We set up a vacuum theory of gravity with an extra dimension of vanishing proper length. The most general solution to the field equations are presented. This formulation is free of Kaluza-Klein modes and does not allow the propagation of gravitons along the invisible fifth direction. Apart from a vacuum energy and radiation, the associated emergent theory exhibits a nonpropagating vector-tensor multiplet which has no analogue in standard Einstein gravity. It is naturally inert, obeys a bounded equation of state and has coupling properties radically different from ordinary matter. Based on these distinctive features, we propose that this geometric multiplet could supercede the hypothetical “dark matter.” As further evidence in support of this possibility, we show that the galactic rotation curves are predicted to be asymptotically flat.

We develop estimators of agreement and disagreement between correlated cosmological data sets. These account for data correlations when computing the significance of both tensions and excess confirmation while remaining statistically optimal. We discuss and thoroughly characterize different approaches commenting on the ones that have the best behavior in practical applications. We complement the calculation of their statistical distribution within the Gaussian model with one estimator that takes non-Gaussianities fully into account. To illustrate the use of our techniques, we apply these estimators to supernovae measurements of the distance-redshift relation, absolutely calibrated by the local distance ladder. The suite of best estimators that we discuss finds results that are in excellent agreement between estimators and find no indications of significant internal inconsistencies in this data set above the 1% probability threshold. This shows the robustness of local determinations of the Hubble constant to features in the distance-redshift relation.

Generation of magnetic fields during inflation is a promising mechanism for the origin of the observed large scale magnetic fields in the universe. Among several attempts, a popular model is one where the inflaton and the electromagnetic field are coupled through a coupling function $f$ leading to a term in the Lagrangian density of the form, $f^2{F}^{\mu \nu }{F}_{\mu \nu }$. A number of potential difficulties with such models have been raised in the literature. In our earlier work, we have suggested viable models of inflationary magnetogenesis which avoid these problems and at the same time can lead to either nonhelical or helical magnetic fields of astrophysical interest. Our models require a low energy scale for inflation and reheating (reheating temperature, $T_R<{10}^4\mathrm{GeV}$) and generate a blue spectrum of electromagnetic (EM) field which peaks around the horizon scale of reheating. We show here that the anisotropic stress associated with these EM fields naturally source the production of a stochastic background of Gravitational waves (GW) with frequencies in the range of tens of nano Hertz to milli Hertz. These two extremes of the range can be probed respectively by pulsar timing arrays (PTA) experiments and the upcoming Laser Interferometric Space Array (LISA). The peak value of the GW spectrum energy represented by $d{\mathrm{\Omega }}_{\mathrm{GW}}/d\ln k$ is ${10}^{-6}$ for the models which lead to nonhelical primordial fields and $2\times {10}^{-6}$ for the helical case for $T_R=100\mathrm{GeV}$. In this case the spectrum peaks at a frequency $30\mu \mathrm{Hz}$ for nonhelical case and at $40\mu \mathrm{Hz}$ for helical case. These values are obtained when the ratio of EM energy density to the cosmological density at reheating $\epsilon \sim 1$ and decrease approximately as ${\epsilon }^2$ for smaller values. The amplitude is similar for a lower value of $T_R$, but the frequency at which the GW spectrum peaks decreases as $T_R$. The gravitational waves generated are unpolarized if the EM fields are nonhelical but are circularly polarized for helical primordial fields. If detected in the future these gravitational waves will provide a unique probe of such models of inflationary magnetogenesis.

We investigate the stability and $e^{+}{e}^{-}$ pair creation of supercritically charged superheavy nuclei, $ud\mathrm{QM}$ nuggets, strangelets, and strangeon nuggets based on the Thomas-Fermi approximation. The model parameters are fixed by reproducing masses and charge properties of these supercritically charged objects reported in earlier publications. It is found that $ud\mathrm{QM}$ nuggets, strangelets, and strangeon nuggets may be more stable than ${}^{56}\mathrm{Fe}$ at the baryon number $A\gtrsim 315$, $5\times {10}^4$, and $1.2\times {10}^8$, respectively. For those stable against neutron emission, the most massive superheavy element has a baryon number $\sim 965$, while $ud\mathrm{QM}$ nuggets, strangelets, and strangeon nuggets need to have baryon numbers larger than 39, 433, and $2.7\times {10}^5$. The $e^{+}{e}^{-}$ pair creation will inevitably start for superheavy nuclei with charge numbers $Z\ge 177$, for $ud\mathrm{QM}$ nuggets with $Z\ge 163$, for strangelets with $Z\ge 192$, and for strangeon nuggets with $Z\ge 212$. A universal relation $Q/R_e=(m_e-{\overline{\mu }}_e)/\alpha$ is obtained at a given electron chemical potential ${\overline{\mu }}_e$, where $Q$ is the total charge and $R_e$ the radius of electron cloud. The maximum number of $Q$ without causing $e^{+}{e}^{-}$ pair creation is then fixed by taking ${\overline{\mu }}_e=-m_e$. For supercritically charged objects with ${\overline{\mu }}_e<-m_e$, the decay rate for $e^{+}{e}^{-}$ pair production is estimated based on the Jeffreys-Wentzel-Kramers-Brillouin (JWKB) approximation. It is found that most positrons are emitted at $t\lesssim {10}^{-15}\mathrm{s}$, while a long lasting positron emission can be observed for large objects with $R\gtrsim 1000\mathrm{fm}$. The emission of positrons and electron-positron annihilation from supercritically charged objects may be partially responsible for the short $\gamma$-ray burst during the merger of binary compact stars, the 511 keV continuum emission, as well as the narrow faint emission lines in x-ray spectra from galaxies and galaxy clusters.

