IOP Materials and Characterisation Christmas Lecture 2019

Ripples of Gravity, Flashes of Light: The Dawn of Multi-Messenger Astrophysics
Prof Martin Hendry, University of Glasgow, UK


The first ever direct detection of gravitational waves by the LIGO and Virgo Scientific Collaborations, from the collision of two massive black holes more than a billion light years away, was widely hailed as the biggest scientific breakthrough of the decade and led to the award of the 2017 Nobel Prize for Physics to three senior LIGO scientists and pioneers: Rai Weiss, Kip Thorne and Barry Barish. Since then, LIGO and Virgo have made many more spectacular discoveries – including the first ever joint detection of gravitational waves and light from the same cosmic source: a pair of colliding neutron stars 130 million light years distant. Join LIGO scientist Professor Martin Hendry as he explores the amazing technology behind the detection of gravitational waves, and what their discovery is telling us about some of the biggest unsolved mysteries in physics and astronomy.


Martin Hendry is Professor of Gravitational Astrophysics and Cosmology at the University of Glasgow, where he is currently Head of the School of Physics and Astronomy. He is a senior member of the LIGO Scientific Collaboration (LSC), for which he chairs the LSC Education and Public Outreach Group.  Martin is a Fellow of the Royal Society of Edinburgh and the Institute of Physics, and was awarded the MBE for his services to the public understanding of science.

My notes from the talk (if they don’t make sense then it is entirely my fault)

Cosmology is interested in how big the universe is, how it began and why it is expanding

Gravitational wave astronomy is interested in what black holes and neutron stars are, what happens when these collide and why Einstein’s picture of gravity is correct.



Albert Einstein was the editor’s choice for Time magazine person of the 20th century.,9171,993017,00.html

Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics). His work is also known for its influence on the philosophy of science. He is best known to the general public for his mass–energy equivalence formula E= mc2, which has been dubbed “the world’s most famous equation”. He received the 1921 Nobel Prize in Physics “for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect”, a pivotal step in the development of quantum theory.

Gravity in Einstein’s universe


Gravity is a property of matter and spacetime.

Simply put, a mass of an object causes the space around it to essentially bend and curve. This is often portrayed as a heavy ball sitting on a rubber sheet, and other smaller balls fall in towards the heavier object because the rubber sheet is warped from the heavy ball’s weight.

You can demonstrate this with a taut rubber sheet and a large and small ball.


Or you can demonstrate this with the above arrangement, pulling down the centre of the cloth instead of using a ball. By pulling the cloth down by different amounts you can mimic objects of different masses.


A 2-dimensional image of how gravity works. Via NASA’s Space Place

In reality, we can’t see curvature of space directly, but we can detect it in the motions of objects. Any object ‘caught’ in another celestial body’s gravity is affected because the space it is moving through is curved toward that object.


Gravity waves are ripples in space and time caused by changing gravitational fields.


Artist’s impression a neutron star merger and the gravitational waves it creates. (Credit: NASA/Goddard Space Flight Center)

Simulations of two neutron stars rotating around each other

The most powerful gravitational waves are created when objects move at very high speeds. Some examples of events that could cause a gravitational wave are:

  • when a star explodes asymmetrically (called a supernova)
  • when two big stars orbit each other
  • when two black holes orbit each other and merge

In other words, gravitational waves are produced when very massive objects move very quickly,

The types of objects that create gravitational waves are far away. And sometimes, these events only cause small, weak gravitational waves. The waves are then very weak by the time they reach Earth. This makes gravitational waves hard to detect.

Gravitational waves were first detected 100 years after Einstein’s prediction.

In 2015, scientists detected gravitational waves for the very first time. They used a very sensitive instrument called LIGO (Laser Interferometer Gravitational-Wave Observatory). These first gravitational waves happened when two black holes crashed into one another. The collision happened 1.3 billion years ago. But the ripples didn’t make it to Earth until 2015!

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. These can detect a change in the 4 km mirror spacing of less than a ten-thousandth the charge diameter of a proton.

The initial LIGO observatories were funded by the National Science Foundation (NSF) and were conceived, built and are operated by Caltech and MIT. They collected data from 2002 to 2010 but no gravitational waves were detected.

LIGO is the largest and most ambitious project ever funded by the NSF. In 2017, the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry C. Barish “for decisive contributions to the LIGO detector and the observation of gravitational waves.” (bottom left)

Rainer (born September 29, 1932) is an American physicist, known for his contributions in gravitational physics and astrophysics. He is a professor of physics emeritus at MIT and an adjunct professor at LSU. He is best known for inventing the laser interferometric technique which is the basic operation of LIGO. He was Chair of the COBE Science Working Group.

He is a member of Fermilab Holometer experiment, which uses a 40m laser interferometer to measure properties of space and time at quantum scale and provide Planck-precision tests of quantum holographic fluctuation.

image (above centre)

Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist and Nobel laureate, known for his contributions in gravitational physics and astrophysics. A longtime friend and colleague of Stephen Hawking and Carl Sagan, he was the Feynman Professor of Theoretical Physics at the California Institute of Technology (Caltech) until 2009 and is one of the world’s leading experts on the astrophysical implications of Einstein’s general theory of relativity. He continues to do scientific research and scientific consulting, most notably for the Christopher Nolan film Interstellar. (above right)


Barry Clark Barish (born January 27, 1936) is an American experimental physicist and Nobel Laureate. He is a Linde Professor of Physics, emeritus at California Institute of Technology. He is a leading expert on gravitational waves.

At Glasgow University, Prof. Sir James Hough was also involved


Sir James Hough OBE FRS FRSE FInstP FRAS (born 6 August 1945) is a British physicist and an international leader in the search for gravitational waves.

Gravitational waves are like ripples spreading out on a pond. By the time they reach the Earth they are very weak. They are an invisible (yet incredibly fast) ripple in space. Gravitational waves travel at the speed of light (300 million metres per second). These waves squeeze and stretch anything in their path as they pass by.

Gravitational waves passing through the Earth have an amplitude of about 10−20m.

LIGO is a km scale interferometer. There are two sites.

LIGO operates two gravitational wave observatories in unison: the LIGO Livingston Observatory (30°33′46.42″N 90°46′27.27″W) in Livingston, Louisiana, and the LIGO Hanford Observatory, on the DOE Hanford Site (46°27′18.52″N 119°24′27.56″W), located near Richland, Washington. These sites are separated by 3,002 kilometres straight line distance through the earth, but 3,030 kilometres over the surface. Since gravitational waves are expected to travel at the speed of light, this distance corresponds to a difference in gravitational-wave arrival times of up to ten milliseconds. Through the use of trilateration, the difference in arrival times helps to determine the source of the wave, especially when a third similar instrument like Virgo, located at an even greater distance in Europe, is added.

Each observatory supports an L-shaped ultra-high vacuum system, measuring 4 kilometres on each side. Up to five interferometers can be set up in each vacuum system.

The LIGO Livingston Observatory houses one laser interferometer in the primary configuration. This interferometer was successfully upgraded in 2004 with an active vibration isolation system based on hydraulic actuators providing a factor of 10 isolation in the 0.1–5 Hz band. Seismic vibration in this band is chiefly due to microseismic waves and anthropogenic sources (traffic, logging, etc.).

The LIGO Hanford Observatory houses one interferometer, almost identical to the one at the Livingston Observatory. During the Initial and Enhanced LIGO phases, a half-length interferometer operated in parallel with the main interferometer. For this 2 km interferometer, the Fabry–Pérot arm cavities had the same optical finesse, and, thus, half the storage time as the 4 km interferometers. With half the storage time, the theoretical strain sensitivity was as good as the full-length interferometers above 200 Hz but only half as good at low frequencies. During the same era, Hanford retained its original passive seismic isolation system due to limited geologic activity in Southeastern Washington.


