A Gamma-ray Infrastructure to Advance Gravitational Wave Astrophysics

Introduction

When galaxies merge, do their central supermassive black holes also merge? How does the merger affect star formation and the evolution of galaxies? How does physics beyond the Standard Model of particles affect the Universe? The detection and characterisation of low-frequency gravitational waves (GWs) will address these fundamental and longstanding questions of astronomy and cosmology.

Supermassive Black Holes and Gravitational Waves

Supermassive black holes at the centres of merging galaxies are expected to form binary systems whose orbital motion generates GWs. A cosmological population of such systems combines to build up a GW background (GWB). Such a GWB is also expected if the Universe went through an inflationary period, providing a GW map just moments after the Big Bang. Pulsar timing arrays (PTAs), which are ensembles of extremely stable millisecond pulsars (rotating neutron stars), can be used to study this GWB.

Limitations of Current Searches

Searches for the GWB have typically used sensitive radio telescopes. However, radio data exhibit complex noise processes, predominantly arising from the interstellar medium (ISM), that limit its sensitivity and introduce bias. Gamma rays are immune to the effects of the ISM, and a gamma-ray PTA can overcome several of the limitations affecting radio data.

Objectives of GIGA

GIGA will:

1. Establish a gamma-ray PTA and independently detect the GWB.
2. Develop advanced inference techniques to distinguish its astrophysical origins.
3. Measure properties of the ISM through multiwavelength studies.
4. Explore energy-dependent couplings of dark matter.

Through these avenues, GIGA will also maximise the sensitivity of radio PTAs and provide crucial validation of their measurements.

Implications of GWB Detection

The detection of the GWB will provide the first stringent constraints on the dynamical evolution of supermassive black holes and their host galaxies. Advanced inference techniques will aid in disentangling weaker astrophysical sources, including cosmic strings and phase transitions, thus probing physics beyond the Standard Model.