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Physics Astrophysics Cosmology and Nongalactic Astrophysics

astro-ph.CO Watch Cosmology and Nongalactic Astrophysics

Friday 21 August

Cosmology and Nongalactic Astrophysics

Diversity of Galaxy Centers from Small-Scale Isocurvature

Jessica N. Lopez-Sanchez and Wen Yin

The observed diversity of galaxy centers motivates the possibility that the local dark matter composition may vary among galaxies. In conventional multicomponent dark matter scenarios, however, large stochastic variations in the relative abundances are not generically expected, with the cold component typically remaining dominant. We perform $N$-body simulations with a subdominant ultralight dark matter component carrying significant large-amplitude small-scale isocurvature perturbations. We find that although nonlinear evolution largely homogenizes its fraction on galactic scales,ultralight dark matter can still be enhanced and even dominate the centers of small halos because large initial ultralight dark matter fluctuations seed early potential wells that later form halo centers. The ultralight-dominated region can extend beyond $0.01R_{\rm vir}$ and account for more than half of the central dark matter density, even for a cosmological ultralight dark matter fraction below $O(10\%)$. This central segregation provides a possible route to stochastic cusp–core diversity, potentially explaining the observed diversity of galaxy centers.
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Cosmology and Nongalactic Astrophysics

Model independent lensing sub-structure detection with multiply-imaged star clusters constellations

Leo W.H. Fung, Tom Broadhurst, Sung Kei Li et al. (+3 more)

A broad class of dark matter (DM) models predicts the existence of sub-structures residing in DM haloes on sub-resolvable angular scales. Techniques to extract such a generic feature from diffraction-limited observations are lacking. In this work, we propose a model-independent 'super-resolving' method applicable to strong gravitational lens systems that is fully data-driven, without reference to any lens models. This method relies on a specific way of applying the optical Liouville theorem across multiple scales in the imaging data, a technique we refer to as geometrical-duality. We test this method using realistic simulations and apply it to a 'constellation' consists of 11 compact star clusters seen in two giant arcs in the lensing cluster SMACS0723 imaged by JWST as a proof-of-concept. We find reasonable self-consistency with the expectation of non-detection given the statistical sensitivity, except for one pair of star clusters. Such an outlier can be explained in the context of CDM as sub-haloes lensing in the mass range $M_{\rm sub} = 10^8 - 10^9\,M_\odot$, for which the corresponding Einstein radius is smaller than the diffraction limit of JWST.
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All authors Leo W.H. Fung, Tom Broadhurst, Sung Kei Li, Jeremy Lim, Giorgio Manzoni, George F. Smoot

Cosmology and Nongalactic Astrophysics

Physical Calibration of a Minimal Effective Field Theory of the Three-Dimensional Lyman-$\alpha$ Forest

Gabriele Autieri, Vid Iršič, Tomáš Šoltinsky et al. (+1 more)

We study a minimal effective field theory description of the three-dimensional Lyman-$\alpha$ forest using the Sherwood and Sherwood–Relics hydrodynamical simulations. We model the Lyman-$\alpha$ flux auto-power spectrum and its cross-correlation power spectrum with the dark matter density field using a tree-level bias model supplemented by the leading counterterms and stochastic contributions. We find that the model describes the simulated auto- and cross-power spectra well up to $k_{\mathrm{max}}=3\,h\,{\rm Mpc}^{-1}$ and $k_{\mathrm{max}}=2\,h\,{\rm Mpc}^{-1}$, respectively. We analyse simulations spanning multiple redshifts, reionisation histories, box sizes and resolutions to assess the robustness of the model. Even within this minimal model, we find strong parameter degeneracies, highlighting the need for independent constraints on nuisance parameters in applications to real data analyses. The redshift evolution of the linear bias parameters is consistent with previous simulation-based studies and is driven primarily by the evolution of the effective optical depth, $\tau_{\mathrm{eff}}$. We also find that the inferred parameters are affected by the resolution of the simulation and, to a lesser extent, by the simulation box size. We also explore the impact of reionisation history, finding that variations mainly affect the linear bias parameters over the range of scales considered. Moreover, we find empirical correlations between model parameters and the Lyman-$\alpha$ forest density bias that show some scatter, indicating that one single parameter is not enough to determine the model parameters. Finally, we compare our results with theoretical predictions from analytical models of the Lyman-$\alpha$ forest.
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All authors Gabriele Autieri, Vid Iršič, Tomáš Šoltinsky, Matteo Viel

Cosmology and Nongalactic Astrophysics · Cross-listed from astro-ph.GA

Ultra-Deep imaging of the starless galaxy candidate Cloud-9

Ignacio Trujillo, Ignacio Ruiz Cejudo, Sergio Guerra Arencibia et al. (+1 more)

We report the non-detection of stellar light associated with the dark galaxy candidate Cloud-9. Ultra-deep imaging obtained with HiPERCAM at the Gran Telescopio Canarias (GTC) reaches surface-brightness limits of 31.4 and 31.0 mag/arcsec$^2$ in g and r, respectively, approximately ten times deeper than previous deep imaging of the region. No stellar emission is detected within the central 1'$\times$1' ($\sim$1.3$\times$1.3 kpc) region of Cloud-9. Assuming an old, metal-poor stellar population, these limits imply a stellar surface mass density of no more than $\sim$0.01 M$_\odot$/pc$^2$ and an upper limit on the total stellar mass of 1.6$\times$10$^4$ M$_\odot$. This independent constraint from integrated light complements previous limits based on resolved-star counts and strengthens the case for Cloud-9 as a candidate starless galaxy.
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All authors Ignacio Trujillo, Ignacio Ruiz Cejudo, Sergio Guerra Arencibia, Mireia Montes

Cosmology and Nongalactic Astrophysics

Probing cosmological phase transitions with SKAO through nano-Hz gravitational-wave portal

