The Cosmological Memory Effect

TL;DR

This paper studies the gravitational memory effect in flat FLRW cosmologies, showing that only tensor perturbations contribute and that the effect is amplified by the universe's expansion, depending solely on the scale factor at source and observation.

Contribution

It demonstrates that in flat FLRW universes, the gravitational memory effect arises only from tensor modes and depends only on the scale factor, not the detailed expansion history.

Findings

Tensor modes are the sole contributors to memory in FLRW cosmology.

Memory effect is enhanced by a factor of (1 + z) at the same luminosity distance.

The effect depends only on the scale factor at source and observation, not the full expansion history.

Abstract

The "memory effect" is the permanent change in the relative separation of test particles resulting from the passage of gravitational radiation. We investigate the memory effect for a general, spatially flat FLRW cosmology by considering the radiation associated with emission events involving particle-like sources. We find that if the resulting perturbation is decomposed into scalar, vector, and tensor parts, only the tensor part contributes to memory. Furthermore, the tensor contribution to memory depends only on the cosmological scale factor at the source and observation events, not on the detailed expansion history of the universe. In particular, for sources at the same luminosity distance, the memory effect in a spatially flat FLRW spacetime is enhanced over the Minkowski case by a factor of $(1 + z)$.