Most Precise Ruler Ever Constructed

Animation created by T. Pyle, Caltech/MIT/LIGO Lab

This animation illustrates how the twin observatories of LIGO work. One observatory is in Hanford, Washington, the other in Livingston, Louisiana. Each houses a large-scale interferometer, a device that uses the interference of two beams of laser light to make the most precise distance measurements in the world.

The animation begins with a simplified depiction of the LIGO instrument. A laser beam of light is generated and directed toward a beam splitter, which splits it into two separate and equal beams. The light beams then travel perpendicularly to a distant mirror, with each arm of the device being 4 kilometres in length. The mirrors reflect the light back to the beam splitter, repeating this process 200 times.

When gravitational waves pass through this device, they cause the length of the two arms to alternately stretch and squeeze by infinitesimal amounts, tremendously exaggerated here for visibility. This movement causes the light beam that hits the detector to flicker.

The second half of the animation explains the flickering, and this is where light interference comes into play. After the two beams reflect off the mirrors, they meet at the beam splitter, where the light is recombined in a process called interference. Normally, when no gravitational waves are present, the distance between the beam splitter and the mirror is precisely controlled so that the light waves are kept out of phase with each other and cancel each other out. The result is that no light hits the detectors.

But when gravitational waves pass through the system, the distance between the end mirrors and the beam splitter lengthen in one arm and at the same time shorten in the other arm in such a way that the light waves from the two arms go in and out of phase with each other. When the light waves are in phase with each other, they add together constructively and produce a bright beam that illuminates the detectors. When they are out of phase, they cancel each other out and there is no signal. Thus, the gravitational waves from a major cosmic event, like the merger of two black holes, will cause the signal to flicker.

The effects of the gravitational waves on the LIGO instrument have been vastly exaggerated in this video to demonstrate how it works. In reality, the changes in the lengths of the instrument’s arms are only 1/1000th the size of a proton. Other characteristics of LIGO, such as the exquisite stability of its mirrors, also contribute to its ability to precisely measures distances. In fact, LIGO can be thought of as the most precise “ruler” in the world.


LIGO in your hands


Dr Borja Sorazu (University of Glasgow) and project student Matthew Wassell have developed a new portable interactive #STEM #Outreach demonstration: LIGO in your hands. It’s a scale model of Gravitational Wave detectors.



Some key results of our analysis of GW150914, comparing the reconstructed gravitational-wave strain (as seen by H1 at Hanford) with the predictions of the best-matching waveform computed from general relativity, over the three stages of the event: inspiral, merger and ringdown. Also shown are the separation and velocity of the black holes, and how they change as the merger event unfolds. Credit: LIGO.

We can follow the process of the gravitational wave forming.

Inspiral phase

The first stage of the life of a binary black hole is the inspiral, a gradually shrinking orbit. The first stages of the inspiral take a very long time, as the gravitation waves emitted are very weak when the black holes are distant from each other. In addition to the orbit shrinking due to the emission of gravitational waves, extra angular momentum may be lost due to interactions with other matter present, such as other stars.

As the black holes’ orbit shrinks, the speed increases, and gravitational wave emission increases. When the black holes are close the gravitational waves cause the orbit to shrink rapidly.

The last stable orbit or innermost stable circular orbit (ISCO) is the innermost complete orbit before the transition from inspiral to merger.


An example signal from an inspiral gravitational wave source. [Image: A. Stuver/LIGO]

Inspiral gravitational waves are generated during the end-of-life stage of binary systems where the two objects merge into one. These systems are usually two neutron stars, two black holes, or a neutron star and a black hole whose orbits have degraded to the point that the two masses are about to coalesce. As the two masses rotate around each other, their orbital distances decrease and their speeds increase, much like a spinning figure skater who draws his or her arms in close to their body. This causes the frequency of the gravitational waves to increase until the moment of coalescence. The sound these gravitational waves would produce is a chirp sound (much like when increasing the pitch rapidly on a slide whistle) since the binary system’s orbital frequency is increasing (any increase in frequency corresponds to an increase in pitch).

Merger phase

This is followed by a plunging orbit in which the two black holes meet, followed by the merger. Gravitational-wave emission peaks at this time.


Immediately following the merger, the now single black hole will “ring” – oscillating in shape between a distorted, elongated spheroid and a flattened spheroid. This ringing is damped in the next stage, called the ringdown, by the emission of gravitational waves. The distortions from the spherical shape rapidly reduce until the final stable sphere is present, with a possible slight distortion due to remaining spin.

Why does LIGO need two sites?

This enables the observer to recognise a gravitational wave if both sites “see it”.

Computer simulation of the black hole binary system GW150914 as seen by a nearby observer, during its final inspiral, merge, and ringdown. The starfield behind the black holes is being heavily distorted and appears to rotate and move, due to extreme gravitational lensing, as space-time itself is distorted and dragged around by the rotating black holes.

The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016. Previously, gravitational waves had only been inferred indirectly, via their effect on the timing of pulsars in binary star systems. The waveform, detected by both LIGO observatories, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of a pair of black holes of around 36 and 29 solar masses and the subsequent “ringdown” of the single resulting black hole. The signal was named GW150914 (from “Gravitational Wave” and the date of observation 2015-09-14). It was also the first observation of a binary black hole merger, demonstrating both the existence of binary stellar-mass black hole systems and the fact that such mergers could occur within the current age of the universe.

The initial LIGO observatories were funded by the National Science Foundation (NSF) and were conceived, built and are operated by Caltech and MIT. They collected data from 2002 to 2010 but no gravitational waves were detected.

This was followed by a multi-year shut-down while the detectors were replaced by much improved “Advanced LIGO” versions. Much of the research and development work for the LIGO/aLIGO machines was based on pioneering work for the GEO600 detector at Hannover, Germany. By February 2015, the detectors were brought into engineering mode in both locations.

GEO600 is a gravitational wave detector located near Sarstedt in the South of Hanover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics, Max Planck Institute of Quantum Optics and the Leibniz Universität Hannover, along with University of Glasgow, University of Birmingham and Cardiff University in the United Kingdom, and is funded by the Max Planck Society and the Science and Technology Facilities Council (STFC). GEO600 is part of a worldwide network of gravitational wave detectors. This instrument, and its sister interferometric detectors, when operational, are some of the most sensitive gravitational wave detectors ever designed. They are designed to detect relative changes in distance of the order of 10−21, about the size of a single atom compared to the distance from the Sun to the Earth. GEO600 is capable of detecting gravitational waves in the frequency range 50 Hz to 1.5 kHz. Construction on the project began in 1995.

Advanced LIGO

By the time the LIGO Laboratory started the first observing run ‘O1’ with the Advanced LIGO detectors in September 2015, the LIGO Scientific Collaboration included more than 900 scientists worldwide.

Simplified diagram of an Advanced LIGO detector (not to scale).

