Tag Archives: gravitational wave

Standby for a New Round of Gravitational Wave Detection

Since late August 2017, the US LIGO 0bservatories in Washington and Louisiana and the European Gravitational Observatory (EGO), Virgo, in Italy, have been off-line for updating and testing.  These gravitational wave observatories were set to start Observing Run 3 (O3) on 1 April 2019 and conduct continuous observations for one year.  All three of these gravitational wave observatories have improved sensitivities and are capable of “seeing” a larger volume of the universe than in Observing Run 2 (O2).

Later in 2019, the Japanese gravitational wave observatory, KAGRA, is expected to come online for the first time and join O3.  By 2024, a new gravitational wave observatory in India is expected to join the worldwide network.

On the advent of this next gravitational wave detection cycle, here’s is a brief summary of the status of worldwide gravitational wave observatories.

Advanced LIGO 

The following upgrades were implemented at the two LIGO observatories since Observing Run 2 (O2) concluded in 2017:

  • Laser power has been doubled, increasing the detectors’ sensitivity to gravitational waves.
  • Upgrades were made to LIGO’s mirrors at both locations, with five of eight mirrors being swapped out for better-performing versions.
  • Upgrades have been implemented 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 quantum “squeezing” (vacuum squeezing), researchers can shift the uncertainty in the laser light 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.  This technique initially was developed for gravitational wave detectors at the Australian National University, and matured and routinely used since 2010 at the GEO600 gravitational wave detector in Hannover, Germany,

In comparison to its capabilities in 2017 during O2, the twin LIGO detectors have a combined increase in sensitivity of about 40%, more than doubling the volume of the observable universe.

You’ll find more news and information on the LIGO website at the following link:

https://www.ligo.caltech.edu/news

GEO600 

GEO600 is a modest-size laser interferometric gravitational wave detector (600 meter / 1,969 foot arms) located near Hannover, Germany. It was designed and is operated by the Max Planck Institute for Gravitational Physics, along with partners in the United Kingdom.

In mid-2010, GEO600 became the first gravitational wave detector to employ quantum “squeezing” (vacuum squeezing) and has since been testing it under operating conditions using two lasers: its standard laser, and a “squeezed-light” laser that just adds a few entangled photons per second but significantly improves the sensitivity of GEO600.  In a May 2013 paper entitled, “First Long-Term Application of Squeezed States of Light in a Gravitational Wave Observatory,” researchers reported the following results of operational tests in 2011 and 2012.

“During this time, squeezed vacuum was applied for 90.2% (205.2 days total) of the time that science-quality data were acquired with GEO600. A sensitivity increase from squeezed vacuum application was observed broadband above 400 Hz. The time average of gain in sensitivity was 26% (2.0 dB), determined in the frequency band from 3.7 to 4.0 kHz. This corresponds to a factor of 2 increase in the observed volume of the Universe for sources in the kHz region (e.g., supernovae, magnetars).”

The installed GEO600 squeezer (in the foreground) inside the GEO600 clean room together with the vacuum tanks (in the background).  
Source: http://www.geo600.org/15581/1-High-Tech

While GEO600 has conducted observations in coordination with LIGO and Virgo, GEO600 has not reported detecting gravitational waves. At high frequencies GEO600 sensitivity is limited by the available laser power. At the low frequency end, the sensitivity is limited by seismic ground motion.

You’ll find more information on GEO600 at the following link:

http://www.geo600.org/3020/About-GEO600

Advanced Virgo, the European Gravitational Observatory (EGO)

At Virgo, the following upgrades were implemented since Observing Run 2 (O2) concluded in 2017:

  • The steel wires used during O2 observation campaign to suspend the four main mirrors of the interferometer have been replaced.  The 42 kg (92.6 pound) mirrors now are suspended with thin fused-silica (glass) fibers, which are expected to increase the sensitivity in the low-medium frequency region.  The mirrors in Advanced LIGO have been suspended by similar fused-silica fibers since those two observatories went online in 2015.
  • A more powerful laser source has been installed, which should improve sensitivity at high frequencies. 
  • Quantum “squeezing” has been implemented in collaboration with the Albert Einstein Institute in Hannover, Germany.  This should improve the sensitivity at high frequencies.
Virgo mirror suspension with fused-silica fibers.  
Source: EGO/Virgo Collaboration/Perciballi

In comparison to its capabilities in 2017 during O2, Virgo sensitivity has been improved by a factor of about 2, increasing the volume of the observable universe by a factor of about 8.

