How can gravitational waves help us to understand the universe ?

The discovery of gravitational waves opens a new era for astronomy and our understanding of the universe. But how ?.

In the beginning, there was light: the sky that we observe with the naked eye or with the first glasses, the astronomy of the visible. The discovery of electromagnetic waves at the end of the 19th century opened the door to a whole range of new techniques for exploring space: infrared, ultraviolet, X-rays, gamma rays, radio waves, microwaves ... the night sky became much less dark, and there were many discoveries.

These slices of universe that we don't see

But electromagnetic waves have a practical limitation: they can be deflected, distorted, and even hidden. We only see a small part of the universe, despite the wide palette offered by the different wave ranges.

Take an example: the Milky Way. When we look at the sky (outside the cities), we can clearly see this white trail, which corresponds to the "slice" of our spiral galaxy. It is also an obstacle to observation: if we can go looking for distant objects in other directions, the brightness and the number of stars, dust or other objects present in the galaxy hide us whole slices of universe, hidden behind them.

Thus, last week, astronomers announced the discovery of several hundred galaxies hitherto unknown. They were simply hidden behind the Milky Way. Who knows how many other objects are hidden from our view?

Gravitation can also be a problem for observation, under certain circumstances. The masses, especially those of stars and galaxies, deflect light rays of all kinds. As a result, some will not reach us, or will come to us distorted. Again, a brake on observation.

Light up the dark side

Gravitational waves, on the other hand, do not have this kind of problem. They are waves in space-time itself: the different objects and forces that are there will not stop them, deflect them, distort them.

The waves captured on September 14, 2015 by the LIGO observatories traveled 1.3 billion light years to give us an image of a fusion of black holes that we would not have been able to obtain otherwise.

What new information could gravitational waves give us? Here is a non-exhaustive list:

-Black holes. Obviously, gravitational waves transmit information to us about them, and that should improve our knowledge of these objects. Not only stellar black holes, resulting from the "death" of a star, but also supermassive black holes located in the center of many galaxies.

- Major catastrophic events: collisions of stars and in general, everything concerning very massive objects, in particular neutron stars, or even explosions of supernovae.


- The beginnings of the universe. The subject is vast: light could not circulate freely in the universe until 380,000 years after the Big Bang. Before that, the universe is opaque to us. Gravitational waves could change that, and help confirm (or deny) certain theories.

For example, the Bicep experiment, which in 2014 believed to detect the imprint of gravitational waves in the fossil radiation of the beginnings of the universe, also hoped to demonstrate the existence of a phase of very rapid expansion that we called "cosmic inflation". In general, the gravitational waves could bring us elements on what could have happened in the first moments of the Big Bang.

- Check certain theories. Or not. Relativity, of course, the Big Bang, but also "string theory". The latter, which aims in particular to unify relativity and quantum mechanics, is for the moment only a sum of mathematical equations. One can imagine that in the future astrophysicists could demonstrate the existence of processes described by this theory, by observing "cracks in the fabric of space-time" which would emit gravitational waves.

A new technological era?

The parallel between electromagnetic waves and gravitational waves can naturally lead us to another question: since we have developed a whole technology around the former, does this mean that the latter will also bring about a technological revolution? After all, those who first discovered the properties of electromagnetic waves did not necessarily imagine medical radiology, microwave ovens or reality TV shows.

If we see here and there flourish some unorthodox studies on the use of gravitational waves for secret communications, armament, nuclear fusion or propulsion systems, skepticism is rather the norm in this area among scientists . To use gravitational waves, you would have to move really very massive objects. The waves so small that were detected by Ligo required the fusion of two large black holes between them like more than 60 suns! While Stephen Hawking recently said that putting a black hole in the mass of a mountain into orbit could provide all the energy the Earth needs, we're not there yet 

But just because the direct technological use of gravitational waves is akin to science fiction doesn't mean they won't bring us anything. Understanding the physical processes at work in the universe alone is a potential mine of practical applications. The many discoveries that will be made in the future thanks to an "astronomy of gravitational waves" most certainly conceal unknown treasures which will advance not only science, but consequently technology.

There is also another direct contribution of gravitational waves to technology: the innovations developed to detect them. The Ligo observatories had to equip themselves with systems to isolate their instruments from seismic vibrations, for example. Like all technological progress, they will probably spread and find other uses. Again, the "window" is wide open.