The Gravity From Entropy (GfE) action posits that gravity is fundamentally given by the information encoded in the interplay between matter and geometry. The GfE Lagrangian is given by the Geometric Quantum Relative Entropy (GQRE) between the physical metric and the metric induced by matter and curvature, leading to modified gravitational field equations with an emergent dynamical effective dark energy term, which reduce to Einstein’s equations in the low-energy, small-curvature limit. Adopting a thermodynamic viewpoint, we identify the GfE energy density with this emergent effective dark energy term. For homogeneous and isotropic Friedmann-Robertson-Walker spacetimes, we show that GfE universes admit a thermal description: locally, they are characterized by $k$-temperatures and $k$-pressures satisfying a first law of GfE thermodynamics. In the low-energy, small-curvature regime with perfect-fluid matter and radiation, GfE solutions are well approximated by Friedmann cosmologies. While the total GQRE per unit volume does not increase, the total entropy of GfE universes is nondecreasing in time. We show that, while the total GQRE per unit volume does not increase, consistent with its nature as a relative entropy, the total entropy of GfE universes is nondecreasing in time. These results provide a thermodynamic interpretation of GfE cosmologies and of general relativity itself, recovered in the low-energy, small-curvature limit of the theory, offering a framework to reconcile local order and complexity with the global increase of entropy in the Universe.

We formulate a first-order thermodynamic description of Jordan-frame tensor-multiscalar gravity. From the Einstein-like field equations we obtain the exact covariant $1+3$ decomposition of the geometric sector and write it as an effective imperfect fluid. The thermodynamic interpretation is understood in the first-order Eckart sense: the effective temperature, conductivity, entropy current, and entropy production are meaningful only on branches where the geometric dissipative variables satisfy the appropriate matching and integrability conditions. In a generic frame, the heat flux admits the exact decomposition $q_a^{(g)}=-\chi (a_a+W_a)$, with $\chi =-\dot{\mathcal{F}}/(8\pi \mathcal{F})$ and with $W_a$ encoding the residual temperature-gradient sector. In the $\mathcal{F}$-comoving frame this gives the inertial variable ${\chi }_{\mathcal{F}}\equiv K_{\mathcal{F}}{T}_{\mathcal{F}}$ together with a generally nonvanishing spatial contribution $W_a^{(\mathcal{F})}$, sourced by scalar directions not aligned with the coupling. Thus the multifield thermal description is not generically reducible to a single $KT$-type quantity. We derive transport equations for ${\chi }_{\mathcal{F}}$, for the field-space thermal vector ${\chi }^A$ and covector ${\chi }_A$, and for the residual gradient sector. We also introduce the diagnostics ${\mathfrak{D}}_{\chi }={\chi }_A{\chi }^A$ and ${\mathfrak{D}}_{\mathrm{grad}}={\mathcal{B}}_{AB}{\mathrm{D}}_a^{(\mathcal{F})}{\varphi }^A{\mathrm{D}}_{(\mathcal{F})}^a{\varphi }^B$. Their interpretation depends on the kinetic matrix ${\mathcal{B}}_{AB}$: they are canonical contractions when ${\mathcal{B}}_{AB}$ is nondegenerate, but become non-negative normlike diagnostics only when ${\mathcal{B}}_{AB}$ is positive definite; if ${\mathcal{B}}_{AB}$ is degenerate, no model-independent inverse field-space metric or full scalar norm is available without extra structure. With this qualification, these diagnostics show that freezing the effective coupling is, in general, weaker than full relaxation to the GR sector. Finally, we construct the entropy current and entropy production in the coupling frame, state the assumptions required for non-negative entropy production, and show that homogeneous cosmology suppresses the spatial sector while retaining nontrivial timelike multiscalar thermal dynamics.

The precise calculation of gravitational waves (GWs) from cosmic string networks is of significant theoretical and experimental interest. The Nambu–Goto (NG) approximation has long been employed to calculate GW emission from such networks; however, its validity has never been systematically verified. We perform large-scale zero-temperature Abelian–Higgs lattice simulations under different gauge couplings and compare them with NG predictions. We find excellent agreement in the power-law region for near-global strings but strong deviation for strongly coupled local strings with $m_v/m_s\sim 1$, quantitatively establishing the breakdown of the NG approximation. Additionally, we confirm that particle emission significantly dominates the energy loss of the string network, with the ratio of GW energy to particle energy approximately ${10}^{-3}$ to ${10}^{-2}$ for both near-global and local string scenarios.

