Imagine having a double, the same in every way, particle by particle, who lives in a world just like ours, also traced at the subatomic level. Now imagine that universe overlapping ours, but unable to interact with it, except in one aspect: gravity. This mirror world image is what has been proposed by scientists from the universities of New Mexico and California, Davis. There is a reason behind this exercise of imagination, and it is to try to solve one of the great enigmas of current cosmology: The discrepancy in the Hubble constant.
The Hubble Riddle.
The Hubble constant is the rate at which the Universe is expanding. We have known for about a century that there is a “force” that pushes galaxies and other distant objects away from us. This “force” is not at all, since it does not push objects along a static stage but rather alters the size of the stage itself: the Universe expands.
One of the questions that would arise after knowing that would be, at what speed does it do it? This speed is called the Hubble constant (which is neither strictly constant nor speed), and we have several ways of calculating it.
One way is to measure the speed at which “nearby” objects in space such as supernovae are moving; the second is through the microwave background radiation (CMB), a remnant radiation from the Big Bang that still swarms around our environment and can be measured.
A small big discrepancy.
Well, after years of measurements, those who try to measure the Hubble constant disagree. Each of these measures yields (albeit quite consistently) one of two results. As cosmologists take more measurements with more precise instruments, they obtain more accurate results, but they continue to go down one of two paths depending on the methodology used. An error in the measurements can practically be ruled out.
A clue to understand the evolution of the Universe
The speed is important since from there we can get an idea of the size of the observable universe and supernovae tell us that the universe should be more than 20% smaller than using the CMB data, according to explica Cyr-Racine, one of the authors of the article. This second measurement also corresponds to what is considered the “standard model” of cosmology.
An example of these discrepancies was recently given by the Hubble telescope. After years of collecting satellite data on galaxies billions of light-years away and others closer, NASA scientists posted just a couple of weeks ago his calculation of the constant, which was within the range of what would be expected within his methodology. But 9% higher than the speed calculated through the CMB.
The new proposal to solve it.
The new proposal has been published in the magazine Physical Review Letters, but it is not entirely new. In 2003 another team of cosmologists proposed a similar alternative, but this new model has refined the calculations based on the mathematical properties of cosmological models. This means that the model they propose can better fit the observed data.
Cyr-Racine admits that the idea of an entire mirror universe of ours may seem crazy “But such mirror worlds have an extensive literature in physics in very different contexts, as they help solve important problems in particle physics.” He goes on to explain that his work is nothing more than the application of these previous studies to the realm of cosmology.
A model that does not explain everything.
The proposal of this “mirror world” Cyr-Racine and his colleagues manages to explain some observations but not all. An example of this is the scattering of photons. The model has sparked the interest of some experts, but they also draw attention to its limitations. Radek Wojtak, from the University of Copenhagen, for example, considers that this is a great effort for a very specific problem. A very concrete problem that could be the greatest current enigma of cosmology.
Image | NASA