Papers

Supermassive black hole binaries (SMBHB) are expected at the center of galaxies as a result of galaxy mergers. The dynamics and evolution of SMBHBs shed light on the evolution and formation of galaxies, the gravitational wave background and the nature of gravity. Many methods have been proposed to identify sub-parsec SMBHBs via the detection of gravitational waves or the modulation of the emission from the surrounding gaseous environment. In a previous study, we proposed a new method of identifying SMBHBs in non-active galactic nuclei via gravitational lensing. The caustics, i.e. the source positions with a divergent magnification, exhibit a complex extended geometry which rotates with the binary period. Each caustic-crossing produces an extreme magnification by up to a factor of $10^{6}$. In this work, we generalize the previous study and describe the rich phenomenology of binary-lens amplification of starlight (BLAST). We explore the electromagnetic transients produced when accounting for the relative proper motion of the source, either a single or a binary star, the eccentricity and precession of the SMBHB, and the relative inclination. Systems where a star rapidly drifts across the caustic produce a single- or double-peaked flare. In the opposite limit, a star with no relative proper motion produces quasi-periodic lensed starlight (QPLS), i.e., repeated peaks that track the system's orbital evolution. We determine the expected magnification and peak duration in different circumstances, and calculate the magnification curves and their Fourier transforms. This study may help construct search pipelines to identify SMBHBs in electromagnetic time-domain surveys.

No: neither through the bosenovae of gravitational atoms formed around stars and compact objects, nor through those of boson stars formed in the galactic dark-matter halo. We assume that all the dark matter consists of ultralight bosons with attractive self-interactions, which can be captured in the gravitational potential of astrophysical objects or condense into self-gravitating configurations. When these systems collapse in a bosenova, part of their mass is turned into relativistic radiation that escapes the galaxy. We bound the mass that a single object can eject and sum it over realistic models of the Milky Way and of other types of galaxies. For gravitational atoms, with realistic stellar populations and stellar evolution, the stars of the Milky Way deplete at most $\epsilon^{\rm GA}_\star\simeq3\times10^{-9}$ of its halo, and the pump is equally negligible in other galaxies. The central black hole, Sgr A$^*$, could reach $\epsilon^{\rm GA}\sim10^{-3}$, but only for a boson mass disfavoured by ultrafaint dwarf galaxies and beyond the validity of the perturbative treatment, where only a geometric bound on the dark matter flux is available. For boson stars feeding on the rest of the halo, these would radiate at most $\sim3\times10^{-5}$ of the Milky-Way halo and $0.1\%$ of a DF2-like one.

How galaxies stop forming stars is a complex and central problem in astrophysics. Numerous physical mechanisms have been proposed, yet a simple, unified framework connecting these mechanisms remains lacking. Here, we derive a step-function quenching response from the fundamental principle of gas mass conservation, governed by the competition between gas-supply and gas-depletion timescales. This response also enables the gas-supply timescale to be inferred from the observed quenched fraction. Applying this method to approximately 350,000 galaxies, we find a characteristic gas-supply timescale of ~1 Gyr, reveal distinct inside-out and outside-in quenching signatures associated with supermassive black holes and dark matter halos, respectively, and identify galaxies with intermediate stellar surface mass densities as occupying a critical state for quenching and rejuvenation. Our work establishes a unified physical framework in which a simple step-function response captures how diverse physical processes regulate galaxy quenching and the resumption of star formation through their effects on the competition between gas-supply and gas-depletion timescales.

Line intensity mapping (LIM) provides a unique window into the high-redshift universe, but the separation of signals from overlapping emission lines and bright continuum foregrounds remains a central analysis challenge. At millimeter wavelengths in particular, [CII], [CI] and numerous CO transitions overlap with comparable brightness, leading to significant spectral confusion. This work presents a joint likelihood framework that incorporates multi-tracer cross-correlations both between distinct LIM tracers and with external galaxy surveys. By jointly fitting all auto- and cross-spectra between the LIM and galaxy data, this approach combines the spatial and spectral information in each dataset to break degeneracies between overlapping lines and recover individual bias-weighted line intensities at fixed cosmology, even at redshifts where no direct cross-correlation is available. We validate this framework on realistic mocks of an extended SPT-SLIM-like survey spanning 125-175 GHz over 100 deg$^2$, combined with an LSST Y1-like photometric galaxy catalog, and recover CO(2-1) through CO(6-5) and [CI](1-0) over $0.3\lesssim z\lesssim4$ in the presence of bright continuum foregrounds. We demonstrate that adding the photometric galaxy data tightens line constraints by a factor of $\sim$3-4 where the galaxies overlap the lines in redshift, and by $\sim$20-50% at higher redshifts beyond the galaxy coverage, where the gain comes indirectly from constraining the low-redshift interlopers. These results highlight external tracer overlap as a primary consideration when optimizing the frequency coverage and footprint of near-future LIM experiments.

