Coupled Dark Energy Research Articles

Little rip and pseudo rip cosmological models with coupled dark energy based on a new generalized entropy

In this paper, we study Little Rip (LR) and Pseudo Rip (PR) cosmological models containing two coupled fluids: dark energy and dark matter. We assume a spatially flat Friedmann–Robertson–Walker (FRW) universe. The interaction between the dark energy and the dark matter fluid components is described in terms of the parameters in the generalized Equation of State (EoS) in presence of the bulk viscosity. We consider entropic cosmology and use a description based on a new generalized entropy function, which was proposed by Nojiri–Odintsov–Faraoni [From nonextensive statistics and black hole entropy to the holographic dark universe, Phys. Rev. D 105(4) (2022) 044042]. Conditions for the appearance of the (LR) and the (PR) in terms of the parameters of the (EoS) are obtained. Introducing an energy density [Formula: see text] corresponding to a specified entropy function [Formula: see text], together with an interaction term [Formula: see text] in the gravitational equations of motion, we derive modified forms of the EoS parameters. We discuss the corrections of the thermodynamic parameters associated with the generalized entropy function. Properties of the late universe as well as in the early universe in this formalism are pointed out.

Finch–Skea dark energy stars with vanishing complexity factor

We make use of the complexity factor formalism for static and self-gravitating objects, recently proposed by L. Herrera (2018), for setting up a relativistic, anisotropic dark energy stellar model satisfying the vanishing complexity condition. The condition serves as an aid in closing the system of field equations for General Relativity. The popular Finch–Skea ansatz is used for the metric component, grr, with the temporal counterpart determined by imposing the vanishing complexity condition. We assess the physical acceptability of our dark energy stellar model according to the standard requirements for static, spherically symmetric compact matter, and we study the associated matter quantities with respect to the dark energy coupling parameter, α. The effect of α on our stellar model has been studied in detail. It is established that our model is regular and stable, and the radial pressure vanishes at the surface boundary as required by selecting a suitable range for α. We find that the coupling parameter influences the baryonic energy density and pressures; however, the pressure anisotropy remains largely unaffected. This is a novel feature of our dark-energy star model, obtained via the vanishing complexity condition. We further demonstrate the robustness of our model in predicting the observed radii of well-known compact objects such as 4U 1820-30, Vela X-1, LMC X-4, PSR J1614-2230, and PSR J1903+0327 by varying a free parameter arising in the Finck–Skea ansatz. Our findings show that our solution predicts masses beyond the standard general relativity limit of 2.0 M⊙ for neutron stars. The prediction of compact objects with masses in the range of 2.5 M⊙ within our solution-generating framework augers well for the observation of gravitational waves arising in binary mergers.

Charged strange star coupled to anisotropic dark energy in Tolman–Kuchowicz spacetime

The concept of dark energy can be used as a possible option to prevent the gravitational collapse of compact objects into singularities. It affects the universe on the largest scale, as it is responsible for our universe’s accelerated expansion. As a consequence, it seems possible that dark energy will interact with any compact astrophysical stellar object [Phys. Rev. D 103, 084042 (2021)]. In this work, our prime focus is to develop a simplified model of a charged strange star coupled to anisotropic dark energy in Tolman–Kuchowicz spacetime (Tolman in Phys Rev 55:364, 1939; Kuchowicz in Acta Phys Pol 33:541, 1968) within the context of general relativity. To develop our model, here we consider a particular strange star object, Her X-1 with observed values of mass =(0.85 pm 0.15)M_{odot } and radius = 8.1_{-0.41}^{+0.41} km. respectively. In this context, we initially started with the equation of state (EoS) to model the dark energy, in which the dark energy density is proportional to the isotropic perfect fluid matter-energy density. The unknown constants present in the metric have been calculated by using the Darmois–Israel condition. We perform an in-depth analysis of the stability and force equilibrium of our proposed stellar configuration as well as multiple physical attributes of the model such as metric function, pressure, density, mass–radius relation, and dark energy parameters by varying dark energy coupling parameter alpha . Thus after a thorough theoretical analysis, we found that our proposed model is free from any singularity and also satisfies all stability criteria to be a stable and physically realistic stellar model.

