We examine the possibility that Universe expansion be made of some $ΛCDM$ expansions repeating periodically, separated by some inflation and radiation dominated phases. This so-called $ΛCDM$ periodic cosmology is motivated by the possibility that inflation and the present phase of accelerated expansion be due to the same dark energy. Then, in a phase space showing the variation of matter density parameter $Ω_m$ with respect to this of the radiation $Ω_r$, the curve $Ω_m(Ω_r)$ looks like a closed trajectory that Universe could run through forever. In this case, the end of the expansion acceleration of the $ΛCDM$ phase is the beginning of a new inflation phase. We show that such a scenario implies the coupling of matter and/or radiation to dark energy. We consider the simplest of these $ΛCDM$ periodic models i.e. a vacuum energy coupled to radiation. From matter domination phase to today, it behaves like a $ΛCDM$ model, then followed by an inflation phase. But a sudden and fast decay of the dark energy into radiation periodically ends the expansion acceleration. This leads to a radiation dominated Universe preceding a new $ΛCDM$ type expansion. The model is constrained with Markov Chain Monte Carlo simulations using supernovae, Hubble expansion, BAO and CMB data and fits the data as well as the $ΛCDM$ one.
We examine gravitational wave memory in the case where sources and detector are in a $Λ$CDM cosmology. We consider the case where the universe can be highly inhomogeneous, but the gravitatational radiation is treated in the short wavelength approximation. We find results very similar to those of gravitational wave memory in an asymptotically flat spacetime; however, the overall magnitude of the memory effect is enhanced by a redshift-dependent factor. In addition, we find the memory can be affected by lensing.
We apply the teleparallelism condition to the Poincaré gauge theory of gravity. The resultant teleparallelized cosmology is completely equivalent to the Friedmann cosmology derived from Einstein's general theory of relativity. The torsion is shown to play the role of the cosmological constant driving the cosmic acceleration. We then extend such theory to include the effect of spin and explore the possibility of accounting for the current accelerating universe by a spinning dark energy.
We point out that $Λ$CDM cosmology has an ignored assumption. That is, the $Λ$ component of the universe moves synchronously with ordinary matters on Hubble scales. If cosmological constant is vacuum energy, this assumption may be very difficult to be understood. We then propose a new mechanism which can explain the accelerating recession of super-novaes. That is, considering the pressures originating from the random moving (including Hubble recession) of galaxy clusters and galaxies. We provide an new analytical solution of Einstein equation which may describe a universe whose pressures origi…
In this study, we investigate the effect of the generalized uncertainty principle on the $Λ$CDM cosmological model. Using quantum corrected Unruh effect and Verlinde's entropic gravity idea, we find Planck-scale corrected Friedmann equations with a cosmological constant. These results modify the $Λ$CDM cosmology.
We show that a cosmology driven by gravitationally induced particle production of all non-relativistic species existing in the present Universe mimics exactly the observed flat accelerating $Λ$CDM cosmology with just one dynamical free parameter. This kind of scenario includes the creation cold dark matter (CCDM) model [Lima, Jesus & Oliveira, JCAP 011(2010)027] as a particular case and also provides a natural reduction of the dark sector since the vacuum component is not needed to accelerate the Universe. The new cosmic scenario is equivalent to $Λ$CDM both at the background and perturbative …
Forecast constraints for a Symmetric Teleparallel Gravity model with a $Λ$CDM background are made using forthcoming ground and space based gravitational waves observatories. A Bayesian analysis resorting to generated mock catalogs shows that LIGO is not expected to be able to distinguish this model from $Λ$CDM, while both LISA and the ET will, with the ET outperforming LISA. We also show that low redshift events are favored in order to improve the quality of the constrains.
