I am very happy to host here today an article by my INFN colleague Alessandro de Angelis, a well-known and authoritative italian astrophysicist. Alessandro has recently published a beautiful new book on this subject, which I invite you to have a look at (see link at the bottom of the article) - T.Dorigo .

In 1610 Galileo Galilei published the astronomical book “Sidereus Nuncius”, the first scientific paper based on observations made with the telescope. In the book Galilei comments and interpret the observation of the mountains of the Moon, of hundreds of stars never seen before, and of Jupiter's satellites. All these observations had been possible thanks to the light emanating from the heavenly bodies: according to Galilei, light is the nuncius (messenger) of the stars. The history of cosmic messengers, thus, begins with the photons, particles of light that represent the quanta of the electromagnetic field, and up to today the photon is the supreme messenger. Being neutral, photons are not diverted by the magnetic fields in the intergalactic space and in the Milky Way, and so it is possible to locate their sources.

Humans can only see a very small part (thus called “visible”) of the electromagnetic waves, i.e., of photons. Waves with a length of the order of a few tenths of micrometer are visible, that is, with energies of the order of an electron volt (eV), about 10-19 joule. These are the energies that atomic electrons release when they move to a lower energy level. And our brain interprets the small energy and wavelength differences of these waves as colors. The blue of the ocean has 10% greater energy than the green of the forests.

From the 1930s, the wave spectrum we can observe in the Universe started to expand. We could see new “colors” invisible to the naked eye: the first radio waves produced by the Sun and by the galaxy, billions and millions of times less energetic than visible light; then microwaves, and finally X-rays and gamma rays, millions to billions of times more energetic than visible light. The history of astronomy between 1930 and 2015 can be interpreted as a long journey to discover the colors of the universe invisible to the human eye. Thanks to the comparison between the signals at various wavelengths, we learned a lot about the origin and evolution of the Universe, and many of the discoveries in recent years are due to the detection of high energy gamma photons. In particular, we have seen that supernova remnants in the Milky Way accelerate cosmic rays up to a few thousand TeV, that binary systems of which one is a compact object (a black hole or a neutron star) can behave as powerful particle accelerators, and we could observe the mechanisms of accretion and radiation of supermassive black holes in other galaxies. We have seen gravitational lenses caused by black holes with billion solar masses, which cause the same gamma-ray signal to be repeated within a few days distance. All this was possible thanks to new detection technologies, mostly coming from accelerator particle physics. But we also confronted with the limits of our knowledge, opening new questions. What happened in the first moments of the life of the Universe? What are dark matter and dark energy? How do black holes grow and evolve?

Knowledge has evolved suddenly and in part unexpectedly in recent years.

In February 2016, the first detection of a gravitational wave was announced. Gravitational waves are produced in the acceleration of masses with spherical asymmetry. They deform space-time and increase and decrease with a constant cadence the distances in space in two directions 90 degrees from each other, perpendicular to the direction of wave motion. The effect is very small: for a released energy corresponding to about 3 solar masses, as in the first event detected by the LIGO instrument in 2015, the relative effect on Earth is about 10-22 - that is, as if the distance between the Earth and the Sun varied by an atom. Einstein’s opinion was that the gravitational radiation was too small to detect it. However, thanks to laser interferometry, the two LIGO detectors, separated by 3002 kilometers, which at light speed imply a time difference of about 10 milliseconds, could do the job., observing two black holes orbiting and then merging while emitting gravitational radiation - the final phase of the collapse lasts just a few seconds. Gravitational waves promise a revolution in astrophysics, opening up a whole new way of observing the most violent events in the Universe: they travel unimpeded at the speed of light and provide unique information about cataclysmic collisions. For the discovery of gravitational waves, the 2017 Nobel Prize for Physics was awarded to Barish, Thorne and Weiss.

Figure 1 - Observation of the gravitational wave called GW170817 and, a few seconds later, of the gamma ray burst GRB 1709817A. Source: ESA.

