Singularities and soft-Big Bang in a viscous ΛCDM model

The universe content is considered as a non-perfect fluid with bulk viscosity and can be described by a general equation of state (endowed some deviation from the conventionally assumed cosmic perfect fluid model). An explicitly bulk viscosity dark energy model is proposed to confront consistently with the current observational data sets by statistical analysis and is shown consistent with (not deviated away much from) the concordant Λ Cold Dark Matter (CDM) model by comparing the decelerating parameter. Also we compare our relatively simple viscosity dark energy model with a more complicated one by contrast with the concordant Λ CDM model and find our model improves for the viscosity dark energy model building. Finally we discuss the perspectives of dark energy probes for the coming years with observations.

The Cold Dark Matter (CDM) model has an elegant simplicitly which makes it very predictive, but when its parameters are fixed at their ‘canonical’ values its predictions are in conflict with observational data. There is, however, much leeway in the initial conditions within the CDM framework. We advocate a re-examination of the CDM model, taking into account modest variation of parameters from their canonical values. We find that CDM models with n = 0.8–0.9 and h = 0.45–0.50 can fit the available data. Our ‘best fitting’ CDM model has n = 0.9, h = 0.45 and |$C_{2}^{\text T}/C_{2}^{\text S}=0.7$|⁠. We discuss the current state of observations which could definitely rule out this model.

We investigate the effects of warm dark matter (WDM) on the cosmic 21-cm signal. If dark matter exists as WDM instead of cold dark matter (CDM), its non-negligible velocities can inhibit the formation of low-mass halos that normally form first in CDM models, therefore delaying star-formation. The absence of early sources delays the build-up of UV and X-ray backgrounds that affect the 21-cm radiation signal produced by neutral hydrogen. With use of the 21CMFAST, code, we demonstrate that the pre-reionization 21-cm signal can be changed significantly in WDM models with a free-streaming length equivalent to that of a thermal relic with mass mx of up to ~10-20 keV. In such a WDM cosmology, the 21-cm signal traces the growth of more massive halos, resulting in a delay of the 21-cm absorption signature and followed by accelerated X-ray heating. CDM models where astrophysical sources have a suppressed photon-production efficiency can delay the 21-cm signal as well, although its subsequent evolution is not as rapid as compared to WDM. This motivates using the gradient of the global 21-cm signal to differentiate between some CDM and WDM models. Finally, we show that the degeneracy between the astrophysics and mx can be broken with the 21-cm power spectrum, as WDM models should have a bias-induced excess of power on large scales. This boost in power should be detectable with current interferometers for models with mx < 3 keV, while next generation instruments will easily be able to measure this difference for all relevant WDM models.

In `entropic cosmology', instead of a cosmological constant $\Lambda$, an extra driving term is added to the Friedmann equation and the acceleration equation, taking into account the entropy and the temperature on the horizon of the universe. By means of the modified Friedmann and acceleration equations, we examine a non-adiabatic-like accelerated expansion of the universe in entropic cosmology. In this study, we consider a homogeneous, isotropic, and spatially flat universe, focusing on the single-fluid (single-component) dominated universe at late-times. To examine the properties of the late universe, we solve the modified Friedmann and acceleration equations, neglecting high-order corrections for the early universe. We derive the continuity (conservation) equation from the first law of thermodynamics, assuming non-adiabatic expansion caused by the entropy and temperature on the horizon. Using the continuity equation, we formulate the generalized Friedmann and acceleration equations, and propose a simple model. Through the luminosity distance, it is demonstrated that the simple model agrees well with both the observed accelerated expansion of the universe and a fine-tuned standard $\Lambda$CDM (lambda cold dark matter) model. However, we find that the increase of the entropy for the simple model is likely uniform, while the increase of the entropy for the standard $\Lambda$CDM model tends to be gradually slow especially after the present time. In other words, the simple model predicts that the present time is not a special time, unlike for the prediction of the standard $\Lambda$CDM model.

