[Paper Review] Astro2020 Decadal Science White Paper: The state of gravitational-wave astrophysics in 2020

Sean T. McWilliams, Robert R. Caldwell+3 more|arXiv (Cornell University)|Mar 11, 2019

Pulsars and Gravitational Waves Research23 references6 citations

TL;DR

This white paper outlines the transformative state of gravitational-wave astrophysics as of 2020, advocating for a multi-band approach spanning ground-based (LIGO/Virgo), space-based (LISA), and low-frequency (Pulsar Timing Arrays, CMB) observatories. It emphasizes that LISA will enable unprecedented tests of general relativity and probe massive black hole dynamics, neutron star equations of state, and early-universe physics through mergers, extreme mass ratio inspirals, and stochastic backgrounds.

ABSTRACT

While still in its infancy, the budding field of gravitational-wave astronomy has so far exceeded most expectations, and the achievements that have already been made bode well for the decade to come. While the discoveries made possible by LIGO have captured the imagination of experts and nonexperts alike, it is important when looking ahead to consider those discoveries in the context of the field as a whole. Just as radio, optical, and x-ray radiation probe different physical phenomena occurring on a range of length and energy scales, the future of gravitational-wave astrophysics depends on our ability to open up the entire spectrum. We will describe the scientific prospects for the field of gravitational-wave astronomy as a whole as we enter the coming decade, and we will place the specific contributions from a future space-based gravitational-wave observatory within this context.

Motivation & Objective

• To assess the current state and future scientific potential of gravitational-wave astrophysics across multiple observational bands.
• To argue for the critical importance of a space-based gravitational-wave observatory (LISA) in completing the full gravitational-wave spectrum.
• To highlight how multi-messenger and multi-band observations will resolve key questions in cosmology, compact object formation, and general relativity.
• To position LISA as essential for probing strong-field gravity, extreme mass ratio inspirals, and the early universe.

Proposed method

• Analyzing data from ground-based detectors (LIGO, Virgo) and their planned upgrades (A+, Voyager, Cosmic Explorer, Einstein Telescope) to assess sensitivity improvements and source detection ranges.
• Evaluating the scientific potential of space-based interferometers like LISA, focusing on its sensitivity to massive black hole binaries, extreme mass ratio inspirals (EMRIs), and galactic binary populations.
• Assessing low-frequency gravitational-wave detection via Pulsar Timing Arrays (PTAs) and Cosmic Microwave Background (CMB) polarization experiments to probe primordial gravitational waves.
• Modeling the complementarity between ground-based and space-based observatories, particularly in sky localization and timing of merger events.
• Using theoretical frameworks from general relativity and alternative gravity models to interpret expected signals from EMRIs and strong-field binaries.
• Quantifying detection rates and signal-to-noise ratios for various sources (e.g., neutron-star binaries, supermassive black hole mergers) across different observatories.

Experimental results

Research questions

• RQ1 How will the next generation of ground-based detectors (e.g., A+) improve the detection range and localization of neutron-star and black-hole binaries?
• RQ2 What unique astrophysical and fundamental physics insights can be gained from observing massive black hole binaries with LISA, particularly in the strong-field regime?
• RQ3 How can LISA’s early warning capability for mergers improve multi-messenger follow-up with electromagnetic observatories?
• RQ4 What constraints can be placed on deviations from general relativity using extreme mass ratio inspirals (EMRIs) observed by LISA?
• RQ5 Can LISA detect a primordial stochastic gravitational-wave background from inflation, and what would that imply for early-universe cosmology?

Key findings

• Advanced LIGO and Virgo are expected to reach design sensitivity by 2024, increasing the detection range for neutron-star binaries to ~1 Gpc and for stellar-mass black-hole binaries beyond a redshift of 1.
• Upgrades to reach the quantum noise limit could extend the observable redshift for stellar- and intermediate-mass black-hole binaries to z ≈ 6, significantly improving sky localization.
• The 2017 detection of GW170817, coinciding with electromagnetic counterparts across the spectrum, confirmed that neutron-star mergers produce short gamma-ray bursts and are the primary source of heavy elements in the universe.
• The multi-messenger observation of GW170817 enabled a new, independent measurement of the Hubble constant, offering a path to resolving the current tension between Planck and supernova-based estimates.
• LISA is expected to detect hundreds of massive black hole binary mergers, many of which will be far louder than any ground-based signal, enabling precise tests of general relativity in the strong-field regime.
• LISA will observe extreme mass ratio inspirals (EMRIs), where stellar-mass black holes spiral into supermassive black holes, providing a unique probe of spacetime geometry and potential deviations from general relativity.