Where do matter and light of cosmic radiation come from? Physicists have had ideas about this since the early 1960s, and these ideas are no stranger to black hole radiation. Some clues, stemming from quantum field theory in curved spacetime, have just been tested for cosmology with a Bose-Einstein condensate in a laboratory on Earth.

The theory of big Bangbig Bang it is a definitive result of the beginning of the 21st^{And} century. But that is so if by Big Bang theory we mean the theory that the observable Universe, which does not mean everything that exists, was in a much denser and hotter state, with no atoms and starsstars, say between 10 and 20 billion years ago. It could therefore be that our observable Universe is just a region of a cosmoscosmos infinite in space and time that one day collapsed gravitationally, like a star giving rise to a black hole, before bouncing back into an expansion phase after reaching a limiting but finite density.

In any case, one can ask the question of the origin of it mattersit matters and the light of cosmic radiation that we observe around us. The developments of Quantum mechanicsQuantum mechanics and in particular of the quantum field theory of the years from 1925 to 1935 allow us to imagine processes not only of 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.

These processes could 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 we are dealing with processes described by a quantum field theory in a space timespace time curve that is not quantified, we knew it from the 60s before Stephen HawkingStephen Hawking did not use this theory at the beginning of the next decade to discover the production of particles by black holesblack holes it now bears his name under the title of Hawking Radiation.

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

There Leonard Parker also explains that some time after passing the thesis he told Fred Hoyle about his discovery of the production of particles by expanding space-time described by the famous family of solutions of equationsequations Einstein called Friedmann-Lemaître-Robertson-Walker (FLRW) for cosmological models isotropicisotropic and homogeneous (therefore appearing identical to any observer anywhere and looking in different directions with regard in particular to the average particle density and the speedspeed expansion at some point in the history of the observable cosmos).

Fred Hoyle, at that time arguably Britain’s best cosmological theorist behind a Stephen Hawking whose star had just begun to shine, was known as the author in 1948, together with Hermann Bondi and Thomas Gold, of the now defunct cosmological model stationary, a 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 the 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 whatever was the place or time in which an observer made measurements on it, he would always see the same things on average, without an evolution of the galaxiesgalaxies or matter is really remarkable.

But for this, 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 the expansion.

Hoyle had developed some equations to explain 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, did not allow enough matter to be created at the rate of expansion measured. But that all changed with a much faster primitive expansion phase.

The theory of quantum fields in curved space-time will develop rapidly during the 1970s under the impetus of numerous researchers both in England and in Russia, for cosmology obviously but above all due to the discovery of Hawking radiation. . A second boost will come at the beginning of the 1980s with the discovery of the theory of cosmological inflation which will allow for the development of a scenario for the creation of matter which today constitutes the observable cosmos and will also lead to the prediction of a production of gravitons, more generally fromgravitational wavesgravitational wavesfrom 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 test the particle production mechanisms due to the expansion of the Universe proposed by Parker and later by his colleagues?

## Space-time simulators with Bose-Einstein condensates

It doesn’t seem directly, but just as in the case of the indirect tests of Hawking’s radiation, the Canadian physicist William Unruh, discoverer of a cousin radiation of that of black holes since then called “Unruh effect” had already demonstrated in the 80s that the equations of quantum field theory in curved spacetime had analogies to phenomena in fluids and that one could therefore test the ideas and calculations involved in the laboratory, failing to really reproduce the creation of particles in the spacetime of relativity.

Indeed, for more than a decade, we have in fact obtained in the laboratory, especially with what are called sonic black holes, analogues 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 publication in *Nature,* and which can be found freely at *arXiv,* precisely signaling a breakthrough in this field that now allows the creation of particles in cosmology to be explored.

The article talks about the 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 using laserlaser to slow them down and lower their 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 a Bose-Einstein condensate. It is possible to manipulate this set 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 space-time of the FLRW family, more precisely an infinite space-time of hyperbolic type to use the slang of physicalphysical relativists.

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

In doing so, the researchers have just carried out the first experiment using ultracold atoms to simulate an expanding, curved universe. The quantum sound waves in the BE condensate thus exhibit the analog of particle pair creation predicted by the work of Parker and his colleagues, which bolsters 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 calculation and reasoning.