THE GRAVITATIONAL WAVE UNIVERSE
For centuries, humankind has observed the sky with the naked eye. In modern times we have learnt to study our universe by capturing light – in a very wide range of wavelengths – as well as charged particles, known as cosmic rays, and neutrinos, which can be emitted by very different astrophysical objects. All these messengers carry different information and travel for long distances before reaching us. After the detection of GW150914 in September, 2015, a new window onto the Universe opened: gravitational waves.
- Astrophysical structures within the detection range of Advanced Virgo
- Galaxies observable by Advanced Virgo, at different stages of progress of the detector
- Sources of Gravitational Waves
- Coalescence of compact binary systems
- Rotating neutron stars
- GW Bursts and Supernovae
- Stochastic background
- Facing the Unexpected
Gravitational waves (GW) are ripples in the fabric of space-time emitted in a wide range of frequencies and amplitudes by different kinds of astrophysical systems, ranging from binary systems of compact objects, to supernovae explosions, to spinning isolated neutron stars and many more; even a relic background of GWs released shortly after the Big Bang is expected.
When accelerated motion of mass and/or energy takes place without spherical or cylindrical symmetry, a GW is generated and propagates at the speed of light. Advanced Virgo, Advanced LIGO, GEO600 and KAGRA constitute the first world-wide network of GW detectors.
Advanced Virgo looks for GWs during dedicated observing campaigns, called observing runs. These correspond to long periods (months) in which the detector continuously gathers data, looking for GWs. Within their first and second observing runs, Advanced Virgo and LIGO detected 10 GWs from the merging of Binary Black Holes (BBH) and 1 GW from a Binary Neutron Star (BNS). These detections are all collected in a catalogue, called the GW Transient Catalogue 1 (GWTC-1).
Today, Advanced Virgo and LIGO have devised a real-time catalogue of GW event candidates: GraceDB. This catalogue provides real-time information relating to the GW events, such as distance from the Earth and position in the sky. This is precious information for the astronomers who would like to point their telescopes in the direction of source of the GW.
Astrophysical structures within the detection range of Advanced Virgo
Advanced Virgo can detect GWs with frequencies from 10 Hz up to 10,000 Hz. The amplitude of the signal is, in general, very small when it arrives at the Earth. For example, the amplitude of the first gravitational wave ever detected, GW150914, was so tiny as to change the relative length of the interferometer arms by a mere 10-18 m. This explains why detecting gravitational waves is so difficult. See this page on how Virgo detects gravitational waves.
The more sensitive the detector becomes, the more the GW horizon we can span increases. This is described in the following animation.
The animation shows some of the astrophysical structures that are within the detection range of Advanced Virgo.
The largest galaxy cluster reached by the Virgo detector is the Virgo Cluster, composed of about 1,500 galaxies. This is the cluster that gave its name to the detector. The vast majority of galaxies do not belong to any cluster and are distributed in more or less uniform manner. This uniformity means that the total number of galaxies within a certain distance of the Earth is proportional to the enclosed volume, approximately following a cubic law. The final image shows the horizon achieved by Advanced Virgo in the second and third observation periods (respectively called O2 and O3) and the expected horizons for the fourth and fifth runs (O4 and O5), in comparison with the local galaxy super-cluster, called Laniakea. The reach of the detector is given in terms of the Binary Neutron Star (BNS) range, i.e. the average distance at which the merger of a BNS system gives a matched filter signal-to-noise ratio (SNR) of 8 in Advanced Virgo with the current sensitivity; the distance is averaged over all the possible sky localisations and binary orientations. Each neutron star in the binary system is assumed to have a mass equal to 1.4 solar masses.
Galaxies observable by Advanced Virgo, at different stages of progress of the detector
Just as the detection horizon of GW detectors increases, the volume of the universe that we can observe expands and the number of galaxies we can reach through GW observation rises, as explained in the following animation.
The animation shows the number of galaxies observable by Advanced Virgo and their volumetric distribution as a function of their distance from the Earth, at different stages of progress of the detector. The distance from the Earth is given both in units of millions of light years (Mly) and Millions of parsec (Mpc; 1Mpc = 3.26 Mly), a popular unit in astrophysics. The sensitivity of Advanced Virgo is given in terms of the Binary Neutron Star (BNS) range, i.e. the average distance at which the merger of a Binary Neutron Star system gives a matched filter signal-to-noise ratio (SNR) of 8 in Advanced Virgo with the current sensitivity; the distance is averaged over all the possible sky localisations and binary orientations. Each neutron star in the binary system is assumed to have a mass equal to 1.4 solar masses. The table at top left of the animation gives the sensitivity of Advanced Virgo also in terms of the Binary Black Hole (BBH) range, i.e. the average distance at which the merger of a Binary Black Hole system gives a matched filter signal-to-noise ratio (SNR) of 8 in Advanced Virgo with the current sensitivity; the distance is averaged over all the possible sky localizations and binary orientations. Each black hole in the binary is assumed to have a mass equal to 30 solar masses. The BBH range extends the detection horizon of Advanced Virgo to much further away than the BNS range: at those distances the galaxy catalogues are still quite incomplete and therefore the number of galaxies within any BBH is not reported here.
Sources of Gravitational Waves
What are the main sources of GWs in the sky? Below you can find a list of astrophysical sources of GWs and even listen to their “gravitational sound”!
Indeed the GW frequencies are the same frequencies as the sound waves that are audible to humans. Therefore, any signal measured by Advanced Virgo can be sent to a loudspeaker (after some filtering), allowing us to hear the symphony of the Universe. However, let us specify that GWs are not sound waves: the analogy with sound is made just to allow us to have a better comprehension of changes in amplitude and frequency of a GW signal.