Roman Pasechnik and António P. Morais

The recent evidence for a stochastic gravitational wave (GW) background at nano-Hz frequencies from pulsar timing arrays (PTAs) provides a strong science case for exploring cosmological histories and phase transitions at the Square Kilometer Array Observatory (SKAO). The latter cosmological events are of primary interest for physics of the early Universe associated with out-of-equilibrium and relaxation phenomena in the vicinity the quantum chromodynamics (QCD) epoch as well as those originating from dark sectors or other phenomena beyond the Standard Model of particle physics. Such a novel GW portal provides a complimentary access to particle physics phenomena that have so far escaped detection at particle colliders, and also probes fundamental aspects of out-of-equilibrium dynamics beyond the perturbation theory and their cosmological implications. This chapter aims at elucidating most interesting physics cases, theoretical models and approaches, as well as cosmological scenarios that are of particular relevance for future GW measurements at SKAO. We elaborate on how such measurements can deepen our understanding of particle interactions and violent processes in the early Universe, and discuss possible competitive advantages and complementarity of SKAO in this research compared to other experiments.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Qubits for Dark Matter Hunting

Takeo Moroi

An introductory review is provided for those who are interested in exploring applications of qubits and other quantum excitations to the detection of dark matter (and any other physics beyond the Standard Model). Topics covered include the fundamental properties of qubits, the excitation mechanism of qubits due to the electric field induced by dark matter (with attention to the effects of the coherence of dark matter), the dynamics of coupled qubit-cavity systems modeled by the Jaynes-Cummings framework, and the influence of noise and decoherence (especially Markovian noise described by the Lindblad equation). In addition, the article introduces essential concepts in quantum sensing, including the operator-sum representation and positive operator-valued measures, the Cramér-Rao bound, the standard quantum limit and the Heisenberg limit, and the potential enhancement of sensitivity to dark matter achievable with entangled states. Throughout, these topics are discussed with particular emphasis on their application to the detection of wave-like dark matter.
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Thursday 20 August

Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Causal Gravitational-Wave Production Does Not Generate Infrared Adiabatic Curvature

Ruth Durrer and Alessandro Melchiorri

Short tensor modes can generate a white noise contribution to the local scalar $K=8\pi G\rho/3-\theta^2/9$, but this does not induce an infrared-enhanced adiabatic curvature spectrum. For a conserved causal source, analyticity of the longitudinal velocity correlator requires $P_V=\mathcal O(k^2)$, hence $P_{\mathcal R}=\mathcal O(k^0)$ and $\Delta_{\mathcal R}^2=\mathcal O(k^3)$, excluding the $\Delta_{\mathcal R}^2\propto k^{-1}$ relic. We also construct a conserved conversion of radiation into gravitational waves: the white field resides in a compensated entropy mode, and the fluid response cancels the slowly decaying $a^{-2}$ relic. Adiabatic bounds on such sources are therefore source- and matching-dependent.
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Cosmology and Nongalactic Astrophysics

One-point matter PDFs beyond TopHat filters

Anton Chudaykin, Alexander M. Kayssi and Sergey Sibiryakov

We study the one-point probability distribution function (PDF) for matter densities averaged with an arbitrary spherically symmetric window function. The PDF is analytically modeled within the path integral framework, enabling a non-perturbative description of large-scale structure. It contains a leading order contribution controlled by the spherically symmetric gravitational collapse dynamics, as well as an order-one factor arising from aspherical fluctuations. We develop a numerical pipeline to compute the leading spherical-collapse part of the PDF and apply it to a family of window functions interpolating between the TopHat and Gaussian filters in coordinate space, as well as to a window function with non-monotonic radial dependence. We find that the PDF weakly depends on the choice of the filter, provided the width of the filter is normalized to yield a fixed linear averaged density variance. For each filter and each value of the averaged density, our pipeline gives the most probable density profile. We find that these profiles vastly differ for different filters in the case of overdensities, but closely follow a universal curve at underdensities. We obtain a perturbative expression for the aspherical part of the PDF valid at small density contrasts. We find from it that all PDFs are equally sensitive to the effective field theory (EFT) corrections accounting for short-scale clustering, regardless of how smooth the filter's boundary is. We test our PDF model against the results of high-resolution N-body simulations. The agreement is excellent for filters with widths larger than 10 Mpc/h. Small discrepancies at a few percent level arise for narrower filters and are interpreted as higher-order perturbative corrections.
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Cosmology and Nongalactic Astrophysics

On what scales does strong lensing robustly constrain the mass profile of low-mass perturbers?

Maryam Tajalli and Simona Vegetti

Galaxy-galaxy strong gravitational lensing is an established method to detect and characterise low-mass haloes, which are otherwise too faint to be directly observed. Recent analyses have shown empirically that there exists a characteristic radius within which the projected enclosed mass of these haloes can be robustly measured. Here we provide a physically and mathematically motivated definition of this robust radius based on the Fisher information matrix. We show that for any spherically symmetric smooth profile with at least two free parameters, there exists a pivot radius at which estimates of the projected mass become decorrelated from the local logarithmic slope and have minimum relative uncertainty. As a result, the projected mass within this radius can be recovered consistently by any profile that fits the data well enough. This robust radius coincides, to leading order, with the weighted geometric mean of the arc pixel distances from the halo, where the weights depend on the perturber deflection angle, the macro-model magnification, and the source surface brightness gradient. Beyond leading order, corrections are of order the weighted log-width of the arc. The data constraining power on the local properties of the halo profile beyond amplitude is shown to depend on the radial leverage of the arc around the perturber, and on the imprint of the perturber deflection angle on high signal-to-noise-ratio pixels. Our definition generalises to both subhaloes and line-of-sight haloes and naturally captures the important physical aspects of the problem and the effect of the data resolution and signal-to-noise ratio.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Tunneling from a Slowly Rolling Multifield Background Using Tadpole Subtraction