In Inflation without an Inflaton (IWI), a de Sitter (dS) background drives inflation, and the graviton bilinear generates scalar perturbations at second order, with no inflaton or additional fundamental scalar field. We investigate how this scenario can account for the observed scalar spectral tilt. The tensor-mode dependence of the scalar power spectrum gives the consistency relation $n_s-1=2n_T$, since it contains four tensor mode functions. The scalar spectral tilt $n_s\simeq0.9649$ at CMB scales therefore gives $n_T\simeq-0.0175$, independent of the tensor-to-scalar ratio $r$, hence different from the standard single-field slow-roll prediction $n_T=-r/8$. A full time-dependent calculation of $\phi_2$ in exact dS leads to an almost scale-invariant scalar power spectrum, with a small blue infrared residual on the order of $n_s-1\simeq+4\times10^{-4}$. In order to account for the observed red tilt, we introduce a slow drift in the Hubble rate, parametrized by $\varepsilon=-\dot H/H^2$. The tensor tilt is then given by $n_T=-2\varepsilon$, implying $n_s-1\simeq-4\varepsilon$, at leading order. The observed tilt requires $\varepsilon_\ast\simeq0.0088$, far exceeding the drift that can be sustained by the classical graviton backreaction, for which we find $|\varepsilon_{\rm self}|\lesssim10^{-11}$. Therefore, we investigate the quantum break-time of dS as the origin of the background evolution. In this picture, the scalar tilt becomes $n_s-1=-4C_qN_{sp}(H/m_{pl})^2$, where $C_q$ is an order-one depletion coefficient, and the tilt is directly connected to the particle content of dS, $N_{sp}$. The evolution of the drift itself further predicts a positive running of $\alpha_s=+\tfrac12(1-n_s)^2\simeq+6\times10^{-4}$. These results conclude the distinctive observational predictions of the IWI scenario, establishing it as a falsifiable alternative to standard inflaton-driven models.

We investigate the global dynamics and cosmological viability of non-singular solutions in an anisotropic Bianchi type I universe featuring an interacting vacuum energy component $V$ and a perfect fluid $\rho$, extending previous work based on a Robertson-Walker geometry. By modeling the energy-momentum transfer via a phenomenological four-vector $Q_\mu$, we reduce the field equations to a four-dimensional autonomous dynamical system for the expansion rate $\Theta$, total shear $\sigma$, energy density $\rho$, and vacuum component $V$. We examine two distinct forms for how the interaction depends on $\rho$ and $V$: a linear coupling $Q_1 \propto \Theta[\xi(V_\Lambda - V) + \chi \rho]$ and a non-linear coupling $Q_2 \propto \Theta \rho (1 - V/V_\Lambda)$. Using a potential-energy analogy and phase-space analysis, we show that the linear model $Q_1$ naturally admits non-singular bouncing trajectories. In contrast, unlike the Robertson-Walker case, for the non-linear interaction $Q_2$, invariant boundary surfaces in phase space strictly constrain the domain of expansion ruling out cosmological bounce scenarios for physical matter configurations.

It is conventionally believed that the Stochastic Gravitational-Wave Background (SGWB) from inflation provides information about the earliest universe. However, in this Letter, we present a different picture. Our analysis starts from the observation that the inflation-predicted spectrum implies gravitons reside in high-occupation states, which significantly enhances their self-interactions and triggers rapid thermalization. We show that approximately $80\%$ of the primordial gravitons participate in this thermalization, forming a Bose–Einstein distribution and thereby erasing the primordial information. Evaluated using typical inflation parameters, this thermalized spectrum yields a present-day peak frequency of $f_{0, \text{peak}}^{(\text{th})} \sim 1.4 \times 10^8 \, \mathrm{Hz}$ in the hundred-megahertz band with a peak amplitude of $\Omega_{0, \text{peak}}^{(\text{th})} \sim 3.5 \times 10^{-13}$. Due to the sharp drop-off away from the peak, the resulting spectrum lies beyond the reach of low-frequency observatories like PTAs, LISA, and LIGO, but remains uniquely accessible to high-frequency detectors such as microwave cavity resonators and dielectric haloscopes.

Forthcoming cosmic microwave background (CMB) experiments are targeting ambitious science goals, including the exploration of the particle content of the Universe across cosmic times and the search for the faint signal from primordial gravitational waves. To pursue these goals, CMB experiments will house an unprecedented high number of detectors that will dramatically enhance their sensitivity. This will in turn require a tight control of instrumental systematics, demanding efficient and precise simulation of systematic effects. Leveraging spin-mapmaking, we present a map-based method to propagate beam convolution systematics to the map-level, especially well-suited for main-beam studies. From the scanning strategy information compressed into precomputed h-maps and the beam and sky harmonic coefficients, our method directly outputs beam convolved maps. We validate against pixel-centered TOD-level 4{\pi} convolution for circular, elliptical, and Planck HFI scanning beams, demonstrating that our method retains agreement to numerical precision, while operating in map space. This formalism enables efficient simulation of beam convolution systematics across the full focal plane of upcoming experiments. As a first practical application, we demonstrate the ability to efficiently produce beam-convolved simulated Planck maps, showing excellent agreement with the QuickPol pipeline at the power-spectrum level. We release a public code, Smarties4Planck, to easily generate such maps for broader use.

