In dark-energy models where a scalar field is nonminimally coupled to the spacetime geometry, gravitational waves are expected to be supplemented with a scalar mode. Such scalar waves may interact with the standard tensor waves, thereby affecting their observed amplitude and polarization. Understanding the role of scalar waves is thus essential in order to design reliable gravitational-wave probes of dark energy and gravity beyond general relativity. In this article, we thoroughly investigate the propagation of scalar and tensor waves in the subset of Horndeski theories in which tensor waves propagate at the speed of light. We work at linear order in scalar and metric perturbations, in the eikonal regime, and for arbitrary scalar and spacetime backgrounds. We diagonalize the system of equations of motion and identify the physical tensor mode, which differs from the metric perturbation. We find that interactions between scalar and tensor waves generally depend on the scalar propagation speed. If the scalar waves are luminal or quasiluminal, then interactions are negligible. In the subluminal case, scalar-tensor interactions are effectively suppressed due to the incoherence of the wave’s phases.

The quintessence field coupled to the cosmic neutrino background (CNB) has been widely discussed as an alternative mechanism to address the coincidence problem. As it is well known, it is possible to extend such models to obtain quintessential inflation, that is, to incorporate inflationary stage as well. Taking an alternative route, one can start from the well-established inflationary models and obtain successful quintessence models at the expense of coupling with the CNB. To follow this route, we use a slightly reformulated model addressed in [H. Mohseni Sadjadi and V. Anari, Phys Rev. D 95, 123521 (2017)]. This particular model assumes ${\mathcal{Z}}_2$ symmetry for both scalar field potential and coupling term, which then breaks down in course of the cosmological evolution. For our discussion, however, the ${\mathcal{Z}}_2$ symmetry of the potential is not mandatory for the model to work. The conventional mechanism of particle production by the oscillating inflaton field (and their subsequent thermalization) remains operative. It is plain to see that the proposed construction can be easily applied for many successful models of inflation to incorporate dark energy at the expense of coupling with the CNB. We address the issue of neutrino nuggets from the quantum field theory point of view. Namely, these nuggets are considered as bound states caused basically by the Yukawa force, which arises in the framework of linear perturbation theory due to exchange of virtual quanta of quintessence field between the neutrinos.

Primordial black holes could have been formed in the early universe from nonlinear cosmological perturbations reentering the cosmological horizon when the Universe was still radiation dominated. Starting from the shape of the power spectrum on superhorizon scales, we provide a simple prescription, based on the results of numerical simulations, to compute the threshold ${\delta }_c$ for primordial black hole formation. Our procedure takes into account both the nonlinearities between the Gaussian curvature perturbation and the density contrast and, for the first time in the literature, the nonlinear effects arising at horizon crossing, which increase the value of the threshold by about a factor two with respect to the one computed on superhorizon scales.

The redshifted 21-cm radio signal has emerged as an important probe for investigating the dynamics of the dark age Universe (recombination to reionization). In the current analysis, we explore the combined effect of primordial black hole (PBH) evaporation and the baryon-dark matter (DM) interaction in the 21-cm scenario. The variation of brightness temperature shows remarkable dependence on the DM masses ($m_{\chi }$) and the baryon-DM cross sections (${\overline{\sigma }}_0$) besides the influences of the PBH parameters (mass ${\mathcal{M}}_{\mathrm{BH}}$ and initial mass fraction ${\beta }_{\mathrm{BH}}$). We address both upper and lower bounds on ${\beta }_{\mathrm{BH}}$ for a wide range of PBH mass in the presence of different $m_{\chi }$ and ${\overline{\sigma }}_0$ by incorporating the observational excess $(-500_{-500}^{+200}\mathrm{mK})$ of EDGES’s experimental results. Finally, we address similar limits in the $m_{\chi }-{\overline{\sigma }}_0$ parameter plane for different PBH masses.

In this work, we study the relativistic quantum kinetic equations in $2+1$ dimensions from Wigner function formalism by carrying out a systematic semiclassical expansion up to $\hslash$ order. The derived equations allow us to explore interesting transport phenomena in $2+1$ dimensions. Within this framework, the parity-odd transport current induced by the external electromagnetic field is self-consistently derived. We also examine the dynamical mass generation by implementing four-fermion interaction with mean-field approximation. In this case, a new kind of transport current is found to be induced by the gradient of the mean-field condensate. Finally, we also utilize this framework to study the dynamical mass generation in an external magnetic field for the ($2+1$)-dimensional system under equilibrium.

