Cosmological Models Research Articles

Fluctuations and correlations in causal set theory

Abstract We study the statistical fluctuations (such as the variance) of causal set quantities, with particular focus on the causal set action. To facilitate calculating such fluctuations, we develop tools to account for correlations between causal intervals with different cardinalities. We present a convenient decomposition of the fluctuations of the causal set action into contributions that depend on different kinds of correlations. This decomposition can be used in causal sets approximated by any spacetime manifold M . Our work paves the way for investigating a number of interesting discreteness effects, such as certain aspects of the Everpresent Λ cosmological model.

Homology reveals significant anisotropy in the cosmic microwave background

We test the tenet of statistical isotropy of the standard cosmological model via a homology analysis of the cosmic microwave background (CMB) temperature maps in galactic coordinates. The map pixels were normalized by subtracting the mean and rescaling by standard deviation, both of which were computed from the relevant unmasked pixels. Examining small sectors of the normalized maps, we find that the results exhibit a dependence on whether we compute the mean and variance locally from the non-masked patch, or from the full masked sky. Assigning local mean and variance for normalization, we find the maximum discrepancy between the data and model in the northern hemisphere, at more than $3.5$ standard deviations (s.d.) for the PR4 dataset at degree scale. For the PR3 dataset, the C-R and SMICA maps display a higher significance than the PR4 dataset at ∼ 4 and $4.1$ s.d., respectively; however, the NILC and SEVEM maps present a lower significance at ∼ 3.4 s.d. The discrepancy is most prominent at scales of roughly a degree, which coincides with the physical scale of the horizon at the epoch of the CMB. The southern hemisphere exhibits a high degree of consistency between the data and the model for both the PR4 and PR3 datasets. Assigning the mean and variance of the full masked sky decreases the significance for the northern hemisphere; in particular, the tails. However, the tails in the southern hemisphere are strongly discrepant at more than $4$ standard deviations at approximately $5$ degrees. The p values obtained from the χ^2-statistic show commensurate significance in both experiments. Examining the quadrants of the sphere, we find the northwest quadrant of the Galactic frame to be the major source of the discrepancy. Prima facie, the results indicate a breakdown of statistical isotropy in the CMB maps; however, more work is needed to ascertain the source of the anomaly. Regardless, these map characteristics may have serious consequences for downstream computations and parameter estimation, and the related problems of Hubble and σ_8 tension.

New exact solutions of two-field inflationary model via Noether symmetries

In this paper, we consider a cosmological model of hyperbolic inflation involving two-scalar fields. By examining the Noether symmetries of the corresponding Lagrangian in the context of a spatially flat Friedmann–Lemaître–Robertson–Walker spacetime, we identify conserved quantities or first integrals corresponding to specific cases. These conserved quantities facilitate the derivation of novel solutions to the field equations, offering new perspectives on the dynamics of two-field inflationary models.

Dust-fluid accelerating flat cosmological models in f(R,T,Lm)-gravity with observational constraints

In this paper, we investigate dust-fluid flat cosmological models in the recently proposed modified [Formula: see text]-gravity theory. We derive the field equations for the flat FLRW spacetime metric for the arbitrary function [Formula: see text], where [Formula: see text] is the Ricci curvature scalar, [Formula: see text] is the matter Lagrangian, [Formula: see text] is the trace of the energy–momentum tensor [Formula: see text], and [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] are the model parameters. We solve these equations to obtain the Hubble function in terms of matter energy density parameters [Formula: see text], [Formula: see text], [Formula: see text], and Hubble constant [Formula: see text]. Then, we use the cosmic chronometer (CC) Hubble points dataset and the Pantheon dataset to do MCMC analysis of the Hubble function and find the best fit model parameters for the lowest [Formula: see text] values. Subsequently, we investigate the effective equation of state parameter [Formula: see text] and deceleration parameter [Formula: see text] for the present past epoch of the universe. We also perform analysis for energy conditions and statefinder parameters to discuss the different stages of the dark energy models.

