Explore the vast range of titles available.

Pulsar timing array (PTA) collaborations in North America, Australia, and Europe, have been exploiting the exquisite timing precision of millisecond pulsars over decades of observations to search for correlated timing deviations induced by gravitational waves (GWs). PTAs are sensitive to the frequency band ranging just below 1 nanohertz to a few tens of microhertz. The discovery space of this band is potentially rich with populations of inspiraling supermassive black hole binaries, decaying cosmic string networks, relic post-inflation GWs, and even non-GW imprints of axionic dark matter. This article aims to provide an understanding of the exciting open science questions in cosmology, galaxy evolution, and fundamental physics that will be addressed by the detection and study of GWs through PTAs. The focus of the article is on providing an understanding of the mechanisms by which PTAs can address specific questions in these fields, and to outline some of the subtleties and difficulties in each case. The material included is weighted most heavily toward the questions which we expect will be answered in the near-term with PTAs; however, we have made efforts to include most currently anticipated applications of nanohertz GWs.

Quantum fluctuations of the gravitational field in the early Universe, amplified by inflation, produce a primordial gravitational-wave background across a broad frequency band. We derive constraints on the spectrum of this gravitational radiation, and hence on theories of the early Universe, by combining experiments that cover 29 orders of magnitude in frequency. These include Planck observations of cosmic microwave background temperature and polarization power spectra and lensing, together with baryon acoustic oscillations and big bang nucleosynthesis measurements, as well as new pulsar timing array and ground-based interferometer limits. While individual experiments constrain the gravitational-wave energy density in specific frequency bands, the combination of experiments allows us to constrain cosmological parameters, including the inflationary spectral index nt and the tensor-to-scalar ratio r . Results from individual experiments include the most stringent nanohertz limit of the primordial background to date from the Parkes Pulsar Timing Array, ΩGW(f)<2.3×10−10 . Observations of the cosmic microwave background alone limit the gravitational-wave spectral index at 95% confidence to nt≲5 for a tensor-to-scalar ratio of r=0.11 . However, the combination of all the above experiments limits nt<0.36 . Future Advanced LIGO observations are expected to further constrain nt<0.34 by 2020. When cosmic microwave background experiments detect a nonzero r , our results will imply even more stringent constraints on nt and, hence, theories of the early Universe.

Axion-like scalar fields can induce temporary deviations from the standard expansion history of the universe. The scalar field's contribution to the energy density of the universe grows while the field is held constant by Hubble friction, but when the scalar field starts to evolve, its energy density decreases faster than the radiation density for some potentials. We explore the observational signatures of such a scalar field that becomes dynamical between big bang nucleosynthesis and matter-radiation equality, which we call very Early Dark Energy (vEDE). If vEDE momentarily dominates the energy density of the universe, it generates a distinctive feature in the matter power spectrum that includes a bump on scales that enter the horizon just after the scalar field starts to evolve. For \\(k 10\\,h\\,Mpc^-1\\), the amplitude of this bump can exceed the amplitude of the standard matter spectrum. The power on scales on either side of this peak is suppressed relative to the standard power spectrum, but only scales that are within the horizon while the scalar field makes a significant contribution to the total energy density are affected. We determine how vEDE scenarios are constrained by observations of the cosmic microwave background, measurements of the primordial deuterium abundance, and probes of the late-time expansion history. We find that these observations are consistent with vEDE scenarios that enhance power on scales \\(k 30\\,h\\,Mpc^-1\\) and nearly double the amplitude of the matter power spectrum around \\(200\\,h\\,Mpc^-1\\). These scenarios also suppress power on scales between \\(0.3\\,h\\,Mpc^-1\\) and \\(30\\,h\\,Mpc^-1\\).

Ever since the Planck satellite measured the the cosmic microwave background (CMB) down to arcminute angular scales, the mismatch between the CMB-inferred value of the Hubble constant and the value inferred from the distance ladder (i.e., the Hubble tension) has been a growing concern and is currently at the \\( 6 \\) level. There are a handful of proposed mechanisms operating in the early universe which have shown some promise in resolving the Hubble tension. These mechanisms are expected to leave a measurable impact on the smallest scale CMB anisotropy, deep in the damping tail. Using current CMB data, baryonic acoustic oscillation data, and the luminosities of Type Ia supernovae as a baseline, we compute the predicted small-scale CMB power spectra for a characteristic set of these models. We find that near-future CMB data should be able to distinguish some but not all of the investigated models from the core cosmological model, \\(\\)CDM.

We derive constraints on the injection of free-streaming dark radiation after big bang nucleosynthesis (BBN) by considering the decay of a massive hidden sector particle into dark radiation. Such a scenario has the potential to alleviate the Hubble tension by introducing a new energy component to the evolution of the early universe. We employ observations of the cosmic microwave background (CMB) from \\(Planck\\) 2018 and the South Pole Telescope (SPT-3G), measurements of the primordial deuterium abundance, Pantheon+ Type Ia supernovae data, and baryon acoustic oscillation (BAO) measurements from BOSS DR12 to constrain these decay scenarios. Pre-recombination decays are primarily restricted by observations of the CMB via their impact on the effective number of relativistic species. On the other hand, long-lived decay scenarios in which the massive particle lifetime extends past recombination tend to decrease the late-time matter density inferred from the CMB and are thus subject to constraints from Pantheon+ and BAO. We find that, when marginalizing over lifetimes of \\(_Y = [10^-12.08, 10^-1.49]\\) Gyr, the decaying particle is limited at \\(2\\) to only contribute a maximum of \\(3\\%\\) of the energy density of the universe. With limits on these decays being so stringent, neither short-lived nor long-lived scenarios are successful at substantially mitigating the Hubble tension.