Abbott, B. P. et al. – Observation of Gravitational Waves from a Binary Black Hole Merger B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration) Phys. Rev. Lett. 116, 061102 doi:10.1103/PhysRevLett.116.061102

Simplified diagram of an Advanced LIGO detector (not to scale). A gravitational wave propagating orthogonally to the detector plane and linearly polarized parallel to the 4-km optical cavities will have the effect of lengthening one 4-km arm and shortening the other during one half-cycle of the wave; these length changes are reversed during the other half-cycle. The output photodetector records these differential cavity length variations. While a detector’s directional response is maximal for this case, it is still significant for most other angles of incidence or polarizations (gravitational waves propagate freely through the Earth). Inset (a): Location and orientation of the LIGO detectors at Hanford, WA (H1) and Livingston, LA (L1). Inset (b): The instrument noise for each detector near the time of the signal detection; this is an amplitude spectral density, expressed in terms of equivalent gravitational-wave strain amplitude. The sensitivity is limited by photon shot noise at frequencies above 150 Hz, and by a superposition of other noise sources at lower frequencies. Narrow-band features include calibration lines (33–38, 330, and 1080 Hz), vibrational modes of suspension fibres (500 Hz and harmonics), and 60 Hz electric power grid harmonics.


The first observing advanced LIGO run operated at a sensitivity roughly 3 times greater than Initial LIGO, and a much greater sensitivity for larger systems with their peak radiation at lower audio frequencies.

On 11 February 2016, the LIGO and Virgo collaborations announced the first observation of gravitational waves. The signal was named GW150914. The waveform showed up on 14 September 2015, within just two days of when the Advanced LIGO detectors started collecting data after their upgrade. It matched the predictions of general relativity for the inward spiral and merger of a pair of black holes and subsequent ringdown of the resulting single black hole. The observations demonstrated the existence of binary stellar-mass black hole systems and the first observation of a binary black hole merger.

On 15 June 2016, LIGO announced the detection of a second gravitational wave event, recorded on 26 December 2015, at 3:38 UTC. Analysis of the observed signal indicated that the event was caused by the merger of two black holes with masses of 14.2 and 7.5 solar masses, at a distance of 1.4 billion light-years.[74] The signal was named GW151226.

The second observing run (O2) ran from 30 November 2016 to 25 August 2017, with Livingston achieving 15–25% sensitivity improvement over O1, and with Hanford’s sensitivity similar to O1. In this period, LIGO saw several further gravitational wave events: GW170104 in January; GW170608 in June; and five others between July and August 2017. Several of these were also detected by the Virgo Collaboration. Unlike the black hole mergers which are only detectable gravitationally, GW170817 came from the collision of two neutron stars and was also detected electromagnetically by gamma-ray satellites and optical telescopes.

The third run (O3) began on 1 April 2019 and is planned to last one year. Future observing runs will be interleaved with commissioning efforts to further improve the sensitivity. It is aimed to achieve design sensitivity in 2021.


Above left is LIGO Hansford and above right is LIGO Livingston

Noise is a big problem so much of the design of the equipment is to reduce the effect of noise.

Design sensitivity curve

Updated Advanced LIGO noise curve


The above figure shows the effect of the different noise sources for the upgraded LIGO detector


The second observing run of Advanced LIGO, and the first observing run of Advanced Virgo, which joined O2 on the 1st of August, 2017.

The release includes over 150 days of recorded data from each of the two LIGO observatories, as well as 20 days of recorded data from Virgo, making this the largest data set of ‘advanced’ gravitational-wave detectors to date. Observations in O2 include seven binary black hole mergers, as well as the first binary neutron star merger observed in gravitational waves, all recently published in the GWTC-1 catalogue. Along with the strain data, the release contains detailed documentation and links to open-source software tools.

The figure above shows the sensitivity achieved during O2 of the three detectors in the network.

Seismic noise occurs at the low-frequency range

Thermal noise of the mirrors occurs at the middle-frequency range

Photon/quantum noise effect occurs at the high-frequency range

In physics and engineering, the quality factor or Q factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the peak energy stored in the resonator in a cycle of oscillation to the energy lost per radian of the cycle. Q factor is alternatively defined as the ratio of a resonator’s centre frequency to its bandwidth when subject to an oscillating driving force. These two definitions give numerically similar, but not identical, results. Higher Q indicates a lower rate of energy loss and the oscillations die out more slowly. A pendulum suspended from a high-quality bearing, oscillating in air, has a high Q, while a pendulum immersed in oil has a low one. Resonators with high-quality factors have low damping so that they ring or vibrate longer.

By using high-Q materials noise can be reduced.

Seismic noise

One of the major obstacles to the detection and study of gravitational waves using ground-based laser interferometers is the effect of seismic noise on instrument sensitivity. Environmental disturbances cause motion of the interferometer optics, coupling as noise in the gravitational wave data output whose magnitude can be much greater than that of an astrophysical signal.

Due to the motion of the mirrors from ground vibrations, earthquakes, wind, ocean waves, and human activities such as vehicle traffic.

To isolate the LIGO detector’s core optics from seismic jitters, multiple layers of springs, actuators, and pendulums counteract vibrations and dissipate seismic noise. Seismic isolation begins with a spring-mounted framework resting on the ground. On it stands the second stage: a double-decker platform, with each deck suspended from springs and other controls. This stage 2 also includes three broadband seismometers and six geophones for monitoring seismic noise.

From the centre of stage 2 hang the mirrors that reflect the lasers used to detect changes in the lengths of the arms of LIGO. The mirrors dangle from the end of a quadruple pendulum, which, as its name implies, hangs in turn from a second pendulum hanging from a third pendulum, all of which are suspended from the second stage platform. Add up all those layers and pendulums, integrate them with computer controls, and you get seven stages of isolation of LIGO’s optics from the Earth’s tremors. That knocks those jitters down by a factor of more than a billion, explained Stanford University’s Brian Lantz, lead scientist for the Advanced LIGO Seismic Isolation subsystem.

Internal Seismic Isolation (ISI)


Made of two stages, and has suspension through stiff blade springs and short pendulum links

Use low noise inertial sensors and provides low-frequency active isolation (0.1 Hz)

Attenuates seismic motion above 10Hz

These also position the optics in the vacuum chambers.

The vibration of each stage is reduced by sensing its motion in 6 degrees of freedom (up/down/left/right/yaw/pitch/roll) and applying forces in feedback loops which reduces motion

Seismic noise, if not corrected, limits sensitivity to below 10Hz



The noise equivalent detector sensitivity is shown in displacement together with identified noise sources that were limiting the sensitivity. The frequency noise was suppressed well and optical shot noise was the dominant contribution in the higher frequency region above 1 kHz. In the lower frequency region below 100 Hz, seismic noise in the vertical direction which couples through the suspension system exceeded direct horizontal seismic noise and had a significant influence on the detector sensitivity.

Reduce seismic noise: Advanced (active) seismic isolation. Seismic wall moved from 40 Hz to ~ 12 Hz.

Reduce seismic and suspension noise: Quadruple pendulum suspensions to filter environmental noise in stages.

Reduce suspension noise: Fused silica fibres, silica welds

Quantum noise

Due to the discrete nature of light (composed of photons) and the statistical uncertainty from the “photon counting” that is performed by the photodetectors.

The system is so sensitive that can pick up quantum fluctuations in empty space. A central property of quantum systems is that they can never be completely pinned down. It’s part of Heisenberg’s Uncertainty Principle. This is true even for a vacuum. This means quantum fluctuations appear within the vacuum. As photons of light travel through these fluctuations, they are jostled a bit. This makes the beams of light move slightly out of phase. Imagine a fleet of small boats sailing across a rough sea, and how difficult it would be to keep them together.


A close up of LIGO’s quantum squeezer. Credit: Maggie Tse

But quantum uncertainty is a funny thing. Although aspects of a quantum system will always be uncertain, parts of it can be extremely precise. The catch is that if you make one part more precise another part becomes less precise. For light, this means you can keep the phase of the beam more aligned by making the brightness of the light more uncertain. This is known as squeezed light because you squeeze one uncertainty smaller at the cost of another.