In this paper, we explain the recently reported a nHz-band gravitational-wave background from NANOGrav 15-year through the merger of binary super-massive black holes with masses of ${10}^9{M}_{\odot }$ formed by the growth of primordial black holes. When a primordial black hole accretes at a high accretion rate, it emits a large number of high-energy photons. These heat the plasma, causing high-redshift cosmological 21 cm line emission. Since this has not been detected, there is a strict upper bound on the accretion rate. We have found that with the primordial black hole abundance ${10}^{-14}\lesssim f_{\mathrm{PBH}}\lesssim {10}^{-12}$ and the mass $1M_{\odot }\lesssim m_{\mathrm{PBH}}\lesssim {10}^3{M}_{\odot }$, we successfully fit the nHz band gravitational wave background from NANOGrav 15-year while avoiding the 21 cm line emission. In addition, we also discuss the implication for the origins of the little red dots. We propose that future observations of the gravitational wave background and the cosmological 21 cm line can test this scenario.

Dark sectors consisting of atomic constituents (electrons, protons, and photons) offer a well-motivated extension to the Standard Model while providing multiple avenues for phenomenological study. In this work, we explore the impact of conversions between the dark and Standard Model photons in the primordial cosmic microwave background (CMB) spectral distortion epoch (${10}^3\lesssim z\lesssim {10}^6$). These conversions are resonantly enhanced when the induced thermal masses of both photonic species are equal, thus leading to the possibility that sizeable distortions can be produced. To this end, we solve the Boltzmann equation at early times to determine the (irreducible) freeze-in or freeze-out abundance of dark photons. This procedure also allows us to update the limits on generic millicharged dark sectors using the ACT DR6 bound on the number of effective radiative degrees of freedom ($N_{\mathrm{eff}}$). By then modeling the evolution of the thermal masses in both sectors, we compute the primordial CMB distortion using the Landau-Zener formalism. We find that when the dark electron and proton are roughly similar in mass (the positronium limit), current spectral distortion data from the COBE/FIRAS instrument is able to rule out novel regions of parameter space. We also forecast bounds from the proposed FOSSIL satellite, finding that spectral distortions can also be used to probe the ultralow dark electric charge regions of parameter space, which are difficult to investigate by other means.

We introduce a new class of interacting dark sector models that couple the intrinsic entropy of dark matter to scalar field dark energy. Using the Lagrangian formulation for relativistic perfect fluids, we construct consistent covariant actions that incorporate algebraic and derivative entropy couplings. These interactions leave the expansion history unchanged, rendering the background cosmology indistinguishable from $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) or uncoupled quintessence. At the level of cosmological perturbations, the entropy couplings generate scale-dependent modifications to the dark matter Euler equation, while the continuity equation remains unaltered at linear order. The resulting interactions correspond to a pure-momentum exchange within the dark sector. We show that intrinsic entropy perturbations can carry primordial scale dependence, and nonminimal couplings can lead to a scale-dependent suppression or enhancement of structure growth. Finally, we demonstrate that these models are generically compatible with current cosmic microwave background observations, while inducing distinctive signatures in large-scale structure. The framework provides a theoretically well-motivated and observationally viable extension to the standard cosmological model, opening new directions to explore novel interactions in the dark sector.

We investigate the cosmological information in higher-order statistics of the cosmic microwave background (CMB) lensing convergence field for a near-term experiment with noise properties similar to the Simons Observatory (SO). Using a fully field-level forward-modeling pipeline based on ray-traced simulations from the massivenus suite and realistic SO-like CMB lensing reconstruction, we naturally include nonlinear structure formation, post-Born effects, and higher-order reconstruction noise. We measure several non-Gaussian statistics, including Minkowski functionals, peak and minima counts, moments, and wavelet-scattering coefficients. We train Gaussian-process emulators to model each statistic’s dependence on the matter density fraction ${\mathrm{\Omega }}_m$, the scalar power spectrum amplitude $A_s$, and the neutrino mass sum $M_{\nu }$. We quantify the relative information gain these statistics provide beyond the lensing power spectrum and identify which are most robust to reconstruction noise. We find that morphology-based statistics, particularly Minkowski functionals and peak/minima counts, offer significant complementary constraining power: combining all non-Gaussian statistics with the power spectrum yields reductions of 40% and 38% in the marginalized uncertainties on ${\mathrm{\Omega }}_m$ and $A_s$, respectively, and a 70% reduction in the one-sided uncertainty on $M_{\nu }$. These gains remain non-negligible even when the power spectrum is extended to larger scales and combined with primary CMB and BAO data, with Minkowski functionals providing an additional 11% improvement in $\sigma (M_{\nu })$ and 35% in $\sigma ({\mathrm{\Omega }}_m)$ beyond the extended power spectrum. By contrast, moments and wavelet-scattering coefficients provide more limited gains at SO noise levels. Our results highlight the potential of non-Gaussian statistics to enhance cosmological constraints from SO and future CMB surveys.

