Excerpted from The Sounds of the Cosmos by Mario Díaz, Gabriela González and Jorge Pullin. Reprinted with permission from The MIT Press. Copyright 2023.
The Network of Detectors
Traditional telescopes can be pointed to any visible direction in the sky to observe a source that is emitting light. Gravitational wave detectors cannot be oriented. They are fixed at the observatories housing them, and can receive signals from almost all directions. Therefore, if one detects a wave with a single detector, it is almost impossible to infer where it came from(1). If the signal is received in two detectors exactly at the same time, one can infer it came from some point in an imaginary ring in space located between the detectors. Each point on the ring has the same distance to both detectors, so it takes the same time to reach both of them. If the signal arrives to one detector before the other, one infers it came from a point on a ring displaced in the direction of the observatory that received it first. For example, the first gravitational wave, GW150914, reached the Livingston Observatory before the Hanford site, and this can give us some idea of where it came from.
A ring in space is still a very poor localization in astronomical terms. In reality, one can do a bit better using other information in addition to the times of detection (differences of amplitudes and phases). This tells us the source is not at any point on the ring but is from a section of it. The diagrams plotting the partial rings on the celestial sphere are colloquially called “bananas” due to their shape; figure 11.1 shows some examples of them. The situation improves qualitatively if we have three detectors. We can then use the three times of arrival to triangulate the position of the source in the sky. This is the same technique used by police to locate a suspect’s cell phone, using the signal’s received by three or more cell towers. In the case of gravitational waves, the various parameters are known with some uncertainty due to noise. That translates, even with three detectors, into not a very good localization in the sky (a blob instead of a banana). More detectors would improve the precision, and fortunately, there will be more detectors joining the network in the near future.
The localization of the source is very important for astronomers. If a precise location in the sky is known, they can point their telescopes in the given direction and look for a signal – optical, gamma rays, radio waves – associated with the source. GW170817, the first collision of neutron stars observed by LIGO and Virgo, was localized within a region that occupied 0.04 percent of the sky. This area is not so small, corresponding to 150 times the area of the sky occupied by a full Moon – but it is the best localized source to date.
Figure 11.1 shows a map with the localization areas of some of the O1 and O2 detections (note their banana shaped contours). This type of image in which the celestial sphere is mapped onto an ellipse, is common in astronomy, but it distorts the geometry quite a bit. That is why a portion of a ring looks like a banana and some rings look even more distorted. It is the same problem as found in world maps of the Earth, where the regions near the north and south poles are very distorted. The map of the figure allows us to compare the most poorly localized source in O1-O2 (GW170823), in violet, obtained with two detectors, and the best localized (GW170817), in pink, measured with three detectors.
Multiple-Messenger Astronomy
The detection of the merger of two neutron stars observed by LIGO and Virgo ushered in a new era in astronomy: multi-messenger astronomy with gravitational waves. It has been dubbed with such an imposing name because it really involves the use of many couriers, all providing information about the same phenomenon but each contributing to paint a more comprehensive picture than their separate messages alone would deliver.
The full Interdialogue with Jorge Pullin.
For example, in 1987, a supernova explosion in the Large Magellanic Cloud (a neighbor galaxy of the Milky Way) was observed with many kinds of telescopes. It also generated neutrinos that were observed in experiments on Earth, giving us rich information about the explosion mechanism. The importance of having multiple messengers is reflected in what happened on August 17th 2017. That day the LIGO and Virgo instruments detected GW170817, a source of gravitational waves emitted by the collision of two small objects that could be neutron stars. It happened at 12:41:04 Coordinated Universal Time, which corresponded to 8:41 AM Eastern time. Independently, the gamma ray detectors of the NASA Fermi satellite detected a gamma ray burst (GRB) of short duration (less than 2 sec) that received the name GRB170817A, approximately 1.7 seconds after the instant when LIGO and Virgo observed the collision. The European gamma ray telescope INTEGRAL confirmed Fermi’s detection in a later reanalysis of its data. Fermi had made its detection public with an automated system, and LIGO/Virgo alerted the astronomical community shortly thereafter. The estimated distance to the source was 130 million light-years, which is close to the Earth by astronomical standards, so electromagnetic waves from the collision could perhaps be observed. This immediately launched an observation campaign with multiple observatories and telescopes including those from the neutrino and cosmic ray community. The campaign ended up involving over 2,000 astronomers in addition to the members of the LIGO/Virgo Scientific Collaboration. The telescopes looking for visible light from the blob in the sky had to wait for a few hours until it was nighttime. They concentrated the observations on about 40 galaxies in the region of interest that LIGO/Virgo had found.
