Understanding Gravitational Waves

Gravitational waves are fluctuations in space-time that convey gravitational forces across distances. Albert Einstein predicted them in 1916 as part of his general theory of relativity, although he doubted their detectability given the minuscule size of expected waves in the context of known astronomical objects.

The narrative changed with the identification of dense and massive entities like neutron stars—remnants of supernovae—and black holes, which have such strong gravitational fields that even light cannot escape. Theoretical models suggested that mergers of these objects would generate gravitational waves detectable by sufficiently sensitive equipment. Economic growth and technological advancements since Einstein’s time enabled the construction of these advanced detectors.

Gravitational-wave detectors operate by splitting a laser beam into two paths, each extending several kilometers and positioned perpendicularly. Mirrors at the ends of these paths reflect the beams back, where they recombine. Normally, this process leads to cancellation of the light waves, resulting in darkness. However, if a gravitational wave passes through, altering the lengths of the arms, the beams will fail to align perfectly, generating an interference pattern that reveals the characteristics of the wave. Despite the significant mass of the sources, the resultant gravitational disturbances are incredibly minute—on the order of a thousandth the width of a proton over a 4 km detector arm. Nevertheless, laser interferometry is capable of detecting such subtle variations.

LIGO successfully recorded its first event—a signal from two merging black holes—in September 2015. Since then, both LIGO and Virgo have confirmed nine additional events, including one merger of neutron stars. If S190814bv is verified as a neutron star-black hole merger, it will facilitate comparisons between different event types.

A New Frontier in Astronomy

The detection of gravitational waves, as illustrated by the messages following S190814bv, is now part of a broader initiative to gather data from various sources. This approach was first effectively implemented after the August 2017 detection of the first neutron star merger. The associated cosmic phenomena commenced with a gravitational wave burst lasting 100 seconds. Within two seconds of its onset, both NASA's Fermi Telescope and the European Space Agency's International Gamma-ray Astrophysics Laboratory detected gamma rays from the galaxy NGC 4993, situated 130 million light-years away in the Hydra constellation. The aftermath included a kilonova—a burst of visible and ultraviolet light driven by the radioactive decay of newly formed heavy elements. For an entire year, the remnants emitted radiation across the spectrum, from X-rays to radio waves.

Dubbed GW170817, this neutron-star merger proved invaluable for astronomers—literally. The spectrum from the kilonova indicated the formation of gold and platinum, confirming that such cosmic explosions are sources of these heavy elements, which cannot be produced through typical stellar nuclear processes. The simultaneous arrival of GW170817's gravitational waves and gamma rays also validated Einstein's prediction that gravitational waves travel at light speed.

What GW170817 lacked, which S190814bv might provide, is a glimpse inside a neutron star, should the black hole disrupt it prior to their merger. While numerous theories exist about neutron star interiors, replicating such conditions in a laboratory remains impossible. It is hypothesized that matter deep within a neutron star's core is compressed into various structures, referred to as "nuclear pasta," owing to their resemblance to pasta shapes. If this material exists, it would likely be the strongest substance in the universe, with estimates suggesting it could be 10 billion times stronger than steel.

Whether S190814bv will reveal neutron stars as cosmic culinary creations remains uncertain. The proximity in mass between the two objects could determine how long the black hole took to disassemble the neutron star, thus exposing its internal structure to the cosmos. Conversely, if the black hole were significantly more massive, the neutron star would likely succumb without much spectacle.

Both black holes and neutron stars form following the fuel depletion and collapse of massive stars. Though both are incredibly dense, their physical characteristics differ markedly. Neutron stars are primarily composed of neutrons, the constituents of ordinary matter found in atomic nuclei, except for hydrogen's lightest isotope, which is a lone proton. In contrast, black holes are singularities, lacking internal structure beyond mass.

As Christopher Berry, an astronomer at Northwestern University, notes, "Neutron stars, being made of matter, can become distorted, whereas black holes cannot." The gravitational waves generated during collisions carry information about such distortions, providing insights into phenomena like nuclear pasta.

In the case of S190814bv, the critical mass ratio that could expose the pasta remains undetermined. Astronomers suspect a neutron star-black hole merger based on the masses involved: the larger object exceeds five solar masses, indicating it must be a black hole, while the smaller one weighs under three solar masses, too light to be a black hole and likely a neutron star. However, the exact relative masses, and the potential for the neutron star to have disintegrated, are yet to be clarified.

Neutron star collisions offer astronomers a deeper understanding of these celestial objects. Additionally, LIGO and Virgo may also detect non-colliding neutron stars, especially rapidly spinning ones known as pulsars. These pulsars emit electromagnetic waves observable only when aligned with an observer, akin to a lighthouse beam. Surface irregularities, even tiny ones, could generate detectable gravitational waves as well, with the height of any millimetric anomalies providing insights into the stiffness of the neutron star's internal nuclear pasta.

