We consider a theory of $N$ self-interacting quantum scalar fields with quartic $O(N)$-symmetric potential, with a coupling constant $\lambda$, in a generic curved spacetime. We analyze the renormalization process of the semiclassical Einstein equations at leading order in the $1/N$ expansion for different renormailzation schemes, namely: the traditional one that sets the geometry of the spacetime to be Minkowski at the renormalization point and other schemes (originally proposed in [1,2]) that set the geometry to be that of a fixed de Sitter spacetime. In particular, we study the quantum backreaction for fields in de Sitter spacetimes with masses much smaller than the expansion rate $H$. We find that the scheme that uses the classical de Sitter background solution at the renormalization point, stands out as the most convenient to study the quantum effects on de Sitter spacetimes. We obtain that the backreaction is suppressed by $H^2/M_{\mathrm{pl}}^2$ with no logarithmic enhancement factor of $\ln \lambda$, giving only a small screening of the classical cosmological constant due to the backreaction of such quantum fields. We point out the use of the new schemes can also be more convenient than the traditional one to study quantum effects in other spacetimes relevant for cosmology.

We explore constraints on $(1+1)d$ unitary conformal field theory with an internal ${\mathbb{Z}}_N$ global symmetry, by bounding the lightest symmetry-preserving scalar primary operator using the modular bootstrap. Among the other constraints we have found, we prove the existence of a ${\mathbb{Z}}_N$-symmetric relevant/marginal operator if $N-1\le c\le 9-N$ for $N\le 4$, with the end points saturated by various Wess-Zumino-Witten models that can be embedded into $({\mathfrak{e}}_8{)}_1$. Its existence implies that robust gapless fixed points are not possible in this range of $c$ if only a ${\mathbb{Z}}_N$ symmetry is imposed microscopically. We also obtain stronger, more refined bounds that depend on the ‘t Hooft anomaly of the ${\mathbb{Z}}_N$ symmetry.

We study stationary black holes in the presence of an external strong magnetic field. In the case where the gravitational backreaction of the magnetic field is taken into account, such an scenario is well described by the Ernst-Wild solution to Einstein-Maxwell field equations, representing a charged, stationary black hole immersed in a Melvin magnetic universe. This solution, however, describes a physical situation only in the region close to the black hole. This is due to the following two reasons: First, Melvin spacetime is not asymptotically locally flat; second, the nonstatic Ernst-Wild solution is not even asymptotically Melvin due to the infinite extension of its ergoregion. All this might seem to be an obstruction to address an scenario like this; for instance, it seems to be an obstruction to compute conserved charges as this usually requires a clear notion of asymptotia. Here, we circumvent this obstruction by providing a method to compute the conserved charges of such a black hole by restricting the analysis to the near horizon region. We compute the Wald entropy, the mass, the electric charge, and the angular momentum of stationary black holes in highly magnetized environments from the horizon perspective, finding results in complete agreement with other formalisms.

In this paper, we analyze the causal aspects of evolving marginally trapped surfaces in a $D$ dimensional spherically symmetric spacetime, sourced by perfect fluid with a cosmological constant. The norm of the normal to the marginally trapped tube is shown to be the product of lie derivatives of the expansion parameter of future outgoing null rays along the incoming and outgoing null directions. We obtain a closed form expression for this norm in terms of principal density, pressure, areal radius and cosmological constant. For the case of a homogeneous fluid distribution, we obtain a simple formula for determining the causal nature of the evolving horizons. We obtain the causal phase portraits and highlight the critical radius. We identify many solutions where the causal signature of the marginally trapped tube or marginally anti-trapped tube is always null despite having an evolving area. These solutions do not comply with the standard inner and outer horizon classification for degenerate horizons. We propose an alternate prescription for this classification of these degenerate horizons.

