Tuesday, August 3

The astrophysics of the new waves | Cosmic Void


On August 17, 2017, LIGO and the Virgo interferometer detected the collision between two neutron stars.
On August 17, 2017, LIGO and the Virgo interferometer detected the collision between two neutron stars.NASA and ESA

For millennia, our ability to learn from the sky has been limited to our eyes’ sensitivity to visible light, a very narrow strip of all electromagnetic radiation. The 20th century, with the development of telescopes in other ranges such as infrared, X-rays, or ultraviolet, meant an expansion of our capabilities. Like superheroes, we acquired abilities that went far beyond the visible and that have allowed us to explore the most energetic universe, but also the coldest, the most distant and the darkest. The 21st century, with the detections of gravitational waves and astrophysical neutrinos, has begun with the promise of a science that will free us, at least astrophysicists, from the tyranny of photons.

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Let’s look at it this way, so far with photons, or electromagnetic waves, we’ve had the chance to explore the particles of the universe and the quantum interactions that govern them. With gravitational waves, a path is opened that allows investigations that go further, how to approach the relationship between matter and energy, space and time. For the advancement of knowledge, the detection of gravitational waves and electromagnetic waves in the same source is explosive, like the kilonova where both were detected for the first time.

With electromagnetic waves we all know what we are talking about, it is enough to simply mention the word light. Gravitational waves are something else. They appear within the context of general relativity and give us information on the behavior of mass in the most extreme phenomena that occur in the universe. For them to occur gravitational waves that we can measure we need systems with strong gravitational fields (which generate a great curvature of space-time) and with a lot of acceleration. With current detectors, the detectable signal from the most intense sources of gravitational waves is in the form of binary systems that contain two compact objects: neutron stars or black holes.

This combination of detections with different “messenger” waves allows a science that is not possible otherwise.

What happened in 2017 will most likely be reflected forever in the history of science books. In August of that year, the LIGO-Virgo interferometers detected the gravitational wave signal that was identified as the merger of two neutron stars located about 130 million light years apart. Less than two seconds late, the first electromagnetic signal arrived: the high-energy gamma rays of the explosive cataclysm from the same region of the sky. The optical counterpart, the one responsible, a kilonova, was detected 11 hours later. It was located in the galaxia NGC 4993 and it faded and reddened very quickly. But it is also that a week later, X-ray and radio emission were measured, allowing an understanding of the event as never before had been possible.

With the reconstruction of the facts that the different types of signals allow us, we now know that humans witnessed the merger of two neutron stars. What we saw with different types of waves allows us to establish that the stars had masses of 1.4 and 1.6 times the mass of the Sun (or 1.2 and 1.4 if you consider that they rotated slowly before the merger). We can determine the geometry of the explosion, a kind of hourglass with high-speed jets, and place the axis of the jet of relativistic particles generated by the fusion in space to determine that it was pointing at about 15-30 degrees from us. From the neutron-rich material that was generated in the event (52 times the mass of Jupiter) we directly identified, and for the first time, the place where heavy elements are synthesized by a process known as fast neutron capture. But it is also that, because we know the distance, and that the gravitational and luminous signals arrived with less than two seconds of difference between them, we can conclude that there is no difference between the speed of gravity (gravity is not instantaneous, but we leave that for another post) and the speed of light.

This combination of detections with different “messenger” waves also allows a science that is not possible otherwise, for example, to measure the Hubble constant (and thus the age of the universe) by independent methods. But it also helps to clarify questions related to fundamental physics such as the constancy of the speed of light and gravitational waves or the equation of state of dense matter.

What is spectacular about this first detection is that it has opened more questions than questions it has settled: how are these binaries formed? Are they frequent? Are these systems the source of all heavy elements such as platinum, gold or iridium In the universe?. This is science. Now we have all the telescopes ready for when, in the middle of next year, the update of the gravitational wave detectors that will provide them with sensitivity to detect events at greater distances will be completed. A detection of gravitational waves will launch an activation campaign that will go through all the observatories in the world to identify the counterpart, including high and low energy ground and space observatories. Although what we detect already belongs to the past (it has already happened), what we learn awaits us in the future.

Eva Villaver She is a researcher at the Astrobiology Center, dependent on the Higher Council for Scientific Research and the National Institute for Aerospace Technology (CAB / CSIC-INTA).

Cosmic Void It is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than 1 atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillion atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up of Pablo G. Pérez González, researcher at the Center for Astrobiology; Patricia Sánchez Blázquez, Professor at the Complutense University of Madrid (UCM); Y Eva Villaver, researcher at the Center for Astrobiology.

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