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Register a new userGeneralized Emergent Dark Energy Model and the Hubble Constant Tension
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We investigate a generalized form of the phenomenologically emergent dark energy model, known as generalized emergent dark energy (GEDE), introduced by Li and Shafieloo [Astrophys. J. {bf 902}, 58 (2020)] in light of a series of cosmological probes and considering the evolution of the model at the level of linear perturbations. This model introduces a free parameter $Delta$ that can discriminate between the $Lambda$CDM (corresponds to $Delta=0$) or the phenomenologically emergent dark energy (PEDE) (corresponds to $Delta=1$) models, allowing us to determine which model is preferred most by the fit of the observational datasets. We find evidence in favor of the GEDE model for Planck alone and in combination with R19, while the Bayesian model comparison is inconclusive when Supernovae Type Ia or BAO data are included. In particular, we find that $Lambda$CDM model is disfavored at more than $2sigma$ CL for most of the observational datasets considered in this work and PEDE is in agreement with Planck 2018+BAO+R19 combination within $1sigma$ CL.
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Holographic dark energy (HDE) describes the vacuum energy in a cosmic IR region whose total energy saturates the limit of avoiding the collapse into a black hole. HDE predicts that the dark energy equation of the state transiting from greater than the $-1$ regime to less than $-1$, accelerating the Universe slower at the early stage and faster at the late stage. We propose the HDE as a new {it physical} resolution to the Hubble constant discrepancy between the cosmic microwave background (CMB) and local measurements. With Planck CMB and galaxy baryon acoustic oscillation (BAO) data, we fit the HDE prediction of the Hubble constant as $H_0^{}!=, 71.54pm1.78,mathrm{km,s^{-1} Mpc^{-1}}$, consistent with local $H_0^{}$ measurements by LMC Cepheid Standards (R19) at $1.4sigma$ level. Combining Planck+BAO+R19, we find the HDE parameter $c=0.51pm0.02$ and $H_0^{}! = 73.12pm 1.14,mathrm{km ,s^{-1} Mpc^{-1}}$, which fits cosmological data at all redshifts. Future CMB and large-scale structure surveys will further test the holographic scenario.
The Hubble tension can be significantly eased if there is an early component of dark energy that becomes active around the time of matter-radiation equality. Early dark energy models suffer from a coincidence problem -- the physics of matter-radiation equality and early dark energy are completely disconnected, so some degree of fine-tuning is needed in order for them to occur nearly simultaneously. In this paper we propose a natural explanation for this coincidence. If the early dark energy scalar couples to neutrinos then it receives a large injection of energy around the time that neutrinos become non-relativistic. This is precisely when their temperature is of order their mass, which, coincidentally, occurs around the time of matter-radiation equality. Neutrino decoupling therefore provides a natural trigger for early dark energy by displacing the field from its minimum just before matter-radiation equality. We discuss various theoretical aspects of this proposal, potential observational signatures, and future directions for its study.
Early dark energy (EDE) offers a particularly interesting theoretical approach to the Hubble tension, albeit one that introduces its own set of challenges, including a new `why then problem related to the EDE injection time at matter-radiation equality, and a mild worsening of the large-scale structure (LSS) tension. Both these challenges center on the properties of dark matter, which becomes the dominant component of the Universe at EDE injection and is also responsible for seeding LSS. Motivated by this, we explore the potential of couplings between EDE and dark matter to address these challenges, focusing on a mechanism similar to chameleon dark energy theories, deeming this chameleon early dark energy (CEDE). We study the cosmological implications of such theories by fitting to the CMB, BAO, supernovae and the local value of $H_0$. We find that the Hubble tension is resolved by CEDE with $H_0 = 71.19(71.85)pm 0.99$ km/s/Mpc. Further, the model provides an excellent fit to all the data, with no change to the CMB $chi^2$ relative to a $Lambda$CDM fit to just the CMB, BAO and SNe (i.e. excluding the $H_0$ tension for $Lambda$CDM). We find a mild preference $(sim 2sigma)$ for the chameleon coupling constant $beta >0$.
Braneworld models with induced gravity exhibit phantom-like behaviour of the effective equation of state of dark energy. They can, therefore, naturally accommodate higher values of $H_0$, preferred by recent local measurements, while satisfying the CMB constraints. We test the background evolution in such phantom braneworld scenarios with the current observational datasets. We find that the phantom braneworld prefers a higher value of $H_0$ even without the R19 prior, thereby providing a much better fit to the local measurements. Although this braneworld model cannot fully satisfy all combinations of cosmological observables, among existing dark energy candidates the phantom brane provides one of the most compelling explanations of cosmic evolution.
The Phenomenologically Emergent Dark Energy model, a dark energy model with the same number of free parameters as the flat $Lambda$CDM, has been proposed as a working example of a minimal model which can avoid the current cosmological tensions. A straightforward question is whether or not the inclusion of massive neutrinos and extra relativistic species may spoil such an appealing phenomenological alternative. We present the bounds on $M_{ u}$ and $N_{rm eff}$ and comment on the long standing $H_0$ and $sigma_8$ tensions within this cosmological framework with a wealth of cosmological observations. Interestingly, we find, at $95%$ confidence level, and with the most complete set of cosmological observations, $M_{ u}sim 0.21^{+0.15}_{-0.14}$ eV and $N_{rm eff}= 3.03pm 0.32$ i.e. an indication for a non-zero neutrino mass with a significance above $2sigma$. The well known Hubble constant tension is considerably easened, with a significance always below the $2sigma$ level.
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