Captures an invisible gravitational wave signal

Two gravitational wave detectors in Europe and the United States have detected a sign of an unprecedented cosmic catastrophe: the collision between black holes and neutron stars.

The two events that were discovered occurred hundreds of millions of years ago. Since then, the waves they created have traveled in space-time toward Earth at the speed of light. Several years ago, physicists used Albert Einstein’s equations of general relativity and calculated what type of gravitational wave such an event would produce. The two signals picked up by the LIGO detectors in the US and Virgo in Europe match the predictions of the German physicist a century ago.

Neutron stars are hallucinogenic things. When a star reaches the end of its life, it can collapse in on itself like a huge building until it forms a ball with a diameter smaller than the diameter of a city like Madrid. Inside, the material is so compact that a teaspoon of a neutron star weighs the same as everyone else on the planet. These objects are subject to stressful conditions that are impossible to reproduce in controlled experiments. It is believed that inside these stars there are huge groups of subatomic particles, the elementary particles that make up atoms. The ability to observe exactly what is going on inside would be one of the greatest discoveries in the history of physics.

Toni Font, a member of the science collaboration between LIGO and Virgo that captured the signals, explains: “This discovery confirms for the first time that there are binary systems consisting of a black hole and a neutron star, and we can observe them thanks to gravitational waves.”

The team acquired the two mergers over a 10-day period in January. In one of them, a hole nine times the mass of the Sun collided with a neutron star of 1.9 solar masses. These two objects may have been orbiting each other for tens of millions of years, but the signal picked up is only the last bit, when the two collide, and lasts for a few seconds. The catastrophe occurred in a place 900 million light years from Earth, that is, it would have to travel at the speed of light for 900 million years to reach it, which is absolutely impossible for human technology.

The second merger occurred between a hole six times the mass of the Sun and a neutron star 1.5 solar masses, which collided about a billion light years ago, or a billion years ago, when single-celled life was just beginning. Land. .

Once the two signals were picked up, the two detectors set off an international warning that other telescopes would try to capture potential light from these disasters. They didn’t see a single flash, which made perfect sense. When the size of a black hole and a neutron star does not differ much, the hole breaks the star into a kind of noodle that continues to spin until it is completely swallowed up. In this case, a flash may be emitted. That may have happened in 2017, when LIGO first detected gravitational waves and light from the merger of two neutron stars.

When a black hole is much larger than a star, the merger is surprising. “The black hole swallows the star at once, without breaking it first,” explains Font. “This appears to be the case in the two events that were recorded,” the researcher adds. Details of these two phenomena were published on Tuesday in Astrophysical Journal Letters.

Gravitational waves are distortions of spacetime, the matter of which the universe is made. They are similar to the waves that a stone makes when it falls into the water of a lake. The ability to measure these fluctuations predicted by Einstein gives humanity a new way of looking at the universe. One of the main goals of the observatories involved in this discovery is to capture more hybrid mergers of the same type, especially those that also emit light, because they provide more information, explains Juan Calderon, a researcher in Galician high energy physics. Institute and co-author of the study. The physicist explains: “In these two cases there was no electromagnetic signal, so we can only say that one of the two objects in question must be a neutron star because, in theory, it is too light to be another black hole.” When the merger emits x-rays, gamma rays, or any other electromagnetic signal, it allows “a better understanding of how matter inside a neutron star behaves, which is one of the greatest open questions in physics today,” Calderon says. These mergers make it possible to verify that gravitational waves and light travel at the same speed, as predicted by Einstein.

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