The standard $\mathrm{\Lambda }\mathrm{CDM}$ model, despite its success, is challenged by persistent observational tensions in the Hubble constant ($H_0$) and the matter clustering amplitude ($S_8$), motivating the exploration of alternative cosmological scenarios. We investigate a dark energy model with a phenomenological interaction in the dark sector, constructed to be exactly degenerate at the background level with the Chevallier-Polarski-Linder (CPL) parametrization. This setup allows us to test whether models with identical expansion histories but distinct physical mechanisms can be distinguished by cosmological data. We perform a Bayesian analysis using a combination of recent datasets: Dark Energy Spectroscopic Instrument data release 2 baryon acoustic oscillation measurements, Dark Energy Survey year 5 supernovae, and cosmic microwave background data from Planck and Atacama Cosmology Telescope. We find that both the interacting model and the CPL model provide significantly better fits to the data than $\mathrm{\Lambda }\mathrm{CDM}$. Although indistinguishable in background observables, the interacting model predicts a distinct matter-sector evolution driven by a late-time sign change in the dark sector interaction at $z\approx 0.8$, corresponding to the $w=-1$ crossing in the CPL description. In this sense, the interacting picture may be considered more physical, since it avoids the problematic crossing by construction. The resulting decay of dark energy into dark matter lowers $S_8$, potentially alleviating the weak-lensing $S_8$ tension. At the same time, it predicts a sharp suppression of the growth rate $f{\sigma }_8(z)$ at $z\lesssim 0.8$, which is in tension with current measurements of structure formation. This indicates that the model may not simultaneously reconcile the expansion history and the observed growth of cosmic structure, highlighting the need for a more comprehensive analysis to fully assess its viability.

Neutral hydrogen (HI) 21-cm intensity mapping is an effective method to track the distribution of baryonic matter, and extract astrophysical and cosmological information. The 21-cm intensity field has a nonvanishing cross-correlation with the kinetic Sunyaev-Zel’dovich (kSZ) effect that traces the velocity and density perturbations of free electrons. By using the linear perturbation theory, in this paper we calculate analytically, for the first time, the cross-correlation between the squared kSZ field and the projection of the squared HI intensity mapping field with the flat-sky approximation. This statistic remains nonvanishing even after the long-wavelength line-of-sight modes ($k_{\parallel }$) are removed due to foreground contamination. We further forecast for the prospects of detection with the SKA-MID 21-cm intensity mapping experiments (redshifts in range of $0.3<z<1$), and the kSZ maps measured by the Atacama Cosmology Telescope (ACT) and Simons Observatory (SO). The predicted cumulative signal-to-noise ratio is 1.92 for SKA-ACT and 3.99 for SKA-SO. These results show a possible on-the-edge detection on the cross-correlation signal at low redshifts, which in turn could serve as a validation step toward using it for the Epoch of Reionization studies.

We model anisotropic neutron stars using three distinct prescriptions for pressure anisotropy—the Horvat, Bowers-Liang, and covariant models—and three equations of state with different particle compositions, each described by a piecewise-polytropic parametrization with continuous sound speed. The stability of these configurations is assessed through their dynamical evolution using a fully nonlinear relativistic code. For stable configurations, we compute the oscillation spectrum and identify the fundamental mode frequency. We found that, while the isotropic and Horvat models become unstable close to the maximum-mass point, the Bowers-Liang and covariant models become unstable at lower central densities, indicating that the standard turning-point criterion may not reliably predict the onset of dynamical instability in anisotropic stars. Based on our results, we also determine the neutral-stability line and verify that configurations lying to the right of this line are indeed unstable under radial perturbations and collapse. Overall, given an equation of state, pressure anisotropy can increase the maximum mass of an stable configuration by up to $\sim 30\%$ compared to the isotropic case. It also allows for more compact stable configurations that may collapse on longer timescales once they become unstable. Finally, we show that these compact stars could initially mimic a black hole’s gravitational-wave ringdown. However, the production of subsequent echoes is not guaranteed by high compactness; instead, it depends critically on the star’s specific internal structure and equation of state.