Reducing motion of particles to a two-dimensional potential problem, we show that there are stable circular orbits around a squashed Kaluza-Klein black hole with a spherical horizon and multi–Kaluza-Klein black holes with two spherical horizons in five dimensions. For a single horizon, we show analytically that the radius of an innermost stable circular orbit monotonically depends on the size of an extra dimension. For two horizons, the radius of an innermost stable circular orbit depends on the separation between two black holes besides the size of an extra dimension. More precisely, the set of the stationary points of the potential is composed of two branches. For a large separation, stable circular orbits exist on the two branches regardless of the size of an extra dimension, and in particular, on one branch, the set of stable circular orbits is connected for the small extra dimension but has two disconnected parts for the large extra dimension. For a small separation, only on one branch it exists, and the radius of an innermost stable circular orbit monotonically increases with an extra-dimension size.

We consider the Hawking-Ellis (Segré-Plebański) classification of stress-energy tensors, both in the test-field limit and in the presence of backreaction governed by the usual Einstein equations. For test fields it is not too difficult to get a type IV stress energy via quantum vacuum polarization effects. (For example, consider the Unruh quantum vacuum state for a massless scalar field in the Schwarzschild background.) However, in the presence of backreaction driven by the ordinary Einstein equations, the situation is often much more constrained. For instance: (1) in any static spacetime the stress energy is always type I in the domain of outer communication and on any horizon that might be present; (2) in any stationary axisymmetric spacetime the stress energy is always type I on any horizon that might be present; (3) on any Killing horizon that is extendable to a bifurcation 2-surface the stress energy is always type I; (4) in any stationary axisymmetric spacetime the stress energy is always type I on the axis of symmetry; (5) some of the homogeneous Bianchi cosmologies are guaranteed to be Hawking-Ellis type I (for example, all the Bianchi type I cosmologies, all the Friedmann–Lemaître–Robertson–Walker cosmologies, and all the “single mode” Bianchi cosmologies). That is, in very many physically interesting situations once one includes backreaction the more unusual stress-energy types are automatically excluded.

Recently, Barrow argued that quantum-gravitational effects may introduce intricate, fractal features on the black hole horizon [J. D. Barrow, , The area of a rough black hole, Phys. Lett. B 808, 135643 (2020)]. In this viewpoint, black hole entropy no longer obeys the area law and instead it can be given by $S\sim A^{1+\delta /2}$, where the exponent $\delta$ ranges $0\le \delta \le 1$, and indicates the amount of the quantum-gravitational deformation effects. Based on this, and using the deep connection between gravity and thermodynamics, we disclose the effects of Barrow entropy on the cosmological equations. For this purpose, we start from the first law of thermodynamics, $dE=TdS+WdV$, on the apparent horizon of the Friedmann-Robertson-Walker universe, and derive the corresponding modified Friedmann equations by assuming that the entropy associated with the apparent horizon has the form of Barrow entropy. We also examine the validity of the generalized second law of thermodynamics for the Universe enclosed by the apparent horizon. Finally, we employ the emergence scenario of gravity and extract the modified Friedmann equation in the presence of Barrow entropy which coincides with one obtained from the first law of thermodynamics. When $\delta =0$, the results of standard cosmology are deduced.

Galactic-scale tests have proven to be powerful tools in constraining fundamental physics in previously underexplored regions of parameter space. The astrophysical regime which they probe is inherently complicated, and the inference methods used to make these constraints should be robust to baryonic effects. Previous analyses have assumed simple empirical models for astrophysical noise without detailed calibration or justification. We outline a framework for assessing the reliability of such methods by constructing and testing more advanced baryonic models using cosmological hydrodynamical simulations. As a case study, we use the Horizon-AGN simulation to investigate warping of stellar disks and offsets between gas and stars within galaxies, which are powerful probes of screened fifth forces. We show that the degree of $\mathsf{\text{U}}$-shaped warping of galaxies is well modeled by Gaussian random noise but that the magnitude of the gas-star offset is correlated with the virial radius of the host halo. By incorporating this correlation we confirm recent results ruling out astrophysically relevant Hu-Sawicki $f(R)$ gravity and identify an $\sim 30\%$ systematic uncertainty due to baryonic physics. Such an analysis must be performed case by case for future galactic tests of fundamental physics.

