São Paulo Advanced School on Multi-Messenger Astrophysics

 

May 29 - June 7, 2023

Principia Institute – São Paulo, Brazil

Target audience of the School: graduate students in physics, astronomy and related fields

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Motivation

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Traditional astronomy based on electromagnetic signals (photons) has been responsible for some of the most groundbreaking discoveries in Science: from the expansion of the Universe to an image of the event horizon of a black hole, from the cosmic microwave background to the onset of star-formation and galaxy formation, all the way to dark energy. Up until very recently, electromagnetic radiation was driving observations and the advances in our understanding of physical theories behind those phenomena.

However, even as far back as 100 years ago there was already some evidence that these light signals did not fully represent the matter content of the Universe, leading to the first cosmological evidence pointing towards the existence of dark matter. Nowadays the prospects for the discovery of dark matter, which is 5-6 times more abundant than baryonic matter, rely mostly on other messengers.

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Moreover, several decades ago there were already clear indications that high-energy cosmic rays from outer space were hitting Earth and producing particle showers that propagated through the atmosphere all the way to our detectors. After decades of efforts, facilities such as the Pierre Auger Observatory, HiRes and the next generation of Imaging Air Cherenkov Telescopes, represented by the Cherenkov Telescope Array, were designed and built to detect these showers, and to link their properties to those of the ultra-high energy charged particles and photons that initiated them, eventually pinpointing their origin. These observations are allowing astronomers to trace the origin of these cosmic rays and high energy photons, and to understand the physical mechanisms responsible for accelerating particles to such high energies.

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On the other hand, over the past 50 years a series of efforts to detect neutrinos produced by the Sun—as well as those produced here on Earth in nuclear power plants and in particle accelerators—started to observe those fleeting particles with better accuracies and over a greater range of energies. These experiments reached a spectacular result with the observation of a burst of neutrinos produced during the explosion of the supernova 1987A, which were detected by the different experiments including Kamiokande. More recently, in 2017, a high-energy neutrino observed by the the IceCube Neutrino Observatory was traced to the blazar TXS 0506 +056, located 5.7 billion light-years from Earth.

 

A more recent scientific triumph was achieved when LIGO (the Laser Interferometer Gravitational Observatory) and three other similar facilities detected for the first time one of the most spectacular theoretical predictions of all time: gravitational waves. These ripples in space and time, which propagate at the speed of light, were already indirectly observed through the decaying orbit of the binary pulsar PSR 1913+16, observed by R. Hulse and J. Taylor: the rate at which the radius of the orbit decays with time as a result of the emission of radiation in the form of gravitational waves can be calculated with exquisite precision and accuracy, and matches the observations of the system carried ever since it was discovered, in 1974. However, a direct observation of gravitational waves had to wait another 40 years. As soon as LIGO became capable of detecting gravitational waves in 2016, it inaugurated a new era of astronomy, starting with the observation of the merger of a pair of massive black holes with a total mass over 55 solar masses which radiated almost 3 solar masses in the form of gravitational waves.

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Perhaps the most spectacular event of all was detected by both LIGO and the Fermi satellite: the merger of a binary pair of neutron stars in 2017 (GW170817). The event was observed at nearly the same instant by LIGO/Virgo as a gravitational wave "chirp", and by the GBM detector aboard Fermi as a burst of gamma-rays. It was in fact the coincidence of the signals in these two messengers (gravitational waves and light), and the subsequent kilonova with its optical counterpart, which allowed astronomers to locate the origin of the event – in the outskirts of the galaxy NGC 4993, at a distance of approximately 40 Mpc from Earth.

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Format of the Advanced School

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All evidence now points to a future where many types of astrophysical objects may be observed in a variety of ways: multiple wavelengths of light, neutrinos, high-energy particles and/or gravitational waves. The possibilities and opportunities for combining these observations are a treasure-trove for new discoveries: what are the physical mechanisms behind the acceleration of particles to energies as high as 1020 eV? What happens as BHs and NSs collide? Is Einstein's General Relativity an accurate description of gravitational phenomena in these cataclysmic events? Can we detect supernovae by measuring the bursts of neutrinos that are produced when their core finally collapses, seconds before the supernova explodes? Is it possible to solve the long-standing dichotomy about the expansion rate of the Universe (the "Hubble tension") using binary NS mergers as standard sirens to measure cosmological parameters? Can we detect the cosmic background of neutrinos, which is believed to be a thermal relic with a temperature of approximately 2K today, and which should have decoupled from the remaining matter and radiation before the first minute after the Big Bang? Can we combine the cosmic microwave background with observations of primordial gravitational waves to show whether or not there was in fact an inflationary phase at the very beginning of the Universe?

 

These are some of the fundamental questions that may be addressed by research in MM Astrophysics, and the São Paulo School of Advanced Science on Multi-messenger Astrophysics will train a new generation of students to start working in these exciting new fields of research.

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