Cosmological Models Research Articles

Accelerating cosmological models in Hoyle–Narlikar gravity with observational constraints

In this paper, we explore the cosmic acceleration and the physical aspects of the dark energy nature in Hoyle–Narlikar’s creation-field theory, taking into account observational constraints. We find analytical solutions to the modified field equations for a specific choice of creation field [Formula: see text] with certain background fluid sources as a barotropic fluid and a radiation fluid in a flat Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime universe. From there, we use observational constraints with data from the cosmic chronometer (CC) Hubble data and the apparent magnitude from the Pantheon SNe sample to get the model parameters’ constrained values with [Formula: see text] and [Formula: see text] confidence levels. We look at the physical properties of the models we’ve made using these estimated values of model parameters. To do this, we look at the deceleration parameter [Formula: see text], the effective equation of state parameter [Formula: see text], the jerk parameter [Formula: see text], the snap parameter [Formula: see text], and the energy conditions. We also examine the cosmic behavior of square sound speed [Formula: see text] for the test of causality and viability of the models. We measure the present age of the universe and transition redshift [Formula: see text] for the signature-flipping point. We obtain two transit-phase accelerating universe models with [Formula: see text] Km/s/Mpc, [Formula: see text], [Formula: see text], and [Formula: see text], respectively, with barotropic and radiation fluid sources.

Dynamical system approach of interacting dark energy models in f(R, Tϕ) gravity

Abstract We have examined an isotropic and homogeneous cosmological model in $f(R,T^{\phi})$ gravity, where $R$ represents the Ricci scalar and $T^{\phi}$ exhibits the energy momentum tensor's trace. We examine the stability criteria by performing the dynamical system analysis for our model $f(R,T^{\phi})=R+2(aT^{\phi}+b)$, where $a$ $\&$ $b$ are the constants. We derive a set of autonomous equations and find their solutions by assuming a flat potential $V_{0}$. We assess the equilibrium points from these equations and find the eigenvalues. We analyze the physical interpretation of the phase space for this system. We obtain three stable equilibrium points. We also examine the interaction between scalar field and dark energy, represented by $Q=\psi H\rho_{de}$ and determine the equilibrium points for this interaction. We identify four stable equilibrium points for this interaction. We calculate the values of the physical parameters for both scenarios at each equilibrium point, indicating the Universe's accelerated expansion.

Measuring the matter fluctuations in the Local Universe with the ALFALFA catalogue

Abstract The standard model of cosmology describes the matter fluctuations through the matter power spectrum, where σ8 ≡ σ8, 0 ≡ σ8(z = 0), defined at the scale of 8 h−1 Mpc, acts as a normalisation parameter. Currently, the literature reports measurements of σ8 analysing different cosmic tracers, where some of these results were obtained assuming a fiducial cosmology. In this study we measure, in a model-independent approach, the matter fluctuations in the Local Universe using HI extragalactic sources mapped by the ALFALFA survey. Our analyses allow us to test the standard cosmological model under extreme conditions in the highly non-linear Local Universe, quantifying the amplitude of the matter fluctuations there. Our work directly measures σ8 using the 3-dimensional distances of the HI sources determined by the ALFALFA survey without assuming a fiducial cosmology, resulting in a robust model-independent measurement of σ8. Our methodology involves the construction of suitable mock catalogues to simulate the large scale structure features observed in the data, applying the 2-point correlation function, and making use of Markov Chain Monte Carlo methods to estimate the parameters. Analysing these data we measure σ8 = 0.78 ± 0.04 for h = 0.6727, σ8 = 0.80 ± 0.05 for h = 0.698, and σ8 = 0.83 ± 0.05 for h = 0.7304. Considering the data pairs (σ8, H0) from the Planck CMB and ACT CMB-lensing analyses, our measurement agrees with them within 1 σ confidence level. From a model-independent perspective, we find that the scale where the matter fluctuation is 1, is R = 7.2 ± 1.5 Mpc.