Finding intermediate-mass black holes (IMBHs) and measuring their masses and spins are key to understanding massive black hole formation. White dwarf (WD)-IMBH binaries provide a unique probe because they emit both electromagnetic radiation and gravitational waves (GWs), thereby conveying richer information. However, such multimessenger sources often enter the regime of strong gravity, where existing models often fail to capture their relativistic dynamics. Here, we develop a fully relativistic model for the tidal response of a WD close to an IMBH, and use it to study the secular orbital evolution as well as the GW signal. We find that for IMBHs more massive than ${10}^5{M}_{\odot }$, tidal interaction becomes relativistic and sensitive to IMBH spin. The interaction generally dissipates binary orbital energy and angular momentum, but due to relativistic frame rotation, which reduces phase coherence across pericenter passages, the orbit-averaged tidal dissipation rate can be suppressed by up to $\sim 50\%$ relative to Newtonian predictions. Including tidal dissipation leads to more rapid damping of the orbital eccentricity, to the extent that the pericenter distance may even increase over time, potentially explaining quasiperiodic eruptions and secular orbital period growth. Such tidal effect accumulates into measurable phase and amplitude deviations in the GW signal. For typical space-based observations, the GW waveform mismatch can reach $\mathcal{O}(0.1)$ within six months. Our results indicate that relativistic tidal dissipation is both dynamically important and observationally essential for reliably predicting the multimessenger signals of WD-IMBH systems.

We estimate the spins of a sample of 58 low-mass AGNs. Analysis of the obtained spins shows that they decrease with increasing SMBH mass, leading us to hypothesize that mergers and/or chaotic accretion are the primary mechanisms for mass growth. In this regard, we propose a more general hypothesis about the evolution of AGNs. We assume that early low-mass SMBHs have high spins; then, during their evolution, the spins initially decrease and then begin to increase, with the rate of increase gradually slowing.

We investigate the gravitational wave spectrum from first-order QCD phase transitions using the parity doublet model at finite baryon chemical potential. The model incorporates the chiral invariant mass $m_0$, representing the portion of nucleon mass that persists even when chiral symmetry is restored. Within the model, we identify two first-order phase transition regions: the nuclear liquid-gas transition and the chiral phase transition. By solving the bounce equation and computing the Euclidean action $S_3/T$, we obtain the gravitational wave spectra from both transitions. The liquid-gas transition yields transition strength $\alpha \sim \mathcal{O}(1)$ and inverse duration of the phase transition $\beta /H\sim \mathcal{O}(10)–\mathcal{O}(100)$ near the end point of the first-order line, producing signals with peak frequencies from the millihertz to the nanohertz band that can fit the existing data. In contrast, the chiral transition produces signals suppressed by approximately five orders of magnitude, well below the sensitivity of all current and planned detectors. These results establish, within the parity doublet model, a quantitative link between first-order QCD phase transitions and their gravitational wave signatures, and provide a concrete framework for relating gravitational wave observations to the chiral invariant mass $m_0$ and thus to the origin of nucleon mass.

Inflationary phase transitions can generate a stochastic gravitational-wave background that probes primordial physics. We study the detectability and parameter reconstruction of such a signal with a space-based gravitational-wave detector. Using a Taiji-like mission as a benchmark, we construct a realistic data-analysis framework that includes instrumental noise, astrophysical foregrounds and backgrounds, and the $A$, $E$, and $T$ time-delay interferometry channels. The target signal is described in a minimal, model-independent form and analyzed using both Fisher-matrix forecasts and Bayesian inference with nested sampling. We quantify detection significance and parameter-recovery thresholds, showing that, while detection is achievable at moderate signal-to-noise ratios, stronger signals provide more reliable parameter reconstruction. These results offer a realistic assessment of the capability of future space-based missions to probe inflationary phase transitions through stochastic gravitational radiation.