Galaxy-to-galaxy stochasticity may be less important for reionization, IGM heating, and the 21-cm signal than commonly assumed. We find that most of its apparent impact arises because introducing or removing scatter changes the mean emissivity of the galaxy population, rather than from redistributing radiation at fixed mean emissivity. We use the Meraxes semi-analytic model to separate these effects. For ionising sources, we vary the scatter in the escape fraction and in the SFR-halo mass relation across three source prescriptions. For the X-ray heating sources, we introduce luminosity scatter and also separately remove SFR scatter. Without recalibration, these modifications shift the mean emissivities, changing the timing of reionization and IGM heating and producing substantial differences in the 21-cm signal. Once the relevant mean emissivities are restored, however, the global ionizing and thermal histories converge and the power-spectrum differences fall below the forecast sensitivity of a deep SKA-Low observation. There are some weak changes in ionisation morphology, depending on which galaxies dominate the ionising emissivity and on their response to photoheating. Within the models and scales considered here, calibrating the mean radiative emissivities is therefore substantially more important than resolving their galaxy-to-galaxy scatter.

We point out that for black holes (BH) of the same mass, despite of arising from two entirely different physical arguments, the peak frequency of the coarse-grained gravitational wave (GW) spectrum of gravitons $f_\text{peak}^\text{graviton}$ is very close to the peak frequency of the GW spectrum produced from binary BH mergers $f_\text{peak}^\text{merger}$. In particular, we find that $f_\text{peak}^\text{graviton}/f_\text{peak}^\text{merger}\approx 2.3$ independent from BH mass or population. If BHs are still evaporating, the peak frequencies scale as $\propto M_i^{-1}$ with $M_i$ is the initial BH mass. If BHs evaporated in the radiation (matter) dominated era, the peak frequencies scale as $\propto M_i^{1/2}$ ($\propto M_i$) respectively after taking into account redshift effect. Any deviation from the aforementioned magical ratio would indicate new BH physics. The sweetest frequency ballpark to look for this correlation is around $10^{14}\ \rm Hz-10^{20}\ \rm Hz$ where both GW spectra are allowed to have maximum amplitude. As a demonstration of what the new physics could be, we consider a phenomenological self-similar regular BH model of Hayward featuring a single hair parameter. The observed ratio of peak frequencies can be used to directly infer the hair parameter, along with crucial properties of the novel BH solution such as Hawking temperature, horizon size, and innermost stable circular orbit. We also comment on the pros and cons of correlating frequencies among other GW sources such as formation, population fluctuation, and sudden transition of adiabatic perturbation.

Higher-order curvature corrections to general relativity arise naturally in many fundamental theories and provide phenomenologically interesting low-energy effective field theories. However, higher-derivative terms can introduce ghost modes, which call the validity of such models into question. Here, we thoroughly investigate the classical linear stability of the most general second-order scalar-tensor theory, namely the Horndeski theory of gravity, augmented by a Weyl-squared term in the action. We consider shift-symmetric de Sitter and power-law FLRW backgrounds and study the evolution of linear perturbations. Vector perturbations are free of gradient and tachyonic instabilities for a positive Weyl-induced mass squared. Tensor perturbations necessarily contain a ghost; their superhorizon branches are bounded, while a frozen-background calculation identifies a separate finite-momentum stability condition. The scalar sector instead contains a growing long-wavelength branch for all expanding backgrounds we consider, which is inherited by at least one of the gauge-invariant Bardeen potentials. We show that this growth enters the Bardeen potentials through an inverse-gradient factor $a^2/k^2$: the dynamical scalar fields remain bounded on the inflationary backgrounds, the direct contribution of the growing shear to the clock-matched tidal field cancels, and the electric part of the Weyl tensor decays. We discuss the implications of our results for perturbative control and for the phenomenological viability of theories including quadratic curvature corrections.

