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Physics Astrophysics Cosmology and Nongalactic Astrophysics

astro-ph.CO Watch Cosmology and Nongalactic Astrophysics

Monday 31 August

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

Building a Better Beta: Nucleation and Timescales in Cosmological Phase Transitions

William Searle and Csaba Balázs

First-order phase transitions in the early universe can generate a stochastic background of gravitational waves, offering a unique probe of high-energy physics. In this work, we investigate aspects of bubble nucleation and transition timescales, which play a central role in shaping the resulting gravitational wave spectrum. Many common approaches characterise the transition rate via a Taylor expansion of the false vacuum decay rate. We argue that a more fundamental description is instead given by the distribution of bubble lifetimes, and define a new timescale, $\beta_\nu$, as the first moment of this distribution. We show that $\beta_\nu$ reproduces the behaviour of previous timescale definitions in the appropriate limits, while avoiding their pathologies, and offers a more natural description of the ensemble of nucleated bubbles. We then quantify the impact of this improved timescale on the predicted gravitational wave spectrum, finding that it shifts the peak amplitude by up to an order of magnitude relative to previous definitions. As next-generation gravitational wave detectors come online, robust theoretical predictions will be essential; we hope this work represents a step in that direction.
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Sunday 30 August

Cosmology and Nongalactic Astrophysics

Host Dependence and Line-of-Sight Effects on Galaxy-Galaxy Strong Lensing in Clusters

Cian Roche, Michael McDonald, Priyamvada Natarajan et al. (+4 more)

The cross section for galaxy-galaxy strong lensing (GGSL) events in galaxy clusters has repeatedly been found to be higher in observations than in cosmological simulations. We revisit this discrepancy using updated simulation methodology and investigate the dependence of the GGSL probability, $P_{\rm GGSL}$, on host-cluster lensing properties, baryonic physics, and correlated and uncorrelated line-of-sight structure. We find that correlated material within $\sim 35$ cMpc of the cluster along the line of sight enhances $P_{\rm GGSL}$ by a few percent for typical systems and by up to $\sim15\%$ for the most efficient lenses. At fixed cluster mass, dark-matter-only simulations yield GGSL probabilities up to an order of magnitude lower than hydrodynamical simulations. We also find a strong dependence on the host-cluster Einstein radius, with an approximate scaling $P_{\rm GGSL} \propto \theta_{\rm E}^{2}$. Matching simulated and observed clusters in both mass and Einstein radius seems to reduce the discrepancy relative to previous comparisons. However, our analysis does not clearly resolve the GGSL discrepancy, as the inferred tension depends strongly on the field-of-view definition: square fields are approximately consistent with simulations, while cluster member-bounded fields yield observed probabilities a factor of $\sim 2-3$ higher. Until observations and simulations share a matched selection function and matched measurement methodology, the residual tension cannot be cleanly attributed to either astrophysics or cosmology.
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All authors Cian Roche, Michael McDonald, Priyamvada Natarajan, Keren Sharon, Isaque Dutra, Massimo Meneghetti, Mark Vogelsberger

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

The Structural Abundance Crisis of Massive Galaxies in Current Cosmological Simulations

Hassen M. Yesuf, Connor Bottrell, Luis C. Ho et al. (+5 more)

The internal structure of galaxies encodes the complex baryon cycle driven by gas accretion, star formation, and feedback, together with secular evolution and environmentally driven processes such as mergers and tidal interactions. We present a population-level census of massive nearby galaxies ($\log(M_\star/M_\odot) > 10$) by comparing Hyper Suprime-Cam Subaru Strategic Program observations with matched mock images from IllustrisTNG, EAGLE, and SIMBA. Using a consistent, like-for-like imaging pipeline, we construct structural abundance functions (SAFs) for key morphological parameters, revealing a structural abundance crisis. Across Sérsic index, concentration, size, and ellipticity, all simulations exhibit large, systematic discrepancies ($>5\sigma$; RMSE $\sim 0.2$–$1.8$\,dex), typically corresponding to abundance differences of factors of several. While individual simulations display diverse failures—underproducing or overproducing compact spheroids, extended or round galaxies —all underproduce highly flattened disks. Although TNG shows the closest agreement and SIMBA the largest offsets, this shared failure indicates that current models—despite matching global demographics such as the stellar mass function—do not uniquely constrain internal galaxy structure. Our results demonstrate that agreement in integrated observables can mask fundamental shortcomings in the modelling of mass and angular momentum redistribution. We therefore establish SAFs as a stringent, multidimensional, and observationally accessible benchmark for testing and calibrating next-generation galaxy formation models in the era of upcoming deep, wide-field surveys.
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All authors Hassen M. Yesuf, Connor Bottrell, Luis C. Ho, Aaron S. G. Robotham, Sabine Bellstedt, Robin H. W. Cook, Lei Hao, Fengshan Liu

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

Seesaw and Axion in No-Scale Gravity

Anamaria Hell, Lincoln da S. Pereira and Tsutomu T. Yanagida

The No-Scale gravity is a compelling framework to describe particle physics, gravity and cosmology, in which all mass scales are an illusion given by the value of a scalar field $\phi$. This theory, however, falls under the umbrella of more general scalar-tensor theories, which propagate different modes depending on the scalar field, and faces the extensive debate regarding the equivalence between its Jordan and Einstein frame descriptions. In this work we advocate that by excluding the singular point in the transformation between the two frames (e.g., $\phi=0$) we can have the classical equivalence between them. To show the merit of removing this singular point, we argue the Jordan frame makes the symmetries of the model manifest, which creates a suitable playground for the particle-physics model building which we illustrate by showing the emergence of discrete symmetries which leads to a high-quality axion and also naturally incorporate the seesaw mechanism.
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Cosmology and Nongalactic Astrophysics · Cross-listed from astro-ph.GA

