The story starts with gravitational waves. The term ‘waves’ makes them sound deceptively simple: flick a light switch or play music through a speaker and almost instantaneously you can detect that light and sound are real things, through using your eyes and ears, and very little energy.
Gravitational waves, on the other hand, need a lot more energy to create detectable waves, since gravity is such a weak ‘force’. Or, more accurately put, spacetime (the fabric of the Universe which depresses when an object is within it) is hard to deform. When there is enough mass and energy though, it can depress, sending out waves to communicate to other parts of the Universe the change in the distribution of mass and energy in that place (since nothing, not even simple information like mass and energy, can travel faster than the speed of light).
And when an event such as a black hole collision or supernova occurs, the spacetime around the object begins to stretch and squash, like mesh being stretched in two directions (with spacetime being a 4 dimensional ‘material’, where there’s height, depth, length as well as an internal structure). If you’re confused, don’t worry! Click here and watch a short explanation by a scientist who has found a novel use of some mesh from a wine bottle .
Black hole merger producing gravitational waves.
Credit: LIGO/T. Pyle
Signatures in Waves
The waves detected at gravitational-wave interferometers like LIGO (an ‘L’ shaped detector that looks for deformations in each arm that are the 1,000th the diameter of a proton ) lead to a graph of the frequency of the waves by time (much like the frequency of ripples moving past a point in a pond, but along two axes). The way in which the frequency of waves rises and falls helps scientists work out the mass and spin of the objects involved in making these waves by comparing them to simulations of events.
LIGO Livingston, at the corner of the L
Credit: Caltech/MIT/LIGO Lab
One example of this is the very first time gravitational waves were detected, on 14th September 2015. The graph of the gravitational waves showed the three main stages expected from black hole mergers: inspiral, merger and ring-down. Inspiral is where the two black holes are orbiting one another, moving faster and faster as they get closer and closer (the frequency of gravitational waves increases). Then, the merger event happens, where the ISCO (Innermost Stable Circular Orbit: the part where nothing can stand still, but where you can still escape) of each black hole touches the other, leading to an ISCO plunge, before the black holes merge to form a Kerr black hole (one with no electric charge): the point where the waves have the highest frequency. Following this, ring-down happens, where the frequency quickly drops, like the ringing of a bell slowly fading away after being struck. The combination of these stages is known as a chirp, due to the characteristic sound made by converting the graph into something audible.
With this general wave form in mind, just a few numbers for the spin and masses of the black holes are needed and a perfect wave form is obtained from a simulation, matching that measured by a detector.
And, with every event, there’s a different signature for the different masses, spins and processes involved .
The first observation of gravitational waves by LIGO (with one detector in Hanford and another in Livingston)
Types of Events
Gravitational waves are most famously formed by the collision of two black holes, as seen in the very first collision measured. These are known as Compact Binary Inspiral Gravitational Waves, formed by two neutron stars colliding (Binary Neutron Stars or BNS), two black holes colliding (Binary Black Hole or BBH) and a neutron star colliding with a black hole (Neutron Star-Black Hole Binary or NSBH) .
As the two objects get closer, their orbits accelerate due to a centripetal force (a force directed into the centre of motion, in this case due to the strong gravitational attraction between the two objects). This then leads to the inspiral, merger and ring-down that was explained earlier .
From the 14th September 2015 detection of gravitational waves from a black hole merger, 53 mergers have now been confirmed by the LIGO and Virgo detectors. This treasure trove of observations led to the first neutron star merger detection on 17th August 2017. It revealed the illusive kilonova: the rapid decay (through the r-process) of neutrons expelled by the neutron stars, creating a burst of energy, detected by LIGO and Virgo as a chirp .
However, the exciting thing about a neutron star merger is that it’s detectable using conventional telescopes, such as the Fermi gamma ray (high energy light) telescope, detecting a burst 1.7 seconds after the chirp detected by LIGO and Virgo. This then led to spectra being taken which showed that elements present in the burst included Lanthanides (heavy elements in the periodic table), hinting at neutron star collisions being responsible for elements such as gold and platinum .