Conformal Solutions of Static Plane Symmetric Cosmological Models in Cases of a Perfect Fluid and a Cosmic String Cloud

In this work, we obtained exact solutions of Einstein’s field equations for plane symmetric cosmological models by assuming that they admit conformal motion. The space-time geometry of these solutions is found to be nonsingular, non-vacuum and conformally flat. We have shown that in the case of a perfect fluid, these solutions have an energy-momentum tensor possessing dark energy with negative pressure and the energy equation of state is ρ+p=0. We have shown that a fluid has acceleration, rotation, shear-free, vanishing expansion, and rotation. In the case of a cosmic string cloud, we found that the tension density and particle density decrease as the fluid moves along the direction of the strings, then vanish at infinity. We shown that the exact conformal solution for a static plane symmetric model reduces to the well-known anti-De Sitter space-time. We obtained that the space-time under consideration admits a conformal vector field orthogonal to the 4-velocity vector and does not admits a vector parallel to the 4-velocity vector. Some physical and kinematic properties of the resulting models are also discussed.

Colonial Entanglements: Rethinking Space, Spatiality, and Spacetime

This paper interrogates the ontological and epistemological transformations of spatial paradigms within the framework of colonial hegemony, elucidating the commodification and annexation of lands as emblematic of deeper ontological fractures. Through a meticulous historiographical analysis, the study foregrounds the aggressive territorial acquisitions characteristic of colonial expansion, positing these not merely as physical dominations but as epistemicide— the erasure of indigenous and first nations spatial understandings wherein space is regarded as a living, spiritual entity, integral to all forms of life. By critically examining the socio-political fabric of colonial dispossession, particularly within the Canadian context, this research serves as a poignant critique of the eurocentric spatial paradigms that dominate modern scientific discourse on space, spatiality, and spacetime. It then extends this analysis to the cosmic realm, employing the ƛCDM model of cosmology as a metaphor for understanding the interconnectedness of the universe and the colonial imprint on our scientific and philosophical outlooks on space. Furthermore, the paper delves into the philosophical underpinnings of space, spatiality, and spacetime, tracing its evolution from ancient Greece to contemporary poststructuralist and postcolonial theories. It challenges the foundational assumptions of logic, math, and physics and encourages a decolonized approach to understanding reality, suggesting that such perspectives could unveil more nuanced insights into the cosmos. The analysis culminates in a call for a holistic reconceptualization of space, spatiality, and spacetime, advocating for a future where decolonization, inclusivity, and equity form the cornerstones of our engagement with spatial realities. It posits that embracing relationality and acknowledging the interconnectedness of all forms of existence can lead to a more just and sustainable spatial praxis, transcending historical legacies of domination and exclusion.

Neural Networks for cosmological model selection and feature importance using Cosmic Microwave Background data

The measurements of the temperature and polarisation anisotropies of the Cosmic Microwave Background (CMB) by the ESA Planck mission have strongly supported the current concordance model of cosmology. However, the latest cosmological data release from ESA Planck mission still has a powerful potential to test new data science algorithms and inference techniques. In this paper, we use advanced Machine Learning (ML) algorithms, such as Neural Networks (NNs), to discern among different underlying cosmological models at the angular power spectra level, using both temperature and polarisation Planck 18 data. We test two different models beyond ΛCDM: a modified gravity model: the Hu-Sawicki model, and an alternative inflationary model: a feature-template in the primordial power spectrum. Furthermore, we also implemented an interpretability method based on SHAP values to evaluate the learning process and identify the most relevant elements that drive our architecture to certain outcomes. We find that our NN is able to distinguish between different angular power spectra successfully for both alternative models and ΛCDM. We conclude by explaining how archival scientific data has still a strong potential to test novel data science algorithms that are interesting for the next generation of cosmological experiments.

