The accelerated expansion of the universe would not need dark energy, according to a study published today in the journal Monthly Notices of the Royal Astronomical Society: Letters. The author of the paper is Enrique Gaztañaga, a researcher from the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC — Institut d'Estudis Espacials de Catalunya). His research, of a theoretical nature, shows that the cosmic expansion can be derived simply from the fact that our universe has a very large, but finite, mass.
On this day in 1879, Albert Einstein was born. One of the greatest icons of science, he is best known for the theory of Relativity, which revolutionised our understanding of space and time, as well as of matter and energy. Applying his equations to the universe, he came to the conclusion that he had to introduce a new term, the so-called cosmological constant, to prevent the universe from collapsing as a result of the gravity exerted by the celestial bodies on each other. The meaning of this constant, however, seemed difficult to interpret. Indeed, what is it that prevents the universe from contracting back on itself?
Since then, attempts at clarification have not ceased, and no wonder. Astronomers discovered not only that the universe would not collapse, but that it is in fact expanding at an accelerating rate. Dark energy has been the concept that cosmologists have turned to in order to explain this issue: there must be an abundant energy that repels galaxies from each other. However, how this energy originates remains a mystery.
In this context, ICE-CSIC and IEEC researcher Enrique Gaztañaga has presented a cosmological model that completely dispenses with dark energy or Einstein's cosmological constant. "The current model, the Big Bang Theory, proposes that our universe has an infinite extension (and therefore an infinite mass). However, useful as they are, infinities are abstract mathematical concepts that are never observed in physics. If we consider that the universe has a finite mass, the problem of dark energy disappears," explains Gaztañaga.
The researcher has been working on the Black Hole Universe (BHU) model for about four years. We normally imagine black holes as very compact masses with a strong gravitational pull, so that not even light can escape from them — hence their name. However, the key aspect that defines them is the latter: that they have a boundary, called an event horizon or gravitational radius, from which nothing can escape. Whether the mass inside them is more or less compact depends on the density of each black hole.
"Imagine a rubber band that stretches (as the universe expands). Since it is elastic, there is a force that opposes its stretching, which gets bigger the more you stretch it. Dark energy (or the cosmological constant) would be a measure of this elasticity," says the researcher. But the rubber has a limit to how much it can be stretched and this produces what is known as a boundary effect. This is due to a fundamental property in Einstein's Theory of Relativity: no event can happen (in this case, stretch) faster than the speed of light. "This indicates that we are inside an event horizon (or gravitational radius) due to the finite mass of our universe, which produces exactly the same effect as dark energy or the cosmological constant. That is why they are unnecessary," explains Gaztañaga.
Cosmology is a field prone to theories or hypotheses that are difficult to validate. However, the virtue of this model is that it gives simpler explanations for already observed phenomena. "There are other cosmological models that do without dark energy, or without other problematic elements (such as the so-called dark matter), but rely on modifying the laws of physics. The model I propose has the advantage that it uses already known laws, but this does not exempt us from trying to find more evidence that this is the correct interpretation of cosmic acceleration," says the expert.
The model allows us to estimate some quantities that can be compared with observations of our universe. For example, we can obtain a value for the mass of the universe, which is 6 · 10 raised to 22 solar masses (a 6 followed by 22 zeros). This is a reasonable number considering the number of stars and galaxies in our universe. We also obtain that the measured density of the universe is higher than the density of a black hole with the same mass. This implies that all the mass is contained within the gravitational radius, which is consistent with the idea that we are in a black hole from which nothing can escape. On the other hand, it is also possible to calculate the time it would take for the universe to expand to its limit or collapse on itself. In the model, this is 14 billion years. This roughly matches the measured age of the oldest galaxies.
"The model could change the idea we currently have about the origin of the universe or the evolution of galaxies, for example. Furthermore, if our universe is in a black hole, who's to say that there can't be other universes in other black holes? In the end, this is part of the Copernican Revolution: we are not in a privileged place in the cosmos," concludes Gaztañaga.
Press release in collaboration with IEEC