Physicists have not been able to unravel the mystery of what happened when a seed that disappeared as a child entered the universe for a long time. Now, a scientist thinks he knows why I 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 assumption that states, in terms of the young universe, “the observer should be protected” from direct observation of the smallest structures in the cosmos.
In other words, by definition physicists will never be able to build a model of inflation using the usual tools and will have to come up with a better way.
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But why not? This new assumption, which is an opinion or a thought based on incomplete information, points the finger at a particular feature of inflation patterns. These models have very, very small fluctuations in space-time and make them bigger. But we do not have a complete physical theory of those small fluctuations, so inflation models that have this characteristic (which is almost all) will never work.
enter string theory, which could be the key to elucidating the secrets of inflation.
Inflate
Observations on the large-scale structure of the universe and the remaining light from The big bang revealed that in the very early universe, our cosmos probably experienced an incredibly rapid period of expansion. This remarkable event, known as inflation, has led the universe to become trillions upon trillions of times larger in the smallest fraction of a second.
In the process of becoming huge, inflation has also made our cosmos a little bumpy. As inflation unfolded, the smallest random quantum fluctuations – fluctuations embedded right in the fabric space time in itself – it became much, much larger, which means that some regions were more densely packed with matter than others. Eventually, those sub-microscopic differences became macroscopic … and even greater, in some cases, stretching from one end of the universe to the other. Millions and billions of years later, those small differences in density have grown 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 happened in the first moments of the universe, when it was less than a second; even so, they do not know what triggered inflation, what fueled it, how long it lasted, or what stopped it. In other words, physicists lack a complete physical description of this important event.
It is added to the mix of mysteries that, in most inflation patterns, fluctuations at extremely small scales swell to become macroscopic differences. How small? Thinner than Planck length, or about 1.6 x 10 ^ minus 35 meters (number 16 preceded by 34 zeros and one decimal point). This is the scale in which power gravity rivals that of the other fundamental forces of nature. At this scale, we need a unified theory of physics to describe reality
We do not have such a theory.
So we have a problem. Most (if not all) inflation patterns require the universe to grow so large that sub-Planckian differences become macroscopic. But we do not understand subplank physics. So how can we build a theoretical model of inflation if we do not understand basic physics?
Beyond the Planck ladder
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 that falls below Planck’s length).
Robert Brandenberger, Swiss-Canadian theoretical cosmologist and professor at McGill University in Montreal, Canada, recently wrote a review of TCC. According to Brandenberger, “CBT is a new principle that constrains viable cosmologies.” In his view, CBT implies that any large-scale observer in our world can never “see” what is happening on the tiny trans-Planckian scale. Even if we had a theory of quantum gravity, TCC states that anything that lives in the subplanckian regime will never “pass” into the macroscopic world. As for what the CBT could mean for inflation patterns, unfortunately it is not good news.
Most inflation theories are based on a technique known as “efficient field theory”. Since we do not have a theory that unifies physics at high energy and on a small scale (also known as conditions such as inflation), physicists are trying to build lower energy versions to make progress. But in CBT, this type of strategy doesn’t work, because when we use it to build inflation patterns, the inflation process happens so fast that it “exposes” the subplanckian regime to macroscopic observation, Brandenberger said.
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In light of this problem, some physicists wonder if we should take a completely different approach to the early universe.
Outside the swamp
String gas cosmology is a possible approach to modeling the early universe within string theory, which is itself a promising candidate for a unified theory of physics that brings classical and quantum physics under one roof. In the string gas model, the universe never suffers from a period of rapid inflation. In contrast, the inflation period is much milder and slower, and fluctuations below Planck’s length are never “exposed” to the macroscopic universe. Physics below the Planck scale never grows to become observable, so TCC is happy. However, gas string models do not yet have enough detail to test against observable evidence of inflation in the universe.
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CBT is linked to another point of connection between inflation and theories of unified physics, such as string theory. String theory predicts a huge number of potential universes, of which our particular cosmos (with its set of forces and particles and the rest of physics) is just one. It seems that most (if not all) inflation patterns are incompatible with string theory at a basic level. Instead, they belong to what string theorists have called the “swamp” – the region of possible universes that are simply not physically realistic.
CBT could be an expression of inflation rejection in the swamp area.
It may still be possible to build a traditional inflation model that satisfies CBT (and that lives outside the swamp of string theory); but if TCC is true, this severely limits the types of models that physicists can build. If inflation manages to unfold for a short enough period of time (imagine throwing the balloon slowly and stopping before it appears), while planting the seeds that will one day grow to be massive structures, inflation theory it might work.
Currently, CBT is not proven – it’s just an assumption. It aligns with other lines of thought of string theory, but string theory is itself unproven (in fact, the theory is not complete and is not even capable of making predictions yet). But still, such ideas are useful, because physicists do not fundamentally understand inflation, and anything that can help clarify this thinking is welcome.
Originally published on Live Science.