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Gravitational waves detected 100 years after Einstein's prediction

On February 11, 2016, the LIGO (Laser Interferometric Gravitational-wave Observatory) Collaboration announced the detection of gravitational waves. For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the Earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein's 1915 general theory of relativity and opens an unprecedented new window onto the cosmos. 1)

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.

The gravitational waves were detected on September 14, 2015 at 09:51 UTC by both of the twin LIGO detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. Each LIGO detector is 4 km long, the observatories are separated by a distance of 3,002 km. As a gravitational wave passes through a detector, it distorts spacetime such that one arm lengthens, and the other shortens. By comparing the disturbances at the two detectors, the scientists can confirm the direct detection of a gravitational wave. - The LIGO Observatories are funded by the NSF (National Science Foundation), and were conceived, built, and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors. 2)

Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. About 3 times the mass of the sun was converted into gravitational waves in a fraction of a second—with a peak power output about 50 times that of the whole visible universe. By looking at the time of arrival of the signals—the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford—scientists can say that the source was located in the Southern Hemisphere.

According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide into each other at nearly one-half the speed of light and form a single more massive black hole, converting a portion of the combined black holes' mass to energy, according to Einstein's formula E=mc2. This energy is emitted as a final strong burst of gravitational waves. It is these gravitational waves that LIGO has observed.

The new LIGO discovery is the first observation of gravitational waves themselves, made by measuring the tiny disturbances the waves make to space and time as they pass through the Earth.

The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed—and the discovery of gravitational waves during its first observation run. The US National Science Foundation leads in financial support for Advanced LIGO. Funding organizations in Germany (Max Planck Society), the STFC (Science and Technology Facilities Council) of the UK, and Australia (Australian Research Council) also have made significant commitments to the project. Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration. Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University, and the University of Wisconsin- Milwaukee. Several universities designed, built, and tested key components for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Florida, Stanford University, Columbia University of the City of New York, and Louisiana State University.

LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration. The LSC detector network includes the LIGO interferometers and the GEO600 detector. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom, and the University of the Balearic Islands in Spain.

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Figure 1: Photo of the LIGO facility on Livingston, Louisiana (image credit: LIGO)

Light is—or has been up until now—the only way to study objects in the universe (actually the entire electromagnetic spectrum). This includes everything from the Moon, all the way out to the most distant objects ever observed. Astronomers and astrophysicists use observatories that can see in not only visible light, but in all other parts of the electromagnetic spectrum, to study objects in the universe. And we've learned an awful lot. But things will change with this announcement. 3) 4)

Gravitational waves are a new way to study notoriously difficult things to observe like black holes and neutron stars. Black holes emit no light at all, and their characteristics and properties are inferred from cause and effect relationships with objects near them. But the detection of gravitational waves holds the promise of answering questions about black holes, neutron stars, and even the early days of our universe, including the Big Bang.

It's almost impossible to overstate the magnitude of this discovery. Once we understand how to better detect and observe gravitational waves, we may come to a whole new understanding of the universe, and we may look back on this day as truly ground-breaking and revolutionary.

Some background: Experiments to detect gravitational waves began with Weber and his resonant mass detectors in the 1960s, followed by an international network of cryogenic resonant detectors. Interferometric detectors were first suggested in the early 1960s and the 1970s (Ref. 2). A study of the noise and performance of such detectors, and further concepts to improve them, led to proposals for long-baseline broadband laser interferometers with the potential for significantly increased sensitivity.

By the early 2000s, a set of initial detectors was completed, including TAMA 300 in Japan, GEO600 in Germany, the LIGO (Laser Interferometer Gravitational-Wave Observatory) in the United States, and Virgo in Italy. Combinations of these detectors made joint observations from 2002 through 2011, setting upper limits on a variety of gravitational-wave sources while evolving into a global network. In 2015, Advanced LIGO became the first of a significantly more sensitive network of advanced detectors to begin observations.

On September 14, 2015 at 09:50:45 UTC, the LIGO Hanford, WA, and Livingston, LA, observatories detected the coincident signal GW150914 shown in Figure 2. The initial detection was made by low-latency searches for generic gravitational-wave transients and was reported within three minutes of data acquisition.

