Gravitational Waves: A New Era in Astronomy's Understanding
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Chapter 1: The Evolution of Gravitational Astronomy
Gravitational astronomy has reached a significant milestone with the observation of a merger between a black hole and a neutron star.
On August 14th, shortly after 9 PM Universal Time, a surge of gravitational waves arrived at Earth. Not long ago, such an event would have gone unnoticed. However, the Laser Interferometer Gravitational-Wave Observatory (LIGO) was upgraded and reopened in 2015, with additional support from Virgo, an enhanced detector in Italy that joined in 2017. Collectively, these three instruments not only detected the passing gravitational waves but also pinpointed their origin in the sky and relayed that information to astronomers worldwide.
This prompted the mobilization of numerous other observational instruments to investigate the wave's source, located near the constellations Cetus and Sculptor. Various telescopes, capable of analyzing different parts of the spectrum—from gamma rays to radio waves—were utilized. IceCube, an instrument located at the South Pole, also scanned for tiny neutrinos that could have been emitted during this monumental cosmic event that disturbed the space-time continuum.
The preliminary result of this "multimessenger" observation indicates that the detectors recorded the merger of a neutron star and a black hole, dubbed S190814bv by LIGO. If confirmed, this discovery will mark the first detection of such a merger, as previous gravitational-wave observations involved either two black holes or two neutron stars colliding. The detection of this third type of event signifies a pivotal moment in the field of gravitational astronomy.
Section 1.1: Understanding Gravitational Waves
Gravitational waves are ripples in space-time that convey the force of gravity from one location to another. Predicted by Albert Einstein in 1916 as part of his general theory of relativity, these waves were initially thought to be too minuscule to detect given the astronomical objects known at the time.
However, the identification of dense objects like neutron stars and black holes—which are remnants of supernovae or extremely concentrated mass—changed this perception. Calculations indicated that mergers between such entities could produce detectable gravitational waves with sufficiently sensitive instruments. The economic and technological advancements since Einstein's era have enabled the development of these sophisticated detectors.
Gravitational-wave detectors operate by splitting a laser beam into two. Each half of the beam travels down separate arms, oriented perpendicular to each other, before reflecting back from mirrors. Under normal circumstances, the overlapping beams would cancel each other out, resulting in darkness. However, if a gravitational wave passes through, it alters the lengths of the arms, creating an interference pattern that reveals the wave's characteristics. Even with massive objects like neutron stars or large black holes, the distortion caused by these waves is incredibly tiny—merely a thousandth of the width of a proton across a 4-km-long arm. Nonetheless, laser interferometry can detect these minute variations.
LIGO achieved its first success in September 2015 with a signal from the merger of two black holes. Since then, it, along with Virgo, has recorded several such events, including the merger of two neutron stars. Should S190814bv prove to be a neutron star/black hole merger, it will provide a framework for comparing these different cosmic occurrences.
Astrophysicist explains big GRAVITATIONAL WAVE discovery! Are they NEW PHYSICS or merging SMBHs?
This video discusses the implications of the recent gravitational wave detection and explores whether it indicates new physics or simply reflects the merging of supermassive black holes.
Section 1.2: The Impact of Multimessenger Astronomy
Gravitational waves are now part of a broader initiative involving data collection from multiple sources. This endeavor was first successfully executed following the detection of the first neutron star merger in August 2017. The cosmic event commenced with a 100-second-long burst of gravitational waves. Within two seconds, NASA's Fermi Telescope and the European Space Agency's International Gamma-ray Astrophysics Laboratory detected a burst of gamma rays from the galaxy NGC 4993, located 130 million light-years away in the Hydra constellation. This event culminated in a kilonova, characterized by optical and ultraviolet radiation due to the radioactive decay of newly formed heavy elements. The aftermath glowed with radiation across various wavelengths for a year.
Dubbed GW170817, this neutron-star merger proved to be a treasure trove for astronomers, revealing that heavy elements like gold and platinum were produced during the explosion, confirming that such cosmic events are responsible for generating these metals. The simultaneous detection of gravitational waves and gamma rays also validated Einstein’s prediction that gravitational waves travel at the speed of light.
What GW170817 lacked, which S190814bv might provide, is an opportunity to peer inside a neutron star if the black hole disrupted it prior to their merger. There are numerous theories about the interior of neutron stars, but recreating those conditions on Earth remains impossible. It is theorized that matter deep within a neutron star’s core may form structures resembling "nuclear pasta," named for its resemblance to various types of pasta. If nuclear pasta exists, it is likely the strongest substance in the universe, estimated to be ten billion times stronger than steel.
