Recent observations of gravitational waves from binary black holes and neutron stars allow us to probe the strong and dynamical field regime of gravity. On the other hand, a collective signal from many individual, unresolved sources results in what is known as a stochastic background. We here consider probing gravity with such a background from stellar-mass binary black hole mergers. We adopt a simple power-law spectrum and carry out a parameter estimation study with a network of current and future ground-based detectors by including both general relativistic and beyond general relativistic variables. For a network of second-generation detectors, we find that one can place meaningful bounds on the deviation parameter in the gravitational-wave amplitude if it enters at a sufficiently negative post-Newtonian order. However, such future bounds from a stochastic background are weaker than existing bounds from individual sources, such as GW150914 and GW151226. We also find that systematic errors due to mismodeling of the spectrum are much smaller than statistical errors, which justifies our use of the power-law model. Regarding a network of third-generation detectors, we find that the bounds on the deviation parameter from statistical errors improve upon the second-generation case, though systematic errors now dominate the error budget and thus one needs to use a more realistic gravitational-wave spectrum model. We conclude that the observation of individual sources seems to be a more powerful way to probe the amplitude of gravitational-wave emission as predicted by general relativity.

We investigate the transformation of initial conditions for primordial curvature perturbations under two types of transformations of the associated action: simultaneous redefinition of time and the field to be quantized, and the addition of surface terms. The latter encompasses all canonical transformations, whilst the time and field redefinition is a distinct, noncanonical transformation since the initial and destination systems use different times. Actions related to each other via such transformations yield identical equations of motion and preserve the commutator structure. They further preserve the time evolution of expectation values of quantum operators unless the vacuum state also changes under the transformation. These properties suggest that it is of interest to investigate vacuum prescriptions that also remain unchanged under canonical transformations. We find that initial conditions derived via minimizing the vacuum expectation value of the Hamiltonian and those obtained using the Danielsson vacuum prescription are not invariant under these transformations, whereas those obtained by minimizing the local energy density are invariant. We derive the range of physically distinct initial conditions obtainable by Hamiltonian diagonalization, and illustrate their effect on the scalar primordial power spectrum and the cosmic microwave background under the “just enough inflation” model. We also generalize the analogy between the dynamics of a quantum scalar field on a curved, time-dependent spacetime and the gauge-invariant curvature perturbation. We argue that the invariance of the vacuum prescription obtained by minimizing the renormalized stress-energy tensor should make it the preferred procedure for setting initial conditions for primordial perturbations. All other procedures reviewed in this work yield ambiguous initial conditions, which is problematic both in theory and in practice.

By using the first-principles approach, we derive a system of three quantum kinetic equations governing the production and evolution of charged scalar particles by an electric field in an expanding universe. Analyzing the ultraviolet asymptotic behavior of the kinetic functions, we found the divergent parts of the electric current and the energy-momentum tensor of the produced particles and determined the corresponding counterterms. The renormalized system of equations is used to study the generation of electromagnetic fields during and after inflation in the kinetic coupling model ${\mathcal{L}}_{\mathrm{EM}}=-(1/4)f^2(\varphi )F_{\mu \nu }{F}^{\mu \nu }$ with the Ratra coupling function $f=\exp (\beta \varphi /M_p)$. It is found that the electric current of created particles is retarded with respect to the electric field. This leads to an oscillatory behavior of both quantities in agreement with the results obtained previously in phenomenological kinetic and hydrodynamical approaches.

We use supernovae measurements, calibrated by the local determination of the Hubble constant $H_0$ by SH0ES, to interpolate the distance-redshift relation using Gaussian process regression. We then predict, independent of the cosmological model, the distances that are measured with strong lensing time delays to test their mutual agreement. We find excellent agreement between these predictions and the measurements. The agreement holds when we consider only the redshift dependence of the distance-redshift relation, independent of the value of $H_0$. Our results disfavor the possibility that lens mass modeling contributes a 10% bias or uncertainty in the strong lensing analysis, as suggested recently in the literature. In general our analysis strengthens the case that residual systematic errors in both measurements are below the level of the current discrepancy with the CMB determination of $H_0$, and supports the possibility of new physical phenomena on cosmological scales. With additional data our methodology can provide more stringent tests of unaccounted for systematics in the determinations of the distance-redshift relation in the late universe.