Animation showing a squeezed state of light. Credit: Wikipedia user Geek3

This squeezed state of light is done through an optical parametric oscillator. It’s basically a set of mirrors around a special kind of crystal. When the light passes through the crystal, it minimizes the fluctuations in phase. The fluctuations in amplitude get larger, but it’s the phase that matters most to the LIGO detectors.

With this upgrade, the sensitivity of LIGO should double. This will help astronomers see black hole mergers more clearly. It could also allow LIGO to see new kinds of mergers. Ones that are fainter or farther away than we’ve ever seen before.

Source: New Instrument extends LIGO’s reach, MIT News.

Thermal noise

From the microscopic fluctuations of the individual atoms in the mirrors and their suspensions.

Atoms are in continuous motion even at low temperatures.

Reduce test mass thermal noise: Last pendulum stage (test mass) is controlled via electrostatic or photonic forces (no magnets).

Reduce test mass thermal noise: High-Q material (40 kg sapphire).

To handle thermal distortions due to beam heating: advanced mirror materials, coatings, thermal de-lensing compensation (heating mirror at edges)

Fluctuations in the mirror coating are due to mechanical loss in which vibrations result in heat generation, causing thermal fluctuations. The noise from the fluctuations interferes with measuring the gravity waves. For the current systems, the level of noise is acceptable, but it must be reduced significantly for more sensitive detectors.

To reduce the noise in the mirror coating, the researchers replaced the fused silica and tantalum oxide currently used with hafnium oxide and amorphous silicon, respectively. Testing showed the replacements to be 25 times less noisy than the present coatings—enough for use on the Einstein Telescope.

The first fused silica mirror suspensions for the Advanced LIGO gravitational wave detectors have been installed at the LIGO Hanford and Livingston sites. These quadruple pendulums use synthetic fused silica fibres produced using a CO2 laser pulling machine to reduce thermal noise in the final suspension stage. The suspension thermal noise in Advanced LIGO is predicted to be limited by internal damping in the surface layer of the fibres, damping in the weld regions, and the strength of the fibres.


Left: Fused silica pulling machine installed at LIGO Hanford. Right: A typical Advanced LIGO fibre.

Close-up of fused silica glass fibres attached to one of LIGO’s primary optics. The bottom of the photo shows the glass welds binding the fibres to the optic. The fibres taper to 0.4 mm. (Caltech/MIT/LIGO Lab)


Glasgow university commissioned, built and installed the fused silica suspensions


The LIGO test masses are suspended in a four-stage pendulum that uses active and passive elements to isolate the masses from ground vibration. The first three pendulums are suspended with metal chains. The last one uses fused silica fibres to help eliminate thermal noise. Image Credit: Caltech/MIT/LIGO Lab.

LIGO employs a passive/active system to hang the test masses by an assemblage of pendulums. Each pendulum has a different length, and so a different frequency of oscillation, and they are stacked, one below the other, into four stages. The test masses holding the interferometer mirrors are suspended on the lowest level. The setup keeps the test masses stable while allowing them to be in effect free-fall along the axes of the laser beams.

On February 11, 2016, the LIGO Scientific Collaboration and Virgo Collaboration announced the first confirmed observation of gravitational waves from colliding black holes. The gravitational-wave signals were observed by the LIGO’s twin observatories on September 14, 2015. This confirms a key prediction of Einstein’s theory of general relativity and provides the first direct evidence that black holes merge.

The inspiral, merger, ringdown was half the speed of light.

Simulation of GW170104

This animation shows the inspiral and merger of two black holes with masses and spins consistent with the GW170104 observation.

The top part of the movie shows the black hole horizons (surfaces of “no return”). The initial two black holes orbit each other, until they merge and form one larger remnant black hole. The shown black holes are spinning, and angular momentum is exchanged among the two black holes and with the orbit. This results in a quite dramatic change in the orientation of the orbital plane, clearly visible in the movie. Furthermore, the spin-axes of the black holes change, as visible through the coloured patch on each black hole horizon, which indicates the north pole.

The lower part of the movie shows the two distinct gravitational waves (called ‘polarizations’) that the merger is emitting into the direction of the camera. The modulations of the polarizations depend sensitively on the orientation of the orbital plane and thus encode information about the orientation of the orbital plane and its change during the inspiral. Presently, LIGO can only measure one of the polarizations and therefore obtains only limited information about the orientation of the binary. This disadvantage will be remedied with the advent of additional gravitational wave detectors in Italy, Japan and India.

Finally, the slowed-down replay of the merger at the end of the movie makes it possible to observe the distortion of the newly formed remnant black hole, which decays quickly. Furthermore, the remnant black hole is “kicked” by the emitted gravitational waves, and moves upward.

Simulation of the binary black-hole coalescence GW170104

Numerical simulation of a black-hole merger consistent with LIGO’s GW170104 observation. The strength of the gravitational wave is indicated by the elevation of the bands, as well as colour, with blue indicating weak fields and yellow, strong fields. The amplitude of the gravitational wave is rescaled during the simulation to show the signal during the entire animation. The sizes of the black holes are increased by a factor of two. The bottom panel in the video shows the gravitational waveform. Simulation Credit: S. Ossokine/A. Buonanno/T. Dietrich (MPI for Gravitational Physics)/R. Haas (NCSA)/SXS project

Analysis of the signal of GW150914 along with the inferred redshift suggested that it was produced by the merger of two black holes with masses of 35 times and 30 times the mass of the Sun (in the source frame), resulting in a post-merger black hole of 62 solar masses. The mass-energy of the missing 3.0 solar masses was radiated away in the form of gravitational waves. Einstein’s famous formula E = mc2 applies. In this case no light was produced but the energy released is equivalent to 50 times the luminosity of every star in the universe.

In physics, spacetime is any mathematical model which fuses the three dimensions of space and the one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why different observers perceive where and when events occur differently.

Until the 20th century, it was assumed that the three-dimensional geometry of the universe (its spatial expression in terms of coordinates, distances, and directions) was independent of one-dimensional time. However, in 1905, Albert Einstein based his seminal work on special relativity on two postulates:

The laws of physics are invariant (i.e., identical) in all inertial systems (i.e., non-accelerating frames of reference)

The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.

Space-time is unbelievably stiff and the creation of black holes are believed to break this.

Computer simulation showing the warping of space and time around two colliding black holes observed by LIGO on September 14, 2015. LIGO detected gravitational waves generated by this black hole merger—humanity’s first contact with gravitational waves and black-hole collisions. Gravitational waves are ripples in the shape of space and flow of time.

It takes a HUGE amount of stress on space-time to produce an appreciable amount of warp or curvature (‘G’). In fact, it takes objects like the Earth (all 6 trillion trillion kilograms of it) to warp space-time to a level that we’re intimately familiar with.

To produce enough warp to create an object like a black hole – with an extreme, maximal amount of space-time curvature – the universe has to concentrate mass and energy to an extraordinary degree. In other words, an immense amount of stress has to be created. For example, producing an Earth-mass black hole would involve squishing all those kilograms into a region roughly the size of a small coin to generate enough local stress. It’s analogous to how the fine point of a nail concentrates enough force to break wood fibres.

It turns out that space-time is very stiff, very resilient. But it can, and does, yield to stress. That’s fortunate, because without a little bit of warping there’d be no stars or planets, and we’d not be here to celebrate Einstein’s wonderful insights.

How LIGO got the word out about gravitational waves


the relevant hashtags got 70 million aggregate impressions,

The image below shows the signal observed by each LIGO detector. These plots show the signals of gravitational waves detected by the twin LIGO observatories at Livingston, Louisiana, and Hanford, Washington. The signals came from two merging black holes, each about 30 times the mass of our sun, lying 1.3 billion light-years away.