Primordial Black Hole Seeds for Little Red Dots in $f(R)$ Gravity

Saeed Fakhry

The discovery by the James Webb Space Telescope (JWST) of an abundant population of compact little red dots (LRDs) and supermassive black holes at high redshifts poses a severe timing challenge for standard Eddington-limited growth from stellar remnants. In this work, we present a unified hybrid assembly framework in which intermediate-mass seed black holes are formed via hierarchical primordial black hole (PBH) mergers within dense clusters in Hu-Sawicki $f(R)$ gravity, followed by galactic gas accretion. We demonstrate that the screened $f(R)$ fifth force nonlinearly accelerates early seed formation through a quadratic enhancement of the gravitational-wave radiation-capture kernel, while environmental chameleon screening dynamically self-regulates the growth. Because subsequent gas accretion scales multiplicatively with seed mass, the $f(R)$ seed enhancement is preserved throughout the accretion history, drastically reducing the time-averaged Eddington ratios required to assemble $10^{6}\text{-}10^{9}\,M_{\odot}$ LRDs by cosmic dawn. Confronting our active mass functions with JWST data reveals that a small clustered PBH fraction ($f_{\rm PBH} \lesssim 10^{-3}$) under a realistic active galactic nucleus duty cycle ($\delta \approx 10^{-2}$) naturally matches observed LRD space densities at $z \sim 5.5\text{-}6.5$ while remaining fully compliant with LIGO-Virgo-KAGRA limits, with modified gravity naturally driving the high-mass tail. Finally, we show that parameter degeneracies between modified gravity and cluster density can be cleanly broken by combining three multi-messenger diagnostics: remnant spin state signatures, a stochastic GW background cutoff, and a scale-dependent inversion in the PBH spatial correlation function.
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Cosmology and Nongalactic Astrophysics

Spheroidal Resampling Analysis of High-Redshift Gamma-Ray Burst Spatial Densities

Istvan Horvath, Zsolt Bagoly, Lajos G. Balazs et al. (+6 more)

Gamma-ray bursts (GRBs) are bright transient sources that can be observed at high redshift. They can therefore be used as tracers of the distant large-scale structure, although the GRB redshift sample is sparse and affected by strong selection effects. We analyze the three-dimensional distribution of 542 GRBs with spectroscopic redshifts. The method extends our earlier spherical-window search by replacing spherical counting volumes with axisymmetric spheroids. The angular positions of the observed GRBs are kept fixed in the Monte-Carlo null samples, while the redshifts are shuffled within each Galactic hemisphere. In the Northern Galactic hemisphere, the overdensity associated with the Hercules–Corona Borealis Great Wall remains significant for a range of spheroidal shapes. This supports the stability of the previously reported signal and shows that it is not only a consequence of using spherical counting volumes. In the Southern Galactic hemisphere, the same method finds no structure with comparable significance. A small candidate grouping is present, but its significance is only marginal, and it is sensitive to the small number of events. We conclude that spheroidal resampling is a useful check of GRB overdensity searches, but the physical nature of any candidate structure still requires confirmation with independent tracers such as galaxy or quasar samples.
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All authors Istvan Horvath, Zsolt Bagoly, Lajos G. Balazs, Jon Hakkila, Janos Horvath, Sandor Pinter, Istvan I. Racz, Peter Veres, Bendeguz Koncz

Saturday 29 August

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

How much of the black hole ringdown is sourced within the light ring?

Mark Ho-Yeuk Cheung

Not much. It has been proposed that applying a rational filter to a black hole ringdown gravitational waveform reveals a direct wave component sourced near the event horizon. By solving the Teukolsky equation for a point particle plunging into a Kerr black hole, I show that the maximum amplitude of the waves sourced within the prograde light ring is $\lesssim 10\%$ that of those sourced by the full trajectory no matter if the waves are filtered or not. This agrees with the interpretation of quasinormal modes as waves orbiting the light ring and the direct wave as the power of these modes redistributed in time. Therefore, both quasinormal modes and direct waves are more a probe of the light ring than the near-horizon region. For GW250114, I estimate that the component of the waves sourced within the light ring have a signal-to-noise ratio $\rho \lesssim 1$ measured after the peak of the filtered waves.
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Cosmology and Nongalactic Astrophysics

The onset of the strong backreaction regime in axion inflation, an analytical study

Aditya Kulkarni and Lorenzo Sorbo

Models where an axion-like inflaton is coupled to a $U(1)$ gauge field are theoretically well motivated and can display a rich phenomenology. The regime in which the quanta of the gauge field strongly backreact on the rolling inflaton has been studied for well more than a decade, and yet we cannot say that it is fully understood. In this paper we present an analytical study of the onset of the strong backreaction regime. Our formalism relies on a Laplace transform to convert a complicated integro-differential equation into the search of the poles of an analytic function. We confirm the existence, previously observed using a completely different formalism, of a stable region of strong backreaction. Our work provides, for the first time, analytical formulae that allow to study the evolution of the system as it transitions from the weak to the strong backreaction regime.
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Cosmology and Nongalactic Astrophysics

Primordial black holes and induced gravitational waves from localized features in DBI inflation

Narges Rashidi

We investigate primordial black hole formation in a Dirac-Born-Infeld inflationary framework in which the background dynamics are determined by explicit functional forms of the inflaton potential $V(\phi)$ and the warp factor $f(\phi)$. The background equations are solved numerically to obtain the evolution of the slow-roll parameters. We show that a localized feature in the potential, accompanied by a correlated structure in the warp factor, dynamically induces a transient suppression of the slow-roll parameter, leading to a short-lived non-attractor phase. This behavior generates an enhancement of the curvature power spectrum on small scales, while preserving consistency with CMB-scale observables. The Mukhanov-Sasaki equation is then solved numerically to compute the resulting power spectrum, which exhibits narrow and localized peaks in the PBH abundance across different mass ranges. We also evaluate the associated stochastic background of induced gravitational waves and find that the predicted signal can fall within the sensitivity bands of future pulsar timing arrays and space-based interferometers, depending on the scale of the inflationary feature. A parameter scan in the $(r,n_s)$ plane shows that the large-scale predictions remain consistent with current observational constraints.
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Friday 28 August

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

Lynx2030 Science Analysis Group: Final Report

Lynx2030 Science Analysis Group: Simon R. Bandler, Laura W. Brenneman, Nico Cappelluti et al. (+50 more)

