Kavli Affiliate: Joshua Frieman
| Summary:
Recent results from Type Ia supernovae (SNe Ia) and baryon acoustic oscillations (BAO), in combination with cosmic microwave background (CMB) measurements, have focused renewed attention on dark energy models with a time-varying equation-of-state parameter, $w(z)$. In this paper, we describe the simplest, physically motivated models of evolving dark energy that are consistent with the recent data, a broad subclass of the so-called thawing scalar field models that we dub $w_φ$CDM. We provide a quasi-universal, quasi-one-parameter functional fit to the scalar-field $w_φ(z)$ that captures the behavior of these models more informatively than the standard $w_0w_a$ phenomenological parametrization; their behavior is completely described by the current value of the equation-of-state parameter, $w_0=w(z=0)$. Combining current data from BAO (DESI Data Release 2), the CMB (Planck and ACT), large-scale structure (DES Year-3 $3times2$pt), SNe Ia (DES-SN5YR), and strong lensing (TDCOSMO + SLACS), for $w_φ$CDM we obtain $w_0=-0.904_-0.033^+0.034$, 2.9$σ$ discrepant from the $Λ$ cold dark matter ($Λ$CDM) model. The Bayesian evidence ratio substantially favors this $w_φ$CDM model over $Λ$CDM. The data combination that yields the strongest discrepancy with $Λ$CDM is BAO+SNe Ia, for which $w_0=-0.837^+0.044_-0.045$, $3.6σ$ discrepant from $Λ$CDM and with a Bayesian evidence ratio strongly in favor. We find that the so-called $S_8$ tension between the CMB and large-scale structure is slightly reduced in these models, while the Hubble tension is slightly increased. We forecast constraints on these models from near-future surveys (DESI-extension and the Vera C. Rubin Observatory LSST), showing that the current best-fit $w_φ$CDM model will be distinguishable from $Λ$CDM at over 9$σ$.
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