A shorter sound horizon scale at the recombination epoch, arising from introducing extra energy components such as extra radiation or early dark energy (EDE), is a simple approach to resolving the so-called Hubble tension. We compare EDE models, an extra radiation model, and a model in which EDE and extra radiation coexist, paying attention to the fit to big bang nucleosynthesis (BBN). We find that the fit to BBN in EDE models is somewhat poorer than that in the $\mathrm{\Lambda }\mathrm{CDM}$ model, because the increased inferred baryon asymmetry leads to a smaller deuterium abundance. We find that an extra radiation–EDE coexistence model gives the largest present Hubble parameter $H_0$ among the models studied. We also examine the differences between the results obtained with and without consideration of the BBN. The difference in the extra radiation model is $3.22<N_{\mathrm{eff}}<3.49(68\%)$ for data sets without BBN and $3.16<N_{\mathrm{eff}}<3.40(68\%)$ for data sets with BBN, which is so large that the $1\sigma$ border of the larger side becomes the $2\sigma$ border.

This study addresses the collapse behavior of neutron star (NS) mergers expressed through the binary threshold mass $M_{\text{thres}}$ for prompt black-hole (BH) formation, which we determine by relativistic hydrodynamical simulations for a set of 40 equation of state (EOS) models of NS matter. $M_{\text{thres}}$ can be well described by various fit formulas involving stellar parameters of nonrotating NSs, which are employed to characterize the EOS models. Using these relations we compute which constraints on NS radii and the tidal deformability are set by current and future merger detections that reveal information about the merger product. We systematically investigate the impact of the binary mass ratio $q=M_1/M_2$ and assemble various fits, which make different assumptions about a-priori knowledge. This includes fit formulas for $M_{\text{thres}}$ for a fixed mass ratio or a range of $q$ if this parameter is not known very well. Also, we construct relations describing the threshold to prompt collapse for different classes of candidate EOSs, which for instance do or do not include models with a phase transition to quark matter. In particular, we find fit formulas for $M_{\text{thres}}$ including an explicit $q$ dependence, which are valid in a broad range of $0.7\le q\le 1$ and which are nearly as tight as relations for fixed mass ratios. For most EOS models except for some extreme cases the threshold mass of asymmetric mergers is equal or smaller than the one of equal-mass binaries. Generally, the impact of the binary mass asymmetry on $M_{\text{thres}}$ becomes stronger with more extreme mass ratios, while $M_{\text{thres}}$ is approximately constant for small deviations from $q=1$, i.e., for $0.85\le q\le 1$. The magnitude of the reduction of $M_{\text{thres}}$ with the binary mass asymmetry follows a systematic EOS dependence. We also describe in more detail that a phase transition to deconfined quark matter can leave a characteristic imprint on the collapse behavior of NS mergers. The occurrence of quark matter can reduce the stability of the remnant and thus the threshold mass relative to a purely hadronic reference model. Comparing specifically the threshold mass and the combined tidal deformability ${\tilde{\mathrm{\Lambda }}}_{\text{thres}}$ of a system with $M_{\text{thres}}$ can yield peculiar combinations of those two quantities, where $M_{\text{thres}}$ is particularly small in relation to ${\tilde{\mathrm{\Lambda }}}_{\text{thres}}$. Since no purely hadronic EOS can yield such a combination of $M_{\text{thres}}$ and ${\tilde{\mathrm{\Lambda }}}_{\text{thres}}$, a combined measurement or a constraint on both quantities can indicate the onset of quark deconfinement. Finally, we point out new univariate relations between $M_{\text{thres}}$ and stellar properties of high-mass NSs, which can be employed for direct EOS constraints or consistency checks in combination with other measurements.

We review the derivation of the pulsations equations for spherically symmetric boson stars and then make a thorough study of the radial oscillation frequencies for the fundamental and first excited modes. We do this for self-interacting boson stars and consider a range of values for the self-coupling constant. We also numerically evolve boson stars and Fourier transform the dynamic solutions. The Fourier transform gives an independent computation of the radial oscillation frequencies and allows us to verify our results obtained from the pulsation equations. We find excellent agreement between the two methods.

