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Dynamical dark energy versus $Λ=$const. in light of observations

Joan Sola Peracaula, Javier de Cruz Perez, Adria Gomez-Valent

TL;DR

The work examines whether cosmic acceleration is due to a constant $Λ$ or dynamical dark energy (DDE) by analyzing a large data ensemble that includes SNIa, BAO, $H(z)$, LSS, and CMB. It compares ΛCDM to DDE scenarios (XCDM, φCDM, and the running vacuum model, RVM) using joint likelihood fits and information criteria, revealing signs of DDE at approximately $3.3σ$ for XCDM and up to $≈3.8σ$ for the RVM. A key finding is that LSS growth data strongly contribute to the DDE signal and that the RVM can alleviate the $σ_8$ tension inherent in ΛCDM. Overall, the results suggest a mild evolution of the dark energy density is favored by current observations, with dynamical vacuum models offering a compelling alternative to a strictly constant $Λ$.

Abstract

After about two decades of the first observational papers confirming the accelerated expansion of the universe, we are still facing the question whether the cause of it is a rigid cosmological constant $Λ$-term or a mildly evolving dynamical dark energy (DDE). While studies focusing mainly on CMB measurements do not perceive signs of physics beyond the $Λ$CDM, in this work we show that if we take a large string $SNIa+BAO+H(z)+LSS+CMB$ of modern cosmological observations, in which not only the CMB but also a rich sample of large scale structure formation data are included, one can extract $\sim 3.3σ$ signs of DDE using a simple XCDM parameterization. These signs can be enhanced up to near $3.8σ$ in the context of the running vacuum model (RVM), in which the vacuum energy density is in interaction with dark matter. Recently the RVM has been shown to provide an efficient and economical solution to the $σ_8$-tension, which is one of the intriguing phenomenological problems that has not been possible to solve within the $Λ$CDM so far. This fact contributes to strengthen the possibility that dynamical vacuum energy, or in general DDE, could be presently favored by the observations.

Dynamical dark energy versus $Λ=$const. in light of observations

TL;DR

The work examines whether cosmic acceleration is due to a constant or dynamical dark energy (DDE) by analyzing a large data ensemble that includes SNIa, BAO, , LSS, and CMB. It compares ΛCDM to DDE scenarios (XCDM, φCDM, and the running vacuum model, RVM) using joint likelihood fits and information criteria, revealing signs of DDE at approximately for XCDM and up to for the RVM. A key finding is that LSS growth data strongly contribute to the DDE signal and that the RVM can alleviate the tension inherent in ΛCDM. Overall, the results suggest a mild evolution of the dark energy density is favored by current observations, with dynamical vacuum models offering a compelling alternative to a strictly constant .

Abstract

After about two decades of the first observational papers confirming the accelerated expansion of the universe, we are still facing the question whether the cause of it is a rigid cosmological constant -term or a mildly evolving dynamical dark energy (DDE). While studies focusing mainly on CMB measurements do not perceive signs of physics beyond the CDM, in this work we show that if we take a large string of modern cosmological observations, in which not only the CMB but also a rich sample of large scale structure formation data are included, one can extract signs of DDE using a simple XCDM parameterization. These signs can be enhanced up to near in the context of the running vacuum model (RVM), in which the vacuum energy density is in interaction with dark matter. Recently the RVM has been shown to provide an efficient and economical solution to the -tension, which is one of the intriguing phenomenological problems that has not been possible to solve within the CDM so far. This fact contributes to strengthen the possibility that dynamical vacuum energy, or in general DDE, could be presently favored by the observations.

Paper Structure

This paper contains 7 sections, 11 equations, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Dynamical dark energy
  3. XCDM and CPL parameterizations
  4. $\phi$CDM models
  5. Dynamical DE and structure formation
  6. Fitting results and discussion
  7. Conclusions

Figures (4)

  • Figure 1: Likelihood contours from $1\sigma$ up to 5$\sigma$ c.l. for the XCDM (left) in the $(\Omega_m,w_0)$-plane, and the RVM (center) in the $(\Omega_m,\nu)$-plane, using all SNIa+BAO+$H(z)$+LSS+CMB data after marginalizing over the rest of the fitting parameters indicated in Table 1. The plot on the right shows the contours for the RVM when only the CMB+BAO+LSS data are used. It also displays the partial contributions of these data sources at $1\sigma$ and $2\sigma$ . Further marginalization over $\Omega_m$ increases the c.l. of DDE up to $3.35\sigma$ (resp. $3.76\sigma$) for the XCDM (resp. RVM). The main contribution to the DDE signal is seen to emerge from the triad of CMB+BAO+LSS data.
  • Figure 2: The prediction of the various models confronted to the LSS data points $f(z)\sigma_8(z)$ for the normal and starred scenarios of Table 1. The plot on the right shows a magnified view and includes the $\phi$CDM prediction as well, which almost overlaps with that of the XCDM. The EoS analysis presented in Fig. 3 explains the possible origin of the large overlap (see also the text).
  • Figure 3: The EoS $w = w(z)$ for the XCDM and $\phi$CDM models within the corresponding $1\sigma$ bands. For the XCDM the EoS is of course "flat" (constant): $w = -0.923\pm 0.023$ (cf. Table 1) and points to quintessence (at $3.35\sigma$ c.l.) As for the $\phi$CDM model, with PR potential (\ref{['eq:PRpotential']}), the EoS evolves with time and is computed through a Monte Carlo analysis (see text). The current value reads as in Eq. (\ref{['eq:wphinow']}), which favors once more the quintessence region (at $3.37\sigma$ c.l.)
  • Figure 4: Contour lines for the $\phi$CDM with PR potential (\ref{['eq:PRpotential']}) (left) and RVM (\ref{['eq:RVMvacuumdadensity']}) (right) using the same CMB+BAO+LSS data as in Table 1 (solid contours); and also when replacing the LSS data (i.e. the $f(z)\sigma_8(z)$ points) with the $S_8$ value obtained from the analysis of the weak gravitational lensing data Joudaki2017 (dashed lines), i.e. the starred scenarios in Tables 1 and 2.