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Tom Siegfried

Context

Cosmic inflation has its flaws, but so do its critics

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DALLAS — Back in the old days, like half a century ago, many astronomers and physicists thought they understood the universe, more or less. It looked pretty much the same everywhere, so it probably had looked that way forever. Sure, it was expanding, but that just meant that somewhere far away new matter was popping into existence, thereby maintaining the cosmos in a steady state, with just enough matter created to compensate for the expanding space.

This “steady state” theory originated in the late 1940s from the collaborative cogitation of Fred Hoyle, Thomas Gold and Hermann Bondi, all working at the time in England. It wasn’t the only explanation for how the universe worked (there was a competing idea involving a Big Bang). But many scientists soon took the steady state idea for granted. It was neat and simple and apparently explained what astronomers saw when they looked at the sky. There was not, however, any actual evidence for it.

Wolfgang Rindler, then a physics graduate student in London, recalls the theory’s great success in acquiring followers.

“It’s amazing how the steady state theory had a life of its own,” says Rindler, a physicist at the University of Texas at Dallas, where he’s been for the last 50 years. “Everybody jumped on the bandwagon. If you went to a meeting, when people talked about what happens in cosmology they didn’t even say ‘on the basis of the steady state theory,’ they just implicitly assumed that the steady state theory is the right one.”

Rindler notices a similar bandwagon effect today in the widespread acceptance of the version of the Big Bang theory known as inflationary cosmology. It proposes that the fiery explosion bringing space and time into existence was accompanied by a brief burst of rapid expansion that sent the universe on its way to today’s vast cosmic grandeur. Inflation theory has become the dominant framework for studying the history of the universe, just as the steady state theory was in the 1950s.

Rindler thinks the steady-state theory was crazy. And that inflation is, too.

 “It’s an equally crazy idea and everybody accepts it, implicitly,” he says.

Well, not quite everybody. 

Some contrarian perspectives showed up on the program this week at the Texas Symposium on Relativistic Astrophysics. Paul Steinhardt, a cosmologist at Princeton University, argued that the inflation explanation for the evolution of the universe should more properly be labeled the inflation “myth.”

He built his case on the observation that the universe is, in essence, a simple place. On the largest scales, matter is pretty much evenly distributed, and things seem to appear the same no matter which direction you look. The geometry of space is very nearly perfectly flat, which means it observes the rules of good old Euclidean geometry. (In principle, because matter and energy can warp space, it could have been drastically curved.) In other words, Steinhardt says, the universe is basically featureless. Simple.

“If you’re trying to explain something simple, it calls for a simple explanation,” he asserted during a plenary talk at the symposium.

At first glance, inflation seems to be just such a simple explanation. That explosive burst of expansion would have smoothed out the baby universe’s wrinkles, leaving the cosmos smooth and almost featureless, except for tiny mites of matter that grew into galaxies. Probes of the leftover radiation from the Big Bang show the imprints of those mites pretty much just as inflation theorists had expected. Inflation has therefore been the prevailing paradigm for something like two decades now.

Steinhardt was even one of the key contributors to inflation’s early success. For the last decade or so, though, he has been waving a red flag in a quest for an instant replay review. Examined from some different angles, the case for inflation is not so conclusive, he contends.

For one thing, Steinhardt points out, inflation would not make just one universe. It would keep making universes, forever. So reality would comprise a multiverse, uncountably numerous pockets of spacetime with differing properties. In fact, it would contain all possible universes with all possible properties. Inflation therefore has no business celebrating its success in predicting our universe’s properties, since it would have “predicted” them no matter what they turned out to be.

“If you ask what inflation predicts, the safest thing to say is just that anything that can happen will happen and will happen an infinite number of times.... So what we have is really, I would say, an unmitigated disaster as a theory, a theory which is maximally unpredictive,” Steinhardt says. “Literally any physically possible cosmic property will occur in this multiverse…, so there’s no way to falsify this theory, or verify it for that matter, because anything could fit.”

Steinhardt and some collaborators have pursued a different approach. In their view, the beginning of the universe was not conditioned by inflation, but by the properties of a previous universe. When that universe collapsed into nothingness, in a big crunch, our universe then emerged like a phoenix from the debris.