Possible combinations of early and late time cosmologies through BAO scales

The theoretical scenarios of early universe and late time accelerated expansion constitute an open problem in cosmology. Relative Baryon Acoustic Oscillation (BAO) scales measured by 6dFGS, mathrm SDSS+2dFGRS, BOSS DR11 Lyalpha and BOSS DR11 QSO Lyalpha are used to investigate the parameter spaces of combinations of pre-recombination physics and late time cosmology. For pre-recombination physics we fix mathrm N_{eff}=3 and 4 and incorporate an Early Dark Energy (EDE) component. For late time cosmologies we consider curved Lambda CDM model, Dynamical Dark Energy (DDE) model with CPL parameterisation (omega _0,omega _1) of equation of state, Coupled Dark Energy (CDE) model with constant coupling and curved DGP brane-world scenario. Combination of a curved Lambda CDM (Omega _{textrm{k}}Lambda CDM) model with mathrm N_{eff}-EDE cosmology is found to be possible. For DDE, the relative BAO scales are achievable through omega _1 preferring the region omega _1< 0. For mathrm N_{eff}-EDE-CDE, the BAO scales are generated for both dark energy-dark matter and dark matter-dark energy conversion, however with exceptionally large value of dark energy density parameter and constant equation of state with omega _0 > -1. Curved DGP cosmology (Omega _textrm{k}DGP) is found to join with N_{textrm{eff}}-EDE cosmology through curvature parameter Omega _{mathrm{k(0)}}in [-0.002, -0.1] and cross-over parameter Omega _{mathrm{r_c}} in [0.08, 0.45]. Cosmic ages are calculated for the derived parameters of Omega _{textrm{k}}Lambda CDM, flat DDE and Omega _{textrm{k}}DGP models and compared with the ages of few globular clusters available in literature. Curved cosmologies are found to be promising.

Global asymptotic dynamics of the cubic galileon interacting with dark matter

In this paper we perform a thorough dynamical systems analysis of the cubic galileon model non-minimally coupled to the dark matter. Three well-known classes of interacting models are considered where the energy exchange between the dark components is a function of the dark matter density and of the dark energy density: $Q_1=3\alpha H\rho_m$, $Q_2=3\beta\rho_m\dot{\phi}$ and $Q_3=3 \epsilon H \rho_\phi$, respectively. We are able to show the global asymptotic dynamics of the model for the exponential potential in a homogeneous and isotropic background. The cosmological implications of the proposed scenarios are explored and it is found that, in addition to the appearance of new equilibrium configurations that do not appear neither in the non-interacting cubic galileon model nor in the interacting quintessence model, there is a significant impact of the non-minimal coupling through modification of the stability properties of the critical points. The resulting cosmological scenario provides a bigbang origin of the cosmic expansion, an early transient stage of inflationary expansion, as well as matter-scaling late time stable state of the universe, among other solutions of lesser cosmological interest. This work extends previous studies of coupled dark energy to a broader class of gravitational theories.

Baryogenesis triggered by Barrow holographic dark energy coupling

Baryogenesis triggered by Barrow holographic dark energy coupling

Constraining constant and tomographic coupled dark energy with low-redshift and high-redshift probes

We consider coupled dark energy (CDE) cosmologies, where dark matter particles feel a force stronger than gravity, due to the fifth force mediated by a scalar field which plays the role of dark energy. We perform for the first time a tomographic analysis of coupled dark energy, where the coupling strength is parametrized and constrained in different redshift bins. This allows us to verify which data can better constrain the strength of the coupling and how large the coupling can be at different epochs. First, we employ cosmic microwave background data from Planck, the Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT), showing the impact of different choices that can be done in combining these datasets. Then, we use a range of low redshift probes to test CDE cosmologies, both for a constant and for a tomographic coupling. In particular, we use for the first time data from weak lensing (the KiDS-1000 survey), galaxy clustering (BOSS survey), and their combination, including 3x2pt galaxy-galaxy lensing cross-correlation data. We do not find evidence for nonzero coupling, either for a constant or tomographic case. A nonzero coupling is however still in agreement with current data. For CMB and background datasets, a tomographic coupling allows for $\ensuremath{\beta}$ values up to one order of magnitude larger than in previous works, in particular at $z&lt;1$. The use of 3x2pt analysis then becomes important to constrain $\ensuremath{\beta}$ at low redshifts, even when coupling is allowed to vary: for 3x2pt we find, at $0.5&lt;z&lt;1$, $\ensuremath{\beta}={0.018}_{\ensuremath{-}0.011}^{+0.007}$, comparable to what CMB and background datasets would give for a constant coupling. This makes upcoming galaxy surveys potentially powerful probes to test CDE models at low redshifts. We find a smaller tension in ${H}_{0}$ and ${S}_{8}$ when the coupling is allowed to vary, although this is rather due to an increase in their uncertainties. We also see that our model is penalized by a Bayesian ratio model comparison with respect to $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$, which is still favored by current data.