This paper treats nonrelativistic matter and a scalar field $ϕ$ with a monotonically decreasing potential minimally coupled to gravity in flat Friedmann-Lemaître-Robertson-Walker cosmology. The field equations are reformulated as a three-dimensional dynamical system on an extended compact state space, complemented with cosmographic diagrams. A dynamical systems analysis provides global dynamical results describing possible asymptotic behavior. It is shown that one should impose \emph{global and asymptotic} bounds on $λ=-V^{-1}\,dV/dϕ$ to obtain viable cosmological models that continuously defo…
We study late-time cosmic accelerating dynamics from Born-Infeld-$f(R)$ gravity in a simplified conformal approach. We find that a variety of cosmic efects such as Little Rip, $Λ$CDM universe and dark energy cosmology with finite-time future singularities may occur. Unlike the convenient Born-Infeld gravity where in the absence of matter only de Sitter expansion may emerge, apparentlly any FRW cosmology maybe reconstructed from this conformal version of the Born-Infeld-$f(R)$ theory. Despite the fact that the explicit form of $f(R)$ is fixed by the conformal ansatz, the relation between the tw…
We provide an $F(R)$ gravity description of a $Λ$CDM bouncing model, without the need for matter fluids or for cosmological constant. As we explicitly demonstrate, the two cosmological eras that constitute the $Λ$CDM bouncing model, can be generated by $F(R)$ gravity which can lead to accelerating cosmologies. The resulting F(R) gravity has Einstein frame inflationary properties that have concordance to the latest Planck observational data. Both the $F(R)$ gravity stability properties are thoroughly investigated and also, the gravitational particle production, a feature necessary for the viabi…
In this paper, we reconstruct the $f(R)$ theory using the viscous $ω$CDM scenario (V-$ω$CDM) constrained by several differently astrophysical observations, including Type Ia supernovae (SNe Ia), observational Hubble parameter data (OHD), Wilkinson Microwave Anisotropy Probe 9-year data (WMAP-9), Planck observations and the single data point from the newest event GW150914 (GW). We find that the joint constraints from SNe Ia+OHD+Planck+GW data-sets give out a tighter restriction for the parameters of the V-$ω$CDM scenario than other constraints. Subsequently, we find that there exists a substant…
Multiple $Λ$CDM cosmology is studied in a way that is formally a classical analog of the Casimir effect. Such cosmology corresponds to a time-dependent dark fluid model or, alternatively, to its scalar field presentation, and it motivated by the string landscape picture. The future evolution of the several dark energy models constructed within the scheme is carefully investigated. It turns out to be almost always possible to choose the parameters in the models so that they match the most recent and accurate astronomical values. To this end, several universes are presented which mimick (multipl…
We reconsider the gravitational wave spectrum induced by scalar perturbations in spatially flat Friedmann-Lemaître-Robertson-Walker spacetimes, focusing on the matter- and $Λ$-dominated epochs. During matter domination, sub-horizon modes are not free and a commonly applied approximation for the derivative of the tensor perturbation is flawed. We show analytically that this leads to a significant overestimation of the energy density spectrum. In addition, we demonstrate that gauge-dependent non-oscillating tensor perturbations appear in the presence of a cosmological constant. Complementing the…
The standard $Λ$CDM model of cosmology is formulated as a simple modified gravity coupled to a single scalar field ("darkon") possessing a non-trivial hidden nonlinear Noether symmetry. The main ingredient in the construction is the use of the formalism of non-Riemannian spacetime volume-elements. The associated Noether conserved current produces stress-energy tensor consisting of two additive parts -- dynamically generated dark energy and dark matter components non-interacting among themselves. Noether symmetry breaking via an additional scalar "darkon" potential introduces naturally an inter…
In this work, which follows a series of studies on the higher-dimensional steady state universe idea and prepared for Professor Tekin Dereli's Festschrift, we show the influence of the dynamical internal (unobservable) space on the evolution of the possible anisotropy of the external (observable) space. We obtain mathematically exactly the same Friedmann equation of the standard $Λ$CDM model for the external space, but with some remarkable physical differences. In particular, the higher-dimensional negative cosmological constant plays the role of the four-dimensional positive cosmological cons…
In this work, we consider the so-called $Λ(α)$CDM cosmology with $Λ\proptoα^{-6}$ while the fine-structure "constant" $α$ is varying. In this scenario, the accelerated expansion of the universe is driven by the cosmological "constant" $Λ$ (equivalently the vacuum energy), and the varying $α$ is driven by a subdominant scalar field $ϕ$ coupling with the electromagnetic field. The observational constraints on the varying $α$ and $Λ\proptoα^{-6}$ models with various couplings $B_F(ϕ)$ between the subdominant scalar field $ϕ$ and the electromagnetic field are considered.
