Mirror QCD phase transition as the origin of the nanohertz Stochastic Gravitational-Wave Background

Several Pulsar Timing Array (PTA) Collaborations have recently provided strong evidence for a nHz Stochastic Gravitational-Wave Background (SGWB). Here we investigate the implications of a first-order phase transition occurring within the early Universe's dark quantum chromodynamics epoch, specifically within the framework of the mirror twin Higgs dark sector model. Our analysis indicates a distinguishable SGWB signal originating from this phase transition, which can explain the measurements obtained by PTAs. Remarkably, a significant portion of the parameter space for the SGWB signal also effectively resolves the existing tensions in both the H0 and S8 measurements in Cosmology. This intriguing correlation suggests a possible common origin of these three phenomena for 0.2<ΔNeff<0.5, where the mirror dark matter component constitutes less than 30% of the total dark matter abundance. Next-generation CMB experiments such as CMB-S4 can test this parameter region.

Search for a Neutron Dark Decay in ^{6}He

Neutron dark decays have been suggested as a solution to the discrepancy between bottle and beam experiments, providing a dark matter candidate that can be searched for in halo nuclei. The free neutron in the final state following the decay of ^{6}He into ^{4}He+n+χ provides an exceptionally clean detection signature when combined with a high efficiency neutron detector. Using a high-intensity ^{6}He^{+} beam at Grand Accélérateur National d'Ions Lourds, a search for a coincident neutron signal resulted in an upper limit on a dark decay branching ratio of Br_{χ}≤4.0×10^{-10} (95% C.L.). Using the dark neutron decay model proposed originally by Fornal and Grinstein, we translate this into an upper bound on a dark neutron branching ratio of O(10^{-5}), improving over global constraints by one to several orders of magnitude depending on m_{χ}.

FLAMINGO: calibrating large cosmological hydrodynamical simulations with machine learning

To fully take advantage of the data provided by large-scale structure surveys, we need to quantify the potential impact of baryonic effects, such as feedback from active galactic nuclei (AGN) and star formation, on cosmological observables. In simulations, feedback processes originate on scales that remain unresolved. Therefore, they need to be sourced via subgrid models that contain free parameters. We use machine learning to calibrate the AGN and stellar feedback models for the FLAMINGO (Fullhydro Large-scale structure simulations with All-sky Mapping for the Interpretation of Next Generation Observations) cosmological hydrodynamical simulations. Using Gaussian process emulators trained on Latin hypercubes of 32 smaller volume simulations, we model how the galaxy stellar mass function (SMF) and cluster gas fractions change as a function of the subgrid parameters. The emulators are then fit to observational data, allowing for the inclusion of potential observational biases. We apply our method to the three different FLAMINGO resolutions, spanning a factor of 64 in particle mass, recovering the observed relations within the respective resolved mass ranges. We also use the emulators, which link changes in subgrid parameters to changes in observables, to find models that skirt or exceed the observationally allowed range for cluster gas fractions and the SMF. Our method enables us to define model variations in terms of the data that they are calibrated to rather than the values of specific subgrid parameters. This approach is useful, because subgrid parameters are typically not directly linked to particular observables, and predictions for a specific observable are influenced by multiple subgrid parameters.

Canonical Hubble-Tension-Resolving Early Dark Energy Cosmologies Are Inconsistent with the Lyman-α Forest

Current cosmological data exhibit discordance between indirect and some direct inferences of the present-day expansion rate H_{0}. Early dark energy (EDE), which briefly increases the cosmic expansion rate prior to recombination, is a leading scenario for resolving this "Hubble tension" while preserving a good fit to cosmic microwave background (CMB) data. However, this comes at the cost of changes in parameters that affect structure formation in the late-time universe, including the spectral index of scalar perturbations n_{s}. Here, we present the first constraints on axionlike EDE using data from the Lyman-α forest, i.e., absorption lines imprinted in background quasar spectra by neutral hydrogen gas along the line of sight. We consider two independent measurements of the one-dimensional Lyα forest flux power spectrum from the Sloan Digital Sky Survey (SDSS eBOSS) and from the MIKE/HIRES and X-Shooter spectrographs. We combine these with a baseline dataset comprised of Planck CMB data and baryon acoustic oscillation (BAO) measurements. Combining the eBOSS Lyα data with the CMB and BAO dataset reduces the 95% confidence level (C.L.) upper bound on the maximum fractional contribution of EDE to the cosmic energy budget f_{EDE} from 0.07 to 0.03 and constrains H_{0}=67.9_{-0.4}^{+0.4}  km/s/Mpc (68% C.L.), with maximum a posteriori value H_{0}=67.9  km/s/Mpc. Similar results are obtained for the MIKE/HIRES and X-Shooter Lyα data. Our Lyα-based EDE constraints yield H_{0} values that are in >4σ tension with the SH0ES distance-ladder measurement and are driven by the preference of the Lyα forest data for n_{s} values lower than those required by EDE cosmologies that fit Planck CMB data. Taken at face value, the Lyα forest severely constrains canonical EDE models that could resolve the Hubble tension.