We extend shapelets for the analysis of astronomical images to be available in a phase space, introducing Wigner function shapelets (WFSs). Whereas conventional shapelets expand images separately in configuration or Fourier space using Hermite-Gaussian or Laguerre-Gaussian modes, WFSs represent images directly in the four-dimensional phase space with symplectic group $\mathrm{Sp}(4,\mathbb{R})$, which is quantized by a phase-space cell $2\pi ƛ$ that determines a resolution limit of a telescope. WFSs consist of a bilinear form of the cross-Wigner function of the Laguerre-Gaussian modes as an orthogonal and complete basis for the Wigner function of an image, carrying out SU(2) irreducible representations of the phase space with the Hopf tori. We introduce a scalar function ${\mathcal{W}}_{k\ell }(Q_0,Q_2)$ from the $\mathrm{U}(1)\times \mathrm{U}(1)$ covariant tori to a two-dimensional space of constants of motion $(Q_0,Q_2)$—the harmonic energy and axial angular momentum—thereby yielding a natural phase-space “band structure,” given a pair of winding numbers $(k,\ell )\in {\mathbb{Z}}^2$. The WFS leverages key properties of the Wigner function for image analysis: (i) it encodes full information of an image in a symmetry-preserving way, (ii) its transport equation naturally evolves with a Liouville equation at $ƛ\to 0$, (iii) it admits positive/negative oscillatory patterns on the $(Q_0,Q_2)$ plane that can be a sensitive spatial coherent structure of galaxy morphology and cosmological imprints, and (iv) systematics and noise can be manipulated as a quantum channel operation. This paper aims to bring together all the formulae related to the Wigner function in the context of astrophysics and cosmology, formally organizing them in terminologies of both astronomy and quantum information theory.

Recent measurements of diffuse sub-PeV $\gamma$ rays by the Tibet ${\mathrm{AS}}_{\gamma }$ and Large High Altitude Air Shower Observatory (LHAASO) collaborations have reshaped our understanding of the $\gamma$-ray sky. Besides uncovering the nature of “PeVatrons,” these measurements can also be used to probe the nongravitational nature of dark matter. PeV-scale decaying dark matter can produce high-energy $\gamma$ rays in the final state and contribute to the measurements made by extensive air-shower detectors such as Tibet ${\mathrm{AS}}_{\gamma }$ and LHAASO. Using the latest Tibet ${\mathrm{AS}}_{\gamma }$ upper limits on diffuse $\gamma$ rays away from the Galactic plane and the LHAASO square kilometer array (LHAASO-KM2A) measurements of diffuse $\gamma$ rays from the Galactic plane, we put stringent constraints on lifetimes of decaying dark matter for masses $\sim {10}^6–{10}^9\mathrm{GeV}$. Future observations of high-energy diffuse $\gamma$-ray emission can thus provide stronger limits or potentially discover heavy decaying dark matter.

Test-mass thermal noise is a limiting noise source for current and next-generation ground-based gravitational-wave observatories in their most sensitive frequency band. In addition to ongoing efforts to reduce coating mechanical loss, complementary approaches such as the use of uniform-intensity laser beams, including higher-order Laguerre–Gaussian (LG) and Hermite–Gaussian (HG) modes, have been proposed as alternatives to the fundamental Gaussian beam due to their thermal-noise advantages. As the interferometer operating power increases toward the megawatt regime, as anticipated for next-generation detectors, thermal aberrations arising from absorption in the high-reflectivity coatings of the test masses become an increasingly severe challenge. In parallel, the spatially more extended intensity profiles of higher-order modes, which enable their thermal-noise benefits, modify the thermal loading of the test masses, making it essential to assess their behavior under absorption-induced self-heating. In this work, we quantify the robustness of higher-order incident beams against thermal deformation. We show that, under identical operating conditions, higher-order modes produce significantly more uniform thermally induced mirror distortions compared to the sharply peaked aberrations generated by the fundamental mode. As a result, substantially less thermal compensation power is required for their active correction, with the required optimal curvature correction reduced to 33% of that of the fundamental mode for the ${\mathrm{LG}}_{2,2}$ mode and to 24% for the ${\mathrm{HG}}_{3,3}$ mode. In addition, we show that the residual thermal deformation for higher-order modes leads to significantly lower optical power loss and enables larger power buildup and enhanced modal purity in an aLIGO-like optical cavity. We further demonstrate that astigmatism compensation enhances the intracavity modal purity of HG modes under self-heating-induced thermal deformation. These results highlight that higher-order laser modes mitigate thermal noise while intrinsically suppressing beam self-heating-induced thermal distortions, making them more thermally robust and well suited for operation in high-power gravitational-wave interferometers.

