Astronomy

What changes are being made to VIRGO and LIGO (if any)?

What changes are being made to VIRGO and LIGO (if any)?


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Gravitational wave detectors VIRGO and LIGO are currently down for some updates, slated to be completed sometime early next year (they hope). What exactly are they doing? I hadn't heard it was down again after the last set of updates awhile ago. What are they updating now-- is that knowledge public domain?

Source: I went to an astronomy club meeting and it was stated in the presentation.


Currently, LIGO and Virgo are in what's known as a commissioning phase, a scheduled stage where the teams work on improving the sensitivity of the detectors. A big part of this is noise reduction, but there are other technical updates (an evolving plan also mentions increasing the power of the lasers and adding a "squeezed vacuum source"). Virgo in particular is replacing the steel fibers that hold up their mirrors with silica ones. The original silica fibers failed and were replaced with steel ones before last year's detection of GW170817, but now new silica ones are being added.

As of October, the Livingston site has reached low noise levels, while Hanford is still a bit away. Virgo isn't quite there yet. Soon, though, the three detectors should begin a couple of joint engineering runs, testing the hardware updates before O3 (the third, year-long, observing session) begins in February.


The First Two Years of Advanced LIGO and Virgo

The “advanced generation” of LIGO’s two and Virgo’s one gravitational wave detectors come online in 2015 and 2016 respectively. If these instruments detect gravitational waves from mergers of neutron stars and/or black holes (let me just call these “compact mergers”), they are expected to alert cooperating electromagnetic telescopes with pointing information.

Why? Because overwhelming evidence indicates that compact mergers involving at least one neutron star are sometimes accompanied by minutes-long x-ray or optical afterglows, days-long optical or infrared kilonovae, and maybe even years-long radio emission (let me just call all of these “short bright flashes”). Detecting and studying short bright flashes requires LIGO/Virgo to give quick and precise pointing information.

But how quickly will these instruments be able to detect a gravitational wave, and how precisely will they be able to tell their partner telescopes where to point? Today’s authors answer these questions for the most promising and best-understood systems, binary neutron star mergers (let me just call these “BNS mergers”). Specifically, they take a realistic look at LIGO/Virgo’s first two years online, including their early sensitivity and expected downtime.

Keep in mind that these detectors will be slowly improving their sensitivity over the next decade. Furthermore, at design sensitivity, which LIGO/Virgo expect to achieve around 2019, the highly-uncertain BNS merger rate implies scenarios ranging from 0.4 to 400 detections per year.

How Quickly?

Fig. 1, taken from today’s paper, displays LIGO/Virgo’s state-of-the-art detection timeline up against the various timescales of light emission from a BNS merger.

Figure 1: A rough timeline for possible electromagnetic counterparts to a BNS merger, and LIGO/Virgo’s current detection capabilities.

How Precisely?

But how precise will the rapid sky localization be? Today’s authors confirm what others have found, that most gravitational wave detections occuring in the next few years will provide pointing information no better than hundreds to a thousand square degrees, even after the days-latent parameter estimation has been computed. Hundreds of square degrees is a big patch of sky, around 1% of it! (The Moon spans about a fifth of a square degree.)

The histograms below show the fraction of all detections that LIGO/Virgo can localize to a given angular region with 90% confidence. The left panel represents 2015, the right panel 2016. Notice that in 2015, the rapid localization (red curve) is approximately as good as the days-later parameter estimation (blue curve). Note, the higher the curve, the more events are localized to small regions.

Figure 2: The fraction of all detections that LIGO/Virgo can localize to a given angular region with 90% confidence. (Note, the full sky spans

40,000 deg 2 .) The bottom axis shows the size of the 90% confidence region, and the right axis shows the percentage of all detections that could be localized to that angular size or smaller. (The left axis translates this percentage to numbers of detections, using very uncertain estimates of BNS merger rates.) The left panel (c) represents 2015, when only LIGO’s two detectors are in use. The right panel (d) represents 2016 when Virgo joins them. The red curve represents the rapid localization that is finished in about a minute. The higher blue curve represents the more accurate parameter estimation that takes hours to days.

How’d they Figure that Out?

