Gravitational wave detectors find 56 potential cosmic collisions

During collaborative measurement campaigns, so-called observation runs, the worldwide gravitational wave detector network listens for signals from space. During the third observation run “O3”, which started on April 1st, 2019, the LIGO detectors (USA), Virgo (Italy), and GEO600 (Germany) recorded a range of promising signals.

An article by Denise Müller-Dum

The epoch of gravitational wave astronomy has begun, concluded David Reitze, Executive Director of the LIGO gravitational wave detectors, according to the scientific journal Nature Reviews Physics in April 2020. While the first two observation runs (O1: September 2015 until Januar 2016, O2: November 2016 until August 2017) found 11 cosmic collisions in total, no less than 56 potential events were detected during O3, which makes, on average, one in six days.

The third observation run of LIGO, Virgo and GEO600 started on April 1st, 2019. Measurements were supposed to be performed in two chunks of six months, with a one month commissioning break in October. But the Covid-19-pandemic forced the campaign to an early end on March 27th, 2020 – one month earlier than initially planned.

Nevertheless, the list of potential events that the detectors recorded during this time is quite impressive – and accessible for everybody. For the first time, possible events were announced through a Public Alert System. It enabled astronomers to react quickly and point their own instruments at the right position in the sky, trying to capture electromagnetic signals from the origin of the gravitational wave signals. As this was a time-sensitive task, the alerts were automatically sent within minutes after the possible event and only later analyzed by gravitational wave experts. Naturally, some of the alerts turned out to be false and were retracted. In any case, proper attribution of the signals to their origin requires a thorough scientific analysis, which takes time. Fifty-six events need to be investigated in detail. And while the to-do list is still long, the first results are indeed very promising.

Overview of the four events published so far

Overview of the first four published events from O3

Results published so far

GW190425

The second neutron star merger after the premier in 2017 was observed in April 2019, soon after O3 started. What was first only seen by LIGO later turned out to also be concealed in the Virgo data. This event, GW190425, happened 290 to 740 million light years away, which is 2 to 6 times farther than the first observed neutron star merger. Researchers were surprised by the remarkable size of the binary of 3.3 to 3.7 solar masses, because this is higher than that of other known neutron star binaries. Therefore, they still aren’t entirely sure if they were dealing with a neutron star merger, or if the two objects may have been two very light black holes. More on GW190425

GW190412

Another April event turned out to be a signal from two coalescing black holes of markedly different masses. It was the first time astronomers observed a binary system with such a mass ratio. The collision revealed itself with a very unique gravitational wave signal: a higher harmonic, that is, an oscillation at a multiple of the fundamental frequency – similar to overtones in music, only as gravitational waves, as predicted by general relativity. More on GW190412

GW190814

Such “humming” of a higher harmonic was also identified in the readings of event GW190814. Researchers had hoped that it could be the merger of a neutron star with a black hole, which would have been the first observation of such a system ever made. After detailed analysis, they were able to confirm that the larger object was indeed a 23-solar-mass black hole. The nature of its mysterious and nine times lighter companion, however, remains unresolved: It seems too heavy to be a neutron star, but also too light for a black hole. More on GW190814

Simulation of gravitational wave overtones, Max-Planck-Institute for Gravitational Physics (Albert-Einstein-Institute)

GW190521

A very short signal from a heavy weight arrived at the detectors in May 2019. During the subsequent analysis, researchers realized that they had observed the most massive and most distant event so far: a coalescence of two black holes with a combined mass of 150 solar masses that happened seven billion years ago when the universe was half as old as it is today. The gravitational wave signals of this event, called GW190521, provide evidence of the birth of an intermediate mass black hole, whose one parent is unusually heavy, having researchers speculate if it may itself have formed during a merger. However, the scientific discussion on this event is still ongoing and other interpretations have been proposed. More on GW190521

Further Information

Colophon

Denise Müller-Dum

is a trained physicist and geoscientist and works as a science communicator in Bremen, Germany.

Citation

Cite this article as:
Denise Müller-Dum, “Gravitational wave detectors find 56 potential cosmic collisions” in: Einstein Online Band 12 (2020), 12-1002

Definitioner

merger

A merger occurs as a consequence of an inspiral, when two black holes collide to form one single bigger black hole. The emission of gravitational waves peaks during a merger.

wave

In a general sense: any travelling pattern, whether or not it involves matter being transported as well. Simple examples are water-waves - wave crests and troughs travelling over a water surface, and a Mexican wave in a football stadium, with fans alternately standing up and sitting down - the pattern moves throught the stadium, not the fans themselves.

