Evolution of Dark Energy Perturbations in Scalar-Tensor Cosmologies

TL;DR

This paper analytically and numerically studies dark energy perturbations in scalar-tensor cosmologies, revealing significant amplification and anti-correlation with matter, which could impact galaxy cluster observations and challenge LCDM predictions.

Contribution

It provides the first detailed comparison of scalar-tensor dark energy perturbations with minimally coupled quintessence, highlighting their amplified and anti-correlated nature.

Findings

Scalar-Tensor dark energy perturbations are amplified by about 10^6 on sub-Hubble scales.

Dark energy perturbations are anti-correlated with matter perturbations on these scales.

Matter perturbations are mildly amplified (~10%) due to dark energy anti-correlation.

Abstract

We solve analytically and numerically the generalized Einstein equations in scalar-tensor cosmologies to obtain the evolution of dark energy and matter linear perturbations. We compare our results with the corresponding results for minimally coupled quintessence perturbations. Our results for natural (O(1)) values of parameters in the Lagrangian which lead to a background expansion similar to LCDM are summarized as follows: 1. Scalar-Tensor dark energy density perturbations are amplified by a factor of about 10^6 compared to minimally coupled quintessence perturbations on scales less than about 100 h^{-1} Mpc. On these scales dark energy perturbations constitute a fraction of about 10% compared to matter density perturbations. 2. Scalar-Tensor dark energy density perturbations are anti-correlated with matter linear perturbations on sub-Hubble scales. Thus clusters of galaxies are predicted to overlap with voids of dark energy. 3. This anti-correlation of matter with negative pressure perturbations induces a mild amplification of matter perturbations by about 10% on sub-Hubble scales. 4. The evolution of scalar field perturbations on sub-Hubble scales, is scale independent and therefore it corresponds to a vanishing effective speed of sound (c_{s\Phi}=0). It also involves large oscillations at early times induced by the amplified effective mass of the field. This amplification is due to the non-minimal coupling of the field to the Ricci curvature scalar and (thus) to matter. No such oscillations are present in minimally coupled quintessence perturbations which are suppressed on sub-Hubble scales (c_{s\Phi}=1). Implications of our results include modified predictions for cluster halo profiles observed properties. These profiles are known to be in some tension with the predictions of LCDM.