First signal of a neutron star collision (GW170817)
Credit: LIGO Scientific Collaboration and Virgo Collaboration, CC BY-SA 4.0
And then, on 5th January 2020 and 15th January 2020 (confirmed on 29th June in a paper), the LIGO and Virgo detectors picked up the signal of a neutron star and black hole colliding, an NSBH: a first for the detectors. The first collision (known as GW200105) expelled 0.34 to 0.41 quattuordecillion Joules of energy (up to 3.4 trillion times the energy output of the Sun in a year) whilst the second (known as GW200115) expelled slightly less, at 0.22-0.34 quattuordecillion Joules. However, this staggering amount of energy wasn’t able to be seen with telescopes like the neutron star mergers, possibly because the black hole gulped up the neutron star before it could through out a burst of light. That said, scientists aren’t totally sure, since the light may just not have been located due to the vastness of the sky is was predicted to have come from (17% of it), and the distance the light would have travelled to get here meaning it may be extremely dim (with the event happening up to 1.3 billion light-years away) .
Simulation of a black hole neutron star merger with a tidal disruption.
Credit: Scientific visualization: T. Dietrich (Potsdam University and Max Planck Institute for Gravitational Physics), N. Fischer, S. Ossokine, H. Pfeiffer (Max Planck Institute for Gravitational Physics), T. Vu. Numerical-relativity simulation: S.V. Chaurasia (Stockholm University), T. Dietrich (Potsdam University and Max Planck Institute for Gravitational Physics)
But it’s not just mergers that create gravitational waves that we can measure. There are also continuous gravitational waves sources, produced by a single object which is both massive and spinning. The irregular shape of the star (with various imperfections) will lead to gravitational waves, but of a single frequency (regularity of the wave) and amplitude (strength of the wave). One method is by looking at pulsars (neutron stars who’s poles regularly point in the direction of the observer: us) to see if the radio pulse, which is sometimes more accurate than atomic clocks, would arrive at irregular intervals due to the warping of spacetime by gravitational waves (which warps light). One such project hoping to observe just this is the Pulsar Timing Array, involving 8 radio telescopes around the world, from the Lovell Telescope a few miles down the road, to the Green Bank Telescope in the USA. It could also be seen as a continuous signal, or pitch, if it were observed by LIGO or Virgo .
But we can’t forget the last, or more correctly first, to the party: stochastic gravitational waves. These waves are small and come from all directions, known as a Stochastic Signal (a signal with a random pattern which can’t be precisely predicted), and are thought to originate from the Big Bang. Unlike the CMBR (Cosmic Microwave Background Radiation) which originate from when light could pass through matter without being reabsorbed 380,000 years after the Big Bang, these stochastic gravitational waves propagated out due to the events that happened during the Big Bang, meaning that, if we can observe them, we could see further back in time than ever before .
There is just one more type that tags on the end, called burst gravitational waves. These don’t have a specific source as such, and aren’t well understood at all. However, we do know that they’re random events with no way to predict how they’ll act. This is simply the ‘other’ category in the list of types of gravitational waves, highlighting just how new this branch of astronomy is and how much we still have to learn about gravitational waves .
The stellar graveyard of black holes and neutron stars detected by LIGO and Virgo, including the two neutron star-black hole mergers highlighted in the centre (the lines show the two objects which then merge to form a product: the one at the top)
Credit: Credits: LIGO-Virgo / Frank Elavsky, Aaron Geller / Northwestern University.
Hopefully, as many more observations are made of these illusive events, we will be able to build a better picture of our Universe, far beyond the constraints of the traditional light based astronomy, so we can understand what really happened at the beginning of the Universe, and more about neutron stars and black holes: objects that we still don’t understand due to the constraints of the electromagnetic spectrum.
by George Abraham, ADAS member.
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Click here to listen to what Stochastic Gravitational Waves are thought to sound like
Click here to watch a simulation of a neutron star black hole merger
Click here to listen to other gravitational wave observations by LIGO and Virgo
Click here to see the interactive LIGO-Virgo Compact Binary Catalogue by the University of Cardiff
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