Recently James Webb Space Telescope (JWST) have observed an excess of luminous galaxies at high redshifts ($z\gtrsim 10$). In this work, we investigate whether supermassive primordial black holes (SMPBHs) can explain it by their influence on the ultraviolet luminosity function (UV LF) of high-redshift galaxies. Through Markov chain Monte Carlo analysis, we constrain the parameters relevant with SMPBHs against current JWST observational data. The results reveal that SMPBHs with masses $M_{\mathrm{PBH}}\sim {10}^{6.3–8.3}{M}_{\odot }$, abundances $f_{\mathrm{PBH}}\sim {10}^{-7}–{10}^{-5}$, and sub-Eddington ratios ${\lambda }_E\ll 1$ can effectively enhance the bright end of the UV LF, consistent with JWST observations.

It was shown that pair luminosity of the newborn strange star with temperature of ${10}^{11}\mathrm{K}$ may be as high as $L_{\pm }\simeq {10}^{52}\mathrm{erg}/\mathrm{s}$. The question remains: can a strange star maintain such a high surface temperature for a long time? To answer this question we studied thermal evolution of newborn strange star made of unpaired quarks taking into account its thermal conductivity and neutrino emission by the URCA processes $d\to u+e+{\overline{\nu }}_e$ and $u+e\to d+{\nu }_e$. Our results show that extremely high luminosity due to the Schwinger process and insufficient thermal conductivity of quarks leads to development of steep temperature gradient at the surface of strange star. As a result, the temperature at the surface and hence its luminosity decreases, reaching ${10}^{43}\mathrm{erg}/\mathrm{s}$ already at ${10}^2$ seconds. This result holds even in the presence of the neutrinosphere.

We investigate the formation of plasma cavities in unmagnetized relativistic pair-plasma shocks and demonstrate that these cavities emerge naturally as a nonlinear consequence of an asymmetric Weibel instability. We provide an intuitive physical picture and a minimal fluid model that captures the essential features of this cavitation instability and compare them with particle-in-cell (PIC) simulation results. The cavitation instability constitutes the initial nonlinear stage that, according to PIC simulations, leads to the development of larger-scale magnetic structures in relativistic shocks.

Supermassive black hole binaries (SMBHBs) are among the most powerful known sources of gravitational waves (GWs). Accordingly, these systems could dominate GW emission in the micro- and millihertz frequency range. Within this domain, SMBHs evolve rapidly and merge with each other. Dynamical friction from stars and gas at the centers of galaxies typically helps to bring together two SMBHs when they are at relatively far separations ($\approx \mathrm{kpc}-100\mathrm{pc}$), but becomes less efficient at smaller separations. However, dark matter (DM) spikes around SMBHs could enhance dynamical friction at close separations and, thus, shorten the evolution times. In this paper, we simulate the effects of DM spikes on GW signals in the micro- to millihertz frequency range and confirm that the GW signals from SMBHBs with DM spikes can be clearly distinguished from those without any additional matter. Making use of the projected sensitivity curve of the Laser Interferometer Space Antenna, we forecast upper limits for the (dark) matter density for given future SMBHB observations. We then compare these thresholds with the theoretical density profiles expected for self-interacting dark matter spikes.

Intermediate mass ratio inspirals (IMRIs) of binary black holes with mass ratios ${10}^{-4}\lesssim q\lesssim 0.1$ are astrophysically interesting sources of gravitational waves. Mergers of intermediate-mass black holes (IMBHs) with stellar-mass black holes would be IMRIs, so their detection can help us probe the formation mechanisms of IMBHs. They can also help us perform precise tests of general relativity due to the presence of strong higher-order mode emission. We perform a search for aligned-spin IMRIs within the data of the two LIGO detectors in the third observing run (O3) of the LIGO-Virgo-KAGRA (LVK) Collaboration, including higher modes in the template banks for the first time. We use the IAS-HM pipeline for our search and construct template banks in the range $1/100<q<1/18$ using the SEOBNRv5HM waveform model. Our banks retain a similar level of effectualness for IMRPhenomXHM and BHPTNRSur2dq1e3 waveforms, making our search results relatively robust against waveform systematics. We show that the sensitivity volume of the search increases by up to $\sim 500\%$ upon inclusion of higher modes. We do not find any significant candidates with inverse false alarm rate (IFAR) $>1\mathrm{year}$ in the O3 data. This gives us upper limits on the IMRI merger rate in the local Universe, ranging from $\sim 30$ to ${10}^3{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ depending on the masses of the black holes in the binary. These constraints are consistent with rate predictions in the literature. Our projections indicate that we would be able to detect IMRIs or constrain some of their proposed formation channels in the fourth (O4) and fifth (O5) observing runs.