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Figure 2: The gravitational-wave event GW150914 observed by the LIGO Hanford (H1, left column panels) and Livingston (L1, right column panels) detectors. Times are shown relative to September 14, 2015 at 09:50:45 UTC (image credit: LIGO consortium).

In 2016, gravitational-wave astronomy is going international, as LIGO India (sometimes referred to as INDIGO) received the green light recently in the wake of the detection announcement. Set to begin science operations around 2019, the third LIGO detector will be constructed in India. This will give LIGO the ‘third vector' it was initially envisioned with, allowing researchers to pin down the source direction in the sky. Other detectors are on the hunt as well, including VIRGO near Pisa, Italy, GEO600 in Germany, and KAGRA (Kamioka Gravitational Wave Detector), University of Tokyo, Japan. 5)

The LISA Pathfinder mission of ESA also started science operations in late February 2016. Launched on December 3rd, 2015 from Kourou, French Guiana, LISA Pathfinder won't detect gravitational waves. It will, however, pave the way for a full-up space based gravitational wave detector, eLISA (evolved Laser Interferometer Space Antenna), set to launch sometime in the 2030s.

• A second gravitational wave source was detected by LIGO as reported on June 15, 2016. The LSC (LIGO Scientific Collaboration)and the Virgo Collaboration used data from the twin LIGO detectors — located in Livingston, Louisiana, and Hanford, Washington — to make the discovery, which is accepted for publication in the journal Physical Review Letters. 6) 7)

From the data of the gravitational wave event, named GW151226, the researchers concluded the second set of gravitational waves were produced during the final moments of the merger of two black holes that were 14 and 8 times the mass of the Sun, and the collision produced a single, more massive spinning black hole 21 times the mass of the Sun. In comparison, the black holes detected in September 2015 were 36 and 29 times the Sun's mass, merging into a black hole of 62 solar masses.

The inferred component masses are consistent with values dynamically measured in X-ray binaries, but are obtained through the independent measurement process of gravitational- wave detection. Although it is challenging to constrain the spins of the initial black holes, we can conclude that at least one black hole had spin greater than 0.2. These recent detections in Advanced LIGO's first observing period have revealed a population of binary black holes that heralds the opening of the field of gravitational-wave astronomy.

The merger occurred approximately 1.4 billion years ago. The detected signal comes from the last 27 orbits of the black holes before their merger. Based on the arrival time of the signals—the Livingston detector measured the waves 1.1 milliseconds before the Hanford detector—researchers can roughly determine the position of the source in the sky.

"GW151226 perfectly matches our theoretical predictions for how two black holes move around each other for several tens of orbits and ultimately merge," said Alessandra Buonanno of UMD (University of Maryland). "Remarkably, we could also infer that at least one of the two black holes in the binary was spinning."

"It is very significant that these black holes were much less massive than those observed in the first detection," said Gabriela Gonzalez, LSC spokesperson and professor of physics and astronomy at Louisiana State University. "Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe."

• June 1, 2017: A third gravitational wave has been detected by the LIGO (Laser Interferometer Gravitational-wave Observatory). An international team announced the detection today, while the event itself was detected on January 4, 2017. 8) 9)

The team, including engineers and scientists from Northwestern University in Illinois, published their results in the journal Physical Review Letters. 10)

Like the previous two detections, this one was created by the merging of two black holes. These two were different sizes from each other; one was about 31.2 solar masses, and the other was about 19.4 solar masses. The combined 50 solar mass event caused the third wave, which is named GW170104. The black holes were about 3 billion light years away.

LIGO is showing us that their is a population of binary black holes out there. "Our handful of detections so far is revealing an intriguing black hole population we did not know existed until now," said Northwestern's Vicky Kalogera, a senior astrophysicist with the LSC (LIGO Scientific Collaboration), which conducts research related to the twin LIGO detectors, located in the U.S.

"Now we have three pairs of black holes, each pair ending their death spiral dance over millions or billions of years in some of the most powerful explosions in the universe. In astronomy, we say with three objects of the same type you have a class. We have a population, and we can do analysis."

This third finding strengthens the case for the existence of a new class of black holes: binary black holes that are locked in relationship with each other. It also shows that these objects can be larger than thought before LIGO detected them. "It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us." – David Shoemaker, MIT.

