The fact that the universe is expanding at an accelerated rate is a known phenomenon that has been confirmed and discussed in numerous pieces of research. Yet, the theoretical explanation of this phenomenon is far from being resolved, because not even the rate at which this expansion is taking place is known. The Gravitation and Cosmology research group of the UPV/EHU’s Department of  Theoretical Physics and History of Science has devoted many years to working on this subject, and right now two researchers in the group have published a piece of work in the journal Physics of the Dark Universe; it was conducted with a researcher from the University of Szczecin (Poland) and puts forward a physical and mathematical model to explain this expansion.

The model proposed belongs to the family of the so-called Chaplygin models, which are characterised by using unified fluids to represent the most abundant components of the universe: dark matter and dark energy. The puzzle would be completed with the matter we already know (baryonic matter), radiation, neutrinos, etc. “Our model is a variant of the Chaplygin models that consider all matter, both baryonic and dark matter, and dark energy within one and the same fluid. Gravitation can cause bodies to attract each other, as dark matter and conventional matter do, but surprising repulsive effects are also possible, such as that which would be produced by dark energy.  What we are proposing is that the state of this Chaplygin fluid has been changing over the course of time, specifically, the proportions of dark matter and dark energy have varied and therefore in early times the dominant effect would be the tendency to attract, in other words, collapse, whereas in later periods, by contrast, it is repulsion that would take place instead,” explained Ruth Lazkoz, researcher in the Gravitation and Cosmology Group and one of the authors of the study.  That would explain that the universe is expanding in the way for which evidence exists, in other words, at an accelerating rate, or to put it another way, the expansion is taking place more and more rapidly.

In the cosmological model proposed, the parameters corresponding to the described fluid, nicknamed umami, have been provided “with greater freedom. In other words, we have proposed a generalization of the Chaplygin model; in ours, the fluid can adopt a broader range of mathematical values. Even though against this the result obtained is not so conclusive statistically, we have been able to compare the results offered by our model with a set of data providing us with various astrophysical phenomena, such as supernovae, quasars, gamma ray bursts, etc., in other words, ones that allow us to read what the universe is telling us about itself”, specified the researcher.

And the result has been positive. “Our model has been capable of unifying the behaviour displayed in the cosmological context by both dark matter and dark energy. In addition, one of the most interesting results has been that the parameter that represents the rate of expansion of the universe, known as the Hubble parameter, in our model has turned out to have a value midway between two values far away from each other, and which produce two large groups of study that are in conflict with each other, and this positions it as a compromise point between these values,” pointed out María Ortiz-Baños, a member of the same research group who is expecting to be able to submit this and other pieces of work in the defence of her PhD thesis shortly.

The researchers believe that the information obtained in this study is showing them that “the direction taken appears to be the right one and is an interesting path to pursue, because we are achieving slightly better results than other, current, more standard theoretical frameworks. Although there is no doubt that the subject must continue to be tackled in depth, there are other aspects of the model that have to be analysed, such as, for example, the perturbations, which would enable us to go into the physics in greater depth on smaller scales. That way, among other important questions, we could find out whether our model could lead to the structures currently present in the universe”, they concluded.