Gravitational Waves

Abstract:
In astronomy, the traditional data collection method is to utilize electromagnetic radiation. While electromagnetic observations have been the cornerstone of astronomy, they are fundamentally limited by photon scattering, absorption, and their inability to escape event horizons. In 2015, the first detection of a gravitational wave was achieved at LIGO (Laser Interferometer Gravitational-Wave Observatory). This marked a new era within the field of astronomy. Gravitational wave physics is a quickly developing field in astronomical research. It allows astronomers to gather data about events previously inaccessible, and to determine information more precisely and directly. This review analyzes three key areas where gravitational waves overcome the limitations of electromagnetic observation: performing strong-field tests of General Relativity, establishing standard siren measurements independent of the cosmic distance ladder, and probing the stochastic background of the radiation-dominated early universe. Synthesizing these developments demonstrates that gravitational waves are a healthy complement to electromagnetic data, resolving long-standing ambiguities in cosmology and physics.

Intro:
Gravitational waves were first predicted by Albert Einstein in his theory of general relativity. He proposed that massive objects undergoing asymmetric acceleration, such as orbiting black holes or neutron stars, generate ripples in the curvature of spacetime. These ripples, known as gravitational waves, propagate outward at the speed of light, stretching and squeezing space itself. Prior to 2015, astronomical data was primarily collected through electromagnetic radiation. Astronomers study radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma-ray emissions to gain a more complete picture of the universe. Radio astronomy involves using ground-based dishes to detect low-energy, long-wavelength signals. Infrared astronomy detects heat signatures to observe cool stars, star-forming regions, and distant galaxies, often requiring space-based telescopes to avoid atmospheric interference. Ultraviolet, X-ray, and gamma-ray astronomy require space-based telescopes to observe high-energy and violent phenomena such as supernova remnants or accreting black holes. While integral to our understanding of the universe, electromagnetic radiation has inherent limitations: it can be absorbed or scattered by matter, it cannot escape from within the event horizon of a black hole, and it cannot provide direct observational access to epochs prior to recombination, when photons decoupled from matter and formed what we now observe as the Cosmic Microwave Background. In 2015, the LIGO Scientific Collaboration achieved the first direct detection of gravitational waves. This milestone marked the beginning of gravitational-wave astronomy and opened a new observational window onto the universe. Gravitational waves complement electromagnetic observations by providing direct access to compact objects such as black holes and neutron stars, probing strong-field gravity, and offering insight into early-universe processes that are inaccessible through electromagnetic radiation alone. They provide dynamical information about gravity and give us access to events inaccessible to photons. This review analyzes how gravitational wave physics overcomes these electromagnetic limitations, focusing on strong-field tests of General Relativity [Abbott et al., 2019], standard siren measurements of cosmic expansion [Abbott et al., 2017], and the search for stochastic backgrounds from the infant universe [Christensen, 2019].

Source 1:
In the publication “Tests of General Relativity with the Binary Black Hole Signals from the LIGO-Virgo Catalog GWTC-1“, Abbott et al. describe their analysis of ten binary black hole mergers, sites of gravitational wave propagation. They employed residual tests to see how consistent the data was with General Relativity. By subtracting the best-fit General Relativity waveform from the observed data, they analyzed the signal that remained, also known as “the residual.” In all cases tested, the residual signals were consistent with the pure instrument noise, showing that General Relativity’s predictions for the merger were consistent with what actually happened. Electromagnetic radiation observations are incapable of providing us with this direct data because light only reveals the matter surrounding a black hole (accretion disks), whereas gravitational waves reveal the mass and gravity of the black hole itself.

Source 2:
To calculate the distance between Earth and some celestial object, there’s no one method; astronomers refer to a system called the “Distance Ladder,” which utilizes multiple complex calculations to come up with an answer. The issue with this method is that not only is it complex, it can often have systematic inaccuracies. In this publication, Abbott et al. used each gravitational wave as a “standard siren.” In astronomy, a standard siren is a way of calculating distance from an object; it’s easiest to think of it as the siren on a police car. If you know exactly how loud the siren is, then you can find your distance from it. Gravitational waves provide a direct luminosity distance based purely on the waveform’s shape and amplitude. By conducting this analysis, the researchers found that this was a much cleaner way to measure distances because they didn’t rely on the complex astrophysics of star brightness. In other words, the traditional distance calculations were dependent on visible light measurements which are subject to extinction and calibration errors. By using gravitational waves as their “standard siren,” astronomers have made the process of finding the distance between the Earth and astronomical objects much more direct.

Source 3:
In this publication, Christensen highlights the usefulness of gravitational waves in studying events from the radiation-dominated era of the early universe, an epoch inaccessible to electromagnetic observation. The Cosmic Microwave Background is the faint thermal radiation that fills the universe. This radiation is the residual heat and first light released approximately 380,000 years after the Big Bang when the universe cooled enough for neutral atoms to form. In the context of the early universe, plasma is the superheated state of matter in which atoms cannot exist due to the sheer heat that prevents electron attachment to nuclei. This means that photons in the early universe plasma would be unable to travel any significant distance without hitting an electron and bouncing off. This trapped light created a thick “fog.” Because of this, electromagnetic observation cannot be used to determine anything about the events of this period of time. Gravitational waves however, pass right through charged particles without scattering. Christensen determines that by studying the combined superposition of weak gravitational events, something known as the Stochastic Gravitational Wave Background, astronomers could gather data about events that happened up to 10^-32 seconds after the Big Bang.