Since the first detection of gravitational waves issued during the collision of two black holes By the end of 2015, nearly 100 events had been detected where two such objects, or two neutron stars or one of each, collided, emitting large amounts of energy in the form of these waves. Gravitational waves are nothing more than disturbance in the gravitational field caused by mass movement. In other words, just moving an arm or dropping a rock is technically creating gravitational waves, but they are so ridiculously weak that are imperceptible even to our most powerful instruments. It is not until we consider the collisions of two of the densest and most extreme objects of the universe that we can consider its detection and study.

Gravitational waves have been predicted by the theory of general relativityby the German physicist Albert Einstein. Since this theory tells us about how mass and energy contribute to space-time warp and about how this deformation determines how mass and energy move, he had something to say about these waves. Nearly a century after his prediction, we have direct evidence the existence of gravitational waves. We emphasize the “direct” because there were already indirect indications since the 70’s, when it was detected as a system composed of two pulsars orbiting each other, they lost energy by emitting, we presume, these waves.

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Since that first detection, the systems in charge of carrying out it have been refined and perfected, with which we have been able to increase the sensitivity and reliability of the detections. What is more LIGO and VIRGO detectorslocated in the United States and Italy, respectively, other new projects have been developed recently, such as KAGRA in Japanthat started operating recently or IndIGO, India, which has not yet started its construction. Plans are also being developed to establish gravitational wave detectors in space, such as the LISA Space Telescope.

Although the detection of gravitational waves emitted during the collision of black holes or neutron stars is interesting in itself, and although there are other less compact binary systems in the spotlight, such as two white dwarfsThe ultimate goal, the holy grail we might call it, of this new branch of astronomy is to detect what is known as the gravitational waves cosmic backgroundthe very echo left by the rapid expansion of the universe after the big bang.

This cosmic background can be compared (and perhaps confused) with the microwave cosmic background, which is the oldest light we can observe in the universe. After the Big Bang, the universe was expanding and consequently cooling. The particles that formed it were losing energy. during the first 370,000 years the protons and electrons that inhabited the universe had enough energy and were close enough together to almost immediately absorb any light, any photon, which was emitted by any of the other particles. During this time the universe was opaque, as no light ray could travel more than a short distance without being reabsorbed. However, that changed around those 370,000 years of the universe, when the average energy of protons and electrons dropped enough to allow them to form. neutral hydrogen atoms. From that moment on, the emitted photons could travel freely, becoming the transparent universe. This light has been losing energy with the expansion of the universe until the present day and we can currently detect it as background radiation in the microwave range.
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Something similar should have happened with gravitational waves, being able to observe the background radiation, a kind of gravitational humwith the difference that this cosmic background of gravitational waves would have been issued shortly after the big bang, without waiting hundreds of thousands of years. This would therefore allow obtaining valuable information about the first moments of the universe, a moment unattainable with the detection of light. We think that the new generation of gravitational wave detectors could detect such a cosmic background. LIGO and VIRGO have been incorporating improvements recently and in March 2023 they will resume their observations, in parallel with the KAGRA detector.
This cosmic background will also be blurred by another cosmic background, less fundamental, but also interesting. Any planet orbiting a star, any pair of stars orbiting each other, or any pair of black holes, neutron stars, white dwarfs, and other exotic objects orbiting some other type of these objects must emit gravitational waves. At the moment we are only able to detect the waves emitted by the most extreme events, but every little contribution must contribute to a kind of cosmic background, to a gravitational noise that can be detected. This noise could, for example, help us in our characterization of dark energy and matter, because depending on whether it has some properties or others, it will indicate the presence of matter or energy in different forms and configurations.

References:

  • JD Romano, NJ Cornish (2017). “Detection methods for stochastic backgrounds of gravitational waves: a unified treatment”. Live Rev Relativ. twenty (1): 2, doi:10.1007/s41114-017-0004-1