Gauging the global $B-L$ (Baryon number minus Lepton number) symmetry in the standard model (SM) is well motivated since anomaly cancellations require the introduction of three right-handed neutrinos which play an essential role in naturally generating tiny SM neutrino masses through the seesaw mechanism. In the context of the $B-L$ extended SM, we propose a pseudo-Goldstone boson dark matter (DM) scenario in which the imaginary component of a complex $B-L$ Higgs field serves as the DM in the universe. The DM relic density is determined by the SM Higgs boson mediated process, but its elastic scattering with nucleons through the exchange of Higgs bosons is highly suppressed due to its pseudo-Goldstone boson nature. The model is therefore free from the constraints arising from direct DM detection experiments. We identify regions of the model parameter space for reproducing the observed DM density compatible with the constraints from the Large Hadron Collider and the indirect DM searches by Fermi Large Area Telescope and Major Atmospheric Gamma Imaging Cherenkov.

The spontaneous muonium-to-antimuonium conversion is one of the interesting charged lepton flavor violation processes. The Muonium-to-Antimuonium Conversion Experiment (MACE) is the next generation experiment to probe such a phenomenon. In models with a triplet Higgs boson to generate neutrino masses, such as the type-II seesaw and its variant, this process can be induced by the doubly charged Higgs boson contained in it. In this article, we study the prospect of MACE to probe these models via the muonium-to-antimuonium transitions. After considering the limits from ${\mu }^{+}\to e^{+}\gamma$ and ${\mu }^{+}\to e^{+}{e}^{-}{e}^{+}$, we find that MACE could probe a parameter space for the doubly charged Higgs boson, which is beyond the reach of LHC and other flavor experiments.

Achieving the highest possible precision for theoretical predictions at the present and future high-energy lepton and hadron colliders requires a precise determination of fragmentation functions (FFs) of light and heavy charged hadrons from a global QCD analysis with great accuracy. We describe a simultaneous determination of unpolarized FFs of charged pions, charged kaons and protons/antiprotons from single-inclusive hadron production in electron-positron annihilation (SIA) data at next-to-leading order and next-to-next-to-leading order accuracy in perturbative QCD. A new set of FFs, called SGKS20, is presented. We include data for identified light charged hadrons (${\pi }^{\pm }$, $K^{\pm }$ and $p/\overline{p}$) as well as for unidentified light charged hadrons, $h^{\pm }$ and show that these data have a significant impact on both size and uncertainties of the fragmentation functions. We examine the inclusion of higher-order perturbative QCD corrections and finite-mass effects. We compare the new SGKS20 FFs with other recent FFs available in the literature and find in general reasonable agreement, but also important differences for some parton species. We show that theoretical predictions obtained from our new FFs are in very good agreement with the analyzed SIA data, especially at small values of $z$. The SGKS20 FF sets presented in this work are available via the LHAPDF interface.

In this paper, we analyze the vertices of doubly heavy spin-$3/2$–spin-$1/2$ baryons with light mesons within the method of light-cone QCD sum rules. These vertices are parametrized in terms of one (three) coupling constant(s) for the pseudoscalar (vector) mesons. The said coupling constants are calculated for all possible transitions. The results presented here can serve as useful information in experimental as well as theoretical studies of the properties of doubly heavy baryons.

The propagators for the Faddeev-Popov (FP) ghosts in Yang-Mills theory and perturbative gravity in the covariant gauge are infrared (IR) divergent in de Sitter spacetime. An IR cutoff in the momentum space to regularize these divergences breaks the de Sitter invariance. These IR divergences are due to the spatially constant modes in the Yang-Mills case and the modes proportional to the Killing vectors in the case of perturbative gravity. It has been proposed that these IR divergences can be removed, with the de Sitter invariance preserved, by first regularizing them with an additional mass term for the FP ghosts and then taking the massless limit. In the Yang-Mills case, this procedure has been shown to correspond to requiring that the physical states, and the vacuum state in particular, be annihilated by some conserved charges in the Landau gauge. In this paper we show that there are similar conserved charges in perturbative gravity in the covariant Landau gauge in de Sitter spacetime and that the IR-regularization procedure described above also correspond to requiring that the vacuum state be annihilated by these charges with a natural definition of the interacting vacuum state.