Studying Thermal and Dynamical Stability of Interacting Rényi and Tsallis Holographic Dark Energy Models in LTB Inhomogeneous Universe

Abstract This work aims to investigate the different stability conditions of two scenarios of the inhomogeneous Lemaitre–Tolman–Bond model of the universe with holographic dark energy. We considered the Rényi and Tsallis holographic models of interacting dark energy. These holographic models are investigated using the IR cutoff that equals the Hubble horizon. Various stability conditions of these models have been investigated to understand how much these models can tell us about the recent and future epochs of the universe in comparison with the cosmological constant model, or ΛCDM model. The conditions of violating the cosmological energy conditions have been studied. The evolution of the entropy and its first and second derivatives have been calculated and plotted for these holographic models. This gives an idea of how far these models satisfy the generalized second law of thermodynamics and hence have thermodynamical stability. The dynamical stability is studied for these evolved models, which give us glimpses of the dynamical stability at different phases of its evolution. We focus on investigating the stability in recent and near future times up to z ≤ −4. Further investigation of stability has been obtained by studying the evolved sound speed squared parameter for these models, which gave us a final and decisive evaluation of the stability of these models.

Chebyshev cosmography in the framework of extended symmetric teleparallel theory

Cosmography has been extensively utilized to constrain the kinematic state of the Universe using measured distances. In this work, we propose a new method to reconstruct coupling theories using the first kind of Chebyshev polynomial for two variables in which the functional form of the f(Q,T) theory has been obtained. Further, the unknowns that appeared in the series are constrained using the cosmographic parameters. We find the explicit form of the luminosity distance in terms of cosmographic parameters to perform MCMC analysis using the PANTHEON+SH0ES data set. Through the distance modulus function, we observe that the result comes out to be an excellent match to the standard cosmological model and data.

A comparison of Bayesian and frequentist confidence intervals in the presence of a late Universe degeneracy

Hubble tension is a problem in one-dimensional (1D) posteriors, since local H0 determinations are only sensitive to a single parameter. Projected 1D posteriors for ΛCDM cosmological parameters become more non-Gaussian with increasing effective redshift when the model is fitted to redshift-binned data in the late Universe. We explain mathematically why this non-Gaussianity arises and show, using observational Hubble data (OHD), that Markov chain Monte Carlo (MCMC) marginalisation leads to 1D posteriors that fail to track the χ2 minimum at 68% confidence level in high redshift bins. To gain a second perspective, we resort to profile likelihoods as a complementary technique. Doing so, we observe that z≳1 cosmic chronometer (CC) data currently prefer a non-evolving (constant) Hubble parameter over a Planck-ΛCDM cosmology at ∼2σ. Within the Hubble tension debate, it is imperative that subsamples of data sets with differing redshifts yield similar H0 values. In addition, we confirm that MCMC degeneracies observed in 2D posteriors are not due to curves of constant χ2. Finally, on the assumption that the Planck-ΛCDM cosmological model is correct, using profile likelihoods we confirm a >2σ2 \\sigma $$\\end{document}]]> discrepancy with Planck-ΛCDM in a combination of CC and baryon acoustic oscillations (BAO) data beyond z∼1.5. This confirms a discrepancy reported earlier with fresh methodology.

Enhancing Cosmological Model Selection with Interpretable Machine Learning

Enhancing Cosmological Model Selection with Interpretable Machine Learning

Metric-affine cosmological models and the inverse problem of the calculus of variations. Part II: Variational bootstrapping of the ΛCDM model

The method of variational bootstrapping, based on canonical variational completion, allows one to construct a Lagrangian for a physical theory depending on two sets of field variables, starting from a guess of the field equations for only one such set. This setup is particularly appealing for the construction of modified theories of gravity, since one can take lessons from GR for an “educated guess” of the metric field equations; the field equations for the other fields are then fixed by the obtained Lagrangian (up to terms that are completely independent from the metric tensor). In the present paper, we apply variational bootstrapping to determine metric-affine models which are, in a variational sense, closest to the ΛCDM model of cosmology. Starting from an “educated guess” that formally resembles the Einstein field equations with a cosmological “constant” (actually, a scalar function built from the metric and the connection) and a dark matter term, the method then allows to find “corrected” metric equations and to “bootstrap” the connection field equations. Lagrangians obtained via this method, though imposing some restricting criteria, still encompass a wide variety of metric-affine models. In particular, they allow for a subclass of quadratic metric-affine theories restricted to linear terms in the curvature tensor.