The Lynx2030 Science Analysis Group (SAG) was convened to reassess the scientific goals and technical drivers of the Lynx mission concept amid a rapidly evolving astrophysics landscape. Building on the original Lynx Concept Study, the SAG examined how recent discoveries, emerging facilities, and advances in instrumentation influence the scientific opportunities for a next-generation flagship X-ray observatory. Through focused working groups, the SAG investigated the scientific impact of enhanced capabilities: (i) improved angular resolution, (ii) broader bandpass coverage, (iii) an enhanced microcalorimeter, (iv) new capabilities and observing modes, and (v) larger fields of view. Across a broad range of topics, from the formation of the first black holes and the evolution of galaxies to the baryon cycle, compact objects, stellar explosions, multi-messenger astrophysics, and the dynamic high-energy Universe, the SAG finds that the scientific motivation for a Lynx-class observatory remains compelling and, in many areas, has significantly strengthened over the past decade, prominently through JWST's discovery of the "Little Red Dots", likely massive accreting black holes in infant galaxies whose nature is fundamentally an X-ray question. This report shows that modest extensions beyond the original Lynx design reference mission can unlock transformative science while preserving the observatory's core architecture. Powerful current and future facilities such as Roman, Rubin, JWST, SKA, ngVLA, LISA, and NewAthena highlight the unique role a high-angular-resolution, high-throughput X-ray observatory would play in the multi-wavelength and multi-messenger ecosystem of the 2030s and beyond. The findings of the Lynx2030 SAG confirm Lynx's central vision: an unprecedented view of the hot and energetic Universe, enabling discoveries that will define high-energy astrophysics in the coming decades.
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All authors Lynx2030 Science Analysis Group: Simon R. Bandler, Laura W. Brenneman, Nico Cappelluti, Daniel Castro, Steven R. Ehlert, W. Peter Maksym, Fabio Pacucci, Scott W. Randall, Grant R. Tremblay, John ZuHone, Steven W. Allen, Antara R. Basu-Zych, Akos Bogdan, Joel N. Bregman, Tamta Burduli, Thomas Connor, Sanskriti Das, Casey DeRoo, Stephen DiKerby, Paul A. Draghis, Martin Elvis, Giuseppina Fabbiano, Ralf K. Heilmann, Jimmy A. Irwin, Amruta Jaodand, Margarita Karovska, Vinay L. Kashyap, Anthony A. Kerr, Caroline Kilbourne, Ralph Kraft, Jiangtao Li, Labani Mallick, Herman L. Marshall, Michael L. McCollough, Anna Ogorzałek, Frederik Paerels, Daniel Patnaude, Paul Plucinsky, David Pooley, Frederick S. Porter, Daniele Rogantini, Roger Romani, Helen R. Russell, Kazuhiro Sakai, Mark Schattenburg, Dan A. Schwartz, Malgorzata Sobolewska, Paolo Soffitta, Alexey Vikhlinin, Daniel R. Wilkins, Scott Wolk, Ka-Wah Wong, Irina Zhuravleva

Cosmology and Nongalactic Astrophysics

Reconstructing stochastic gravitational-wave signals from flavour deconstruction with LISA

Noemi Fabri, Raphael Bertrand-Delgado, Danny Laghi et al. (+1 more)

We investigate the reconstruction of a stochastic gravitational-wave background generated by a first-order phase transition in flavour-deconstruction models using the Laser Interferometer Space Antenna (LISA). In these scenarios, the spontaneous breaking of an extended flavour-non-universal gauge symmetry at the TeV scale can both generate the observed hierarchies of Standard Model fermion masses and mixing angles and induce a strong first-order phase transition. We consider representative benchmark points of the model, compute the corresponding thermodynamic transition parameters, and construct the resulting gravitational-wave spectra using a state-of-the-art sound-wave template. We then inject these spectra into simulated LISA data and perform Bayesian inference with SGWBinner, jointly reconstructing the cosmological signal, instrumental noise, and astrophysical foregrounds. We find that the strongest benchmark signal can be successfully reconstructed, whereas weaker signals are substantially degenerate with the unresolved extragalactic compact-binary foreground. We further show that tighter prior information on this foreground, motivated by observations with ground-based detectors, can reduce this degeneracy and improve signal reconstruction. Our results demonstrate that flavour-deconstruction models can produce signals accessible to LISA, while highlighting the importance of astrophysical-foreground modelling for their identification and characterization.
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All authors Noemi Fabri, Raphael Bertrand-Delgado, Danny Laghi, Lucio Mayer

Cosmology and Nongalactic Astrophysics

Comment on `Measuring the Hubble Constant Using Strongly Lensed Gravitational Wave Signals'

Jason S.C. Poon and Otto A. Hannuksela

In Huang et al (JCAP08(2023)003), the work claimed that the Hubble constant can be constrained from lensed gravitational waves, despite the absence of information on GW source redshift or Einstein radius (angular size of the lens), as long as lens redshift is provided independently. To demonstrate, the authors set up a singular isothermal sphere lens model and applied Fisher information matrix and Bayesian parameter estimation on injected lensed GWs. However, their conclusion contradicts our previous work on degeneracies of GW lens system parameters. In this comment, we illustrate that Hubble constant cannot be measured with lens redshift alone, in the absence of source redshift or (Einstein radius).
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Cosmology and Nongalactic Astrophysics

Investigating the magnetic field in the inter-cluster filament between Abell 3667 and Abell 3651 with POSSUM

C. Stuardi, S. P. O'Sullivan, L. Rudnick et al. (+19 more)

[Abridged abstract] The objective of this study is to measure the magnetic field within the prominent inter-cluster filament recently detected in X-rays by the extended ROentgen Survey with an Imaging Telescope Array (eROSITA). This filament spans over 13 Mpc projected on the sky, connecting the galaxy clusters Abell 3667 and Abell 3651. We employed the Polarisation Sky Survey of the Universe's Magnetism (POSSUM) Rotation Measure (RM) grid to isolate the RM dispersion and median value of background polarised sources induced by the filament's magnetised plasma, and to infer the strength of this magnetic field. The filament region is sampled by 54 background polarised sources. After subtracting the foreground Galactic RM, we detected a marginal residual RM dispersion in the filament region of $6.9\pm3.6$ rad/m$^{2}$, together with a coherent residual RM signal with median $6.3\pm1.3$ rad/m$^{2}$. Assuming simplified single-scale magnetic-field models and adopting informed priors on the thermal electron density distribution derived from the X-ray analysis, we constrained the magnetic field strength to the range 0.1-3.5 $\mu$G within 95$\%$ confidence, with preferred values around 0.2-0.3 $\mu$G depending on the assumed magnetic-field coherence scale. However, we also found that the Galactic foreground RM in this region is highly structured on angular scales comparable to the extent of the filament itself, representing a major source of uncertainty for the RM analysis. Our results provide the first magnetic field constraints based on Faraday rotation measurements in an individual X-ray-detected inter-cluster filament, with field strengths consistent with theoretical expectations for gas in bridges and cluster outskirts. Our analysis also highlights the critical importance of accurately modelling Galactic RM foregrounds for future studies of extra-galactic magnetism with POSSUM and the SKA.
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All authors C. Stuardi, S. P. O'Sullivan, L. Rudnick, G. Bernardi, A. Bonafede, J. Dietl, T. Akahori, D. Alonso-López, C. Anderson, E. Carretti, B. M. Gaensler, G. Heald, F. Loi, Y. K. Ma, E. Osinga, G. Pignataro, C. Riseley, X. Sun, A. Thomson, C. L. Van Eck, T. Vernstrom, J. L. West