The top two plots show data received at Livingston and Hanford, along with the predicted shapes for the waveform. These predicted waveforms show what two merging black holes should look like according to the equations of Albert Einstein’s general theory of relativity, along with the instrument’s ever-present noise. Time is plotted on the X-axis and strain on the Y-axis. Strain represents the fractional amount by which distances are distorted. As the plots reveal, the LIGO data very closely match Einstein’s predictions.

The final plot compares data from both detectors. The Hanford data have been inverted for comparison, due to the differences in orientation of the detectors at the two sites. The data were also shifted to correct for the travel time of the gravitational-wave signals between Livingston and Hanford (the signal first reached Livingston, and then, travelling at the speed of light, reached Hanford seven-thousandths of a second later). As the plot demonstrates, both detectors witnessed the same event, confirming the detection. (Image Credit: Caltech/MIT/LIGO Lab.)


The image below shows the approximate location of GW150914 on a sky map of the southern hemisphere. The coloured lines represent different probabilities for where the signal originated: the purple line defines the region where the signal is predicted to have come from with a 90 per cent confidence level; the inner yellow line defines the target region at a 10 per cent confidence level. The gravitational waves were produced by a pair of merging black holes located 1.3 billion light-years away.

A small galaxy near our own, called the Large Magellanic Cloud, can be seen as a fuzzy blob underneath the marked area, while an even smaller galaxy, called the Small Magellanic Cloud, is below it.

Researchers were able to home in on the location of the gravitational-wave source using data from the LIGO observatories in Livingston, Louisiana, and Hanford, Washington. The gravitational waves arrived at Livingston 7 milliseconds before arriving at Hanford. This time delay revealed a particular slice of sky, or ring, from which the signal must have arisen. Further analysis of the varying signal strength at both detectors ruled out portions of the ring, leaving the remaining patch shown on this map. (Image credit: LIGO/Axel Mellinger.) Trigonometry was used in the analysis.


The facility was able to pinpoint the location of the event to within a 600 square degree area of the sky. (The full moon takes up about half a degree on the sky.)

LIGO researchers were awarded a Special Breakthrough Prize in Fundamental Physics in 2016. The team that detected gravitational waves shared the $3 million prize.

The first joint catch by LIGO and VIRGO: another black hole merger detected on the 14th August 2017. It was the fourth detection of a merging binary black hole system. Three such events were detected by the twin LIGO detectors previously (first two events in 2015 and the third one in January 2017). It was the first time that such a detection was being confirmed by a third detector. The Virgo detector started collecting data on 1 August 2017, and was soon bestowed with a detection, jointly with LIGO.


The Virgo interferometer is a large interferometer designed to detect gravitational waves predicted by the general theory of relativity. Virgo is a Michelson interferometer that is isolated from external disturbances: its mirrors and instrumentation are suspended and its laser beam operates in a vacuum. The instrument’s two arms are three kilometres long and located in Santo Stefano a Macerata, near the city of Pisa, Italy.

The first detection of gravitational waves by Virgo is known as GW170814, which was announced on 27 September 2017 in a G7 science meeting conference in Turin, Italy.

The figure below shows the localizations of some of the gravitational wave detections; most were made only by the two LIGO antennae, but the two most recent, GW 170814 and GW170817, include VIRGO as well.


On December 1, 2018, the LIGO Scientific Collaboration and the Virgo Collaboration announced the full results of their searches for gravitational-waves from stellar-mass coalescing compact binaries with an advanced detector network. In addition to the six previously announced binary black hole and single binary neutron star detections, this includes four new binary black hole mergers: GW170729, GW170809, GW170818, and GW170823.

Spectrograms and waveforms for the gravitational-wave transient catalogue. The insets of this image below show the gravitational waveform (bottom) and time-frequency spectrogram for each confident detection in the gravitational-wave transient catalogue. The spectrogram colour indicates a measure of signal strength; the increase of signal frequency with time clearly shows the characteristic “chirp” signature of a binary inspiral. The waveforms shown at the bottom of each panel are produced from either a range of models based on Einstein’s theory (orange) or a wavelet decomposition of the observed signal (grey).


LIGO/Virgo Binary-Black-Hole Orrery. A visualization of the merging black holes that LIGO and Virgo have observed so far.

The evidence from gravitational waves shows that relativity really does fit.


Cataclysmic Collision Artist’s illustration of two merging neutron stars. The rippling space-time grid represents gravitational waves that travel out from the collision, while the narrow beams show the bursts of gamma rays that are shot out just seconds after the gravitational waves. Swirling clouds of material ejected from the merging stars are also depicted. The clouds glow with visible and other wavelengths of light.  Image credit: NSF/LIGO/Sonoma State University/A. Simonnet

GW170817 Press Release

LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars

Discovery marks first cosmic event observed in both gravitational waves and light.

Neutron star merger (Credit: Christopher W. Evans/Georgia Tech)

GW 170817 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, originating from the shell elliptical galaxy NGC 4993. The GW was produced by the last minutes of two neutron stars spiralling closer to each other and finally merging, and is the first GW observation which has been confirmed by non-gravitational means. Unlike the five previous GW detections, which were of merging black holes not expected to produce a detectable electromagnetic signal, the aftermath of this merger was also seen by 70 observatories on 7 continents and in space, across the electromagnetic spectrum, marking a significant breakthrough for multi-messenger astronomy. The discovery and subsequent observations of GW 170817 were given the Breakthrough of the Year award for 2017 by the journal Science.

The first electromagnetic signal detected was GRB 170817A, a short gamma-ray burst, detected 1.74±0.05 s after the merger time and lasting for about 2 seconds.

GRB 170817A was discovered by the Fermi Gamma-ray Space Telescope, with an automatic alert issued just 14 seconds after the GRB detection. After the LIGO/Virgo circular 40 minutes later, manual processing of data from the INTEGRAL gamma-ray telescope also detected the same GRB. The difference in arrival time between Fermi and INTEGRAL helped to improve the sky localization.

This GRB was relatively faint given the proximity of the host galaxy NGC 4993, possibly due to its jets not being pointed directly toward Earth, but rather at an angle of about 30 degrees to the side.

The Fermi Gamma-ray Space Telescope (FGST), formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope (LAT), with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor (GBM; formerly GLAST Burst Monitor), is being used to study gamma-ray bursts.

Fermi was launched on 11 June 2008 at 16:05 UTC aboard a Delta II 7920-H rocket. The mission is a joint venture of NASA, the United States Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden, becoming the most sensitive gamma-ray telescope on-orbit, succeeding INTEGRAL.

This animation captures phenomena observed over the course of nine days following the neutron star merger known as GW170817, detected on Aug. 17, 2017. They include gravitational waves (pale arcs), a near-light-speed jet that produced gamma rays (magenta), expanding debris from a kilonova that produced ultraviolet (violet), optical and infrared (blue-white to red) emission, and, once the jet directed toward us expanded into our view from Earth, X-rays (blue).

Credit: NASA’s Goddard Space Flight Center/CI Lab

The first sign of the Aug. 17, 2017, neutron star merger was a brief burst of gamma-rays seen by NASA’s Fermi Gamma-ray Space Telescope (top). Shortly after, LIGO scientists reported detecting gravitational waves that arrived 1.7 seconds before the Fermi burst (middle). A short time later, scientists analyzing gamma-ray data from the European Space Agency’s INTEGRAL spacecraft also reported seeing the burst (bottom).

