Where does the matter and light from the cosmic radiation come from? Physicists have had ideas about this since the early 1960s, and these ideas are not independent of black hole radiation. Certain leads stemming from quantum field theory in curved spacetime have just been tested for cosmology with a Bose-Einstein condensate in the laboratory on Earth.

The theory of big bangbig bang is a final achievement from the beginning of the 21st century^{e} century. But this is the case if by Big Bang theory is meant the theory that the observable universe, which does not mean everything that exists, was in a much denser and hotter state without atoms and starsstarsbetween 10 and 20 billion years ago. It could therefore be that our observable universe is only a region of one cosmoscosmos infinite in space and time, which one day gravitationally collapsed, like a star giving rise to a black hole, before jumping back into an expansion phase after reaching a limit but finite density.

In any case, one can ask the question about the origin of fabricfabric and the light from the cosmic radiation that we observe all around us. The development of Quantum mechanicsQuantum mechanics and especially the quantum field theory from the years 1925 to 1935 makes it possible to imagine processes, not only for the creation of light quanta, but also of matter quanta, electronselectrons atoms and quarksquarks forms protonsprotons and neutronsneutrons be then cousins to photonsphotons.

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

## A creation of matter produced by dynamic spacetimes

The answer is yes and paradoxically enough, when these are in fact processes described by a quantum field theory in one space timespace time curve which is not quantified, we have known that since the 1960s before Stephen HawkingStephen Hawking did not use this theory at the beginning of the following decade to discover the production of particles by black holesblack holes now bears his name under the title Hawking Radiation.

We owe the discovery of the quantum creation of particles in cosmology to an American physicist who began working on this issue in 1962 in his thesis under the direction of the legendary Sydney Colman (see on this topic the article in Futura on Jean-Pierre Luminet’s latest book about black holes). The physicist in question is named Leonard Parker and can be found at *arXiv, *in the form of an interview, a fascinating story about 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 passing his thesis he talked with Fred Hoyle about his discovery of the production of particles by expanding spacetime described by the famous family of solutions of equationsequations of Einstein called by Friedmann-Lemaître-Robertson-Walker (FLRW) for cosmological models isotropicisotropic and homogeneous (therefore acting identically to any observer everywhere and looking in different directions with regard to especially the density of average particles and velocityvelocity expansion at some point in the history of the observable cosmos).

Fred Hoyle, at the time arguably Britain’s best cosmological theorist behind a Stephen Hawking whose star was just beginning to shine, was known as the author in 1948, along with Hermann Bondi and Thomas Gold, of the now defunct stationary cosmological model, model that denied 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 from fossil radiation, that the cosmos was infinite in time and space, though paradoxically expanding. It was therefore absolutely homogeneous in space and time, since regardless of the place or time at which an observer made measurements of it, he would always see the same things on average without a development of galaxiesgalaxies or the case is really noticeable.

But for that Hoyle had to assume that there must be a continuous creation of matter, leading to an 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 creation of matter, but they were more or less rudimentary. Parker’s work provided a much more accurate description and unfortunately, as he explained to Hoyle, it did not allow sufficient creation of matter at the measured rate of expansion. But everything changed with a much faster primitive expansion phase.

Quantum field theory in curved spacetime will develop rapidly during the 1970s under the impetus of several researchers both in England and Russia, obviously for cosmology, but especially due to the discovery of Hawking radiation. . Another impulse will come in the early 1980s with the discovery of the theory of cosmological inflation, which will make it possible to develop a scenario for the creation of the matter that today constitutes the observable cosmos and also lead to prediction of a production of gravitations, more generally ofgravitational wavesgravitational wavesby the incredibly exponentially rapid expansion phase of the early history of the universe in the theory of inflation.

These gravitational waves could leave traces that can be observed today in the fossil radiation.

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

## Space-time simulators with Bose-Einstein condensates

Directly it does not appear, but as in the case of indirect tests of Hawking radiation, Canadian physicist William Unruh, discoverer of a radiation cousin to black holes since called the “Unruh effect”, had already shown in the 1980s that quantum field theory’s equations in curved space-time had analogues to phenomena in fluids, and that one could therefore test the ideas and calculations involved in the laboratory without being able to truly reproduce the creation of particles in the space-time of relativity.

In fact, for more than a decade we have actually obtained in the laboratory, especially with what are 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. We will therefore not be surprised by a recent release in *Nature,* and which can be freely found on *arXiv,* just reports a breakthrough in this field, which now allows exploring the creation of particles in cosmology.

The article reports on work done by Markus Oberthaler of the University of Heidelberg, Germany, who, together with his colleagues, started by obtaining about 20,000 ultracold atoms of potassiumpotassium 39 users laserslasers to slow them down and lower their temperature to about 60 nanokelvin, 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 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 of a curved spacetime of the expanding FLRW family, more precisely a space-time infinity of a hyperbolic type to use the jargon physicistsphysicists relativists.

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

The researchers have thus just completed the first experiment that used ultracold atoms to simulate a curved and expanding universe. The quantum sound waves in the BE condensate then exhibit the analogue to the creation of particle pairs predicted by the work of Parker and his colleagues, strengthening confidence in the theory of quantum fields in curved spacetime.

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

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