78,338 papers in this slice of arXiv.
Ariel Zhitnitsky
Transient star-like objects of unknown origin have been identified in the first Palomar Observatory Sky Survey (POSS-1) as part of the Vanishing and Appearing Sources during a Century of Observations (VASCO) project. The source of the transients recorded by POSS-1 remains unknown, which is the warrant to coin the observed phenomena as Mysterious Transients (MT). We advocate an idea that the dark matter (DM) in form of the axion quark nuggets (AQN) made of standard model quarks (or antiquarks) and gluons, similar to the old idea of the Witten's strangelets, could simultaneously explain all the observed MT signals (including very short time scale for flash itself, association with nuclear test timing, observed alignments of several MT events, correlation with UAP reports, etc) collected or recorded for many years. Essentially we argue that the MT is a cousin of Ball Lightning (BL) events, also observed for centuries, without commonly accepted physics explanation. The basic parameters of this model (such as the typical baryon charge of the nuggets) had been fixed long ago by explaining the observed excess of radiation at variety of scales: from galactic to the solar, to local Earth's environments. In this work we use the same framework with the same set of parameters to study the observed MT phenomena. We also suggest several tests which substantiate or refute our proposal. We also present some suggestions on type of instruments required to study this specific (and well defined) type of the UAP events representing the cousins of BL and MT events in the AQN framework.
Meng-Xiang Lin, Adam Lidz, Chung-Pei Ma
Shot-noise anisotropies in the nHz gravitational wave background (GWB) are a promising target for pulsar timing arrays (PTAs). If the nHz GWB is sourced by merging supermassive black hole binaries (SMBHBs), as current evidence suggests, the shot-noise signal is expected to be large, potentially of order unity at observing frequencies of f∼1yr−1. In this regime, the signal is dominated by rare bright binaries, and Poisson fluctuations in the discrete SMBHB population produce significant spatial anisotropies. Here, we use Monte Carlo simulations to model the realization-to-realization scatter in the shot-noise, sampling from empirically calibrated models of the SMBHB source populations. We find that the probability distribution of shot-noise amplitudes is broad, spanning a factor of ∼50 (95% interval) at fixed frequency, with a long tail towards high amplitudes. The most probable and median amplitudes lie significantly below the ensemble means by factors of ∼2−3, implying that the shot-noise in typical realizations is smaller than the mean. The ensemble-averaged shot-noise also differs from simple estimates based on moments of the strain, ⟨h4⟩/⟨h2⟩2, because the average of a ratio is not equal to the ratio of the averages (i.e., ⟨X/Y⟩=⟨X⟩/⟨Y⟩). This difference is a factor of ∼3 at f=0.1yr−1, growing to larger than two orders of magnitude by f∼1yr−1, where the GWB is dominated by low abundance, high-strain sources. Shot-noise nevertheless provides a powerful diagnostic for understanding the GWB and SMBHB populations; interpreting PTA measurements, however, requires modeling its full probability distribution.
Aurora Ireland
Working to second order in cosmological perturbation theory, we reconsider the Large Scale White Noise (LSWN) effect proposed in [2511.13866] and [2511.15803]. We demonstrate that the kurvature density variable Δρ does indeed develop LSWN at second order. However, contrary to the claims of [2511.15803], we show that this is not inherited as an infrared (IR) pole in the second-order comoving curvature perturbation R2. This apparent enhancement was an artifact of using a linear Poisson equation to relate two genuinely second order quantities. Further, we show that R2 is protected from developing any such IR enhancement: limk→0(k2R2)=0, which follows from a conservation law in the soft limit. The constraint proposed in [2511.15803] and its implications for the small-scale primordial power spectrum are therefore invalid.