view Abstract Citations (15) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Are SP 91 and COBE Inconsistent with Cold Dark Matter? Bunn, Emory ; White, Martin ; Srednicki, Mark ; Scott, Douglas Abstract We present results on the consistency of standard cold dark matter (CDM) models with both the COBE normalization and the data from the 4th channel of the 9-point South Pole scan. We find that CDM models are consistent with both experiments, a conclusion which is at odds with some other analyses. This is partly due to our careful treatment of the temperature autocorrelation function, but also derives from a realization that the statistical conclusions depend strongly on assumptions about the prior distribution for the normalization $Q$. Publication: The Astrophysical Journal Pub Date: July 1994 DOI: 10.1086/174296 arXiv: arXiv:astro-ph/9308021 Bibcode: 1994ApJ...429....1B Keywords: Astronomical Models; Cosmology; Dark Matter; Infrared Radiation; Microwave Emission; Sky Surveys (Astronomy); Bayes Theorem; Cosmic Background Explorer Satellite; Cosmic Noise; Distribution Functions; Fluctuation Theory; Astrophysics; COSMOLOGY: COSMIC MICROWAVE BACKGROUND; COSMOLOGY: DARK MATTER; COSMOLOGY: THEORY; Astrophysics; High Energy Physics - Phenomenology E-Print: 8 pages in Plain TeX plus 2 PS figures full text sources arXiv | ADS |

We describe an algorithm for constructing N-body realizations of equilibrium spherical systems. A general form for the mass density rho(r) is used, making it possible to represent most of the popular density profiles found in the literature, including the cuspy density profiles found in high-resolution cosmological simulations. We demonstrate explicitly that our models are in equilibrium. In contrast, many existing N-body realizations of isolated systems have been constructed under the assumption that the local velocity distribution is Maxwellian. We show that a Maxwellian halo with an initial r^{-1} central density cusp immediately develops a constant-density core. Moreover, after just one crossing time the orbital anisotropy has changed over the entire system, and the initially isotropic model becomes radially anisotropic. These effects have important implications for many studies, including the survival of substructure in cold dark matter (CDM) models. Comparing the evolution and mass-loss rate of isotropic Maxwellian and self-consistent Navarro, Frenk, & White (NFW) satellites orbiting inside a static host CDM potential, we find that the former are unrealistically susceptible to tidal disruption. Thus, recent studies of the mass-loss rate and disruption timescales of substructure in CDM models may be compromized by using the Maxwellian approximation. We also demonstrate that a radially anisotropic, self-consistent NFW satellite loses mass at a rate several times higher than that of its isotropic counterpart on the same external tidal field and orbit.

This article questions the common assumption of cold dark matter (DM) by exploring the possibility of a nonzero equation of state (EOS) without relying on any parametric approach. In standard cosmological analyses, DM is typically modeled as pressureless dust with w DM = 0 , an assumption that aligns with large-scale structure formation, supports the empirical success of the Λ -cold dark matter model, and simplifies cosmological modeling. However, there is no fundamental reason to exclude a nonzero w DM from the cosmological framework. In this work, we explore this possibility through nonparametric and parametric reconstructions based on Gaussian process regression. The reconstructions use Hubble parameter measurements from cosmic chronometers, the Pantheon + sample of Type Ia supernovae, and baryon acoustic oscillation (BAO) data from DESI DR1 and DR2. Our findings suggest that a dynamical EOS for DM, although only mildly supported statistically, cannot be conclusively ruled out. Notably, we observe a mild tendency ( ∼ 1 σ ) toward a negative w DM at the present epoch, which is most likely due to inconsistencies between the BAO data from DESI and other datasets.

In this paper we evaluate the spectrum of the pregalactic density field on scales 1 h-1 Mpc < r < 100 h-1 Mpc from a variety of astronomical data. We start with the APM data on the projected angular correlation function, w(θ), in six narrow magnitude bins and check whether possible evolutionary effects can affect inversion of the w(θ) data in terms of the underlying power spectrum. This is done by normalizing to the angular correlation function on small scales where the underlying three-dimensional galaxy correlation function, ξ(r) is known. Using the Automatic Plate Measuring Facility (APM) data in narrow magnitude bins allows us to test the various fits to the APM data power spectrum more accurately. We find that for linear scales r > 10 h-1 Mpc, the Baugh & Efstathiou spectrum of galaxy distribution gives the best fit to the data at all depths. Fitting power spectra of cold dark matter (CDM) models to the data at all depths requires Ωh = 0.2 if the primordial index n = 1 and Ωh = 0.3 if the spectrum is tilted with n = 0.7. Next we compare the peculiar velocity field predicted by the APM spectrum of galaxy (light) distribution with the actual velocity data. The two fields are consistent, and the comparison suggests that the bias factor is scale independent with Ω0.6/b ≃ (0.2–0.4). These steps enable us to fix the pregalactic mass-density field on scales between 10 and ~100 h-1 Mpc. The next data set we use to determine the pregalactic density field comes from the cluster correlation data. We calculate in detail the amplification of the cluster correlation function due to gravitational clustering and use the data on both the slope of the cluster correlation function and its amplitude-richness dependence. Cluster masses are normalized using the Coma Cluster. We find that no CDM model can fit all three data sets: APM data on w(θ), the data on cluster correlation function, and the data on the latter's amplitude-richness dependence. Next we show that the data on the amplitude-richness dependence can be used directly to obtain the spectrum of the pregalactic density field. Applying the method to the data, we recover the density field on scales between 5 and 25 h-1 Mpc whose slope is in good agreement with the APM data on the same scales. Requiring the two amplitudes to coincide fixes the value of Ω to be 0.25–0.3 in agreement with observations of the dynamics of the Coma Cluster. We then use the data on high-z objects to constrain the small-part, (1–5) h-1 Mpc of the pregalactic density field. We argue that the data at high redshifts require more power than given by CDM models normalized to the APM and cluster data. Then we reconstruct the pregalactic density field out of which modern-day galaxies have formed. We use the data on blue absolute luminosities, the fundamental plane relations, and the latest X-ray data on the halo velocity dispersion. From this we recover the pregalactic density field on comoving scales between 1 and 5 h-1 Mpc, which is in reasonable agreement with the simple power-law extrapolation from the larger scales.

