In this paper we study a model of interacting dark energy - dark matter where the ratio between these components is not constant, changing from early to late times in such a way that the model can solve or alleviate the cosmic coincidence problem (CP). The interaction arises from an assumed relation of the form $ρ_x \propto ρ_d^α$, where $ρ_x$ and $ρ_d$ are the energy densities of dark energy and dark matter components, respectively, and $α$ is a free parameter. For a dark energy equation of state parameter $w=-1$ we found that, if $α=0$, the standard $Λ$CDM model is recovered, where the coincidence problem is unsolved. For $0<α<1$, the CP would be alleviated and for $α\sim 1$, the CP would be solved. The dark energy component is analyzed with both $w=-1$ and $w \neq -1$. Using Supernovae type Ia and Hubble parameter data constraints, in the case $w=-1$ we find $α=0.109^{+0.062}_{-0.072}$ at 68% C.L., and the CP is alleviated. For $w\neq -1$, a degeneracy arises on the $w$ - $α$ plane. In order to break such degeneracy we add cosmic microwave background distance priors and baryonic acoustic oscillations data to the constraints, yielding $α=-0.075\pm 0.046$ at 68% C.L.. In this case we find that the CP is not alleviated even for 2$σ$ interval for $α$. Furthermore, this last model is discarded against flat $Λ$CDM according to BIC analysis.
In this work, we apply the smooth deformation concept in order to obtain a modification of Friedmann equations. It is shown that the cosmic coincidence can be at least alleviated using the dynamical properties of the extrinsic curvature. We investigate the transition from nucleosynthesis to the coincidence era obtaining a very small variation of the ratio $r=\frac{ρ_{m}}{ρ_{ext}}$, that compares the matter energy density to extrinsic energy density, compatible with the known behavior of the deceleration parameter. We also show that the calculated "equivalence" redshift matches the transition r…
In the framework of a phenomenological cosmological model with the assumption of $ρ_{X} \propto ρ_{m} a^ξ$ ($ρ_{X}$ and $ρ_{m} $ are the energy densities of dark energy and matter, respectively.), we intend to diagnose the cosmic coincidence problem by using the recent samples of Type Ia supernovae (SNe Ia), baryon acoustic oscillation (BAO) and cosmic microwave background (CMB). $ξ$ is a key parameter to characterize the severity of the coincidence problem, wherein $ξ=3$ and $0$ correspond to the $Λ$CDM scenario and the self-similar solution without the coincidence problem, respectively. The …
It is said that there are no accidents or coincidences in physics. Within the minimal standard model combined with general relativity we point out that there are three exceptionally-long lifetimes which are consistent with being equal, and hence that there are two unexplained coincidences.
In this paper, we use electromagnetic wave data (H0LiCOW, $H(z)$, SNe) and gravitational wave data (Tianqin) to constrain the interacting dark energy (IDE) model and investigate the Hubble tension problem and coincidences problem. By combining these four kinds of data (Tianqin+H0LiCOW+SNe+$H(z)$), we obtained the parameter values at the confidence interval of $1σ$: $Ω_m=0.36\pm0.18$, $ω_x=-1.29^{+0.61}_{-0.23}$, $ξ=3.15^{+0.36}_{-1.1}$, and $H_0=70.04\pm0.42$ $kms^{-1}Mpc^{-1}$. According to our results, the best valve of $H_0$ show that the Hubble tension problem can be alleviated to some ext…
Using the quantum corrected Friedmann equation, obtained from the quantum Raychudhuri equation, and assuming a small mass of the graviton (but consistent with observations and theory), we propose a resolution of the smallness problem(why is observed vacuum energy so small?) and the coincidence problem(why does it constitute most of the universe, about 70%, in the current epoch?).