We propose a Michelson-type interferometric protocol for testing the quantum nature of gravity, in the sense of experimentally probing whether gravity can consistently remain classical when coupled to quantum systems. This is achieved by testing the phenomenology of semiclassical gravity theory in the nonrelativistic limit, which predicts a state-dependent Schrödinger-Newton (SN) evolution of the test mass. The protocol exploits the asymmetry of the SN self-gravity term to generate crosstalk between the common and differential motion of the test masses. This crosstalk can be detected via correlation measurements of the outgoing light field, providing a clean binary signature. Our results show that, when assisted by 10 dB squeezed input states, three hours of aggregated measurement data suffice to achieve an SNR sufficient to conclusively test the SN theory in a 1 Kelvin environment. These findings demonstrate a relatively strong feasibility of using interferometric protocols to experimentally probe whether gravity can remain classical or must exhibit quantum features.

The infrared properties of QED are investigated within the framework of the Dyson-Schwinger equations. Our study finds that, independently of the value of the coupling constant, requiring the photon self-energy to be finite for any momenta, combined with a smooth behavior for the photon-fermion vertex, is equivalent to stating that the photon is massless and that the photon propagator diverges at low momenta as $1/k^2$. Furthermore, the Schwinger mechanism to generate, in a gauge invariant way, a photon mass is investigated and the form factors that can be at the origin of a possible photon mass are identified. For the Schwinger mechanism the link between the finiteness of the photon self-energy and the masslessness of the photon is lost. The infrared behavior of the fermion gap equation and the vertex equation are found to be infrared safe integral equations.

We explore exclusive scalar meson electroproduction off a scalar target in the ($3+1$)-dimensional scalar field model. This model analysis is a straightforward extension of the previous ($1+1$)-dimensional model analysis presented in Choi et al. [Phys. Rev. D 105, 096014 (2022)]. In contrast to the ($1+1$)-dimensional model, the ($3+1$)-dimensional model allows us to compute the beam spin asymmetry (BSA), which is proportional to the imaginary part of the product of the two Compton form factors (CFFs) that appear in the hadronic current of the present scalar meson electroproduction process. We compute both real and imaginary parts of the CFFs and note that the BSA is detectable for $-t/Q^2\gtrsim 0.1$, although it gets quite small in the kinematic region $-t/Q^2\ll 0.1$, where the factorization of the generalized parton distribution (GPD) is attainable. We find the analytic forms of the leading-twist GPD for the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) and Efremov-Radyushkin-Brodsky-Lepage (ERBL) regions in the ($3+1$)-dimensional scalar field model, confirming its uniqueness independent of the hadronic current component. While we verify that the GPD sum rule for the total result of summing the DGLAP and ERBL regions holds for all components of the hadronic current, we note that the respective correspondence of the DGLAP and ERBL regions to the valence and nonvalence parts of the electromagnetic form factor holds only for the light-front plus component of the hadronic current but not for any other components of the hadronic current. We discuss the polynomiality of the GPD up to the second moments and remark on accessible ranges of kinematics to measure the BSA and CFFs with respect to the future experimental efforts of extracting the leading-twist GPDs.

