Questions about whether other universes might exist as part of a larger Multiverse, and if they could harbour life, are burning issues in modern cosmology.
Now new research led by Durham University, UK, and Australia’s University of Sydney, Western Sydney University and the University of Western Australia, has shown that life could potentially be common throughout the Multiverse, if it exists.
The key to this, the researchers say, is dark energy, a mysterious “force” that is accelerating the expansion of the Universe.
Previously it was thought that if dark energy were a lot stronger, the Universe would have been driven apart not only before the first stars and galaxies formed, but even before the first stable atoms could form. If it were stronger in the opposite (negative) direction, the Universe would have recollapsed before anything interesting could have formed. The fact that dark energy is as weak as we observe it to be is one of the greatest cosmic coincidences of all, and one that’s seemingly necessary for our existence.
The dark matter fine-tuning problem is shown to be less severe than previously thought. It turns out that dark energy might not matter very much at all for allowing life in the Universe. If the dark matter value were higher by about a factor of 10 or 100 — or lower by an arbitrary amount — the overall Universe itself would hardly change at all. Even increasing the amount of dark energy by a factor of three, ten, or even fifty will only change the number of stars you form by about 15%.
Researchers investigate the effect of the accelerated expansion of the Universe due to a cosmological constant, Λ, on the cosmic star formation rate. They utilize hydrodynamical simulations from the EAGLE suite, comparing a ΛCDM (cold dark matter) Universe to an Einstein–de Sitter model with Λ = 0. Despite the differences in the rate of growth of structure, they find that dark energy, at its observed value, has negligible impact on star formation in the Universe. They study these effects beyond the present day by allowing the simulations to run forward into the future (t over 13.8 Gyr). We show that the impact of Λ becomes significant only when the Universe has already produced most of its stellar mass, only decreasing the total comoving density of stars ever formed by ≈15 percent. They develop a simple analytic model for the cosmic star formation rate that captures the suppression due to a cosmological constant. The main reason for the similarity between the models is that feedback from accreting black holes dramatically reduces the cosmic star formation at late times. Interestingly, simulations without feedback from accreting black holes predict an upturn in the cosmic star formation rate for t over 15 Gyr due to the rejuvenation of massive (over 1011 M⊙) galaxies. They briefly discuss the implication of the weak dependence of the cosmic star formation on Λ in the context of the anthropic principle.
Models of the very early Universe, including inflationary models, are argued to produce varying universe domains with different values of fundamental constants and cosmic parameters. Using the cosmological hydrodynamical simulation code from the EAGLE collaboration, they investigate the effect of the cosmological constant on the formation of galaxies and stars. They simulate universes with values of the cosmological constant ranging from Λ = 0 to Λ0 × 300, where Λ0 is the value of the cosmological constant in our Universe. Because the global star formation rate in our Universe peaks at t = 3.5 Gyr, before the onset of accelerating expansion, increases in Λ of even an order of magnitude have only a small effect on the star formation history and efficiency of the universe. They use their simulations to predict the observed value of the cosmological constant, given a measure of the multiverse. Whether the cosmological constant is successfully predicted depends crucially on the measure. The impact of the cosmological constant on the formation of structure in the universe does not seem to be a sharp enough function of Λ to explain its observed value alone.
Scientists say that current theories of the origin of the Universe predict much more dark energy in our Universe than is observed. Adding larger amounts would cause such a rapid expansion that it would dilute matter before any stars, planets or life could form.
The Multiverse theory, introduced in the 1980s, can explain the “luckily small” amount of dark energy in our Universe that enabled it to host life, among many universes that could not.
Using huge computer simulations of the cosmos, the new research has found that adding dark energy, up to a few hundred times the amount observed in our Universe, would actually have a modest impact upon star and planet formation.
This opens up the prospect that life could be possible throughout a wider range of other universes, if they exist, the researchers said.
The simulations were produced under the EAGLE (Evolution and Assembly of GaLaxies and their Environments) project – one of the most realistic simulations of the observed Universe.
Jaime Salcido, a postgraduate student in Durham University’s Institute for Computational Cosmology, said: “For many physicists, the unexplained but seemingly special amount of dark energy in our Universe is a frustrating puzzle.
“Our simulations show that even if there was much more dark energy or even very little in the Universe then it would only have a minimal effect on star and planet formation, raising the prospect that life could exist throughout the Multiverse.”
Dr Luke Barnes, a John Templeton Research Fellow at Western Sydney University, said: “The Multiverse was previously thought to explain the observed value of dark energy as a lottery – we have a lucky ticket and live in the Universe that forms beautiful galaxies which permit life as we know it.
“Our work shows that our ticket seems a little too lucky, so to speak. It’s more special than it needs to be for life. This is a problem for the Multiverse; a puzzle remains.”
Dr Pascal Elahi, Research Fellow at the University of Western Australia, said: “We asked ourselves how much dark energy can there be before life is impossible? Our simulations showed that the accelerated expansion driven by dark energy has hardly any impact on the birth of stars, and hence places for life to arise. Even increasing dark energy many hundreds of times might not be enough to make a dead universe.”
However, the researchers said their results were unexpected and could be problematic as they cast doubt on the ability of the theory of a Multiverse to explain the observed value of dark energy.
According to the research, if we live in a Multiverse, we’d expect to observe much more dark energy than we do – perhaps 50 times more than we see in our Universe.
Although the results do not rule out the Multiverse, it seems that the tiny amount of dark energy in our Universe would be better explained by an, as yet, undiscovered law of nature.
New law of physics
Professor Richard Bower, in Durham University’s Institute for Computational Cosmology, said: “The formation of stars in a universe is a battle between the attraction of gravity, and the repulsion of dark energy.
“We have found in our simulations that universes with much more dark energy than ours can happily form stars. So why such a paltry amount of dark energy in our Universe?
“I think we should be looking for a new law of physics to explain this strange property of our Universe, and the Multiverse theory does little to rescue physicists’ discomfort.”