We are approaching a new era to probe the 21-cm neutral hydrogen signal from the period of cosmic dawn. This signal offers a unique window to the virgin Universe, e.g., to study dark matter models with different small-scale behaviors. The EDGES Collaboration recently published the first results of the global 21-cm spectrum. We demonstrate that such a signal can be used to set, unlike most observations concerning dark matter, both lower and upper limits for the mass of dark matter particles. We study the 21-cm signal resulting from a simple warm dark matter model with a sharp-$k$ window function calibrated for high redshifts. We tie the PopIII star formation to Lyman-alpha and radio background production. Using Markov Chain Monte Carlo to sample the parameter space, we find that to match the EDGES signal, a warm dark matter particle must have a mass of ${7.3}_{-3.3}^{+1.6}\mathrm{keV}$ at 68% confidence interval. This translates to ${2.2}_{-1.7}^{+1.4}\times {10}^{-20}\mathrm{eV}$ for fuzzy dark matter and ${63}_{-35}^{+19}\mathrm{keV}$ for Dodelson-Widrow sterile neutrinos. Cold dark matter is unable to reproduce the signal due to its slow structure growth.

Signal estimation in the presence of background noise is a common problem in several scientific disciplines. An “on/off” measurement is performed when the background itself is not known, being estimated from a background control sample. The “frequentist” and Bayesian approaches for signal estimation in on/off measurements are reviewed and compared, focusing on the weakness of the former and on the advantages of the latter in correctly addressing the Poissonian nature of the problem. In this work, we devise a novel reconstruction method, Bayesian analysis including single-event likelihoods (dubbed BASiL), for estimating the signal rate based on the Bayesian formalism. It uses information on event-by-event individual parameters and their distribution for the signal and background population. Events are thereby weighted according to their likelihood of being a signal or a background event and background suppression can be achieved without performing fixed fiducial cuts. Throughout the work, we maintain a general notation that allows us to apply the method generically and provides a performance test using real data and simulations of observations with the MAGIC telescopes, as a demonstration of the performance for Cherenkov telescopes. BASiL allows one to estimate the signal more precisely, avoiding loss of exposure due to signal extraction cuts. We expect its applicability to be straightforward in similar cases.

A nonminimal coupling $\eta$ has been attracting growing interest particularly in the context of inflation models, though its quantum nature is not clear yet. We study the renormalization of $\eta$ in the scalar quantum electrodynamics (QED). There are one-loop diagrams, which could give the finite correction to $\eta$ in the scalar QED. We find that they cancel with each other. We also find no inhomogeneous term of the renormalization group equation (RGE) at the two-loop level, while there could be the two-loop order finite correction. This is similar to other theories, where an inhomogeneous term of the RGE appears only at a higher-loop order: e.g., four-loop order in $\lambda {\varphi }^4$ theory.

Meson decays offer a good opportunity to probe new physics. The rare kaon decay $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ is one of the cleanest of them and, for this reason, is rather sensitive to new physics, in particular, vector mediators. The NA62 Collaboration, running a fixed-target experiment at CERN, recently reported an unprecedented sensitivity to this decay, namely a branching fraction of $\mathrm{BR}(K^{+}\to {\pi }^{+}\nu \overline{\nu })=(11_{-3.5}^{+4.0})\times {10}^{-11}$ at 68% C.L. Vector mediators that couple to neutrinos may yield a sizeable contribution to this decay. Motivated by the new measurement, we interpret this result in the context of a concrete $Z^{\prime }$ model, and put our findings into perspective with the correlated $K_L\to {\pi }^0\nu \overline{\nu }$ decay measured by the KOTO Collaboration, current, and future colliders, namely the High-Luminosity and High-Energy Large Hadron Collider.

Alice and Boojums are both representative characters created by Lewis Carroll. We show that they possibly meet in cores of rotating neutron stars. Recent studies of quark-hadron continuity suggest that neutron superfluid matter can connect smoothly to two-flavor symmetric quark matter at high densities. We study how this can be maintained in the presence of the vortices. In the neutron matter, quantized superfluid vortices arise. In the two-flavor dense quark matter, vortices carrying color magnetic fluxes together with fractionally quantized superfluid circulations appear as the most stable configuration, and we call these as the non-Abelian Alice strings. We show that three integer neutron superfluid vortices and three non-Abelian Alice strings of different color magnetic fluxes with total color flux canceled out are joined at a junction called a Boojum.