Compact star with coupled dark energy in Karmarkar connected relativistic space–time

Compact star with coupled dark energy in Karmarkar connected relativistic space–time

Coupled and uncoupled early dark energy, massive neutrinos, and the cosmological tensions

Some cosmological models with non-negligible dark energy fractions in particular windows of the pre-recombination epoch are capable of alleviating the Hubble tension quite efficiently, while keeping the good description of the data that are used to build the cosmic inverse distance ladder. There has been an intensive discussion in the community on whether these models enhance the power of matter fluctuations, leading {\it de facto} to a worsening of the tension with the large-scale structure measurements. We address this pivotal question in the context of several early dark energy (EDE) models, considering also in some cases a coupling between dark energy and dark matter, and the effect of massive neutrinos. We fit them using the Planck 2018 likelihoods, the supernovae of Type Ia from the Pantheon compilation and data on baryon acoustic oscillations. We find that ultra-light axion-like (ULA) EDE can actually alleviate the $H_0$ tension without increasing the values of $\sigma_{12}$ with respect to those found in the $\Lambda$CDM, whereas EDE with an exponential potential does not have any impact on the tensions. A coupling in the dark sector tends to enhance the clustering of matter, and the data limit a lot the influence of massive neutrinos, since the upper bounds on the sum of their masses are too close to those obtained in the standard model. We find that in the best case, namely ULA, the Hubble tension is reduced to $\sim 2\sigma$.

Compact star coupled with dark energy in the background of Tolman–Kuchowicz spacetime

This paper provides a simplified description for simulating the coupling of dark energy with baryonic matter by considering a super-dense pulsar PSRJ1614-2230 as the model star. The starting equation of state for modeling the dark energy is motivated by the MIT Bag model for spherically confined hadrons and the observational evidences of its repulsive nature. Einstein field equations are solved in the stellar interior using the generalized framework of Tolman–Kuchowicz spacetime metric. The solutions are then analyzed for various physical parameters such as metric potential, pressure, density, energy conditions, etc. The physical analysis of multiple parameters indicates stable star formation. The proposed stellar model is free from all singularities and also meets the requisite stability criteria. The numerical results obtained for relativistic adiabatic index indicate that the model star is stiff and stable against radially induced adiabatic perturbations.

Hybrid star model in Tolman-Buchdahl metric potentials with coupled dark energy and baryonic matter

Hybrid star model in Tolman-Buchdahl metric potentials with coupled dark energy and baryonic matter

Cosmic evolution in DHOST theory with scaling solutions

We study cosmic evolution based on the fixed points in the dynamical analysis of the degenerate higher-order scalar-tensor (DHOST) theories. We consider the DHOST theory in which the propagation speed of gravitational waves is equal to the speed of light, the tensor perturbations do not decay to dark energy perturbations, and the scaling solutions exist. The scaling fixed point associated with late-time acceleration of the universe can be either stable or saddle depending on the parameters of the theory. For some ranges of the parameters, this scaling fixed point and field-dominated fixed point can be simultaneously stable. Cosmic evolution will reach either the scaling attractor or the field-dominated attractor depending on the sign of the time derivative of the scalar field in the theory during the matter domination. The density parameter of dark matter can be larger than unity before reaching the scaling attractor if the deviation from Einstein’s theory of gravity is too large. For this DHOST theory, stabilities of {phi }-matter-dominated epoch (phi MDE) and field-dominated solutions are similar to the coupled dark energy models in Einstein gravity even though gravity is described by different theories. In our consideration, the universe can only evolve from the phi MDE regime to the field-dominated regime. The ghost and gradient instabilities up to linear order in cosmological perturbations have been investigated. There is no gradient instability, while the ghost instability can be avoided for some range of the model parameters.