Masaki Yamada

We study semiclassical tunneling in scalar field theories with slowly evolving homogeneous backgrounds. We formulate the problem by combining tadpole subtraction with an adiabatic expansion: the former maps the rolling system to an instantaneous multi-field tunneling problem, while the latter systematically tracks the slow evolution of the background. This formulation is particularly useful for tunneling during slow-roll inflation, where the tunneling direction may be transverse to the rolling inflaton direction. For a time-reflection-symmetric bounce on a fixed on-shell geometry, we show that the real tunneling exponent receives no direct correction linear in the rolling velocity. The tunneling rate nevertheless drifts along the rolling trajectory because the frozen tadpole-subtracted potential evolves. We also find that the exponent approaches a finite but nonanalytic limit as the tangential curvature vanishes, suggesting that a weakly tachyonic tangential direction can yield an exponent of the same order.
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Cosmology and Nongalactic Astrophysics · Cross-listed from cond-mat.quant-gas

Imaging the vacuum fluctuations of a quantum field

Yansheng Zhang, Feiyang Wang, Yi Jiang et al. (+5 more)

Heisenberg uncertainties lead to inevitable fluctuations in the measurement outcomes for quantum-mechanical observables. For quantum fields, these uncertainties result in random spatial structures in snapshots of a field, even when the field is in its ground (vacuum) state. Such `vacuum fluctuations' are at the heart of a wide range of phenomena, from spontaneous decay processes to the Casimir force and Hawking radiation. Their existence is a key manifestation of the quantumness of the physical world, but usually it is only their consequences that are directly observed. Here, we directly observe spatial vacuum fluctuations of a bosonic quantum field. Our experiments are based on a homogeneous planar atomic Bose–Einstein condensate. The condensate comprises two coherently coupled interacting components (spin states), and the quantum field describes its spin degrees of freedom. In the regime where the interactions dominate over the coherent coupling, our system emulates a (massive relativistic) sine-Gordon field. Images of the field reveal simultaneous fluctuations on different length scales, with scale-dependent amplitudes consistent with theoretical predictions for a vacuum state. Observing such fluctuations in the sine-Gordon limit opens many possibilities for laboratory simulations of relativistic fields in regimes that are presently not theoretically tractable.
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All authors Yansheng Zhang, Feiyang Wang, Yi Jiang, Alexander C. Jenkins, Paul H. C. Wong, Christoph Eigen, Gehrig Carlse, Zoran Hadzibabic

Cosmology and Nongalactic Astrophysics

A blind spot in transverse BAO calibration

Domenico Sapone

Transverse baryon acoustic oscillation (BAO) measurements are increasingly used for cosmological inference, and carry a calibration that no such inference can constrain. A constant error in the transverse BAO scale is exactly degenerate with the combination $r_{\rm d} h$: it leaves the goodness of fit unchanged and the recovered parameters plausible, and is therefore invisible to any analysis that uses these measurements alone. The radial BAO sector removes this degeneracy, supplying $D_{\rm M}/r_{\rm d}$ by integration without reference to $H_0$ or to any model for the expansion rate. In flat Friedmann–Lemaître–Robertson–Walker (FLRW) geometry the relation between the two sectors is an identity, so the sound horizon and the dark-energy equation of state cancel as well. An integrated form of this identity reduces the consistency test to a straight line, whose slope measures a relative transverse calibration $\varepsilon$. A departure from $\varepsilon = 1$ cannot be produced by any dark-energy model, nor by spatial curvature: it indicates an inconsistency in the measurement chain rather than in the cosmology. We apply the test to the two main SDSS transverse BAO compilations, which give $\varepsilon = 1.073 \pm 0.021$ and $1.021 \pm 0.029$. The first differs from unity at $3.8\sigma$ using the published independent errors, while the second is consistent with unity. The two compilations themselves differ by $(5.4 \pm 1.4)\%$, or $3.9\sigma$, and the offset is constant in redshift. The test provides a direct diagnostic for current and future angular BAO measurements.
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Cosmology and Nongalactic Astrophysics

Non-Minimally Coupled Warm Inflation in the Defining Frame

Adrián Casado-Turrión, Paulo B. Ferraz, Mindaugas Karčiauskas et al. (+1 more)

Warm inflation in $F(\Phi)R$ scalar-tensor theories of gravity is investigated in the `defining' frame, where the theory and its parameter values are specified. Translating the resulting dynamics to the Einstein frame, we find that the dissipation ratio is suppressed by the modified-gravity effects. Thus, although the effective warm-inflation dynamics can be consistently analysed in either frame, the dissipative regimes need not coincide between them. In particular, we find that quantum perturbations can dominate over thermal fluctuations in the scalar power spectrum even in a high-temperature, strong-dissipation regime in the defining frame. Finally, we compute the scalar spectral index and tensor-to-scalar ratio for a non-minimal coupling function $F(\Phi) = 1+\xi (\Phi/m_{\rm P})^2$ with a quartic potential and both constant and quadratic field-dependent dissipation coefficients, and identify benchmark points compatible with current CMB constraints.
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All authors Adrián Casado-Turrión, Paulo B. Ferraz, Mindaugas Karčiauskas, José Jaime Terente Díaz

Cosmology and Nongalactic Astrophysics

Transient Early Dark Energy-Like Dynamics as a Mechanism for Enhanced Early Structure Formation in the JWST Era

Abhik Bhattacharjee, Amlan Chakraborty, Subinoy Das et al. (+2 more)