D. Giataganas, G. F. Giudice, A. Kehagias +2
When a black hole rings after a merger, it emits gravitational waves at characteristic frequencies known as quasinormal modes (QNMs). In the eikonal limit, these modes are governed by the unstable circular light orbits that form the photon ring. In this work, we demonstrate that the ringing of a black hole has a precise thermal interpretation. A probe string propagating in the near-ring geometry acquires an induced Rindler horizon on its worldsheet, with a temperature set by the Lyapunov exponent of the photon ring. Out of this structure, the black hole QNMs emerge as thermal excitations, so that the characteristic ringing of a black hole is the retarded response of a thermal system living on the photon ring. We explicitly derive the QNM spectrum from two complementary perspectives: microscopically, via unstable transverse worldsheet fluctuations, and macroscopically, through the pole structure of the causal response function of an open thermal quantum system.
S. D. Odintsov, V. K. Oikonomou
We study the adiabaticity of the cosmological perturbations in the context of inflationary Einstein-Gauss-Bonnet theories. We focus on viable inflationary Einstein-Gauss-Bonnet theories which are compatible with the current Cosmic Microwave Background radiation experiments and also are compatible with the GW170817 observations. We derive the effects of the adiabaticity requirement on the Einstein-Gauss-Bonnet physical parameters and we show that the sound speed of the scalar perturbations and the propagation speed of the tensor perturbations are constrained. We consider two classes of inflationary viable and GW170817-compatible theories, and in the first class the adiabaticity is not violated during inflation, while in the second class the adiabaticity is violated only at the end of inflation. We discuss the effects of the adiabaticity violation in the second class of models. From our analysis, it seems that only one class of viable EGB inflationary theories, which is also compatible with the GW170817 event, is free from adiabaticity pathologies.
Wayne Hu
Kurvature, a recently identified curvature invariant, has been argued to acquire superhorizon, or large-scale, white noise from hard-hard momentum coupling even when matter nonlinearities are small. If kurvature were related directly to cosmological curvature perturbations R through a Poisson equation, this white noise would cause an infrared-divergent variance sensitive to ultraviolet hard-mode physics. However, this relation does not generically hold. Kurvature is not intrinsic 3-curvature: on comoving slices it contains extrinsic-curvature terms, and intrinsic curvature is not related to R by a Poisson equation beyond linear order. We test this inference with second-order perturbation theory in radiation domination, relevant to CMB observables. Quadratic hard-hard composites do generate large-scale white noise in the kurvature density, but the Hamiltonian constraint separates it into intrinsic curvature and extrinsic shear, or equivalently density and expansion. Only the extrinsic terms carry the growing dimensionless kurvature density that mimics an ordinary density fluctuation above the horizon. The direct hard-hard curvature power is ultraviolet convergent, dominated by horizon-scale modes at evaluation, and leaves no IR relic in R from purely ultraviolet modes. By contrast, the Poisson construction of a curvature potential from kurvature is infrared divergent and cutoff sensitive; its white noise arises from extrinsic curvature associated with nonlinear acoustic beat modes in a radiation fluid.
Li-E Qiang, Peng Xu
Compact spatial topology restricts the eigenmodes of primordial tensor perturbations, and the resulting discreteness can render the primordial stochastic gravitational-wave background (SGWB) anisotropic. Here we treat the CMB tensor B-mode covariance as a transfer-filtered measurement of that ultra-low-frequency anisotropy. Writing the normalized angular tensor-power measure as F(k,k^)=1+Q(k,k^) and its nonmonopole moments as qLM(k), we obtain an explicit kernel that maps qLM(k) onto the off-diagonal covariance δCℓm,ℓ′m′BB. The kernel factorizes into tensor transfer functions and a spin-weighted Gaunt coefficient and obeys the parity rule L+ℓ+ℓ′ even for BB and odd for TB/EB. It is an exact source--response representation of the full compact covariance rather than an additional observable. For a cubic three-torus the geometry pins down a common cubic angular subspace and orientation across frequency bands, although the amplitudes of the allowed multipoles still depend on the radial shell and source spectrum. The same topology-restricted template can therefore be read out either through the CMB B-mode kernel or through the anisotropy response of PTA/LISA/Taiji/TianQin searches. Using CAMB transfer functions and an invariant anisotropic-template statistic, we contrast this tensor channel with the scalar T/E covariance. Independent direct angular-shell sums and qLM--Gaunt contractions agree at Lqmax=2ℓmax to relative Frobenius residuals of 1.4×10−14--3.0×10−14. The scalar sector holds most of the practical CMB topology information; a fixed-template scan places the combined full-sky S/N=1 transition between L/χ∗=2.34 and 2.36, while the B-mode channel remains subthreshold but isolates the primordial SGWB contribution.
