turns in field space encompassing a phase of tachyonic instability for the isocurvature perturbations and a transfer of power from isocurvature to curvature modes. Furthermore, we highlight a degeneracy between the fine-tuning of the feature in the inflaton potential and the parameters of the spectator, leading to an overall resilience of model predictions to parameter variations. This makes it easier for the underlying PBH model to accommodate both high-precision CMB constraints and production of PBHs in the asteroid-mass range.

astro-ph.CO↗

arXiv2025

Light Scalar Fields Foster Production of Primordial Black Holes

Scalar fields are ubiquitous in theories of high-energy physics. In the context of cosmic inflation, this suggests the existence of spectator fields, which provide a subdominant source of energy density. We show that spectator fields boost the inflationary production of primordial black holes, with single-field ultra-slow roll evolution supplanted by a phase of evolution along the spectator direction, and primordial perturbations amplified by the resulting multifield dynamics. This generic mechanism is largely free from the severe fine-tuning that afflicts single-field inflationary PBH models.

astro-ph.CO↗

arXiv2024

Close encounters of the primordial kind: a new observable for primordial black holes as dark matter

Primordial black holes (PBHs) remain a viable dark matter candidate in the asteroid-mass range. We point out that in this scenario, the PBH abundance would be large enough for at least one object to cross through the inner Solar System per decade. Since Solar System ephemerides are modeled and measured to extremely high precision, such close encounters could produce detectable perturbations to orbital trajectories with characteristic features. We evaluate this possibility with a suite of simple Solar System simulations, and we argue that the abundance of asteroid-mass PBHs can plausibly be probed by existing and near-future data.

astro-ph.CO↗

arXiv2023

Planck Constraints and Gravitational Wave Forecasts for Primordial Black Hole Dark Matter Seeded by Multifield Inflation

We perform a Markov Chain Monte Carlo (MCMC) analysis of a simple yet generic multifield inflation model characterized by two scalar fields coupled to each other and nonminimally coupled to gravity, fit to Planck 2018 cosmic microwave background (CMB) data. In particular, model parameters are constrained by data on the amplitude of the primordial power spectrum of scalar curvature perturbations on CMB scales $A_s$, the spectral index $n_s$, and the ratio of power in tensor to scalar modes $r$, with a prior that the primordial power spectrum should also lead to primordial black hole (PBH) production sufficient to account for the observed dark matter (DM) abundance. We find that $n_s$ in particular controls the constraints on our model. Whereas previous studies of PBH formation from an ultra-slow-roll phase of inflation have highlighted the need for at least one model parameter to be highly fine-tuned, we identify a degeneracy direction in parameter space such that shifts by $\sim 10\%$ of one parameter can be compensated by comparable shifts in other parameters while preserving a close fit between model predictions and observations. Furthermore, we find this allowed parameter region produces observable gravitational wave (GW) signals in the frequency ranges to which upcoming experiments are projected to be sensitive, including Advanced LIGO and Virgo, the Einstein Telescope (ET), Cosmic Explorer (CE), DECIGO, and LISA.

astro-ph.CO↗

arXiv2022

Primordial Black Holes from Multifield Inflation with Nonminimal Couplings

Primordial black holes (PBHs) provide an exciting prospect for accounting for dark matter. In this paper, we consider inflationary models that incorporate realistic features from high-energy physics -- including multiple interacting scalar fields and nonminimal couplings to the spacetime Ricci scalar -- that could produce PBHs with masses in the range required to address the present-day dark matter abundance. Such models are consistent with supersymmetric constructions, and only incorporate operators in the effective action that would be expected from generic effective field theory considerations. The models feature potentials with smooth large-field plateaus together with small-field features that can induce a brief phase of ultra-slow-roll evolution. Inflationary dynamics within this family of models yield predictions for observables in close agreement with recent measurements, such as the spectral index of primordial curvature perturbations and the ratio of power spectra for tensor to scalar perturbations. As in previous studies of PBH formation resulting from a period of ultra-slow-roll inflation, we find that at least one dimensionless parameter must be highly fine-tuned to produce PBHs in the relevant mass-range for dark matter. Nonetheless, we find that the models described here yield accurate predictions for a significant number of observable quantities using a smaller number of relevant free parameters.

hep-th↗

arXiv2018

Mass of a Patch of an FRW Universe

In 1963, Zel'dovich devised a method to define the mass of a closed Friedmann-Robertson-Walker (FRW) universe, showing that by this definition it is exactly zero. Rounding out this result, we show that the masses of flat and open universes are (unsurprisingly) divergent. We also present closed-form solutions for the trajectory of the boundary of a finite spherical patch of homogeneous pressureless dust for each class of curvature, exploring the dynamics of the boundary in detail. In all cases, the FRW patch emerges from a white hole. In the closed case, the patch expands to a maximum radius before contracting and entering a black hole, while flat and open FRW patches expand without bound. We compare our results to the classical expectations of Newtonian cosmology, showing that for small radii the Newtonian energy gives the leading correction to the rest mass energy.

gr-qc↗