Gravitational wave luminosity distance for Starobinsky gravity in viscous cosmological models

In this paper, in the Starobinsky gravity (SG) model, the gravitational wave (GW) luminosity distance dLGW(z) is analytically and numerically investigated by considering the dark matter as the two different kinds of shear viscous fluids in universe. Concretely, on the basis of the propagation equation of the metric perturbation derived by using transverse-traceless gauge for the linear perturbation of the metric in FRW background, we obtain the analytical expressions of the modified friction term δ(z) and the ratio dLGW(z)/dLEM(z) of GW to electromagnetic (EM) wave luminosity distance, and find that they only depend on the SG model parameter α and the shear viscous coefficient η, regardless of the bulk viscous coefficient ζ. Evidently, both of them are affected by α and η. It worth stressing that the results given by us can reduce to ones in documents [13, 22, 30]. It follows that we can extend the previous results as the special cases in our model. Furthermore, by combining the latest observational data, we more tightly constrain the values of related parameters and numerically analyze the influence of both α and η on δ(z) and dLGW(z)/dLEM(z). We observe that, in the range of 0<z≤8.2, the evolutionary curves of δ(z) and dLGW(z)/dLEM(z) exhibit that δ(z) (dLGW(z)/dLEM(z)) monotonically decrease (increase) as redshift rises, moreover there are always δ(z)<0 and dLGW(z)≥dLEM(z), specially dLGW(0)=dLEM(0) in the limit of z→0, which means that detecting GW signals might represent a more efficient tool than detecting EM signals to test modified gravity model on the given scale of cosmic distances. It is worth noting that the numerical analyses to δ(z) and dLGW(z)/dLEM(z) illustrate once again that the GW luminosity distance dLGW(z) is indeed influenced by α and η, in particular more sensitive to α than η in our model. In addition, by comparing the GW and EM luminosity distances of SG model with dL(GR) of Einstein GR, we find that the GW and EM signals in SG1 and SG2 change from stronger to weaker than the ones in GR as redshift increases and more difficultly to be detected in the same distance and detection sensitivity. Meanwhile, the EM signals change from strong to weak earlier than GW signals do. Finally, we confirm that δ(z) and dLGW(z) of our model meet the constraints imposed by the standard siren GW170817 and its electromagnetic companion GRB170817A.

Modified power law cosmology: theoretical scenarios and observational constraints

Abstract This research paper examines a cosmological model in flat space-time via $f(R,G)$ gravity where $R$ and $G$ are respectively the Ricci scalar and Gauss-Bonnet invariant. Our model assumes that $f(R,G)$ is an exponential function of $G$ combined with a linear combination of $R$. We scrutinize the observational limitations under a power law cosmology that relies on two parameters - $H_0$, the Hubble constant, and $q$, the deceleration parameter, utilizing the 57-point $H(z)$ data, 8-point BAO data, 1048-point Pantheon data, joint data of $H(z)$ + Pantheon, and joint data of $H(z)$ + BAO + Pantheon. The outcomes for $H_0$ and $q$ are realistic within observational ranges. We have also addressed Energy Conditions, $Om(z)$ analysis and cosmographical parameters like Jerk, Lerk and Snap. Our estimate of $H_0$ is remarkably consistent with various recent Planck Collaboration studies that utilize the $\Lambda$CDM model. According to our study, the power law cosmology within the context of $f(R,G)$ gravity provides the most comprehensive explanation for the important aspects of cosmic evolution.

A new approach in classical Klein-Gordon cosmology: “Small Bangs”, inflation and Dark Energy

Abstract In this work, we analyze the cosmological model in which the expansion is driven by a classical, free Klein-Gordon field on a flat, four-dimensional Friedmann-Lemaitre-Robertson-Walker spacetime. The model allows for arbitrary mass, non-zero cosmological constant and coupling to curvature. We find that there are strong restrictions to the parameter space, due to the requirement for the reality of the field values. At early cosmological times, we observe Big Bang singularities, solutions where the scale factor asymptotically approaches zero, and Small Bangs. The latter are solutions for which the Hubble parameter diverges at a finite value of the scale factor. They appear generically in our model for certain curvature couplings. An early inflationary era is observed for a specific value of the curvature coupling without further assumptions (unlike in many other inflationary models). A late-time Dark Energy period is present for all solutions with positive cosmological constant, numerically suggesting that a “cosmic no-hair” theorem holds under more general assumptions than the original Wald version which relies on classical energy conditions. The classical fields in consideration can be viewed as resembling one-point functions of a semiclassical model, in which the cosmological expansion is driven by a quantum field.&amp;#xD;