We investigate the possibility of secondary production as the source of cosmic ray (CR) positrons, taking into account observed electron and antiproton spectra and allowing that the reported steepening in the AMS-02 positron spectrum may be due to radiative losses. Using a simple argument based on the antiproton spectrum, we show that positrons cannot be purely secondary. Namely, the antiproton to positron ratio requires that positrons be uncooled up to a few hundred GeV. This requirement implies that CR protons escape from the Galaxy too quickly to reproduce the observed antiproton flux. We further show that this result rules out also more complex secondary production scenarios such as reacceleration and the Nested Leaky Box. Thus, we conclude that, while antiprotons can be produced exclusively in secondary interactions, a primary source of CR positrons—be it dark matter or pulsars—is required to explain their spectrum.

I have demonstrated the existence of a tight correlation between the mass, radius, central density, and pressure of maximum-mass neutron stars modeled using diverse baryonic equations of state. A possible explanation for these correlations is provided. Simple analytic forms of such correlations are suggested and compared with observational constraints on the maximum mass of neutron stars and their radii. This gives a valuable tool to constrain maximum pressure and density that could be reached in stable neutron stars, assuming their equation of state is baryonic.

It is well known that helical magnetic fields undergo a so-called inverse cascade by which their correlation length grows due to the conservation of magnetic helicity in classical ideal magnetohydrodynamics (MHD). At high energies above approximately 10 MeV, however, classical MHD is necessarily extended to chiral MHD and then the conserved quantity is $⟨\mathcal{H}⟩+2⟨{\mu }_5⟩/\lambda$ with $⟨\mathcal{H}⟩$ being the mean magnetic helicity and $⟨{\mu }_5⟩$ being the mean chiral chemical potential of charged fermions. Here, $\lambda$ is a (phenomenological) chiral feedback parameter. In this paper, we study the evolution of the chiral MHD system with the initial condition of nonzero $⟨\mathcal{H}⟩$ and vanishing ${\mu }_5$. We present analytic derivations for the time evolution of $⟨\mathcal{H}⟩$ and $⟨{\mu }_5⟩$ that we compare to a series of laminar and turbulent three-dimensional direct numerical simulations. We find that the late-time evolution of $⟨\mathcal{H}⟩$ depends on the magnetic and kinetic Reynolds numbers ${\mathrm{Re}}_{\mathrm{M}}$ and ${\mathrm{Re}}_{\mathrm{K}}$. For a high ${\mathrm{Re}}_{\mathrm{M}}$ and ${\mathrm{Re}}_{\mathrm{K}}$ where turbulence occurs, $⟨\mathcal{H}⟩$ eventually evolves in the same way as in classical ideal MHD where the inverse correlation length of the helical magnetic field scales with time $t$ as $k_{\mathrm{p}}\propto t^{-2/3}$. For a low Reynolds numbers where the velocity field is negligible, the scaling is changed to $k_{\mathrm{p}}\propto t^{-1/2}\ln (t/t_{\log })$. After being rapidly generated, $⟨{\mu }_5⟩$ always decays together with $k_{\mathrm{p}}$, i.e., $⟨{\mu }_5⟩\approx k_{\mathrm{p}}$, with a time evolution that depends on whether the system is in the limit of low or high Reynolds numbers.

We study the prospect of using TianQin to detect stellar-mass binary black holes (SBBHs). We estimate the expected detection number as well as the precision of parameter estimation on SBBH inspirals, using five different population models. We note TianQin can possibly detect a few SBBH inspirals with signal to noise ratios greater than 12; lowering the threshold and combining multiple detectors can both boost the detection number. The source parameters can be recovered with good precision for most events above the detection threshold. For example, the precision of the merger time most likely occurs near 1 s, making it possible to guide the detection of the ground-based detectors, the precision of the eccentricity $e_0$ most likely occurs near ${10}^{-4}$, making it possible to distinguish the formation channels, and the precision of the mass parameter is better than ${10}^{-6}$ in general and most likely occurs near ${10}^{-7}$. We note, in particular, that for a typical merger event, the error volume is likely to be small enough to contain only the host galaxy, which could greatly help in the study of gravitational wave cosmology and relevant studies through the multimessenger observation.

