New calculations shed light on one of science’s great puzzlesby Philip Ball / June 12, 2018 / Leave a comment
New calculations of “model universes” different from our own have shown that our existence might not be as special as has been supposed. According to these results, just published by a team of astrophysicists in the Monthly Notices of the Royal Astronomical Society, the problem of so-called “dark energy” in our universe becomes more puzzling than we thought. The work juggles with several speculative ideas in cosmology, but goes to the heart of one of the deepest problems in science.
At the centre of the issue is the so-called “fine-tuning problem,” which confronts the fact that our universe seems oddly “designed” for life to exist. Everything we see—atoms, matter, planets, stars and galaxies—can be largely explained by physical laws that govern, say, the forces of gravity and electromagnetism But those laws have within them a handful of numbers (“fundamental constants”) that just happen to have particular values—so that, for example, the strength of the electromagnetic force is much greater than that of gravity. It’s in these numbers that the fine-tuning is manifested.
For decades, physicists dreamed of finding a “Theory of Everything” that would not only unify our understanding of such forces in one over-arching theory but would also reveal why the fundamental constants of nature have to have the values we observe and not some other.
The trouble is, efforts to develop such a unified theory have led in the opposite direction. Many (though by no means all) physicists working on those problems regard string theory—which posits that all particles are made up of incredibly tiny one-dimensional vibrating entities called strings—as the best candidate. But string theory seems to imply that there is an absurdly huge range of possible “ways things could be,” all with different values of the fundamental constants.
That looked discouraging, but in fact it fits quite nicely with one of the favourite cosmological theories of how the Big Bang occurred that led to our expanding universe. This theory posits something called inflation: a period of incredibly rapid expansion very early on in the Big Bang, in which our universe ballooned suddenly from microscopic to cosmic size. Inflation potentially solves some puzzles about the large-scale distribution of galaxies and radiation that we observe. But if the theory of inflation is right, it seems to imply that this wasn’t a one-off event. There could have been a profusion of inflationary expansions, each one producing a self-contained universe, perhaps with physical laws potentially quite different to those in our own. This is the inflationary multiverse.
It might seem like a terribly profligate idea, and this multiverse was long regarded askance by many scientists. But it offers a way to rationalise why our fundamental constants have the values they do. What seemed particularly disconcerting about them is not just that they had particular, seemingly arbitrary values, as though chosen by God, but that if some of those values are altered only a little, everything falls apart: our theories then fail to predict stable atoms, and so there could be no stars, planets or us. It was as if the universe were fine-tuned for our existence.
“The multiverse offers a way to rationalise why our fundamental constants have the values they do”
But if there’s a multiplicity of universes with other values of the fundamental constants, most of which don’t contain such stable matter and therefore life, it stands to reason that ours has the values it does without any need to invoke mysterious “cosmic fine-tuning.” In that case, of course our universe is like it is—because in most of the others there’s no one to ask such a question. We should be no more amazed at this than we should that, of all the sperm that could have fertilised the egg cell from which we grew, the one that did so was precisely that which would produce us.
This explanation for the state of our universe is called anthropic reasoning: our very existence in effect selects an apparently fine-tuned universe.
The validity of this sort of argument is still disputed—to some it seems circular. But discoveries in the past few decades have only added to the fine-tuning puzzle. When in 1998 it was found from observations of exploding stars in very distant galaxies that the universe is not just expanding but doing so at an accelerating rate, cosmologists and astrophysicists needed to add a new “ingredient” to the mix that makes up the cosmos.
That mix was already puzzling enough, because it has long been considered to include a mysterious substance called dark matter. This dark matter seems required, for example, to explain how galaxies rotate without spinning apart, but it makes itself felt only through its gravitational effects, otherwise interacting with ordinary matter and light not at all (hence “dark”). On current estimates, the amount of dark matter outweighs the total amount of ordinary matter in the universe by a factor of around five. But the cosmic acceleration required now the further addition of “dark energy”: a kind of energy that somehow opposes gravity.
Like dark matter, dark energy is really just a place-holding name for something unknown. It looks as though there’s a lot of it, though: bearing in mind the equivalence that Einstein drew between matter and energy (E=mc2), we can think of it as a kind of “stuff”—and this stuff constitutes around 70 per cent of all there is in our universe, considerably exceeding the total amount of both ordinary and dark matter.
Even so, the curious thing is that if dark energy exists at all, there are good reasons to be puzzled about why there isn’t much, much more of it. A very plausible rationale for dark energy is that it is the intrinsic energy of empty space—of the vacuum—that quantum theory predicts it should have. But it’s one of the dirty secrets of physics that, on this basis, the energy of the vacuum should be absolutely enormous: far too great for matter to be stable. No one knows why this isn’t so.
“Cosmic acceleration required the further addition of ‘dark energy’: a kind of energy that somehow opposes gravity”
It’s actually easier to imagine why this vacuum energy might vanish entirely than why there should be some tiny residue of it left as the dark energy we observe Calculations of the vacuum energy imply that it should be greater than the observed amount of dark energy by a factor of a million billion billion billion billion billion billion. That it isn’t suggests that there must be some other unknown factor, some hidden symmetry in the physics, that cancels it out. Which would be fine if the cancellation was exact, but to be left with this absurdly small remnant strikes physicists as somehow not “natural”—as a weird and unlikely contingency.
In other words, there seems to be another fine-tuning in the amount of dark energy, which somehow pares it down to the minuscule level needed for the universe to host atoms and life. But once again we can invoke anthropic reasoning here: of course we’re in that highly specific member of the multiverse, because otherwise we wouldn’t be asking the question.
But the new computer simulations of virtual universes cast doubt on that reasoning. The researchers created many computer models of possible universes identical to our own except for the amount of dark energy they contain, and watched how they evolved from their primordial conditions after a big bang. These simulations were part of the EAGLE project, an international initiative based at Durham University that uses massive computing resources to understand how galaxies form and evolve.
As expected, the universes with more dark energy accelerated more rapidly as they expanded. But to their surprise, the researchers found that this didn’t necessarily prevent stars and planets from forming. Even those with a hundred times more dark energy could support such objects: they didn’t look so very different from our universe. Only rather late in the process, after more than ten billion years or so—does the larger amount of dark energy start to exert a strong influence. By that time the universes have already made plenty of stars and galaxies. The team says that their results make life much more likely—perhaps about a million billion billion billion billion times more—in the putative multiverse. We’re not so special.
If that’s right, anthropic reasoning doesn’t get us far in explaining why our universe seems to have the relatively tiny amount of dark energy that it does. The researchers say that we might have to think about the possibility that some other, unknown physical law limits the amount of dark energy—or perhaps that the explanation for it is different altogether.