Revisiting the cosmological time dilation of distant quasars: Influence of source properties and evolution

Abstract After decades of searching, cosmological time dilation was recently identified in the timescale of variability seen in distant quasars. Here, we expand on the previous analysis to disentangle this cosmological signal from the influence of the properties of the source population, specifically the quasar bolometric luminosity and the rest-frame emission wavelength at which the variability was observed. Furthermore, we consider the potential influence of the evolution of the quasar population over cosmic time. We find that a significant intrinsic scatter of 0.288 ± 0.021 dex in the variability timescales, which was not considered in the previous analysis, is favoured by the data. This slightly increases the uncertainty in the results. However, the expected cosmological dependence of the variability timescales is confirmed to be robust to changes in the underlying assumptions. We find that the variability timescales increase smoothly with both wavelength and bolometric luminosity, and that black hole mass has no effect on the variability timescale once rest wavelength and bolometric luminosity are accounted for. Moreover, if the standard cosmological model is correct, governed by relativistic expansion, we also find very little cosmological evolution in the intrinsic variability timescales of distant quasars.

Implications of the Intriguing Constant Inner Mass Surface Density Observed in Dark Matter Halos

It has long been known that the observed mass surface density of cored dark matter (DM) halos is approximately constant, independently of the galaxy mass (i.e., ρcrc≃constant, with ρc and rc being the central volume density and the radius of the core, respectively). Here, we review the evidence supporting this empirical fact as well as its theoretical interpretation. It seems to be an emergent law resulting from the concentration–halo mass relation predicted by the current cosmological model, where the DM is made of collisionless cold DM particles (CDM). We argue that the prediction ρcrc≃constant is not specific to this particular model of DM but holds for any other DM model (e.g., self-interacting) or process (e.g., stellar or AGN feedback) that redistributes the DM within halos conserving its CDM mass. In addition, the fact that ρcrc≃constant is shown to allow the estimate of the core DM mass and baryon fraction from stellar photometry alone is particularly useful when the observationally expensive conventional spectroscopic techniques are unfeasible.

Model-agnostic assessment of dark energy after DESI DR1 BAO

Baryon acoustic oscillation measurements by the Dark Energy Spectroscopic Instrument (Data Release 1) have revealed exciting results that show evidence for dynamical dark energy at ∼ 3σ when combined with cosmic microwave background and type Ia supernova observations. These measurements are based on the w 0 w a CDM model of dark energy. The evidence is less in other dark energy models such as the wCDM model. In order to avoid imposing a dark energy model, we reconstruct the distance measures and the equation of the state of dark energy independent of any dark energy model and driven only by observational data. Our results show that the model-agnostic (in terms of late-time models) evidence for dynamical dark energy from DESI is not significant. Our analysis also provides model-independent constraints on cosmological parameters such as the Hubble constant and the matter-energy density parameter at present. Although we used CMB distance priors (not full CMB data) from a ΛCDM early-time model, our results remain largely similar for other cosmological models, provided that these models do not differ significantly from the standard model.