Jens Chluba, Atsuhisa Ota, Nicola Bartolo
In this paper, we evaluate the precise distortion source and transfer functions caused by mixing of blackbodies of different temperatures using the recently developed frequency hierarchy (FH) treatment of CosmoTherm. With this we are able to evaluate the effects of primordial non-Gaussianity (PNG) on the μT, μE, yT and yE cross-power spectra including the coupled spectro-spatial evolution and important photon-transport effects retained by the FH treatment. For local-type PNG, we compare our results with those from previous works, illustrating new aspects that were previously not captured. We then demonstrate how the μT and μE signals change in the presence of enhanced curvature perturbations at small scales. For a nearly scale-invariant primordial spectrum, our results agree broadly with previous estimates on large angular scales but exhibit additional small-scale damping and modified y-distortion correlations arising from the scale dependence of the heating source and the distinct transport of distortion perturbations. Tight-coupling and monopole-source approximations accurately reproduce the μ-distortion spectra, while the y-distortion signals remain more sensitive to the detailed source evolution. For enhanced small-scale power, anisotropies generated by propagation of the distorted average spectrum can become comparable to those from anisotropic dissipation. Their different angular dependences and parameter scalings principally allow the small-scale power amplitude and primordial non-Gaussianity to be constrained separately. The results presented here thus pave the path for studying PNG in new regimes using existing and upcoming high precision CMB anisotropy data to measure primordial distortion correlations.
Ava Shahbazi Sooraki, Ahmad Sheykhi
We investigate Big-Bang Nucleosynthesis (BBN) in the context of Yukawa cosmology. We first derive the modified Friedmann equations by starting from the first law of thermodynamics on the apparent horizon. Using observational data on 4He, deuterium, and 7Li abundances, we place stringent bounds on the Yukawa coupling α. The 4He and deuterium constraints are mutually consistent (−0.24≲α≲0.12), while 7Li requires α∈[−0.76,−0.72]. This indicate that Yukawa cosmology cannot resolve the Lithium Problem. We then extend our analysis to WIMP freeze-out, and show that the modified Hubble parameter alters the relic abundance, yielding an independent constraint −0.017≲α≲0.018 from ΩCDMh2=0.120±0.001. We also derive the modified time-temperature relation, and show that the positive α raises the early Universe temperature. Our analysis demonstrates that BBN and dark matter relic abundance serve as complementary probes of modified gravity. Our studies confirm that Yukawa cosmology is a testable framework for early-Universe physics.
Qihong Huang, Yuchen Zhang, Bing Xu +1
Based on the fractional entropy from fractional quantum mechanics, fractional holographic dark energy (FHDE) has been proposed with the Hubble horizon as the IR cutoff (FHDEH). We extend this framework by adopting the future event horizon and the particle horizon as the IR cutoff, proposing the FHDEF and FHDEP models. Using the SN+OHD+DESI DR2 dataset to constrain these models, we find that all three models provide a marginally lower χmin2 compared to ΛCDM but without significant preference according to AIC and BIC. When CMB distance priors are included, the FHDEH and FHDEP models are strongly ruled out. We further analyze the cosmological evolution for these models, and find that only the FHDEF model predicts nearly identical evolutions of Ωm and Ωde to those of the ΛCDM model across cosmic history, but its deceleration parameter q deviate from the ΛCDM model in the future, indicating richer late time dynamics beyond the standard ΛCDM cosmology.