Slightly under 11 hours after the alerts went out, the Swope telescope2, operated by the 1M2H Collaboration,3 observed a very bright object that did not appear on previously archived images they had from the same region of the sky (see figure 11.2).
The International Astronomical Union assigned this object the identification AT217gfo, located near the galaxy NGC4993, at about 130 million light years from Earth. This object was independently detected by various groups within the hour (see figure 11.3). Observations continued for several days (e.g., figure 11.2), and for some instruments, months. In the following days, the brightness dimmed in the blue but it increased in redder hues. Nine days later X-ray emissions were detected and 16 days in the wake of it, radio waves were detected as well. They were all coming from the same direction in the sky. Such emissions lasted longer than a year in the radio band.
The 2017 observation of gravitational waves and a gamma-ray burst produced by the same event provided another confirmation of Einstein’s prediction: gravitational waves travel at the speed of light. Since the gamma rays and gravitational waves were detected only 1.7 sec apart from each other after traveling for 130 million years, that provides a very stringent bound on any difference between the speed of gravitational waves and the speed of light (one part in 1016).
The Mystery of Gamma Rays
The emission of gamma rays in a collision of neutron stars was not completely unexpected. The origin of GRBs had been a mystery that was still being unraveled, and GW170817 provided important clues.
During October of 1963 the US Air Force launched the first of a series of satellites known as Vela (“velar” means “to watch” in Spanish). They received this name since their central mission was to monitor the compliance by the Soviet Union of the 1963 Partial Test Ban Treaty4. These satellites had sensors to detect X-rays, gamma rays and other high-energy particles that could result from a nuclear explosion. During all their years in operation they never observed a violation of the treaty. However, with the fourth series of these satellites, which had enhanced sensitivity, a very intense source of gamma rays was detected in 1969. But it did not come from the ground: it came from outer space. Aside from the fact that the source was unknown, and the distance to it could not be inferred, one thing was certain: it originated in an extremely energetic event.
This event motivated the design of space missions to start the study of the phenomenon. In 1991, NASA launched the Compton Gamma Ray Observatory, named in honor of the American physicist Arthur Holly Compton, who received the Nobel Prize in 1927. It was the heaviest satellite launched by NASA at the time. Its mission was part of the NASA series of Great Observatories, which also included the famous Hubble Space Telescope. Several other missions launched by NASA and by the European Space Agency (ESA) were dedicated to studying these events.
The first years of cosmic gamma ray observations allowed a preliminary classification of the various GRBs detected. GRBs could be divided into two categories: long and short duration. In those of long duration, most of the burst energy is emitted over a period longer than 2 sec, and they have a “soft” (i.e., concentrated in longer wavelengths) emission of energy. The short-duration bursts lasted less than two seconds and emitted most of their energy with a “harder” (shorter wavelength) emission. The origin of the two groups appeared to be different.
In February 1997, it was possible for the first time to identify with certainty the distance to a GRB source: the BeppoSAX satellite – an Italian–Dutch collaboration – detected GRB9702285 and observed an associated emission of X-rays. Some 20 hours later, the William Herschel6 telescope in the Canary Islands identified the optical counterpart of the event. It was located in a galaxy so distant that it could barely be observed, and for many years the distance to it could not be determined precisely. But one thing was clear: these explosions originate outside our galaxy. Later that year, the same satellite identified another source: GRB970508. This was the second GRB observed with an associated emission of X-rays and visible light, but its distance was determined to be 6 billion light years away.
The following year, the connection of GRB980425 with the explosion of a supernova was established. The first years of the twenty-first century allowed astronomers to determine that the GRBs of long duration and soft energy were associated with the collapse of a massive star, a process in which either a neutron star or a black hole is created. But until 2017, no conclusive evidence that identified the progenitors of the short GRBs. These have such high energy levels that they emit in less than 2 sec all the energy the Sun will produce in its life. Various elements in the observations have led to the belief that the emission of these GRBs is beamed as a narrow jet. It is expected that statistically, only a small fraction of these jets will point towards Earth and be detectable. But it is also estimated that the events are sufficiently infrequent as to happen only twice every million years per galaxy. We should not be so sad about not witnessing such an event in our Milky Way. Their energy is so huge that if a burst like this did occur in our galaxy and the beam were directed toward us, it could most likely extinguish life on Earth. Some scientists have speculated that the massive Ordovician extinction that happened 440 million years ago was provoked by a GRB emission striking straight at our Earth.
