We revisit the epoch of cosmic speed-up characterized by the redshift of transition from a decelerated to an accelerated phase. This redshift is termed the transition redshift ($z_t$). We use the spatially Flat and Non-Flat variants of the most common $Λ$CDM and XCDM models to put constraints on the transition redshift along with the other model parameters. The data for this analysis comes from the recent and updated Pantheon+ Supernova dataset and the Hubble parameter measurements obtained from Cosmic Chronometers. We consider both datasets with their respective covariance matrices incorporating all kinds of statistical and systematic uncertainties. We observe that using the combined datasets of H(z) and SNe, the best fit value of transition redshift lies in the range $0.61 < z_t < 0.82$ for all four dark energy models. Incidentally, we observe a positive curvature for the Non-Flat models and correlations between several model parameters.
We investigate the transition from primordial Population III (Pop III) star formation to normal Pop II star formation in the first galaxies using new cosmological hydrodynamic simulations. We find that while the first stars seed their host galaxies with metals, they cannot sustain significant outflows to enrich the intergalactic medium, even assuming a top-heavy initial mass function. This means that Pop III star formation could potentially continue until z~6 in different unenriched regions of the universe, before being ultimately shut off by cosmic reionization. Within an individual galaxy, t…
One of the strongest arguments against the cosmological constant as an explanation of the current epoch of accelerated cosmic expansion is the existence of an earlier, dynamical acceleration, i.e. inflation. We examine the likelihood that acceleration is an occasional phenomenon, putting stringent limits on the length of any accelerating epoch between recombination and the recent acceleration; such an epoch must last less than 0.05 e-fold (at z>2) or the matter power spectrum is modified by more than 20%.
Cosmic backreaction as an additional source of the expansion of the universe has been a debate topic since the discovery of cosmic acceleration. The major concern is whether the self interaction of small-scale nonlinear structures would source gravity on very large scales. Gregory Ryskin argued against the additional inclusion of gravitational interaction energy of astronomical objects, whose masses are mostly inferred from gravitational effects and hence should already contain all sources with long-range gravity forces. Ryskin proposed that the backreaction contribution to the energy momentum…
The cosmographic expansion history of the universe is investigated by using the 557 type Ia supernovae from the Union2 Compilation set along with the current estimates involving the product of the CMB acoustic scale $\ell_{A}$ and the BAO peak at two different redshifts. Using a well-behaved parameterization for the deceleration parameter, $q(z) = q_0 + q_1z/(1 + z)$, we estimate the accelerating redshift $z_{acc}=-q_0/(q_0 + q_1)$ (at which the universe switches from deceleration to acceleration) and investigate the influence of a non-vanishing spatial curvature on these estimates. We also us…
The M101 galaxy contains the best-known example of an ultraluminous supersoft source (ULS), dominated by a thermal component at kT ~ 0.1 keV. The origin of the thermal component and the relation between ULSs and standard (broad-band spectrum) ultraluminous X-ray sources (ULXs) are still controversial. We re-examined the X-ray spectral and timing properties of the M101 ULS using archival Chandra and XMM-Newton observations. We show that the X-ray time-variability and spectral properties are inconsistent with standard disk emission. The characteristic radius R_{bb} of the thermal emitter varies …
Standard cosmological models based on general relativity (GR) with dark energy predict that the Universe underwent a transition from decelerating to accelerating expansion at a moderate redshift $z_{acc} \sim 0.7$. Clearly, it is of great interest to directly measure this transition in a model-independent way, without the assumption that GR is the correct theory of gravity. We explore to what extent supernova (SN) luminosity distance measurements provide evidence for such a transition: we show that, contrary to intuition, the well-known "turnover" in the SN distance residuals $Δμ$ relative to …
In this work we study the theory of extended quasidilaton massive gravity together with the presence of matter fields. After discussing the homogeneous and isotropic fully dynamical background equations, which governs the exact expansion history of the universe, we consider small cosmological perturbations around these general FLRW solutions. The stability of tensor, vector and scalar perturbations on top of these general background solutions give rise to slightly different constraints on the parameters of the theory than those obtained in the approximative assumption of the late-time asymptot…
Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the most accurate measurements have been obtained with Type Ia Supernovae and Baryon Acoustic Oscillations, providing evidence for the existence of a transition epoch at which the expansion rate changes from decelerated to accelerated. However, these results have been obtained within the framework of specific cosmological models that must be implicitly or explicitly assumed in the measurement. It is therefore crucial to obtain measurements of the accelerated expansion of the Universe independently of a…