Such a recycled-universe scenario has some problems, though. A universe with repeating life-death cycles is an old idea, long frowned upon because it didn’t seem compatible with the law of entropy. Entropy increases as a universe ages, so a baby universe should start out with relatively low entropy. If a universe is born from the ashes of an old one, entropy would start out unacceptably high.

But hey, if you’re going to redesign how universes get born, why not fiddle with the laws of nature? Roger Penrose, the distinguished British mathematician-physicist, thinks you can avoid the entropy problem. You just need to ignore one of the requirements of quantum mechanics (I wouldn’t try that at home if I were you).

Let’s skip some details and simply point out that entropy is very closely related to information. Black holes swallow lots of information stored in the light and matter they suck in; entropy is, in a sense, a measure of how much information they have swallowed. So in an old universe, Penrose points out, most of the entropy will be concealed within black holes. And black holes, as Stephen Hawking figured out decades ago, evaporate. Ultimately they go poof, disappear, and everything they swallowed disappears too.

In this scenario, the information that black holes had swallowed gets destroyed. So you have to recalibrate your entropy calculations and that means a very old universe ends up with very little entropy, Penrose says. Then the new universe it gives birth to can start out life sufficiently low on the entropy scale to live to a ripe old age.

Other physicists object that quantum mechanics does not permit the destruction of information, though, and other analyses conclude that the information swallowed by black holes somehow survives their evaporation. But Penrose doesn’t think so.

Now, if this were a simple report of presentations at a physics meeting, that’s all I’d have to say. But since it’s a blog, I feel compelled to embellish it with some further observations potentially worth tweeting.

For one thing, all of these machinations leave that quest for a simple explanation for the simple universe in a very strange place. Instead of a superquick simple flash of inflation, you need to explain the universe by first finding a preexisting universe that figures out a way to dodge the rules of quantum mechanics. Doesn’t actually sound so simple.

That doesn’t mean that the inflationary theory should be considered immune to criticism. Its advocates acknowledge that despites its successes in accounting for cosmic phenomena, it still poses theoretical challenges. As cosmologist (and inflation aficionado) Michael Turner says, “inflation has captured much of the truth, but it might not be true.”

So perhaps a sort of cyclic universe scenario will someday explain the universe better than inflation does. But in the meantime, dismissing inflation as an unmitigated disaster reflects more philosophical prejudice than physical judgment.

Steinhardt’s lament that inflation fails to make “falsifiable” predictions rests on a philosophical view that not all scientists share. Must a scientific theory make falsifiable predictions, or can it simply provide an explanation that relates observations to the operation of underlying physical principles? It’s a nuanced distinction, but the “falsifiable prediction” philosophy of what counts as science isn’t quite the same as the “explaining the observations” approach, and many scientists prefer the latter. It remains to be seen which of those philosophies proves must fruitful in making progress on cosmological problems.

A related philosophical issue is the implicit dislike of anthropic explanations among many expressing anti-inflation sentiment. It’s a philosophical prejudice, widely shared by many scientists, that anthropic reasoning — appealing to the existence of life — has no business being part of a scientific explanation. But historically, similar reasoning has been used, appropriately, to answer questions that had been misposed when assuming that life’s existence was irrelevant to the answer. (Why is the planet we live on so comfy, for instance? No theory predicts that planets must have temperatures in this range. It just turns out that there are a bunch of planets, and people live on one that happens to occupy the comfort zone.)

An important known unknown is whether the universe is analogous to the Earth in this respect. Maybe the conditions in our universe allow life not because some deep theory predicts these conditions precisely, but because there’s a whole bunch of universes and we live in one with the properties that allow us to exist. If that’s the case, then inflation does explain why the universe we live in is the way it is, even if it does not predict the properties uniquely.

Still, these questions are open. What’s the best philosophy, what’s the best picture of the cosmos, what is the relationship of life to reality? Can’t say for sure. So it’s OK to question the conventional cosmological wisdom. But it might be worth remembering that a simple universe does not necessarily call for a simple explanation, or a philosophically pleasing explanation — it calls for a correct explanation.

Follow me on Twitter: @tom_siegfried

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