Physicists have long been unable to crack the mystery of what happened in the moments when a vanishing little seed penetrated the universe. One scientist now thinks he knows why they cannot invent a physical description of the phenomenon, inflation: the universe does not allow us.
The scientist specifically describes a new conjecture which, with respect to the young universe, ‘must protect the observer’ from direct observation of the smallest structures in the cosmos.
In other words, physicists by definition can never build a model of inflation using the usual tools, and they will have to think of a better way.
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But why not? This new presumption, which is an opinion or thought based on incomplete information, points the blame at a specific feature of inflation models. These models assume very, very small fluctuations in space-time and make them larger. But we do not have a complete physical theory about the small fluctuations, and so inflation models that have that feature (which is almost everyone) will never work.
Tap in string theory, which may be the key to explaining the secrets of inflation.
Blow away
Observations of the large-scale structure of the universe and the remaining light of the Big explosion revealed that our cosmos experienced a period of incredibly rapid expansion very early in the universe. This remarkable event, known as inflation, has driven the universe to become trillions a thousand times larger in the smallest fraction of a second.
In the process of becoming big, inflation has also made our cosmos a bit bumpy. As inflation unfolded, the smallest random quantum fluctuations – fluctuations were built into the tissue of space-time itself – has become much, much larger, which means that some regions are more densely packed with matter than others. Eventually, these submicroscopic differences became macroscopic … and even larger, and in some cases they extended from one side of the universe to the other. Millions and billions of years later, these small differences in density have become the seed 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; yet they do not know what caused inflation, what drove it, how long it lasted or what turned it off. In other words, physicists have a complete physical description of this important event.
An addition to the mix of puzzles is that in most inflation models fluctuations are inflated on extremely small scales to become macroscopic differences. How small? Smaller than the Plank length, or about 1.6 x 10 ^ minus 35 meters (the number 16 preceded by 34 zeros and a decimal point). This is the scale where the strength of gravity compete with those of the others fundamental forces of nature. On the scale, we need a unified physical theory to describe reality
We do not have such a theory.
So we have a problem. Most (if not all) inflation models require that the universe be so large that the differences between Planckia become macroscopic. But we do not understand sub-Planque physics. How then can we build a theoretical model of inflation if we do not understand the underlying physics?
Outside the Planck scale
Maybe the answer is: we can not. Ever. This concept is called the trans-Planckian censorship assumption (TCC) (in this name, “trans-Planckian” means anything that extends below the Planck length).
Robert Brandenberger, a Swiss-Canadian theoretical cosmologist and professor at McGill University in Montreal, Canada, recently wrote a review about the TCC. According to Brandenberger, “The TCC is a new principle that limits viable cosmologies.” According to him, the TCC implies that any observer in our large-scale world can never ‘see’ what is happening on the small trans-Planckian scale. Even if we had a theory of quantum gravity, the TCC says that anything that lives in the sub-Planckian regime will never “pass” in the macroscopic world. What the TCC may mean for inflation models is unfortunately not good news.
Most theories about inflation are based on a technique known as ‘effective field theory’. Since we do not have a theory that unites high-energy, small-scale physics (including conditions such as inflation), physicists are trying to build lower-energy versions to make progress. But under the TCC, that kind of strategy does not work, because if we use it to draw up inflation models, the process of inflation happens so fast that it ‘exposes’ the sub-Planckian regime to macroscopic observation, Brandenberger said.
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In light of this issue, some physicists wonder if we should approach the early universe completely differently.
From the marshland
String gas cosmology is a possible approach to modeling the early universe under string theory, which is itself a hopeful candidate for a unified physics theory that brings classical and quantum physics under one roof. In the flue gas model, the universe never undergoes a period of rapid inflation. Instead, the inflation period is softer and slower, and fluctuations below the Planck length are never “exposed” to the macroscopic universe. Physics below the Planck scale never becomes observable, and so the TCC is satisfied. However, greenhouse gas models do not yet have enough details to test the observable evidence of inflation in the universe.
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The TCC relates to another sticking point between inflation and theories of unified physics, such as string theory. String theory predicts an enormous number of potential universes, of which our specific cosmos (with its set of forces and particles and the rest of physics) represents only one. It would seem that most (if not all) inflation models are incompatible with string theory at a basic level. Instead, they belong to the string theorists called the ‘swamp country’ – the region of possible universes that are simply not physically realistic.
The TCC may be an expression of the rejection of inflation in the wetland.
It is perhaps still possible to build a traditional model of inflation that complies with the TCC (and lives outside the heaviness of string theory); but if the TCC is true, it severely limits the types of models physicists can build. If inflation manages to continue for a short enough time (suggest you inflate a balloon slowly and stop before it pops up) while still planting the seeds that will one day turn into massive structures, inflation theory may work .
Right now, the TCC is unproven – it’s just a suspicion. This is in line with other schools of thought of the string theory, but the string theory itself has not been proven either (in fact, the theory is not yet complete and is not even capable of making predictions). Yet ideas like these are useful because physicists fundamentally do not understand inflation, and anything that can help sharpen thinking is welcome.
Originally published on Live Science.