In October 2017, another revolutionary announcement: for the first time gravitational waves were detected together with gamma rays in the merging of two neutron stars. Two neutron stars in orbit lose energy emitting gravitational waves; such energy loss brings them closer and closer until they merge. Some scientists had suggested that the fusion of neutron stars could produce most of the elements heavier than iron in the periodic table and therefore is central to the evolution of life as we know it - of course these elements must be formed in an environment rich in neutrons. It was suspected that the signal of such a merging could quickly cover the electromagnetic spectrum, from gamma rays to X-rays, visible light and infrared. Some thought that one of the consequences could be the formation of a very energetic gamma-ray burst. In the end, all the pieces of the puzzle fit the observation of the gravitational wave by the LIGO/Virgo collaboration and, almost simultaneously, of a flash of photons along the entire electromagnetic spectrum by astronomers and astrophysicists all around the world (figure 1). On the one hand, this confirms the hypothesis that the fusion of neutron stars produces short gamma ray bursts; on the other hand, it confirmed nuclear fusion models supported by theoretical physics and real-world observations. It is not so common to see something new, and even rarer that this confirms old theories.

Figure 2 - The building of the IceCube observatory at the South Pole and the muon coming from from the 300 TeV neutrino generated by blazar TXS 0506 +056. Source: NSF.

Finally, in July 2018, another “big bang” in science. The collaborations IceCube (figure 2), Fermi and MAGIC (figure 3) announced the simultaneous detection of a signal of gamma rays and a neutrino from blazar TXS 0506 +056, a supermassive black hole (billions of solar masses) placed in the center of a galaxy and accreting at the expense of the surrounding mass, with jets of energy pointing to Earth. Once again, we could solve a mystery: comparing neutrino and gamma ray energies revealed that in the vicinity of blazar matter (hydrogen nuclei) is accelerated to energies tens of thousands times higher than those of the LHC, and unveiled the mechanism of gamma ray and neutrino production: subnuclear collisions between protons and a “sea” of low energy (ultraviolet) photons.

Gravitational waves and neutrinos, like photons, point directly to their source of production: the simultaneous observations of two or more of these messengers opened the field of multi-messenger astrophysics, further integrating particle physics and astrophysics. Today we can start answering some fundamental questions that seemed to be beyond our reach. After building instruments capable of observing new colors, we are developing new “senses” and we start to know them. As touch, smell, hearing, and taste give us information about the reality that surrounds us, completing what appears through vision, we are now beginning to collect and analyze new information from remote regions of the universe transmitted not by light, but by different messengers.

Figure 3 - The two 17-meter telescopes MAGIC in the Canary island of La Palma and, in the middle, the first CTA Lasge Size Telescope (23 m), in construction. Source: N. Giglietto.

What did we learn in the first three years of this new astronomy? We explained the mechanism that generates short (neutron star coalescence) and long (collapse of large mass supernovae) gamma-ray bursts, until a few years ago an astrophysical puzzle. We revealed a galaxy that simultaneously emits neutrinos and gamma rays, discovering at its center an accelerator of tens of millions of GeV. We saw how atoms heavier than iron are created, and this is important for understanding life. We have discovered that the universe is surprisingly full of “small” black holes of a few tens solar masses.

What do we plan to do and we hope to understand in the next ten years? New satellites sensitive to photons in the energy region between MeV and GeV and new instruments on Earth sensitive to photons in the energy region from hundreds of GeV (such as the CTA telescope, Figure 4) and beyond will pave the way for understanding the highest energies in the Universe and will probably lead to the discovery of new physical phenomena, particularly in synergy with neutrino and gravitational wave detectors. The colors of the photons will be enriched with by new flavors in the multi-messenger context.

Figure 4 - Rendering of the multi-telescope array CTA, in construction. Source: CTA.

This decade will be remembered in the history of science for the birth of multi-messenger astronomy, and new detectors promise new sensational discoveries. We will build ever larger and technologically advanced instruments. IceCube, which revealed the first neutrino signal, is an instrument a cubic kilometer in volume located in the Antarctic ice: we will enlarge it to 10 cubic kilometers. The Cherenkov Telescope Array (CTA) will cover surfaces hundreds of times higher than current telescopes. New satellites will require new technologies in the field of silicon trackers and light detectors sensitive to a single photon.

Bibliography - A. De Angelis and M. Pimenta, Introduction to Particle and Astroparticle Physics: Multimessenger Astronomy and its Particle Physics Foundations, Springer-Nature, Heidelberg 2018.