They came to this conclusion by simulating a suite of realistic detection scenarios. Using

1000 imaginary detections from an astrophysically-motivated source population. That is, for each event, they pick a point in the 3-dimensional detectable volume, draw two neutron star masses from the observed neutron star mass distribution, and pick a randomly-oriented spin for each star and an orientation of the orbital plane. From these parameters they construct a theoretical waveform and bury it into simulated detector noise. Then they “search” the noise blindly with the state-of-the-art LIGO/Virgo detection algorithms. Then they apply LIGO/Virgo’s data analysis pipeline to draw sky maps, or probability distributions over the whole sky. Here’s a typical sky map:

Figure 3: An example of a sky map for a 2015 detector scenario. The map is fixed to the Earth just as the detectors are. The darkest red pixels represent the most likely directions to look for an electromagnetic signal. The star symbol indicates the actual direction of the simulated source. Notice that there are two distinct regions of high probability. This is a common feature of gravitational waves detected by two nearly-aligned interferometers. The long arcs represent the great circle of equal distance between the LIGO detectors in Washington and Louisiana.

The strange double swath of probability is actually quite common in this early detector era. This type of sky map arises because the two LIGO detectors (one in Washington, one in Louisiana) were built with nearly identical alignments. This means they cannot distinguish crucial polarization information which would clue them in to whether a source is nearly straight up or nearly straight down.

These authors have simulated a suite of detection scenarios to give their partner telescopes a realistic picture of the pointing information LIGO/Virgo will be able to provide if they makes any detections in 2015-2016. Many of the detections involve sources localized to long arcs of the sky occupying hundreds to a thousand deg 2 . And many of the sky localizations will come in the form of two or more equally-likely patches of sky, up to 180 deg apart. This is helpful (though perhaps disheartening) information for collaborators planning to point x-ray, optical, infrared, or radio telescopes at gravitational wave sources.

A final note about this paper. The authors have made their code and their extensive data available online. I appreciate the effort they have made to make the data very usable! Check it out.


Northwestern Now

Object lies between heaviest known neutron star and lightest known black hole

An international research collaboration, including Northwestern University astronomers, has detected a mystery object inside the puzzling area known as the “mass gap” -- the range that lies between the heaviest known neutron star and the lightest known black hole. The finding has important implications for astrophysics and the understanding of low-mass compact objects.

When the most massive stars die, they collapse under their own gravity and leave behind black holes when stars that are a bit less massive than this die, they explode in a supernova and leave behind dense, dead remnants of stars called neutron stars. The heaviest known neutron star is no more than 2.5 times the mass of our sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses. For decades, astronomers have wondered: Are there any objects in this mass gap?

Now, in a new study from the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European Virgo observatory, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap.

The intriguing object was found on Aug. 14, 2019, as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo. A paper about the detection was published today (June 23) by The Astrophysical Journal Letters.

“Mergers of a mixed nature -- black holes and neutron stars -- have been predicted for decades, but this compact object in the mass gap is a complete surprise,” said Northwestern’s Vicky Kalogera, who coordinated writing of the paper. “We are really pushing our knowledge of low-mass compact objects. Even though we can’t classify the object with conviction, we have seen either the heaviest known neutron star or the lightest known black hole. Either way, it breaks a record.”

Kalogera, a leading astrophysicist in the LIGO Scientific Collaboration (LSC), is an expert in the astrophysics of compact object binaries and analysis of gravitational-wave data. She is the Daniel I. Linzer Distinguished University Professor of Physics and Astronomy and director of CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics) in Northwestern’s Weinberg College of Arts and Sciences.

“Whereas we are not sure about the nature of the low-mass compact object, we have obtained a very robust measure of its mass, which falls right into the so-called mass gap,” said Mario Spera, a co-author of the paper who studies the formation of merging binaries. He is a Virgo collaboration member and a European Union Marie Curie Postdoctoral Fellow at CIERA and the University of Padova.

“This exciting and unprecedented finding, combined with the unique mass ratio of the merger event, challenges all the astrophysical models that try to shed light on the origins of this event,” Spera said. “However, we are quite sure that the universe is telling us, for the umpteenth time, that our ideas on how compact objects form, evolve and merge are still very fuzzy.”

The cosmic merger described in the study, an event dubbed GW190814, resulted in a final black hole about 25 times the mass of the sun. (Some of the merged mass was converted to a blast of energy in the form of gravitational waves). The newly formed black hole lies about 800 million light-years away from Earth.

Before the two objects merged, their masses differed by a factor of nine, making this the most extreme mass ratio known for a gravitational-wave event. Another recently reported LIGO-Virgo event, called GW190412, occurred between two black holes with a mass ratio of about 4:1.

In addition to Kalogera and Spera, the other Northwestern researchers involved in the study are Chase Kimball, Christopher Berry and Mike Zevin. The three are authors of the paper and members of CIERA.

Kimball, an astronomy Ph.D. student and LSC member, assessed how often mergers such as GW190814 occur in the universe. Berry, the CIERA Board of Visitors Research Professor, is a member of the LSC Editorial Board for all LSC publications and served as the lead representative for this study. Zevin, an astronomy Ph.D. student and LSC member, contributed to the astrophysical interpretation and also to writing the GW190412 discovery paper.