Recent analyses combining DESI baryon acoustic oscillation (BAO) measurements with other cosmological observations have reported indications of departures from the $\Lambda$CDM model within dynamical dark energy scenarios. We investigate whether such deviations are reproduced consistently in $f(T)$ gravity and whether they persist when different cosmological observations are combined. Using the DESI DR2 BAO measurements, Cosmic Microwave Background (CMB), and Pantheon+ Type Ia Supernovae (SNe Ia) sample, we constrain three representative $f(T)$ models: the power-law, exponential, and hyperbolic-tangent models. We compare the constraints obtained from different combinations of data, distinguishing changes in marginalized parameter distributions from changes in the relative statistical performance of the models. For the power-law model, the mild deviation from the $\Lambda$CDM model obtained from BAO+CMB disappears after the SNe Ia data are included. In contrast, the exponential model retains a preference for a nonzero deformation parameter in the full analysis, while the allowed parameter space of the hyperbolic-tangent model changes substantially after the CMB distance priors are included. Importantly, the parameter-level deviation found for the exponential model does not lead to a clear statistical preference over the $\Lambda$CDM model according to the information criteria, whereas the hyperbolic-tangent model shows a poorer statistical performance for the combined data. Despite these differences in the model constraints, the inferred transition redshift $z_T$ remains relatively stable among the three models and data combinations. Our results show that the departures from the $\Lambda$CDM model indicated by DESI analyses are not a generic feature of $f(T)$ gravity, but depend strongly on the specific modified gravity parameterization and on the combination of cosmological observations.

Machine learning can build near-optimal summary statistics for extracting cosmological information from the large-scale structure of the Universe. Training them, however, requires large numbers of costly simulations. We aim to build a neural summary trained on fast, approximate low-resolution simulations that remains informative when applied to higher-resolution ones, reducing the cost of the training set. We train an ensemble of convolutional neural networks with different architectures to compress the matter overdensity fields of low-resolution particle-mesh simulations. The outputs of a subset of networks are concatenated into a summary statistic, which we use to constrain $\Omega_{\rm m}$ and $\sigma_8$ from high-resolution particle-mesh simulations, each about a thousand times more costly than their low-resolution counterparts. Theoretical predictions are provided by a Gaussian process emulator built from the high-resolution simulations. We test three strategies for selecting the smallest informative subset of networks. Constraints are compared with those from a reference power spectrum analysis. We also consider dark matter halo catalogues from independent high-resolution $N$-body simulations. The neural summary extracts information beyond the binned power spectrum. Using multiple networks is essential: a single network can be less informative than the power spectrum. A greedy search based on posterior volume provides the tightest constraints with the fewest networks. The neural summary also outperforms the power spectrum for dark matter haloes, at different number densities and redshifts, despite being trained on low-resolution matter fields at $z=0$. This work demonstrates that an ensemble of networks trained on approximate simulations yields a summary statistic that remains informative on higher-fidelity data, including dark matter haloes. (Abridged)

We present specwizard, a publicly available modular Python package for generating synthetic absorption spectra from hydrodynamical simulations. The code combines atomic data from the National Institute of Standards and Technology database with ionisation fractions computed using the cloudy spectral synthesis code for multiple ionising backgrounds, enabling consistent modelling of bound–bound and bound–free absorption for resonant transitions of all ions. specwizard employs a column-density–conserving projection scheme, includes natural line broadening for the Lyman series, and offers optional treatments of instrumental resolution and noise for direct comparison with high-resolution quasar spectra. As example applications of the code, we shoot sightlines through the eagle cosmological simulation to generate a z = 3.2 synthetic quasar spectrum containing H i Lyman-series transitions, including damping wings and Lyman-limit absorption. Additionally, we compute metal-absorption lines probing the circumgalactic medium of simulated galaxies, and investigate the correlated absorption of H i and Si iii. We demonstrate that specwizard applied to outputs from eagle accurately reproduces key observational diagnostics of the Lyman-alpha forest, including the redshift evolution of the mean transmission, the transmission PDF and the transmission power spectrum. We take advantage of the fact that the eagle simulation also reproduces the measured evolution of the temperature-density relation to infer the evolution of the amplitude of the ionizing background, finding excellent agreement with recent theoretical models and measurements.

Three of the most pressing open questions in cosmology – the nature of dark energy and dark matter, and the growing tension in the value of the Hubble constant H$_0$ – call for cosmological probes that are independent of, and complementary to, traditional methods. Strong gravitational lensing offers exactly that: a purely gravitational probe of the universe's expansion history and matter distribution, sensitive to dark matter's presence regardless of whether it interacts with light. Time-delay lenses and multiplane lenses can measure H$_0$ and the dark energy equation of state directly, while the distortion of lensed images can reveal the abundance and internal structure of dark matter subhalos. After a brief theoretical and historical introduction, I review recent breakthroughs enabled by JWST and by advances in lens modeling and stellar dynamics. Examples include the recent measurement of H$_0$ with 3% precision in flat $\Lambda$ cold dark matter cosmology, and a tentative detection of dark matter self-interaction. Strong lensing is entering a pivotal era: wide-field surveys with Euclid, Rubin, and Roman, and radio surveys with the Square Kilometer Array, are poised to increase the number of known strong lenses by orders of magnitude, positioning strong lensing to play a central role in answering these open questions in the decade ahead.