We discuss axion dark matter detection via two mechanisms: spontaneous decays and resonant conversion in neutron star magnetospheres. For decays, we show that the brightness temperature signal, rather than flux, is a less ambiguous measure for selecting candidate objects. This is owing principally to the finite beam width of telescopes which prevents one from being sensitive to the total flux from the object. With this in mind, we argue that the large surface-mass-density of the galactic center or the Virgo cluster center offer the best chance of improving current constraints on the axion-photon coupling via spontaneous decays. For the neutron star case, we first carry out a detailed study of mixing in magnetized plasmas. We derive transport equations for the axion-photon system via a controlled gradient expansion, allowing us to address inhomogeneous mass-shell constraints for arbitrary momenta. We then derive a nonperturbative Landau-Zener formula for the conversion probability valid across the range of relativistic and nonrelativistic axions and show that the standard perturbative resonant conversion amplitude is a truncation of this result in the nonadiabatic limit. Our treatment reveals that infalling dark matter axions typically convert nonadiabatically in magnetospheres. We describe the limitations of one-dimensional mixing equations and explain how three-dimensional effects activate new photon polarizations, including longitudinal modes and illustrate these arguments with numerical simulations in higher dimensions. We find that the bandwidth of the radio signal from neutron stars could be dominated by Doppler broadening from the oblique rotation of the neutron star if the axion is nonrelativistic in the conversion region. Therefore, we conclude that the radio signal from the resonant decay is weaker than previously thought, which means one relies on local density peaks to probe weaker axion-photon couplings.

The ratio between the CMB lensing/galaxy counts and the galaxy shear/galaxy counts cross-correlations combines the information from different cosmological probes to infer cosmographic measurements that are less dependent on astrophysical uncertainties and can constrain the geometry of the Universe. We discuss the future perspectives for the measurement of this lensing ratio as previously introduced, i.e., with the use of the Limber and flat-sky approximations and neglecting all the effects on the galaxy survey from observing on the past light cone. We then show how the cosmological information in this estimator is affected by the Limber approximation and by the inclusion of the redshift space distortions (RSD) and lensing magnification contributions to the galaxy number counts. We find that the lensing magnification contribution induces a multipole dependence of the lensing ratio that we show to be detectable at a statistical significant level combining post-Planck CMB surveys and a Euclid-like experiment. We propose an improved estimator which takes into account this angular scale dependence. Using this extended formalism, we present forecasts for upcoming and future cosmological surveys, and we show at which extent the lensing ratio information can improve the CMB constraints on cosmological parameters. We get that for extended cosmological models where the neutrino mass, the spatial curvature and the dark energy equation of state are allowed to vary, the constraints from Planck on these parameters and on $H_0$ can be reduced by $\sim 40\%$ with the inclusion of a single lensing ratio and by $\sim 60\%$–70% adding the joint measurement of 9 lensing ratios with a Euclid-like survey. We also find that neglecting the contribution from lensing magnification can induce a bias on the derived cosmological parameters in a combined analysis.