“It’s a challenge for current theoretical models to form merging pairs of compact objects with such an extreme mass ratio in which the low-mass partner resides in the mass gap,” Kalogera said. “This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object.

“The mystery object may be a neutron star merging with a black hole -- an exciting possibility expected theoretically but not yet confirmed observationally,” she said. “However, at 2.6 times the mass of our sun, it exceeds modern predictions for the maximum mass of neutron stars and may instead be the lightest black hole ever detected.”

“Whether or not the object is a heavy neutron star or a light black hole, the discovery is the first in a new class of binary mergers,” Kimball added. “Models of binary populations will have to account for how often we now can infer that these sort of events occur.”

When the LIGO and Virgo scientists spotted this merger, they immediately sent out an alert to the astronomical community. Dozens of ground- and space-based telescopes followed up in search of light waves generated in the event, but none picked up any signals.

So far, such light counterparts to gravitational-wave signals have been seen only once, in an event called GW170817. The event, discovered by the LIGO-Virgo network in August of 2017, involved a fiery collision between two neutron stars that was subsequently witnessed by dozens of telescopes on Earth and in space. Neutron star collisions are messy affairs with matter flung outward in all directions and are thus expected to shine with light. Conversely, black hole mergers, in most circumstances, are thought not to produce light.

According to the LIGO and Virgo scientists, the August 2019 event was not seen in light for a few possible reasons. First, this event was six times farther away than the merger observed in 2017, making it harder to pick up any light signals. Secondly, if the collision involved two black holes, it likely would have not shone with any light. Thirdly, if the object was in fact a neutron star, its nine-fold more massive black-hole partner might have swallowed it whole a neutron star consumed whole by a black hole would not give off any light.

“I think of Pac-Man eating a little dot,” says Kalogera. “When the masses are highly asymmetric, the smaller compact object can be eaten by the black hole in one bite.”

How will researchers ever know if the mystery object was a neutron star or black hole? Future observations with LIGO and possibly other telescopes may catch similar events that would help reveal whether additional objects exist in the mass gap.

“The mass gap has been an interesting puzzle for decades, and now we’ve detected an object that fits just inside it,” said Pedro Marronetti, program director for gravitational physics at the National Science Foundation (NSF). “That cannot be explained without defying our understanding of extremely dense matter or what we know about the evolution of stars. This observation is yet another example of the transformative potential of the field of gravitational-wave astronomy, which brings novel insights to light with every new detection.”

Additional information about the gravitational-wave observatories:

LIGO is funded by the NSF and operated by Caltech and MIT, which conceived of LIGO and lead the project. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available.

The Virgo Collaboration is currently composed of approximately 520 members from 99 institutes in 11 different countries including Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy and Nikhef in the Netherlands. A list of the Virgo Collaboration groups is available.

Webinar Series:

For those wishing for a deeper dive into these LIGO-Virgo results and other research from the latest observing run, the team has scheduled a webinar intended for a scientific audience. Called the LIGO-Virgo-KAGRA Webinar Series, this will be the first in a series of webinars discussing the gravitational-wave network’s results in-depth. The one-hour Zoom webinar will be on June 25 at 14:00 Universal Time Coordinated (7:00am Pacific Daylight Time 10:00am Eastern Daylight Time 16:00 Central European Summer Time 23:00 Japan Standard Time).


Search and follow-up coordination

Coordinated synoptic searches using a network of telescopes placed strategically around the globe could address these large sky localizations by obtaining observations of different portions of the sky maps or cadenced epochs of the same region to increase the probability of detecting a counterpart. Using an iterative strategy, one telescope could cover regions of the sky map that were not observed by another telescope of comparable sensitivity 14 . In addition, using common online platforms in which observed candidates are ingested, astronomers could directly trigger accessible photometric and spectroscopic facilities all over the world to expedite the counterpart discovery process. This vision has been supported by the rapid, public release of candidates in GCNs and on the Transient Name Server (TNS), and public access to data such as that from the Dark Energy Camera (DECam) 9 or the Canada–France–Hawaii Telescope (CFHT) 10 , which has allowed for more science to be accomplished by the astronomy community, but there is further to go.

