We develop a unified description, via the Boltzmann equation, of damping of gravitational waves by matter, incorporating collisions. We identify two physically distinct damping mechanisms—collisional and Landau damping. We first consider damping in flat spacetime, and then generalize the results to allow for cosmological expansion. In the first regime, maximal collisional damping of a gravitational wave, independent of the details of the collisions in the matter is, as we show, significant only when its wavelength is comparable to the size of the horizon. Thus damping by intergalactic or interstellar matter for all but primordial gravitational radiation can be neglected. Although collisions in matter lead to a shear viscosity, they also act to erase anisotropic stresses, thus suppressing the damping of gravitational waves. Damping of primordial gravitational waves remains possible. We generalize Weinberg’s calculation of gravitational wave damping, now including collisions and particles of finite mass, and interpret the collisionless limit in terms of Landau damping. While Landau damping of gravitational waves cannot occur in flat spacetime, the expansion of the universe allows such damping by spreading the frequency of a gravitational wave of given wave vector.

We provide the first explicit proposal for all microstates of generic black holes in three dimensions (of Bañados-Teitelboim-Zanelli type): black hole microstates, termed “horizon fluff,” are a particular class of near horizon soft hairs which have zero energy as measured by the horizon observer and cannot be distinguished by observers at a finite distance from the horizon. These states are arranged in orbits of the two-dimensional conformal algebra associated with the asymptotic black hole geometry. We count these microstates using the Hardy-Ramanujan formula for the number of partitions of a given integer into non-negative integers, recovering the Bekenstein-Hawking entropy. We discuss possible extensions of our black hole microstate construction to astrophysical Kerr-type black holes.

We consider a nonlocal gravity model motivated by nonperturbative quantum gravity studies. This model, if correct, suggests the existence of strong IR relevant effects which can lead to an interesting late time cosmology. We implement the IR modification directly in the effective action. We show that, upon some assumptions on initial conditions, this model describes an observationally viable background cosmology being also consistent with local gravity tests.

Earlier it was shown that the entropy of an ideal gas, contained in a box and moving in a gravitational field, develops an area dependence when it approaches the horizon of a static, spherically symmetric spacetime. Here we extend the above result in two directions; viz., to (a) the stationary axisymmteric spacetimes and (b) time-dependent cosmological spacetimes evolving asymptotically to the de Sitter or the Schwarzschild–de Sitter spacetimes. While our calculations are exact for the stationary axisymmetric spacetimes, for the cosmological case we present an analytical expression of the entropy when the spacetime is close to the de Sitter or the Schwarzschild–de Sitter spacetime. Unlike the static spacetimes, there is no hypersurface orthogonal timelike Killing vector field in these cases. Nevertheless, the results hold, and the entropy develops an area dependence in the appropriate limit.

Observations indicate that large-scale anomalies exist in the fluctuations of the cosmic microwave background. In these anomalies, the hemispherical power amplitude asymmetry has a correlation length comparable to that of the observable universe. We propose that a topological defect created by spontaneous breaking of the U(1) symmetry prior to inflation generated an initial phase variation, $\delta \theta$, across the observable region of the universe. The amplitude of this phase fluctuation is protected by topology if the defect is inside the horizon, and is frozen by causality if the defect exits the horizon. After inflation, the phase-corresponding boson field started to oscillate, when the Hubble rate decreased to a level comparable to the mass of the boson field. The energy density of the newly created boson particles varied across the observable universe. The bosons subsequently decayed into radiation prior to the big bang nucleosynthesis epoch, and the resulting fluctuations in the energy density produced the observed power asymmetry. This scenario predicts a scale-dependent modulation amplitude power asymmetry and in addition, as topological defects created by phase transitions are a very general phenomenon, the observed hemispherical asymmetry may be seen as an evidence for the cosmological inflation.

