Invisible radiation of the universe

Original author: Ethan Siegel
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The pages are still empty, but in a strange way it is clear that all the words are already written in invisible ink and only pray for visibility.
- Vladimir Nabokov

Beautiful images of deep space - from distant galaxies to stars, clusters, nebulae in our Galaxy - have one thing in common.

image

Shine! Specifically, electromagnetic radiation. This light does not always fall into the visible part of the spectrum, but it is to it that we are most familiar. No wonder: the greatest source of energy for us is the same as for the cluster above, NGC 3603 .



The light emanating from these stars - like the light emanating from all stars - is highly dependent on the temperature of the stars. The hotter it is, the more blue, or even ultraviolet light will come from it, and the colder it is, the more it will go into the red, or even into the infrared region.


Colors exaggerated

But not every star resembles our Sun, either a little colder or a little hotter than it. Some stars are thousands of times more massive, while others make up only a tiny fraction of the mass of the sun.

Therefore, the fate of the stars there is a huge variety.



The vast majority of stars known to us get their energy from the same place where the Sun takes it - from nuclear fusion - this is not the only source of energy for the stars of the Universe.

In addition to the nuclear reactions that give off this energy, a huge amount of energy is stored in gravity.



When a large mass is compressed or collapsed, several interesting things happen, and also do not happen. Spacetime outside the mass - that which was outside the original star, before the collapse - does not change. Its energy does not change, the curvature does not change, the gravitational potential does not change, etc.

But in the space-time that was originally inside the object, and after collapse or compression it was outside, the negative gravitational potential energy increases modulo. And this energy must go somewhere.



It, for example, can turn into light - this is what happens with white dwarfs. They are comparable in mass with the Sun, but in size with the Earth, and a large amount of light emanates from them, for which only gravitational compression serves as an energy source.

For example, if a white dwarf appeared on the spot of the Sun, it would still be 400 times brighter than our full moon!



But not every shrunken or collapsed object was one star in its solar system. Many of them, like the brightest star in our sky , are binary systems. In a binary system, two stars, or star-shaped objects, rotate around each other. Over time, these orbits do not remain stable, due to gravity they decrease, and stars fall in a spiral to each other.



But this time, not with light decreasing gravitational energy. And I'm not only talking about visible light - they do not emit any light. No x-rays, no infrared, no radio waves, nothing.

And what kind of radiation should such a system emit?



Gravitational radiation, also known as gravitational waves! These waves must propagate through space-time, and we can detect them not as light, but as a deformation of the dimensions of objects when a gravitational wave passes through them!

With a spiral approach of two objects, a constantly accelerating emission of waves should be observed. The closer they are to each other, the shorter the period becomes. A catastrophic emission of both light (and, in the case of two white dwarfs, the appearance of a supernova) and gravitational waves, followed by a calming phase of the waves, should occur.



This is a bold prediction of Einstein's general theory of relativity. But we have already observed, though not directly, one of the important aspects of this phenomenon.

Observing two pulsars (collapsed neutron stars) orbiting each other, we can predict a decrease in the orbital period of these stars. And for more than 30 years since the opening of the first double pulsar, we did just that.



But we really needed to detect these waves directly! So what do we do to discover them? For example, you can shoot from high-precision lasers at a known wavelength over long distances in different directions. This light is reflected from the mirrors and sent back, you collect the light received from both directions and look at the picture of interference.



Gravitational waves are extremely weak, so you need a very long base (to get a large number of wavelengths - you need to detect a change of 1/10 28 ) to detect a small shift of one of the two distances.

And on Earth there is such a project: a laser-interferometric gravitational-wave observatory , LIGO .



But LIGO on Earth, where we are not only limited in the possibilities of passing laser beams, but also in protecting the experiment from surface vibrations.

It will be much easier to detect gravitational waves in space!



It is for this that they are developing a project for an improved space antenna using the principle of a laser interferometer, eLISA (formerly LISA) Unfortunately, due to the high cost of space projects, problems with NASA's budget and the inability of the ESA to afford such an object alone, a set of three spacecraft designed to be in Earth orbit at distances of 5 million km from each other will not be launched in the next ten years [ current estimated launch time is 2034; approx. perev. ].

The universe speaks to us all the time in a language that we did not understand. And, as soon as we heard him, we immediately began to understand him!

So what does she tell us? How many, and where, white dwarfs approaching in a spiral. How many black holes merge in distant galaxies. What does the catastrophic emission of gravitational waves look like when two bodies combine. The universe is telling us this right now. We only need to listen, and we can catch this invisible radiation of the Universe: gravitational waves!

Note perev: the translation has been slightly revised due to the fact that after its appearance, the LIGO ground project detected gravitational waves . On February 11, 2016, LIGO and Virgo collaborations announced the discovery of gravitational waves on September 14, 2015 at LIGO installations. Signal detected proceeded from the merger of two black holes with masses of 36 and 29 solar masses at a distance of about 1.3 billion light-years from Earth, while three solar masses were emitted.

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