Just in the case of the Zwicky Transient Facility 15 , we have more than 100,000 objects changing brightness at detectable levels during a given night, and the goal is to find the one associated with the GW event. While observations within around the last three nights are especially desirable to strongly constrain explosion times, there are often still tens of transients to characterize. With changes in their brightness and colour occuring on the order of hours, it is essential to have photometric and spectroscopic observations every few hours to understand the event in detail, including the chemical elements synthesized and material surrounding the remnant. Given that half of the battle is classifying objects found by surveys, there should be more coordination between astronomers performing counterpart identification and those performing classification. Follow-ups to O3 have made it clear that robotic spectroscopic classification resources are incredibly useful for classifying many objects as quickly as possible given that it is unlikely that such large time allocations will be awarded in future without more identified counterparts, medium-sized robotic systems devoted to spectroscopic follow-up will need to be the future.

Finding balance in the amount of data taken is crucial when their eventual significance is still unknown. Incentivizing coordination between those using more telescopes, including the sharing of information and credit, will simplify the process of splitting up the responsibilities of getting spectroscopic or long-term photometric follow-up. While high-cadence spectroscopy of the eventual kilonova counterpart is highly desirable, this would prevent, for example, the over-abundance of classifications for specific candidates such as ZTF19aarzaod 12 , a potential counterpart to BNS candidate GW190425 with at least five spectra and more than twice as many photometric observations. It would also prevent other candidates from falling through the cracks, perhaps disfavoured for being relatively faint or not having a known host redshift. Isolated follow-up programmes on large aperture systems, without support from other resources, is simply not the most efficient use of time, and emphasizes the need for data reduction and dissemination of results as quickly as possible.


LIGO and Virgo are back in business to search for more gravitational waves

The National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory) is set to resume its hunt for gravitational waves – ripples in space and time – on 1 April 2019, after receiving a series of upgrades to its lasers, mirrors, and other components. LIGO—which consists of twin detectors located in Washington and Louisiana, both in the United States – now has a combined increase in sensitivity of about 40 percent over its last run, which means that it can survey an even larger volume of space than before for powerful, wave-making events, such as the collisions of black holes.

Joining the search will be Virgo, the European-based gravitational-wave detector, located at the European Gravitational Observatory (EGO) in Italy, which has almost doubled its sensitivity since its last run and is also starting up 1 April 2019.

“For this third observational run, we achieved significantly greater improvements to the detectors’ sensitivity than we did for the last run,” says Peter Fritschel, LIGO’s chief detector scientist at Massachusetts Institute of Technology (MIT). “And with LIGO and Virgo observing together for the next year, we will surely detect many more gravitational waves from the types of sources we’ve seen so far. We’re eager to see new events too, such as a merger of a black hole and a neutron star.”

In 2015, after LIGO began observing for the first time in an upgraded program called Advanced LIGO, it soon made history by making the first direct detection of gravitational waves.The ripples traveled to Earth from a pair of colliding black holes located 1.3 billion light years away. For this discovery, three of LIGO’s key players—Caltech’s Barry C. Barish, the Ronald and Maxine Linde Professor of Physics, Emeritus, and Kip S. Thorne, the Richard P. Feynman Professor of Theoretical Physics, Emeritus, along with MIT’s Rainer Weiss, professor of physics, emeritus—were awarded the 2017 Nobel Prize in Physics.

Since then, the LIGO-Virgo detector network has uncovered nine additional black hole mergers and one explosive smashup of two neutron stars. That event, dubbed GW170817, generated not just gravitational waves but light, which was observed by dozens of telescopes in space and on the ground.

“With our three detectors now operational at a significantly improved sensitivity, the global LIGO-Virgo detector network will allow more precise triangulation of the sources of gravitational waves,” says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration. “This will be an important step toward our quest for multi-messenger astronomy.”

Virgo and LIGO have increased their equipment ahead of the beginning of another gravitational wave hunt on 1 April 2019. Image credit: EGO/Virgo Collaboration/Perciballi

Now, with the start of the next joint LIGO-Virgo run, the observatories are poised to detect an even greater number of black hole mergers and other extreme events, such as additional neutron-neutron star mergers or a yet-to-be-seen black hole-neutron star merger. One of the metrics the team uses for measuring increases in sensitivity is to calculate how far out they can detect neutron-neutron star mergers. In the next run, LIGO will be able to see those events out to an average of 550 million light years away, or more than 190 million light years farther out than before.

A key to achieving this sensitivity involves lasers. Each LIGO installation consists of two long arms that form an L shaped interferometer. Laser beams are shot from the corner of the “L” and bounced off mirrors before traveling back down the arms and recombining. When gravitational waves pass by, they stretch and squeeze space itself, making imperceptibly tiny changes to the distance the laser beams travel and thereby affecting how they recombine. For this next run, the laser power has been doubled to more precisely measure these distance changes, thereby increasing the detectors’ sensitivity to gravitational waves.

