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Register a new userInteracting Quintessence, Cosmic Acceleration and the Coincidence Problem
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Faced by recent evidence for a flat universe dominated by dark energy, cosmologists grapple with deep cosmic enigmas such as the cosmological constant problem, extreme fine-tuning and the cosmic coincidence problem. The extent to which we observe the dimming of distant supernovae suggests that the cosmic acceleration is as least as severe as in cosmological constant models. Extrapolating this to our cosmic future implies terrifying visions of either a cold and empty universe or an explosive demise in a ``Big Rip. We construct a class of dynamical scalar field models of dark energy and dark matter. Within this class we can explain why supernovae imply a cosmic equation of state $wlesssim-1$, address fine tuning issues, protect the universe from premature acceleration and predict a constant fraction of dark energy to dark matter in the future (thus solving the coincidence problem), satisfy the dominant energy condition, and ensure that gravitationally bound objects remain so forever (avoid a Big Rip). This is achieved with a string theory inspired Lagrangian containing standard kinetic terms, exponential potentials and couplings, and parameters of order unity.
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It is argued that cosmological models that feature a flow of energy from dark energy to dark matter may solve the coincidence problem of late acceleration (i.e., why the energy densities of both components are of the same order precisely today?). However, much refined and abundant observational data of the redshift evolution of the Hubble factor are needed to ascertain whether they can do the job.
We study the effect of an explicit interaction between two scalar fields components describing dark matter in the context of a recent proposal framework for interaction. We find that, even assuming a very small coupling, it is sufficient to explain the observational effects of a cosmological constant, and also overcome the problems of the $Lambda$CDM model without assuming an exotic dark energy.
It is a mystery why the density of matter and the density of vacuum energy are nearly equal today when they scale so differently during the expansion of the Universe. We suggest a paradigm that might allow for a non-anthropic solution to this cosmic coincidence problem. The fact that the half life of Uranium 238 is very near to the age of the solar system is not considered a coincidence since there are many nuclides with half lives ranging over a huge range of time scales implying that there is likely to be some nuclide with a half life near to any given time scale. Likewise it may be that the vacuum field energy causing the universal acceleration today is just one of a large ensemble of scalar field energies, which have dominated the Universe in the past and then faded away. Given that in standard cosmology and particle physics there are already several scalar fields that probably contribute to universal vacuum energy (the Higgs field, the inflaton, and whatever quintessence/dark energy field causes the current acceleration), the idea of a large ensemble of fields does not seem far fetched. Predictions of the idea include: 1) The current vacuum energy driving the acceleration is not constant and will eventually fade away, 2) The ratio w of scalar field pressure to density is currently changing and is not precisely negative one, 3) There were likely periods of vacuum dominance and acceleration in the past, 4) the current phase of acceleration will end but there may be additional periods of acceleration in the future, 5) the ultimate fate of the Universe cannot be decided until the nature of these fields is known, but the eventual sum of densities from all scalar fields could be zero, as was usually assumed before the discovery of the current universal acceleration.
In the framework of a phenomenological cosmological model with the assumption of $rho_{X} propto rho_{m} a^{xi}$ ($rho_{X}$ and $rho_{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). $xi$ is a key parameter to characterize the severity of the coincidence problem, wherein $xi=3$ and $0$ correspond to the $Lambda$CDM scenario and the self-similar solution without the coincidence problem, respectively. The case of $xi = Constant$ has been investigated in the previous studies, while we further consider the case of $xi(z) = xi_{0} + xi_{z}*frac{z}{1+z}$ to explore the possible evolution. A joint analysis of the Pantheon SNe Ia sample with the recent BAO and CMB data figures out that $xi=3.75_{-0.21}^{+0.13}$ in the case of $xi = Constant$ at $68%$ confidence level (CL), in addition, $xi_{0} = 2.78_{-1.01}^{+0.28}$ and $xi_{z} = 0.93_{-0.91}^{+1.56}$ in the case of $xi(z) = xi_{0} + xi_{z}*frac{z}{1+z}$ at $68%$ CL . It implies that the temporal evolution of the scaling parameter $xi$ is supported by the joint sample at $68%$ CL; moreover, the $Lambda$CDM model is excluded by the joint sample at $68%$ CL in both cases, and the coincidence problem still exists. In addition, according to the model selection techniques, the $Lambda$CDM model is the favorite one in terms of the AIC and BIC techniques, however, the scenario of $xi(z)$ is most supported in term of the DIC technique.
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 $rho_xproptorho_d^alpha$, where $rho_x$ and $rho_d$ are the energy densities of dark energy and dark matter components, respectively, and $alpha$ is a free parameter. For a dark energy equation of state parameter $w=-1$ we found that, if $alpha=0$, the standard $Lambda$CDM model is recovered, where the coincidence problem is unsolved. For $0<alpha<1$, the CP would be alleviated and for $alphasim 1$, the CP would be solved. The dark energy component is analyzed with both $w=-1$ and $w eq -1$. Using Supernovae type Ia and Hubble parameter data constraints, in the case $w=-1$ we find $alpha=0.109^{+0.062}_{-0.072}$ at 68% C.L., and the CP is alleviated. This model is also slightly favoured against nonflat $Lambda$CDM model by using a Bayesian Information Criterion (BIC) analysis. For $w eq-1$, a degeneracy arises on the $w$ - $alpha$ plane. In order to break such degeneracy we add cosmic microwave background distance priors and baryonic acoustic oscillations data to the constraints, yielding $alpha=-0.075pm 0.046$ at 68% C.L.. In this case we find that the CP is not alleviated even for 2$sigma$ interval for $alpha$. Furthermore, this last model is discarded against nonflat $Lambda$CDM according to BIC analysis.
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