If, as some propose, our universe is just one of many in a multiverse, all operating with slightly different physics, then more of them than previously suspected have the potential to host life. Specifically, the quantity of dark energy is not as constraining as proposed.
One of the puzzles of physics is how well the universe appears suited to us. Modest changes to any of the fundamental forces would make it impossible to form elements heavier than helium.
Without assuming a supreme being tailoring things for our benefit, this seems an astonishing stroke of luck. One explanation is that whatever process formed the universe spawned many others, scrambling the fundamental constants each time. In which case, there could be a lot of universes very unsuited to life, just as the galaxy hosts many barren planets. Rather than getting lucky, we appeared where our existence was possible.
This has led some physicists to invest great effort into modeling how things would look if one fundamental force or another was a little different. Professor Geraint Lewis of the University of Sydney, with colleagues across Australia and in Europe, has extended this to the universe's quantity of dark energy. However, there is an important difference to the dark energy study. We have no reason to think electromagnetism, for example, should be stronger or weaker than it is, just that it could be. However, Lewis told IFLScience that we expect most other universes, if they exist, to have more dark energy.
When astronomers realized the universe's rate of expansion is accelerating, they concluded this must be driven by something they called dark energy. Unfortunately, if our understanding of dark energy is right, there ought to be a lot more of it, and we mean a LOT. According to Lewis, our best estimates are for there to be about 10120 times as much dark energy in the universe as there appears to be.
Assuming cosmologists haven't made some huge mistake, this suggests, Lewis said, “Some unknown mechanism has sliced most of it off.” The problem, he continued; “Is we can imagine something that canceled out all of dark energy, but it is hard to explain what would get rid of most and leave this tiny residue.”
Yet if most of the expected dark energy existed, the universe would be lifeless, flying apart so fast that matter would be too diffuse to form stars. Back-of-the-envelope estimates suggested this would happen at only slightly higher dark energy levels than exist.
However, Lewis' modeling, published in the Monthly Notices of the Royal Astronomical Society, concludes a universe could have 50-100 times the dark energy of our own and still allow the multiple generations of stars needed for rocky planets with the elements required for life. It would be a lonely place, with galaxies separating so quickly each once would lose touch with others, but the fundamentals within galactic groups wouldn't change.
A difference of 102 is tiny compared to 10120, so most values of dark energy would still prevent life. The finding creates new puzzles because, Lewis said, “We expect to be close to the maximal acceptable value of dark energy.” As co-author Dr Luke Barnes of Western Sydney University put 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.” The fact that there appear to be so many possible values above ours could be a strange quirk, but might also suggest our understanding remains incomplete.
Sadly, however, it's unlikely that anywhere in the multiverse the laws of physics are so different they allow a habitable disk resting on the back of a turtle.