Haomin Rao, Yunlong Zheng, Qi-Zhe Hou +2
Parity violation in the gravitational and electromagnetic sectors has been extensively investigated, yet the two are conventionally treated as independent phenomena. This separation, however, may be a four-dimensional prejudice. In higher-dimensional spacetime, gravity and electromagnetism may share a common geometric origin---and so, perhaps, does their parity violation. In this paper, we pursue this idea by constructing parity-violating Kaluza-Klein models in both Riemannian and teleparallel geometries. In Riemannian geometry, the simplest five-dimensional parity-violating term reduces to several complicated four-dimensional terms, including the familiar gravitational Chern-Simons term, which suffers from a ghost instability. In teleparallel geometry, however, the result is strikingly simple. The simplest five-dimensional parity-violating term reduces to only two ghost-free terms---the familiar Nieh-Yan term and the standard electromagnetic Chern-Simons term. Remarkably, the model predicts that the helicity-dependent dispersion shift for electromagnetic waves is exactly six times that for gravitational waves, ΔωEM2=6ΔωGW2, a background-independent relation. This relation offers a falsifiable test of unification through joint cosmic microwave background and gravitational wave birefringence observations.
Shifan Zuo, Xuelei Chen, Yi Mao
We introduce a native tensor-based framework for foreground mitigation in 21 cm intensity mapping (IM), utilizing the Oriented Singular Value Decomposition (O-SVD) algorithm. While 21 cm IM is a powerful probe of the large-scale structure of the Universe, its efficacy is severely limited by astrophysical foregrounds that are orders of magnitude brighter than the cosmological signal. Traditional mitigation strategies often necessitate flattening multidimensional data cubes into two-dimensional matrices, a process that potentially compromises the intrinsic spatial-spectral correlations by treating distinct spatial pixels as independent samples. By treating multi-frequency sky maps and angular power spectra as third-order tensors, the O-SVD method performs decomposition directly on the multilinear manifold, preserving the underlying physical topology and leveraging the distinct coherence properties of astrophysical foregrounds across different dimensions. We demonstrate the performance and versatility of the O-SVD framework through its application to high-fidelity simulations from the SKA Science Data Challenge 3a (SDC3a) and real-world observational data from the Tianlai Cylinder Pathfinder Array. Our results indicate that O-SVD provides a robust and universal approach for foreground subtraction, achieving high-fidelity signal recovery while offering superior performance compared to conventional matrix-based Singular Value Decomposition (SVD) methods.
Youcai Zhang, Xiaohu Yang, Hong Guo +1
Cosmic voids provide a unique environment for studying the relationship between galaxies, subhaloes, and dark matter in the underdense Universe. Using the SDSS galaxy catalogue and the ELUCID constrained simulation, we establish an observationally anchored framework for measuring multi-tracer mass bias within matched cosmic voids. A sample of 102 matched void pairs is constructed to directly compare galaxy, subhalo, and dark matter mass distributions within an observationally constrained realisation of the local Universe. We find that both the galaxy-to-dark matter and subhalo-to-dark matter mass ratios decrease toward void centres, indicating that luminous and halo tracers become increasingly depleted relative to the underlying matter distribution in the deepest underdensities. In contrast, the galaxy-to-subhalo mass ratio exhibits substantially larger statistical uncertainties within the inner void regions (r/Rv≲0.5). By comparing measurements obtained using independent and common coordinate frameworks, we show that coordinate offsets contribute to the observed scatter but cannot fully account for the large uncertainties. The remaining uncertainty primarily arises from the severe scarcity of massive subhaloes (log10(Msub/h−1M⊙)≥11.8) within void interiors, which greatly reduces the number of statistically valid measurements near void centres. Our results provide a direct measurement of multi-tracer mass bias in observationally constrained cosmic environments and highlight the fundamental statistical limitations of multi-tracer studies in extreme underdense regions.