The junction conditions for the infinite derivative gravity theory $R+RF(\square )R$ are derived under the assumption that the conditions can be imposed by avoiding the “ill-defined expressions” in the theory of distributions term by term in infinite summations. We find that the junction conditions of such nonlocal theories are much more restrictive than in local theories, since the conditions comprise an infinite number of equations for the Ricci scalar. These conditions can constrain the geometry far beyond the matching hypersurface. Furthermore, we derive the junction field equations which are satisfied by the energy-momentum tensor on the hypersurface. It turns out that the theory still allows some matter content on the hypersurface (without external flux and external tension) but with a traceless energy-momentum tensor. We also discuss the proper matching condition where no matter is concentrated on the hypersurface. Finally, we explore the possible applications and consequences of our results to the braneworld scenarios and star models. Particularly, we find that the internal tension is given purely by the trace of the energy-momentum tensor of the matter confined to the brane. Consequences of the junction conditions are illustrated on two simple examples of static and collapsing stars. It is demonstrated that, even without solving the field equations, the geometry on one side of the hypersurface can be determined to a great extent by the geometry on the other side if the Ricci scalar is analytic. We further show that some usual star models in the general relativity are no longer solutions of the infinite derivative gravity.

Recent work has argued that the strong cosmic censorship conjecture is violated by near-extremal Reissner-Nordström–de Sitter (RNdS) black holes but respected by Kerr–de Sitter (Kerr-dS) black holes. It has also been shown that the conjecture is violated by near-extremal Bañados-Teitelboim-Zanelli (BTZ) black holes. The latter result relies on a coincidence between “exterior” and “interior” quasinormal frequencies. If this coincidence were to occur also for RNdS or Kerr-dS, then it would significantly modify the conclusions of earlier work. In this paper, it is demonstrated that this coincidence does not occur for RNdS or Kerr-dS, and so the conclusions of the earlier work remain valid.

We continue our study of the optical properties of the solar gravitational lens. Taking the next step beyond representing it as an idealized monopole, we now characterize the gravitational field of the Sun using an infinite series of multipole moments. We consider the propagation of electromagnetic (EM) waves in this gravitational field within the first post-Newtonian approximation of the general theory of relativity. The problem is formulated within the Mie diffraction theory. We solve Maxwell’s equations for the EM wave propagating in the background of a static gravitational field of an extended gravitating body, while accounting for multipole contributions. Using a wave-theoretical approach and the eikonal approximation, we find an exact closed form solution for the Debye potentials and determine the EM field at an image plane in the strong interference region of the lens. The resulting EM field is characterized by a new diffraction integral. We study this solution and show how the presence of multipoles affects the optical properties of the lens, resulting in distinct diffraction patterns. We identify the gravitational deflection angle with the individual contributions due to each of the multipoles. Treating the Sun as an extended, axisymmetric, rotating body, we show that each zonal harmonics causes light to diffract into an area whose boundary is a caustic of a particular shape. The appearance of the caustics modifies the point-spread function of the lens, thus affecting its optical properties. The new wave-theoretical solution allows the study of gravitational lensing by a realistic lens that possesses an arbitrary number of gravitational multipoles. This angular eikonal method represents an improved treatment of realistic gravitational lensing. It may be used for a wave-optical description of many astrophysical lenses.

We construct a large class of spacetimes that are smoothly matched to homogeneous, spherically symmetric clouds of matter. The evolution of the clouds is left arbitrary to allow for the incorporation of modifications by quantum effects, which can in particular lead to bounces. We further discuss two simple yet illustrative examples of these spacetimes, both in general terms and for a specific form of the bounce, with a focus on horizon behavior and relevant time scales.

We consider holography of two $pp$-wave metrics in conformal gravity, their one-point functions, and asymptotic symmetries. One of the metrics is a generalization of the standard $pp$ waves in Einstein gravity to conformal gravity. The holography of this metric shows that within conformal gravity one can have a realized solution which has a nonvanishing partially massless response tensor even for a vanishing subleading term in the Fefferman-Graham expansion (i.e., Neumann boundary conditions) and vice versa.

We present an implementation of the dual foliation generalized harmonic gauge (DF-GHG) formulation within the pseudospectral code bamps. The formalism promises to give greater freedom in the choice of coordinates that can be used in numerical relativity. As a specific application we focus here on the treatment of black holes in spherical symmetry. Existing approaches to black hole excision in numerical relativity are susceptible to failure if the boundary fails to remain outflow. We present a method, called DF-excision, to avoid this failure. Our approach relies on carefully choosing coordinates in which the coordinate lightspeeds are under strict control so that the excision boundary must remain outflow. These coordinates are then combined with the DF-GHG formulation. After performing a set of validation tests in a simple setting, we study the accretion of large pulses of scalar field matter on to a spherical black hole. We compare the results of DF-excision with a naive setup. DF-excision proves reliable even when the previous approach fails.