We examine scattering amplitudes for an arbitrary number of photons in a class of nonnull background electromagnetic fields, studying tree-level and one-loop amplitudes in scalar and spinor quantum-electrodynamics in backgrounds defined by a gauge field $A_{\mu }(\mathfrak{n}·x)$ for ${\mathfrak{n}}^2\ne 0$. Motivated to account for more physically realistic laser-plasma dispersive properties, our approach overcomes prior work studying such amplitudes in a constant background field and relaxes the familiar null criterion assumed for plane waves. Master formulas for the $N$-photon amplitudes dressed by the nonnull background are constructed using the first-quantized worldline formalism, which can systematically account for all orders in the nonnull parameter, ${\mathfrak{n}}^2$, treated here as an expansion parameter. These are derived from worldline representations of the coordinate and momentum space propagators (and their LSZ-truncated amplitudes) and the effective action, each incorporating the nonnull background nonperturbatively. We then outline a partial resummation of their expansions in ${\mathfrak{n}}^2$. A special exactly solvable case of nonnull constant crossed fields without photon insertion in the effective action is explored to test the master formulas that result. The validity of the presented master formulas is further checked against known expressions for the wave function and nonlinear Compton scattering in a nonnull background to lowest order in the nonnull parameter.

We investigate Nelson-Barr (NB) solutions to the strong $CP$ problem in which spontaneous $CP$ violation is transmitted to the Standard Model (SM) through mixing with a vectorlike partner of the SM quark doublet. We show that these constructions constitute compelling and phenomenologically viable alternatives to the more widely studied singlet-based NB models. A key result of our analysis is that an accidental symmetry of the renormalizable theory delays the leading contributions to $\overline{\theta }$ until three loops, naturally suppressing hadronic $CP$ violation. We outline the main phenomenological constraints, including future electric dipole moment experiments, as well as the main differences between these scenarios and generic models with doublet vectorlike quarks.

Asymmetric dark matter is a well-motivated approach to explain the apparent coincidence between the relic densities of visible and dark matter, ${\mathrm{\Omega }}_D\simeq 5.4{\mathrm{\Omega }}_b$. A complete explanation requires two components, a relation between the particle masses of the dark and visible matter, and a second relation between the number densities in each sector. In this work, we propose a new mechanism to address the former. We consider an extended $SU(3{)}_1\times SU(3{)}_2$ color group in the visible sector, with QCD embedded as the diagonal subgroup. A ${\mathbb{Z}}_2$ exchange symmetry then relates $SU(3{)}_2$ to a dark, confining $SU(3{)}_D$ sector. The dark matter is a composite state of dark fermions transforming in the fundamental representation of $SU(3{)}_D$. The spontaneously broken ${\mathbb{Z}}_2$ symmetry ultimately leads to a relation between the QCD and dark gauge couplings which, for suitable field content, gives rise to confinement scales of the same order of magnitude. The mechanism leads to a rich particle spectrum above the TeV scale which could be probed at future experiments. The model also naturally includes an axion solution to the strong $CP$ problem.

We propose the nuclear interferometer—a single-photon interferometry experiment based upon the thorium-229 nuclear clock transition—as a novel detector for ultralight dark matter. Thanks to the enhanced sensitivity of this transition to the variation of fundamental constants, we find that possible realizations of such an experiment deploying either single ions or clouds of atoms have the potential to complement advanced very-long-baseline terrestrial clock atom interferometers in the search for ultralight dark matter with scalar couplings to photons in the future. Nuclear interferometry may also offer an unparalleled window to new physics coupling to the QCD sector via quarks or gluons, with a discovery reach that could enhance existing and proposed experiments over a range of frequencies in the direction of well-motivated parameter space.

Ultralight dark matter can behave as a coherent background field and induce time-dependent modifications of Standard Model parameters. We study a scenario in which a real ultralight scalar $\varphi$ couples off-diagonally to down-type quarks, linking ultralight dark sectors to flavor physics. Working within an effective field theory, we diagonalize the quark mass matrix in a coherent $\varphi$ background and derive analytic expressions for oscillatory shifts in down-type quark masses and Cabibbo-Kobayashi-Maskawa parameters. These effects lead to signatures in both the classical regime, where $\varphi$ acts as a background field, and the quantum (particle) regime, where it contributes through on-shell production or off-shell mediation. Using precision flavor measurements, nuclear $\beta$ decays, atomic-clock data, compiled NANOGrav-derived limits, and meson observables, we derive constraints on the flavor-violating couplings ${\lambda }_{ij}$ for $m_{\varphi }\sim {10}^{-24}–{10}^{-12}\mathrm{eV}$, highlighting the complementarity of time-domain and flavor probes of ultralight dark sectors.