Efficient Computation of Overlap Reduction Functions for Pulsar Timing Arrays.

Pulsar timing arrays seek and study gravitational waves (GWs) through the angular two-point correlation function of timing residuals they induce in pulsars. The two-point correlation function induced by the standard transverse-traceless GWs is the famous Hellings-Downs curve, a function only of the angle between the two pulsars. Additional polarization modes (vector or scalar) that may arise in alternative-gravity theories have different angular correlation functions.

Insights from HST into Ultramassive Galaxies and Early-Universe Cosmology.

The early-science observations made by the James Webb Space Telescope (JWST) have revealed an excess of ultramassive galaxy candidates that appear to challenge the standard cosmological model (ΛCDM). Here, we argue that any modifications to ΛCDM that can produce such ultramassive galaxies in the early Universe would also affect the UV galaxy luminosity function (UV LF) inferred from the Hubble Space Telescope (HST). The UV LF covers the same redshifts (z≈7-10) and host-halo masses (M_{h}≈10^{10}-10^{12}M_{⊙}) as the JWST candidates, but tracks star-formation rate rather than stellar mass.

Cosmic Optical Background Excess, Dark Matter, and Line-Intensity Mapping.

Recent studies using New Horizons's Long Range Reconnaisance Imager (LORRI) images have returned the most precise measurement of the cosmic optical background to date, yielding a flux that exceeds that expected from deep galaxy counts by roughly a factor of 2. We investigate whether this excess, detected at ∼4σ significance, is due to axionlike dark matter that decays to monoenergetic photons. We compute the spectral energy distribution from such decays and the contribution to the flux measured by LORRI.

Searching for the Radiative Decay of the Cosmic Neutrino Background with Line-Intensity Mapping.

We study the possibility to use line-intensity mapping (LIM) to seek photons from the radiative decay of neutrinos in the cosmic neutrino background. The Standard Model prediction for the rate for these decays is extremely small, but it can be enhanced if new physics increases the neutrino electromagnetic moments. The decay photons will appear as an interloper of astrophysical spectral lines.

Chiral Photons from Chiral Gravitational Waves.

We show that a parity-breaking uniform (averaged over all directions on the sky) circular polarization of amplitude V_{00}≃2.6×10^{-17}Δχ(r/0.06) can be induced by a chiral gravitational-wave (GW) background with a tensor-to-scalar ratio r and chirality parameter Δχ (which is ±1 for a maximally chiral background).

Early Dark Energy can Resolve the Hubble Tension.

Early dark energy (EDE) that behaves like a cosmological constant at early times (redshifts z≳3000) and then dilutes away like radiation or faster at later times can solve the Hubble tension. In these models, the sound horizon at decoupling is reduced resulting in a larger value of the Hubble parameter H_{0} inferred from the cosmic microwave background (CMB). We consider two physical models for this EDE, one involving an oscillating scalar field and another a slowly rolling field.

Lensing of Fast Radio Bursts as a Probe of Compact Dark Matter.

The possibility that part of the dark matter is made of massive compact halo objects (MACHOs) remains poorly constrained over a wide range of masses, and especially in the 20-100  M_{⊙} window. We show that strong gravitational lensing of extragalactic fast radio bursts (FRBs) by MACHOs of masses larger than ∼20  M_{⊙} would result in repeated FRBs with an observable time delay. Strong lensing of a FRB by a lens of mass M_{L} induces two images, separated by a typical time delay ∼few×(M_{L}/30  M_{⊙})  msec.

Did LIGO Detect Dark Matter?

We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 20M_{⊙}≲M_{bh}≲100M_{⊙} where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they radiate enough energy in gravitational waves to become gravitationally bound.

Statistical diagnostics to identify galactic foregrounds in B-mode maps.

Recent developments in the search for inflationary gravitational waves in the cosmic microwave background polarization motivate the search for new diagnostics to distinguish the Galactic foreground contribution to B modes from the cosmic signal. We show that B modes from these foregrounds should exhibit a local hexadecapolar departure in power from statistical isotropy (SI). We present a simple algorithm to search for a uniform SI violation of this sort, as may arise in a sufficiently small patch of sky.

Reheating constraints to inflationary models.

Evidence from the BICEP2 experiment for a significant gravitational-wave background has focused attention on inflaton potentials V(ϕ)∝ϕ(α) with α = 2 ("chaotic" or "m(2)ϕ(2)" inflation) or with smaller values of α, as may arise in axion-monodromy models. Here we show that reheating considerations may provide additional constraints to these models. The reheating phase preceding the radiation era is modeled by an effective equation-of-state parameter w(re).

21-cm lensing and the cold spot in the cosmic microwave background.

An extremely large void and a cosmic texture are two possible explanations for the cold spot seen in the cosmic microwave background. We investigate how well these two hypotheses can be tested with weak lensing of 21-cm fluctuations from the epoch of reionization measured with the Square Kilometer Array. While the void explanation for the cold spot can be tested with Square Kilometer Array, given enough observation time, the texture scenario requires significantly prolonged observations, at the highest frequencies that correspond to the epoch of reionization, over the field of view containing the cold spot.

Clustering fossils from the early universe.

Many inflationary theories introduce new scalar, vector, or tensor degrees of freedom that may then affect the generation of primordial density perturbations. Here we show how to search a galaxy (or 21-cm) survey for the imprint of primordial scalar, vector, and tensor fields. These new fields induce local departures to an otherwise statistically isotropic two-point correlation function, or equivalently, nontrivial four-point correlation functions (or trispectra, in Fourier space), that can be decomposed into scalar, vector, and tensor components.

Lensing of 21-cm fluctuations by primordial gravitational waves.

Weak-gravitational-lensing distortions to the intensity pattern of 21-cm radiation from the dark ages can be decomposed geometrically into curl and curl-free components. Lensing by primordial gravitational waves induces a curl component, while the contribution from lensing by density fluctuations is strongly suppressed. Angular fluctuations in the 21-cm background extend to very small angular scales, and measurements at different frequencies probe different shells in redshift space.

Do baryons trace dark matter in the early universe?

Baryon-density perturbations of large amplitude may exist if they are compensated by dark-matter perturbations such that the total density is unchanged. Primordial abundances and galaxy clusters allow these compensated isocurvature perturbations (CIPs) to have amplitudes as large as ~10%. CIPs will modulate the power spectrum of cosmic microwave background (CMB) fluctuations--those due to the usual adiabatic perturbations--as a function of position on the sky.

Early annihilation and diffuse backgrounds in models of weakly interacting massive particles in which the cross section for pair annihilation is enhanced by 1/upsilon.

Recent studies have considered modifications to the standard weakly interacting massive particle scenario in which the pair annihilation cross section (times relative velocity v) is enhanced by a factor 1/upsilon to approximately 10(-3) in the Galaxy, enough to explain several puzzling Galactic radiation signals. We show that in these scenarios a burst of weakly interacting massive particle annihilation occurs in the first collapsed dark-matter halos. We show that severe constraints to the annihilation cross section derive from measurements of the diffuse extragalactic radiation and from ionization and heating of the intergalactic medium.

Nonlinear evolution of anisotropic cosmological power.

There has been growing interest in the possibility of testing more precisely the assumption of statistical isotropy of primordial density perturbations. If it is to be tested with galaxy surveys at distance scales < or = 10 Mpc, then nonlinear evolution of anisotropic power must be understood. To this end, we calculate the angular dependence of the power spectrum to third order in perturbation theory for a primordial power spectrum with a quadrupole dependence on the wave vector direction.