Credit: NASA’s Goddard Space Flight Center, Caltech/MIT/LIGO Lab and ESA


On Aug. 17, 2017, gravitational waves from a neutron star merger produced a signal detected by LIGO. 1.7 seconds later, a brief burst of gamma-rays was seen by NASA’s Fermi Gamma-ray Space Telescope (top). This video contains the actual sound of the LIGO detection.

Credit: NASA’s Goddard Space Flight Center, Caltech/MIT/LIGO Lab

INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a space telescope for observing gamma rays. It was launched by the European Space Agency into Earth orbit in 2002 and is designed to detect some of the most energetic radiation that comes from space. It was the most sensitive gamma-ray observatory in space before NASA’s Fermi was launched in 2008.

INTEGRAL is an ESA mission in cooperation with the Russian Space Agency and NASA. It has had some notable successes, for example in detecting a mysterious ‘iron quasar’. It has also had success in investigating gamma-ray bursts and evidence for black holes.

Multi-messenger Astronomy with Gravitational Waves


Fermi and INTEGRAL could identify 30 possible host galaxies

NGC 4993 (also catalogued as NGC 4994) is a lenticular galaxy located about 140 million light-years away in the constellation Hydra. It was discovered on 26 March 1789 by William Herschel and is a member of the NGC 4993 Group.

NGC 4993 is the site of GW170817, the first astronomical event detected in both electromagnetic and gravitational radiation, the collision of two neutron stars, a discovery given the Breakthrough of the Year award for 2017 by the journal Science. Detecting a gravitational wave event associated with the gamma-ray burst provided direct confirmation that binary neutron star collisions produce short gamma-ray bursts.

About 5000 scientists involved in the investigation of gravitational waves. LIGO has over 1000


GW 170817 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, originating from the shell elliptical galaxy NGC 4993. The GW was produced by the last minutes of two neutron stars spiralling closer to each other and finally merging, and is the first GW observation which has been confirmed by non-gravitational means.

The discovery and subsequent observations of GW 170817 were given the Breakthrough of the Year award for 2017 by the journal Science.

The gravitational wave signal, designated GW 170817, had a duration of approximately 100 seconds and showed the characteristics in intensity and frequency expected of the inspiral of two neutron stars. Analysis of the slight variation in arrival time of the GW at the three detector locations (two LIGO and one Virgo) yielded an approximate angular direction to the source. Independently, a short (~2 seconds’ duration) gamma-ray burst, designated GRB 170817A, was detected by the Fermi and INTEGRAL spacecraft beginning 1.7 seconds after the GW merger signal.

An intense observing campaign then took place to search for the expected emission at optical wavelengths. An astronomical transient designated AT 2017gfo (originally, SSS 17a) was found, 11 hours after the gravitational wave signal, in the galaxy NGC 4993[15] during a search of the region indicated by the GW detection. It was observed by numerous telescopes, from radio to X-ray wavelengths, over the following days and weeks, and was shown to be a fast-moving, rapidly-cooling cloud of neutron-rich material, as expected of debris ejected from a neutron-star merger.

As it is a result of the merger of neutron stars it is believed to be an unknown kilonova transient.

A kilonova (also called a macronova or r-process supernova) is a transient astronomical event that occurs in a compact binary system when two neutron stars or a neutron star and a black hole merge into each other. Kilonovae are thought to emit short gamma-ray bursts and strong electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected fairly isotropically during the merger process.

The neutron star merger event is thought to result in a kilonova, characterized by a short gamma-ray burst followed by a longer optical “afterglow” powered by the radioactive decay of heavy r-process nuclei. Kilonovae are candidates for the production of half the chemical elements heavier than iron in the Universe. [A total of 16,000 times the mass of the Earth in heavy elements is believed to have formed, including approximately 10 Earth masses just of the two elements gold and platinum].


Scientists use two primary methods to measure the Hubble constant. One involves monitoring nearby objects whose properties scientists understand well, such as stellar explosions known as supernovas and pulsating stars known as Cepheid variables, in order to estimate their distances and then deduce the expansion rate of the universe. The other focuses on the cosmic microwave background, the leftover radiation from the Big Bang, and examines how it has changed over time to calculate how quickly the cosmos has expanded.

However, this pair of techniques has yielded two different results for the value of the Hubble constant. Data from the cosmic microwave background suggests the universe is currently expanding at a rate of about 67 kilometres per second per 3.26 million light-years, while data from supernovas and Cepheids in the nearby universe suggests a rate of about 73 km per second per 3.26 million light-years.

This discrepancy suggests that the standard cosmological model — scientists’ understanding of the universe’s structure and history— could be wrong. Resolving this debate, known as the Hubble constant conflict, could shed light on the evolution and ultimate fate of the cosmos.

In the new study, physicists suggest that future data from the ripples in the fabric of space and time known as gravitational waves might help break this deadlock.

As mentioned earlier scientists, in 2017, detected gravitational waves from colliding neutron stars, remnants of stars that perished in catastrophic explosions known as supernovas.

Neutron stars emit visible light, and so do their collisions. The gravitational waves from these mergers, dubbed “standard sirens,” will help scientists pinpoint their distance from Earth, while the light from these collisions will help determine the speed at which they were moving relative to Earth. Researchers can then use both these sets of data to calculate the Hubble constant.

However, that estimate depends on how often neutron-star collisions occur. There is considerable uncertainty in the rate of neutron star mergers as so far we have only seen one.

On 1 April 2019, the start of the third observation run was announced with a circular published in the public alerts tracker. The first O3/2019 binary black hole detection alert was broadcast on 8 April 2019. A significant percentage of O3 candidate events detected by LIGO were accompanied by corresponding triggers at Virgo. False alarm rates were mixed, with more than half of events assigned false alarm rates greater than 1 per 20 years, contingent on the presence of glitches around signal, foreground electromagnetic instability, seismic activity, and operational status of any one of the three LIGO-Virgo instruments. For instance, events S190421ar and S190425z weren’t detected by Virgo and LIGO’s Hanford site, respectively.

The detection rates and signal qualities of gravitational waves will improve when the Kamioka Gravitational Wave Detector (KAGRA) in Japan becomes operational.

The Kamioka Gravitational Wave Detector (KAGRA), formerly the Large Scale Cryogenic Gravitational Wave Telescope (LCGT), is a project of the gravitational wave studies group at the Institute for Cosmic Ray Research (ICRR) of the University of Tokyo. It will be the world’s first gravitational wave observatory in Asia, and that is built underground, and whose detector uses cryogenic mirrors. The design calls for an operational sensitivity equal to, or greater than LIGO

A proposal for a gravitational-wave detector made of two space-based atomic clocks has been unveiled by physicists in the US. The scheme involves placing two atomic clocks in different locations around the Sun and using them to measure tiny shifts in the frequency of a laser beam shone from one clock to the other. The designers claim that the detector will complement the LISA space-based gravitational-wave detector, which is expected to launch in 2034.

ESA’s potential future mission, LISA, will detect and observe gravitational waves that are emitted during the most powerful events in the universe. LISA will detect gravitational radiation from astronomical sources, observing galaxies far back in time and testing the fundamental theories of gravitation.

41 candidate O3 events – its raining gravitational waves

The document is intended for both professional astronomers and science enthusiasts who are interested in receiving alerts and real-time data products related to gravitational-wave (GW) events.

Allows you to keep track of the latest gravitational wave alerts.


This artist’s depiction illustrates a black hole devouring a neutron star. As the neutron star circles the black hole, the black hole’s immense gravity shreds it to pieces, a phenomenon called tidal disruption.