We consider General Relativity with matter, radiation and a minimally coupled dark energy defined by an equation of state w. Using dynamical system method, we find the equilibrium points of such a theory assuming an expanding Universe and a positive dark energy density. Two of these points correspond to classical radiation and matter dominated epochs for the Universe. For the other points, dark energy mimics matter, radiation or accelerates Universe expansion. We then look for possible sequences of epochs describing a Universe starting with some radiation dominated epoch(s) (mimicked or not by…
In this paper, we revisit the Cardassian model in which the radiation energy component is included. It is important for early epoch when the radiation cannot be neglected because the equation of state (EoS) of the effective dark energy becomes time variable. Therefore, it is not equivalent to the quintessence model with a constant EoS anymore. This situation was almost overlooked in the literature. By using the recent released Union2 557 of type Ia supernovae (SN Ia), the baryon acoustic oscillation (BAO) from Sloan Digital Sky Survey and the WiggleZ data points, the full information of cosmic…
We investigate the effect of backreaction due to inhomogeneities on the evolution of the present universe by considering a two-scale model within the Buchert framework. Taking the observed present acceleration of the universe as an essential input, we study the effect of inhomogeneities in the future evolution. We find that the backreaction from inhomogeneities causes the acceleration to slow down in the future for a range of initial configurations and model parameters. The present acceleration ensures formation of the cosmic event horizon, and our analysis brings out how the effect of the eve…
Observations indicate that the evolution of our universe can be divided into three epochs consisting of early time inflation, radiation (and matter) domination and the late time acceleration. One can associate with each of these epochs a number N which is the phase space volume of the modes which cross the Hubble radius during the corresponding epoch. This number turns out to be (approximately) the same for the cosmologically relevant ranges of the three epochs. When the initial de Sitter space is characterized by the Planck length, the natural value for N is 4π. This allows us to determine th…
Weighted mean and median statistics techniques are used to combine 23 independent lower redshift, $z<1.04$, Hubble parameter, $H(z)$, measurements and determine binned forms of $H(z)$. When these are combined with 5 higher redshift, $1.3\leqslant z \leqslant 2.3$, $H(z)$ measurements the resulting constraints on cosmological parameters, of three cosmological models, that follow from the weighted-mean binned data are almost identical to those derived from analyses using the 28 independent $H(z)$ measurements. This is consistent with what is expected if the lower redshift measurements errors are…
The cosmic expansion history proceeds in broad terms from a radiation dominated epoch to matter domination to an accelerated, dark energy dominated epoch. We investigate whether intermittent periods of acceleration are possible in the early universe -- between Big Bang nucleosynthesis (BBN) and recombination and beyond. We establish that the standard picture is remarkably robust: observations of anisotropies in the cosmic microwave background exclude any extra period of accelerated expansion between 1 \leq z \lesssim 10^5 (corresponding to 5\times10^{-4}\ {\rm eV} \leq T \lesssim 25\ {\rm eV})…
A novel, interesting class of scalar-tensor gravity theories is those with a limit on the field motion, where the scalar field either goes to a constant acceleration or stops accelerating and goes to a constant velocity. We combine these with the ability to dynamically cancel a high energy cosmological constant, e.g. through the well tempered or self tuning approaches. One can successfully have a cosmic expansion history with a matter dominated epoch and late time acceleration despite a large cosmological constant, although the late time de Sitter limit may be unstable. Pole models, such as in…
Revisiting the Matter Creation Process: Observational Constraints on Gravitationally Induced Dark Energy and the Hubble Tension
SISSA-International School for Advanced Studies（国际高等研究学院）；INFN, Sezione di Trieste（意大利国家核物理研究所的里雅斯特分部）；Institute of Space Sciences (ICE,CSIC)（空间科学研究所）；Universidad Complutense de Madrid（马德里康普顿斯大学）；Istanbul Technical University（伊斯坦布尔理工大学）；ENEA, Nuclear Department（意大利国家新能源与可再生能源署核能部）；Sapienza University of Rome（罗马智慧大学）；University of Sheffield（谢菲尔德大学）
In this paper we show that a pseudo-Nambu-Goldstone boson (PNGB) quintessence of spontaneous symmetry breaking (SSB) is responsible for an epoch of the late time cosmic acceleration. We suggest that an N=2 PNGB quintessence with SSB can give rise to the fast-roll cosmic acceleration in evolution of the Universe. In the N=2 PNGB quintessence model, the standard slow-roll condition in which the absolute value of the tachyonic mass squared $|m_q^2|$ of this model is much less than the square of the Hubble constant $H^2$, is broken down to the fast-roll condition, $|m_q^2|=O(H^2)$. However, it is …
We introduce a new asymptotic ansatz for spherical perturbations of the Standard Model of Cosmology (SM) which applies during the $p=0$ epoch, and prove that these perturbations trigger instabilities in the SM on the scale of the supernova data. These instabilities create a large, central region of uniform under-density which expands faster than the SM, and this central region of accelerated uniform expansion introduces into the SM {\it precisely} the same range of corrections to redshift vs luminosity as are produced by the cosmological constant in the theory of Dark Energy. A universal behav…