We establish comprehensive theoretical benchmarks for weakly interacting massive particles (WIMPs) accessible to ultimate direct detection experiments, focusing on the challenging parameter space between current experimental limits and the irreducible neutrino background. We systematically examine both thermal freeze-out and freeze-in production mechanisms across a range of simplified dark matter models, including $s$-channel scalar and vector portals, $t$-channel mediator scenarios, and electroweakly interacting multiplets. For thermal relics, we identify parameter regions where suppressed direct detection cross sections naturally arise through momentum-dependent interactions and blind-spot configurations, while maintaining the correct relic abundance. We extensively investigate freeze-in scenarios, demonstrating how feebly interacting massive particles in portal models can populate experimentally accessible parameter space despite their ultraweak couplings. Additionally, we explore how nonstandard cosmological histories—including early matter domination and fast-expanding Universe scenarios—can dramatically alter the relationship between relic density and detection prospects, opening new avenues for discovery. Our analysis provides a roadmap for next-generation experiments approaching the neutrino floor, highlighting complementary detection strategies and identifying the most promising theoretical targets for ultimate sensitivity dark matter searches. These benchmarks establish the theoretical foundation for the final push toward comprehensive coverage of well-motivated WIMP parameter space.

We present a new dark matter candidate, the inelastically decoupling relic (iELDER), which is a cold thermal relic whose abundance is determined by the freeze out of its inelastic scattering off of bath particles in the presence of $3\to 2$ self-annihilations. The dark matter is predicted to be light, in the $\mathcal{O}(\mathrm{MeV}-\mathrm{GeV})$ range, with significant self-annihilations and very weak inelastic couplings to ordinary matter. We demonstrate iELDER dark matter using a $Z_3$-symmetric toy model as well as quantum chromodynamics (QCD)-like pion theories—the latter showing promising prospects for detection and providing a new benchmark for future searches.

We investigate the interactions of the deuteron with light mesons during the hadronic phase in heavy-ion collisions. We treat the deuteron as a weakly bound state and employ the quasifree approximation to describe the $d\pi$ interaction. The underlying elementary $N\pi$ amplitudes are described by a hybrid effective model, combining the nonresonant background from chiral perturbation theory with resonant contributions via Breit-Wigner parametrizations. These amplitudes are used to calculate the vacuum and thermally averaged cross sections for deuteron dissociation and production, namely, $d+\pi \to N+N^{\prime }+\pi$ and the corresponding inverse reaction. We then use these cross sections in a rate equation to estimate the time evolution of the deuteron multiplicity. For the initial conditions, we consider two models: the statistical hadronization model and the coalescence model, where the deuteron is treated as a hadronic molecule. Our findings suggest that the final deuteron yield does not retain a memory of its initial production mechanism.

Radio phoenixes are extended filamentary sources in the intracluster medium (ICM) of galaxy clusters, associated with fossil radio lobes. Their soft, concave-curved spectra, commonly attributed to aged relativistic electrons recently accelerated/compressed, are shown to approach a power-law at high frequencies. Moreover, secondary $e^{\pm }$ produced by a pure power-law spectrum of relativistic ions explain all phoenix observations with only three free parameters. Hadronic phoenixes require strong magnetization and, for high-curvature, also $e^{\pm }$ heating, by a factor $\gtrsim 15$ if at ICM pressure.

I derive a new charge constraint in $\frac{1}{16}$-Bogomol’nyi-Prasad-Sommerfield sectors of the Berenstein, Maldacena, and Nastase supersymmetric, gauged $U(N)$ matrix quantum mechanics. It was previously shown that the supersymmetric index of the model exhibits an exponential dependence on $N^2$ from the vacuum that preserves the full gauge group. I show that the reality of the associated entropy implies that the Cartan of $SO(3)$ vanishes for any $\frac{1}{16}$-Bogomol’nyi-Prasad-Sommerfield sector with $U(N)$ gauge symmetry. The Berenstein-Maldacena-Nastase charge constraint could be useful to determine whether there exists a dual supersymmetric black hole solution asymptoting to a plane-wave background.