Other upgrades were made to LIGO’s mirrors at both locations, with a total of five of eight mirrors being swapped out for better-performing versions.

“We had to break the fibres holding the mirrors and very carefully take out the optics and replace them,” says Calum Torrie, LIGO’s mechanical-optical engineering head at Caltech. “It was an enormous engineering undertaking.”

This next run also includes upgrades designed to reduce levels of quantum noise. Quantum noise occurs due to random fluctuations of photons, which can lead to uncertainty in the measurements and can mask faint gravitational-wave signals. By employing a technique called “squeezing,” initially developed for gravitational-wave detectors at the Australian National University, and matured and routinely used since 2010 at the GEO600 detector, researchers can shift the uncertainty in the photons around, making their amplitudes less certain and their phases, or timing, more certain. The timing of photons is what is crucial for LIGO’s ability to detect gravitational waves.

Torrie says that the LIGO team has spent months commissioning all of these new systems, making sure everything is aligned and working correctly. “One of the things that is satisfying to us engineers is knowing that all of our upgrades mean that LIGO can now see farther into space to find the most extreme events in our Universe.”

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LIGO and Virgo observatories jointly detect black hole collision

In August, detectors on two continents recorded gravitational wave signals from a pair of black holes colliding. This discovery, announced today, is the first observation of gravitational waves by three different detectors, marking a new era of greater insights and improved localization of cosmic events now available through globally networked gravitational-wave observatories.

The collision was observed Aug. 14 at 10:30:43 a.m. Coordinated Universal Time (UTC) using the two National Science Foundation (NSF)-funded Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors located in Livingston, Louisiana, and Hanford, Washington, and the Virgo detector, funded by CNRS and INFN and located near Pisa, Italy.

The detection by the LIGO Scientific Collaboration (LSC) and the Virgo collaboration is the first confirmed gravitational wave signal recorded by the Virgo detector. A paper about the event, a collision designated GW170814, has been accepted for publication in the journal Physical Review Letters.

"Little more than a year and a half ago, NSF announced that its Laser Interferometer Gravitational Wave Observatory had made the first-ever detection of gravitational waves, which resulted from the collision of two black holes in a galaxy a billion light-years away," said NSF Director France Córdova. "Today, we are delighted to announce the first discovery made in partnership between the Virgo gravitational-wave observatory and the LIGO Scientific Collaboration, the first time a gravitational wave detection was observed by these observatories, located thousands of miles apart. This is an exciting milestone in the growing international scientific effort to unlock the extraordinary mysteries of our universe."

The detected gravitational waves -- ripples in space and time -- were emitted during the final moments of the merger of two black holes, one with a mass about 31 times that of our sun, the other about 25 times the mass of the sun. The event, located about 1.8 billion light-years away resulted in a spinning black hole with about 53 times the mass of our sun -- that means about three solar masses were converted into gravitational-wave energy during the coalescence.

"This is just the beginning of observations with the network enabled by Virgo and LIGO working together," says LSC spokesperson David Shoemaker of the Massachusetts Institute of Technology (MIT). "With the next observing run planned for fall 2018, we can expect such detections weekly or even more often."

LIGO has transitioned into a second-generation gravitational-wave detector, known as Advanced LIGO, that consists of two identical interferometers. Beginning operations in September 2015, Advanced LIGO has conducted two observing runs. The second observing run, "O2," began Nov. 30, 2016, and ended Aug. 25, 2017.

The Virgo detector, also now a second-generation detector, joined the O2 run Aug. 1, 2017 at 10 a.m. UTC. The real-time detection Aug. 14 was triggered with data from all three LIGO and Virgo instruments.

"It is wonderful to see a first gravitational-wave signal in our brand new Advanced Virgo detector only two weeks after it officially started taking data," says Jo van den Brand of Nikhef and Vrije Universiteit Amsterdam, spokesperson of the Virgo collaboration. "That's a great reward after all the work done in the Advanced Virgo project to upgrade the instrument over the past six years."

When an event is detected by a three-detector network, the area in the sky likely to contain the source shrinks significantly, improving distance accuracy. The sky region for GW170814 has a size of only 60 square degrees, more than 10 times smaller than the size using data available from the two LIGO interferometers alone.

"Being able to identify a smaller search region is important, because many compact object mergers -- for example those involving neutron stars -- are expected to produce broadband electromagnetic emissions in addition to gravitational waves," says Georgia Tech's Laura Cadonati, deputy spokesperson for the LIGO Scientific Collaboration. "This precision pointing information enabled 25 partner facilities to perform follow-up observations based on the LIGO-Virgo detection, but no counterpart was identified -- as expected for black holes."