In high energy electron-ion colliders, a new way to probe nucleon structure becomes available through diffractive reactions, where the incident particle produces a very energetic almost forward particle. QCD describes these reactions as due to the exchange of a Pomeron which may be perturbatively described as a dressed two-gluon state, provided a hard scale allows the factorization of the amplitude in terms of two impact factors convoluted with a Pomeron propagator. We consider here a process where such a description allows us to access hadronic structure in terms of the generalized parton distributions, namely the electroproduction of a forward $\rho$ meson and a timelike deeply virtual photon, separated by a large rapidity gap. We explore the dependence of the cross section on the kinematic variables and study the dependence on the nonperturbative inputs (generalized parton distributions, distribution amplitude). Our leading order studies show the cross section is mainly sensitive to the GPD model input, but the small size of the cross sections could prohibit straightforward analysis of this process at planned facilities.

The hadronic production of $D$-wave states of $\overline{b}c$ quarkonium is studied. The relative yield of such states is estimated for kinematic conditions of LHC experiments.

We present an $\mathcal{O}(\alpha )$ Standard Model calculation of the inner radiative corrections to Gamow-Teller $\beta$ decays. We find that a priori contributions arise from the photonic vertex correction and $\gamma W$ box diagram. Upon evaluation most elastic contributions vanish due to crossing symmetry or cancellation between isoscalar and isovector photonic contributions, leaving only the polarized parity-odd contribution, i.e., the Gamow-Teller equivalent of the well-known axial $\gamma W$ box contribution for Fermi decays. We show that weak magnetism contributes significantly to the Born amplitude, and consider additional hadronic contributions at low energy using a holomorphic continuation of the polarized Bjorken sum rule constrained by experimental data. We perform the same procedure for the Fermi inner radiative correction through a combination of the running of Bjorken and Gross-Llewellyn Smith sum rules. We discuss heavy flavor, higher-twist, and target mass corrections and find a significant increase at low momentum from the latter. We find ${\mathrm{\Delta }}_R^A=0.02532(22)$ and ${\mathrm{\Delta }}_R^V=0.02473(27)$ for axial and vector inner radiative corrections, respectively, resulting in ${\mathrm{\Delta }}_R^A-{\mathrm{\Delta }}_R^V=0.60(5)\times {10}^{-3}$, which allows us to extract $g_A^0$ for the first time to our knowledge. We discuss consequences for comparing experimental data to lattice calculations in beyond Standard Model fits. Further, we show how some traditional $\beta$ decay calculations contain part of this effect but fail to account for cancellations in the full $\mathcal{O}(\alpha )$ result. Finally, we correct for a double-counting instance in the isospin $T=1/2$ mirror decay extraction of $|V_{ud}|$, the up-down matrix element of the Cabibbo-Kobayashi-Maskawa matrix element, resolving a long-standing tension and leading to increased precision.

We present an analysis of the exclusive $B^{+}\to {\pi }^{+}{\pi }^{-}{\ell }^{+}{\nu }_{\ell }$ decay, where $\ell$ represents an electron or a muon, with the assumption of charge-conjugation symmetry and lepton universality. The study exploits the full $\mathrm{\Upsilon }(4S)$ data sample collected by the Belle detector, corresponding to $711{\mathrm{fb}}^{-1}$ of integrated luminosity. Events are selected by fully reconstructing one $B$ meson in hadronic decay modes, subsequently determining the properties of the other $B$ meson. We extract the signal yields using a binned maximum-likelihood fit to the missing-mass squared distribution in bins of the invariant mass of the two pions or the momentum transfer squared. We measure a total branching fraction of $\mathcal{B}(B^{+}\to {\pi }^{+}{\pi }^{-}{\ell }^{+}{\nu }_{\ell })=[{22.7}_{-1.6}^{+1.9}(\mathrm{stat})\pm 3.5(\mathrm{syst})]\times {10}^{-5}$, where the uncertainties are statistical and systematic, respectively. This result is the first reported measurement of this decay.