We update the search for features in the cosmic microwave background (CMB) power spectrum due to transient reductions in the speed of sound, using Planck 2015 CMB temperature and polarization data. We enlarge the parameter space to much higher oscillatory frequencies of the feature, and define a robust prior independent of the ansatz for the reduction, guaranteed to reproduce the assumptions of the theoretical model. This prior exhausts the regime in which features coming from a Gaussian reduction are easily distinguishable from the baseline cosmology. We find a fit to the $\ell \approx 20–40\mathrm{minus}/\mathrm{plus}$ structure in Planck TT power spectrum, as well as features spanning along higher $\ell$’s ($\ell \approx 100–1500$). None of those fits is statistically significant, either in terms of their improvement of the likelihood or in terms of the Bayes ratio. For the higher-$\ell$ ones, their oscillatory frequency (and their amplitude to a lesser extent) is tightly constrained, so they can be considered robust, falsifiable predictions for their correlated features in the CMB bispectrum. We compute said correlated features, and assess their signal to noise and correlation with the secondary bispectrum of the correlation between the gravitational lensing of the CMB and the integrated Sachs-Wolfe effect. We compare our findings to the shape-agnostic oscillatory template tested in Planck 2015, and we comment on some tantalizing coincidences with some of the traits described in Planck’s 2015 bispectrum data.

We point out a unique mechanism to produce the relic abundance for the glueball dark matter from a gauged $\mathrm{SU}(N{)}_d$ hidden sector which is bridged to the standard model sector through heavy vectorlike quarks colored under gauge interactions from both sides. A necessary ingredient of our assumption is that the vectorlike quarks, produced either thermally or nonthermally, are abundant enough to dominate the universe for some time in the early universe. They later undergo dark color confinement and form unstable vectorlike-quarkonium states which annihilate decay and reheat the visible and dark sectors. The ratio of entropy dumped into two sectors and the final energy budget in the dark glueballs is only determined by low energy parameters, including the intrinsic scale of the dark $\mathrm{SU}(N{)}_d$, ${\mathrm{\Lambda }}_d$, and number of dark colors, $N_d$, but depend weakly on parameters in the ultraviolet such as the vectorlike quark mass or the initial condition. We call this a cosmic selection rule for the glueball dark matter relic density.

Prior change is discussed in observational constraints studies of nonlocally modified gravity, where a model characterized by a modification of the form $\sim m^{2}R{\square }^{-2}R$ to the Einstein-Hilbert action was compared against the base $\mathrm{\Lambda }\mathrm{CDM}$ one in a Bayesian way. It was found that the competing modified gravity model is significantly disfavored (at $22:1$ in terms of betting-odds) against $\mathrm{\Lambda }\mathrm{CDM}$ given $\mathrm{CMB}+\mathrm{SNIa}+\mathrm{BAO}$ data, because of a tension appearing in the $H_0–{\mathrm{\Omega }}_M$ plane. We identify the underlying mechanism generating such a tension and show that it is mostly caused by the late-time, quite smooth, phantom nature of the effective dark energy described by the nonlocal model. We find that the tension is resolved by considering an extension of the initial baseline, consisting in allowing the absolute mass of three degenerated massive neutrino species $\sum m_{\nu }/3$ to take values within a prior interval consistent with existing data. As a net effect, the absolute neutrino mass is inferred to be nonvanishing at $2\sigma$ level, best-fitting at $\sum m_{\nu }\approx 0.21\mathrm{eV}$, and the Bayesian tension disappears rendering the nonlocal gravity model statistically equivalent to $\mathrm{\Lambda }\mathrm{CDM}$, given recent $\mathrm{CMB}+\mathrm{SNIa}+\mathrm{BAO}$ data. We also discuss constraints from growth rate measurements $f{\sigma }_8$, whose fit is found to be improved by a larger massive neutrino fraction as well. The $\nu$-extended nonlocal model also prefers a higher value of $H_0$ than $\mathrm{\Lambda }\mathrm{CDM}$, therefore in better agreement with local measurements. Our study provides one more example suggesting that the neutrino density fraction ${\mathrm{\Omega }}_{\nu }$ is partially degenerated with the nature of the dark energy. This emphasizes the importance of cosmological and terrestrial neutrino research and, as a massive neutrino background impacts structure formation observables non-negligibly, proves to be especially relevant for future galaxy surveys.

Dipolar dark matter is an alternative model motivated by the challenges faced by the standard cold dark matter model to describe the right phenomenology at galactic scales. A promising realization of dipolar dark matter was recently proposed in the context of massive bigravity theory. The model contains dark matter particles as well as a vector field coupled to the effective composite metric of bigravity. This model is completely safe in the gravitational sector thanks to the underlying properties of massive bigravity. In this work, we investigate the exact decoupling limit of the theory, including the contribution of the matter sector, and prove that it is free of ghosts in this limit. We conclude that the theory is acceptable as an effective field theory below the strong coupling scale.

