A model of the universe explains its accelerated expansion without the need for dark energy

The model provides simpler explanations for the observed cosmic phenomena

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 Studies of Catalonia (IEEC — Institut d'Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE-CSIC). 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, IEEC researcher at ICE-CSIC 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 · 10^22 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 prepared in collaboration with the Communication office of the Institute of Space Sciences (ICE-CSIC).

Main Image

Caption: Our universe could have formed like the first stars: collapsing and exploding into a supernova (a Big Bang). The image on the left shows the Crab Nebula, a remnant of a supernova. This could be a small analog of our universe today, represented by a simulation (MICE) in the image on the right.

Credits: NASA/ESA (left) and MICE (right)

Links

– IEEC
– ICE-CSIC
– MICE simulation
– Dark Cosmos (Enrique Gaztañaga’s blog)

More information

This research is presented in a paper entitled “The Mass of Our Observable Universe”, by Enrique Gaztañaga, to appear in the journal Monthly Notices of the Royal Astronomical Society: Letters on 14 March 2023.

The Institute of Space Studies of Catalonia (IEEC  — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of 25 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centers, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors.

IEEC is a private non-profit foundation, governed by a Board of Trustees composed of Generalitat de Catalunya and four other institutions that each have a research unit, which together constitute the core of IEEC R&D activity: the Universitat de Barcelona (UB) with the research unit ICCUB — Institute of Cosmos Sciences; the Universitat Autònoma de Barcelona (UAB) with the research unit CERES — Center of Space Studies and Research; the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) with the research unit CTE — Research Group in Space Sciences and Technologies; the Spanish Research Council (CSIC) with the research unit ICE — Institute of Space Sciences. The IEEC is a CERCA (Centres de Recerca de Catalunya) centre.

Contacts

IEEC Communication Office
Barcelona, Spain
E-mail: comunicacio@ieec.cat 

Lead Researcher at IEEC
Barcelona, Spain
Enrique Gaztañaga
Institute of Space Studies of Catalonia (IEEC)
Institute of Space Sciences (ICE-CSIC)
E-mail: gazta@ieec.cat