Photograph by illustration by Dana Berry, NASA

Some 900 million years ago, a black hole released a terrible burp that echoed through the cosmos. On August 14 2019, the resulting ripples in the fabric of spacetime passed through Earth—giving us the best evidence yet of a never-before-seen type of cosmic collision that could offer new insights on how the universe works.

The detection, called S190814bv, was likely triggered by the merging of a black hole and a neutron star, the ultra-dense leftovers of an exploded star. Though astronomers have long expected such binary systems to exist, they’ve never been seen by telescopes scanning the heavens for different wavelengths of light.

However, astronomers also expect such systems to create ripples known as gravitational waves if and when the black hole and neutron star merge. These spacetime ripples were predicted more than a century ago by Einstein’s general theory of relativity, which suggested that the collision of two extremely massive bodies would cause the very fabric of the universe to wrinkle.

The Future

The Laser Interferometer Gravitational-Wave Observatory (LIGO) – India is a planned advanced gravitational-wave observatory to be located in India as part of the worldwide network, whose concept proposal is now under active consideration in India and the USA. LIGO-India is envisaged as a collaborative project between a consortium of Indian research institutions and the LIGO Laboratory in the USA, along with its international partners.

LIGO-India received the Indian Government’s in-principle approval in February 2016. Since then the project reached several milestones towards selecting and acquiring a site and building the observatory.

Sapphire mirror for the KAGRA gravitational wave detector

KAGRA, the Japanese interferometric gravitational wave detector currently under construction, will employ sapphire test masses for its cryogenic operation. Sapphire has an advantage in its higher thermal conductivity near the operating temperature 20 K compared to fused silica used in other gravitational wave detectors, but there are some uncertain properties for the application such as hardness, optical absorption, and birefringence. Test polish of sapphire substrate has especially proven that specifications on the surface are sufficiently met. Recent measurements of absorption and inhomogeneity of the refractive index of the sapphire substrate indicate that the other properties are also acceptable to use sapphire crystal as test masses.

Gravitational-wave observatory LIGO set to double its detecting power

A planned US$35-million upgrade could enable LIGO to spot one black-hole merger per hour by the mid-2020s.

New instrument extends LIGO’s reach

Technology “squeezes” out quantum noise so more gravitational wave signals can be detected.

The major improvement for ALIGO+ — requiring the 300-metre pipes — will introduce ‘frequency-dependent squeezing’. This will enable the interferometers to reduce both the pressure on the mirrors and the photon fluctuations at the same time. Other improvements will include new mirrors with state-of-the-art coatings, which is expected to reduce thermal noise fourfold.

Improvements of the ground-based detectors would increase the sensitivity by 10 times and the event rate by 1000 times.

Einstein Telescope (ET) or Einstein Observatory, is a proposed third-generation ground-based gravitational wave detector, currently under study by some institutions in the European Union. It will be able to test Einstein’s general theory of relativity in strong field conditions and realize precision gravitational wave astronomy.

The ET is a design study project supported by the European Commission under the Framework Programme 7 (FP7). It concerns the study and the conceptual design for a new research infrastructure in the emergent field of gravitational-wave astronomy.

Gravitational-wave detectors may benefit from an alternative coating material that is less noisy at low temperatures than currently used materials.

To detect the faint “chirps” of gravitational waves, researchers at the upgraded facilities of Advanced LIGO and Virgo must minimize noise as much as possible. One of the most troublesome sources of noise in these detectors is the thermal fluctuations in the mirror coatings. Iain Martin from the University of Glasgow, UK, and colleagues now propose a new coating material that could help reduce thermal noise at temperatures of around 10 K, where the next generation of gravitational-wave detectors plan to operate.

Gravitational-wave detectors use kilometre-long interferometer arms formed by laser beams bouncing off highly reflective mirrors. To maximize reflection, the mirrors are coated with alternating layers of materials that have high and low refractive indices. The coatings create an interference effect that reduces light absorption to less than 5 parts per million.

In current detectors, the coatings are made of doped tantala (Ta2O5) and silica (SiO2). But both of these materials exhibit strong mechanical losses—which means vibrations are readily converted to heat and thermal fluctuations—at the low temperatures planned for next-generation detectors, such as the Einstein Telescope.

Martin and colleagues propose replacing the silica with a low-index material called hafnia (HfO2). In tests, the team placed thin films of hafnia (doped with silica) on a cantilever and measured the material’s mechanical loss to be a factor of 2 lower than that of pure silica. They also calculated the expected performance of a coating design consisting mainly of hafnia and amorphous silicon (a tantala replacement). The researchers found that this model coating would be 25 times less noisy than current LIGO coatings for low-frequency (10-Hz) gravitational waves, the detection of which is one of the design goals for the Einstein Telescope.

Summaries of LSC scientific publications

Gravitational Wave Open Science Center

Like electromagnetic waves, gravitational waves travel at the speed of light. And like the electromagnetic spectrum, the gravitational wave spectrum is extremely broad, with the different parts classified according to frequency. In general, gravitational wave frequencies are much lower than those of the electromagnetic spectrum (a few thousand hertz at most, compared to some 1016 to 1019 Hz for X-rays). Consequently, they have much larger wavelengths – ranging from hundreds of kilometres to potentially the span of the Universe.

Gravitational waves offer a unique view into the very early universe because they can allow us to see “behind” the Cosmic Microwave Background Radiation (CMBR). The CMBR gives us an image of the universe about 380,000 years after the beginning of the universe, the ‘Big Bang’. This is because, before that time, the universe was filled with hot ionized gas (meaning the electrons and nuclei were separate, just free electrons flying around), so photons of light would be scattered wildly by these free electrons, rather like what happens on a foggy day, so they don’t carry much information about their original source. Because the universe had been expanding since the beginning, it was cooling, and at around the 380,000-year mark, the universe became cool enough that the gas stopped being ionized: the free electrons combined with protons to form neutral hydrogen (we call this recombination), which is much less effective at scattering photons — in other words the fog clears! For us, this marks the beginning of the period when photons actually can free-stream directly towards us from the early universe, with minimal scattering, so they can carry information about their origins.



Image credit: Chris Henze, NASA. Advanced Supercomputing Division, NASA Ames Research Center

2-nd ISSI Workshop on Clocks, Spacetime Metrology and Geodesy (Bern, Switzerland)

The horizontal axis shows the frequency (and the wave period, which is the inverse of frequency) on a logarithmic scale, with the colours representing the corresponding wavelengths (red = longer, blue = shorter). The detectors shown are those existing or planned, while the sources are those known to exist and expected to produce detectable gravitational waves.

On the other hand, gravitational radiation does not care about this epoch of recombination, because atoms and molecules of gas — whether ionized or not — have minimal effect on gravitational waves. This means that gravitational waves created before recombination can still stream right towards us without being disturbed along the way, and so could in principle tell us something about that very early phase in the history of the cosmos. The LIGO detectors may not be sensitive to these ‘primordial’ gravitational waves, but there are other ways we can search for their signature — e.g. by looking for polarized light in the CMBR.

Dark matter and dark energy have had a big role in the history of the universe expanding (in fact we think dark energy is now causing that expansion to speed up!) and in the formation of galaxies and clusters of galaxies. But we don’t expect dark matter to exist in nearly dense enough ‘clumps’ to produce gravitational waves that could be detected by LIGO.

In the future, however, astronomers hope to use gravitational-wave sources such as compact binary coalescence to map out the cosmos, completely independently of the methods available right now — using e.g. the CMBR or distant supernovae. So it’s possible that future gravitational wave observations could help us to better understand the effects of dark matter and dark energy on the expansion of the universe.

At the moment there is too much noise for the lower frequencies at the moment.