We discuss the feasibility of using astrophysical observations of white dwarfs as probes of fundamental physics. We quantify the effects of varying fundamental couplings on the white dwarf mass-radius relation in a broad class of unification scenarios, both for the simple case of a polytropic stellar structure model and for more general models. Independent measurements of the mass and radius, together with direct spectroscopic measurements of the fine-structure constant in white dwarf atmospheres lead to constraints on combinations of the two phenomenological parameters describing the underlying unification scenario (one of which is related to the strong sector of the theory while the other is related to the electroweak sector). While currently available measurements do not yet provide stringent constraints, we show that forthcoming improvements, expected for example from the Gaia satellite, can break parameter degeneracies and lead to constraints that ideally complement those obtained from local laboratory tests using atomic clocks.

The analysis of signals in directional dark matter (DM) detectors typically assumes that the directions of nuclear recoils can be measured in the Galactic rest frame. However, this is not possible with all directional detection technologies. In nuclear emulsions, for example, the recoil events must be detected and measured after the exposure time of the experiment. Unless the entire detector is mounted and rotated with the sidereal day, the recoils cannot be reoriented in the Galactic rest frame. We examine the effect of this “time integration” on the primary goals of directional detection, namely: (1) confirming that the recoils are anisotropic; (2) measuring the median recoil direction to confirm their Galactic origin; and (3) probing below the neutrino floor. We show that after time integration the DM recoil distribution retains a preferred direction and is distinct from that of Solar neutrino-induced recoils. Many of the advantages of directional detection are therefore preserved and it is not crucial to mount and rotate the detector. Rejecting isotropic backgrounds requires a factor of 2 more signal events compared with an experiment with event time information, whereas a factor of 1.5–3 more events are needed to measure a median direction in agreement with the expectation for DM. We also find that there is still effectively no neutrino floor in a time-integrated directional experiment. However to reach a cross section an order of magnitude below the floor, a factor of $\sim 8$ larger exposure is required than with a conventional directional experiment. We also examine how the sensitivity is affected for detectors with only 2D recoil track readout, and/or no head-tail measurement. As for non-time-integrated experiments, 2D readout is not a major disadvantage, though a lack of head-tail sensitivity is.

We consider a baryogenesis scenario via the oscillation of right-handed neutrinos with Majorana masses of the order of GeV, which are also responsible for neutrino masses by the seesaw mechanism. We study how the initial condition alters the prediction of the present baryon asymmetry by this mechanism. It is usually assumed that the abundance of right-handed neutrinos is zero after the reheating of the inflationary universe and they are produced in scattering processes by the renomalizable Yukawa interaction. However, the higher-dimensional operator with right-handed neutrinos may provide an additional production which is most effective at the reheating epoch. It is shown that such an initial abundance of right-handed neutrinos can significantly modify the prediction when the strong washout of the asymmetry is absent. This leads to the parameter space of the model for the successful baryogenesis being enlarged.

In hierarchical searches for continuous gravitational waves, clustering of candidates is an important post-processing step because it reduces the number of noise candidates that are followed up at successive stages [J. Aasi et al., Phys. Rev. Lett. 88, 102002 (2013); B. Behnke, M. A. Papa, and R. Prix, Phys. Rev. D 91, 064007 (2015); M. A. Papa et al., Phys. Rev. D 94, 122006 (2016)]. Previous clustering procedures bundled together nearby candidates ascribing them to the same root cause (be it a signal or a disturbance), based on a predefined cluster volume. In this paper, we present a procedure that adapts the cluster volume to the data itself and checks for consistency of such volume with what is expected from a signal. This significantly improves the noise rejection capabilities at fixed detection threshold, and at fixed computing resources for the follow-up stages, this results in an overall more sensitive search. This new procedure was employed in the first Einstein@Home search on data from the first science run of the advanced LIGO detectors (O1) [LIGO Scientific Collaboration and Virgo Collaboration, arXiv:1707.02669 [Phys. Rev. D (to be published)]].