Rather than obtaining cosmic acceleration with a scalar field potential (quintessence) or noncanonical kinetic term (k-essence), we can do it purely through a modified gravity braiding of the scalar and metric, i.e. the $G_3$ Horndeski action term. Such "Horndessence" allows an exact $Λ$CDM cosmological expansion without any cosmological constant, and by requiring shift symmetry we can derive the exact form of $G_3$. We find that this route of deriving $G_3(X)$ leads to a functional form far from the usual simple assumptions such as a power law. Horndessence without any kinetic term or potenti…
Exact Integrable $Λ$CDM-Mimicking $f(Q)$ Cosmology: Background, Stability, and Perturbations in the First Connection Branch
Department of Mathematics, St. Anthony’s College（圣安东尼学院数学系）；SGT University（SGT大学）；Centre For Cosmology and Science Popularization (CCSP), SGT University（宇宙学与科学普及中心（CCSP），SGT大学）；Institute of Research and Development, Duy Tan University（得台大学研发院）；Faculty of Natural Sciences, Duy Tan University（得台大学自然科学学院）；Center for Space Research, North-West University（西北大学太空研究中心）；Mathematics Division, Department of Basic Sciences and Social Sciences, North Eastern Hill University（东北山大学基础科学与社会科学系数学分部）；Inter University Centre for Astronomy and Astrophysics（大学间天文与天体物理中心）
On the basis of a previously established scalar-tensor extension of the $Λ$CDM model we develop an effective fluid approach for the matter growth function. This extended $Λ$CDM (henceforth $e_ΦΛ$CDM) cosmology takes into account deviations from the standard model both via a modified background expansion and by the inclusion of geometric anisotropic stresses as well as of perturbations of the geometric dark-energy equivalent. The background dynamics is governed by an explicit analytic expression for the Hubble rate in which modifications of the standard model are given in terms of a single cons…
The coupled quintessence model with disformal couplings is treated here to mimic the $Λ$CDM background. Using this approach, the quintessence potential does not have to be specified. The model considers a generic fluid coupled to the quintessence, which is specified to be either dark matter or a relativistic fluid. The background consists of a cosmological constant and another uncoupled generic fluid, to cater for three studied scenarios. The cosmological dynamics is investigated for the coupled quintessence model, whose disformal couplings depend on the equation of state of both generic fluid…
We construct observational Hubble $H(z)$ and angular diameter distance $D_{A}(z)$ mock data with baseline Planck $Λ$CDM input values, before fitting the $Λ$CDM model to study evolution of probability density functions (PDFs) of best fit cosmological parameters $(H_0, Ω_m, Ω_k)$ across redshift bins. We find that PDF peaks only agree with the input parameters in low redshift ($z \lesssim 1$) bins for $H(z)$ and $D_{A}(z)$ constraints, and in all redshift bins when $H(z)$ and $D_{A}(z)$ constraints are combined. When input parameters are not recovered, we observe that PDFs exhibit non-Gaussian t…
We investigate the growth of large-scale structure in the superfluid Chaplygin gas (SCG) model. Both linear and non-linear growth, such as $σ_8$ and the skewness $S_3$, are discussed. We find the growth factor of SCG reduces to the EdS case at early times while differs from the $Λ$CDM case in the large $a$ limit. We also find there will be much stricture growth on large scales in the SCG scenario than in $Λ$CDM and the variations of $σ_{8}$ and $S_3$ between SCG and $Λ$CDM can not be discriminated.
Soft cosmology is an extension of standard cosmology allowing for a scale-dependent equation-of-state (EoS) parameter in the dark sectors, which is one of the properties of soft materials in condensed-matter physics, that may arise either intrinsically or effectively. We use data from Cosmic Microwave Background (CMB), Baryonic Acoustic Oscillations (BAO), Supernovae Type Ia (SNIa), and Redshift space distrotion (RSD) probes, in order to impose observational constraints on soft dark energy and soft dark matter. We examine three simple models, corresponding to the minimum extensions of $Λ$CDM s…
We wish to investigate whether there is an extension to the base $Λ$CDM cosmology that can resolve the tension between the Planck observation of the cosmic microwave background anisotropies and the local measurement of the Hubble constant. We consider various plausible extended models in this work, and we use the Planck 2015 observation, combined with the baryon acoustic oscillation data, the JLA type Ia supernovae data, and the local measurement of the Hubble constant (by Riess et al. in 2016), to make an analysis. We find that the holographic dark energy plus sterile neutrino model can reduc…
Exponential $f(R)$ cosmology with massive neutrinos as a dynamical dark energy framework
Institute of Space Sciences (ICE, CSIC)（空间科学研究所）；ICREA（加泰罗尼亚高级研究学院）；SISSA-International School for Advanced Studies（国际高等研究学校）；INFN, Sezione di Trieste（意大利国家核物理研究所的里雅斯特分部）
Effective $Λ$CDM model emerging from $f(Q,T)$ under a special EOS limit in symmetric cosmology with Bayesian and ANN observational constraints
United College of Engineering and Research（联合工程与研究学院）；S.P.M. Science and Gilani Arts, Commerce College（SPM科学与Gilani文理学院）；L N Gumilyov Eurasian National University（L·N·古米廖夫欧亚国立大学）；GLA University（GLA大学）；King Khalid University（国王哈立德国立大学）
Our intuitive understanding of cosmic structure formation works best in scales small enough so that bound, relaxed gravitating systems are no longer adjusting their radius; and large enough so that space and matter follow the average expansion of the Universe. Yet one of the most robust predictions of $Λ$CDM cosmology concerns the scale that separates these limits: the turnaround radius, which is the non-expanding shell furthest away from the center of a bound structure. The maximum possible value of the turnaround radius within the framework of the $Λ$CDM model is, for a given mass $M$, equal…
A time-dependent phenomenological model of $Λ$, viz. $\dot Λ\sim H^3$ is selected to investigate the $Λ$-CDM cosmology. Time-dependent form of the equation of state parameter $ω$ is derived and it has been possible to obtain the sought for flip of sign of the deceleration parameter q. Present age of the Universe, calculated for some specific values of the parameters agrees very well with the observational data.