By integration of generalized BSBM and Brans-Dicke cosmological models, in this article, we investigate the theoretical framework of fine structure constant variation and current cosmic acceleration. We first develop a mathematical formalism to analyse the stability of the model. By employing observational data to constrain the model parameters, phase space is performed and the attractor solutions of the model are detected. We then examine the model against observational data such as observational Hubble parameter dataset and quasar absorption spectra. The results confirms current universe acc…
Redshifts of an astronomical body measured at multiple epochs (e.g., separated by 10 years) are different due to the cosmic expansion. This so-called Sandage-Loeb test offers a direct measurement of the expansion rate of the Universe. However, acceleration in the motion of Solar System with respect to the cosmic microwave background also changes redshifts measured at multiple epochs. If not accounted for, it yields a biased cosmological inference. To address this, we calculate the acceleration of Solar System with respect to the Local Group of galaxies to quantify the change in the measured re…
By phenomenologically assuming a slow temporal variation of the percent acceleration rate $\ddot S S^{-1}$ of the cosmic scale factor $S(t)$, it is shown that the orbit of a local binary undergoes a secular expansion. To first order in the power expansion of $\ddot S S^{-1}$ around the present epoch $t_0$, a non-vanishing shift per orbit $\left\langleΔr\right\rangle$ of the two-body relative distance $r$ occurs for eccentric trajectories. A general relativistic expression, which turns out to be cubic in the Hubble parameter $H_0$ at the present epoch, is explicitly calculated for it in the cas…
A late epoch cosmic acceleration may be naturally entangled with cosmic coincidence -- the observation that at the onset of acceleration the vacuum energy density fraction nearly coincides with the matter density fraction. In this Letter we show that this is indeed the case with the cosmology of a Friedmann-Lamaître-Robertson-Walker (FLRW) 3-brane in a five-dimensional anti-de Sitter spacetime. We derive the four-dimensional effective action on a FLRW 3-brane, which helps define a general reduction formula, namely, $M_P^{2}=ρ_{b}/|Λ_5|$, where $M_{P}$ is the effective Planck mass, $Λ_5$ is the…
Detecting the line-intensity mapping (LIM) signal from the galaxies of the Epoch of Reionization is an emerging tool to constrain their role in reionization. Ongoing and upcoming experiments target the signal fluctuations across the sky to reveal statistical and astrophysical properties of these galaxies via signal statistics, e.g., the power spectrum. Here, we revisit the [C II]$_{158 μ\text{m}}$ LIM power spectrum under non-uniform line-luminosity scatter, which has a halo-mass variation of statistical properties. Line-luminosity scatter from a cosmological hydrodynamic and radiative transfe…
In this (second) part of the work we present the results of numerical and qualitative analysis, based on a new model of the Archimedean-type interaction between dark matter and dark energy. The Archimedean-type force is linear in the four-gradient of the dark energy pressure and plays a role of self-regulator of the energy redistribution in a cosmic dark fluid. Because of the Archimedean-type interaction the cosmological evolution is shown to have a multistage character. Depending on the choice of the values of the model guiding parameters,the Universe's expansion is shown to be perpetually ac…
We apply the $Om$ diagnostic to models for dark energy based on scalar fields. In case of the power law potentials, we demonstrate the possibility of slowing down the expansion of the Universe around the present epoch for a specific range in the parameter space. For these models, we also examine the issues concerning the age of Universe. We use the $Om$ diagnostic to distinguish the $Λ$CDM model from non minimally coupled scalar field, phantom field and generic quintessence models. Our study shows that the $Om$ has zero, positive and negative curvatures for $Λ$CDM, phantom and quintessence mod…
We propose in this letter a relativistic coordinate independent interpretation for Milgrom's acceleration $a_{0}=1.2 \times 10^{-8} \hbox{cm/s}^{2}$ through a geometric constraint obtained from the product of the Kretschmann invariant scalar times the surface area of 2--spheres defined through suitable characteristic length scales for local and cosmic regimes, described by Schwarzschild and Friedman--Lema\^ıtre--Robertson--Walker (FLRW) geometries, respectively. By demanding consistency between these regimes we obtain an appealing expression for the empirical (so far unexplained) relation betw…
The review of basic cosmological properties of four-dimensional F(R)-gravity, including FRW equations of motion and its accelerating solutions, generalized fluid and scalar-tensor representation of the theory is done. Cosmological reconstruction equation is written and conditions for stability of cosmological solution are discussed. The overview of realistic F(R)-models unifying inflation with dark energy epoch is made. The avoidance of finite-time future singularities in such theories via the introduction of R^2-term is studied. New realistic non-singular F(R)-gravity unifying early-time infl…