MUSEQuBES: Unveiling Cosmic Web Filaments at z ≈ 3.6 through Dual Absorption and Emission Line Analysis

Abstract According to modern cosmological models, galaxies are embedded within cosmic filaments, which supply a continuous flow of pristine gas, fueling star formation and driving their evolution. However, due to their low density, the direct detection of diffuse gas in cosmic filaments remains elusive. Here, we report the discovery of an extremely metal-poor ([X/H] ≈ −3.7), low-density ( log 10 n H / cm − 3 ≈ −4, corresponding to an overdensity of ≈5) partial Lyman limit system (pLLS) at z ≈ 3.577 along the quasar sightline Q1317–0507, probing cosmic filaments. Additionally, two other low-metallicity ([X/H] ≲ −2) absorption systems are detected at similar redshifts, one of which is also a pLLS. Very Large Telescope (VLT) MUSE observations reveal a significant overdensity of Lyα emitters (LAEs) associated with these absorbers. The spatial distribution of the LAEs strongly suggests the presence of an underlying filamentary structure. This is further supported by the detection of a large Lyα-emitting nebula with a surface brightness of ≥ 1 0 − 19 erg cm − 2 s − 1 arcsec − 2 , with a maximum projected linear size of ≈260 pkpc extending along the LAEs. This is the first detection of giant Lyα emission tracing cosmic filaments, linked to normal galaxies and likely powered by in situ recombination.

Constraints on cosmological models with gamma-ray bursts in cosmology-independent way

In this paper, we present a cosmology-independent method to constrain cosmological models from the latest 221 gamma-ray bursts (GRBs) sample, including 49 GRBs from Fermi catalog with the Amati relation (the [Formula: see text] correlation), which are calibrated by using a Gaussian process from the Pantheon+ type Ia supernovae (SNe Ia) sample. With 182 GRBs at [Formula: see text] in the Hubble diagram and the latest observational Hubble data (OHD) by the Markov Chain Monte Carlo (MCMC) method, we obtained [Formula: see text] and [Formula: see text] for the flat [Formula: see text]CDM model and [Formula: see text], [Formula: see text], [Formula: see text] for the flat wCDM model. These results are consistent with those in which the coefficients of the Amati relation and the cosmological parameters fitted simultaneously.

Model - Independent Probe of Cosmic Distance Duality Relation

Abstract In this paper, cosmic distance duality relation is probed without considering any background cosmological model. The only \textit{a priori} assumption is that the Universe is described by the Friedmann-Lema$\hat{i}$tre-Robertson-Walker (FLRW) metric 
The strong gravitational lensing (SGL) data is used to construct the dimensionless co-moving distance function $d(z)$ and latest type Ia supernovae (SNe Ia) Pantheon+ data is used to estimate luminosity distances at the corresponding redshifts $z$. Using the distance sum rule along null geodesics of the FLRW metric, the CDDR violation is probed in both flat and non-flat space-time by considering two parametrizations for $\eta(z)$, the function generally used to probe the possible deviations from CDDR. The results show that, CDDR is compatible with the observations at a very high level of confidence for linear parametrization in flat Universe. In non-flat Universe too, CDDR is valid within $1\sigma$ confidence interval with a mild dependence of $\eta$ on the curvature density parameter $\Omega_{K}$ . The results for non-linear parametrization also show no significant deviation from CDDR.

Stability analysis of cosmological model in f(T) gravity

In this paper, we examine the stability condition of [Formula: see text] gravity theory where T is the Torsion scalar using interacting and non-interacting models with the help of dynamical system. We let the [Formula: see text] function be [Formula: see text], where T is Torsion scalar and [Formula: see text] and [Formula: see text] are the arbitrary constants. We calculated the critical points and study the stability behavior for this model. We analyze the system’s phase graphs and examined the physical interpretation. We demonstrate all of the cosmological parameters including [Formula: see text], [Formula: see text], q, and [Formula: see text] at each critical point and contrast them with values from observations. Then, we assume hybrid scale factor and transform it into the equation of redshift and time. Using this equation, we convert all the parameters into terms of redshift and analyze their behavior. Our model illustrates the Universe’s accelerating expansion. The energy conditions are examined in terms of redshift for the model. As a result, we conclude that our [Formula: see text] model is stable and all the observed values are in agreement with observations.