Grand Unified models of particle physics inevitably give rise to magnetic monopoles. Often the model will also contain a weakly broken discrete symmetry that leads to the formation of biased domain walls that survive for a limited period in cosmology. For a range of the discrete symmetry breaking parameter, domain walls can collapse to form magnetically charged extremal black holes lighter than $\simeq 1\, {\rm kg}$ but these are not expected to survive until the present epoch due to Hawking evaporation and Schwinger pair production of monopoles. If the parameter is smaller still, the black holes may be astrophysical, with mass greater than $10^{15}\, {\rm g}$ and with sub-extremal magnetic charge: $Q \simeq 10^8$ (for our canonical choice of parameters). The cosmological energy density in residual magnetic monopoles depends on the discrete symmetry breaking parameter and for sufficiently small values of the parameter, the magnetic monopole density can satisfy all observational constraints.

We study the relative contributions of mergers and accretion to the assembly of dark matter haloes within the framework of Excursion Set Theory (EST). We extend a variance-discretized merger-tree algorithm, which recursively reconstructs all branches of a complete merger tree and introduces a merger-ratio criterion for distinguishing mergers from accretion. The method is validated by comparing the resulting progenitor distributions with the analytical conditional first up-crossing distribution, showing excellent agreement over a wide range of halo masses and redshifts. We find that the accretion contribution depends strongly on the adopted merger-ratio threshold. For large merger ratios ($MR=0.1$–$0.25$), accretion accounts for up to $\sim70\%$ of the mass growth of massive haloes, whereas including all resolved mergers ($MR=10^{-6}$) reduces the accretion contribution to $\sim40\%$. These values are in good agreement with previous N-body simulation results. The accreted mass exhibits a nearly self-similar distribution in the mass-redshift plane, with the dominant contribution arising from progenitors in the range $10^{-3}M_0 \lesssim M \lesssim 10^{-1}M_0$ and predominantly at $0.5 \lesssim z \lesssim 2$. Our results demonstrate that EST-based merger trees provide an accurate and computationally efficient framework for investigating halo assembly and quantifying the relative roles of mergers and accretion in hierarchical structure formation.

Axion-like particles (ALPs) with sufficiently low masses are dark matter candidates because they could account for much or all of the observed dark matter energy density and help explain observations of galactic substructure. Such pseudoscalar fields could couple to photons, causing a time-dependent rotation of the polarization of the light with an oscillation frequency equal to the ALP mass. We use measurements of the polarization of the cosmic microwave background (CMB) by the Atacama Cosmology Telescope (ACT) between 2017 and 2022 to search for this effect using an EB power spectrum estimator. By probing oscillation periods from days to years, the data constrain the coupling between ALPs and photons for ALP masses between $2.5\times 10^{-23}$ eV and $4.8\times 10^{-20}$ eV. We find no evidence for such a signal and set a 95% upper bound on the axion-photon coupling constant, $g_{\phi\gamma} < 1.1\times10^{-11}\left({m_\phi}/{10^{-21}\,\mathrm{eV}}\right)\,\mathrm{GeV}^{-1} .$ This limit assumes that ALPs make up all of the dark matter and that the ALP field amplitude is stochastically distributed, which leads to more conservative bounds than the deterministic-field assumption used in some previous CMB polarization analyses. Among searches for ALP-photon coupling using CMB polarization, we set the strongest bounds to date under the stochastic-field assumption. This work develops a methodology to analyze over 10,000 time-sliced "depth-1" polarization maps from ACT, a data product that will be standard for time-domain astrophysics with the Simons Observatory, enabling searches with a higher sensitivity and observing span.

Understanding how galaxies populate and grow within dark matter haloes is essential for linking the observed evolution of galaxies to the hierarchical growth of structure. We present a galaxy clustering analysis using optical and near-infrared data from the UltraVISTA and VIDEO surveys covering $9.3$ deg$^2$ across the COSMOS, XMM-LSS and ECDF-S fields, $\sim 5\times$ larger than previous comparable studies, providing sufficient volume to determine the halo occupation of the most massive galaxies to $z\sim 4$. We measure the angular two-point correlation function for stellar-mass-selected galaxies over $0.25<z<4$ and by combining the three fields, we reduce the cosmic variance by a factor of $\sim2$ compared to the individual fields. The stellar-to-halo mass ratio peaks at a halo mass of $M_{h}\approx10^{12} {\rm M}_{\odot}$ and is a maximum at $z\sim 1$. For the most massive galaxies, the peak halo mass ($M_{h}^{\rm peak}$) increases from $z=1\rightarrow 0$, while the peak stellar mass in such haloes remains approximately constant, indicating that at $z<1$ massive galaxy formation becomes less efficient. We also find a steady increase in the satellite fraction since $z\sim 1$, from which we infer that massive halos continue to grow hierarchically while their central galaxies experience suppressed stellar-mass growth. We suggest this is due to the central massive galaxies having their growth controlled by feedback from active galactic nuclei feedback, likely from low-power stochastic radio emission.