Model-independent gamma-ray bursts constraints on cosmological models using machine learning

Model-independent gamma-ray bursts constraints on cosmological models using machine learning

Infrared measurements of the JWST suggest that our dynamic universe is spatially closed

Abstract Global geometry and shape of the physical universe may be revealed by observing objects at large cosmological redshift z, since for small z the universe seems almost flat. Recent infrared measurements of the James Webb Space Telescope (JWST) indicate that there exist very luminous galaxies at distances z ≥ 13 that should not exist according to the standard ΛCDM cosmological model for the flat universe with curvature index k = 0. We introduce a spacetime-lens principle that could explain why these very distant galaxies shine so much. We show that the observed large flux luminosities may be mere optical effects due to the positive curvature index k = 1 of an expanding 3-sphere modeling our physical universe in time. For Euclidean or hyperbolic geometries such large flux luminosities seem implausible. This suggests that the right model of a homogeneous and isotropic physical universe for each fixed time instant is a 3-sphere. The standard cosmological model is based on the normalized Friedmann equation ΩM + ΩΛ + Ω k = 1, where ΩM + ΩΛ = 1 by measurements. We also show that this does not imply that Ω k = 0 and k = 0 as it is often claimed.

The concept of God as a Contemplative: cosmological, anthropological, and ethical-soteriological aspects

In this publication, the authors offer an experience in the theoretical construction of a cosmological concept based on the idea of God as a Contemplative. The specifics of this model are revealed in comparison with similar schemes from the history of philosophy. The author's approach is consistent with the principles of the New Testament revelation, as well as modern scientific data. The article shows a way to overcome the typical problems of classical cosmogonic concepts for European philosophy, starting from Antiquity. The article presents the problems of typical solutions to the cosmological question in European philosophy and offers the experience of a consistent model of the coexistence of God as the Absolute and the worlds generated by him, including material ones similar to the observable universe, based on the category of "contemplation". This category is briefly considered by the authors in a historical and theological context, then a general description of the new theological and cosmological model is given. The authors reveal the paradigmatic limitations of the philosophical understanding of God, focusing on the need to take into account the influence of natural philosophical beliefs of a particular era, which are reflected in religious doctrines (along with the personal experience of a particular thinker). In the second half of the article, specific conclusions from the proposed theological and philosophical concept are indicated: 1) cosmological conclusions (divine ideas and the way they are implemented in the world; the question of theodicy; understanding divine Providence); 2) anthropological (human phenomenon; changing approach to human rationality; characterization of God's will in relation to individual freedom); 3) ethical-soteriological (communion with God, divine revelation and salvation; comprehension of certain ethical categories within the framework of a new concept; ontological meaning of heaven and hell). The presented philosophical and religious concept of the "Contemplative God" represents a fundamentally new direction within the framework of religious and philosophical thought. This model is distinguished by its uniqueness and internal logical consistency, being free from the limitations and disadvantages inherent in previous philosophical systems. Its theoretical significance and originality can arouse wide interest in the scientific and philosophical community, opening up new perspectives for research in this field.

Testing the thermal Sunyaev-Zel’dovich power spectrum of a halo model using hydrodynamical simulations

Statistical properties of large-scale cosmological structures serve as powerful tools for constraining the cosmological properties of our Universe. Tracing the gas pressure, the thermal Sunyaev-Zel’dovich (tSZ) effect is a biased probe of mass distribution and, hence, can be used to test the physics of feedback or cosmological models. Therefore, it is crucial to develop robust modelling of hot gas pressure for applications to tSZ surveys. Since gas collapses into bound structures, it is expected that most of the tSZ signal is within halos produced by cosmic accretion shocks. Hence, simple empirical halo models can be used to predict the tSZ power spectra. In this study, we employed the HMx halo model to compare the tSZ power spectra with those of several hydrodynamical simulations: the Horizon suite and the Magneticum simulation. We examine various contributions to the tSZ power spectrum across different redshifts, including the one- and two-halo term decomposition, the amount of bound gas, the importance of different masses, and the electron pressure profiles. Our comparison of the tSZ power spectrum reveals discrepancies between the halo model and cosmological simulations that increase with redshift. We find a 20% to 50% difference between the measured and predicted tSZ angular power spectrum over the multipole range ℓ = 103 − 104. Our analysis reveals that these differences are driven by the excess of power in the predicted two-halo term at low k and in the one-halo term at high k. At higher redshifts (z ∼ 3), simulations indicate that more power comes from outside the virial radius than from inside, suggesting a limitation in the applicability of the halo model. We also observe differences in the pressure profiles, despite the fair level of agreement on the tSZ power spectrum at low redshift with the default calibration of the halo model. In conclusion, our study suggests that the properties of the halo model need to be carefully controlled against real or mock data to be proven useful for cosmological purposes.