"We have further confirmation of the existence of black holes that are heavier than 20 solar masses, objects we didn't know existed before LIGO detected them," said David Shoemaker of MIT, spokesperson for the LIGO Scientific Collaboration . "It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us."

"With the third confirmed detection of gravitational waves from the collision of two black holes, LIGO is establishing itself as a powerful observatory for revealing the dark side of the universe," said David Reitze of Caltech, executive director of the LIGO Laboratory and a Northwestern alumnus. "While LIGO is uniquely suited to observing these types of events, we hope to see other types of astrophysical events soon, such as the violent collision of two neutron stars."


Further detections of gravitational waves

• December 4, 2018: Scientists with the LIGO and Virgo gravitational wave observatories report four new sets of these ripples in spacetime. Those additions bring the total count to 11, the researchers say in a study published December 3 at, marking major progress since the first gravitational wave detection in 2015 (SN: 3/5/16, p. 6). 11)

- All but one of the 11 sets of waves were stirred up in violent collisions of two black holes. The one remaining detection, reported in October 2017, instead came from the smashup of two stellar corpses called neutron stars (SN: 11/11/17, p. 6).

• December 4, 2018: Researchers from the University of Portsmouth (UoP) have made vital contributions to the observations of four new gravitational waves, which were announced this weekend (1 December). 12)

- The new results are from the National Science Foundation's LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based VIRGO gravitational-wave detector. The results were announced at the Gravitational Wave Physics and Astronomy Workshop in College Park, Maryland, USA.

- Three years ago LIGO made the first observation of a binary black hole merger. Today, there have been observations of 11 gravitational-wave signals (10 stellar-mass binary black hole mergers and one merger of two neutron stars, which are the dense, spherical remains of stellar explosions).

- These observations are revolutionizing our understanding of the processes by which high mass stars (10 – 100 times as heavy as our sun) are formed, how they evolve, and the method by which black holes are produced.

- The new events are known as GW170729, GW170809, GW170818, and GW170823, in reference to the dates they were detected. All of the events are included in a new catalogue, also released Saturday, with some of the events breaking records.

- Researchers from the newly-formed Gravitational-Wave Physics Group in the University's ICG (Institute of Cosmology and Gravitation) in the University of Portsmouth have played a significant role in the observation of the first 11 gravitational-wave events.

- Dr Laura Nuttall, a senior lecturer in the ICG, made the initial observation of GW170729. This event, detected in the second observing run on July 29 2017, is the most massive and distant gravitational-wave source ever observed. Any theoretical work trying to understand the mechanisms by which black holes form, now must allow for black holes as massive as this one to be produced.

- Dr Andrew Lundgren, a reader at the ICG, was one of the main developers of the noise subtraction scheme, which was necessary to increase the sensitivity of the LIGO observatories to be able to confirm that GW170729, GW170809 and GW170818 were genuine gravitational-wave signals. Without this work, we would only be talking about eight observed gravitational-wave signals today. 13)

- Dr Ian Harry, a senior lecturer at the ICG, is one of the two main developers of the PyCBC algorithm, which is responsible for searching for merging black holes and neutron stars in Advanced LIGO and Advanced Virgo data. This code made the first observation of many of the 11 gravitational-wave signals seen so far.

- Dr Harry said: "I'm happy to see the vital contributions our researchers have made to the observation of the first 11 gravitational-wave events. Gravitational-wave observations offer us a way to observe astrophysical sources that have never been seen before, including the collision of two black holes. These observations allow us to begin to understand the processes by which black holes are produced and explore the environments in which they are formed.

- "However, this is only the beginning of gravitational-wave astronomy, and as our observatories become more sensitive we expect to observe hundreds of sources in the coming years. My personal hope is that we observe something truly unexpected in the next years, which would help us to better understand the Universe that we live in. Gravitational-wave astronomy is one of the fastest growing fields in astronomy and collaboration between the new Gravitational-Wave Physics Group and existing ICG researchers offers tremendous possibilities for future world-leading research."

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Figure 3: These observations allow us to begin to understand the processes by which black holes are produced and explore the environments in which they are formed (image credit: UoP Team)


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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (