Even a cyclical Big Bang could have a beginning in time

Une vue d'artiste du Big Bang. © santa pa design, Fotolia

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[EN VIDÉO] Interview: What did we really know before the Big Bang? The Big Bang is a singularity that is often described as embodying the origin of the universe, but it is possible that it is only one episode in its existence. Futura-Sciences went to meet physicist Etienne Klein to find out if something could have preceded it. In 1965, Nobel laureate in physics Roger Penrose published a theorem showing that the equations of Einstein’s theory of general relativity implied that the gravitational collapse of a sufficiently massive star must necessarily lead to a singularity of the space. -time surrounded by an event horizon and therefore inside a black hole. Before him, this fact was thought to be an artifact of idealized solutions to Einstein’s equations. In 1969, he published with Stephen Hawking an improved version of a theorem produced by the latter that had applied Penrose’s methods and mathematical ideas to relativistic cosmology to demonstrate the occurrence of the same phenomenon with the Big Bang. Indeed, many of the solutions to Einstein’s equations describing an expanding Universe could be considered more or less equivalent, by reversing the direction of the flow of time, to those describing a gravitationally collapsing star. In very general terms, Einstein’s equations therefore implied that space and time had a beginning, a beginning during which the density and temperature of its contents, as well as the curvature of space- time, they tended to infinity as time went on, as we asymptotically approached instant zero.Nobel laureate Roger Penrose explaining his work in 2021. For a fairly faithful French translation, click the white rectangle in the lower right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © Tencent WE Summit But already, in the minds of Hawking and his colleagues at the time, this must have been just an artifact of a non-quantum treatment of spacetime and perhaps also simply Einstein’s equations. In fact, it is possible to consider different equations that govern a curved space-time and when a volume of the observable cosmos was much smaller at the beginning of the expansion, its contents had to behave like a quantum atom. However, we know that the laws of quantum mechanics precisely guarantee a finite size to an atom by suppressing any collapse of its electron shells into its nucleus. In fact, since the end of the 1960s, Bryce DeWitt had laid the foundations of a quantum theory of gravitation applicable to cosmology, they were going to revise questions already addressed in the twenties and thirties by Alexandre Friedmann, Georges Lemaître and Richard Tolman for the most part. In the hands of these men, it had become clear that Einstein’s equations contained models of the universe in which the expansion of space eventually slowed before reversing, returning its contents to an infinite density . A new phase of expansion could then begin, and thus nature could be considered to have perhaps “chosen” to manifest itself in the form of a cyclical cosmology without beginning or end, perpetually oscillating between a Big Bang and a Big Crunch -to use a terminology. which would only appear after the Second World War and which is now well known to the general public.The thermodynamics of the Big Bang career in physical chemistry before becoming a world authority on statistical mechanics, whether classical or quantum, and on relativity , had laid the groundwork for reflections that would show that there was potentially a problem with the thermodynamics of relativistic cyclic cosmology. A problem that was to worsen after the discovery of fossil radiation in 1965. In fact, Tolman had thus succeeded in transposing the laws of thermodynamics and, in particular, those closely related to entropy, a of the most fundamental state functions of thermodynamics. Finally, from here it turned out that with each new phase of a cyclical cosmology, the entropy of its content in matter and radiation had to increase (it can be estimated by measuring the ratio between the number of photons and the number of baryons in the observable cosmos as well as its black hole content). It was difficult to reconcile with the observation that the entropy measured today is not only finite but far from maximal, if one believes that there are both an infinite number of cycles in the past and in the future, as explained by Nobel laureate Steven Weinberg. at the end of his famous book The First Minutes of the Universe, he thought that Tolman is more complex than we have just explained, but we continue to ponder the difficulties he poses. A few years ago, the famous cosmologist and theoretical physicist Paul Steinhardt revisited these questions with his colleague Anna Ijjas, an interaction between this field and the expansion of the observable cosmos. This scalar field, sometimes called quintessence, can be used to describe the nature of dark energy and allows the acceleration of the expansion of the cosmos to turn into a deceleration with contraction, but in the similar scenarios studied so far followed of a rebound phase. , a Big Bounce as we say in English, the contraction phase resulted in the Planck density and, shortly before reaching it, a merger of the black holes formed during the phase. This fusion could make the rebound impossible and above all, the passage through a quantum phase should cause the next phase to begin with a very high state of the entropy of the observable cosmos, which is not observed. Anna Ijjas explains her work with Paul J. Steinhardt on a cyclical cosmology. For a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © Dr Brian KeatingA cyclical cosmology without quantum rebound Anna Ijjas and Paul J. Steinhardt then showed that with the scalar field model they introduced, the contraction stops long before it reaches the Planck density and the cosmos rebounds. But it rebounds with a larger expansion factor than during the previous phase, while this factor periodically oscillates back to its values ​​in previous models of cyclic cosmology. In doing so, the additional entropy produced by the previous phase it is diluted and pushed in some way. outside what is called the cosmological horizon. To an observer below this horizon, there is no longer a continuous increase in each phase of the expansion of the observable cosmos, and there is no longer a contradiction between measured entropy and an already infinitely old Universe with an infinite number of cycles in the past. But two other cosmologists from the University of Buffalo in the United States, William Kinney and Nina K. Stein, have just thrown a stone into the pond. As they explain in an open access publication on arXiv, even the cyclic universe of Ijjas and Steinhardt must have a beginning in time with an initial singularity several years by Arvind Borde, Alan H. Guth, and Alexander Vilenkin , inspired by those of Penrose and Hawking, who showed that even the famous theory of inflation that was also supposed to avoid an initial singularity and avoid asking about the concept of the beginning of the Universe, in reality I could not spare these two ideas. .geodesics of a space-time. These geodesics are the trajectories that light rays and matter particles must take under the sole effect of the curvature of space-time. William Kinney and Nina K. Stein, as well as Arvind Borde, Alan H. Guth, and Alexander Vilenkin concluded that the geodesic in the cosmology of Ijjas and Steinhardt could not be parameterized by a variable that could go to infinity , which showed in the jargon of differential geometry that these geodesics are incomplete in the past. That’s why in a press release from the University at Buffalo, Kinney explains that: “People have proposed bouncing universes to make the Universe infinite in the past, but what we” What it shows is that one of the types newer of these models does not work. In this new kind of model, which deals with entropy issues, even though the universe has cycles, it still has to have a beginning. Despite specifying that: “Unfortunately, it has been known for almost 100 years that these cyclical models do not work because disorder, or entropy, builds up in the Universe over time, so each cycle is different from the ‘previous. It’s not really cyclical. A recent cyclical model circumvents this problem of entropy accumulation by proposing that the Universe expands with each cycle, diluting entropy. You stretch everything to get rid of cosmic structures like black holes, which return the Universe to its original homogeneous state before another bounce begins, but in short, we’ve shown that by solving the entropy problem, a situation is created in which the Universe had to have a beginning. Our proof shows in general that any cyclical pattern that eliminates entropy by expansion must have a beginning. Kinney admits, however: “Our test does not apply to a cyclical model proposed by Roger Penrose, in which the Universe expands infinitely with each cycle. We are working on that issue. »You are interested in what you just to read?
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