the Earth-Moon pair could serve as a giant detector

Every time in the field electromagnetic waves for’astrophysicswhich led to new discoveries. In this regard, we can refer to the origins of radio astronomy andastronomy X which made it possible to discover quasars, neutron stars and the first candidates for the title black hole. Therefore, we can expect similar discoveries in gravitational astronomy.

Detectors Ligo as well as Virgo will allow you to explore range from gravitational waves with some frequencies approximately between 1 and 1000 Hz. By combining these observations with electromagnetic wave instruments to perform multi-messenger astronomy, it has also become possible to test certain models that can explain certain gamma-ray bursts, in the case of those associated with neutron star collisions, also manifesting as kilonovae.

Another spectral band with lower frequencies in the range 10−5 Hertz at a frequency of 1 hertz will be available using the eLisa mission in space, and this should speak of emissions gravitational waves involving supermassive black holes. But that will have to wait until the 2030s…

…Or maybe not for part of this group because we think we can use pulsar belonging Milky Way in the range from 1 to 100 nHz, including within the framework of the collaboration North American Nanohertz Gravitational Wave Observatory (NANOGrav) orEuropean Pulsar Array (this).

Another similar point of view, in the region near the microhertz () is also looming on the horizon, as we can be convinced by two publications that are freely available on arXiv. They are somewhat reminiscent of an idea put forward many years ago and which Futura already talked about in a previous article below.

Fifty years after Neil Armstrong’s first step, the instruments deployed on the Moon by the Apollo 11 mission are still being used by French scientists. Thanks to reflective panels placed on the lunar soil, they measure the distance separating our planet from its satellite. The key is valuable lessons about the moon’s rotation or the composition of its core. © CNRS

From supermassive black holes to primordial cosmology

To understand what we are talking about, we must remember that gravitational waves are periodic deformationsspace-time which behaves like an elastic medium. Therefore, the passage of a gravitational wave will oscillatoryly stretch and compress space, and therefore the distances over which light as well as simply stretching and contracting the material body.

However, it so happened that in about 50 years and after the Apollo program and the arrival Soviet moon roversthere are retroreflectors on the surface of our satellite that can reflect directly in their directioninfluence laser impulses. In this way, we can calculate the distance from the Earth to the Moon very accurately by measuring the round trip time of laser pulses on Earth.

This is what they have been doing for many years telemetry laser moon Observatory Côte d’Azur, located on the Calerne Plateau. This allows them to count motion orbit and rotation of the Moon with an uncertainty of the order of a centimeter over 10 years.

But it turns out that astrophysicists relativists have shown by calculation that a random combination of gravitational waves in the region of about a microhertz, coming from many sources in the observable space and forming a kind of background noise. stochastic as experts call it, can lead to a change in the parameters of the orbit of the Earth-Moon system.

Thus, the passage of these waves would turn these two celestial bodies into a kind of material body, oscillating in a partially random way, but with a characteristic feature that allows us to claim that we can clearly see the influence of gravitational waves and nothing else that disturbs the parameters of the orbit. these heavenly bodies. To do this, you need to combine equations celestial mechanics in the theory of relativity with gravitational waves and the famous Fokker-Planck equation, originally used to describe Brownian motion, but has other long-known applications in astrophysics, as shown in the famous article of the great Indian astrophysicist Chandrasekhar (Stochastic problems in physics and astronomy).

Not just gravitational waves from supermassive black holes binaries thus could be detected, but other sources are more exotic like phase transition first order in contentUniverse very important.

Earth can be used to detect gravitational waves

Article Laurent Sacco published on 03/18/2014

Theoretically, some of the Earth-scale seismic noise could come from a background of gravitational waves generated by sources scattered throughout space. Therefore, it would be possible to observe and measure this background using the global network seismometers. The idea was put forward several decades ago physicist Freeman Dyson. It has recently been put into practice again.

Freeman Dyson recently celebrated its 90th birthday. He is one of the most original minds of the 20th well as century. Famous mathematician’s student Godfrey Hardy in Cambridge and an admirer Logical treatisephilosophical from Ludwig Wittgenstein, he first made a name for himself in quantum field theory. He was indeed the first to understand the importance and dignity of work Richard Feynman on thequantum electrodynamics relativist, to which he gave a more rigorous form. This allowed him to secure a lifetime position at Princeton University without even having a Ph.D.

Then his scientific contribution was concentrated in a wide variety of areas. For example, he was an important contributor to the project Orionspacecraft that would be propelled by nuclear explosions, and we owe the concept dyson sphere. Two researchers have just revived a brilliant idea that Dyson put forward in 1969. It’s about theory general theory of relativity.

Weber bars

At that time, relativistic astrophysics received its letters of nobility with the discovery quasarspulsars and cosmic radiation. We are experiencing a renaissance in research on general theory of relativity, and we begin to take the concept of a black hole very seriously. One of the most important predictions of general relativity is the prediction of the existence of gravitational waves. The physicist Josef Weber took up their discovery in the 1960s, for which he used metal rods in aluminum weighing approximately one ton, placed under vacuum and isolated from sources as much as possible vibration ground. Now they are called Weber bars.

In principle, if a strong gravitational wave resulting from a strong astrophysical phenomenon (for example, the collision of two black holes), crossed solar system, it must make material objects vibrate, distorting the structure of space-time. The effect is very weak and you have to make sure that the metal rods you are using are really well insulated. Weber repeatedly thought he had discovered gravitational waves, but they were wrong. We are currently hunting them with giant detectors such as Virgo and Ligowhich are based on a different detection principle: the measurement of fringesintervention with lasers.

The results are currently negative and they set limits on intensity and frequency bands where we could detect gravitational waves. It will likely take you through the eLisa project for gravitational astronomy to really take off. However, in 1968 Dyson pointed out that there was a giant natural gravitational wave detector: the Earth.

Cosmic gravity wave background

The earth can indeed be compared to an elastic rotating body that can vibrate in response to the passage of a gravitational wave. Dyson wondered if these vibrations could give a clear signal in the formseismic waves recordable seismometers. Given the uncertainty of the time, his calculations showed that this might not be impossible with frequencies in the hertz range.

Michael Coughlin of Harvard University (Cambridge, Massachusetts) and Ian Harms ofNational Institute of Nuclear Physics (INFN) in Florence, Italy, used a state-of-the-art global network of seismometers to reexamine the issue. They had to evaluate this time the background noisegravitational waves from all over space in the frequency band from 0.05 to 1 Hz.

Unfortunately they didn’t find anything. All they did was set a new limit for background noise in that frequency band. It’s not very restrictive if we compare it with those posted on other bands. But as the researchers explain in an article on archivethis represents an improvement on the order of a billion times over the previous limit for the same frequency band.

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