The GRB observations during this century raised the suspicion that the short GRBs involved small progenitors compared to the supernovas, like collisions of binary neutron stars, or of a neutron star and a black hole. Immediately before the collision the stars rupture due to the mutual forces generated7. The emission produced by the rupture is very fast given the small volume of the neutron stars (radius of about 10 km) and because in the final moments, matter is moving close to the speed of light. But the evidence was not conclusive. Only with GW170817 was it unambiguously verified that a collision of neutron stars produces gravitational waves and that less than 2 sec later, a gamma ray jet reaches the Earth. Almost immediately, several observations showed a series of effects that had been predicted by various theoretical models proposed by astrophysicists for collisions of neutron stars. The birth of the multi-messenger astronomy had solved a mystery that was more than 50 years old. For both supernova collapses (long GRBs) and the mergers of two neutron stars (short GRBs), the most likely outcome is the creation of a compact object heavier than a neutron star. GRBs are the wailing sounds of black holes being born.
All That Glitters…
The collision of these two neutron stars helped solve another mystery besides that of short GRBs: the origin of the heavy elements that we find in the universe. The heaviest elements that can be formed in stars or novas (or even supernovas) in significant quantities are iron and cobalt. Heavier elements also get produced, but not in quantities large enough to explain, for instance, the abundance of precious metals like gold and platinum found on the Earth.
Listen to Jorge Pullin speak about LIGO and future planned advanced gravitational wave observatories, the first detections of gravitational waves via black hole and neutron star collisions, applications of multi-messenger astronomy, gamma-ray bursts, and the cosmic origins of heavy elements like gold.
The full Interdialogue with Jorge Pullin.
When the two neutron stars collide, a gigantic explosion ejects matter in an expanding shell of colliding neutrons and atoms moving at 1/3 of the speed of light. This material is compressed at densities that are hard to imagine and completely unachievable on Earth. The blast generates a nuclear process called the r-process. The r-process entails a succession of rapid neutron captures (hence the “r” in the name) by one or more heavy seed nuclei, such as iron (recall that neutron stars are made of more than neutrons). Most of the elements that are formed through this process of fusion are unstable isotopes that decay radioactively.
It is through this process that electromagnetic waves in the ultraviolet, visible and infrared region of the spectrum are emitted. A cocoon of fireworks shines intensely: it is known as a “kilonova”, due to the amount of energy being released, which falls between the amount emitted by novas and that emitted by supernovas. Novas are also the product of stellar collisions, but they involve stars that are not so compact (e.g., white dwarfs). There is not as much mass involved as with kilonovas, and the energy released is about 1,000 times lower. Figure 11.4 shows an artist’s conception of a kilonova.
All chemical elements consist of ensembles of neutrons and protons tightly packed in the atomic nucleus with a surrounding cloud of electrons that balances its electric charge (the electric charge of a proton is the same as that of an electron but is of the opposite sign). The simplest of all the elements is a hydrogen atom with only one proton and one electron.
After the Big Bang, the universe expands, cools and matter starts to form, beginning with elementary particles, followed by atoms and molecules, in a process known as nucleosynthesis. About a few hundred thousand years after the big explosion the universe is cool enough for the nuclei of atoms formed by neutrons and protons to start capturing electrons and forming atoms. The process starts with hydrogen and a much smaller quantity of the next heavier element, helium. That is why hydrogen and helium are called “primordial” elements. These elements form molecules and group together in big clouds, where stars are created. It is in stars that more complex chemical elements are formed in a fusion process. In particular, all of the elements of the periodic table up to cobalt and iron, with others in much smaller quantities. When the stars explode, the created elements eventually make up a newly formed star system like the solar system hosting our Earth and are enough to explain the evolution of life on the planet, including human life. Paraphrasing Carl Sagan, we are made of stardust. Figure 11.5 shows a periodic table listing the origins of the various elements.
In nova and supernova explosions, the process of creation of heavier elements – like gold and platinum – continues but without producing sufficient quantities of them to explain their abundance ratios in the universe. With the neutron star collisions, the process of creation of all chemical elements is complete. This is what has been learned from the analysis of the observations subsequent to the detection of the collision made by LIGO and Virgo, and it is a good example of what the new discoveries from multi-messenger astronomy can achieve. Although the spectra obtained from AT2017gfo (the kilonova associated with GW170817) were approximately consistent with an outflow of radioactive heavy elements, there was no clear identification of any particular element. But in October 2019, a team of scientists, led by Darach Watson and Camilla J. Hansen from the Niels Bohr Institute, in Denmark, reported that after a reanalysis of the spectra obtained in 2017, they managed to identify the presence of the element strontium. This element is known to be the result of a neutron-capture process. It was the smoking gun proving the long-suspected speculation that neutron star mergers, through their kilonova explosions, are where heavy elements are manufactured in the universe. It is estimated that in AT2017gfo, gold and platinum equivalent to 10 times the mass of the Earth were produced and ejected into space during the burst, together with 16,000 Earth masses of other heavy elements. We may be tempted to go and pick up that much gold, but that gold is spread out in the universe. The gold we find on Earth probably comes from similar events.
