Searching For Stochastic Gravitational Waves Below a Nanohertz

The stochastic gravitational-wave background is imprinted on the times of arrival of radio pulses from millisecond pulsars. Traditional pulsar timing analyses fit a timing model to each pulsar and search the residuals of the fit for a stationary time correlation. This method breaks down at gravitational-wave frequencies below the inverse observation time of the array; therefore, existing analyses restrict their searches to frequencies above 1 nHz. An effective method to overcome this challenge is to study the correlation of secular drifts of parameters in the pulsar timing model itself. In this paper, we show that timing model correlations are sensitive to sub-nanohertz stochastic gravitational waves and perform a search using existing measurements of binary spin-down rates and pulsar spin-decelerations. We do not observe a signal at our present sensitivity, constraining the stochastic gravitational-wave relic energy density to $\Omega_\text{GW} ( f ) < 3.9 \times 10 ^{ - 9}$ at 450 pHz with sensitivity which scales as the frequency squared until approximately 10 pHz. We place additional limits on the amplitude of a power-law spectrum of $A_\star \lesssim 8\times10^{-15}$ for the spectral index expected from supermassive black hole binaries, $\gamma = 13/3$. If a detection of a supermassive black hole binary signal above 1 nHz is confirmed, this search method will serve as a critical complementary probe of the dynamics of galaxy evolution.Comment: 13 pages, 2 figure

Pulsar Timing Probes of Primordial Black Holes and Subhalos

Pulsars act as accurate clocks, sensitive to gravitational redshift and acceleration induced by transiting clumps of matter. We study the sensitivity of pulsar timing arrays (PTAs) to single transiting compact objects, focusing on primordial black holes and compact subhalos in the mass range from $10^{-12} M _{\odot}$ to well above $100~M_\odot$. We find that the Square Kilometer Array can constrain such objects to be a subdominant component of the dark matter over this entire mass range, with sensitivity to a dark matter sub-component reaching the sub-percent level over significant parts of this range. We also find that PTAs offer an opportunity to probe substantially less dense objects than lensing because of the large effective radius over which such objects can be observed, and we quantify the subhalo concentration parameters which can be constrained.Comment: 18 pages, 6 figure

Cosmological Tension of Ultralight Axion Dark Matter and its Solutions

A number of proposed and ongoing experiments search for axion dark matter with a mass nearing the limit set by small scale structure (${\cal O} ( 10 ^{ - 21 } {\rm eV} )$). We consider the late universe cosmology of these models, showing that requiring the axion to have a matter-power spectrum that matches that of cold dark matter constrains the magnitude of the axion couplings to the visible sector. Comparing these limits to current and future experimental efforts, we find that many searches require axions with an abnormally large coupling to Standard Model fields, independently of how the axion was populated in the early universe. We survey mechanisms that can alleviate the bounds, namely, the introduction of large charges, various forms of kinetic mixing, a clockwork structure, and imposing a discrete symmetry. We provide an explicit model for each case and explore their phenomenology and viability to produce detectable ultralight axion dark matter.Comment: 11 pages, 3 figure

New Insights Into Axion-Lepton Interactions

We revisit the theory and constraints on axion-like particles (ALPs) interacting with leptons. We clarify some subtleties in the constraints on ALP parameter space and find several new opportunities for ALP detection. We identify a qualitative difference between weak-violating and weak-preserving ALPs, which dramatically change the current constraints due to possible ``energy enhancements'' in various processes. This new understanding leads to additional opportunities for ALP detection through charged meson decays (e.g., $\pi^+\to e^+ \nu a$, $K^+\to e^+ \nu a$) and $W$ boson decays. The new bounds impact both weak-preserving and weak-violating ALPs and have implications for the QCD axion and addressing experimental anomalies using ALPs.Comment: 10 pages, 2 figure