If primordial black holes (PBHs) contribute more than 10 percent of the dark matter (DM) density, their energy density today is of the same order as that of the baryons. Such a cosmic coincidence might hint at a mutual origin for the formation scenario of PBHs and the baryon asymmetry of the Universe. Baryogenesis can be triggered by a sharp transition of the rolling rate of inflaton from slow-roll to (nearly) ultraslow-roll phases that produces large curvature perturbations for PBH formation in single-field inflationary models. We show that the baryogenesis requirement drives the PBH contribu…
Dynamical Resolution of the Cosmic Coincidence Problem in Non-Interacting Holographic Dark Energy via Einstein-Cartan Torsion
Graduate School Department of Physics, Daejin University（大真大学物理学院研究生院）；School of Liberal Arts and Sciences, Korea National University of Transportation（韩国交通大学本科文理学院）；School of Physics, Korea Institute for Advanced Study（韩国高等研究院物理学院）
The induced gravitational wave (GW) background from enhanced primordial scalar perturbations is one of the most promising observational consequences of primordial black hole (PBH) formation from inflation. We investigate the induced GW spectrum $Ω_{\textrm{IGW}}$ from single-field inflation in the general ultra-slow-roll (USR) framework, restricting the peak frequency band to be inside $10^{-3}$-$1$ Hz and saturating PBH abundance to comprise all dark matter (DM) in the ultralight asteroid-mass window. By invoking successful baryogenesis driven by USR inflation, we verify the viable parameter …
We investigate several varying-mass dark-matter particle models in the framework of phantom cosmology. We examine whether there exist late-time cosmological solutions, corresponding to an accelerating universe and possessing dark energy and dark matter densities of the same order. Imposing exponential or power-law potentials and exponential or power-law mass dependence, we conclude that the coincidence problem cannot be solved or even alleviated. Thus, if dark energy is attributed to the phantom paradigm, varying-mass dark matter models cannot fulfill the basic requirement that led to their co…
The cosmic coincidence problem is a serious challenge to dark energy model. We suggest a quantitative criteria for judging the severity of the coincidence problem. Applying this criteria to three different interacting models, including the interacting quintessence, interacting phantom, and interacting Chaplygin gas models, we find that the interacting Chaplygin gas model has a better chance to solve the coincidence problem. Quantitatively, we find that the coincidence index C for the interacting Chaplygin gas model is smaller than that for the interacting quintessence and phantom models by six…
A cosmological constant fits all current dark energy data, but requires two extreme fine tunings, both of which are currently explained by anthropic arguments. Here we discuss anti-anthropic solutions to one of these problems: the cosmic coincidence problem- that today the dark energy density is nearly equal to the matter density. We replace the ensemble of Universes used in the anthropic solution with an ensemble of tracking scalar fields that do not require fine-tuning. This not only does away with the coincidence problem, but also allows for a Universe that has a very different future than …
We model the evolution of galaxy clustering through cosmic time to investigate the nature of the power-law shape of xi(r), the galaxy two-point correlation function. Departures from a power law are mainly governed by galaxy pair counts on small scales, subject to non-linear dynamics. We assume that galaxies reside within dark matter halos and subhalos and use a semi-analytic substructure evolution model to study subhalo populations within host halos. We find that tidal mass loss and, to a lesser degree, dynamical friction deplete the number of subhalos within larger host halos over time by ~90…
The observational fact that the present values of the densities of dark energy and dark matter are of the same order of magnitude, $ρ_{de0}/ρ_{dm0} \sim \mathcal{O}(1)$, seems to indicate that we are currently living in a very special period of the cosmic history. Within the standard model, a density ratio of the order of one just at the present epoch can be seen as coincidental since it requires very special initial conditions in the early Universe. The corresponding "why now" question constitutes the cosmological "coincidence problem". According to the standard model the equality $ρ_{de} = ρ…
We perform a full dynamical analysis by considering the interactions between dark energy and radiation, and dark energy and dark matter. We find that the interaction helps alleviate the coincidence problem for the quintessence model.