You’ll find more information on Virgo at the following link:

http://www.virgo-gw.eu

Japan’s KAGRA 

KAGRA is a cryogenically-cooled laser interferometer gravitational wave detector that is sited in a deep underground cavern in Kamioka, Japan.  This gravitational wave observatory is being developed by the Institute for Cosmic Ray Research (ICRR) of the University of Tokyo.  The project website is at the following link:

https://gwcenter.icrr.u-tokyo.ac.jp/en/

One leg of the KAGRA interferometer.  
Source: ICRR, University of Tokyo

The cryogenic mirror cooling system is intended to cool the mirror surfaces to about 20° Kelvin (–253° Celsius) to minimize the motion of molecules (jitter) on the mirror surface and improve measurement sensitivity.   KAGRA’s deep underground site is expected to be “quieter” than the LIGO and VIRGO sites, which are on the surface and have experienced effects from nearby vehicles, weather and some animals.

The focus of work in 2018 was on pre-operational testing and commissioning of various systems and equipment at the KAGRA observatory. In December 2018, the KAGRA Scientific Congress reported that, “If our schedule is kept, we expect to join (LIGO and VIRGO in) the latter half of O3…”   You can follow the latest news from the KAGRA team here:

https://gwcenter.icrr.u-tokyo.ac.jp/en/category/latestnews

LIGO-India

IndIGO, the Indian Initiative in Gravitational-wave Observations, describes itself as an initiative to set up advanced experimental facilities, with appropriate theoretical and computational support, for a multi-institutional Indian national project in gravitational wave astronomy.  The IndIGO website provides a good overview of the status of efforts to deploy a gravitational wave detector in India.  Here’s the link:

http://www.gw-indigo.org/tiki-index.php?page=Welcome

On 22 January 2019, T. V. Padma reported on the Naturewebsite that India’s government had given “in-principle” approval for a LIGO gravitational wave observatory to be built in the western India state of Maharashtra. 

“India’s Department of Atomic Energy and its Department of Science and Technology signed a memorandum of understanding with the US National Science Foundation for the LIGO project in March 2016. Under the agreement, the LIGO Laboratory — which is operated by the California Institute of Technology (Caltech) in Pasadena and the Massachusetts Institute of Technology (MIT) in Cambridge — will provide the hardware for a complete LIGO interferometer in India, technical data on its design, as well as training and assistance with installation and commissioning for the supporting infrastructure. India will provide the site, the vacuum system and other infrastructure required to house and operate the interferometer — as well as all labor, materials and supplies for installation.”

India’s LIGO observatory is expected to cost about US$177 million.  Full funding is expected in 2020 and the observatory currently is planned for completion in 2024.  India’s Inter-University Centre for Astronomy and Astrophysics (IUCAA), also in Maharashtra  state, will lead the project’s gravitational-wave science and the new detector’s data analysis.

For T. V. Padma’s complete article, refer to: 

https://www.nature.com/articles/d41586-019-00184-z.

Spatial resolution of gravitational wave events

Using only the two US LIGO detectors, it is not possible to localize the source of gravitational waves beyond a broad sweep through the sky.  On 1 August 2017, Virgo joined LIGO during the second Observation Run, O2. While the LIGO-Virgo three-detector network was operational for only three-and-a-half weeks, five gravitational wave events were observed.  As shown in the following figure, the spatial resolution of the source was greatly improved when a triple detection was made by the two LIGO observatories and Virgo. These events are labeled with the suffix “HLV”.  