The discovery of massive galaxies at redshifts $z\gtrsim10$ by the James Webb Space Telescope (JWST) has renewed interest in cosmological mechanisms capable of enhancing early structure formation while preserving the successful large-scale predictions of the standard $\Lambda$CDM model. We investigate a phenomenological scenario in which an exotic dark matter species can undergo a transient early dark energy-like phase during the radiation-dominated era ($10^{-7}\lesssim a\lesssim10^{-5}$) before reverting to pressureless cold dark matter. We utilize the generalized dark matter framework to model this species, which is restricted to a sub-percent fraction of the total dark matter component by CMB, BAO and Type Ia supernova data. Its background and perturbation dynamics are characterized by a time-dependent equation of state, $w(a)$, and a time- as well as scale-dependent sound speed, $c_s^2(a,k)$. The temporary negative equation of state, combined with our phenomenological pressure-response prescription, induces a finite interval of negative effective sound speed squared. This triggers an instability-driven growth of density perturbations over a limited range of comoving scales, thereby enhancing the formation of early dark matter halos. We find that the enhanced halo abundance can substantially reduce the star-formation efficiencies required to reproduce the observed abundance of JWST galaxies relative to the standard $\Lambda$CDM scenario, especially at higher redshifts. Our results demonstrate that transient early dark energy-like dynamics in a subdominant dark matter component provide a viable mechanism for enhancing early structure formation and offer a new framework for interpreting the abundance of high-redshift galaxies observed by JWST and future surveys.
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All authors Abhik Bhattacharjee, Amlan Chakraborty, Subinoy Das, Anshuman Maharana, Priyank Parashari

Cosmology and Nongalactic Astrophysics

SPT-3G+: A Cosmic Microwave Background Experiment for the South Pole Telescope

T. Natoli, Z. Ahmed, H. Athreya et al. (+72 more)

SPT-3G+ is the next survey receiver planned to be installed in early 2029 on the 10-meter South Pole Telescope (SPT). This new receiver will feature 6,020 polarization-sensitive dichroic pixels with transition-edge sensors observing in frequency bands centered at 90 GHz and 150 GHz. The 24,080 detectors in the SPT-3G+ receiver will be cooled to 100 mK by a dilution refrigerator and read out using microwave SQUID multiplexing. The optical design of the receiver enables a 4 degree diameter field of view, which is broken up into 14 individual optics tubes each containing cryogenic alumina, silicon, and nylon lenses. These technology choices will allow the SPT-3G+ receiver to improve on the mapping speed of the currently operating SPT-3G receiver by nearly an order of magnitude. Once deployed, the SPT-3G+ receiver will observe for 6-years an area overlapping with the BICEP survey to achieve a combined (90 GHz and 150 GHz) CMB map depth of 0.5 uK-arcmin. Data from these observations will be used to create unprecedentedly deep CMB lensing maps, discover new galaxy clusters, and detect astrophysical transients. The lensing map produced by SPT-3G+ will be used to remove or "delens" foreground B modes, where large-scale structure gravitationally lenses the CMB and converts E modes into B-mode polarization, with the goal of revealing inflationary B modes. Together with data from the BICEP Array as part of the South Pole Observatory, SPT-3G+ data will be used to constrain the tensor-to-scalar ratio $r$ with a goal of achieving a measurement of $\sigma(r) = 0.001$
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All authors T. Natoli, Z. Ahmed, H. Athreya, J. E. Austermann, K. Bae, A. Bapat, D. R. Barron, P. S. Barry, A. N. Bender, B. A. Benson, L. E. Bleem, J. E. Carlstrom, T. W. Cecil, C. L. Chang, S. Cisneros, A. Coerver, J. Cornelison, T. M. Crawford, R. Datta, K. R. Dibert, W. Dominguez, S. M. Duff, K. Fichman, J. P. Filippini, L. Gades, P. A. Gallardo, S. Galli, N. W. Halverson, Q. Hao, S. Henderson, R. Herbst, W. L. Holzapfel, A. Hryciuk, J. Hubmayr, D. Jones, V. Kabra, K. S. Karkare, C. King, M. A. Koc, A. M. Kofman, R. A. Lew, M. J. Link, T. J. Lucas, A. Mangu, E. S. Martsen, J. J. McMahon, J. Montgomery, J. M. Nagy, H. Nguyen, V. Novosad, S. Padin, T. Pinsonneault-Marotte, S. Raghunathan, A. S. Rahlin, C. L. Reichardt, L. Ruckman, J. E. Ruhl, M. S. Sarwar, S. Simon, R. Singh, J. A. Sobrin, A. A. Stark, Q. Taylor, C. Tucker, J. Ullom, J. Van Lanen, J. D. Vieira, A. G. Vieregg, M. R. Vissers, G. Wang, W. L. K. Wu, V. Yefremenko, E. Yilmaz, C. Yu, J. Zivick