Logarithmic Duality of the Curvature Perturbation

We study the comoving curvature perturbation R in the single-field inflation models whose potential can be approximated by a piecewise quadratic potential V(φ) by using the δN formalism. We find a general formula for R(δφ,δπ), consisting of a sum of logarithmic functions of the field perturbation δφ and the velocity perturbation δπ at the point of interest, as well as of δπ_{*} at the boundaries of each quadratic piece, which are functions of (δφ,δπ) through the equation of motion. Each logarithmic expression has an equivalent dual expression, due to the second-order nature of the equation of motion for φ. We also clarify the condition under which R(δφ,δπ) reduces to a single logarithm, which yields either the renowned "exponential tail" of the probability distribution function of R or a Gumbel-distribution-like tail.

Constraints on the Cosmic Expansion Rate at Redshift 2.3 from the Lyman-α Forest

We determine the product of the expansion rate and angular-diameter distance at redshift z=2.3 from the anisotropy of Lyman-α (Lyα) forest correlations measured by the Sloan Digital Sky Survey (SDSS). Our result is the most precise from large-scale structure at z>1. Using the flat Λ cold dark matter model we determine the matter density to be Ω_{m}=0.36_{-0.04}^{+0.03} from Lyα alone. This is a factor of 2 tighter than baryon acoustic oscillation results from the same data due to our use of a wide range of scales (25<r<180  h^{-1} Mpc). Using a nucleosynthesis prior, we measure the Hubble constant to be H_{0}=63.2±2.5  km/s/Mpc. In combination with other SDSS tracers, we find H_{0}=67.2±0.9  km/s/Mpc and measure the dark energy equation-of-state parameter to be w=-0.90±0.12. Our Letter opens a new avenue for constraining cosmology at high redshift.

What It Takes to Solve the Hubble Tension through Modifications of Cosmological Recombination

We construct data-driven solutions to the Hubble tension which are perturbative modifications to the fiducial ΛCDM cosmology, using the Fisher bias formalism. Taking as proof of principle the case of a time-varying electron mass and fine structure constant, and focusing first on Planck CMB data, we demonstrate that a modified recombination can solve the Hubble tension and lower S_{8} to match weak lensing measurements. Once baryonic acoustic oscillation and uncalibrated supernovae data are included, however, it is not possible to fully solve the tension with perturbative modifications to recombination.

Testing general relativity with cosmological large scale structure

In this paper I investigate the possibility to test Einstein's equations with observations of cosmological large scale structure. I first show that we have not tested the equations in observations concerning only the homogeneous and isotropic Universe. I then show with several examples how we can do better when considering the fluctuations of both, the energy momentum tensor and the metric. This is illustrated with galaxy number counts, intensity mapping and cosmic shear, three examples that are by no means exhaustive.

On the Degeneracy between fσ8 Tension and Its Gaussian Process Forecasting

In this Article, we reconstruct the growth and evolution of the cosmic structure of the Universe using Markov chain Monte Carlo algorithms for Gaussian processes. We estimate the difference between the reconstructions that are calculated through a maximization of the kernel hyperparameters and those that are obtained with a complete exploration of the parameter space. We find that the difference between these two approaches is of the order of 1%. Furthermore, we compare our results with those obtained by Planck Collaboration 2018 assuming a ΛCDM model and we do not find a statistically significant difference in the redshift range where the reconstructions of fσ8 have been made.

A Short Review on the Latest Neutrinos Mass and Number Constraints from Cosmological Observables

We review the neutrino science, focusing on its impact on cosmology along with the latest constraints on its mass and number of species. We also discuss its status as a possible solution to some of the recent cosmological tensions, such as the Hubble constant or the matter fluctuation parameter. We end by showing forecasts from next-generation planned or candidate surveys, highlighting their constraining power, alone or in combination, but also the limitations in determining neutrino mass distribution among its species.

How the Big Bang Ends Up Inside a Black Hole

The standard model of cosmology assumes that our Universe began 14 Gyrs (billion years) ago from a singular Big Bang creation. This can explain a vast range of different astrophysical data from a handful of free cosmological parameters. However, we have no direct evidence or fundamental understanding of some key assumptions: Inflation, Dark Matter and Dark Energy. Here we review the idea that cosmic expansion originates instead from gravitational collapse and bounce. The collapse generates a Black Hole (BH) of mass M≃5×1022M⊙ that formed 25 Gyrs ago. As there is no pressure support, the cold collapse can continue inside in free fall until it reaches atomic nuclear saturation (GeV), when is halted by Quantum Mechanics, as two particles cannot occupy the same quantum state. The collapse then bounces like a core-collapse supernovae, producing the Big Bang expansion. Cosmic acceleration results from the BH event horizon. During collapse, perturbations exit the horizon to re-enter during expansion, giving rise to the observed universe without the need for Inflation or Dark Energy. Using Ockham’s razor, this makes the BH Universe (BHU) model more compelling than the standard singular Big Bang creation.