Source:  http://www.virgo-gw.eu, 3 December 2018

The greatly reduced areas of the triple event localizations demonstrate the capabilities of the current global gravitational wave observatory network to resolve the source of a gravitational-wave detection.  The LIGO and Virgo Collaboration reports that it can send Open Public Alerts within five minutes of a gravitational wave detection.

With timely notification and more precise source location information, other land-based and space observatories can collaborate more rapidly and develop a comprehensive, multi-spectral (“multi-messenger”) view of the source of the gravitational waves.

When KAGRA and LIGO-India join the worldwide gravitational wave detection network, it is expected that source localizations will become 5 to 10 times more accurate than can be accomplished with just the LIGO and Virgo detectors.

For more background information on gravitational-wave detection, see the following Lyncean posts:

Linking Gravitational Wave Detection to the Rest of the Observable Spectrum

The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S. reported the first ever detection of gravitational waves on 14 September 2015 and, to date, has reported three confirmed detections of gravitational waves originating from black hole coalescence events. These events and their corresponding LIGO press releases are listed below.

  • GW150914, 14 September 2015

https://www.ligo.caltech.edu/page/press-release-gw150914

  • GW151226, 26 December 2015

https://www.ligo.caltech.edu/page/press-release-gw151226

  • GW170104, 4 January 2017

https://www.ligo.caltech.edu/page/press-release-gw170104

The following figure illustrates how these black hole coalescence events compare to our knowledge of the size of black holes based on X-ray observations. The LIGO team explained:

“LIGO has discovered a new population of black holes with masses that are larger than what had been seen before with X-ray studies alone (purple). The three confirmed detections by LIGO (GW150914, GW151226, GW170104), and one lower-confidence detection (LVT151012), point to a population of stellar-mass binary black holes that, once merged, are larger than 20 solar masses—larger than what was known before.”

Image credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

On 1 August 2017, the Advanced VIRGO detector at the European Gravitational Observatory (EGO) in Cascina, Italy (near Pisa) became operational, using wire suspensions for its interferometer mirrors instead of the fragile glass fiber suspensions that had been delaying startup of this detector.

On 17 August 2017, the LIGO – VIRGO team reported the detection of gravitational waves from a new source; a collision of two neutron stars. In comparison to black holes, neutron stars are low-mass objects, yet the neutron star collision was able to generate gravitational waves that were strong enough and in the detection frequency range of the LIGO and VIRGO. You’ll find the LIGO press release for that event, GW170817, at the following link.

https://www.ligo.caltech.edu/page/press-release-gw170817

The following figure from this press release illustrates the limits of localizing the source of a gravitational wave using the gravitational wave detectors themselves. The localization of GW180817 was much better than the previous gravitational wave detections because the detection was made by both LIGO and VIRGO, which have different views of the sky and a very long baseline, allowing coarse triangulation of the source.

Gravitational wave sky map. Credit__LIGO_Virgo_NASA_Leo_Singer__Axel_Melli

Unlike the previous gravitational wave detections from black hole coalescence, the neutron star collision that produced GW180817 also produced other observable phenomena. Gravitational waves were observed by LIGO and VIRGO, allowing coarse localization to about 31 square degrees in the sky and determination of the time of the event. The source of a two-second gamma ray burst observed at the same time by the Fermi and INTEGRAL gamma ray space telescopes (in Earth orbit) overlapped with the region of the sky identified by LIGO and VIRGO. An optical transient (the afterglow from the event) in that overlap region was first observed hours later by the 1 meter (40 inch) Swope Telescope on Cerro Las Campanas in Chile. The results of this localization process is shown below and is described in more detail in the following LIGO press release:

https://www.ligo.caltech.edu/news/ligo20171016

The sky map created by LIGO-Virgo (green) showing the possible location of the source of gravitational waves, compared with regions containing the location of the gamma ray burst source from Fermi (purple) and INTEGRAL (grey). The inset shows the actual position of the galaxy (orange star) containing the “optical transient” that resulted from the merger of two neutron stars (Credit: NASA/ESO)

The specific source initially was identified optically as a brilliant blue dot that appeared to be in a giant elliptical galaxy. A multi-spectral “afterglow” persisted at the source for several weeks, during which time the source became a dim red point if light. Many observatories were involved in detailed observations in the optical and infra-red ranges of the spectrum.