Another eagerly awaited gravitational wave source is a supernova, the explosive end of a massive star's life. Observing such an event with current instruments, including LIGO and Virgo, alongside neutrino detectors like IceCube, is challenging. Supernova gravitational waves are expected to be weak, necessitating proximity within the Milky Way for detection. Estimates suggest one to three supernova events occur within the Milky Way each century, with the last observable instance occurring early in the 20th century, hidden from view by dust and gas but later detected via radio astronomy.

Gravitational waves from supernovae could reveal the dynamics of dense matter during the explosion and determine whether the explosion was symmetrical. After shedding much of their stellar material, the remnants often become neutron stars or black holes. By studying supernova evolution, astronomers could witness the transition as dense cosmic bodies are formed.

Testing the Limits of Physics

While some astronomers aim to utilize gravitational waves to understand cosmic structures, others seek to use this new astronomy phase to evaluate the boundaries of general relativity. Thus far, every prediction made by relativity has been confirmed, yet physicists recognize that the theory cannot fully explain gravitational phenomena as it doesn't integrate with quantum mechanics, which encompasses all other universal explanations. Szabolcs Marka, a physicist at Columbia University, believes gravitational astronomy could bridge this gap. He suggests that the best approach is to investigate deviations from relativistic predictions in the waves emitted by orbiting black holes.

A long-term ambition for gravitational-wave astronomers is to explore earlier epochs of the universe than electromagnetic radiation allows. For approximately 400,000 years following the Big Bang, the universe was too hot and dense for light to escape, thus leaving no electromagnetic signals. However, gravitational waves would have traversed through this dense environment. Detecting these cosmological waves could illuminate the moment when the universe began its expansion from the initial singularity.

After 13.8 billion years of cosmic expansion, these gravitational waves would now be exceedingly faint, obscured by background noise from various astrophysical processes. However, if detected, they would provide insights into the universe's formative moments, addressing longstanding questions about the speed and uniformity of its early expansion.

Following these explorations, astronomers will pursue highly theoretical concepts. Gravitational waves could assist in the search for cosmic strings—hypothetical, superdense filament-like structures in space. "If they exist, cosmic strings can wriggle and wiggle, and occasionally, this movement leads to a whip-like crack," states Patrick Brady, an astronomer at the University of Wisconsin-Milwaukee and spokesperson for the LIGO Scientific Collaboration. "And," he adds, "this whipcrack would generate gravitational waves detectable by our instruments."

The true excitement, as Dr. Brady notes, would arise if astronomers identified a signal that could not be explained by known phenomena such as neutron stars, black holes, supernovae, or even cosmic strings. "We're constantly on the lookout for such anomalies—termed unmodelled bursts of gravitational waves—because we lack theoretical models for them. Discovering a confident gravitational-wave detection that defies explanation would be incredibly thrilling."

The Future of Gravitational Astronomy

If all progresses well, the current generation of gravitational-wave observatories will soon be complemented by the Kagra interferometer in Japan and, by 2024, the LIGO-India facility currently under construction near Mumbai. Such global detector networks will enhance astronomers' ability to accurately locate the origins of future gravitational-wave discoveries and provide independent confirmation of individual detections.

LIGO itself is slated for a significant upgrade in the coming years, expected to nearly double its sensitivity, allowing it to observe a volume of space seven times larger than currently possible. Beyond that, the European Space Agency's Laser Interferometer Space Antenna (LISA), set for 2034, will introduce the first space-based gravitational-wave observatory. Its detectors will form an equilateral triangle with sides measuring 2.5 million kilometers, sensitive to low-frequency waves often lost in noise.

Looking even further ahead, next-generation ground-based observatories are vying to succeed LIGO as it approaches the end of its operational lifespan. Europe proposes the Einstein Telescope, an underground interferometer with three arms arranged in an equilateral triangle and cooled to within ten degrees of absolute zero to enhance sensitivity. Meanwhile, the United States is developing the Cosmic Explorer, an extended version of LIGO with 40 km long arms. Either project would have the capability to detect black hole mergers across the universe.

The potential of gravitational astronomy is vast. It promises to illuminate the processes behind heavy element creation, provide answers to long-standing questions regarding the early universe, and possibly reconcile general relativity with quantum theory. From Copernicus to Kepler to Newton, the quest to understand gravity and its role in the cosmos has been a cornerstone of physics. The latest advancements from LIGO and Virgo indicate that this journey of discovery is far from over.