The Speed of Expansion of the Universe
Observations using different astronomical instruments of the collision of two neutron stars with different astronomical instruments have contributed to our understanding of the origin of short GRBs and the genesis of heavy elements in the Universe. Remarkably, these observations have also allowed astronomers to measure in a completely novel way the rate of expansion of the Universe. This is a contentious topic, because the two other existing methods to measure this rate yield results disagreeing with each other.
The full Interdialogue with Jorge Pullin.
In section 3.4, we described how in 1929, Hubble discovered that the speed at which all galaxies recede from the Earth is proportional to their distance from Earth. That is, if you divide the numerical value of the velocity of a given galaxy by the numerical value of its distance from Earth, the result always gives the same number, no matter which galaxy you use for the calculation. The numerical value of the result is known as the Hubble–Lemaître constant. This is what astronomers wish to measure.
The velocity of galaxies and other objects can be determined in a relatively straightforward manner through their redshifts. Measuring distances, however, is more complicated. As we discussed in section 2.2, parallax can be used, but only for nearby objects. As the distances increase, the parallax angles decrease and eventually are too small to be measured. For objects farther away, astronomers typically use relations between the objects’ brightness and other properties of them, like their spectra or how their intensity varies with time. Hubble used the relationship between the time variation of light in a type of stars known as Cepheids.
The two modern methods were developed by two different collaborations. One is the SH0ES collaboration. Its name is an acronym for “Supernova H0 for the Equation of State” (H0 is the Hubble–Lemaître constant). It is based on using the relationship between how much light supernovas emit and other properties of the emission, such as how it decays with time (the “light curve”). Once we know how much light they emit, we can infer how far away they are by noting how luminous they appear when observed from the Earth.
The other method is pursued by the Planck collaboration and uses data from the European Space Agency’s Planck satellite, named after German Nobel Prize laureate Max Planck, who is one of the creators of quantum theory. This satellite measures very precisely the cosmic microwave background. As already mentioned, this background is composed of electromagnetic waves that have traveled through the expansion of the universe. From the details of this travel, the collaboration can infer the value of H0.
The collaborations get different results. At the time of writing of this book, the most recent result from SH0ES is 73 km/sec per megaparsec, while Planck obtains a value of 67 km/sec per megaparsec. The observation of systems like GW170817 with electromagnetic and gravitational waves adds another method for measuring H0 and could eventually help settle the debate. The amplitude and frequency of a gravitational wave contain information about the distance between the source and the Earth. If the system can also be observed optically and its host galaxy identified, we could also infer its velocity (through the redshift of the galaxy). Combining these observations, we could compute H0, using a method completely independent from those used by SHOES and Planck. The detection of GW170817 and its localization in the galaxy NGC4993 allowed LIGO/Virgo scientists to do the computation. The value obtained has a large degree of uncertainty. The likely value is between 43 and 115 km/sec per megaparsec. This range includes both the SHOES and Planck values, so it cannot settle the debate. However, further detections will narrow that range. It is estimated that with 20 observations, one could end up with an accuracy of 2 percent. Multi-messenger astronomy with gravitational waves will help elucidate these great cosmological mysteries.
Banner image: “Forces” – Vannessa Circe – Oil on Canvas – 2020
1In reality the detector has different sensitivities in different directions, so some localization is possible, but with great uncertainty, roughly 1/4 of the sky.
2Named in honor of the American astronomer Henrietta Swope and located in Las Campanas, Chile.
31M for the diameter of the mirror of the telescopes and 2H for two hemispheres, since it consists of two instruments, one in the north and one in the south.
4Its full name was Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water, and it prohibited all test detonations of nuclear weapons except the underground ones.
5This is the same date convention that we mentioned before for gravitational waves; in fact it was modeled after the GRB convention.
6Named after the 18th century German-born British astronomer who discovered Uranus.
7Something similar happens in the collision of a neutron star with a black hole; in that case only the star ruptures.