We model the large scale late time universe as a Lambda-CDM cosmology driven by cosmological constant and perfect dust fluid. Our aim is to find new solutions in the matter and Lambda epoch consistent with inflationary initial conditions, namely that to the far past in the matter era the cosmology tends to a flat FLRW solution. We identify the moduli degrees of freedom that parametrize the flat Lambda-dust FLRW solution and then promote these moduli to slowly varying functions of the spatial coordinates and show how to solve the Einstein equations in a comoving gradient expansion, controlled b…
The absence of a physically motivated model for large scale profiles of cosmic voids limits our ability to extract valuable cosmological information from their study. In this paper, we address this problem by introducing the spherically compensated cosmic regions, named CoSpheres. Such cosmic regions are identified around local extrema in the density field and admit a unique compensation radius $R_1$ where the internal spherical mass is exactly compensated. Their origin is studied by extending the standard peak model and implementing the compensation condition. Since the compensation radius ev…
Using a new recently compiled milliarcsecond compact radio data set of 120 intermediate-luminosity quasars in the redshift range $0.46< z <2.76$, whose statistical linear sizes show negligible dependence on redshifts and intrinsic luminosity and thus represent standard rulers in cosmology, we constrain three viable and most popular $f(T)$ gravity models, where $T$ is the torsion scalar in teleparallel gravity. Our analysis reveals that constraining power of the quasars data (N=120) is comparable to the Union2.1 SN Ia data (N=580) for all three $f(T)$ models. Together with other standard ruler …
In this manuscript, we present a number of fascinating explicit reconstructions for the $f(Q)$ gravity from the background of Friedmann-Laîmatre-Robertson-Walker (FLRW) evolution history. We find the more general functions of non-metricity scalar $Q$ that admit exact $Λ$CDM expansion history. Adding extra degrees of freedom to the matter sector is the only method to get the scale factor to behave in this manner for more generic functions of $Q$. In addition, a cosmological reconstruction for modified $f(Q)$ gravity is constructed in terms of e-folding. It is shown how any FLRW cosmology can ar…
In this paper we seek for relevant information on the asymptotic cosmological dynamics of the Brans--Dicke theory of gravity for several self-interaction potentials. By means of the simplest tools of the dynamical systems theory, it is shown that the general relativity de Sitter solution is an attractor of the Jordan frame (dilatonic) Brans--Dicke theory only for the exponential potential $U(\vphi)\propto\exp\vphi$, which corresponds to the quadratic potential $V(ϕ)\proptoϕ^2$ in terms of the original Brans--Dicke field $ϕ=\exp\vphi$, or for potentials which asymptote to $\exp\vphi$. At the st…
The standard $Λ$CDM model can be mimicked at the background and perturbative levels (linear and non-linear) by a class of gravitationally induced particle production cosmology dubbed CCDM cosmology. However, the radiation component in the CCDM model follows a slightly different temperature-redshift $T(z)$-law which depends on an extra parameter, $ν_r$, describing the subdominant photon production rate. Here we perform a statistical analysis based on a compilation of 36 recent measurements of $T(z)$ at low and intermediate redshifts. The likelihood of the production rate in CCDM cosmologies is …
We propose a new framework for studying the cosmology of $f(R)$ gravity which completely avoids using the reconstruction programme. This allows us to easily obtain a qualitative feel of how much the $Λ$CDM model differs from other $f(R)$ theories of gravity at the level of linear perturbation theory for theories that share the same background dynamics. This is achieved by using the standard model independent cosmographic parameters to develop a new dynamical system formulation of $f(R)$ gravity which is free from the limitation of having to first specify the functional form of $f(R)$. By consi…
We present a full-fledged analysis of Brans-Dicke cosmology with a cosmological constant and cold dark matter (BD-$Λ$CDM for short). We extend the scenarios where the current cosmological value of the BD-field is restricted by the local astrophysical domain to scenarios where that value is fixed only by the cosmological observations, which should be more natural in view of the possible existence of local screening mechanims. Our analysis includes both the background and perturbations equations in different gauges. We find that the BD-$Λ$CDM is favored by the overall cosmological data as compar…