The Effective Field Theory of Large Scale Structure (EFTofLSS) provides a coarse-grained description of the non-linear gravitational clustering of dark matter, in which the effects of short-scale physics are encoded in an effective stress tensor and a set of counterterms. In this work, we develop a framework to investigate the physical origin of these counterterms by relating the effective stress tensor to the dynamics of individual dark matter halos in cosmological N-body simulations. We find that halo accretion contributes to the trace of the halo-based effective stress tensor, contributing to the background pressure and effective sound speed, while anisotropic accretion generates the traceless part, consequently contributing to the shear viscosity. For the halo population with $M_\mathrm{vir}>10^{12}\,M_\odot/h$, we find that halos account for $\sim\!95\%$, $\sim\!85\%$, and $\sim\!70\%$ of the full box background pressure, effective sound speed, and shear viscosity, respectively, depending on the EFT cutoff scale. The contributions to the counterterms are dominated by cluster-scale halos, $M_\mathrm{vir} \sim 10^{14} - 10^{15}\,M_\odot/h$ and halos with larger inflow rates, supporting an interpretation in terms of accretion-driven, out-of-equilibrium dynamics. Our results provide a short-scale, kinetic-theory interpretation of the EFTofLSS counterterms and establish a framework for connecting the properties and dynamics of short-scale structures to physics-informed priors for large-scale cosmological observables.

We present jax-SNANA, a JAX reimplementation of the supernova analysis framework SNANA. jax-SNANA enables GPU-accelerated transient simulations and light curve fitting, supernova standardisation and cosmological inference. Running in single precision on an NVIDIA A100 GPU, jax-SNANA simulates more than 100,000 LSST-observed supernovae per second and fits 5,000 light curves per second after JIT compilation, a two orders-of-magnitude increase over current tools, essential for analysing samples at the scale expected from LSST. jax-SNANA currently supports the SALT and BayeSN spectral energy distribution (SED) models as well as physical models from redback-jax and a variety of SN SED templates; light curve fitting is supported with both photometric and spectroscopic redshifts. We fully validate the accuracy of jax-SNANA by comparing to SNANA, finding excellent consistency between the two across a variety of tasks. The code provides a modular JAX end-to-end supernova cosmology pipeline, user-extensible to support new features including different SED templates and photometric classification algorithms. jax-SNANA has been designed with future applications of simulation-based inference in mind, providing fast simulations and enabling fully compiled on-the-fly simulations to be generated within the training loop of downstream SBI tasks. jax-SNANA extends beyond SNANA by supporting simulations of generic SN-host galaxy relationships and multiple sub-populations of events, for example originating from young and old progenitors. jax-SNANA is publicly available at https://github.com/mattgrayling/jax-snana and installable through pip.

We study a phenomenological cosmological model of black-hole growth in which a finite baryonic reservoir and a bosonic scalar cloud feed the black hole simultaneously. The baryonic component accretes through an Eddington-like channel, while the scalar cloud transfers mass through an effective quasibound-state decay rate. Evolving the system directly in redshift within a flat $\Lambda$CDM background, we scan over $\mu$, $M_0$, $M_{c,0}$, and $M_{b,0}$ to assess whether scalar-assisted accretion can accelerate the formation of $10^9M_\odot$ supermassive black holes. We find that the scalar contribution is highly selective in parameter space. For the mass scales explored here, scalar transfer is most efficient for $\mu\sim10^{-19}-10^{-16} \mathrm{eV}$, producing rapid cloud depletion and significantly earlier black-hole growth, while outside this interval the evolution remains mainly baryonic. More generally, this efficient window reflects a preferred range of the dimensionless coupling $\alpha=\mu M_{\rm BH}$ and therefore shifts approximately as $\mu\propto M_{\rm BH}^{-1}$. Comparison with observed quasar masses and luminosities instead favors lower boson masses, broadly around $\mu\sim10^{-21}-10^{-20}\mathrm{eV}$. These results are not necessarily in tension, since the boson mass that maximizes early growth depends on the characteristic black-hole mass and evolutionary stage. This separation between dynamical efficiency and observational compatibility leaves open complementary roles for different bosonic mass scales in supermassive black hole growth and dark-matter phenomenology.