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

Inspiralling Binary merger parameter reconstruction with lunar and satellite Laser Ranging

Miguel Vanvlasselaer

Precision tracking of Earth–Moon and Earth–satellite ranges offers a novel, low-frequency avenue for detecting gravitational waves from inspiralling massive black hole binaries, complementary to space-based interferometers. In this paper, we propose a first study of the reconstruction of the binary parameters from a synthetic set of range observations. Set-up: Assuming an ideal unperturbed Keplerian motion for the earth-satellite binary, we develop a framework for reconstructing the chirp mass $\mathcal M_c$ and luminosity distance $D_L$ of chirping sources from the perturbations they induce on ranging observables, using both Lunar Laser Ranging (LLR) and a two-satellite ranging configuration (GUEST) as concrete case studies. Result: We find that chirp-mass recovery tightly converges as a power law in SNR ($\sigma_{M_c}/M_c \propto \mathrm{SNR}^{\alpha}$, $\alpha \simeq -1$ to $-1.8$) for both LLR and GUEST. In contrast, the luminosity distance suffers from a severe, SNR-independent degeneracy with the sky-orientation angles in the LLR configuration, yielding fractional biases and uncertainties of tens to over a hundred percent regardless of signal strength. The two-satellite GUEST configuration breaks this degeneracy and restores a convergent, SNR-dependent recovery of $D_L$ comparable in scaling to that of the chirp mass. These results demonstrate that multi-baseline ranging architectures are essential for extracting luminosity-distance information from ranging-based gravitational wave searches, while chirp-mass measurements remain reliably accessible even with a single ranging baseline such as the Earth–Moon system.
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Cosmology and Nongalactic Astrophysics · Cross-listed from astro-ph.GA

Super-Eddington Little Blue Dots May Reionize Helium Too Early

Christopher Cain, Brent Smith, Gibson B. Bowling et al. (+5 more)

The James Webb Space Telescope (JWST) has identified an abundant population of faint Active Galactic Nuclei (AGN) at $z \gtrsim 4$ which display broad emission lines, compact morphologies, and blue ultraviolet–optical continua. These “Little Blue Dots' (LBDs) have been suggested to belong to the same family of objects as the more controversial “Little Red Dots' (LRDs), with differences between the two largely owing to viewing angle effects. In this scenario, super-Eddington accretion resulting in high Extreme UV (EUV) and weak X-ray emission is invoked to explain the properties of both populations. We study the consequences of this super-Eddington scenario for the timing of Helium reionization. We find that observations which support an end to helium reionization no earlier than $z \approx 3$ disfavor scenarios in which the majority of observed $3 \lesssim z \lesssim 7$ LBDs are highly super-Eddington. For our fiducial super-Eddington accretion scenario, we find that the fraction of the LBD population in this state must be $\lesssim 10\%$, assuming LBDs make up $5\%$ of the $M_{ m UV} < -18$ galaxy population and have average escape fractions of $15\%$, comparable to recent observations. Our constraint assumes that LBDs dominate the Helium reionization budget, and would be tighter if bright quasars also contributed significantly. For a majority of LBDs to be super-Eddington, they would need to have small escape fractions ($\lesssim 1.5\%$) and/or be less abundant than observations suggest. Our conclusions are sensitive to the shape of the EUV spectra of super-Eddington black holes, motivating further study.
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All authors Christopher Cain, Brent Smith, Gibson B. Bowling, Yongda Zhu, Lily Whitler, Rafael Ortiz III, Anson D'Aloisio, Rogier Windhorst

Cosmology and Nongalactic Astrophysics

AXIS Could Have Accessed Dark Matter Decays

Joshua W. Foster, Inci Karaaslan, Gordan Krnjaic et al. (+1 more)

The Advanced X-ray Imaging Satellite (AXIS) was a mission concept designed to improve upon the sensitivity and spatial resolution of the Chandra X-ray Observatory and XMM-Newton. Although AXIS was not selected for implementation, the concept study defined a mature instrument design with low-background, arcsecond imaging over the $0.3$-$10$ keV energy range, a large effective area, and a wide field of view. These capabilities provide a useful benchmark for the dark matter decay sensitivity of future X-ray observatories. We estimate the reach of a future AXIS-like instrument for narrow photon lines from decaying keV-scale dark matter, including axion-like particles and sterile neutrinos. For Galactic center observations, we find projected lifetime sensitivities of order $10^{31}$ s, improving upon existing limits by up to an order of magnitude over part of the keV mass range.
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All authors Joshua W. Foster, Inci Karaaslan, Gordan Krnjaic, Nimrod Shapir

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

Fast and efficient nested sampling with BEST

Andreas Nygaard

Nested sampling is widely used for Bayesian evidence computation, but its intrinsically sequential structure limits how efficiently it can exploit modern vectorised likelihoods and emulators. We present a new nested-sampling implementation in best, written entirely in TensorFlow and designed for efficient XLA compilation on both CPUs and GPUs. The sampler combines clustering and slice sampling with the possibility of updating several live points simultaneously. Since batching breaks the strict ordering of conventional nested sampling, we introduce sorting and history-based corrections to reduce the resulting bias in the evidence estimate. We test the sampler on Gaussian, Rosenbrock, and multimodal likelihoods and compare its performance with JAXNS and UltraNest. The results show that accurate evidence estimates can be retained for moderate batch sizes, with $m/N_{\rm live}\lesssim 0.1$ providing a useful practical regime. Finally, using a 27-dimensional cosmological likelihood emulator, we show that batched live-point updates can substantially reduce the wall-clock time while remaining consistent with sequential sampling within the reported uncertainties. The new implementation therefore extends best with an efficient nested-sampling method tailored to fast, vectorised likelihoods and emulator-based inference.
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Cosmology and Nongalactic Astrophysics

Enhancing Cosmological Constraints from Foreground-Cleaned CMB Maps Using Large-Scale Structure Surveys