Alternative cosmologies

Abstract A few remarkable examples of alternative cosmological theories are shown, ranging from a compilation of variations on the Standard Model (inhomogeneous universe, Cold Big Bang, varying physical constants or gravity law, zero-active mass, Milne cosmology, cyclical models), through the more distant quasi-steady-state cosmology, plasma cosmology, or universe models as a hypersphere such as the Dynamic Universe, to the most exotic cases including static models with non-cosmological redshifts of galaxies. Most cosmologists do not usually work within the framework of alternative cosmologies very different from the standard one because they feel that these are not at present as competitive as the standard model. It is true that they are not so developed, but that is because cosmologists do not work on them. This vicious circle is to a great extent due to a sociological phenomenon known as the “snowball effect”, in which resources are distributed to the most successful theory at a given time; the effect acts as a potential in a field that attracts cosmologists, causing funds, research positions, prestige, telescope time, publication in top journals, citations, conferences, and other resources to be dedicated almost exclusively to standard cosmology.

On the structural stability of a simple cosmological model in + theory of gravity

On the structural stability of a simple cosmological model in + theory of gravity

Euclid preparation

The Euclid mission of the European Space Agency will provide weak gravitational lensing and galaxy clustering surveys that can be used to constrain the standard cosmological model and its extensions, with an opportunity to test the properties of dark matter beyond the minimal cold dark matter paradigm. We present forecasts from the combination of the Euclid weak lensing and photometric galaxy clustering data on the parameters describing four interesting and representative non-minimal dark matter models: a mixture of cold and warm dark matter relics; unstable dark matter decaying either into massless or massive relics; and dark matter undergoing feeble interactions with relativistic relics. We modelled these scenarios at the level of the non-linear matter power spectrum using emulators trained on dedicated N-body simulations. We used a mock Euclid likelihood and Monte Carlo Markov chains to fit mock data and infer error bars on dark matter parameters marginalised over other parameters. We find that the Euclid photometric probe (alone or in combination with cosmic microwave background data from the Planck satellite) will be sensitive to the effect of each of the four dark matter models considered here. The improvement will be particularly spectacular for decaying and interacting dark matter models. With Euclid, the bounds on some dark matter parameters can improve by up to two orders of magnitude compared to current limits. We discuss the dependence of predicted uncertainties on different assumptions: the inclusion of photometric galaxy clustering data, the minimum angular scale taken into account, and modelling of baryonic feedback effects. We conclude that the Euclid mission will be able to measure quantities related to the dark sector of particle physics with unprecedented sensitivity. This will provide important information for model building in high-energy physics. Any hint of a deviation from the minimal cold dark matter paradigm would have profound implications for cosmology and particle physics.

Testing the robustness of the BAO determination in the presence of massive neutrinos

We study the robustness of the Baryon Acoustic Oscillation (BAO) feature in the large-scale structure in the presence of massive neutrinos. In the standard BAO analysis pipeline a reference cosmological model is assumed to boost the BAO peak through the so-called reconstruction technique and in the modelling of the BAO feature to extract the cosmological information. State-of-the art spectroscopic BAO measurements, such as the Dark Energy Spectroscopic Instrument claim an aggregate precision of 0.52% on the BAO scale, with a systematic error of 0.1% associated to the assumption of a reference cosmology when measuring and analyzing the BAO feature. While the systematic effect induced by this arbitrary choice of fiducial cosmology has been studied for a wide range of ΛCDM-like models, it has not yet been tested for reference cosmologies with massive neutrinos with the precision afforded by next generation surveys. In this context, we employ the Quijote high-resolution dark-matter simulations with haloes above a mass of M ∼ 2×1013 h -1 M ⊙, with different values for the total sum of neutrinos masses, ∑mν [eV] = 0, 0.1, 0,2, 0.4 to study and quantify the impact of the pipeline's built-in assumption of massless neutrinos on the measurement of the BAO signal, with a special focus on the BAO reconstruction technique. We determine that any additional systematic bias introduced by the assumption of massless neutrinos is no greater than 0.1% (0.2%) for the isotropic (anisotropic) measurement. We expect these conclusions also hold for galaxies provided that neutrino properties do not alter the galaxy-halo connection.