Post-reionization 21 cm intensity mapping provides a powerful technique for efficiently mapping the Universe on large scales. The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a compact and highly redundant radio interferometer designed to exploit this technique. We present a forward modelling pipeline for forecasting a statistical detection of the cosmological 21 cm signal with HIRAX through cross-correlation with an external source catalogue. We simulate the cosmological signal using the halo model and propagate it, together with Galactic and extragalactic foreground emission and instrumental noise, through a model of an early-stage 36-dish HIRAX array configuration. We generate simulated visibilities, apply delay-domain foreground filtering, and construct cross-correlation-ready ringmaps for an observing time of 120 days with a single zenith pointing over the frequency range from 700 to 800 MHz, covering approximately 3547 deg$^2$ of the sky. We investigate the prospects for an early statistical signal detection by stacking these ringmaps at the known three-dimensional positions of sources from an overlapping external survey. Specifically, we quantify how delay-domain foreground filtering, the number of stacking sources, catalogue completeness, and source redshift uncertainties affect the detection significance. We find that redshift precision is the most critical requirement: while spectroscopic redshifts with $\sigma_z / (1+z) \sim 0.001$ preserve a significant stacking signal even under realistic foreground filtering and substantial catalogue incompleteness, photometric redshift uncertainties of $\sigma_z / (1+z) \sim 0.01$ strongly suppress the signal for realistic delay cutoffs. Applying the framework to a Euclid-like spectroscopic galaxy sample, we recover a minimum signal-to-noise ratio of 11 for our most conservative delay cutoff of 100 ns.

We propose a post-inflationary first-order phase transition modulated by a light spectator field as the origin of primordial large-scale curvature perturbations. Spectator fluctuations acquired during inflation modulate the bubble nucleation history, generating curvature perturbations together with correlated intrinsic gravitational-wave (GW) anisotropies. We show that this mechanism can reproduce the observed amplitude of curvature perturbations while producing a loud stochastic GW background in the frequency bands of space-based detectors such as LISA, Taiji, and TianQin. A concrete benchmark yields strongly enhanced intrinsic GW anisotropies and a local primordial non-Gaussianity signal within reach of future large-scale-structure surveys.

The Little Red Dots (LRDs) are compact, red sources, abundant at $4 \lesssim z \lesssim 9$ and likely powered by massive black holes. Photometric studies have found blue companions around them, but none has yet been confirmed spectroscopically. This Letter presents the only two LRD-companion pairs with spectroscopy of both members in the literature: "Waltz" at $z = 3.9$ and "Tango" at $z = 5.5$, with projected separations of $3.6$ and $2.5$ kpc. Both companions lie at the redshift of their LRD, within $132 \, \rm km \, s^{-1}$ and $57 \, \rm km \, s^{-1}$, and are physically identical, but $524$ Myr apart: starbursts $\lesssim 50$ Myr old, metal-poor ($0.2 \, \rm Z_\odot$), dust-free, dynamically cold, and as bright as the LRDs themselves in the rest-frame UV. The LRD in Waltz hosts a $\approx 3 \times 10^{6} \, \rm M_\odot$ black hole accreting at half the Eddington rate, a factor of a few above the birth mass of a direct collapse black hole, and is embedded in gas as metal-poor as its companion. Orbital modeling shows that Tango is bound in $81\%$ of the allowed orbits and that Waltz is bound if its host halo exceeds $5 \times 10^{10} \, \rm M_\odot$. Both blue companions then merge with their respective LRD within a few hundred Myr. We conclude that we are observing what heavy seeding requires: a young black hole beside the metal-poor starburst that the synchronized-pair scenario predicts, in the last steps of a cosmic dance that ends in a merger, and in a galaxy seeded by its black hole.

Mass distributions of binary black hole mergers inferred from gravitational-wave observations offer clues to how binaries form. These distributions conventionally label the heavier component as the primary, with mass $m_1$, and the lighter as the secondary, with mass $m_2$, such that $m_1 \geq m_2$. This ordering folds the physically labeled mass distribution along the equal-mass line, potentially reshaping the underlying population structure. In this Letter, we determine which population features can arise from mass ordering and which require genuine astrophysical structure. We show that ordering alone can produce (1) an $m_2$ distribution that declines more rapidly than the $m_1$ distribution, without intrinsic differences between the physically labeled components, and (2) an apparent concentration along equal masses, without correlations that preferentially pair black holes of equal mass. Moreover, different physically labeled populations can produce the same ordered mass distribution. Crucially, however, a preferred companion mass that remains constant as $m_1$ increases requires a characteristic mass scale already present in the physically labeled population. Applied to binary black hole mergers in the fifth Gravitational-Wave Transient Catalog, our model recovers such a scale near $m_2 \simeq 50\,\mathrm{M}_\odot$. Around this scale, the merger-rate density declines much more rapidly toward higher $m_2$ than toward lower $m_2$, ruling out an equal or weaker high-$m_2$ decline at 99.45% credibility. This structure therefore requires a characteristic mass scale in at least one physically labeled component, but masses alone cannot determine which component carries it. Any formation model reproducing the observed high-mass binary black hole merger population must contain such a scale.

