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Register a new userEarly dark energy from massive neutrinos -- a natural resolution of the Hubble tension
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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.
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New Early Dark Energy (NEDE) is a component of vacuum energy at the electron volt scale, which decays in a first-order phase transition shortly before recombination [arXiv:1910.10739]. The NEDE component has the potential to resolve the tension between recent local measurements of the expansion rate of the Universe using supernovae (SN) data and the expansion rate inferred from the early Universe through measurements of the cosmic microwave background (CMB) when assuming $Lambda$CDM. We discuss in depth the two-scalar field model of the NEDE phase transition including the process of bubble percolation, collision, and coalescence. We also estimate the gravitational wave signal produced during the collision phase and argue that it can be searched for using pulsar timing arrays. In a second step, we construct an effective cosmological model, which describes the phase transition as an instantaneous process, and derive the covariant equations that match perturbations across the transition surface. Fitting the cosmological model to CMB, baryonic acoustic oscillations and SN data, we report $H_0 = 69.6^{+1.0}_{-1.3} , textrm{km}, textrm{s}^{-1}, textrm{Mpc}^{-1}$ $(68 %$ C.L.) without the local measurement of the Hubble parameter, bringing the tension down to $2.5, sigma$. Including the local input, we find $H_0 = 71.4 pm 1.0 , textrm{km}, textrm{s}^{-1}, textrm{Mpc}^{-1}$ $(68 %$ C.L.) and strong evidence for a non-vanishing NEDE component with a $simeq 4, sigma$ significance.
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 $beta >0$.
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.
New physics increasing the expansion rate just prior to recombination is among the least unlikely solutions to the Hubble tension, and would be expected to leave an important signature in the early Integrated Sachs-Wolfe (eISW) effect, a source of Cosmic Microwave Background (CMB) anisotropies arising from the time-variation of gravitational potentials when the Universe was not completely matter dominated. Why, then, is there no clear evidence for new physics from the CMB alone, and why does the $Lambda$CDM model fit CMB data so well? These questions and the vastness of the Hubble tension theory model space motivate general consistency tests of $Lambda$CDM. I perform an eISW-based consistency test of $Lambda$CDM introducing the parameter $A_{rm eISW}$, which rescales the eISW contribution to the CMB power spectra. A fit to Planck CMB data yields $A_{rm eISW}=0.988 pm 0.027$, in perfect agreement with the $Lambda$CDM expectation $A_{rm eISW}=1$, and posing an important challenge for early-time new physics, which I illustrate in a case study focused on early dark energy (EDE). I explicitly show that the increase in $omega_c$ needed for EDE to preserve the fit to the CMB, which has been argued to worsen the fit to weak lensing and galaxy clustering measurements, is specifically required to lower the amplitude of the eISW effect, which would otherwise exceed $Lambda$CDMs prediction by $approx 20%$: this is a generic problem beyond EDE and likely applying to most models enhancing the expansion rate around recombination. Early-time new physics models invoked to address the Hubble tension are therefore faced with the significant challenge of making a similar prediction to $Lambda$CDM for the eISW effect, while not degrading the fit to other measurements in doing so.
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.
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