Spatially resolved spectroscopic data from the CIRPASS integral field unit (IFU) on Gemini are used to measure the gravitational lensing of the four-image quasar Q2237+0305 on different size scales. A method for measuring the substructure present in the lens using observations at multiple wavelengths is demonstrated to be very effective and independent of many of the degeneracies inherent in previous methods. The magnification ratios of the QSO's narrow-line region (NLR) and broad-line region (BLR) are measured and found to disagree with each other and with the published radio and mid-infrared magnification ratios. The disagreement between the BLR ratios and the radio/mid-infrared ratios is interpreted as the result of microlensing by stars in the lens galaxy of the BLR. This implies that the mid-infrared emission region is larger than the BLR and that the BLR is 0.1 pc in size. We find a small difference between the shape of the Hβ line in image A and that in the other images. We consider this difference too small and symmetric to be strong evidence for rotation or large-scale infall in the Hβ emission region. The disagreement between the radio/mid-infrared ratios and the NLR ratios is interpreted as a signature of substructure on a larger scale, possibly the missing small-scale structure predicted by the standard cold dark matter (CDM) model. Extensive lensing simulations are performed to obtain a lower limit on the amount of substructure that is required to cause this discrepancy as a function of its mass and the radial profile of the host lens. The substructure surface density is degenerate with the radial profile of the host lens, but if the expectations of the CDM model are taken into account, certain radial profiles and substructure surface densities can be ruled out. A substructure mass scale as large as 108 M☉ is strongly disfavored, while 104 M☉ is too small if the radio and mid-infrared emission regions have the expected sizes of ~10 pc. The standard elliptical isothermal lens mass profile is not compatible with a substructure surface density of Σsub < 280 M☉ pc-2, at the 95% confidence level. This is 4%-7% of the galaxy's surface density (depending on which image position is used to evaluate this). The required substructure surface density at the required mass scale is high in comparison with the present expectations within the CDM model. Lens mass profiles that are flatter than isothermal—where the surface density in dark matter is higher at the image positions—are compatible with smaller quantities of substructure.

We present a new derivation of the cosmic microwave background anisotropy spectrum from the Ostriker-Vishniac effect for an open, flat, or closed Universe, and calculate the anisotropy expected in cold dark-matter (CDM) models. We provide simple semi-analytic fitting formulas for the Vishniac power spectrum that can be used to evaluate the expected anisotropy in CDM models for any arbitrary ionization history. In a flat Universe, CDM models normalized to cluster abundances produce rms temperature anisotropies of 0.8--2.4 $\mu$K on arcminute angular scales for a constant ionization fraction of unity, whereas an ionization fraction of 0.2 yields rms anisotropies of 0.3--0.8 $\mu$K. In an open and/or high-baryon-density Universe, the level of anisotropy is somewhat higher. The signal in some of these models may be detectable with planned interferometry experiments. The damping of the acoustic peaks in the primary-anisotropy spectrum at degree angular scales depends primarily on the optical depth and only secondarily on the epoch of reionization. On the other hand, the amplitude of Ostriker-Vishniac anisotropies depends sensitively on the epoch of reionization. Therefore, when combined with the estimate of the reionization optical depth provided by maps of degree-scale anisotropies, the Ostriker-Vishniac effect can provide a unique probe of the epoch of reionization.

