ID01: The New Exploration of the Universe Through Gravitational-Wave Observations

Artists illustration of merging neutron stars

Artist’s conception of merging neutron stars producing a kilonova explosion detectable in gravitational waves and electromagnetic radiation from the radio to the gamma-ray regime. (NSF / LIGO / Sonoma State University / A. Simonnet)

Gravitational-wave experiments, in development for decades, have at last become mature facilities able to detect transient gravitational-wave signals from the cosmos. In rapid succession they have produced a series of groundbreaking discoveries, starting on 14 September 2015 with the first detection of gravitational waves from the coalescence of a binary system of surprisingly large stellar-mass black holes by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO). It came as no surprise when the Nobel Prize in Physics 2017 was shared by Rainer Weiss, Barry C. Barish and Kip S. Thorne “for decisive contributions to the LIGO detector and the observation of gravitational waves.”

Another epochal discovery happened on 17 August 2017, with the first observations of gravitational waves from the inspiral and merger of a binary neutron-star system by the Advanced LIGO and Virgo network, followed 1.7 seconds later by a weak short gamma-ray burst detected by the Fermi and INTEGRAL satellites. A worldwide observing campaign involving about 100 instruments led to the detection of multi-wavelength electromagnetic signals associated with this event. This inaugurated a new era of “multi-messenger” exploration of the most energetic transients in the sky.

Detection of GW170817

Multi-messenger detection of GW170817, gravitational waves from the merger of two neutron stars (middle), and its electromagnetic counterpart, GRB170817A, a short gamma-ray burst from the same source (top and bottom). Additional detections followed across the electromagnetic spectrum. (LIGO/Virgo / Fermi / Integral / NASA/DOE / NSF / EGO / ESA)

I will begin my Invited Discourse with the astrophysical implications of the binary black hole coalescences. Through gravitational-wave observations, we know today that black holes live in binary systems, merge within a Hubble time and are frequent in the local Universe, occurring at a rate of hundreds to a few thousand per year in a cubic gigaparsec volume. And there are heavy black holes (> 25 solar masses) that form from low-metallicity massive stars. Taking into account the rate of detections expected in the coming years as gravitational-wave observatories become more and more sensitive, we will undoubtedly learn how black holes form and evolve and understand the role played by their environment — sparse field versus dense star cluster.

The only neutron-star merger detected so far has impacted many astrophysical fields. It confirmed the association of neutron stars with short gamma-ray bursts and provided new details about relativistic astrophysics. The radioactively powered transient associated with the merger of two neutron stars shows the signature of nucleosynthesis able to explain the Universe’s enrichment with heavy elements. Multi-messenger astrophysics makes it possible to probe nuclear matter under extreme conditions and to study the expansion of the Universe through gravitational-wave cosmology.

These remarkable achievements are the results of pioneering scientists who worked for years to reach the reality of detecting gravitational waves, along with the efforts of thousands of scientists worldwide, working together and contributing expertise ranging from fundamental physics to astronomy, experiments, observations, data analysis and theory. Now, more than ever, the importance of different scientific communities collaborating to overcome technological, observational and theoretical challenges with the aim of expanding our knowledge of the cosmos is evident.

This is only the beginning! Many new detections and discoveries are coming. We have instruments now able to observe what was invisible before, and new observatories with deeper sensitivity and exploring different frequency bands are expected in the next decades.

MARICA BRANCHESI is Assistant Professor at the Gran Sasso Science Institute in L’Aquila, Italy, and a member of the Virgo Collaboration, where she helps coordinate electromagnetic follow-up to LIGO-Virgo gravitational-wave alerts. She is also President of IAU Commission D1, Gravitational Wave Astrophysics.