"With this first joint detection by the Advanced LIGO and Virgo detectors, we have taken one step further into the gravitational-wave cosmos," says Caltech's David H. Reitze, executive director of the LIGO Laboratory. "Virgo brings a powerful new capability to detect and better locate gravitational-wave sources, one that will undoubtedly lead to exciting and unanticipated results in the future."


LIGO and Virgo announce new detections in updated gravitational-wave catalog

BATON ROUGE &ndash Scientists who study black holes and their impact on the universe are making strides in their research at Louisiana's Laser Interferometer Gravitational-Wave Observatory (LIGO).

After several months of thorough analysis, the LIGO Scientific Collaboration and the Virgo Collaboration have released an updated catalog of gravitational wave detections.

The catalog contains 39 new signals from black-hole or neutron-star collisions detected between April 1-Oct. 1, 2019, which more than triples the number of confirmed detections. The new set includes some of the most interesting systems seen so far, and will lead to additional research related to the new findings.

The sharp increase in the number of detections was made possible by significant improvements to the instruments used. Improvements included increased laser power, better mirrors and, perhaps most significantly the use of quantum squeezing technology.

According to a July 2020 article in Physics World, when physicists make measurements on a laser signal, uncertainties arise from quantum fluctuations in the numbers of photons detected and the times at which the photons arrive at the detector. The relationship between this pair of uncertainties is described by the uncertainty principle, which dictates that a decrease in uncertainty in photon number must be accompanied by an increase in uncertainty in timing and vice versa. Reducing the uncertainty of one measurement at the expense of increasing the other can be advantageous in some experiments &mdash and is called quantum squeezing.

So, use of quantum squeezing tech and other innovations resulted in a roughly 60 percent improvement in the range to which signals can be detected. The detectors were also able to operate without interruption more often than in the past, with an improved duty cycle of about 75 percent, versus about 60 percent previously.

&ldquoAs an instrument scientist, your focus is on making the instrument better all the time. So even when the instrument is observing, we&rsquore looking at the data coming out of it to see if there is noise somewhere we don&rsquot expect there to be,&rdquo said LSU Department of Physics & Astronomy PhD candidate Corey Austin.

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The new signals pave the way for members of the scientific community to implement research projects that will lead to a better understanding of the populations of black holes and neutron stars.

LIGO says that by analyzing the entire population of binary black hole mergers simultaneously, astrophysical information extracted is maximized. It is inferred that the distribution of black hole masses does not follow a simple power-law distribution. Measuring the deviations from this power law will improve the ability to learn about the formation of these black holes, either as the result of stellar deaths or previous collisions.

Considering the entire population together also helps make stronger measurements of difficult-to-measure properties such as black hole spin. Some merging black holes have spins which are misaligned with their orbital angular momentum. This will help probe the regimes in which these binaries formed.

LIGO says the many signals in the updated catalog will put Einstein&rsquos theory of gravity to the test in more and better ways than before. This was done by comparing the data against predictions from the theory and constraining possible deviations. The results from multiple signals were combined using new statistical methods to obtain the tightest constraints so far on the properties of gravity in the strong, highly-dynamical regime of black hole mergers. With the new catalog, LIGO and Virgo were also able to directly study the properties of the remnant objects produced during the mergers: by measuring the vibrations of these objects, and by ruling out potential &ldquoechoes&rdquo after the main signals, LIGO and Virgo confirmed that the remnants behaved as expected from black holes in Einstein&rsquos theory.

&ldquoWe still need to know more about the universe. Just think of how we discovered new elements, which are now common knowledge,&rdquo said LSU Post-doctoral Researcher Guillermo Valdes, who works at LIGO Livingston as part of the team, led by LSU Boyd Professor Gabriela González, constantly improving the sensitivity of the instrument.


LIGO and Virgo observatories detect gravitational wave signals from black hole collision

Aerial view of the Virgo site showing the Mode-Cleaner building, the Central building, the three kilometer-long west arm and the beginning of the north arm. The other buildings include offices, workshops, computer rooms and the control room of the interferometer. Credit: The Virgo collaboration/CCO 1.0

In August, detectors on two continents recorded gravitational wave signals from a pair of black holes colliding. This discovery, announced today, is the first observation of gravitational waves by three different detectors, marking a new era of greater insights and improved localization of cosmic events now available through globally networked gravitational-wave observatories.

The collision was observed Aug. 14 at 10:30:43 a.m. Coordinated Universal Time (UTC) using the two National Science Foundation (NSF)-funded Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors located in Livingston, Louisiana, and Hanford, Washington, and the Virgo detector, funded by CNRS and INFN and located near Pisa, Italy.