Shu-Fan Chen and J. Colin Hill

Extragalactic foregrounds contaminate cosmic microwave background (CMB) temperature maps at small angular scales and limit their utility for precision cosmology. The internal linear combination (ILC) is a well-known technique for suppressing these contaminants, but residual foreground power remains a limiting factor. Kusiak et al. (2023) proposed adding galaxy number-density maps as additional ILC channels, exploiting their correlation with the large-scale structure sourcing these foregrounds to suppress contamination. Here we apply this framework to forecast the gains in CMB-based cosmological parameter constraints from near- and next-generation experiments. Using a halo-model foreground pipeline and a Fisher forecast from joint TT+TE+EE power spectra, we quantify the improvement from galaxy-tracer-assisted ILC cleaning across three configurations: enhanced Simons Observatory (SO) with unWISE or Rubin-like galaxy catalogs, and a futuristic CMB-HD configuration with a hypothetical deep galaxy survey. We find that adding galaxy tracers reduces the residual foreground power in the cleaned temperature map by $\sim4\%$, $\sim22\%$, and $\sim32\%$ at $\ell\sim10,000$ for the unWISE, Rubin-like, and futuristic samples, respectively. For the overall variance of the cleaned map at $\ell\sim10,000$, it provides $8\%$, $24\%$, and $17\%$ improvements for each combination. The resulting reduction in marginalized parameter error bars is modest for the base six-parameter $\Lambda$CDM model: sub-percent for SO+unWISE, rising to $\sim2\%$ for SO+Rubin-like tracer. Including the effective number of relativistic species $N_{\rm eff}$, we find at most $2.2\%$ improvements for both SO+Rubin-like and CMB-HD+Futuristic tracer. These results establish the expected gains from combining near-term CMB experiments with current and forthcoming large-scale-structure surveys.
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Cosmology and Nongalactic Astrophysics

Follow-up of SN 2025wny V: Lens Modelling and Cosmography of a Strongly Lensed Superluminous Supernova at $z = 2.015$ using Space Data

Edvard Mörtsell, Joel Johansson, Ariel Goobar et al. (+12 more)

We present a lensing and cosmographic analysis of the strongly lensed Type I superluminous supernova SN 2025wny at redshift $z=2.015$, multiply imaged by two foreground galaxies at $z=0.376$. Using imaging obtained with the Hubble Space Telescope and the James Webb Space Telescope, we model the lens system with two elliptical power-law mass distributions and an external shear component. In addition to the supernova image positions, the modelling incorporates the surface-brightness distribution of the lensed host galaxy. The inferred Einstein radii are $\theta_{\rm E,1}\simeq 1.6$" and $\theta_{\rm E,2}\simeq 0.7$ - $0.8$", with broadly consistent results across all filters. After accounting for microlensing by stars in the lens galaxies, the posterior distribution spans total magnifications of approximately $\mu_{\rm tot}\sim 5$–$50$, with flux ratios of the multiple images consistent with observations. Combining the lens models with spectroscopically and photomerically measured time delays yields a filter-marginalized constraint of \[ H_0 = 66.7^{+7.6}_{-6.3}\; {\rm km\,s^{-1}\,Mpc^{-1}}, \] for a fiducial model with isothermal mass profiles. Allowing the density slopes of the lens galaxies to vary over a broad range results in \[ H_0 = 70.8^{+8.2}_{-6.1}\; {\rm km\,s^{-1}\,Mpc^{-1}}. \] These values are conditional on the adopted parameterization of the lens mass distribution, the assumed priors on the density slopes, and possible additional lensing contributions from the surrounding large-scale environment. We find that incorporating the currently available stellar kinematic measurements has only a modest effect on the inferred value of $H_0$. Future measurements of the lens-galaxy kinematics and a detailed characterization of the lens environment will further strengthen the utility of SN 2025wny as a cosmological probe.
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All authors Edvard Mörtsell, Joel Johansson, Ariel Goobar, Alice Townsend, Hannah C.Turner, Suhail Dhawan, Cameron Lemon, Peter Nugent, Thomas E. Collett, Stephen Thorp, Jacob Osman Hjortlund, Jakob Nordin, Lin Yan, Graham P. Smith, Christoffer Fremling

Cosmology and Nongalactic Astrophysics

Follow-up of SN 2025wny IV: Photometric Time-delay Measurements of a Strongly Lensed Superluminous Supernova

Alice Townsend, Suhail Dhawan, Erin E. Hayes et al. (+34 more)

We present photometric time-delay measurements of SN 2025wny, the first strongly lensed Type I superluminous supernova (SLSN-I), discovered at $z = 2.015$. Time-delay measurements from strongly lensed supernovae provide an independent probe of cosmology and the Hubble constant, $H_0$, without reliance on the local distance ladder. Using multi-facility imaging data, we performed scene-modelling photometry to deblend four of the lensed images (A-D) and construct $grizJ$-band light curves. We modelled the resolved light curves with Gaussian process regression using GausSN (Hayes et al. 2024) to infer relative time delays and magnifications between the lensed images. We found that a constant magnification model provides a suboptimal description of the data, motivating a time-dependent sigmoid magnification model to account for evolving relative magnification of image A. We measured time delays of $\Delta t_{AB} = -10.6^{+2.2}_{-2.5}$ days and $\Delta t_{AC} = 1.2^{+2.7}_{-2.6}$ days (68% credible intervals), consistent with independent spectroscopic measurements from Johansson et al. (2026). Combining the photometric time delays with the lens model of Mörtsell et al. (2026) gives $H_{0,\:\rm photo} = 80.5^{+26.4}_{-16.7}\;\rm km\,s^{-1}\,Mpc^{-1}$, while including the spectroscopic time delays as well yields $H_{0,\:\rm comb} = 70.8^{+8.2}_{-6.1}\;{\rm km\,s^{-1}\,Mpc^{-1}}$. Our results further demonstrate the potential of strongly lensed supernovae as independent probes of $H_0$.
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All authors Alice Townsend, Suhail Dhawan, Erin E. Hayes, Maggie L. Li, Joel Johansson, Edvard Mörtsell, Ariel Goobar, Lin Yan, Charlotte Ward, Veena Krishnaraj, Steve Schulze, Jacob Osman Hjortlund, Yu-Jing Qin, Hannah C. Turner, Peter Massey, Jakob Nordin, Jule Augustin, Aleksandra Bochenek, Malte Busmann, Christoffer Fremling, Daniel Gruen, Xander J. Hall, K.-R. Hinds, Ezequiel J. Marchesini, Zoë McGrath, Conor M. B. Omand, Eliana Palazzi, Daniel A. Perley, Andrea Rossi, Killa Santer, Jesper Sollerman, Chiara Ventura, Jacob L. Wise, Tracy X. Chen, Steven L. Groom, Mansi M. Kasliwal, Josiah Purdum

Cosmology and Nongalactic Astrophysics

Follow-up of SN 2025wny III: Spectroscopic Time-delay Measurements of a Strongly Gravitationally Lensed Superluminous Supernova

