Gravitational Waves and Their Memory in General Relativity

TL;DR

This paper reviews gravitational waves in general relativity, focusing on the memory effect, its different types, and how various fields influence this phenomenon, with implications for detectors like LIGO, LISA, and pulsar timing arrays.

Contribution

It clarifies the distinction between linear and nonlinear gravitational wave memory and discusses how electromagnetic and neutrino fields affect this effect.

Findings

Electromagnetic and neutrino fields enlarge the gravitational wave memory.

Linear and nonlinear memory are distinct phenomena with different origins.

Understanding memory effects enhances interpretation of gravitational wave detector data.

Abstract

General relativity explains gravitational radiation from binary black hole or neutron star mergers, from core-collapse supernovae and even from the inflation period in cosmology. These waves exhibit a unique effect called memory or Christodoulou effect, which in a detector like LIGO or LISA shows as a permanent displacement of test masses and in radio telescopes like NANOGrav as a change in the frequency of pulsars' pulses. It was shown that electromagnetic fields and neutrino radiation enlarge the memory. Recently it has been understood that the two types of memory addressed in the literature as `linear' and `nonlinear' are in fact two different phenomena. The former is due to fields that do not and the latter is due to fields that do reach null infinity.