Distribution-free uncertainty quantification for inverse problems: Application to weak lensing mass mapping

In inverse problems, the aim of distribution-free uncertainty quantification (UQ) is to obtain error bars in the reconstruction with coverage guarantees that are independent of any prior assumptions about the data distribution. This allows for a better understanding of how intermediate errors introduced during the process affect subsequent stages and ultimately influence the final reconstruction. In the context of mass mapping, uncertainties could lead to errors that affect how the underlying mass distribution is understood or that propagate to cosmological parameter estimation, thereby impacting the precision and reliability of cosmological models. Current surveys, such as Euclid or Rubin will provide new weak lensing datasets of very high quality. Accurately quantifying uncertainties in mass maps is therefore critical to fully exploit their scientific potential and to perform reliable cosmological parameter inference. In this paper, we extend the conformalized quantile regression (CQR) algorithm, initially proposed for scalar regression, to inverse problems. We compared our approach with another distribution-free approach based on risk-controlling prediction sets (RCPS). Both methods are based on a calibration dataset, and they offer finite-sample coverage guarantees that are independent of the data distribution. Furthermore, they are applicable to any mass mapping method, including black box predictors. In our experiments, we applied UQ to three mass-mapping methods: the Kaiser-Squires inversion, iterative Wiener filtering, and the MCALens algorithm. Our experiments reveal that RCPS tends to produce overconservative confidence bounds with small calibration sets, whereas CQR is designed to avoid this issue. Although the expected miscoverage rate is guaranteed to stay below a user-prescribed threshold regardless of the mass mapping method, selecting an appropriate reconstruction algorithm remains crucial for obtaining accurate estimates, especially around peak-like structures, which are particularly important for inferring cosmological parameters. Additionally, the choice of mass mapping method influences the size of the error bars.

Quantum Entanglement Asymmetry and the Cosmic Matter–Antimatter Imbalance: A Theoretical and Observational Analysis

We propose a distinct mechanism to explain the matter–antimatter imbalance observed in the universe, rooted in quantum entanglement asymmetry (QEA). Our concept of QEA differs from its usage in the recent literature, where it typically measures how much symmetry is broken within a subsystem of a larger quantum system. Here, we define QEA as an intrinsic asymmetry in the entanglement properties of particle–antiparticle pairs in the early universe, leading to a preferential survival of matter over antimatter. We develop a theoretical framework incorporating QEA into the standard cosmological model, providing clear justification for the asymmetry in entangled states and corresponding modifications to the Hamiltonian. Numerical simulations using lattice Quantum Chromodynamics (QCD) demonstrate that QEA can produce a net baryon asymmetry consistent with observations. We also predict specific signatures in Cosmic Microwave Background (CMB) anisotropies and large-scale structure formation, offering potential avenues for empirical verification. This work aims to deepen the understanding of cosmological asymmetries and highlight the significance of quantum entanglement in the universe’s evolution.

Local peculiar motions in McVittie and LTB spacetimes

We consider two inhomogeneous cosmological models, namely, the flat McVittie spacetime and a simple specific LTB spacetime. Relative to the world line of a reference comoving observer that remains spatially at rest, we study the local deviations of the world lines of free test particles. These local peculiar motions can be invariantly characterized within the framework of a quasi-inertial Fermi normal coordinate system established along the world line of the reference comoving observer. Tidal dynamics in the McVittie model involves the sum of the curvature due to the inhomogeneity, the curvature due to the background FLRW spacetime and a mixed term, while tidal dynamics in the particular LTB model turns out to be qualitatively the same as in the Einstein-de Sitter universe. Peculiar motions in the two cosmological models are briefly compared and contrasted.

A Scale Invariant Fully Conformal Cosmological Model and Its Support by Astrophysical Data

A Scale Invariant Fully Conformal Cosmological Model and Its Support by Astrophysical Data