Since the beginning of astronomy, when it was used to mark the passing of days and seasons, or as the basis for myths and legends, all observation of the night sky (or daytime, of course) has been based on the same principle: interpret the light that came to us from those tiny flickering dots . The light that our eyes were capable of seeing, specifically, the part of the electromagnetic spectrum that we know as visible light.
Over the centuries and with the advancement of technology, we were able to cover more and more of that spectrum, being able to detect infrared or ultraviolet light, even observing the sky by detecting radio waves and X-rays. All these are nothing more than the names we give to different parts of the electromagnetic spectrum, differentiated only by the wavelength (or frequency, since they are directly related) of said light. It is as if, among all the possible sounds, we called some “audible sounds” and others infradoes or ultrasíes (for the musical notes Do and Si). We are not able to perceive them due to the limitations of our natural instruments (eyes and ears), but they are there.
By adding all these wavelengths to our toolbox, we managed to open up a window that we had been looking through only between gaps and gaps in the wood. A window with which to observe the universe .
In the last decades we are opening other new windows. Windows that overlook different sides of the house and with which we can discover completely new landscapes. These windows are based on the observation of neutrinos on the one hand and gravitational waves on the other. This is one of those places where we can see the intimate relationship between astrophysics (which studies the largest objects in the universe) and particle physics (which studies the smallest).
Neutrinos are tremendously elusive particles , which are very difficult to detect, but precisely because of these properties they can give us information that photons of light never could. You see, neutrinos are only affected by 2 of the 4 fundamental interactions, the forces that govern the behavior of all particles in the universe. One of these forces is gravity, from which no one escapes, and the other is the weak interaction. This interaction, as its name suggests, is relatively weak. Also, for two particles to interact weakly, they must be very close to each other . At distances on the order of the size of a proton, in fact.
It is often said that a neutrino could pass through a sheet of lead 1 light year thick and would only have a 50% chance of colliding with any of the lead atoms that make it up. There are no sheets of lead that thick in the universe, but what there are are stars with diameters of more than a million kilometers and that are basically invisible to the neutrinos generated in their cores.
The Sun, like the rest of the stars in the universe, stays hot thanks to the nuclear fusion processes that take place inside it. During these processes , a large amount of energy and a large number of neutrinos are released . These neutrinos, as they are barely capable of interacting with matter, leave the Sun as if nothing happened, passing through its almost 700,000 kilometer radius, its temperatures of millions of degrees and its multiple layers. Some of those neutrinos make their way to Earth where we are able to detect them. These neutrinos, which have so much energy that they travel at almost the speed of light, take just over 2 seconds to travel from the core to the surface of the Sun. By comparison, photons produced inside the star can take hundreds of thousands of years to reach its surface, after countless absorption and emission processes.
But we have not only detected neutrinos coming from the Sun. Neutrinos emitted during supernova explosions have also been detected. In fact, these particles serve as an announcement before an imminent supernova, since their production increases just before the explosion takes place. Other neutrinos that we hope to be able to detect are those that were produced during the first second of the life of the universe . Detecting them would help us to see the universe at an age to which, observing only photons, we do not have access.
The detection of gravitational waves also allows us to study processes that would otherwise go unnoticed. These waves are nothing more than the disturbance in the gravitational field produced by the movement of any massive object . You could understand them as the wake left by the gravity of the object, which travels very fast (at the speed of light) but not infinitely fast. When the object in question is as massive as a neutron star or a black hole and moving as fast as when two of these objects are about to collide, after orbiting each other, we are able to detect such waves. These objects are usually detectable also by observing the light they emit when doing so, but the gravitational waves they emit give us information that we could not obtain otherwise , such as the mass of the pair of objects that have collided.
By detecting these waves we can once again reach regions whose light we are unable to observe, such as the galactic nuclei, or the first moments of the universe.
These two branches of observational astronomy are relatively new (the first detection of gravitational waves occurred in September 2015) but they have already given spectacular results and have helped us improve our understanding of the universe. Future advances will undoubtedly bring great discoveries still unsuspected today.
REFERENCES:
Spiering, C. 2012, Towards high-energy neutrino astronomy. A historical review, The European Physical Journal H
LIGO and Virgo Science Collaboration, 2017, Multi-messenger Observations of a Binary Neutron Star Merger*, The Astrophysical Journal Letters