Euclid and KiDS-1000: Quantifying the impact of source-lens clustering on cosmic shear analyses

Cosmic shear is a powerful probe of cosmological models and the transition from current Stage-III surveys such as the Kilo-Degree Survey (KiDS) to the increased area and redshift range of Stage IV surveys such as Euclid will significantly increase the precision of weak lensing analyses. However, with increasing precision, the accuracy of model assumptions needs to be evaluated. In this study, we quantify the impact of the correlated clustering of weak lensing source galaxies with the surrounding large-scale structure, known as source-lens clustering (SLC), which is commonly neglected. We include the impact of realistic scatter in photometric redshift estimates, which impacts the assignment of galaxies to tomographic bins and increases the SLC. For this, we use simulated cosmological datasets with realistically distributed galaxies and measure shear correlation functions for both clustered and uniformly distributed source galaxies. Cosmological analyses are performed for both scenarios to quantify the impact of SLC on parameter inference for a KiDS-like and a Euclid-like setting. We find for Stage III surveys such as KiDS, SLC has a minor impact when accounting for nuisance parameters for intrinsic alignments and shifts of tomographic bins, as these nuisance parameters absorb the effect of SLC, thus changing their original meaning. For KiDS (Euclid), the inferred intrinsic alignment amplitude AIA changes from 0.11+0.44−0.46 (−0.009+0.079−0.080) for data without SLC to 0.28+0.42−0.44 (0.022+0.081−0.082) with SLC. However, fixed nuisance parameters lead to shifts in S8 and Ωm, emphasizing the need for including SLC in the modelling. For Euclid we find that σ8, Ωm, and w0 are shifted by 0.19, 0.12, and 0.12σ, respectively, when including free nuisance parameters, and by 0.20, 0.16, and 0.32σ when fixing the nuisance parameters. Consequently, SLC on its own has only a small impact on the inferred parameter inference when using uninformative priors for nuisance parameters. However, SLC might conspire with the breakdown of other modelling assumptions, such as magnification bias or source obscuration, which could collectively exert a more pronounced effect on inferred parameters.

Neutrino cosmology after DESI: tightest mass upper limits, preference for the normal ordering, and tension with terrestrial observations

The recent DESI Baryon Acoustic Oscillation measurements have led to tight upper limits on the neutrino mass sum, potentially in tension with oscillation constraints requiring ∑ mν ≳ 0.06 eV. Under the physically motivated assumption of positive ∑ mν , we study the extent to which these limits are tightened by adding other available cosmological probes, and robustly quantify the preference for the normal mass ordering over the inverted one, as well as the tension between cosmological and terrestrial data. Combining DESI data with Cosmic Microwave Background measurements and several late-time background probes, the tightest 2σ limit we find without including a local H 0 prior is ∑ mν < 0.05 eV. This leads to a strong preference for the normal ordering, with Bayes factor relative to the inverted one of 46.5. Depending on the dataset combination and tension metric adopted, we quantify the tension between cosmological and terrestrial observations as ranging between 2.5σ and 5σ. These results are strenghtened when allowing for a time-varying dark energy component with equation of state lying in the physically motivated non-phantom regime, w(z) ≥ -1, highlighting an interesting synergy between the nature of dark energy and laboratory probes of the mass ordering. If these tensions persist and cannot be attributed to systematics, either or both standard neutrino (particle) physics or the underlying cosmological model will have to be questioned.

Bianchi-type HDE Cosmological Models with Emergent Scenarios of Scale Factors under Gravity

Bianchi-type HDE Cosmological Models with Emergent Scenarios of Scale Factors under Gravity