Will we ever know exactly how the universe ballooned into existence?

Physicists have long been unable to crack the mystery of what happened in the moments when a vanishingly small seed ballooned into the universe. Now, one scientist thinks he knows why they can't come up with a physical description of this phenomenon called inflation: The universe won't let us. 

Specifically, the scientist describes a new conjecture that states, regarding the young universe, "the observer should be shielded" from directly observing the smallest structures in the cosmos.

In other words, by definition physicists may never be able to build a model of inflation using the usual tools, and they will have to come up with a better way. 

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But why not? This new conjecture, which is an opinion or thought based on incomplete information, points the finger of blame at a particular feature of inflation models. These models take very, very small fluctuations in spacetime and make them bigger. But we don't have a complete physical theory of those small fluctuations, and so models of inflation that have that feature (which is almost all of them) will never work.

Enter string theory, which could be the key to elucidating the secrets of inflation.

Inflate away

Observations of the large-scale structure of the universe and the leftover light from the Big Bang have revealed that in the very early universe, our cosmos likely experienced a period of incredibly rapid expansion. This remarkable event, known as inflation, drove the universe to become trillions upon trillions of times larger in the tiniest fraction of a second.

In the process of getting huge, inflation also made our cosmos a little bit bumpy. As inflation unfolded, the tiniest random quantum fluctuations — fluctuations built into the very fabric of space-time itself — got much, much larger, meaning some regions were more densely packed with matter than others. Eventually, those sub-microscopic differences grew to become macroscopic … and even bigger, in some cases stretching from one end of the universe to the other. Millions and billions of years later, those tiny differences in density grew to become the seeds of stars, galaxies and the largest structures in the cosmos.

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Astronomers strongly suspect that something like this inflation story happened in the early moments of the universe, when it was less than a second old; even so, they don't know what triggered inflation, what powered it, how long it lasted or what shut it off. In other words, physicists lack a complete physical description of this momentous event.

Adding to the mix of mysteries is that in most models of inflation, fluctuations at exceedingly tiny scales get inflated to become macroscopic differences. How tiny? Tinier than the Planck length, or roughly 1.6 x 10^minus 35 meters (the number 16 preceded by 34 zeroes and a decimal point). That's the scale where the strength of gravity rivals that of the other fundamental forces of nature. At that scale, we need a unified theory of physics in order to describe reality

We have no such theory.

So we have a problem. Most (if not all) models of inflation require the universe to grow so large that sub-Planckian differences become macroscopic. But we don't understand sub-Planckian physics. So how could we possibly build a theoretical model of inflation if we don't understand the underlying physics?

Beyond the Planck scale

Maybe the answer is: We can't. Ever. This concept is called the trans-Planckian Censorship Conjecture, or TCC (in this name, "trans-Planckian" means anything reaching below the Planck length).

Robert Brandenberger, a Swiss-Canadian theoretical cosmologist and a professor at McGill University in Montreal, Canada, recently wrote a review of the TCC. According to Brandenberger, "The TCC is a new principle which constrains viable cosmologies." In his view the TCC implies that any observer in our large-scale world can never "see" what happens at the tiny trans-Planckian scale. Even if we had a theory of quantum gravity, the TCC states that anything living in the sub-Planckian regime will never "cross over" into the macroscopic world. As to what the TCC might mean for models of inflation, unfortunately it's not good news. 

Most theories of inflation rely on a technique known as "effective field theory." Since we don't have a theory that unifies physics at high energy and small scales (a.k.a. conditions like inflation), physicists try to build lower-energy versions to make progress. But under the TCC, that kind of strategy doesn't work, because when we use it to build models of inflation, the process of inflation happens so rapidly that it "exposes" the sub-Planckian regime to macroscopic observation, Brandenberger said.

Related: What happened before the Big Bang?

In light of this issue, some physicists wonder if we should take a completely different approach to the early universe.

Out of the swampland

String gas cosmology is a possible approach to modeling the early universe under string theory, which is itself a hopeful candidate for a unified theory of physics that brings classic and quantum physics under the same roof. In the string gas model, the universe never undergoes a period of rapid inflation. Instead, the inflation period is much gentler and slower, and fluctuations below the Planck length never get "exposed" to the macroscopic universe. Physics below the Planck scale never grows up to become observable, and so the TCC is satisfied. However, string gas models don't yet have enough detail to test against the observable evidence of inflation in the universe.

Related: What is the smallest thing in the universe?

The TCC is related to another sticking point between inflation and theories of unified physics like string theory. String theory predicts an enormous number of potential universes, of which our particular cosmos (with its set of forces and particles and the rest of physics) represents only one. It seems as if most (if not all) models of inflation are incompatible with string theory at a basic level. Instead, they belong to what string theorists called the "swampland" — the region of possible universes that simply aren't physically realistic.

The TCC could be an expression of the swampland rejection of inflation.

It may still be possible to build a traditional model of inflation that satisfies the TCC (and lives outside string theory's swampland); but if the TCC is true, this severely limits the kinds of models that physicists can build. If inflation manages to proceed for a short enough period of time (imagine blowing up a balloon slowly and stopping before it pops), while still planting the seeds that will someday grow up to be massive structures, inflation theory might work.

Right now, the TCC is unproven — it's just a conjecture. It lines up with other lines of thinking of string theory, but string theory is itself also unproven (in fact, the theory isn't complete and isn't even able to make predictions yet). But still, ideas like this are useful, because physicists fundamentally don't understand inflation, and anything that can help sharpen that thinking is welcome.

Originally published on Live Science.