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Register a new userGraduated dark energy: Observational hints of a spontaneous sign switch in the cosmological constant
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Added by
\\\"Ozg\\\"ur Akarsu
Publication date
2019
fields
Physics
and research's language is
English
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We study the cosmological constant ($Lambda$) in the standard $Lambda$CDM model by introducing the textit{graduated dark energy} (gDE) characterised by a minimal dynamical deviation from the null inertial mass density of the $Lambda$ in the form $rho_{rm inert}propto rho^{lambda}<0$ with $lambda<1$ being a ratio of two odd integers, for which its energy density $rho$ dynamically takes negative values in the finite past. For large negative values of $lambda$, it creates a phenomenological model described by a smooth function that approximately describes the $Lambda$ spontaneously switching sign in the late universe to become positive today. We confront the model with the latest combined observational data sets of PLK+BAO+SN+$H$. It is striking that the data predict bimodal posterior probability distributions for the parameters of the model along with large negative $lambda$ values; the new maximum significantly excludes the $Lambda$, and the old maximum contains the $Lambda$. The improvement in the goodness of fit for the $Lambda$ reaches highly significant levels, $Deltachi_{rm min}^2=6.4$ for the new maxima, while it remains at insignificant levels, $Deltachi_{rm min}^2lesssim0.02$, for the old maxima. We show that, in contrast to the old maxima, which do not distinguish from the $Lambda$, the new maxima agree with the model-independent $H_0$ measurements, high-precision Ly-$alpha$ data, and model-independent $Omh^2$ diagnostic estimates. Our results provide strong hints of a spontaneous sign switch in the cosmological constant and lead us to conjecture that the universe has transitioned from AdS vacua to dS vacua, at a redshift $zapprox 2.32$ and triggered the late-time acceleration, and suggests looking for such mechanisms in string theory constructions.
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In this work, we first discuss the possibility that dark energy models with negative energy density values in the past can alleviate the $H_0$ tension, as well as the discrepancy with the baryon acoustic oscillation (BAO) Lyman-$alpha$ data, both which prevail within the $Lambda$CDM model. We then investigate whether two minimal extensions of the $Lambda$CDM model, together or separately, can successfully realize such a scenario: (i) the spatial curvature, which, in the case of spatially closed universe, mimics a negative density source and (ii) simple-graduated dark energy (gDE), which promotes the null inertial mass density of the usual vacuum energy to an arbitrary constant--if negative, the corresponding energy density decreases with redshift similar to the phantom models, but unlike them crosses below zero at a certain redshift. We find that, when the Planck data are not included in the observational analysis, the models with simple-gDE predict interesting and some significant deviations from the $Lambda$CDM model. In particular, a spatially closed universe along with a simple-gDE of positive inertial mass density, which work in contrast to each other, results in minor improvement to the $H_0$ tension. The joint dataset, including the Planck data, presents no evidence for a deviation from spatial flatness but almost the same evidence for a cosmological constant and the simple-gDE with an inertial mass density of order $mathcal{O}(10^{-12}),rm eV^4$. The latter case predicts almost no deviation from the $Lambda$CDM model up until today--so that it results in no improvement regarding the BAO Ly-$alpha$ data--except that it slightly aggravates the $H_0$ tension. We also study via dynamical analysis the history of the Universe in the models, as the simple-gDE results in futures different than the de Sitter future of the $Lambda$CDM model.
We consider an interacting field theory model that describes the interaction between dark energy - dark matter interaction. Only for a specific interaction term, this interacting field theory description has an equivalent interacting fluid description. For inverse power law potentials and linear interaction function, we show that the interacting dark sector model is consistent with $textit{four cosmological data sets}$ -- Hubble parameter measurements (Hz), Baryonic Acoustic Oscillation data (BAO), Supernova Type Ia data (SN), and High redshift HII galaxy measurements (HIIG). More specifically, these data sets prefer a negative value of interaction strength in the dark sector and lead to the best-fit value of Hubble constant $H_0 = 69.9^{0.46}_{1.02}$ km s$^{-1}$ Mpc$^{-1}$. Thus, the interacting field theory model $textit{alleviates the Hubble tension}$ between Planck and these four cosmological probes. Having established that this interacting field theory model is consistent with cosmological observations, we obtain quantifying tools to distinguish between the interacting and non-interacting dark sector scenarios. We focus on the variation of the scalar metric perturbed quantities as a function of redshift related to structure formation, weak gravitational lensing, and the integrated Sachs-Wolfe effect. We show that the difference in the evolution becomes significant for $z < 20$, for all length scales, and the difference peaks at smaller redshift values $z < 5$. We then discuss the implications of our results for the upcoming missions.
It is well known that string theories naturally compactify on anti-de Sitter spaces, and yet cosmological observations show no evidence of a negative cosmological constant in the early Universes evolution. In this letter we present two simple nonlocal modifications of the standard Friedmann cosmology that can lead to observationally viable cosmologies with an initial (negative) cosmological constant. The nonlocal operators we include are toy models for the quantum cosmological backreaction. In Model I an initial quasiperiodic oscillatory epoch is followed by inflation and a late time matter era, representing a dark matter candidate. The backreaction in Model II quickly compensates the negative cosmological term such that the Ricci curvature scalar rapidly approaches zero, and the Universe ends up in a late time radiation era.
We consider the models of vacuum energy interacting with cold dark matter in this study, in which the coupling can change sigh during the cosmological evolution. We parameterize the running coupling $b$ by the form $b(a)=b_0a+b_e(1-a)$, where at the early-time the coupling is given by a constant $b_{e}$ and today the coupling is described by another constant $b_{0}$. We explore six specific models with (i) $Q(a)=b(a)H_0rho_0$, (ii) $Q(a)=b(a)H_0rho_{rm de}$, (iii) $Q(a)=b(a)H_0rho_{rm c}$, (iv) $Q(a)=b(a)Hrho_0$, (v) $Q(a)=b(a)Hrho_{rm de}$, and (vi) $Q(a)=b(a)Hrho_{rm c}$. The current observational data sets we use to constrain the models include the JLA compilation of type Ia supernova data, the Planck 2015 distance priors data of cosmic microwave background observation, the baryon acoustic oscillations measurements, and the Hubble constant direct measurement. We find that, for all the models, we have $b_0<0$ and $b_e>0$ at around the 1$sigma$ level, and $b_0$ and $b_e$ are in extremely strong anti-correlation. Our results show that the coupling changes sign during the evolution at about the 1$sigma$ level, i.e., the energy transfer is from dark matter to dark energy when dark matter dominates the universe and the energy transfer is from dark energy to dark matter when dark energy dominates the universe.
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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