Our universe is one among many. But the total number of universes is finite. And the multiple existing universes are similar among them. This is the final vision of the cosmos that Stephen Hawking developed in his last months before his death and that was published yesterday posthumously in the Journal of High Energy Physics.
The work, done in collaboration with his pupil Thomas Hertog, of the University of Leuven (Belgium), is based on the string theory of physics (which postulates that elementary particles are actually tiny strings that vibrate) and on the multiverse concept (which postulates that there are multiple universes).
Our objective has been to transfer the study of the origin of the universe entirely to the field of natural sciences
Hawking and Hertog defend that their model of the universe has the advantage, in comparison with other models, of allowing predictions about phenomena that could be observed. Therefore, it could be checked experimentally if it is valid or erroneous.
“Our objective has been to transfer the study of the origin of the universe entirely to the field of natural sciences. This means that we develop theories of the universe that are at the same time mathematically consistent and verifiable with observations, “Hertog states by email.
The observations that may indicate in the future if the Hawking and Hertog model is valid are those of ” gravitational waves originating in the initial stages of the universe”, says the cosmologist. These waves, which have never been observed yet, could be detected directly with instruments more advanced than the current ones. Or they could be detected indirectly through the signal they had to leave in the cosmic background radiation, popularly known as the big bang echo, says Carlos Sopuerta, a physicist at the Institute of Space Studies of Catalonia (IEEC).
Current theories about the big bang predict that our local universe was born with a brief episode of inflation. According to this view, the universe expanded exponentially for a fraction of a second after the big bang. These theories propose that “once inflation starts, there are regions where it never stops,” explains a statement released by the University of Cambridge, where Hawking worked., on the occasion of the publication of his posthumous article. “It is believed that quantum effects can keep inflation going forever in some regions of the universe, so globally inflation is eternal. The observable part of our universe would, therefore, be just a hospital bag, a region where inflation has ended and where stars and galaxies have formed. ”
I’ve never been a fan of the multiverse
“I’ve never been a fan of the multiverse,” Hawking said last fall in an interview. “The usual theory of eternal inflation predicts that globally our universe is like an infinite fractal, with a mosaic of different universes separated by an ocean in inflation. The laws of physics and chemistry may differ from one universe to another, which together form a multiverse. But […] if the scale of the different universes in the multiverse is large or infinite, the theory cannot be verified experimentally “.
Paradoxically, this multiverse that Hawking was not a fan of was derived from his own research. Specifically, in 1983 he developed a model of the universe together with the physicist James Hartle who proposed that there was no initial moment in a way similar to how a sphere – the Earth, for example – does not have an initial place. The so-called Hartle-Hawking model, based on the mathematics of quantum mechanics, eliminated the problem of how the universe could be formed from nothing. However, it implied that they had to have created an unlimited number of universes with very varied characteristics. Some – few – would be similar to ours and many others would be completely different.
“The problem with multiverses is that everything is possible somewhere. Therefore, it does not allow us to predict much about what we should observe in our local universe, “explains Hertog. “Our new theory solves this question by reducing the vastness of the multiverse to a much smaller, homogeneous and manageable universe.”
The key point of our theory is that the range of possible universes is restricted
Hertog started working with Hawking in the late 1990s as a Ph.D. student and continued to collaborate with him ever since. About three years ago, he explains, they realized that if they applied the mathematics of string theory to the study of the origin of the universe, they could correct the aspects of the multiverse theory that bothered them.
“The key point of our theory is that the range of possible universes is restricted,” Hertog explains. “This makes cosmology based on this theory more predictive, scientifically more robust and hopefully ultimately verifiable. This final article is the culmination of the work that Stephen and I have done together throughout these years. ”
This final article is the culmination of the work that Stephen and I have done together over these years
For the theory to be proven, however, it will be necessary to develop it to the point of making concrete predictions about the gravitational waves emitted in the initial stages of the universe. One could predict, for example, what kind of irregularity the gravitational waves should have left in the cosmic background radiation. If the experimental observations coincide with the predictions derived from the theory, they could even offer indications of the existence of parallel universes. But these are predictions that Hawking and Hertog fail to make in the article published yesterday.
“There are many models of the universe,” warns Carlos Sopuerta, of IEEC, for whom there is no reason to think a priori that Hawking and Hertog is more accurate than others. “As long as we do not have concrete predictions about the phenomena we can expect to observe, we can not prove it.”