In this paper we fit two models of Early Dark Energy (EDE) (an increase in the expansion rate before recombination) to the combination of Atacama Cosmology Telescope (ACT) measurements of the Cosmic Microwave Background (CMB) with data from either the WMAP or the Planck satellite, along with measurements of the baryon acoustic oscillations and uncalibrated supernovae luminosity distance. We study a phenomenological axion-like potential ('axEDE') and a scalar field experiencing a first-order phase-transition ('NEDE'). We find that for both models the 'Planck-free' analysis yields non-zero EDE at > 2 sigma and an increased value for \\(H_0 70-74\\) km/s/Mpc, compatible with local measurements, without the inclusion of any prior on \\(H_0\\). On the other hand, the inclusion of Planck data restricts the EDE contribution to an upper-limit only at 95% C.L. For axEDE, the combination of Planck and ACT leads to constraints 30% weaker than with Planck alone, and there is no residual Hubble tension. On the other hand, NEDE is more strongly constrained in a Planck+ACT analysis, and the Hubble tension remains at \\( 3\\), illustrating the ability for CMB data to distinguish between EDE models. We explore the apparent inconsistency between the Planck and ACT data and find that it comes (mostly) from a slight tension between the temperature power spectrum at multipoles around \\( 1000\\) and \\( 1500\\). Finally, through a mock analysis of ACT data, we demonstrate that the preference for EDE is not driven by a lack of information at high-\\(\\) when removing Planck data, and that a LCDM fit to the fiducial EDE cosmology results in a significant bias on \\(\\H_0,_ cdm\\\). More accurate measurements of the TT power spectra above \\( 2500\\) and EE between \\( 300-500\\) will play a crucial role in differentiating EDE models.

We test the assumption of entropy conservation between Big Bang nucleosynthesis and recombination by considering a massive particle that decays into a mixture of photons and other relativistic species. We employ Planck temperature and polarization anisotropies, COBE/FIRAS spectral distortion bounds, and the observed primordial deuterium abundance to constrain these decay scenarios. If between \\(56\\%\\) and \\(71\\%\\) of the decaying particle's energy is transferred to photons, then \\(N_eff\\) at recombination is minimally altered, and Planck data alone allows for significant entropy injection. If photons are injected by the decay, the addition of spectral distortion bounds restricts the decay rate of the particle to be \\(_Y > 1.9110^-6 s^-1\\) at \\(95\\%\\) confidence level (C.L.). We find that constraints on the energy density of the decaying particle are significantly enhanced by the inclusion of bounds on the primordial deuterium abundance, allowing the particle to contribute at most \\(2.35\\%\\) (\\(95\\%\\) C.L.) of the energy density of the universe before decaying.

We present the steps to forecast the sensitivity of the Laser Interferometer Space Antenna (LISA) to both a stochastic gravitational wave background and deterministic wave sources. We show how to use these expressions to estimate the precision with which LISA can determine parameters associated with these sources. Tools are included to enable easy calculation of the signal-to-noise ratio and draw sensitivity curves. Benchmark values are given for easy comparison and checking of methods in the case of two worked examples. The first benchmark is the threshold stochastic gravitational wave background \\(_GW h^2\\) that LISA can observe. The second is the signal-to-noise ratio that LISA would observe for a binary black hole system identical to GW150914, radiating 4 years before merger.

We investigate the use of harmonic analysis techniques to perform measurements of the angular power spectrum on mock pulsar timing data for an isotropic stochastic gravitational-wave background (SGWB) with a dimensionless strain amplitude \\(A_gw=2 10^-15\\) and spectral index \\(_gw=13/3\\). We examine the sensitivity of our harmonic analysis to the number of pulsars (50, 100, and 150) and length of pulsar observation time (10, 20, and 30 years) for an isotropic distribution of pulsars. We account for intrinsic pulsar red noise and use an average value of white noise of ~100 ns. We are able to detect the quadrupole for all our mock harmonic analyses, and for the analysis with 150 pulsars observed for 30 years, we are able to detect up to the \\( = 5\\) multipole. We provide scaling laws for the SGWB amplitude, the quadrupole, and \\( = 3\\) as a function of pulsar observation time and as a function of number of pulsars. We estimate the sensitivity of our harmonic approach to deviations of general relativity that produce subluminal gravitational wave propagation speeds.

The SPT-3G 2018 TT/TE/EE cosmic microwave background (CMB) data set (temperature and polarization) is used to place constraints on an axion-like model of early dark energy (EDE). These data do not favor axion-like EDE and place an upper limit on the maximum fraction of the total energy density \\(f_ EDE< 0.172\\) (at the 95% confidence level, CL). This is in contrast with ACT DR4 which gives \\(f_ EDE=0.150^+0.050_-0.078\\). When combining CMB measurements with measurements of the baryon acoustic oscillations and luminosity distance to Type Ia supernovae, we show that the tension with the S\\(H_0\\)ES measurement of the Hubble parameter goes up from 2.6\\(\\) with Planck to 2.9\\(\\) with Planck+SPT-3G 2018. The additional inclusion of ACT DR4 data leads to a reduction of the tension to \\(1.6\\), but the discrepancy between ACT DR4 and Planck+SPT-3G 2018 casts some doubt on the statistical consistency of this joint analysis. The importance of improved measurements of the CMB at both intermediate and small scales (in particular the shape of the damping tail) as well as the interplay between temperature and polarization measurements in constraining EDE are discussed. Upcoming ground-based measurements of the CMB will play a crucial role in determining whether EDE remains a viable model to address the Hubble tension.