The detection by the LIGO Scientific Collaboration (LSC) and the Virgo collaboration is the first confirmed gravitational wave signal recorded by the Virgo detector. A paper about the event, a collision designated GW170814, has been accepted for publication in the journal Physical Review Letters.

"Little more than a year and a half ago, NSF announced that its Laser Interferometer Gravitational Wave Observatory had made the first-ever detection of gravitational waves, which resulted from the collision of two black holes in a galaxy a billion light-years away," said NSF Director France Córdova. "Today, we are delighted to announce the first discovery made in partnership between the Virgo gravitational-wave observatory and the LIGO Scientific Collaboration, the first time a gravitational wave detection was observed by these observatories, located thousands of miles apart. This is an exciting milestone in the growing international scientific effort to unlock the extraordinary mysteries of our universe."

LIGO operates two detector sites -- one near Hanford in eastern Washington, and another near Livingston, Louisiana. The Livingston detector site is pictured here. Credit: LIGO Collaboration

The detected gravitational waves—ripples in space and time—were emitted during the final moments of the merger of two black holes, one with a mass about 31 times that of our sun, the other about 25 times the mass of the sun. The event, located about 1.8 billion light-years away resulted in a spinning black hole with about 53 times the mass of our sun—that means about three solar masses were converted into gravitational-wave energy during the coalescence.

"This is just the beginning of observations with the network enabled by Virgo and LIGO working together," says LSC spokesperson David Shoemaker of the Massachusetts Institute of Technology (MIT). "With the next observing run planned for fall 2018, we can expect such detections weekly or even more often."

LIGO has transitioned into a second-generation gravitational-wave detector, known as Advanced LIGO, that consists of two identical interferometers. Beginning operations in September 2015, Advanced LIGO has conducted two observing runs. The second observing run, "O2," began Nov. 30, 2016, and ended Aug. 25, 2017.

The Virgo detector, also now a second-generation detector, joined the O2 run Aug. 1, 2017 at 10 a.m. UTC. The real-time detection Aug. 14 was triggered with data from all three LIGO and Virgo instruments.

View of the LIGO detector in Hanford, Washington. LIGO research is carried out by the LIGO Scientific Collaboration, a group of more than 1,000 scientists from universities around the US and 14 other countries. Credit: LIGO Laboratory

"It is wonderful to see a first gravitational-wave signal in our brand new Advanced Virgo detector only two weeks after it officially started taking data," says Jo van den Brand of Nikhef and Vrije Universiteit Amsterdam, spokesperson of the Virgo collaboration. "That's a great reward after all the work done in the Advanced Virgo project to upgrade the instrument over the past six years."

When an event is detected by a three-detector network, the area in the sky likely to contain the source shrinks significantly, improving distance accuracy. The sky region for GW170814 has a size of only 60 square degrees, more than 10 times smaller than the size using data available from the two LIGO interferometers alone.

"Being able to identify a smaller search region is important, because many compact object mergers—for example those involving neutron stars—are expected to produce broadband electromagnetic emissions in addition to gravitational waves," says Georgia Tech's Laura Cadonati, deputy spokesperson for the LIGO Scientific Collaboration. "This precision pointing information enabled 25 partner facilities to perform follow-up observations based on the LIGO-Virgo detection, but no counterpart was identified—as expected for black holes."

"With this first joint detection by the Advanced LIGO and Virgo detectors, we have taken one step further into the gravitational-wave cosmos," says Caltech's David H. Reitze, executive director of the LIGO Laboratory. "Virgo brings a powerful new capability to detect and better locate gravitational-wave sources, one that will undoubtedly lead to exciting and unanticipated results in the future."


LIGO-Virgo Finds Mystery Object in "Mass Gap"

When the most massive stars die, they collapse under their own gravity and leave behind black holes when stars that are a bit less massive die, they explode in supernovas and leave behind dense, dead remnants of stars called neutron stars. For decades, astronomers have been puzzled by a gap that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses. The question remained: does anything lie in this so-called mass gap?

Now, in a new study from the National Science Foundation's (NSF's) Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo detector in Europe, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap. The object was found on August 14, 2019, as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo. A paper about the detection is being published today, June 23, in The Astrophysical Journal Letters.

"Discoveries such as this are puzzles, and force us to scratch our heads a bit," says David Reitze, executive director of the LIGO Laboratory at Caltech. "Have we observed the most massive neutron star or the lightest mass black hole? Discerning the true nature of this 'mass gap' object will require more observations, but those observations will undoubtedly shine new light on a part of the universe that has previously been inaccessible to us."

