Albert Einstein, one of the greatest icons of science, is known above all for the theory of Relativity, which revolutionized our way of understanding space and time, as well as matter and energy. Applying his equations to the universe, he came to the conclusion that he should introduce a new term, the so-called cosmological constant, to prevent the universe from collapsing as a result of the gravity that the stars exert on each other. The meaning of this constant, however, seemed difficult to interpret. Indeed, what is it that prevents the universe from contracting in on itself?
Since then, the attempts to clarify have not stopped, and it is not for less. The astronomers discovered not only that the universe was not going to collapse in on itself, but that it is in fact expanding at an accelerating rate. Dark energy has been the concept that cosmologists have turned to to explain this question: there must be abundant energy that repels the galaxies between them. However, how this energy originates remains a mystery.
In this context, Enrique Gaztañaga has presented a cosmological model that completely ignores dark energy or Einstein’s cosmological constant. Gaztañaga is a researcher at the Institute of Space Studies of Catalonia (IEEC) at the Institute of Space Sciences (ICE) of Cerdanyola del Vallès in Barcelona, attached to the Higher Council for Scientific Research (CSIC) in Spain.
“The current model, the Big Bang theory, proposes that our universe has infinite extension (and therefore infinite mass). However, as 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 model of a universe in a black hole (Black Hole Universe, BHU) for about four years. We normally imagine black holes as very compact masses with great gravitational attraction, so that not even light can escape them—hence their name. However, the key aspect that defines them is the latter: that they have a border, called the event horizon or gravitational radius, from which nothing can escape. Whether the mass inside it is more or less compact will depend on the density of each black hole.
“Imagine a rubber band stretching (as the universe expands). Since it is elastic, there is a force that opposes its stretching, greater the more you stretch it. Dark energy (or the cosmological constant) would be a measure of this elasticity”, comments the researcher. But the rubber has a stretch limit and that produces what is known as an edge 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 within an event horizon (or gravitational radius) due to the finite mass of our universe, which produces exactly the same edge effect as dark energy or the cosmological constant. That is why they are unnecessary,” explains Gaztañaga.
Sector of our current universe, represented by a simulation (MICE). (Image: MICE)
Cosmology is a prone ground for theories or hypotheses that are difficult to validate. However, the virtue of this model is that it gives simpler explanations to already observed phenomena. «There are other cosmological models that do without dark energy, or other problematic elements (such as the so-called dark matter), but are based on the modification of the laws of physics. The model that 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”, the expert clarifies.
The model allows estimating some quantities that can be compared with observations made of our universe. For example, we can obtain a value of the mass of the universe, which is located in a number of solar masses that is 6 followed by 22 zeros. This is a reasonable number considering the stars and galaxies in our universe. We also obtain that the measured density of the universe is greater 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, one can also calculate the time it would take for the universe to expand to its limit or to collapse on itself. In the model, this is 14 billion years. This figure 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. Without going any further, if our universe is in a black hole, who tells us that there can’t be other universes in other black holes? Deep down, this is part of the Copernican Revolution: we are not in a privileged place in the cosmos”, concludes Gaztañaga.
Gaztañaga presents his cosmological model in the academic journal Monthly Notices of the Royal Astronomical Society: Letters, under the title «The Mass of Our Observable Universe». (Source: IEEC / ICE / CSIC)