We study parametric instability of compact axion dark matter structures decaying to radiophotons. Corresponding objects—Bose (axion) stars, their clusters, and clouds of diffuse axions—form abundantly in the postinflationary Peccei-Quinn scenario. We develop general description of parametric resonance incorporating finite-volume effects, backreaction, axion velocities, and their (in)coherence. With additional coarse graining, our formalism reproduces kinetic equation for virialized axions interacting with photons. We derive conditions for the parametric instability in each of the above objects, as well as in collapsing axion stars, evaluate photon resonance modes and their growth exponents. As a by-product, we calculate stimulated emission of Bose stars and diffuse axions, arguing that the former can give larger contribution into the radio background. In the case of QCD axions, the Bose stars glow and collapsing stars radioburst if the axion-photon coupling exceeds the original Kim-Shifman-Vainshtein-Zakharov value by 2 orders of magnitude. The latter constraint is alleviated for several nearby axion stars in resonance and absent for axionlike particles. Our results show that the parametric effect may reveal itself in observations, from fast radio bursts to excess radio background.

We show that gravitational wave emission from neutron star binaries can be used to discover any generic long-ranged muonic force due to the large inevitable abundance of muons inside neutron stars. As a minimal consistent example, we focus on a gauged ${\mathrm{U}(1)}_{L_{\mu }-L_{\tau }}$ symmetry. In pulsar binaries, such ${\mathrm{U}(1)}_{L_{\mu }-L_{\tau }}$ vectors induce an anomalously fast decay of the orbital period through the emission of dipole radiation. We study a range of different pulsar binaries, finding the most powerful constraints for vector masses below $\mathcal{O}({10}^{-18}\mathrm{eV})$. For merging binaries, the presence of muons in neutron stars can result in dipole radiation as well as a modification of the chirp mass during the inspiral phase. We make projections for a prospective search using both the GW170817 and S190814bv events and find that current data can discover light vectors with masses below $\mathcal{O}({10}^{-10}\mathrm{eV})$. In both cases, the limits attainable with neutron stars reach gauge coupling $g^{\prime }\lesssim {10}^{-20}$, which are many orders of magnitude stronger than previous constraints. We also show projections for next generation experiments, such as Einstein Telescope and Cosmic Explorer.

We describe the signatures of a circularly polarized gravitational-wave background on the timing residuals obtained with pulsar-timing arrays. Most generally, the circular polarization will depend on the gravitational-wave direction, and we describe this angular dependence in terms of spherical harmonics. While the amplitude of the monopole (the overall chirality of the gravitational-wave background) cannot be detected, measures of the anisotropy are theoretically conceivable. We provide expressions for the minimum-variance estimators for the circular-polarization anisotropy. We evaluate the smallest detectable signal as a function of the signal-to-noise ratio with which the isotropic gravitational wave (GW) signal is detected and the number of pulsars (assumed to be roughly uniformly spread throughout the sky) in the survey. We find that the overall dipole of the circular polarization and a few higher overall multipoles, are detectable in a survey with $\gtrsim 100$ pulsars if their amplitude is close to maximal and once the isotropic signal is established with a signal-to-noise ratio $\gtrsim 400$. Even if the anisotropy can be established, though, there will be limited information on its direction. Similar arguments apply to astrometric searches for gravitational waves.

The spatial and temporal discreteness of gravitational wave sources leads to shot noise that may, in some regimes, swamp any attempts at measuring the anisotropy of the gravitational wave background. Cross-correlating a gravitational wave background map with a sufficiently dense galaxy survey can alleviate this issue, and potentially recover some of the underlying properties of the gravitational wave background. We quantify the shot noise level and we explicitly show that cross-correlating the gravitational wave background and a galaxy catalog improves the chances of a first detection of the background anisotropy with a gravitational wave observatory operating in the frequency range (10 Hz, 100 Hz), given sufficient sensitivity.

I derive a system of pulsation equations for compact stars made up of an arbitrary number of perfect fluids that can be used to study radial oscillations and stability with respect to small perturbations. I assume spherical symmetry and that the only interfluid interactions are gravitational. My derivation is in line with Chandrasekhar’s original derivation for the pulsation equation of a single-fluid compact star and keeps the contributions from the individual fluids manifest. I illustrate solutions to the system of pulsations equations with one-, two-, and three-fluid examples.