Cosmology and Nongalactic Astrophysics · Cross-listed from astro-ph.IM

The SPT-3G+ receiver design

H. Athreya, Z. Ahmed, J. E. Austermann et al. (+71 more)

We present the thermo-mechanical design of the cryostat and camera optics for SPT-3G+, a new receiver being developed for the South Pole Telescope (SPT). The receiver consists of 14 detector arrays of 90/150 GHz dichroic polarization-sensitive pixels, totaling 24,080 transition-edge sensor detectors. Each detector array lies at the end of an optics tube, each approximately 240 mm in diameter and 772 mm in length, which are arranged in a hexagonal close-packed configuration to achieve a 4 degree diameter field of view. Each optics tube contains four anti-reflection coated lenses fabricated from different materials (alumina, silicon, and nylon) that are designed to also provide infrared filtering that reduces the radiative loading on the cryogenic stages. The optics and detectors are cooled by a combination of a pulse tube cooler for the 40 K and 4 K stages, and a dilution refrigerator for the 1 K and 100 mK stages. Thermal modeling predicts the heat load to be less than 26 W and 1 W for the 40 K and 4 K stages, respectively. The 1,550 kg cryostat has a 1.1 meter diameter at the vacuum window, which is located near the telescope Gregorian focus, and 1.75 meters in height and length. Fabrication of the cryostat will begin in 2026, with installation on the SPT scheduled for the 2028-29 austral summer, ahead of the 2029 winter observing season.
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All authors H. Athreya, Z. Ahmed, J. E. Austermann, K. Bae, A. Bapat, D. R. Barron, P. S. Barry, A. N. Bender, B. A. Benson, L. E. Bleem, J. E. Carlstrom, T. W. Cecil, C. L. Chang, S. Cisneros, A. Coerver, J. Cornelison, R. Datta, K. R. Dibert, W. Dominguez, S. M. Duff, K. Fichman, J. P. Filippini, L. Gades, P. A. Gallardo, S. Galli, N. W. Halverson, Q. Hao, S. Henderson, R. Herbst, W. L. Holzapfel, A. Hryciuk, J. Hubmayr, D. Jones, V. Kabra, K. S. Karkare, C. King, M. A. Koc, A. M. Kofman, R. A. Lew, M. J. Link, T. J. Lucas, A. Mangu, E. S. Martsen, J. J. McMahon, J. Montgomery, J. M. Nagy, T. Natoli, H. Nguyen, V. Novosad, S. Padin, T. Pinsonneault-Marotte, S. Raghunathan, A. S. Rahlin, C. L. Reichardt, L. Ruckman, J. E. Ruhl, M. S. Sarwar, S. Simon, R. Singh, J. A. Sobrin, A. A. Stark, Q. Taylor, C. Tucker, J. Ullom, J. Van Lanen, J. D. Vieira, A. G. Vieregg, M. R. Vissers, G. Wang, W. L. K. Wu, V. Yefremenko, E. Yilmaz, C. Yu, J. Zivick

Cosmology and Nongalactic Astrophysics

Inflation on the lattice: scalar Gauss-Bonnet single field inflation

Fei-Yu Chen, Jing-Zhi Zhou, Zhi-Chao Li et al. (+1 more)

We use lattice methods to study inflation in the scalar Gauss-Bonnet (SGB) gravity theory. We focus on the ultra-slow-roll scenario with the peak frequency falls into the PTA band. In the parameter range we consider, we find that the lattice results exceed the perturbative predictions, which differs from the case in Einstein gravity. We further find that lattice corrections become significant when the peak of the primordial curvature spectrum reaches $\sim 10^{-2}$. We calculate the energy density spectra of second-order scalar induced gravitational waves (SIGWs) using the primordial power spectra obtained from both the lattice method and the traditional perturbative method, and analyze the SGB model in light of current pulsar timing array (PTA) observations. Our results indicate that lattice corrections enhance the ability of the SGB model to dominate the PTA observations.
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All authors Fei-Yu Chen, Jing-Zhi Zhou, Zhi-Chao Li, Di Wu

Cosmology and Nongalactic Astrophysics

Testing Statistical Isotropy in the FRB Sky Distribution: A Selection-Function-Aware Framework

Bruno W. N. Ribeiro, Thais Lemos, Carlos A. P. Bengaly et al. (+5 more)

We perform a test of statistical isotropy in the Universe using the sky distribution of fast radio bursts (FRBs), based on a compilation of $4066$ events detected by multiple surveys. Our method is based on the two-point angular correlation function $w(\theta)$ as in the Landy–Szalay estimator, together with a tomographic absolute-anisotropy statistic, and estimates their observational uncertainties from complementary jackknife and bootstrap resampling. Both estimators are confronted with hierarchical ensembles of isotropic mock catalogs that propagate the uncertainties of empirically reconstructed survey selection functions, as well as the Poisson fluctuations of the isotropic realizations. The significances are obtained from a covariance-aware, SVD-regularized $\chi^2$ statistic calibrated empirically against the mock ensemble, and we evaluate four nested scenarios that progressively incorporate a Galactic-plane mask and the survey selection functions. As for our results, we find that the raw FRB sky is strongly inconsistent with isotropy; Galactic masking alone reduces the tension by only a factor of $\sim 3$, whereas the selection functions reduce it by nearly four orders of magnitude, showing that the apparent anisotropy is driven by the highly non-uniform sky coverage of the contributing surveys, overwhelmingly dominated by CHIME. Only when both effects are combined we obtain that the observed distribution is fully consistent with statistical isotropy. This result is independently corroborated by the absolute-anisotropy estimator, and is stable under variations of the analysis parameters. Therefore, we find that the FRB sky distribution is consistent with statistical isotropy, helping confirm one of the main predictions of the standard model scenario.
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All authors Bruno W. N. Ribeiro, Thais Lemos, Carlos A. P. Bengaly, Rodrigo Gonçalves, K. E. L. de Farias, Lázaro L. Sales, Amilcar R. Queiroz, Jailson Alcaniz

Cosmology and Nongalactic Astrophysics

Non-minimal Unimodular Inflation

Manda Malekpour, Kourosh Nozar, Fateme Rajabi et al. (+1 more)

We study an extension of the unimodular cosmological inflation in the context of the non-minimal coupling of a generic scalar field with gravitational sector. We consider the non-minimal coupling of the scalar field and gravity as the only source of energy-momentum tensor in this setup. Without introducing new particles other than those already existing in electroweak theory, the generic scalar field's non-minimal coupling is responsible for the generation of the seeds of perturbations for structure formation and the observed Cosmic Microwave Background anisotropies in this scenario. We calculate inflation parameters in both the Jordan and Einstein frames, and then we study primordial spectral indices for slow-roll parameters in Einstein's frame at the first and second orders. The numerical results of this model are consistent with the Planck2018 and BICEP/Keck joint data sets in some subspaces of the model parameters space. By considering a sufficient amount of inflation, we estimate the strength of the non-minimal coupling parameter, $\xi$, to find appropriate new constraints on the values of this parameter. By comparing the numerical values of the inflation observables in two frames and also with observation, we comment on the issue of frames in this framework.
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All authors Manda Malekpour, Kourosh Nozar, Fateme Rajabi, Narges Rashidi

Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Effective Field Theory for Freezing Gravity with Minimally Coupled Matter