Joel Johansson, Edvard Mörtsell, Ariel Goobar et al. (+30 more)

We present spatially resolved spectra and infer the time-delays between the multiple images of the strongly gravitationally lensed superluminous supernova (SLSN) 2025wny at z=2.015. SN 2025wny is the first known spatially resolved strongly lensed SLSN and provides a unique opportunity to measure lensing delays through the temporal evolution of supernova spectra. We present a spectroscopic dataset spanning several months, including spatially resolved spectra of images A, B, C, D, and E. We identify and measure the wavelength evolution of spectral features using Gaussian-process modeling. The time delays are inferred by jointly fitting the temporal evolution of the spectral features, yielding $\Delta t_{AB}=-10.3 \pm 2.3$, $\Delta t_{AC}=0.1 \pm 3.6$, $\Delta t_{AD}=-65.7 \pm 3.5$, and $\Delta t_{AE}=3.7 \pm 8.8$ days (68% confidence intervals). These are the among most precise time-delay measurements obtained for a lensed supernova to date, whether from spectroscopic or photometric methods. The longest delay ($\Delta t_{AD}$) is particularly well constrained, with a ~5% precision. Combined with the lens model presented by Mörtsell et al. (2026), the spectroscopic time-delays give a Hubble constant $H_0 = 70.2^{+8.2}_{-6.1}$ km/s/Mpc. Our analysis demonstrates that spectroscopic evolution provides an independent and complementary route to time-delay measurements in lensed supernova systems, avoiding reliance on photometric light curves alone. As future surveys discover larger samples of lensed supernovae, spectroscopic time-delay measurements will provide an important avenue for precision cosmography.
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All authors Joel Johansson, Edvard Mörtsell, Ariel Goobar, Steve Schulze, Maggie L. Li, Yu-Jing Qin, Lin Yan, Hannah C. Turner, Suhail Dhawan, Alice Townsend, Jakob Nordin, Aleksandra Bochenek, Malte Busmann, Kaustav K. Das, Christoffer Fremling, Lluis Galbany, Alexa C. Gordon, Daniel Gruen, Mansi M. Kasliwal, Chang Liu, Zoë McGrath, Christopher Martin, Peter Massey, Martijn S. S. L. Oei, Daniel A. Perley, Andrés I. Ponte Pérez, Francisco Prada, Nikolaus Z. Prusinski, Nabeel Rehemtulla, R. Michael Rich, Surya Shivaprasad, Jesper Sollerman, Jacob L. Wise

Cosmology and Nongalactic Astrophysics

Follow-up of SN 2025wny II: Superluminous Supernova Physics at Cosmic Noon

Maggie L. Li, Lin Yan, Anamaria Gkini et al. (+46 more)

SN 2025wny is a gravitationally lensed, hydrogen-poor superluminous supernova (SLSN-I) at z = 2.015. To date, it is the most extensively observed high-redshift core-collapse SN and has the most detailed rest-frame UV observations of any SLSN. We present densely sampled rest-frame UV-to-optical photometry and spectroscopy out to +80 d post-peak (rest frame) from several facilities, including JWST, Keck, VLT, Gemini, the Palomar 200-inch, the Fraunhofer Telescope at Wendelstein, and the Liverpool Telescope. Correcting for lensing magnification, SN 2025wny reaches a peak pseudo-bolometric luminosity of $L_{\rm peak}\gtrsim4\times10^{44}$ erg s$^{-1}$ over rest-frame 1500-4230 Å, placing it within the luminosity range of typical SLSNe-I. SN 2025wny exhibits several unusual features, including a continuum excess and sharp spectral features in the FUV from +20-60 d that coincide with an FUV light-curve plateau and higher inferred blackbody temperatures. SN 2025wny's spectra also show little to no UV line blanketing, no obvious O II absorption despite high temperatures, and evidence for C II, H$\alpha$, and possible He I. Light-curve modeling suggests that SN 2025wny may require a hybrid or non-standard power source. This work provides some of the first detailed constraints on high-redshift SLSNe and establishes SN 2025wny as an essential spectral and photometric reference for identifying and interpreting high-redshift SLSNe discovered by Rubin and Roman.
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All authors Maggie L. Li, Lin Yan, Anamaria Gkini, Alice Townsend, Yu-Jing Qin, Steve Schulze, Nikhil Sarin, Suhail Dhawan, Joel Johansson, Ariel Goobar, Jacob Osman Hjortlund, Edvard Mörtsell, Jesper Sollerman, Ragnhild Lunnan, Daniel A. Perley, Ehud Nakhar, Avinash Singh, Avishay Gal-Yam, Mansi M. Kasliwal, Conor M. B. Omand, Aleksandra Bochenek, Malte Busmann, Kaustav K. Das, Christoffer Fremling, Alexa C. Gordon, Daniel Gruen, Wynn Jacobson-Galán, K-Ryan Hinds, Chang Liu, Zoë McGrath, Ezequiel J. Marchesini, Christopher Martin, Peter Massey, Martijn S. S. L. Oei, Eliana Palazzi, Francisco Prada, Nikolaus Z. Prusinski, Nabeel Rehemtulla, Andrea Rossi, R. Michael Rich, Sam Rose, Killa Santer, Surya Shivaprasad, Hannah C. Turner, Chiara Ventura, Jacob L. Wise, Michael Coughlin, Argyro Sasli, Niharika Sravan

Cosmology and Nongalactic Astrophysics

Follow-up of SN 2025wny I: Space-based Observations of the First Multiply-imaged Superluminous Supernova

Ariel Goobar, Joel Johansson, Edvard Mörtsell et al. (+21 more)

We present space-based follow-up observations of the superluminous Type I supernova (SLSN-I) SN 2025wny at redshift $z_{SN}=2.0151$, gravitationally lensed by two galaxies at redshifts $z_{G1}=0.3755$ and $z_{G2}=0.3766$ into five resolved images. SN 2025wny is the first strongly lensed SLSN discovered and the first galaxy-scale lensed supernova for which both photometric and spectroscopic time-delay measurements are feasible. As such, it opens a new observational window for precision cosmology and the study of stellar explosions near the epoch of peak cosmic star formation. Our follow-up observations comprise two epochs of Hubble Space Telescope (HST) imaging, together with near-infrared imaging and spectroscopy obtained with the James Webb Space Telescope (JWST). From these data, we measure precise astrometry and multi-band photometry for the five resolved supernova images, the host galaxy, and the two deflecting galaxies. HST provides accurate relative image positions and rest-frame ultraviolet photometry, while JWST delivers complementary near-infrared imaging and spectroscopy probing the rest-frame optical at high signal-to-noise ratio. Together they yield a detailed characterization of both the lensing configuration and the supernova spectral energy distribution over a broad wavelength range. The data presented here provide the observational foundation for the accompanying analyses of the supernova properties, lens modeling, and time-delay cosmography, including the astrometric, photometric, and spectroscopic information required to measure $H_0$.
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All authors Ariel Goobar, Joel Johansson, Edvard Mörtsell, Cameron Lemon, Steve Schulze, Suhail Dhawan, Alice Townsend, Maggie L. Li, Yu-Jing Qin, Lin Yan, Christoffer Fremling, Mansi M. Kasliwal, Peter Nugent, Graham P. Smith, Jesper Sollerman, Igor Andreoni, Nikki Arendse, Thomas E. Collett, Jakob Nordin, Jacob Osman Hjortlund, Daniel A. Perley, Mickael Rigault, Stephen Thorp, Jacob L. Wise