Characterizing the epoch of reionization (EoR) at $z\gtrsim 6$ via the redshifted 21 cm line of neutral Hydrogen (H i) is critical to modern astrophysics and cosmology, and thus a key science goal of many current and planned low-frequency radio telescopes. The primary challenge to detecting this signal is the overwhelmingly bright foreground emission at these frequencies, placing stringent requirements on the knowledge of the instruments and inaccuracies in analyses. Results from these experiments have largely been limited not by thermal sensitivity but by systematics, particularly caused by the inability to calibrate the instrument to high accuracy. The interferometric bispectrum phase is immune to antenna-based calibration and errors therein, and presents an independent alternative to detect the EoR H i fluctuations while largely avoiding calibration systematics. Here, we provide a demonstration of this technique on a subset of data from the Hydrogen Epoch of Reionization Array (HERA) to place approximate constraints on the brightness temperature of the intergalactic medium (IGM). From this limited data, at $z=7.7$ we infer “$1\sigma$” upper limits on the IGM brightness temperature to be $\le 316$ “pseudo” mK at ${\kappa }_{\parallel }=0.33$ “pseudo” $h{\mathrm{Mpc}}^{-1}$ (data-limited) and $\le 1000$ “pseudo” mK at ${\kappa }_{\parallel }=0.875$ “pseudo” $h{\mathrm{Mpc}}^{-1}$ (noise-limited). The “pseudo” units denote only an approximate and not an exact correspondence to the actual distance scales and brightness temperatures. By propagating models in parallel to the data analysis, we confirm that the dynamic range required to separate the cosmic H i signal from the foregrounds is similar to that in standard approaches, and the power spectrum of the bispectrum phase is still data-limited (at $\gtrsim 10^6$ dynamic range) indicating scope for further improvement in sensitivity as the array build-out continues.

Many low-frequency radio interferometers are aiming to detect very faint spectral signatures from structures at cosmological redshifts, particularly of neutral Hydrogen using its characteristic 21 cm spectral line. Due to the very high dynamic range needed to isolate these faint spectral fluctuations from the very bright foregrounds, spectral systematics from the instrument or the analysis, rather than thermal noise, are currently limiting their sensitivity. Failure to achieve a spectral calibration of the instrument with fractional inaccuracy $\lesssim 10^{-5}$ will make the detection of the critical cosmic signal unlikely. The bispectrum phase from interferometric measurements is largely immune to this calibration issue. We present a basis to explore the nature of the bispectrum phase in the limit of small spectral fluctuations. We establish that these fluctuations measure the intrinsic dissimilarity in the transverse structure of the cosmic signal relative to the foregrounds, expressed as rotations in the underlying phase angle. Their magnitude is related to the strength of the cosmic signal relative to the foregrounds. Using a range of sky models, we detail the behavior of the bispectrum phase fluctuations using standard Fourier-domain techniques and find it comparable to existing approaches, with a few key differences. Foreground contamination from mode-mixing between the transverse and line-of-sight dimensions is more pronounced than in existing approaches because the bispectrum phase is a product of three individual interferometric phases. The multiplicative coupling of foregrounds in the bispectrum phase fluctuations results in the mixing of foreground signatures with that of the cosmic signal. We briefly outline a variation of this approach to avoid extensive mode-mixing. Despite its limitations, the interpretation of results using the bispectrum phase is possible with forward-modeling. Importantly, it is an independent and a viable alternative to existing approaches.