With a reinterpretation of recent results, the GRACE Follow-On mission can be applied to gravitational wave astronomy. Existing GRACE Follow-On data constrain the stochastic gravitational wave background to ${\mathrm{\Omega }}_{\mathrm{GW}}<3.3\times {10}^7$ at 100 mHz. With a dedicated analysis, GRACE Follow-On may be able to detect the inspiral of local neutron star binaries, inspiral of subgalactic stellar-mass black hole binaries, or mergers of intermediate-mass black hole binaries within the Milky Way.

We revisit the chiral anomaly in the quantum kinetic theory in the Wigner function formalism under the background field approximation. Our results show that the chiral anomaly is actually from the Dirac sea or the vacuum contribution in the un-normal-ordered Wigner function. We also demonstrate that this contribution modifies the chiral kinetic equation for antiparticles.

Nonstandard interactions (NSI) of neutrinos with matter mediated by a scalar field would induce medium-dependent neutrino masses which can modify oscillation probabilities. Generating observable effects requires an ultralight scalar mediator. We derive general expressions for the scalar NSI using techniques of quantum field theory at finite density and temperature, including the long-range force effects, and discuss various limiting cases applicable to the neutrino propagation in different media, such as the Earth, Sun, supernovae and early Universe. We also analyze various terrestrial and space-based experimental constraints, as well as astrophysical and cosmological constraints on these NSI parameters, applicable to either Dirac or Majorana neutrinos. By combining all these constraints, we show that observable scalar NSI effects, although precluded in terrestrial experiments, are still possible in future solar and supernovae neutrino data, and in cosmological observations such as cosmic microwave background and big bang nucleosynthesis data.

Using the general formalism presented in [Phys. Rev. D 94, 013008 (2016); Phys. Rev. D 100, 034511 (2019)], we study the finite-volume effects for the $\mathbf{2}+\mathcal{J}\to \mathbf{2}$ matrix element of an external current coupled to a two-particle state of identical scalars with perturbative interactions. Working in a finite cubic volume with periodicity $L$, we derive a $1/L$ expansion of the matrix element through $\mathcal{O}(1/L^5)$ and find that it is governed by two universal current-dependent parameters, the scalar charge and the threshold two-particle form factor. We confirm the result through a numerical study of the general formalism and additionally through an independent perturbative calculation. We further demonstrate a consistency with the Feynman-Hellmann theorem, which can be used to relate the $1/L$ expansions of the ground-state energy and matrix element. The latter gives a simple insight into why the leading volume corrections to the matrix element have the same scaling as those in the energy, $1/L^3$, in contradiction to Phys. Rev. D 91, 074509 (2015), which found a $1/L^2$ contribution to the matrix element. We show here that such a term arises at intermediate stages in the perturbative calculation, but cancels in the final result.

We study three-body decays of ${\mathrm{\Lambda }}_c^{*}(2765)\to {\mathrm{\Lambda }}_c^{+}{\pi }^{+}{\pi }^{-}$ by using effective Lagrangians in a nonrelativistic framework. We consider the sequential decays through ${\mathrm{\Sigma }}_c(2455)\pi$ and ${\mathrm{\Sigma }}_c^{*}(2520)\pi$ in intermediate states which are dominant contributions. The coupling constants in the effective Lagrangians are computed in the quark model. We demonstrate that the ratio $R=\mathrm{\Gamma }({\mathrm{\Lambda }}_c^{*}\to {\mathrm{\Sigma }}_c^{*}(2520)\pi )/\mathrm{\Gamma }({\mathrm{\Lambda }}_c^{*}\to {\mathrm{\Sigma }}_c(2455)\pi )$ and angular correlations are sensitive to the spin and parity of ${\mathrm{\Lambda }}_c^{*}(2765)$. Thus, the measurement of these observables in experimental facilities such as Belle and LHCb can provide useful constraints to determine the spin and parity of ${\mathrm{\Lambda }}_c^{*}(2765)$.

We discuss the general properties of the $B$-meson decay form factors $F(q^2,q^{\prime 2})$, describing $B$-meson decays induced by two currents, e.g., $B$ decays into four leptons in the final state. We study the analytic properties of these complicated objects and identify those regions of $q^2$ and $q^{\prime 2}$, where perturbative QCD can be applied for obtaining predictions for the physical form factors.