By focusing on a simple extension of Lambda cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) in which the dark energy equation of state is allowed to vary, we assess which epoch(s) possibly source the $H_0$-tension. We consider cosmic microwave background (CMB) data in three possible ways: (i) complete CMB data; (ii) excluding the low-$l+\mathrm{lowE}$ ($l<30$ temperature and polarization) likelihoods; (iii) imposing early Universe priors, which allow us to disentangle early- and late-time physics. Through a joint analysis with low-redshift supernovae type-Ia and gravitationally lensed time delay datasets, and neglecting galaxy clustering baryonic acoustic oscillation (BAO) data, we find that the inclusion of early Universe CMB priors is consistent with local estimate of $H_0$, while excluding the low-$l+\mathrm{lowE}$ likelihoods mildly relaxes the tension. This is in contrast to joint analyses with the complete CMB data. Our simple implementation of contrasting the effect of different CMB priors on the $H_0$ estimate shows that the early Universe information from the CMB data when decoupled from late-time physics could be in agreement with a higher value of $H_0$, even for $\mathrm{\Lambda }\mathrm{CDM}$ model with no necessary modification. We also find no evidence for the early dark energy model using only the early Universe physics within the CMB data. Finally, using the BAO data in different redshift ranges to perform inverse distance ladder analysis, we find that the early Universe modifications, while perfectly capable of alleviating the $H_0$-tension when including the BAO galaxy clustering data, would be at odds with the Ly-$\alpha$ BAO data due to the difference in $r_{\mathrm{d}}$ vs $H_0$ correlation between the two BAO datasets. We therefore infer and speculate that source for the $H_0$-tension between CMB and local estimates could possibly originate in the modeling of late-time physics within the CMB analysis. This in turn recasts the $H_0$-tension as an effect of late-time physics in CMB, instead of the current early-time CMB vs local late-time physics perspective.

We present model independent reconstructions of quintessence and the swampland conjectures (SC) using both machine learning (ML) and cosmography. In particular, we demonstrate how the synergies between theoretical analyses and ML can provide key insights on the nature of dark energy and modified gravity. Using the Hubble parameter $H(z)$ data from the cosmic chronometers we find that the ML and cosmography reconstructions of the SC are compatible with observations at low redshifts. Finally, including the growth rate data $f{\sigma }_8(z)$ we perform a model independent test of modified gravity cosmologies through two phase diagrams, namely $H-f{\sigma }_8$ and $\eta -f{\sigma }_8$, where the anisotropic stress parameter $\eta$ is obtained via the $E_g$ statistics, which is related to gravitational lensing data. While the first diagram is consistent within the errors with the $\mathrm{\Lambda }\mathrm{CDM}$ model, the second one has a $\sim 2\sigma$ deviation of the anisotropic stress from unity at $z\sim 0.3$ and a $\sim 4\sigma$ deviation at $z\sim 0.9$, thus pointing toward mild deviations from general relativity, which could be further tested with upcoming large-scale structure surveys.

In determining the gravitational signal of cusps from a network of cosmic string loops, a number of key parameters have to be assumed. These include the typical number of cusps per period of string oscillation and the typical values of the sharpness parameters of left- and right-moving waves on the string, evaluated at the cusp event. Both of these are important, as the power stored in the gravitational waves emitted from the loops of string is proportional to the number of cusps per period and inversely proportional to the product of the sharpness parameters associated with the left- and right-moving modes on the string. In suitable units, both of these quantities are usually thought to be of order unity. To try and place these parameters on a more robust footing, we analyze in detail a large number of randomly chosen loops of string that can have high harmonics associated with them, such as one might expect to form by chopping off an infinite string in the early Universe. This allows us to analyze tens of thousands of loops and obtain detailed statistics on these crucial parameters. While we find in general the sharpness parameters are indeed close to unity, as assumed in previous work [with occasional exceptions where they can become $O({10}^{-2})$], the cusp number per period scales directly with the number of harmonics on the loop and can be significantly larger than unity. This opens up the possibility of larger signals than would have otherwise been expected, potentially leading to tighter bounds on the dimensionless cosmic string tension $G\mu$.

We devise a novel mechanism and for the first time demonstrate that the Higgs model in particle physics can drive the inflation to satisfy the cosmic microwave background observations and simultaneously enhance the curvature perturbations at small scales to explain the abundance of dark matter in our universe in the form of primordial black holes. The production of primordial black holes is accompanied by the secondary gravitational waves induced by the first order Higgs fluctuations which is expected observable by space-based gravitational wave detectors. We propose possible cosmological probes of Higgs field in the future observations for primordial black holes dark matter or stochastic gravitational waves.