A late-time transition in the cosmic dark energy?

TL;DR

This study tests a phenomenological dark-energy scenario in which the equation of state $w(z)$ experiences a sharp late-time transition to a final value $w_f$ at redshift $z_t$, motivated by vacuum metamorphosis. By fitting to a joint dataset of CMB, LSS, and SN1a observations using a modified CMBFAST pipeline, the authors constrain $(\Omega_Q, z_t, w_f)$ and compare to ΛCDM. The results prefer a late transition with best-fit values around $z_t \sim 1.5$–$2.0$, $\Omega_Q \approx 0.73$, and $w_f \approx -1$, yielding a slightly better but not decisively preferred fit relative to ΛCDM. The analysis highlights the ISW-driven sensitivity of the CMB to $z_t$, the limited clustering of the scalar field on small scales, and the potential of future high-precision measurements (e.g., of scalar-field perturbations) to distinguish metamorphosis from a true cosmological constant.

Abstract

We study constraints from the latest CMB, large scale structure (2dF, Abell/ACO, PSCz) and SN1a data on dark energy models with a sharp transition in their equation of state, w(z). Such a transition is motivated by models like vacuum metamorphosis where non-perturbative quantum effects are important at late times. We allow the transition to occur at a specific redshift, z_t, to a final negative pressure -1 < w_f < -1/3. We find that the CMB and supernovae data, in particular, prefer a late-time transition due to the associated delay in cosmic acceleration. The best fits (with 1 sigma errors) to all the data are z_t = 2.0^{+2.2}_{-0.76}, Ω_Q = 0.73^{+0.02}_{-0.04} and w_f = -1^{+0.2}.

Figures (5)

• Figure 1: Left: The total-data best-fitting model (right curve) vs $\Lambda$CDM (left curve) which both have $\Omega_Q = 0.73$ and $w_f = -1$ but $z_t = 1.5$ for the best-fit. Right:The power spectrum for our total best-fit model with $(\Omega_Q, z_t, w_f) = (0.73, 1.5,-1)$ compared with the LSS best-fit ($0.7, 6.5,-0.55)$. The LSS data shown are the linear transfer functions inferred from the 2df and PSCz galaxy surveys and the Abell/ACO cluster survey. We do not show the lyman-$\alpha$ data.
• Figure 2: Variation of the $C_{\ell}$'s with $z_t$ and $w_f$. Left: The $C_{\ell}$ curves increase monotonically with decreasing $w_f$ starting with $-0.6$ (bottom), $-0.7, -0.8,-0.95$ and ending with $-1$ (top). The primary change is in the normalisation of the spectrum through the change in the ISW effect. Right: The $C_{\ell}$'s for $z_t = 0.5$ (bottom), $1.5, 3,5$ and $10$. The ISW contribution to the COBE normalisation changes very rapidly for small $z_t$ which allows for delayed acceleration. However, the CMB is insensitive to $z_t > 3$ since the scalar field is dynamically irrelevant at those redshifts.
• Figure 3: The redshift dependence of the luminosity distance (as magnitudes) minus an empty ($\Omega = 0$) universe for four different metamorphosis models. The $z_t = 0.5$ model fits the data best, mainly due to the single data point at $z = 1.7$. The solid line effectively coincides with a $\Lambda$CDM model. The redshift-binned SN1a data is from Riess et al (2001); the four dashed data points are experimental and were not included in the fit.
• Figure 4: The marginalised 1-d likelihood plots for our variables $(\Omega_Q, z_t, w_f)$. Left column: CMB + LSS, middle column: SN1a, right column: Total data set. Both the CMB and SN1a data favour a late transition (small $z_t$) due to the corresponding delay in cosmic acceleration. We computed these marginalised likelihoods using both integration and maximisation and the results were similar, as they should be for Gaussian likelihoods.
• Figure 5: The marginalised 2-d likelihood plots for the combined CMB, LSS and SN1a data sets showing 1 and 2$\sigma$ contours defined as where the integral of the normalised two-dimensional likelihoods are equal to $0.68$ and $0.95$ respectively.