Zhibang Yao

Freezing gravity (FG), a phantom-crossing dark energy model, was recently proposed in the absence of matter. In this work, we study FG minimally coupled to a perfect fluid through a detailed cosmological perturbation analysis and derive the conditions for the absence of ghost and gradient instabilities. The ghost stability conditions are obtained analytically without taking the high-$k$ limit, yielding the ghost-free range of scales and a finite cutoff when it exists. In the branch where a finite cutoff is already present in vacuum, we find that matter coupling lowers it relative to its vacuum value. Moreover, in the high-$k$ regime, the gradient stability condition guarantees a positive effective gravitational coupling for cold dark matter. We further show that two characteristic properties of FG persist in the presence of matter. First, the background and perturbation sectors remain separated, in the sense that they are controlled by independent sets of parameters, allowing FG to realize arbitrary background evolutions, including phantom crossing, while maintaining stable perturbation dynamics. Second, the scalar degree of freedom of FG becomes non-dynamical in the large-scale limit at linear order, while propagating with a finite speed of sound on small scales. Whether this behavior persists to arbitrarily high orders in perturbation theory, thereby avoiding the potential strong-coupling issue, requires further investigation through a nonlinear perturbation analysis. To make contact with large-scale structure and gravitational lensing observables, we also derive the effective gravitational coupling and gravitational slip within the quasi-static regime and express our results in terms of the EFT parameters.
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Cosmology and Nongalactic Astrophysics

A Prediction for DESI Full-Shape: Increasing Tomographic $\Omega_m(z)$ Trend

Eoin Ó Colgáin and M. M. Sheikh-Jabbari

To confirm $\Lambda$CDM deviations are due to missing physics (not systematics), one should demonstrate that the model fitting parameters exhibit qualitatively similar redshift drift across independent observables. This is the only way one guarantees new physics. Here, we show that a recent Dark Energy Spectroscopic Instrument (DESI) DR2 Full-Shape (FS) modelling Lyman-$\alpha$ constraint at $z_{\rm eff} = 2.33$ combined with earlier DR1 FS modelling constraints with $0.295 \leq z_{\rm eff} \leq 1.491$ leads to a straight line $\Omega_m(z) = m z + c$ with slope $m = 0.022 \pm 0.012$, $1.8 \sigma$ removed from constant $\Omega_m$. Akaike Information Criterion and Bayesian evidence confirm that constant $\Omega_m$ and increasing $\Omega_m(z)$ are statistically indistinguishable. Through the $Om(z)$ diagnostic, we review how increasing and decreasing $\Omega_m(z)$ trends map to phantom and quintessence dark energy (DE) regimes, respectively. While FS modelling constraints map to phantom DE, the decreasing and increasing $\Omega_m(z)$ trends in DESI BAO and DESI with external data make a phantom crossing inevitable. Since dynamical DE is but one interpretation for $\Omega_m(z)$ trends, it is imperative that different datasets converge on their $\Omega_m(z)$ trends before one jumps to physical conclusions. We forecast how DESI FS modelling $\Omega_m$ constraints will improve up to the final data release and explore the implications for model selection.
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Cosmology and Nongalactic Astrophysics

Quintessential \alpha-attractors, updated

Renata Kallosh, Andrei Linde, Marina Shmakova et al. (+1 more)

Quintessential $\alpha$-attractor models of single-field inflation and evolving dark energy were constructed about a decade ago. Recently, it was pointed out that some of them might be disfavored due to dark-radiation constraints on gravitational waves and the higher values of $n_s$ favored by ACT. Here we present a class of updated quintessential $\alpha$-attractor models in which a single field describes both inflation and evolving dark energy, yields higher values of $n_s$, and admits reheating scenarios consistent with the dark-radiation bound on $\Delta N_{\rm eff}$. Depending on the value of the cosmological constant $\Lambda$, these models interpolate between $\Lambda$CDM with $\Lambda > 0$ (future dS universe), dynamical dark energy with $\Lambda = 0$ (future Minkowski universe), and dynamical dark energy with $\Lambda < 0$ (future cosmological collapse). We also study quintessential $\alpha$-attractor models based on an axion-inflaton complex scalar field with hyperbolic geometry, which describe inflation and dark energy of a “phantom illusion” type compatible with DESI DR2.
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All authors Renata Kallosh, Andrei Linde, Marina Shmakova, Yusuke Yamada

Cosmology and Nongalactic Astrophysics

Cluster finding with outskirt stellar masses and percolation

Pablo Avalos, Conghao Zhou, Tesla Jeltema et al. (+5 more)

The abundance of galaxy clusters is a powerful cosmological probe, but optical cluster cosmology is limited by selection systematics, in particular the projection effects that affect cluster finders based on galaxy populations such as the red sequence. The outer stellar mass ($M_\mathrm{out}$) of cluster central galaxies – e.g., the stellar mass in a 50-100 kpc annulus – offers an alternative selection that relies only on the central galaxy and is therefore largely free from projection effects. Its primary systematic is instead satellite contamination, since massive clusters can host more than one galaxy with high outer stellar mass. Using the IllustrisTNG300 simulation at $z=0.4$, we quantify this contamination and investigate a simple, proximity-based percolation method to mitigate it, in which galaxies with lower outer stellar mass lying within a given radius of a more massive galaxy are removed from the sample. We find that the satellite fraction defined by the friends-of-friends (FoF) algorithm is modest even without percolation ($\leq 15\%$ for $M_\mathrm{out} > 10^{10}\,\mathrm{M}_\odot$ and $<10\%$ for $M_\mathrm{out} > 10^{11}\,\mathrm{M}_\odot$), and that percolation reduces it further, with the improvement increasing for percolation radii up to $3.0\,R_{200c}$. For a moderately high outer stellar mass cut ($\sim 4\times10^{10}\,\mathrm{M}_\odot$) and percolation radius ($\sim 2.0\,R_{200c}$), we recover a cluster sample that is both highly complete and pure for halo masses $\gtrsim 10^{14}\,\mathrm{M}_\odot$. These results indicate that outer stellar mass, combined with simple percolation, has the potential to provide a clean and readily calibratable selection of massive galaxy clusters.
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All authors Pablo Avalos, Conghao Zhou, Tesla Jeltema, Katya Leidig, Shuo Xu, Benedikt Diemer, Song Huang, Alexie Leauthaud

Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

How Inflation Might Explain Large-Scale Anisotropy

Brett McInnes

Several recent observational analyses have suggested the possible existence of anisotropy in our Universe. It is often thought that this would contradict the inflationary picture. We have argued that this is not the case: if we relax the artificial restriction that spacetime torsion must be zero, then such anisotropies arise naturally, indeed almost inevitably, in a theory of torsional Inflation. This is seen most clearly if one uses a “$1\,+\,3\,$” analysis of a torsional spacetime, leading naturally to concepts of intrinsic torsion and extrinsic torsion. An anisotropic quantum fluctuation of the inflaton leads to an extrinsic torsion field which is anisotropic and which grows throughout the inflationary era, even though the intrinsic geometry of the spatial sections is isotropised in the conventional manner. This leads to an anisotropic Hubble field at the end of Inflation. The anisotropy takes a very specific and characteristic form, which in principle could be observable on relatively short cosmic scales. On the other hand, an unusual feature of torsional geometry leads to a short interval, early in the inflationary era, during which there is a torsion-induced energy flux, which leaves an anisotropic signal in superhorizon modes and thus ultimately at very large scales in the present Universe. Thus in principle torsion can explain anisotropies both on relatively short cosmic scales and also at the largest scales.
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Cosmology and Nongalactic Astrophysics · Cross-listed from astro-ph.IM

Towards independent event horizon imaging of the supermassive black holes in M87 and the Milky Way

Nithyanandan Thyagarajan, Samuel Lai, Ivy Wong et al. (+1 more)

The Event Horizon Telescope (EHT) Collaboration's images of the supermassive black holes in M87 and the Milky Way have provided the first event-horizon-scale views of these objects, opening new avenues for studies of gravitation, accretion physics, and black hole astrophysics. Achieving these results, however, requires imaging under some of the most challenging conditions in radio astronomy, including low signal-to-noise ratios, severe calibration uncertainties, and sparse aperture coverage. With the aim of presenting independent analyses of the public EHT datasets for M87* and Sgr A*, we adopt an approach that is independent in observables, and reconstruction methodology. Our framework is based on closure invariants, a class of interferometric observables that are intrinsically immune to station-based calibration errors and therefore provide robust constraints on source structure. We combine these observables with Generative Deep learning Image Reconstruction with Closure Terms (GenDIReCT), a diffusion-based image reconstruction framework that operates in the latent space of images conditioned on closure invariants. We present independent reconstructions obtained using GenDIReCT on synthetic challenge data sets as well as real EHT data on 3C279 and Centaurus A, and compare them with previously reported results. This work demonstrates the potential of closure-invariant-driven generative imaging as a calibration-resilient framework for Very Long Baseline Interferometry (VLBI) and provides an independent and complementary avenue for interpreting horizon-scale black hole observations.
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All authors Nithyanandan Thyagarajan, Samuel Lai, Ivy Wong, Foivos Diakogiannis

Cosmology and Nongalactic Astrophysics · Cross-listed from astro-ph.GA

Diversity of Ionized Gas Structures in Nearby Metal-poor Dwarf Galaxies

Yuki Takagishi, Takuya Hashimoto, Kazuya Matsubayashi et al. (+13 more)

We investigate whether optical and far-infrared [O III] emission from nearby metal-poor dwarf galaxies can be represented by a homogeneous one-zone ionized-gas model with a single electron temperature and density. Our sample comprises five galaxies from the Herschel Dwarf Galaxy Survey: HS1222+3741, SBS0335-052E, POX186, Haro11, and IZw18. We combine galaxy-integrated or nearly galaxy-integrated [O III] 4363 and 5007 measurements from Seimei/KOOLS-IFU observations and published or archival spectroscopy with Herschel/PACS [O III] 88um measurements. Because [O III] 4363 is not detected in HS1222+3741, the analysis is based on the remaining four galaxies. SBS0335-052E and Haro11 lie near or slightly beyond the low-density boundary of the one-zone diagnostic. Their nominal line ratios favor effective densities of ne<1cm-3, while conservative treatment of the uncertainties allows values up to 40 and 10cm-3, respectively. These remain substantially below densities inferred from independent diagnostics. By contrast, POX186 and IZw18 show no significant discrepancy between the optical–far-infrared [O III] and low-ionization optical diagnostics. Additional optical and ultraviolet diagnostics show that inferred densities can span several orders of magnitude within a galaxy. Representative two-zone models reproduce the [O III] 4363, 5007, and 88um emission in SBS0335-052E and Haro11 by combining relatively dense gas with cooler, low-density gas. The low-density component contributes approximately 61% and 72% of the 88um luminosity, but only 14% and 23% of the 5007 luminosity, respectively. These solutions are not unique and may represent a broader unresolved distribution of gas conditions. Our results show that temperatures and densities inferred from integrated one-zone analyses are effective quantities and that similar diagnostic discrepancies can arise in nearby metal-poor galaxies.
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All authors Yuki Takagishi, Takuya Hashimoto, Kazuya Matsubayashi, Matthew Hayes, Aida Wofford, Masato Hagimoto, Akio K. Inoue, Ken Mawatari, Yurina Nakazato, Wataru Osone, Yuma Sugahara, Yoshiki Toba, Hidenobu Yajima, Shunsuke Honda, Hiroshi Matsuo, Naoki Yoshida

Wednesday 19 August

Cosmology and Nongalactic Astrophysics

Viability of Big Bang Nucleosynthesis in $f(R,L_m)$ Gravity

Kajal Phukan, Rajdeep Mazumdar, Kalyan Malakar et al. (+1 more)

