Gravitational-wave implications for structure formation: A second-order approach

TL;DR

This paper investigates how second-order gravitational waves influence large-scale structure formation in the universe, revealing a new faster-growing mode in density perturbations that could become significant in the distant future.

Contribution

It introduces a second-order perturbation analysis of gravitational waves' impact on density fluctuations during the post-recombination era, highlighting a novel growth mode.

Findings

Gravitational waves add a faster-growing mode to density perturbations.

The effect of gravitational waves is currently subdominant but may become significant in the future.

Late-time structure evolution might be influenced more by Weyl curvature than local gravitational effects.

Abstract

Gravitational waves are propagating undulations in the spacetime fabric, which interact very weakly with their environment. In cosmology, gravitational-wave distortions are produced by most of the inflationary scenarios and their anticipated detection should open a new window to the early universe. Motivated by the relative lack of studies on the potential implications of gravitational radiation for the large-scale structure of the universe, we consider its coupling to density perturbations during the post-recombination era. We do so by assuming an Einstein-de Sitter background cosmology and by employing a second-order perturbation study. At this perturbative level and on superhorizon scales, we find that gravitational radiation adds a distinct and faster growing mode to the standard linear solution for the density contrast. Given the expected weakness of cosmological gravitational waves, however, the effect of the new mode is currently subdominant and it could start becoming noticeable only in the far future. Nevertheless, this still raises the intriguing possibility that the late-time evolution of large-scale density perturbations may be dictated by the long-range (the Weyl), rather than the local (the Ricci) component of the gravitational field.