Coupled dark energy model inspired from general conformal transformation

We study the coupled dark energy model constructed from the general conformal transformation in which the coefficient of the conformal transformation depends on both the scalar field and its kinetic term. Under this conformal transformation, the action for subclass of Degenerate Higher-Order Scalar-Tensor (DHOST) theories is related to the Einstein-Hilbert action. The evolution of the background universe has the scaling fixed point which corresponds to acceleration of the universe at late time. For the choices of parameters which make the late-time scaling point stable, the fixed point corresponding to $\phi$-matter-dominated-era ($\phi$MDE) is a saddle point, and the universe can evolve from radiation dominated epoch through $\phi$MDE before reaching the scaling point at late time with the cosmological parameters which satisfy the observational bound. During the $\phi$MDE, the effective equation of state parameter is slightly positive, so that one of possible mechanisms for alleviating the $H_0$ tension can be achieved. In this coupled dark energy model, the effective gravitational coupling for dark matter perturbations on small scales can be smaller than that in the $\Lambda$CDM model. Therefore a growth rate of the dark matter perturbations is suppressed compare with the $\Lambda$CDM model, which implies that the $\sigma_8$ tension could be alleviated.

Direct detection of dark energy: The XENON1T excess and future prospects

We explore the prospects for direct detection of dark energy by current and upcoming terrestrial dark matter direct detection experiments. If dark energy is driven by a new light degree of freedom coupled to matter and photons then dark energy quanta are predicted to be produced in the Sun. These quanta free-stream towards Earth where they can interact with Standard Model particles in the detection chambers of direct detection experiments, presenting the possibility that these experiments could be used to test dark energy. Screening mechanisms, which suppress fifth forces associated with new light particles, and are a necessary feature of many dark energy models, prevent production processes from occurring in the core of the Sun, and similarly, in the cores of red giant, horizontal branch, and white dwarf stars. Instead, the coupling of dark energy to photons leads to production in the strong magnetic field of the solar tachocline via a mechanism analogous to the Primakoff process. This then allows for detectable signals on Earth while evading the strong constraints that would typically result from stellar probes of new light particles. As an example, we examine whether the electron recoil excess recently reported by the XENON1T collaboration can be explained by chameleon-screened dark energy, and find that such a model is preferred over the background-only hypothesis at the $2.0\sigma$ level, in a large range of parameter space not excluded by stellar (or other) probes. This raises the tantalizing possibility that XENON1T may have achieved the first direct detection of dark energy. Finally, we study the prospects for confirming this scenario using planned future detectors such as XENONnT, PandaX-4T, and LUX-ZEPLIN.

Little rip phenomena from coupled dark energy with quadratic equation of state with time-dependent parameters

Little rip phenomena from coupled dark energy with quadratic equation of state with time-dependent parameters

Model independent perspectives on coupled dark energy and the swampland

We present a general model-independent approach to study the coupled dark energy and the string Swampland criteria. We show how the dark sector interaction is degenerated with the equation of state of dark energy in the context of the expansion of the Universe. With priors for either of them, the dynamics of dark energy and the dark sector interactions can be reconstructed together with the bounds of the Swampland criteria. Combining cosmic chronometers, baryon acoustic oscillation (BAO), and Type Ia supernovae our results suggest a mild $1 \sigma$ significance of dark sector interactions at low redshift for the coupled quintessence. The Lyman-$\alpha$ BAO at $z=2.34$ leads a $2 \sigma$ signal of nonzero interactions at high redshift. The implications for coupled quintessence are discussed.

Suppressed cosmic growth in coupled vector-tensor theories

We study a coupled dark energy scenario in which a massive vector field $A_{\mu}$ with broken $U(1)$ gauge symmetry interacts with the four-velocity $u_c^{\mu}$ of cold dark matter (CDM) through the scalar product $Z=-u_c^{\mu} A_{\mu}$. This new coupling corresponds to the momentum transfer, so that the background vector and CDM continuity equations do not have explicit interacting terms analogous to the energy exchange. Hence the observational preference of uncoupled generalized Proca theories over the $\Lambda$CDM model can be still maintained at the background level. Meanwhile, the same coupling strongly affects the evolution of cosmological perturbations. While the effective sound speed of CDM vanishes, the propagation speed and no-ghost condition of a longitudinal scalar of $A_{\mu}$ and the CDM no-ghost condition are subject to nontrivial modifications by the $Z$ dependence in the Lagrangian. We propose a concrete dark energy model and show that the gravitational interaction on scales relevant to the linear growth of large-scale structures can be smaller than the Newton constant at low redshifts. This leads to the suppression of growth rates of both CDM and total matter density perturbations, so our model allows an interesting possibility for reducing the tension of matter density contrast $\sigma_8$ between high- and low-redshift measurements.