Coalescence of compact binary systems
A compact binary system is composed of two compact stellar objects, such as a Binary system of two Neutron Stars (BNS) or Binary system of two Black Holes (BBH) or a mixture of a Neutron Star (NS) and a Black Hole (BH) orbiting around each other; this is a typical source of GWs. As the system evolves with time, the two compact objects get closer and closer to one another until they eventually merge, because the system loses energy through the emission of GWs. This phenomenon is known as coalescence. As the two bodies approach one another, the GWs that are generated increase in frequency and amplitude. This type of signal is called a “chirp”. You can hear an example of this kind of signal here.
During the final stage orbital decay and before the merging of the two bodies the GWs are strong enough to be observed in the frequency sensitive band of Advanced Virgo, the two bodies orbit around one another many times per second.
The first announced GW to be detected jointly by Advanced LIGO and Virgo (called GW170814) is a signal of this kind, coming from a BBH merger. The GW signal revealed two BHs of 25 and 30 solar masses, coalescing at a distance of about 2.2 billion light-years from us. Just before their merger into a single BH of about 53 solar masses, the two objects were spinning around one another about 200 times per second. Advanced Virgo was crucial in pinpointing the source sky-location within a narrow sky area of 60 square degrees. A significant improvement with respect to the 1160 square-degree area obtained by analysing only the LIGO data.
On the 17th of August, 2017, another GW arrived at the Advanced Virgo and LIGO detectors: GW170817. This was a very long GW, lasting for about a minute in our detectors. Shortly after its arrival, the Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst coming from the same direction of the Sky, with a time delay of ~1.7 seconds with respect to the merger time. An extensive observing campaign was launched across the electromagnetic spectrum, leading to the discovery of the corresponding optical and infrared emission. Combining these observations, we were able to understand that GW170817 was emitted during the merger of two NS of about 1.4 solar masses, merging into a single body of ~2.8 solar masses.
The arrival of GW170817 opened a new era in the new-born GW astronomy: multi-messenger astronomy with Gravitational Waves, thanks to the observation of both GWs and light emitted by the same astrophysical source.
Rotating neutron stars
A neutron star is a very compact, rapidly rotating and highly magnetised star, which represents the remnant core of a massive star which has undergone a supernova explosion. They have roughly the same mass as our Sun, but concentrated into a sphere with a radius of 10 km, and they can spin up to 1000 times per second! If the star is not perfectly spherical, with only a tiny “mountain” on its surface, it will generate GWs. This type of GW is different from those emerging from compact binary coalescence, since they are emitted continuously by spinning neutron stars. We refer to this type of GW as continuous Gravitational Waves .
Pulsars are special cases of magnetically asymmetric neutron stars, from which we receive periodic light flashes each time the star completes a rotation. Until now, Advanced Virgo has not detected any signals from known pulsars.
This allowed us to conclude that Neutron stars are almost perfect spherical stars. For instance, we know that the pulsar J1400-6325 should not have “mountains” higher than 4 centimetres, otherwise we would observe a GW from this source.
Observations of neutron stars and pulsars will provide important information about their internal structure. Advanced Virgo will search for GWs emitted by neutron stars in the neighbourhood of the Sun, and in the rest of our own galaxy, the Milky Way.
GW Bursts and Supernovae
“GW bursts” is a generic name given to short GW signals that last from a few milliseconds to a few seconds. This type of GWs has not yet been observed and are expected to be associated with short-lived and violent astrophysical events such as supernova explosions. Supernova explosions are one of the possible outcomes of a dying massive star. During this phase, the star collapses on itself due to the fact that the star’s gravitational attraction can no longer be sustained by the thermonuclear reactions within it.
Eventually, the collapsing material becomes so dense that the star’s core becomes a hot neutron star. The newly-born neutron star will cool by intense neutrino emission and the rest of the collapsing star will be disrupted in a supernova explosion. Advanced Virgo will be able to observe the GW emission from the collapse and explosion of such massive stars at the end of their life.
In a galaxy like the Milky Way there are only a few such supernovae per century. Their observation will allow us to better understand what is hidden behind the matter ejected during the explosion and how black holes are created.
Most cosmological models predict that we are bathed in a random background of GWs generated by sources that individually can not be detected.
Eventually the combination of all of the GWs emitted by these sources will form a stochastic GW background that we will be able to observe.
For instance we expect that a background of cosmological GWs were emitted in the first moments after the Big Bang. Detection of such a background would give us insight into the evolution of the Universe, providing a unique probe of the very early Universe.
In addition, an astrophysical background of GWs must result from the superposition of all the faint and distant sources that have emitted GWs since the beginning of the Universe. Detection of this background would help elucidate the star formation history and the evolution of astrophysical sources.
Facing the Unexpected
Every time humans have observed the sky with a new instrument, utilised new light wavelengths or messengers, unexpected objects and phenomena have been discovered.
When looking for GWs, we are also expecting surprises!
Cosmic strings are “defects” in the fabric of spacetime which might be formed during the evolution of the universe. These strings have never been observed, but are expected to emit GWs. Maybe Advanced Virgo will allow us to detect these kinds of physical phenomena for the first time.
Cosmic strings are just an example of an unexpected exotic GW source. There are many other theoretical GW sources that might reveal an exotic astrophysical source: GWs leaking out from black hole mimickers, ultra-light boson clouds forming around spinning black holes and many others.