Merging galaxy clusters provide an ideal laboratory for testing the self-interacting nature of dark matter. In this paper, we present a pilot study of magnetohydrodynamical cosmological zoom-in simulations of two cluster mergers selected from TNG-Cluster, with $M_{\rm 200c} \sim 9\times10^{14}~M_\odot$ and $M_{\rm 200c}\sim 2\times10^{14}~M_\odot$. The simulations are performed with both standard collisionless cold dark matter (CDM) and self-interacting dark matter (SIDM), assuming an isotropic, velocity-independent cross-section of $\sigma_{\rm T}/m_\chi = 1~\rm cm^2~g^{-1}$, with an additional $0.5~\rm cm^2~g^{-1}$ run for the low-mass system, and the mass resolution and galaxy formation model are unchanged from TNG-Cluster. We show how dark matter self-interactions produce an isothermal core in the dark matter halo, generate unique substructures in the intracluster light, and introduce variation in the multi-wavelength observables of the intracluster plasma. Using these simulations, we test the validity of observational probes for dark matter self-interactions in cluster mergers. We find that the shock-to-shock separation shows similar evolution in the self-interaction models tested here, providing a reference for distinguishing the $\sim100-200~\rm kpc$-scale difference in halo separation induced by self-interactions. Moreover, the dark matter halo centroid of the main cluster in the massive cluster merger is offset from the brightest cluster galaxy by as much as $\sim20~\rm kpc$, larger than the offset in the sub-cluster. This offset persists beyond 4 Gyr after the first pericenter passage. Our results indicate that sample selection plays a key role in testing the possible self-interacting nature of dark matter using cluster mergers, and that double radio relic cluster mergers and massive clusters with evidence of an ancient major merger offer a promising testbed for this purpose.

Current and upcoming galaxy-redshift surveys offer a promising path for constraining inflationary physics. These constraints are often imposed by relating correlations of the primordial curvature perturbation to the late-time galaxy correlation functions. The primordial correlators can either be computed analytically, or via realizations of the primordial curvature perturbations that are initialized on a grid and evolved numerically according to the classical equations of motion. In this work we take a hybrid approach, initializing the fields in the Bunch-Davies vacuum; and then imposing time evolution through direct numerical integration. Our method efficiently generates realizations of the inverse decay primordial curvature perturbations for axion inflation—an inflationary model in which a pseudoscalar inflaton field couples to a $U(1)$ gauge field via a Chern-Simons interaction. Depending on the choice of parameter $\xi\in\{2, 2.5, 3\}$, which controls gauge field particle production in this model, our method generates realizations with an average runtime of a few hours on 1 CPU core. We compute the power spectrum and bispectrum from these realizations and find a surprisingly large discrepancy between our result and previous work that used an approximation for the gauge field mode functions—up to a factor of $\sim4$ difference for the bispectrum in the case of $\xi=2$. This new method enables not only rapid computation of higher order polyspectra, but also generation of initial conditions for large-scale structure analyses.

We investigate parity violation searches with the 4-Point Correlation Function (4PCF) using the Quijote and Quijote-Odd halo catalogs. Specifically, we explore the auto- and cross-correlation searching methodologies that have been previously used in the Baryon Oscillation Spectroscopic Survey (BOSS) and the Dark Energy Spectroscopic Instrument (DESI) searches. Although the Quijote-Odd simulations are initialized with a parity-violating primordial trispectrum, we find no statistically detectable parity-odd signal in the halo 4PCF. Instead, we find that the halo measurements are dominated by Poisson noise. We are able to distinguish this noise-dominated behavior from the response expected for a parity-odd signal. These results clarify how parity-sensitive 4PCF statistics respond to noise in a controlled simulation setting.

Transients in galactic nuclei arise from distinct processes, only some of which trace the supermassive black hole (SMBH). These include tidal disruption events (TDEs), changing-look active galactic nuclei, Bowen fluorescence flares (BFFs) and ambiguous nuclear transients, alongside supernovae that appear nuclear but are unrelated to the SMBH. Reliable classification requires spectroscopy, which will not scale to the alert volumes of the Legacy Survey of Space and Time (LSST). We aim to characterise the population of nuclear transients, to identify early observables capable of distinguishing SMBH-related flares, and TDEs in particular, from the dominant supernova background, and to assess the biases this selection introduces. We implemented a real-time filter on the Lasair broker, selecting transients from the Zwicky Transient Facility (ZTF) alert stream by nuclear coincidence with the host galaxy, with no photometric cuts, to retain SMBH-related flares in both quiescent and active hosts. The filter ran over 16 months, with candidates reported for classification through ePESSTO+ and other facilities. The filter returned 476 candidates, of which 202 were spectroscopically classified, yielding predominantly supernovae, 10 confirmed TDEs (5%) and one BFF. The mean g-r colour and the rise time separate TDEs from the supernova background. Our filter recovers 10 of the 25 TDEs classified in the same period and declination range; most misses are technical or observational rather than physical. Simple real-time filtering combined with basic photometric criteria can already isolate TDEs efficiently from nuclear transient alert streams, and high completeness is achievable. In the LSST era, where samples will be predominantly photometric, selection bias will become as important as contamination, and retaining flares in active hosts is necessary if unusual events are not to be excluded.