Tag Archives: gravitational wave

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


  • GW151226, 26 December 2015


  • GW170104, 4 January 2017


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.


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:


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:


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:


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:


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:


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:


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:


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:


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:


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.