In QCD, at high density, the color-octet quark-antiquark condensate $⟨\overline{\psi }{\gamma }_0({\lambda }^A{)}_C({\lambda }^A{)}_F\psi ⟩$ is generally nonzero and dynamically breaks the $\mathrm{SU}(3{)}_C\times \mathrm{SU}(3{)}_L\times \mathrm{SU}(3{)}_R$ symmetry down to the diagonal $\mathrm{SU}(3{)}_V$. We evaluate this condensate in the mean-field approximation and find that it is of order $\mu {\mathrm{\Delta }}^2\log (\mu /\mathrm{\Delta })$ where $\mathrm{\Delta }$ is the Bardeen–Cooper–Schrieffer (BCS) gap of quarks. Next, we propose a novel non-Hermitian chiral random matrix theory that describes the formation of colorful quark-antiquark condensates. We take the microscopic large-$N$ limit and find that three phases appear depending on the parameter of the model. They are the color-flavor-locked phase, the polar phase, and the normal phase. We rigorously derive the effective theory of Nambu-Goldstone modes and determine the quark-mass dependence of the partition function.

We have applied isovector and isoscalar tensor current to evaluate the tensor form factors of the $N(1535)\to N$ transition with the help of the light-cone QCD sum rule method. In numerical computations, have used the most general forms of the interpolating current for the nucleon and the tensor current together with two different sets of the input parameters in the distribution amplitudes (DAs) of the $N(1535)$ state. We have obtained that the values of $N(1535)\to N$ transition tensor form factors are very sensitive to the input parameters of the DAs of the $N(1535)$ state. We have acquired that the $Q^2$ dependence of $N(1535)\to$ transition tensor form factors is well defined by a $p$-pole fit function.

The rare pion decays ${\pi }^{+}\to {\mu }^{+}{\nu }_{\mu }\nu \overline{\nu }$ and ${\pi }^{+}\to e^{+}{\nu }_e\nu \overline{\nu }$ are allowed in the Standard Model but highly suppressed. These decays were searched for using data from the PIENU experiment. A first result for $\mathrm{\Gamma }({\pi }^{+}\to {\mu }^{+}{\nu }_{\mu }\nu \overline{\nu })/\mathrm{\Gamma }({\pi }^{+}\to {\mu }^{+}{\nu }_{\mu })<8.6\times {10}^{-6}$ and an improved measurement $\mathrm{\Gamma }({\pi }^{+}\to e^{+}{\nu }_e\nu \overline{\nu })/\mathrm{\Gamma }({\pi }^{+}\to {\mu }^{+}{\nu }_{\mu })<1.6\times {10}^{-7}$ were obtained.

We propose a new non-Hermitian chiral random matrix model that describes single-flavor spin-one Cooper pairing of quarks. For three colors the model shows spontaneous breaking of color $\mathrm{SU}(3{)}_C$ and spin $\mathrm{SO}(3{)}_J$ symmetries down to the diagonal $\mathrm{SO}(3{)}_{C+J}$ subgroup, in striking analogy to the color-spin locked phase of one-flavor QCD at high density. For two colors, color-singlet spin-one diquarks condense and trigger symmetry breaking $\mathrm{U}(1)\times \mathrm{SO}(3{)}_J\to \mathrm{SO}(2{)}_J$. In both cases the microscopic large-$N$ limit is rigorously taken and the effective theory of Nambu-Goldstone modes is derived.

We study the tricritical Ising universality class using conformal bootstrap techniques. By studying bootstrap constraints originating from multiple correlators on the conformal field theory (CFT) data of multiple operator product expansions (OPEs), we are able to determine the scaling dimension of the spin field ${\mathrm{\Delta }}_{\sigma }$ in various noninteger dimensions $2\le d\le 3$. Here, ${\mathrm{\Delta }}_{\sigma }$ is connected to the critical exponent $\eta$ that governs the (tri)critical behavior of the two-point function via the relation $\eta =2-d+2{\mathrm{\Delta }}_{\sigma }$. Our results for ${\mathrm{\Delta }}_{\sigma }$ match with the exactly known values in two and three dimensions and are a conjecture for noninteger dimensions. We also compare our CFT results for ${\mathrm{\Delta }}_{\sigma }$ with $\epsilon$-expansion results, available up to ${\epsilon }^3$ order. Our techniques can be naturally extended to study higher-order multicritical points.