We examine the viability of Big Bang Nucleosynthesis (BBN) constraints in $f(R,L_m)$ gravity, where the gravitational Lagrangian is an arbitrary function of the the Ricci scalar $R$ and matter Lagrangian density $L_m$. We derive stringent bounds on the underlying model parameters of four different $f(R,L_m)$ gravity models by examining both the primordial abundances of helium-4 ($^4\text{He}$) and the fractional variations in the neutron–proton freeze out temperature. Our results reveal that $f(R,L_m)$ gravity remains viable under the BBN constraints. With the freeze-out condition offering the most dominat constraint on the parameter space, whereas the bounds from $^4\text{He}$ abundance act as a separate abundance level consistency checks on the modified expansion rate. In summary, the presence of these feasible parameter regions demonstrates that $f(R,L_m)$ gravity models can successfully accommodate primordial element abundances without disrupting standard early Universe dynamics.
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All authors Kajal Phukan, Rajdeep Mazumdar, Kalyan Malakar, Kalyan Bhuyan

Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

WIMP Freeze-Out in Diffusive Unimodular Gravity

Cesar Bonilla, Esteban Gonzalez and Carlos Maldonado

We study a non-standard cosmology (NSC) scenario within the Unimodular Gravity (UG) framework, sourced by a scalar field $\phi$ that undergoes energy diffusion, parametrized by the diffusion parameter $x$, the initial energy densities rate $\kappa\equiv\rho_\phi/\rho_\gamma|_{\text{ini}}$, and the end-of-domination temperature $T_{\text{end}}$. We compare this UG+NSC scenario with standard NSC and $\Lambda$CDM cosmologies for WIMP Dark Matter (DM) production via the freeze-out mechanism. We find that energy diffusion reshapes the allowed $(m_\chi, \langle\sigma v\rangle)$ parameter space, where $m_\chi$ is the DM mass and $\langle \sigma v \rangle$ is the thermally averaged annihilation cross section, opening regions otherwise excluded by DM overproduction in $\Lambda$CDM, and shifting the mass and cross-section ranges accessible to WIMP candidates depending on $x$, $\kappa$, $T_{\text{end}}$, and the barotropic index $\omega$ of $\phi$. As a concrete application, we implement this framework for the Real Singlet Scalar WIMP and test its $(m_\chi, \lambda_{HS})$ parameter space against current direct detection bounds from the LZ experiment, showing that energy diffusion opens previously unconstrained regions in the Higgs-portal coupling $\lambda_{HS}$ and thereby alters the detectability prospects of this benchmark model in future searches.
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Cosmology and Nongalactic Astrophysics

Does Early Dark Energy Absorb the DESI Late-Time Dynamics Signal? A Combined Analysis

Mohammad Malekjani and Saeed Pourojaghi

While the recent Baryon Acoustic Oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) collaboration are largely consistent with a flat $\Lambda$CDM cosmology, the preferred parameters are in mild tension with those determined from the cosmic microwave background (CMB). A late-time dynamical dark energy (DDE) solution has been proposed by the DESI collaboration to address this tension. In this work, we investigate whether the statistical preference for DDE is a genuine late-time phenomenon or an artifact of unresolved early-universe physics. To do so, we simultaneously allow for both early- and late-time modifications to the expansion history by combining the Early Dark Energy (EDE) framework with the Chevallier-Polarski-Linder (CPL) parametrization. Excluding the DESI BAO measurements, our joint analysis of the CMB+Pantheon+ datasets demonstrates that within an EDE-extended framework, the CPL parameters remain statistically consistent with the standard $\Lambda$CDM model. This supports the hypothesis that a DDE signal at low redshifts can be effectively accounted for by an EDE component within the $\Lambda$CDM background. However, upon the inclusion of the DESI BAO measurements in the joint analysis, a statistically significant deviation from a cosmological constant emerges. Within this combined framework, the best-fit CPL parameters robustly indicate a departure from the standard $\Lambda$CDM model, favoring a phantom-to-quintessence transition in the DE equation of state. This demonstrates that the DESI preference for the late-time DDE is a robust signature that cannot be absorbed by modifying the physics of the early Universe.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Bayesian Forecasts on Cosmic Superstring Searches with LISA

Satyabrata Datta and Rome Samanta

Cosmic superstrings are well-motivated early-Universe sources of stochastic gravitational waves, with phenomenology controlled by the string tension $G\mu$ and the intercommutation probability $P$. We study whether LISA can reconstruct these parameters and distinguish different superstring signal models in the presence of instrumental noise and astrophysical foregrounds. We consider two phenomenological models. In Model I, reduced intercommutation acts only as an amplitude enhancement of a cusp-dominated spectrum, $\Omega_{\rm SS}^{\rm I}=P^{-\beta}\Omega_{\rm cusp}$. In Model II, the signal is a cusp–kink mixture, $\Omega_{\rm SS}^{\rm II} =P^{-\beta}[p_c\Omega_{\rm cusp}+(1-p_c)\Omega_{\rm kink}]$, so that $P$ controls both amplitude and spectral shape. Using simulated LISA data, we perform Bayesian inference with noise uncertainties, unresolved extragalactic compact-binary backgrounds, and a flexible Galactic double-white-dwarf foreground. We compare the models using Bayesian evidences and map the $(G\mu,P)$ posterior geometry with marginalized widths, correlations, covariance anisotropy, principal eigenvalues, posterior area, and bias. We find that reduced intercommutation generically produces strong $G\mu$–$P$ correlations, because the data often constrain an amplitude-like combination. However, when the cusp–kink spectral difference lies in the LISA band and the signal is sufficiently loud, Model II can be favored and the posterior can retain genuine shape information. As an optimistic foreground scenario, we also compute the Bayes factor using a reduced tanh Galactic foreground template, finding improved model discrimination when the foreground shape is constrained.
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