Among the available perturbative approaches in quantum field theory, heat kernel techniques provide a powerful and geometrically transparent framework for computing effective actions in nontrivial backgrounds. In this work, resummation patterns within the heat kernel expansion are examined as a means of systematically extracting nonperturbative information. Building upon previous results for Yukawa interactions and scalar quantum electrodynamics, we extend the analysis to spinor fields, demonstrating that a recently conjectured resummation structure continues to hold. The resulting formulation yields a compact expression that resums invariants constructed from the electromagnetic tensor and its spinorial couplings, while preserving agreement with known proper-time coefficients. Beyond its immediate computational utility, the framework offers a unified perspective on the emergence of nonperturbative effects (such as Schwinger pair creation) in relation to perturbative heat kernel data, and provides a basis for future extensions to curved spacetimes and non-Abelian gauge theories.

Dual-phase xenon time projection chambers achieve optimal sensitivity for dark matter in the 10 to $1000{\mathrm{GeVc}}^{-2}$ mass range, but sub-GeV dark matter (DM) particles lack sufficient energy to produce nuclear recoils above detection thresholds in these detectors. Blazar-boosted dark matter offers a way to overcome this limitation. Relativistic jets in active galactic nuclei can accelerate light dark matter in their host-galaxy halos to energies that can leave detectable nuclear recoil signals in xenon-based detectors on Earth. We present the first blazar-boosted dark matter search that incorporates detector response modeling, using public data from XENON1T and LZ for the blazar TXS $0506+056$. We model dark matter-proton scattering in the jet environment, covering the full process from jet acceleration through to detector response. We explore how the host galaxy dark matter density profile impacts our analysis. We set model-dependent exclusion regions on the dark-matter–nucleon scattering cross section for $m_{\chi }\simeq 1\mathrm{MeV}{\text{c}}^{-2}$ dark matter, between $5.8\times {10}^{-31}{\mathrm{cm}}^2$ and $6.3\times {10}^{-29}{\mathrm{cm}}^2$ using XENON1T data, and between $9.9\times {10}^{-32}{\mathrm{cm}}^2$ and $2.5\times {10}^{-28}{\mathrm{cm}}^2$ from LZ effective field theory dark matter searches. Our results show that astrophysical uncertainties, especially those in the dark-matter distribution near the supermassive black hole, are the main limitation of this search rather than detector effects. The limits are therefore model-dependent and should be seen as exploratory. They highlight both the potential and the present uncertainties of blazar-boosted dark matter as a probe of light dark matter.

We investigate the unique potential of a high-energy muon collider to probe lepton-flavor-violating signals arising from physics beyond the Standard Model (SM). Low-energy, precision searches for charged lepton flavor violation (LFV) are projected to dramatically improve their sensitivity in the coming years and could provide the first evidence of new physics. We interpret the sensitivity of these searches in terms of a set of LFV operators in the SM effective field theory. The same operators are then probed at the TeV scale via new, high-energy processes only available at a high-energy muon collider, such as $\mu \mu \to \mu \tau$ or the scattering of a muon of an electroweak gauge boson into LFV final states. We find that, for most operators, a muon collider could confirm signals if they are seen at future low-energy experiments, whereas for certain flavor combinations it extends the reach to scales well beyond those accessible at lower energies. We also project the sensitivity of a muon collider to lepton-flavor-violating decays of the SM Higgs boson and demonstrate improved sensitivity to $h\to e\tau$ and $h\to \mu \tau$ by an order of magnitude compared to the High-Luminosity LHC. The importance of having multiple, complementary probes is illustrated by considering both various combinations of operators and relative sizes of flavor-violating transitions between generations under various assumptions for the flavor structure of new physics.

In conventional relativistic quantum field theory, the discrete operators $C$, $P$, and $T$ are matrix operators with no renormalization scale dependence. However, in a Lorentz-violating theory with a fermion $f^{\mu }$ term in the action, these operators may acquire nontrivial renormalization group behavior. Because the $f^{\mu }$ term may actually be exchanged in the action for an equivalent $c^{\nu \mu }$ term, the scale dependence depends explicitly on the renormalization scheme, even at one-loop order. The scheme dependence means it is always possible to set the scale dependence parameter $1-X$ to zero, but for analyses of some high-energy electron-photon processes, using a scheme with $X=0$—and thus definite scale dependences for $C$, $P$, and $T$—may nonetheless be more convenient.