Dao-Hong Zhai, F. Y. Wang, Zi-Gao Dai +1
Traditional astronomical censuses in the late-time Universe can only account for a fraction of the baryonic matter budget. Hydrodynamical simulations predict that the missing baryons reside in the vast filamentary structures of the cosmic web as a highly diffuse, warm-hot intergalactic medium (WHIM). Observing the WHIM directly has remained a long-standing challenge due to its typical temperature. In this study, we report the first detection of spatial cross-correlations between the dispersion measures (DMs) of fast radio bursts (FRBs) from the second CHIME/FRB catalog and the thermal Sunyaev-Zel'dovich (tSZ) Compton-y map from the Planck satellite. By masking virialized galaxy clusters to isolate the diffuse signal, we find a positive correlation with a probability >99.77% between FRBs and tSZ maps. Our joint parameter inference constrains the fraction of cosmic baryons in the WHIM to be fWHIM=0.48 with a 68% confidence interval of 0.27<fWHIM<0.61, anchored at a mean WHIM temperature of 2.4×106 K. More rigorous masking strategies confirm the signal originates from the WHIM instead of galaxy clusters. Our result demonstrates that the missing baryons are residing in the diffuse gas within the cosmic web, closing the cosmic baryon budget in the local Universe.
S. -H. Henry Tye
A natural extension of the successful D3-Dˉ3-brane inflation model in string theory is one with two D3-Dˉ3-brane pairs, giving rise to a novel feature in string-theory. When the first pair annihilates, D1-strings and F1-strings are produced. Following the non-commutative geometric properties in string theory, it is shown by Myers that the presence of the D3-charge field strength of the remaining D3-Dˉ3-brane pair leads to the formation of dielectric 3-branes -- neutral, finite-size bound states of 3-branes with D1-strings. They behave as matter with little or no pressure. Spanning a wide range of masses, they seed primordial black holes in the early universe.
M. Fernández-Torreiro, J. A. Rubiño-Martín, R. T. Génova-Santos +16
We present the commissioning and first results of the Thirty and Forty Gigahertz Instrument (TFGI), which observes the sky at 31 and 41 GHz with angular resolutions of 21' and 18' from the second QUIJOTE telescope at the Teide Observatory. Its primary goal is to conduct a deep cosmological survey in selected regions of the Northern sky with high-sensitivity polarization measurements. The commissioning phase covered Nov2021-Oct2022, during which the instrument operated with a configuration of 7 receivers, 4 at 31 GHz and 3 at 41 GHz. Over this period, approximately 1200 h of data were acquired. Of these, 380 h were dedicated to calibration sources, used to characterize the instrumental properties of TFGI, including the pointing model, beam response, gain stability, polarimetric performance, and instantaneous sensitivity. We provide a detailed characterization of these properties and describe how they are being improved for future observing runs. We use 230 h of observations from bright Galactic regions (Cygnus, W43, W44, and W47) to further validate the instrument performance. As an illustrative example, we present the intensity and polarization spectral energy distributions of W44, finding good agreement with existing measurements. From the noise map of these observations, we measure a polarization sensitivity of 8.3 μK deg−1 after an effective observing depth of 0.57 h deg−2. This performance, achieved considering only 2 detectors at 31 GHz, is already comparable to that achieved by WMAP (with almost 3 times the integration time per unit area, 1.61 h deg−2). Extrapolating these results to the full TFGI array, with up to 29 detectors, we show that the instrument is expected to reach the target sensitivity of 1 μK deg−1 at both 31 and 41 GHz over three cosmological fields covering a total area of 3600 deg2 after an effective integration time of 5.7 years.
Ke Wang, Jia-Yi Feng, Jianbo Lu
In this paper, to reconstruct the equation of state (EOS) of dark energy (DE) w(z) with the redshift binning method, we first introduce a wiCDM model with a piecewise-constant EOS in 7 redshift bins. Then, we turn to the Learning the Universe Implicit Likelihood Inference (LtU-ILI) pipeline to perform a multi-round ILI of wi from the cosmological data combination, including TT, TE, EE and lensing power spectra of Planck 2018, distance ratios of DESI DR2 and corrected apparent magnitudes of SNIa from Pantheon+ sample. More precisely, we build the Cosmic Microwave Background (CMB) power spectrum, Baryon Acoustic Oscillation (BAO) distance ratio and Type Ia Supernovae (SNIa) apparent magnitude simulators by CLASS and embed them into the LtU-ILI pipeline. And, using Sequential Neural Likelihood Estimation (SNLE), we sequentially train neural networks with 6 rounds of total 6×20000 simulations to target a ``black box'' likelihood of our forward model wiCDM. Finally, with the estimated posteriors of wi, we find that except for the unconstrained w5 and w6 (the last two bins), our reconstruction of w(z) marginally favors dynamical DE in the first bin and is consistent with the cosmological constant at 68% C.L. in the other bins.
