What if our Universe were the "mirror" of another?

The fate of our Universe is enclosed in a number. A constant, to be precise, which bears the name of the English astronomer Edwin Hubble, and which is contained in an equation known as the Hubble-Lemaître law. The law essentially expresses the relationship between the red shift of the light emitted by galaxies (a phenomenon similar to that in which ambulance sirens emit a high-pitched sound when approaching the observer and increasingly severe as they approach the observer. away) and the distance of the galaxies themselves; in the equation, the Hubble constant indicates the speed at which our Universe is expanding, linked to the age of the Universe itself and its evolution over time. Despite the name, however, the Hubble constant is anything but constant, in the sense that its measurements, carried out with different methods, so far are not all perfectly in agreement with each other. Small discrepancies, of course, but sufficient to change, precisely, the fate of the Universe: for certain values ​​of the Hubble constant our cosmos would in fact undergo an infinite expansion; for others, however, the expansion is destined to slow down and even reverse (it is the so-called Big Crunch) until all the matter returns to contract in a single point - to then perhaps trigger a new Big Bang. For these reasons, it is understandable that the measurement of the Hubble constant is one of the most important problems of modern cosmology: today a new study, published in the journal Physical Review Letters by a team of scientists from the Department of Physics and Astronomy at the University of New Mexico and the University of California, Davis, puts forward a new suggestive hypothesis, according to which the discrepancies in the measurement of the Hubble constant could be postulating that our Universe is the "mirror" of another that contains exotic particles of matter dark.

Past and future If everything sounds rather complicated and abstruse, it is because it is. Let's take a step back and go back to 1929, when the English astronomer Edwin Hubble noticed that the galaxies were moving away uniformly in all directions, formulating the above law. The discovery actually gave rise to modern cosmology, and led to new hypotheses by Albert Einstein on the cosmological constant, the value of which, as we said, has long been the subject of controversy.

The methods adopted by astronomers for evaluating the value of the constant, up to this moment, are essentially two. The first uses objects of known brightness (the so-called standard candles) to measure their distances and relate them to their speed of departure. The second, on the other hand, is based on the observation of the cosmic background radiation, that is, the "residue" of the Big Bang. These two methods led to the estimation of two slightly discordant values, namely 73 km / s / Mpc (kilometers per second per megaparsec) for the first and 67 km / s / Mpc for the second. This discrepancy is known as the voltage on the Hubble constant and, at the moment, there is no theory that explains it: as explained by the National Institute of Astrophysics (Inaf), "perhaps the error is not in the measurements but in the cosmological models used for the calculation starting from the cosmic microwave background data, which could imply the need for a so-called 'new physics': we do not know. What we do know is that, as the measurements have gradually become more and more precise and the margins of error have consequently reduced, the two values ​​are no longer compatible with each other ".

To complicate matters further there is the fact that in recent years measurements have also been carried out with other methods, different from those described. One of these, based on the observation of red giants (a type of star), led to an estimate of 69.8 km / s / Mpc; the hope is that the observations of NASA's new super telescope, the James Webb Space Telescope, will help dissolve the tension on the Hubble constant once and for all. Waiting for the James Webb data (which could take a few more years), the scientific community, as anticipated above, is still working on alternative hypotheses. The last of which is even inconvenient the possibility that our Universe is the "mirror" of another.

A hypothesis that returns What does this have to do with the Hubble constant? The idea of ​​the mirror Universe was introduced in the 1990s to address another problem in physics, that of matter-antimatter asymmetry, that is, the fact that, despite each known particle corresponding to an analogous antiparticle (equal in mass but with opposite electric charge), matter is much more present today than antimatter, although the Big Bang theory predicts that at the beginning of the Universe particles and antiparticles were created in equal sizes. The physicists, therefore, tried to explain the problem by hypothesizing that all the "excess" antimatter ended up in a Universe mirror of ours, with the same asymmetry, but in reverse. For various reasons this hypothesis did not have great luck, but today it has suddenly returned to the headlines as a possible solution to the problem of the Hubble constant: the authors of the work, in fact, say that they have discovered an invariance in the so-called dimensionless parameters, a series of numbers which is part of the equations of cosmological models. Basically, the idea is that when you "adjust" (or, more precisely, "rescaling") the values ​​of these parameters to ensure that the theoretical predictions of cosmological models match the rates of expansion of the Universe measured experimentally , a mathematical model is obtained in which a sort of cosmic "symmetry" seems to emerge, and in which the value of the Hubble constant coincides with that measured by standard candles. The rescaling of the parameters, at least from a mathematical point of view, the authors say, could be justified by the existence of a mirror Universe that "teems" with exotic dark matter particles, which interact gravitationally with conventional matter. “We were very surprised - explained the authors of the study -“ noting that mirror Universe models can very naturally explain the symmetry we have identified ”. There remains one (another) problem, however: in their current form, these models fail to estimate the concentrations of deuterium and helium currently observed in the Universe. But according to the authors this is a much simpler question to solve than the one related to the value of the Hubble constant. And to the fate of our Universe.