In the standard cosmological model the dark energy (DE) and nonrelativistic (NR) matter densities are observationally determined to be comparable at the present time, in spite of their greatly different evolution histories. This `cosmic coincidence' enigma -- also referred to as the `why now? problem' -- relies, by its very definition, on the implicit prior expectation for our `typicality' in the cosmic (expanding) spacetime volume. Otherwise, this conundrum does not exist in the first place. It is shown here that this apparent coincidence could be explained as a non-anthropic observational se…
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…
Over the last nine years of cosmic microwave background observations, the Wilkinson Microwave Anisotropy Probe ($WMAP$) results were consistent with a $Λ$CDM cosmological model in which the age of the Universe is one Hubble time, and the time-averaged value of the deceleration parameter is consistent with zero. This curious observation has been put forward as a new coincidence problem for the $Λ$CDM concordance cosmology, which is in fact a `greater' coincidence than the near equality of the density parameters of matter and the cosmological constant. At the moment of writing these conference p…
We propose a new dark energy model for solving the cosmological fine-tuning and coincidence problems. A default assumption is that the fine-tuning problem disappears if we do not interpret dark energy as vacuum energy. The key idea to solving the coincidence problem is that the Universe may have several acceleration phases across the whole cosmic history. The specific example we study is a quintessence model with approximately repeated double exponential potential, which only introduces one Planck scale parameter and three dimensionless parameters of order unity. The cosmological background ev…
In this paper, we use three different kinds of observational data, including 130 strong gravitational lensing (SGL) systems, type Ia supernovae (SNeIa: Pantheon and Union2.1) and 31 Hubble parameter data points ($H(z)$) from cosmic chronometers to constrain the phenomenological model ($ρ_x\varproptoρ_m a^ξ$). By combining these three kinds of data (Union2.1+SGL+$H(z)$), we get the parameter value at the confidence interval of $2σ$, $Ω_{X,0} = 0.69\pm0.34$, $ω_x = -1.24\pm0.61$, $ξ= 3.8\pm3.9$ and $H_0 = 70.22\pm0.86$ kms$^{-1}$Mpc$^{-1}$. According to our results, we find that the $Λ$CDM model…
A mechanism capable to provide a natural solution to two major cosmological problems, i.e. the cosmic acceleration and the coincidence problem, is proposed. A specific brane-bulk energy exchange mechanism produces a total dark pressure, arising when adding all normal to the brane negative pressures in the interior of galactic core black holes. This astrophysically produced negative dark pressure explains cosmic acceleration and why the dark energy today is of the same order to the matter density for a wide range of the involved parameters. An exciting result of the analysis is that the recent …
In a recent paper (Vigoureux et al. Int. J. Theor. Phys. 47:928, 2007) it has been suggested that the velocity of light and the expansion of the universe are two aspects of one single concept connecting space and time in the expanding universe. It has then be shown that solving Friedmann's equations with that interpretation (and keeping c = constant) can explain number of unnatural features of the standard cosmology (for example: the flatness problem, the problem of the observed uniformity in term of temperature and density of the cosmological background radiation, the small-scale inhomogeneit…
By assuming a phenomenological form for the ratio of the DE and DM densities $ρ_X\propto ρ_{m} a^ξ$ (Dalal et al.2001), we discuss the cosmic coincidence problem in light of current observational data. $ξ$ is a key parameter to denote the severity of the coincidence problem. In this scenario, $ξ=3$ and $ξ=0$ correspond to $Λ$CDM and the self-similar solution without coincidence problem, respectively. Hence, any solution with a scaling parameter $0<ξ<3$ makes the coincidence problem less severe. In addition, the standard cosmology without interaction between DE and DM is characterized by $ξ+3ω_…
A novel idea is proposed for a natural solution of the dark energy and its cosmic coincidence problem. The existence of local antigravity sources, associated with astrophysical matter configurations distributed throughout the universe, can lead to a recent cosmic acceleration effect. Various physical theories can be compatible with this idea, but here, in order to test our proposal, we focus on quantum originated spherically symmetric metrics matched with the cosmological evolution through the simplest Swiss cheese model. In the context of asymptotically safe gravity, we have explained the obs…