Rayne Liu, Wayne Hu, Daniel Grin
If the Peccei-Quinn symmetry of an ultralight axion is broken before the end of inflation, axion quantum fluctuations seed isocurvature perturbations, linking them to the tensor-to-scalar ratio r. We extend the effective time average (ETA) approach of the Boltzmann code AxiECAMB to accurately evolve these perturbations across the full axion mass range from dark energy (ma≲H0) to dark matter (ma≫10−28 eV) types. We provide analytic fitting formulae for the axion abundance given the initial field value φini, accurate at sub-percent level for ma≫H0 and allowed dark matter fraction fdm. In the dark matter regime, the Planck bound on CDM isocurvature requires r fdm<0.08(ma/10−27eV)−1/2, which becomes stronger than the current BICEP tensor bound for mafdm2≳10−26.4eV. For 10−32≲ma/eV≲10−28, Jeans suppression breaks the degeneracy with CDM isocurvature, leaving unique signatures, and in the dark energy regime (ma≲H0), the isocurvature signal is even more highly suppressed, peaking only at the CMB quadrupole. We provide analytic scalings for both signatures. Given the tensor bound, any primary CMB detection in these two lightest regimes would indicate a non-inflationary origin of the isocurvature modes or a breakdown of the standard frozen-field misalignment scenario. A window of coexistence opens near ma∼10−25 eV and fdm≳0.1 where both axion isocurvature and tensor modes could be discovered just below current bounds while simultaneously alleviating the S8 tension.
Hernan Rincon, Idit Zehavi, Sergio Contreras +2
The clustering of galaxies is affected by the assembly history of their underlying dark matter halos. This complex phenomenon, known as galaxy assembly bias, has been extensively studied, but the exact physical origin remains unclear. Splashback halos, typically low-mass halos that have traversed larger neighboring halos, have been suggested to be associated with halo assembly bias. Using a semi-analytic galaxy-formation model applied to the Millennium simulation, we explicitly explore the role that splasback galaxies play in galaxy assembly bias. We identify splashbacks as present-day central galaxies that were formerly satellites of a more massive host, and construct stellar-mass selected galaxy samples with the splashbacks either removed or reclassified as satellites of their former host halo. We find that splashbacks tend to reside in low-mass, highly concentrated halos and in dense environments, and that they have relatively high stellar-to-halo mass ratios. Splashbacks appear to be largely responsible for the low-mass tail of the occupancy variation for highly concentrated halos and for halos in dense environments. Finally, when computing the impact of assembly bias on galaxy clustering, we find that while removing the splashbacks significantly reduces the signal, reassigning them has little effect on its amplitude but shifts the transition scale. We repeat the analysis with the hydrodynamical simulation TNG300, confirming the robustness of our results. Our results provide insight into assembly bias and have potential implications for modeling the galaxy-halo connection.
Morgan Lynn, Ankur Agrawal, Arjun Ghosh +4
We report the development of a new widely tunable cavity and demonstrate its use in a search for dark matter axions. We achieve unloaded quality factors above 105, roughly 25× larger than a bare copper cavity at the same frequency, using concentric sapphire shells to reduce Ohmic losses on the cavity barrel. A rotating sapphire rod tunes our cavity mode over the 10.1−11.7 GHz range, approximately 16% of its resonant frequency. Using an amplified receiver chain, we demonstrate sensitivity to new axion parameter space by tuning the cavity over the 200 MHz range between 10.25−10.45 GHz (42.4 - 43.2, μeV) to constrain the axion-to-photon coupling to ∣gaγγ∣ ≤ 1 × 10−12 GeV−1. This cavity can scan its tuning range about 9 times faster compared to a bare copper cavity when paired with a photon counting device, laying the groundwork for a definitive search for the QCD axion over 10.1−11.7 GHz.