The nature of the merger involving S190814bv will determine whether we gain insights into this cosmic "pasta." If the two objects have similar masses, the neutron star would take longer to be torn apart by the black hole, allowing for a more extended observation of its interior. Conversely, if the black hole is significantly more massive, the neutron star would likely merge with less observable activity.
Chapter 2: The Future of Gravitational Wave Astronomy
Think Space: Gravitational waves, a new window into nature
This video delves into how gravitational waves are revolutionizing our understanding of the universe and opening new avenues for research.
The study of gravitational waves continues to shed light on the fundamental properties of neutron stars. LIGO and Virgo are also capable of detecting non-colliding neutron stars, as long as they are spinning rapidly. These rapidly spinning neutron stars, known as pulsars, emit beams of electromagnetic waves visible only when aligned with an observer, akin to a lighthouse. They may also produce detectable gravitational waves. Even minor imperfections on a pulsar’s surface could result in gravitational waves detectable on Earth, providing insights into the internal structure of neutron stars.
Another anticipated source of gravitational waves is a supernova, marking the death of a massive star. Observing such explosions with contemporary instruments like LIGO, Virgo, and IceCube will be challenging, as gravitational waves from supernovae are expected to be weak. The rate of supernova events in our galaxy is estimated to be one to three per century, with the last known occurrence hidden from view by cosmic dust and gas in the early 20th century.
Unlike electromagnetic radiation, gravitational waves from supernovae could inform astronomers about the dynamics of dense matter during the explosion. They could also reveal whether the explosion was symmetric. Following the explosion, the remaining material often evolves into a neutron star or black hole, allowing astronomers to witness the birth of these extreme cosmic entities in real-time.
A fair crack of the whip
As astronomers leverage gravitational waves to explore cosmic structures, others aim to test the boundaries of general relativity. Although every prediction made by this theory has been validated, physicists recognize that it does not encompass quantum theory, which explains many phenomena in the universe. Szabolcs Marka, a physicist at Columbia University, suggests that gravitational astronomy may bridge this gap. He believes the most promising avenue is to search for deviations from relativity’s predictions in the gravitational waves emitted by orbiting black holes.
A longer-term ambition for gravitational-wave researchers is to delve further back in time than electromagnetic radiation currently allows. The universe was opaque to light for its first 400,000 years, yet it would have been permeable to gravitational waves. Detecting these so-called cosmological waves could illuminate the moment the singularity that birthed the universe began its Big Bang expansion.
After 13.8 billion years of cosmic expansion, these waves would be faint, obscured by background noise from various astrophysical processes. However, if detected, they could provide critical insights into the universe's early seconds, addressing longstanding questions about its initial expansion rate and uniformity.
Looking ahead, astronomers will also investigate theoretical constructs like cosmic strings—hypothetical superdense filamentary structures in space. Patrick Brady, a spokesperson for the LIGO Scientific Collaboration, explains that if cosmic strings exist, their movement could generate detectable gravitational waves akin to a cracking whip.
The ultimate thrill, according to Dr. Brady, would be discovering gravitational wave signals that cannot be explained by known phenomena such as neutron stars, black holes, or supernovae. Researchers are actively searching for these unexplained bursts, which they refer to as unmodelled gravitational wave events, as they could herald groundbreaking discoveries.
The once and future subject
If all proceeds as planned, the existing gravitational-wave observatories will be complemented by the Kagra interferometer in Japan by the end of the year and LIGO-India by 2024. This global network will enhance astronomers' capacity to pinpoint the origins of future gravitational wave detections and validate individual findings.
LIGO is also scheduled for an upgrade within the coming years, nearly doubling its sensitivity and enabling it to observe a larger volume of space. Additionally, the European Space Agency’s Laser Interferometer Space Antenna (LISA), slated for 2034, will be the first space-based gravitational-wave detector, sensitive to lower-frequency waves currently lost in the noise.
Looking even further into the future, new ground-based observatories are competing to succeed LIGO once its operational life concludes. Europe is proposing the Einstein Telescope, an underground interferometer designed to enhance sensitivity, while America is introducing the Cosmic Explorer, a LIGO variant with 40-km-long arms. Both projects aim to detect black hole mergers across the universe.
The future of gravitational astronomy is immensely promising. It may enhance our understanding of heavy element formation, elucidate the early universe, and potentially reconcile general relativity with quantum theory. The quest to comprehend gravity and its role in the cosmos has been a cornerstone of physics, and the latest advancements from LIGO and Virgo demonstrate that this journey continues to evolve.