Cosmology and Nongalactic Astrophysics

PIFFLE: Characterizing the Foreground Contributions from 4 Decades in Halo Mass to the FRB20230907D Dispersion Measure

Qi Guo, Khee-Gan Lee, Sunil Simha et al. (+10 more)

We characterize the foreground environment of FRB20230907D, localized to a galaxy at $z=0.464$, which has an observed dispersion measure of ${\rm DM}_{\rm obs}=1031~{\rm pc~cm^{-3}}$. At its redshift, FRB20230907D lies above the Macquart relation, the expected relation between cosmological dispersion measure and the source redshift, indicating a substantial excess DM along this line of sight. We use Subaru/PFS and SDSS spectroscopy, published group catalogs, Rubin/LSST imaging, and eROSITA X-ray data to characterize the foreground structures that may account for this excess. A friends-of-friends search identifies a massive foreground system at $z\simeq0.09$ with $M_{200}\simeq5.2\times10^{14}~M_\odot$, while low redshift catalogs reveal an additional group at $z\simeq0.02565$. Assuming that the halo gas follows a modified-NFW halo density profile, we estimate observer frame contributions of $150^{+110}_{-70}~{\rm pc~cm^{-3}}$ and $80^{+60}_{-40}~{\rm pc~cm^{-3}}$ from these systems, respectively. Together with the Milky Way, diffuse intergalactic medium, Virgo cluster, M49 group, and host galaxy contributions, these foreground structures can account for the excess dispersion measure of FRB20230907D within uncertainties. This highlights the importance of dense foreground spectroscopy and multi-wavelength data.
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All authors Qi Guo, Khee-Gan Lee, Sunil Simha, Chenze Dong, Ilya S. Khrykin, Nicolas Tejos, J. Xavier Prochaska, Kevin McCarthy, John D. Silverman, Ines Pastor-Marazuela, Ben Stappers, K. M. Rajwade, Manisha Caleb

Cosmology and Nongalactic Astrophysics

Explicit gauge-invariant variables in multifield inflation beyond linear order and Hamiltonian dynamics

Julien Grain, Hugo Holland and Lucas Pinol

General relativity coupled to multiple scalar fields is a diffeomorphism-invariant constrained system. Consequently, a naive counting of the perturbative degrees of freedom unavoidably overestimates the true number of physical modes propagating in the theory, as gauge redundancies and constraint equations remove non-dynamical ones. While this problem has been solved for linear fluctuations, this work presents the first explicit calculation of all large-scale gauge-invariant phase-space variables in multifield inflation and at second order in perturbation theory, in a Hamiltonian language. Building upon the well-known Sasaki-Mukhanov variables, we show how to construct a finite-dimensional basis of quadratic corrections which are invariant under gauge transformations. Although our procedure is generic to any number of fields and at any scale, we restrict to super-Hubble scales for their explicit solution, which we deliver. Henceforth, we prove that it is possible to recover the usual flat-gauge and comoving-gauge fluctuations as large-scale gauge-invariant combinations, making for a robust consistency check of the gauge-fixed procedure to connect theoretical predictions above the horizon to observations. We derive the quadratic and cubic Hamiltonian of multifield inflation in a gauge independent manner, then we gauge fix our theory by going into the flat gauge, and we show perfect agreement with the literature on this topic, usually based on a Lagrangian approach. After these concrete steps, we propose a more formal proof of the existence of gauge-invariant variables at quadratic order, and we provide a sketch of the procedure that should allow to go to higher orders in perturbation theory.
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Cosmology and Nongalactic Astrophysics

Guide for the Hubble Tension: Mid-Time Solutions and Generalized PAge Parameterizations

Jing-Yi Jia, Da-Chun Qiang and Hao Wei

In cosmology, the Hubble tension has become one of the most serious challenges in the last decade. Assuming that all the observational data are right, one of the ways out is to modify the standard $\Lambda$CDM cosmology. In the literature, the theoretical modifications are mainly made in the early and the late/local universes. But on both sides, some no-go arguments have been proposed recently. In particular, the general no-go arguments for the late-time modifications are mainly based on the PAge parameterization. In the present work, we note that there are some flaws in the (original) PAge parameterization, and then propose various generalized PAge (GPAge) parameterizations, which can be extended to $z>z_{\rm CMB}$ and hence the cosmic microwave background (CMB) can be taken into account, while they are also more accurate at low redshifts. With these GPAge parameterizations, we revisit the Hubble tension by using not only the late-time observations but also the observational data of CMB. We find that the Hubble tension could be alleviated (or even resolved) in GPAges significantly different from the standard $\Lambda$CDM cosmology, while they are overwhelmingly preferred over $\Lambda$CDM by the observational data in terms of all the information criteria. Surprisingly, we find that the new physics in the mid-time might be the key to resolve the Hubble tension.
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Cosmology and Nongalactic Astrophysics · Cross-listed from astro-ph.IM

The Canadian Galactic Emission Mapper: A New 8-10 GHz Telescope to Map the Polarization of the Northern Sky

Joshua MacEachern, Mandana Amiri, Charles L. Bennett et al. (+12 more)