LISA Pathfinder, formerly Small Missions for Advanced Research in Technology-2 (SMART-2), was an ESA spacecraft that was launched on 3 December 2015 onboard Vega flight VV06. The mission tested technologies needed for the Laser Interferometer Space Antenna (LISA), an ESA gravitational wave observatory planned to be launched in 2034. The scientific phase started on 8 March 2016 and lasted almost sixteen months. In April 2016 ESA announced that LISA Pathfinder demonstrated that the LISA mission is feasible.

It paved the way for future missions by testing in flight the very concept of gravitational wave detection. It put two test masses in a near-perfect gravitational free-fall and control and measured their motion with unprecedented accuracy. LISA Pathfinder used the latest technology to minimise the extra forces on the test masses, took measurements. The inertial sensors, the laser metrology system, the drag-free control system and an ultra-precise micro-propulsion system made this a highly unusual mission. LISA Pathfinder was an ESA mission, which also carries a NASA payload.

It didn’t detect gravitational waves but the noise level could be controlled. It could see an inspiral before a merger (allowed preparation for the merger).

The Chinese are developing a space gravitational wave detector.

The TianQin Project is a proposed space-borne gravitational-wave observatory (gravitational-wave detector) consisting of three spacecrafts in Earth orbit.

Inflationary theory and pulsar timing investigations of primordial black holes and gravitational waves

A pulsar timing array (PTA) is a set of pulsars which is analysed to search for correlated signatures in the pulse arrival times. There are many applications for pulsar timing arrays. The most well-known is to use an array of millisecond pulsars to detect and analyse gravitational waves. Such a detection would result from a detailed investigation of the correlation between arrival times of pulses emitted by the millisecond pulsars as a function of the pulsars’ angular separations.

Gravitational waves are ripples in space-time, predicted by Einstein’s theory of general relativity, which stretch and compress spacetime. As pulsars emit pulses with such amazing regularity, organisations such as the European Pulsar Timing Array can use pulsars as extremely accurate clocks, at distances of light-years from the Earth. By comparing the measured pulse arrival times to the expected arrival times, the distortion of space caused by a passing gravitational wave should be detectable as a deviation from the timing model, correlated across all pulsars. Pulsar timing arrays are sensitive to extremely low-frequency gravitational radiation generated by supermassive black hole binaries, cosmic strings, and the inflationary era.

Questions at the end of the talk

1) What can we learn from the spin and mass of a black hole?

Once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum; the black hole is otherwise featureless.

These properties are special because they are visible from outside a black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using a gravitational analogue.

If the spins of supermassive black holes are as high as some have found then these black holes are likely to have formed from rare, major mergers between colliding galaxies, in which a large quantity of material falls into the central black hole from one direction. If the spins are lower then the black holes may have formed from many minor mergers, with bite-sized lumps of material coming from various directions. The distribution of black hole spins could therefore inform researchers about the history of galactic evolution, particularly if astronomers can eventually chart the change in spin over cosmic time by looking at ever-more-distant black holes.

The greater the mass of black holes the more likely their behaviour could deviate from the three properties. These potential differences could help describe some mysteries of the Universe, like dark matter. At the moment there is no evidence of deviation.

The behaviour of black holes is evidence that Albert Einstein’s theory of general relativity holds true.

2) Can the work measure Newton’s gravitational constant, G more accurately?

No because there are too many uncertainties in instruments, distances etc.

The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are “close enough” (within about a thousand parsecs) to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung.

Gravitational waves originating from the inspiral phase of compact binary systems, such as neutron stars or black holes, have the useful property that energy emitted as gravitational radiation comes exclusively from the orbital energy of the pair, and the resultant shrinking of their orbits is directly observable as an increase in the frequency of the emitted gravitational waves. By observing the waveform, the chirp mass can be computed and thence the power (rate of energy emission) of the gravitational waves. Thus, such a gravitational wave source is a standard siren of known loudness.

In astrophysics, the chirp mass of a compact binary system determines the leading-order orbital evolution of the system as a result of energy loss from emitting gravitational waves. Because the gravitational wave frequency is determined by orbital frequency, the chirp mass also determines the frequency evolution of the gravitational wave signal emitted during a binary’s inspiral phase. In gravitational wave data analysis, it is easier to measure the chirp mass than the two-component masses alone.

Just as with standard candles, given the emitted and received amplitudes, the inverse-square law determines the distance to the source.

A standard candle is an astronomical object that has a known absolute magnitude. They are extremely important to astronomers since by measuring the apparent magnitude of the object we can determine its distance.

The most commonly used standard candles in astronomy are Cepheid Variable stars and RR Lyrae stars. In both cases, the absolute magnitude of the star can be determined from its variability period.

Discovery of Accelerating Universe Wins 2011 Nobel Prize in Physics

Dark energy wins out in the end: Three U.S. scientists have been honoured for their observations that type Ia supernovae indicate that the expansion of the universe is accelerating

3) Can the work on gravitational waves indicate dark matter?

Dark matter is believed to be five times as prevalent as visible matter. Its gravitational effects are seen throughout the universe. Scientists think they have yet to definitively see gravitational waves caused by dark matter, but they can think of numerous ways this might happen.

Scientists have seen the gravitational effects of dark matter, so they know it must be there—or at least, something must be going on to cause those effects. But so far, they’ve never directly detected a dark matter particle, so they’re not sure exactly what dark matter is like.

4) Could dark matter be primordial black holes

One idea is that some of the dark matter could actually be primordial black holes.

Imagine the universe as an infinitely large petri dish. In this scenario, the Big Bang is the point where matter-bacteria begins to grow. That point quickly expands, moving outward to encompass more and more of the petri dish. If that growth is slightly uneven, certain areas will become more densely inhabited by matter than others.

These pockets of dense matter—mostly photons at this point in the universe—might have collapsed under their own gravity and formed early black holes.

By using gravitational waves to learn about the properties of black holes, LIGO might be able to prove or constrain this dark matter theory.

Unlike normal black holes, primordial black holes don’t have a minimum mass threshold they need to reach in order to form. If LIGO were to see a black hole less massive than the sun, for example, it might be a primordial black hole.

Even if primordial black holes do exist, it’s doubtful that they account for all of the dark matter in the universe. Still, finding proof of primordial black holes would expand our fundamental understanding of dark matter and how the universe began.

5) Has LIGO seen dark matter?


The basics of gravitational wave theory

The basic physics of the binary black hole merger GW150914

Observational data indicates that the bodies were orbiting each other (roughly Keplerian dynamics) up to at least an orbital angular frequency.

In astronomy, Kepler’s laws of planetary motion are three scientific laws describing the motion of planets around the Sun, published by Johannes Kepler between 1609 and 1619. These improved the heliocentric theory of Nicolaus Copernicus, replacing its circular orbits with epicycles with elliptical trajectories, and explaining how planetary velocities vary. The laws state that:

The orbit of a planet is an ellipse with the Sun at one of the two foci (the orbit of one black hole is an ellipse with another black hole at one of the two foci).

A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time (A line segment joining the two black holes sweeps out equal areas during equal areas of time).

The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit (The square of the orbital period of a black hole is directly proportional to the cube of the semi-major axis of its orbit).

The discovery of gravitational waves is evidence that there is conservation of mass and energy.

Mass was lost when the two black holes collapsed. One of the black holes was 36 times the mass of our sun and the other was 29 times its mass. When they collapsed, the resulting black hole was only 62 solar masses. That means that a mass three times the mass of the sun was lost!

That mass was turned into energy, which caused the ripples in spacetime – gravitational waves – that LIGO detected 1.3 billion years later.

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