An especially simple form for a wave is a sinus wave, a regular pattern of wave crests and troughs.

time

It is a fact of life that not all events in our universe happen concurrently - instead, there is a certain order. Defining a time coordinate or defining time, the way physicists do it, is to define a prescription to associate with each event a number so as to reflect that order - if event B happens after event A, then the number associated with B should be larger than that associated with A. The first step of this definition is to construct a clock: Choose a simple process that repeats regularly. (What is "regular"? Luckily, in our universe, all elementary processes such as a swinging pendulum, the oscillations of atoms or of electronic circuits lead to the same concept of regularity.) As a second step, install a counter: A mechanism that, with every repetition of the chosen process, raises the count by one. With this definition, one can at least assign a time (the numerical value of the counter) to events happening at location of the clock. For events at different locations, an additional definition is necessary: One needs to define simultaneity. After all, the statement that some far-away event A happens at 12 o'clock is the same as saying that event A and "our clock counter shows 12:00:00" are simultaneous. The how and why of defining simultaneity - a centre-piece of Einstein's special theory of relativity - are described in the spotlight topic Defining "now". With all these preparations, physicists can, in principle, assign a time coordinate value ("a time") to any possible events, and describes how fast or how slow processes happen, compared to that time coordinate.

star

A cosmic gas ball that is massive enough for pressure and temperatur in its core to reach values where self-sustained nuclear fusion reactions set in. The energy set free in these reactions makes stars into very bright sources of light and other forms of electromagnetic radiation. Once the nuclear fuel is exhausted, the star becomes a white dwarf, a neutron star or a black hole.

second

In the International System of units: the basic unit of time. Defined as a certain multiple of the oscillation period of electromagnetic radiation set free in a certain transition within the electron shell of atoms of the type Cesium-133.

neutron

Particle that is electrically neutral and comparatively massive; the atomic nuclei consist of neutrons and protons. Neutrons are not elementary particles, they are compound particles consisting of quarks that are bound together through the strong nuclear interaction . Collectively, neutrons, protons and a number of similar particles are called baryons. Neutron stars are mainly made of neutrons.

neutron star

Final stage of massive stars that explode as a supernova. In the explosion process, the core of the star collapses to form a compact object with roughly 1.4 solar masses that mostly consists of nuclear matter, predominantly of neutrons. For astronomers, neutron stars are of interest as there exists a variety called pulsars from which they receive highly regular pulses of electromagnetic radiation. For relativists, they are interesting as the typical effects of general relativity are very pronounced in objects that compact (compare PSR 1913+16, double pulsar PSR J0737-3029A/B).

mass

In classical physics, mass plays a triple role. First of all, it is a measure for how easy it is to influence the motion of a body. Imagine that you're drifting in emtpy space. Drifting by are an elephant and a mouse, and you give each of them a push of equal strength. The fact that the mouse abruptly changes its path, while the elephant's course is as good as unaltered, is a sure sign that the mass (or, in the language of physics, the inertia or inertial mass) of the elephant is much greater than that of the mouse. Secondly, mass is a measure of how many atoms there are in a body, and of what type they are. All atoms of one and the same type have the same mass, and adding up all those tiny component masses, the total mass of the body results. Thirdly, in Newton's theory of gravity, mass determines how strongly a body attracts other bodies via the gravitational force, and how strongly these bodies attract it (in this sense, mass is the charge associated with the gravitational force). In special relativity, one can also define a mass that is a measure for a bodies resistance to changing its motion. However, the value of this relativistic mass depends on the relative motion of the body and the observer. The relativistic mass is the "m" in Einstein's famous E=mc² (cf. equivalence of mass and energy). The relativistic mass has a minimum for an observer that is at rest relative to the body in question. This value is the so-called rest mass of the body, and when particle physicists talk of mass, this is usually what they mean. Just as in classical physics, the rest mass is a kind of measure for how much matter the body is made up of - with one caveat: For composite bodies, the energies associated with the forces holding the body together contribute to the total mass, as well (another consequence of the equivalence of mass and energy). In general relativity, mass still plays a role as a source of gravity; however, it has been joined by physical quantities such as energy, momentum and pressure.

light

Light in the strict sense of the word is electromagnetic radiation the human eye can detect, with wave-lengths between 400 and 700 nano metres. In relativity theory and in astronomy, the word is often used in a more general sense, encompassing all kinds of electromagnetic radiation. For instance, astronomers might talk about "infrared light" or "gamma light"; in this context, light in the stricter sense is referred to as "visible light". Within classical physics, the properties of light are governed by Maxwell's equations; in quantum physics, it turns out that light is a stream of energy packets called light quanta or photons. In the context of relativistic physics, light is of great interest, and for a number of reasons. First of all, the speed of light plays a central role in both special and general relativity. Also, there are a number of interesting effects in general relativity which are associated with the propagation of light, namely deflection, the Shapiro effect and the gravitational redshift.

event

In the context of special or general relativity: Anything that is defined by a single point in time and a single point in space, i.e. something that happens at a definite time at a definite location. Synonym: spacetime point.

black hole

Region in space where a sufficient amount of mass is concentrated so that it forms a gravitational prison - a region into which matter or light can enter from the outside, but from which nothing that has once entered can ever leave. Basic information on this key phenomenon of Einstein's general theory of relativity can be found in the chapter Black holes & Co. of Elementary Einstein. Selected aspects of the physics of black holes and neutron stars are described in the category Black holes & Co. of our Spotlights on relativity. In Einstein's theory, black holes are truly black, due to the fact that no radiation or light can ever escape them. Once quantum theory is taken into account, this assumption no longer holds - on the contrary, it seems as if black holes should emit so-called Hawking radiation. However, for astrophysical black holes (that typically have more or even much more than one solar mass), that radiation would be undetectable, even if we could transport today's most sensitive sensors into the immediate vicinity of the black hole.

black holes (black hole)

binary (binary star)

A system consisting of two stars in orbit around each other. From a relativistic point of view, there are binaries that are of special interest, namely those in which at least one of the partners is a neutron star or a black hole. Potentially, such systems are effective sources of gravitational waves.