The Canadian Galactic Emission Mapper (CGEM) radio telescope is mapping polarized Galactic foregrounds from 8-10 GHz to aid in the search for B-modes in the Cosmic Microwave Background (CMB). Here we describe the design and early on-sky characterization of the CGEM optics. CGEM employs an on-axis, hat feed optical design that is a body of revolution (BOR) and exhibits excellent intrinsic polarization purity. We describe how we have optimized the optics with a novel framework that can directly minimize simulated intensity to polarized intensity ($T\rightarrow P$) leakage in angular power spectrum space. The optimized optics exhibit simulated $T\rightarrow P$ leakage that is orders of magnitude below anticipated $C_\ell^{BB}$ for $r = 10^{-3}$ when scaled from 8-10 GHz to the CMB observing window near 95 GHz. We go on to describe the mechanical design of the optics, including a novel secondary mirror support made from Astroquartz composite that has low loss, low dielectric constant, and preserves the BOR symmetry of the optics. We then showcase the early on-sky performance of the optics with observations of the Sun and satellites, which probe the beam to 40 dB down from the peak. A beam model for the deployed CGEM optics based on electromagnetic simulations is in excellent agreement with the on-sky data.
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All authors Joshua MacEachern, Mandana Amiri, Charles L. Bennett, Guinevere Berg, Mark Halpern, Gary Hinshaw, Gordon Lacy, Thomas J. Rennie, Shuyu van Kerkwijk, Bruce Veidt, Pedro Villalba-González, Janet Weiland, Don Wiebe, Edward J. Wollack, Parham Zarei

Cosmology and Nongalactic Astrophysics

Learning the averaged history of an inhomogeneous universe from its present day density field

Jonas Broe Bendtsen and Sofie Marie Koksbang

In this work, we investigate whether machine learning can be used to infer averaged cosmological quantities directly from the present day matter density distribution. Using an implementation of the simplified silent universe framework, we generate a dataset consisting of 92610 independent relativistic, simplified cosmological simulations spanning a range of initial conditions and average cosmological parameters. We train a convolutional neural network to take the present-time matter density field as input and predict the averaged matter, cosmological constant, curvature, and kinematical backreaction density parameters as well as the Hubble parameter, at both initial and present time. The network achieves coefficients of determination exceeding 0.9 for all predicted quantities, with error distributions showing that only a small fraction of predictions reach percent-level errors or above. Although our use of the simplified silent universe approximation precludes direct application of the trained model to observational data, the results provide a proof-of-principle that neural networks can successfully recover averaged cosmological properties, including backreaction, at different epochs, simply from the late-time matter distribution. More broadly, our findings demonstrate that information about the averaged cosmological history of a universe is encoded in its present-time density field and can be extracted using machine learning techniques. This opens the possibility of applying similar approaches to more realistic cosmological simulations and observational probes such as N-body simulations and weak-lensing maps, ultimately providing a new avenue for constraining the large-scale evolution of the Universe.
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Cosmology and Nongalactic Astrophysics

A Unified Tracer Analysis of DESI DR2 Baryon Acoustic Oscillations

N. Sanders, H. Seo, M. Rashkovetskyi et al. (+65 more)

We improve upon previous efforts to optimally combine overlapping galaxy samples in the DESI baryon acoustic oscillation analysis. By weighting each galaxy by its linear bias, overlapping galaxies are combined into a single, unified catalog, naturally avoiding double counting of cosmic volume and including all auto- and cross- information at the catalog level. Improvements over the previous effort include the addition of QSO out to $z=1.6$ to account for all overlapping DR2 tracers and redshift-dependent bias treatment to improve reconstruction. We report distance measurements using this unified tracer, and find them to be highly consistent with the baseline DR2 BAO analysis. We also test for tracer-dependent systematics within the DESI data, and find no evidence of tracer-dependent systematics within $0.8<z<1.6$. Finally, we take advantage of the unified tracer to rebin the analysis in redshift in order to more finely resolve the redshift-to-distance relation. Dynamical dark energy results on this finer redshift binning indicate that there is no missed feature in the expansion history in the redshifts $0.8<z<1.6$, and reproduces DESI's preference for an evolving dark energy equation of state.
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All authors N. Sanders, H. Seo, M. Rashkovetskyi, U. Andrade, D. Valcin, E. Paillas, P. McDonald, J. Aguilar, S. Ahlen, O. Alves, E. Armengaud, A. Aviles, F. Beutler, D. Bianchi, D. Brooks, A. Carnero Rosell, E. Chaussidon, T. Claybaugh, S. Cole, A. Cuceu, A. de la Macorra, Biprateep Dey, Z. Ding, P. Doel, S. Ferraro, A. Font-Ribera, D. Forero-Sánchez, J. E. Forero-Romero, E. Gaztañaga, Satya Gontcho A Gontcho, A. X. Gonzalez-Morales, G. Gutierrez, C. Hahn, H. K. Herrera-Alcantar, K. Honscheid, S. Juneau, T. Karim, D. Kirkby, A. Kremin, O. Lahav, C. Lamman, M. Landriau, L. Le Guillou, M. Manera, A. Meisner, R. Miquel, J. Moustakas, A. Muñoz-Gutiérrez, S. Nadathur, J. A. Newman, H. E. Noriega, N. Palanque-Delabrouille, W. J. Percival, F. Prada, I. Pérez-Ràfols, C. Ravoux, A. J. Ross, G. Rossi, L. Samushia, E. Sanchez, C. Saulder, D. Schlegel, M. Schubnell, J. Silber, M. Siudek, G. Tarlé, M. Vargas-Magaña, B. A. Weaver

Thursday 27 August

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

Creation of an inflationary epoch from an Emergent Universe through quantum tunneling

Eduardo I. Guendelman, Ramon Herrera and Pedro Labraña

We consider that the very early Universe was well described by an Emergent Universe, whose geometry was that of a static and classically stable Einstein Universe, which is possible in certain Two Measures Theories (TMT). These solutions do not have a big bang, but rather they extend to arbitrarily early times. The TMT we consider are theories with spontaneously broken scale invariance and contain a dilaton field, crucial for the implementation of the scale invariance, that after the symmetry breaking acquires a nontrivial effective potential, which we study in the Einstein frame. Using the conserved quantity associated with the dilaton, we recast the cosmological evolution in terms of an effective potential $V(\dot{\phi})$, which exhibits a divergent barrier separating the static solution from the subsequent expanding evolution. Quantizing the corresponding minisuperspace Hamiltonian, we evaluate the tunneling probability in the WKB approximation. The tunneling action is well defined and finite once the branch of the square root is fixed by requiring a consistent semiclassical interpretation of the tunneling probability. We find that the barrier gives rise to a logarithmic contribution to the tunneling exponent, yielding a power-law rather than the usual exponential dependence of the tunneling probability, which for the parameter values considered is close to unity. The Universe thus emerges with a finite scale factor into a superinflationary phase that develops into slow-roll inflation. This quantum creation of an inflationary Universe does not represent a quantum creation of spacetime, which exists before the tunneling.
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