Update on coupled dark energy and the H0 tension

In this work we provide updated constraints on coupled dark energy (CDE) cosmology with Peebles-Ratra (PR) potential and constant coupling strength $\beta$. This modified gravity scenario introduces a fifth force between dark matter particles, mediated by a scalar field that plays the role of dark energy. The mass of the dark matter particles does not remain constant, but changes with time as a function of the scalar field. Here we focus on the phenomenological behavior of the model, and assess its ability to describe updated cosmological data sets that include the Planck 2018 cosmic microwave background (CMB) temperature, polarization and lensing, baryon acoustic oscillations, the Pantheon compilation of supernovae of Type Ia, data on $H(z)$ from cosmic chronometers, and redshift-space distortions. We also study which is the impact of the local measurement of $H_0$ from SH0ES and the strong-lensing time delay data from the H0LICOW collaboration on the parameter that controls the strength of the interaction in the dark sector. We find a peak corresponding to a coupling $\beta > 0$ and to a potential parameter $\alpha > 0$, more or less evident depending on the data set combination. We show separately the impact of each data set and remark that it is especially CMB lensing the one data set that shifts the peak the most towards $\Lambda$CDM. When a model selection criterion based on the full Bayesian evidence is applied, however, $\Lambda$CDM is still preferred in all cases, due to the additional parameters introduced in the CDE model.

Weak cosmic growth in coupled dark energy with a Lagrangian formulation

We investigate a dark energy scenario in which a canonical scalar field ϕ is coupled to the four velocity ucμ of cold dark matter (CDM) through a derivative interaction ucμ∂μϕ. The coupling is described by an interacting Lagrangian f(X,Z), where f depends on X=−∂μϕ∂μϕ/2 and Z=ucμ∂μϕ. We derive stability conditions of linear scalar perturbations for the wavelength deep inside the Hubble radius and show that the effective CDM sound speed is close to 0 as in the standard uncoupled case, while the scalar-field propagation speed is affected by the interacting term f. Under a quasi-static approximation, we also obtain a general expression of the effective gravitational coupling felt by the CDM perturbation. We study the late-time cosmological dynamics for the coupling f∝X(2−m)/2Zm and show that the gravitational coupling weaker than the Newton constant can be naturally realized for m>0 on scales relevant to the growth of large-scale structures. This allows the possibility for alleviating the tension of σ8 between low- and high-redshift measurements.

Nonminimal dark sector physics and cosmological tensions

We explore whether non-standard dark sector physics might be required to solve the existing cosmological tensions. The properties we consider in combination are an interaction between the dark matter and dark energy components, and a dark energy equation of state $w$ different from that of the canonical cosmological constant $w=-1$. In principle, these two parameters are independent. In practice, to avoid early-time, superhorizon instabilities, their allowed parameter spaces are correlated. We analyze three classes of extended interacting dark energy models in light of the 2019 Planck CMB results and Cepheid-calibrated local distance ladder $H_0$ measurements of Riess et al. (R19), as well as recent BAO and SNeIa distance data. We find that in quintessence coupled dark energy models, where $w > -1$, the evidence for a non-zero coupling between the two dark sectors can surpass the $5\sigma$ significance. On the other hand, in phantom coupled dark energy models, there is no such preference for a non-zero dark sector coupling. All the models we consider significantly raise the value of the Hubble constant easing the $H_0$ tension. The addition of low-redshift BAO and SNeIa measurements leaves some residual tension with R19 but at a level that could be justified by a statistical fluctuation. Bayesian evidence considerations mildly disfavour both the coupled quintessence and phantom models, while mildly favouring a coupled vacuum scenario, even when late-time datasets are considered. We conclude that non-minimal dark energy cosmologies, such as coupled quintessence, phantom, or vacuum models, are still an interesting route towards softening existing cosmological tensions, even when low-redshift datasets and Bayesian evidence considerations are taken into account. (abstract severely abridged)