The momentum signatures of created pairs in the multiphoton regime in strong fields are investigated and obvious vortex patterns that are constituted of the quasi-Archimedian spirals are found when considering two opposite-handedness circularly polarized fields. Distinguished from the familiar Ramsey interference fringes with the concentric rings under two identical-handedness fields, however, the finding vortices are associated to the interference between the pair production amplitudes of these two opposite-handedness fields. Moreover, the sensitive dependence of vortex patterns on the relative carrier envelope phase, the time delay, and the handedness between two fields imply also a new way to detect the applied laser field characteristics.

A next-to-leading order correction to the high-energy factorization limit of radiation spectrum from an ultra-relativistic electron scattering in an external field is evaluated. Generally, it does not express through scattering characteristics, and accounts for smoothness of the crossover between the initial and final electron asymptotes. A few examples of application of this formula are given, including bremsstrahlung in amorphous matter and undulator radiation.

The $K_3$ quantity, introduced in a context of the Leggett-Garg inequality violation, is studied for the neutrino oscillations in matter with a phenomenologically modeled dissipative environment. It is shown that the $K_3$ function acquires different values depending on whether the neutrino is a Dirac or Majorana particle, provided that there is a dissipative interaction between matter and neutrinos. The difference occurs for various matter densities and can serve as a potential quantifier verifying the neutrino nature. Moreover, working within a phenomenological model one can suggest the values of the matter density and dissipation for which the difference is the most visible. There exist also special conditions in which the violation of the Leggett-Garg inequality, to a different extent for both kinds of neutrino, is observed.

We study the effect of nonstandard interactions (NSIs) on the propagation of neutrinos through the Earth’s matter and how it affects the hierarchy sensitivity of the DUNE experiment. We emphasize the special case when the diagonal NSI parameter ${\epsilon }_{ee}=-1$, nullifying the standard matter effect. We show that if, in addition, $CP$ violation is maximal then this gives rise to an exact intrinsic hierarchy degeneracy in the appearance channel, irrespective of the baseline and energy. Introduction of the off diagonal NSI parameter, ${\epsilon }_{e\tau }$, shifts the position of this degeneracy to a different ${\epsilon }_{ee}$. Moreover the unknown magnitude and phases of the off diagonal NSI parameters can give rise to additional degeneracies. Overall, given the current model independent limits on NSI parameters, the hierarchy sensitivity of DUNE can get seriously impacted. However, a more precise knowledge of the NSI parameters, especially ${\epsilon }_{ee}$, can give rise to an improved sensitivity. Alternatively, if a NSI exists in nature, and still DUNE shows hierarchy sensitivity, certain ranges of the NSI parameters can be excluded. Additionally, we briefly discuss the implications of ${\epsilon }_{ee}=-1$ (in the Earth) on the Mikheyev-Smirnov-Wolfenstein effect in the Sun.

We identify the differential cross sections for $t\overline{t}$ production and the total cross section for Higgs production through gluon fusion as the processes in which the two effective operators describing the leading nonstandard interactions of the top quark with the gluon can be disentangled and studied in an independent fashion. Current data on the Higgs production and the $\mathrm{d}\sigma /\mathrm{d}{p}_T^t$ differential cross section provide limits comparable to, but not more stringent than, those from the total $t\overline{t}$ cross sections measurements at the LHC and Tevatron, where however the two operators enter on the same footing and can only be constrained together. We conclude by stating the (modest) reduction in the uncertainties necessary to provide more stringent limits by means of the Higgs production and $t\overline{t}$ differential cross section observables at the LHC with the future luminosity of 300 and $3000{\mathrm{fb}}^{-1}$.

We did a model independent phenomenological study of baryogenesis via leptogenesis, neutrinoless double beta decay (NDBD) and charged lepton flavor violation (CLFV) in a generic left-right symmetric model (LRSM) where neutrino mass originates from the type $\mathrm{I}+\text{type II}$ seesaw mechanism. We studied the new physics contributions to NDBD coming from the left-right gauge boson mixing and the heavy neutrino contribution within the framework of LRSM. We have considered the mass of the RH gauge boson to be specifically 5 TeV, 10 TeV, and 18 TeV and studied the effects of the new physics contributions on the effective mass and baryogenesis and compared with the current experimental limit. We tried to correlate the cosmological baryon asymmetry of the universe from resonant leptogenesis with the low energy observables, notably, NDBD and lepton flavor violation with a view to finding a common parameter space where they coexist.