We investigate the dynamical behaviour of a general class of interacting models in the dark sector in which the phenomenological coupling between cold dark matter and dark energy is a power law of the cosmic scale factor. From numerical simulations we show that, in this background, dark energy always dominates the current composition cosmic. This behaviour may alleviate substantially the coincidence problem. By using current type Ia supernovae, baryonic acoustic oscillations and cosmic microwave background data, we perform a joint statistical analysis and obtain constraints on free parameters …
The abundance of lithium-7 confronts cosmology with a long lasting problem between the predictions of standard big bang nucleosynthesis and the baryonic density determined from the cosmic microwave background observations. This article investigates the influence of the existence of a mirror world, focusing on models in which neutrons can oscillate into mirror neutrons. Such a mechanism allows for an effective late time neutron injection, which induces an increase of the destruction of beryllium-7, due to an increase of the neutron capture, and then a decrease of the final lithium-7 abundance. …
Dark energy interacting with dark matter is a promising model to solve the cosmic coincidence problem. We study the signature of such interaction on large scale cosmic microwave background (CMB) temperature anisotropies. Based on the detail analysis in perturbation equations of dark energy and dark matter when they are in interaction, we find that the large scale CMB, especially the late Integrated Sachs Wolfe effect, is a useful tool to measure the coupling between dark sectors. We also discuss the possibility to detect the coupling by cross-correlating CMB maps with tracers of the large scal…
In order to test if there is energy transfer between dark energy and dark matter, we investigate cosmological constraints on two forms of nontrivial interaction between the dark matter sector and the sector responsible for the acceleration of the universe, in light of the newly revised observations including OHD, CMB, BAO and SNe Ia. More precisely, we find the same tendencies for both phenomenological forms of the interaction term $Q=3γHρ$, i.e., the parameter $γ$ to be a small number, $|γ|\approx 10^{-2}$. However, concerning the sign of the interaction parameter, we observe that $γ>0$ when …
The present standard model of cosmology, $Λ$CDM, contains some intriguing coincidences. Not only are the dominant contributions to the energy density approximately of the same order at the present epoch, but we note that contrary to the emergence of cosmic acceleration as a recent phenomenon, the time averaged value of the deceleration parameter over the age of the universe is nearly zero. Curious features like these in $Λ$CDM give rise to a number of alternate cosmologies being proposed to remove them, including models with an equation of state w = -1/3. In this paper, we examine the validity…
We study the cosmic history of the scalaron in $F(R)$ gravity with constructing the time evolution of the cosmic environment and discuss the chameleonic dark matter based on the chameleon mechanism in the early and current Universe. We then find that the scalaron can be a dark matter. We also propose an interesting possibility that the $F(R)$ gravity can address the coincidence problem.
Interactions between dark matter and dark energy which result in a power-law behavior (with respect to the cosmic scale factor) of the ratio between the energy densities of the dark components (thus generalizing the LCDM model) have been considered as an attempt to alleviate the cosmic coincidence problem phenomenologically. We generalize this approach by allowing for a variable equation of state for the dark energy within the CPL-parametrization. Based on analytic solutions for the Hubble rate and using the Constitution and Union2 SNIa sets, we present a statistical analysis and classify diff…
Possibility of gravitational repulsion in General Relativity is discussed and astronomical data in favor of cosmological acceleration are described. The problem of vacuum energy is emphasized and possible ways of its solution are indicated. The main attention is payed to adjustment mechanism which in principle could compensate originally huge vacuum energy down to cosmologically acceptable value and to solve the coincidence problem of a close magnitudes of the non-compensated remnants of vacuum energy and the energy density of the universe at the present time. Finally possible modifications of…
Decaying cold dark matter (CDM) has been considered as a mechanism to tackle the tensions in the Hubble expansion rate and the clustering of matter. However, polarization measurements of the cosmic microwave background (CMB) severely constrain the fraction of dark matter decaying before recombination, and lensing of the CMB anisotropies by large-scale structure set strong constraints on dark matter decaying after recombination. Together, these constraints make an explanation of the Hubble tension in terms of decaying dark matter unlikely. In response to this situation, we investigate whether a…