Important findings relate to the formation of large quantities of heavy elements (i.e., gold to uranium) in the aftermath of this event, which is known as a “kilonova.” This class of events likely plays an important role in seeding the universe with the heaviest elements, which are not formed in ordinary stars or novae. You’ll find more details on this matter in Lee Billing’s article, “Gravitational Wave Astronomers Hit the Mother Lode,” on the Scientific American website at the following link:

https://www.scientificamerican.com/article/gravitational-wave-astronomers-hit-mother-lode1/

The ability to localize gravitational wave sources will improve as additional gravitational wave detectors become operational and capabilities of existing detectors continue to be improved. The current status of worldwide gravitational wave detector deployment is shown in the following figure.

Source: LIGO

The ability to take advantage of “multi-messenger” (multi-spectral) observations will depend on the type of event and timely cueing of observatories worldwide and in orbit. The success of the GW170817 detection and subsequent multi-spectral observations of “kilonova” demonstrates the rich scientific potential for such coordinated observations

 

Perspective on the Detection of Gravitational Waves

On 14 September 2015, the U.S. Laser Interferometer Gravitational-Wave Observatory (LIGO) became the first observatory to detect gravitational waves. With two separate detector sites (Livingston, Louisiana, and Hanford, Washington) LIGO was able to define an area of space from which the gravitational waves, dubbed GW150914, are likely to have originated, but was not able to pinpoint the source of the waves. See my 11 February 2016 post, “NSF and LIGO Team Announce First Detection of Gravitational Waves,” for a summary of this milestone event.

You’ll find a good overview on the design and operation of LIGO and similar laser interferometer gravity wave detectors in the short (9:06) Veratisium video, “The Absurdity of Detecting Gravitational Waves,” at the following link:

https://www.youtube.com/watch?v=iphcyNWFD10

The LIGO team reports that the Advanced LIGO detector is optimized for “a range of frequencies from 30 Hz to several kHz, which covers the frequencies of gravitational waves emitted during the late inspiral, merger, and ringdown of stellar-mass binary black holes.”

First observing run (O1) of the Advanced LIGO detector

The LIGO team defines O1 as starting on 12 September 2015 and ending on 19 January 2016. During that period, the LIGO team reported that it had, “unambiguously identified two signals, GW150914 and GW151226, with a significance of greater than 5σ,” and also identified a third possible signal, LVT151012. The following figure shows the time evolution of the respective gravitational wave signals from when they enter the LIGO detectors’ sensitive band at 30 Hz.

LIGO GW signals screenshot

Source: B. P. Abbot et al., PHYS. REV. X 6, 041015 (2016)

The second detection of gravitational waves, GW151226, occurred on 26 December 2015. You’ll find the 16 June 2016 LIGO press release for this event at the following link:

https://www.ligo.caltech.edu/news/ligo20160615

At the following link, you can view a video showing a simulation of GW151226, starting at a frequency of 35 Hz and continuing through the last 55 gravitational-wave cycles before the binary black holes merge:

https://www.ligo.caltech.edu/video/ligo20160615v3

GW151226 simularion screenshotSource: Max Planck Institute for Gravitational Physics/ Simulating eXtreme Spacetime (SXS) project

In their GW151226 press release, the LIGO team goes out on a limb and makes the following estimate:

“….we can now start to estimate the rate of black hole coalescences in the Universe based not on theory, but on real observations. Of course with just a few signals, our estimate has big uncertainties, but our best right now is somewhere between 9 and 240 binary black hole coalescences per cubic Gigaparsec per year, or about one every 10 years in a volume a trillion times the volume of the Milky Way galaxy!”