We formulate the basic framework of thermodynamical entropic force cosmology which allows variation of the gravitational constant $G$ and the speed of light $c$. Three different approaches to the formulation of the field equations are presented. Some cosmological solutions for each framework are given and one of them is tested against combined observational data (supernovae, BAO, and CMB). From the fit of the data it is found that the Hawking temperature numerical coefficient $γ$ is two to four orders of magnitude less than usually assumed on the geometrical ground theoretical value of $O(1)$ …
The phenomenology of the modified Newtonian dynamics (MOND) can be recovered from a mechanism of "gravitational polarization" of some dipolar medium playing the role of dark matter. We review a relativistic model of dipolar dark matter (DDM) within standard general relativity to describe, at some effective level, a fluid polarizable in a gravitational field. At first order in cosmological perturbation theory, this model is equivalent to the concordance cosmological scenario, or Lambda-cold dark matter (CDM) model. At second order, however, the internal energy of DDM modifies the curvature pert…
The statistical properties of cosmic structures are well known to be strong probes for cosmology. In particular, several studies tried to use the cosmic void counting number to obtain tight constrains on Dark Energy. In this paper we address this question by using the CoSphere model as introduced in de Fromont & Alimi (2017a). We derive their exact statistics in both primordial and non linearly evolved Universe for the standard $Λ$CDM model. We first compute the full joint Gaussian probability distribution for the various parameters describing these profiles in the Gaussian Random Field. We re…
Assuming homogeneous isotropic Lambda-CDM cosmology allows Lambda, spatial curvature and dark matter density to be inferred from large scale structure observations such as supernovae. The purpose of this paper is to extend this to allow observations to measure or constrain inhomogeneity and anisotropy. We obtain the general inhomogeneous anisotropic Lambda-CDM solution which is locally asymptotic to an expanding de Sitter solution as a late time expansion using Starobinsky's method (analogous to the `holographic renormalization' technique in AdS/CFT) together with a resummation of the series. …
The Higgs-Dilaton model is a scale-invariant extension of the Standard Model non-minimally coupled to gravity and containing just one additional degree of freedom on top of the Standard Model particle content. This minimalistic scenario predicts a set of measurable consistency relations between the inflationary observables and the dark-energy equation-of-state parameter. We present an alternative derivation of these consistency relations that highlights the connections and differences with the $α$-attractor scenario. We study in how far these constraints allow to distinguish the Higgs-Dilaton …
Recently Kaloper, Kleban and Martin reexamined the McVittie solution and argued, contrary to a very widely held belief, that the solution contains a black hole in an expanding universe. Here we corroborate their main conclusion but go on to examine, in some detail, a specific solution that asymptotes to the $Λ$CDM cosmology. We show that part of the boundary of the solution contains the inner bifurcation two - sphere of the Schwarzschild - de Sitter spacetime and so both the black and white hole horizons together form a partial boundary of this McVittie solution. We go on to show that the null…
The cosmological principle states that the Universe is statistically homogeneous and isotropic at large distance scales. There currently exist many observations which indicate a departure from this principle. It has been shown that many of these observations can be explained by invoking superhorizon cosmological perturbations and may be consistent with the Big Bang paradigm. Remarkably, these modes simultaneously explain the observed Hubble tension, i.e., the discrepancy between the direct and indirect measurements of the Hubble parameter. We propose several tests of the cosmological principle…
Notoriously, the two main problems of the standard $Λ$CDM model of cosmology are the cosmological constant $Λ$ and the cold dark matter, CDM. This essay shows that both the $Λ$ and the CDM arise as integration constants in a careful derivation of Einstein's equations from first principles in a Lorentz gauge theory. The dark sector of the universe might only reflect the geometry of a spontaneous symmetry breaking that is necessary for the existence of a spacetime and an observer therein.
We analyze the claimed tension between redshift space distorsions measurements of $f(z)σ_8(z)$ and the predictions of standard $Λ$CDM (Planck 2015 and 2018) cosmology. We consider a dataset consisting of 17 data points extending up to redshift $z=1.52$ and corrected for the Alcock-Paczynski effect. Thus, calculating the evolution of the growth factor in a $w$CDM cosmology, we find that the tension for the best fit parameters $w$, $Ω_m$ and $σ_8$ with respect to the Planck 2018 $Λ$CDM parameters is below $2σ$ in all the marginalized confidence regions.