We study the non-linear growth of cosmic structure in different dark energy models, using large volume N-body simulations. We consider a range of quintessence models which feature both rapidly and slowly varying dark energy equations of state, and compare the growth of structure to that in a universe with a cosmological constant. We use a four-parameter equation of state for the dark energy which accurately reproduces the quintessence dynamics over a wide range of redshifts. The adoption of a quintessence model changes the expansion history of the universe, the form of the linear theory power spectrum and can alter key observables, such as the horizon scale and the distance to last scattering. We incorporate these effects into our simulations in stages to isolate the impact of each on the growth of structure. The difference in structure formation can be explained to first order by the difference in growth factor at a given epoch; this scaling also accounts for the non-linear growth at the 15 per cent level. We find that quintessence models that are different from Λ cold dark matter (ΛCDM) both today and at high redshifts (z∼ 1000), and which feature late (z &lt; 2), rapid transitions in the equation of state, can have identical baryonic acoustic oscillation (BAO) peak positions to those in ΛCDM. We find that these models have higher abundances of dark matter haloes at z &gt; 0 compared to ΛCDM and so measurements of the mass function should allow us to distinguish these quintessence models from a cosmological constant. However, we find that a second class of quintessence models, whose equation of state makes an early (z &gt; 2) rapid transition to w=−1, cannot be distinguished from ΛCDM using measurements of the mass function or the BAO, even if these models have non-negligible amounts of dark energy at early times.

The James Webb Space Telescope (JWST) will revolutionize our understanding of early galaxy formation, and could potentially set stringent constraints on the nature of dark matter. We use a semi-empirical model of galaxy formation to investigate the extent to which uncertainties in the implementation of baryonic physics may be degenerate with the predictions of two different models of dark matter – cold dark matter (CDM) and a 7 keV sterile neutrino, which behaves as warm dark matter (WDM). Our models are calibrated to the observed UV luminosity function at z = 4 using two separate dust attenuation prescriptions, which manifest as high and low star formation efficiency in low-mass haloes. These efficiencies capture the net effect of processes that regulate star formation. We find that while at fixed star formation efficiency, ε, there are marked differences in the abundance of faint galaxies in the two dark matter models at high-z; these differences are mimicked easily by varying ε in the same dark matter model. We find that a high ε WDM and a low ε CDM model – which provide equally good fits to the z = 4 UV luminosity function – exhibit nearly identical evolution in the cosmic stellar mass and star formation rate densities. We show that differences in the star formation rate at fixed stellar mass are larger for variations in ε in a given dark matter model than they are between dark matter models; however, the scatter in star formation rates is larger between the two models than they are when varying ε. Our results suggest that JWST will likely be more informative in constraining baryonic processes operating in high-z galaxies than it will be in constraining the nature of dark matter.

We present a new limit on the cosmological constant based on the absence of correlations between the cosmic microwave background (CMB) and the distribution of distant radio sources. In cosmological constant, cold dark matter ( LambdaCDM) models, such correlations should have been produced via the integrated Sachs-Wolfe effect, assuming that radio sources trace the local ( z approximately 1) matter density. We find no evidence of correlations and obtain a 95% C.L. upper limit on the cosmological constant of Omega(Lambda)< or =0.74. If 0.6<Omega(Lambda)<0.7 as suggested by recent CMB anisotropy and supernovae observations, then the integrated Sachs-Wolfe effect should be detectable with upcoming CMB maps and radio surveys.

We systematically study the evolution of the Friedmann-Robertson-Walker (FRW) universe coupled with a cosmological constant $\Lambda$ and a perfect fluid that has the equation of state $p=w\rho$, where $p$ and $\rho$ denote, respectively, the pressure and energy density of the fluid, and $w$ is an arbitrary real constant. Depending on the specific values of $w,\; \Lambda$, and the curvature $k$ of 3-dimensional space, we separate all of the solutions into various cases. In each case the main properties of the evolution are given in detail, including the periods of deceleration and/or acceleration, and the existence of big bang, big crunch, and big rip singularities. In some cases, errors in classification and interpretation appearing in standard textbooks have been corrected.

In this paper we report the conclusions of a systematic analysis of the dependence of the amplitude, Ap, and position, lp, of the primary acoustic peak in the CMB angular power spectrum with the cosmological parameters. Following the analysis made by Line weaver et al in [2] for flat Cold Dark Matter (CDM) models, we obtain theoretical contour maps of Ap and lp, in various regions of the parameter space, and discuss their behaviour in terms of the relevant cosmological parameters for Flat CDM, A-CDM and open CDM (with no Λ) models.