"This is going to change how scientists talk about neutron stars and black holes," says co-author Patrick Brady, a professor at the University of Wisconsin, Milwaukee, and the LIGO Scientific Collaboration spokesperson. "The mass gap may in fact not exist at all but may have been due to limitations in observational capabilities. Time and more observations will tell."

The cosmic merger described in the study, an event dubbed GW190814, resulted in a final black hole about 25 times the mass of the sun (some of the merged mass was converted to a blast of energy in the form of gravitational waves). The newly formed black hole lies about 800 million light-years away from Earth.

Before the two objects merged, their masses differed by a factor of 9, making this the most extreme mass ratio known for a gravitational-wave event. Another recently reported LIGO-Virgo event, called GW190412, occurred between two black holes with a mass ratio of about 4:1.

"It's a challenge for current theoretical models to form merging pairs of compact objects with such a large mass ratio in which the low-mass partner resides in the mass gap. This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object," says co-author Vicky Kalogera, a professor at Northwestern University. "The mystery object may be a neutron star merging with a black hole, an exciting possibility expected theoretically but not yet confirmed observationally. However, at 2.6 times the mass of our sun, it exceeds modern predictions for the maximum mass of neutron stars, and may instead be the lightest black hole ever detected."

When the LIGO and Virgo scientists spotted this merger, they immediately sent out an alert to the astronomical community. Dozens of ground- and space-based telescopes followed up in search of light waves generated in the event, but none picked up any signals. So far, such light counterparts to gravitational-wave signals have been seen only once, in an event called GW170817. That event, discovered by the LIGO–Virgo network in August of 2017, involved a fiery collision between two neutron stars that was subsequently witnessed by dozens of telescopes on Earth and in space. Neutron-star collisions are messy affairs with matter flung outward in all directions and are thus expected to shine with light. Conversely, black hole mergers, in most circumstances, are thought not to produce light.

According to the LIGO and Virgo scientists, there are a few possible reasons why the August 2019 event was not seen by light-based telescopes. First, this event was six times farther away than the merger observed in 2017, making it harder for instruments to pick up any light signals. Secondly, if the collision involved two black holes, it likely would have not shone with any light. Thirdly, if one object was in fact a neutron star, its 9-fold-more-massive black-hole partner might have swallowed it whole a neutron star consumed whole by a black hole would not give off any light.

"I think of Pac-Man eating a little dot," says Kalogera. "When the masses are highly asymmetric, the smaller neutron star can be eaten in one bite."

How will researchers ever know if the mystery object was a neutron star or a black hole? Future observations with LIGO, Virgo, and possibly other telescopes may catch similar events that would help reveal whether additional objects exist in the mass gap.

"This is the first glimpse of what could be a whole new population of compact binary objects," says Charlie Hoy, a member of the LIGO Scientific Collaboration and a graduate student at Cardiff University. "What is really exciting is that this is just the start. As the detectors get more and more sensitive, we will observe even more of these signals, and we will be able to pinpoint the populations of neutron stars and black holes in the universe."

"The mass gap has been an interesting puzzle for decades, and now we've detected an object that fits just inside it," says Pedro Marronetti, program director for gravitational physics at the NSF. "That cannot be explained without defying our understanding of extremely dense matter or what we know about the evolution of stars. This observation is yet another example of the transformative potential of the field of gravitational-wave astronomy, which brings novel insights with every new detection."


What changes are being made to VIRGO and LIGO (if any)? - Astronomy

History is replete with turning points, moments that are looked back on as being pivotal to the flow of events that led to our civilization today. Scientific history is equally full of turning points — defining discoveries that transformed our understanding of the Universe and how it works. In 2015 such a discovery was made with the first observation of gravitational waves, which let us listen to the Universe and study it in a completely new way, founding the field of gravitational-wave astronomy.

The National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based Virgo instruments have now detected gravitational waves from more than 10 cosmic sources, including stellar-mass binary black hole mergers and one merger of neutron stars, which are the dense, spherical remains of stellar explosions.

Northwestern University has faculty, students, and postdocs in CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics) working in international collaborative teams and leading work in the new fields of gravitational-wave astrophysics and multi-messenger astronomy. Explore some of the exciting moments of these discoveries with them.

LIGO & Gravitational Waves

LIGO’s first detection of gravitational waves and merging black holes occurred on September 14, 2015, an event that made headlines worldwide and confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity. The field of gravitational-wave astronomy was born with a little chirp “heard” on Earth that forever changed the way we see the universe. Additional content available on CIERA’s Gallery. View the story from Northwestern News, “Gravitational Waves Detected 100 Years After Einstein’s Prediction.”