More details on the GW151226 detection are available in the paper “GW151266: Observation of Gravitational Waves from a 22-Solar Mass Black Hole Coalescence,” at the following link:

https://dcc.ligo.org/public/0124/P151226/013/LIGO-P151226_Detection_of_GW151226.pdf

LIGO releases its data to the public. Analyses of the LIGO public data already are yielding puzzling results. In December 2016, researchers reported finding “echoes” in the gravitational wave signals detected by LIGO. If further analysis indicates that the “echoes” are real, they may indicate a breakdown of Einstein’s general theory of relativity at or near the “edge” of a black hole. You can read Zeeya Marali’s 9 December 2016 article, “LIGO black hole echoes hint at general relativity breakdown,” at the following link:

http://www.nature.com/news/ligo-black-hole-echoes-hint-at-general-relativity-breakdown-1.21135

Second observing run (O2) of the Advanced LIGO detector is in progress now

Following a 10-month period when they were off-line for modifications, the Advanced LIGO detectors returned to operation on 30 November 2016 with a 10% improvement in the sensitivity of their interferometers. The LIGO team intends to further improve this sensitivity by a factor of two during the next few years.

VIRGO will add the capability to triangulate the source of gravitational waves

In my 16 December 2015 post, “100th Anniversary of Einstein’s General Theory of Relativity and the Advent of a New Generation of Gravity Wave Detectors,” I reported on other international laser interferometer gravitational wave detectors. The LIGO team has established a close collaboration with their peers at the European Gravitational Observatory, which is located near Pisa, Italy. Their upgraded detector, VIRGO, in collaboration with the two LIGO detectors, is expected to provide the capability to triangulate gravitational wave sources. With better location information on the source of gravitational waves, other observatories can be promptly notified to join the search using other types of detectors (i.e., optical, infrared and radio telescopes).

VIRGO is expected to become operational in 2017, but technical problems, primarily with the mirror suspension system, may delay startup. You’ll find a 16 February 2017 article on the current status of VIRGO at the following link:

http://www.sciencemag.org/news/2017/02/european-gravitational-wave-detector-falters

Perspective on gravitational wave detection

Lyncean member Dave Groce recommends the excellent video of an interview of Caltech Professor Kip Thorne (one of the founders of LIGO) by “Einstein” biographer Walter Issacson. This 2 November 2016 video provides a great perspective on LIGO’s first detection of gravitational waves and on the development of gravitational wave detection capabilities. You’ll find this long (51:52) but very worthwhile video at the following link:

https://www.youtube.com/watch?v=mDFF27Nr-EU

Dr. Thorne noted that, at the extremely high sensitivity of the Advanced LIGO detectors, we are beginning to see the effects of quantum fluctuations in “human sized objects,” in particular, the 40 kg (88.2 pound) mirrors in the LIGO interferometers. In each mirror, the center of mass (the average position of all the mass in the mirror) fluctuates due to quantum physics at just the level of the Advanced LIGO noise.

In the interview, Dr. Thorne also discusses several new observatories that will be become available in the following decades to expand the spectrum of gravitational waves that can be detected. These are shown in the following diagram.

Spectrum for gravitational wave detection screenshotSource: screenshot from Kip Thorne / Walter Issacson interview

  •  LISA = Laser Interferometer Space Antenna
  • PTA = Pulsar Timing Array
  • CMB = Cosmic microwave background

See my 27 September 2016 post, “Space-based Gravity Wave Detection System to be Deployed by ESA,” for additional information on LISA.

Clearly, we’re just at the dawn of gravitational wave detection and analysis. With the advent of new and upgraded gravitational wave observatories during the next decade, there will be tremendous challenges to align theories with real data.   Through this process, we’ll get a much better understanding of our Universe.