The creation of particles by the Big Bang has been simulated in the laboratory

The creation of particles by the Big Bang has been simulated in the laboratory

Where do matter and light from cosmic radiation come from? Physicists have had ideas about this since the early 1960s, and these ideas are not unrelated to radiation from black holes. Certain clues, derived from quantum field theory in curved space-time, have just been tested for cosmology with a Bose-Einstein condensate in the Earth laboratory.

the theory of big Bangbig Bang is a definite achievement of the early 21st centuryme century. But this is so if by Big Bang theory one understands the theory that the observable Universe, which does not mean everything that exists, was in a much denser and hotter state, without atoms and starsstars, say between 10 and 20 billion years. So it could be that our observable Universe is just one region of a cosmoscosmos infinite in space and time that one day gravitationally collapsed, like a star giving rise to a black hole, before rebounding into an expansion phase after having reached a limiting but finite density.

In any case, the question of the origin of the mattermatter and the light from cosmic radiation that we observe around us. The developments of the Quantum mechanicsQuantum mechanics and in particular of the quantum theory of fields from the years 1925 to 1935 allow us to imagine processes not only for the creation of quanta of light but also of quanta of matter, the electronselectrons atoms and quarksquarks forming the protonsprotons and the neutronsneutrons being then cousins ​​of photonsphotons.

Could these processes be used as part of the cosmologycosmology Einstein’s relativist to explain the origin of matter?

A creation of matter produced by dynamic space-times.

The answer is yes, and paradoxically, when in reality it is about processes described by a quantum field theory in a space timespace time curve that is not quantized, we have known about it since the 1960s before Stephen HawkingStephen Hawking did not use this theory early in the next decade to discover the production of particles by black holesblack holes it now bears his name under the title of Hawking Radiation.

We owe the discovery of the quantum creation of particles in cosmology to an American physicist who began working on this question in 1962 in his thesis topic under the direction of the legendary Sydney Colman (see on this subject the article in Futura on Jean- Pierre Luminet’s last book on black holes). The physicist in question is called Leonard Parker and can be found at archive, in interview form, a fascinating history of the quantum theory of particles in curved space-time. We learn, for example, that in fact the first quantum calculations of these effects date back to 1939 and that we owe them to… Erwin Schrödinger!

Leonard Parker also explains there that some time after having approved his thesis, he spoke with Fred Hoyle about his discovery of the production of particles through the expansion of space-times described by the famous family of solutions of equationsequations of Einstein called Friedmann-Lemaître-Robertson-Walker (FLRW) for cosmological models isotropicisotropic and homogeneous (hence it appears identical to any observer everywhere and looks in different directions with respect in particular to the average particle density and the speedspeed expansion at some point in the history of the observable cosmos).

Fred Hoyle, at the time arguably Britain’s greatest cosmological theorist behind a Stephen Hawking whose star had just begun to shine, was known as the author in 1948, along with Hermann Bondi and Thomas Gold, of the now-defunct stationary model of cosmology, model that denies the Big Bang Theory of Lemaître and Gamow.

Hoyle, Bondi, and Gold had proposed in this model, which then dominated cosmology before the discovery of quasarsquasars and above all due to fossil radiation, that the cosmos was infinite in time and space, although paradoxically expanding. Therefore, it was absolutely homogeneous in space and time, since it did not matter where or when an observer made measurements on it, they would always see the same things on average, without an evolution of it. galaxiesgalaxies or matter is actually perceptible.

But for that, Hoyle had to assume that a continuous creation of matter must occur, leading to the equally continuous birth of galaxies. Without this assumption, the cosmos would become more and more diluted with expansion.

Hoyle had developed some equations to account for certain aspects of this matter creation, but they were more or less rudimentary. Parker’s work gave a much more accurate description, and unfortunately, as he explained to Hoyle, he did not allow for sufficient creation of matter at the rate of expansion measured. But everything changed with a much faster primitive expansion phase.

Quantum field theory in curved space-time would develop rapidly during the 1970s under the impetus of multiple researchers in both England and Russia, for cosmology, of course, but especially because of the discovery of Hawking radiation. A second impulse will come at the beginning of the 1980s with the discovery of the theory of cosmological inflation that will allow the development of a scenario for the creation of the matter that today constitutes the observable cosmos and will also lead to the prediction of a production of gravitons, more generally ofgravitational wavesgravitational wavesby the prodigiously exponentially rapid expansion phase of the early history of the Universe in the theory of inflation.

These gravitational waves could leave traces observable today in fossil radiation.

Can we prove the mechanisms of particle production due to the expansion of the Universe proposed by Parker and later by his colleagues?

Spacetime simulators with Bose-Einstein condensates

Directly, it does not seem so, but as in the case of the indirect evidence of Hawking radiation, the Canadian physicist William Unruh, discoverer of a radiation cousin to that of black holes since then called the “Unruh effect”, had shown as early as the 1980s that the equations of quantum field theory in curved spacetime had analogies with phenomena in fluids, and therefore the ideas and calculations involved could be tested in the laboratory, without being able to actually reproduce the creation of particles in the space-time of relativity. .

In fact, for more than a decade, we have obtained in the laboratory, in particular with so-called sonic black holes, analogs not only of Hawking radiation but also of the Unruh effect. Famous examples have been obtained in Bose-Einstein condensates. Therefore, we will not be surprised by a recent publication in Nature, and which can be freely found at archive, precisely reporting a breakthrough in this field that now allows exploring the creation of particles in cosmology.

The article talks about the work carried out by Markus Oberthaler of the University of Heidelberg, Germany, who together with his colleagues began by obtaining some 20,000 ultracold atoms of potassiumpotassium 39 using laserslasers to slow down and lower its temperature to about 60 nanokelvins, or 60 billionths of a degree KelvinKelvin above absolute zero.

These atoms then undergo a phase transitionphase transition which makes them behave like a single quantum wave and, more precisely, therefore, like a Bose-Einstein condensate. It is possible to manipulate this collection of atoms in such a way as to give rise to processes described by equations analogous to those governing the creation of quantum particles by an expanding curved spacetime of the FLRW family, more precisely an infinite spacetime. hyperbolic type to use the jargon of physicalphysical relativists.

Of course, the BE condensate is not infinite but part of it is described by equations related to what is called the Poincaré disk, that is, a set of points in a disk in relation by a mathematical transformation to the points of a space with a hyperbolic geometry. . So, there is a kind of dictionary between the two spaces, so that we can study among ourselves what it is that allows us to translate quantum field theory in curved spacetime in hyperbolic space into a quantum theory with sound wavessound waves quantized cousins ​​that contain photons, phonons.

In doing so, the researchers have just performed the first experiment that used ultracold atoms to simulate an expanding, curved universe. The quantum sound waves in the BE condensate then exhibit the analogy of the creation of particle pairs predicted by the work of Parker and his colleagues, reinforcing confidence in the theory of quantum fields in curved spacetime. .

As a bonus, we now have a laboratory to explore the unknown consequences of the equations of this theory that we have not yet been able to discover in the equations by calculus and reasoning.

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