We study and reveal the relation between the electroweak sphaleron energy and the gravitational wave signals from a first-order electroweak phase transition. For the first time, we find that a larger sphaleron energy corresponds to a stronger gravitational wave signal for a sufficiently strong phase transition. We also present the correlation between the sphaleron and the properties of the gravitational waves, such as the signal-to-noise ratio, peak frequency, and amplitude, with the intention that traces of the sphaleron can be obtained with detailed measurement and studies of the stochastic gravitational wave at future space-based detectors. This study paves the way for a more practical revelation of the sphaleron which is otherwise unprobable at high energy colliders.

The adiabatic regularization method was originally proposed by Parker and Fulling to renormalize the energy-momentum tensor of scalar fields in expanding universes. It can be extended to renormalize the electric current induced by quantized scalar fields in a time-varying electric background. This can be done in a way consistent with gravity if the vector potential is considered as a variable of adiabatic order one. Assuming this, we further extend the method to deal with Dirac fields in four spacetime dimensions. This requires a self-consistent ansatz for the adiabatic expansion, in presence of a prescribed time-dependent electric field, which is different from the conventional expansion used for scalar fields. Our proposal is consistent, in the massless limit, with the conformal anomaly. We also provide evidence that our proposed adiabatic expansion for the fermionic modes parallels the Schwinger-DeWitt adiabatic expansion of the two-point function. We give the renormalized expression of the electric current and analyze, using numerical and analytical tools, the pair production induced by a Sauter-type electric pulse. We also analyze the scaling properties of the current for a large field strength.

In $CP$-violating conformal field theories in four dimensions, the Pontryagin density can appear in the Weyl anomaly. The Pontryagin density in the Weyl anomaly is consistent, but it has a peculiar feature that the parent three-point function of the energy-momentum tensor can violate $CP$ only (semi)locally. In this paper, we study the supersymmetric completion of the Pontryagin density in the Weyl anomaly, where the central charge $c$ effectively becomes a complex number. The supersymmetry suggests that it accompanies the graviphoton $\theta$ term associated with the R-symmetry gauging in the Weyl anomaly. It also accompanies new $CP$-violating terms in the R-current anomaly. While there are no conclusive perturbative examples of $CP$-violating super Weyl anomaly, we construct explicit supersymmetric dilaton effective action which generates these anomalies.

It has been argued that the symmetries of gravity at null infinity should include a $\mathrm{Diff}(S^2)$ factor associated with diffeomorphisms on the celestial sphere. However, the standard phase space of gravity does not support the action of such transformations. Building on earlier work by Laddha and one of the authors, we present an extension of the phase space of gravity at null infinity on which $\mathrm{Diff}(S^2)$ acts canonically. The Poisson brackets of supertranslation and $\mathrm{Diff}(S^2)$ charges reproduce the generalized BMS algebra introduced in [Campiglia and Laddha Phys. Rev. D 90, 124028 (2014)].

We consider the observation of stellar-mass black holes binaries with the Laser Interferometer Space Antenna (LISA). Preliminary results based on Fisher information matrix analyses have suggested that gravitational waves from those sources could be very sensitive to possible deviations from the theory of general relativity and from the strong equivalence principle during the low-frequency binary inspiral. We perform a full Markov Chain Monte Carlo Bayesian analysis to quantify the sensitivity of these signals to two phenomenological modifications of general relativity, namely a putative gravitational dipole emission and a nonzero mass for the graviton, properly accounting for the detector’s response. Moreover, we consider a scenario where those sources could be observed also with Earth-based detectors, which should measure the coalescence time with precision better than 1 ms. This constraint on the coalescence time further improves the bounds that we can set on those phenomenological deviations from general relativity. We show that tests of dipole radiation and the graviton’s mass should improve respectively by seven and half an order(s) of magnitude over current bounds. Finally, we discuss under which conditions one may claim the detection of a modification to general relativity.

We derive the equations of motion of a test particle in the equatorial plane around a static and spherically symmetric wormhole influenced by a radiation field including the general relativistic Poynting-Robertson effect. From the analysis of this dynamical system, we develop a diagnostic to distinguish a black hole from a wormhole, which can be timely supported by several and different observational data. This procedure is based on the possibility of having some wormhole metrics, which smoothly connect to the Schwarzschild metric in a small transition surface layer very close to the black hole event horizon. To detect such a metric change, we analyze the emission proprieties from the critical hypersurface (stable region where radiation and gravitational fields balance) together with those from an accretion disk in the Schwarzschild spacetime toward a distant observer. Indeed, if the observational data are well fitted within such a model, it immediately implies the existence of a black hole; while in the case of strong departures from such a description it means that a wormhole could be present. Finally, we discuss our results and draw conclusions.