In the standard model of cosmology, at the end of the Big Bang and the era of Planck, the early Universe entered a brief and violent phase of inflation during which it grew disproportionately. Although this hypothesis resolves certain phenomena observed in the Universe (such as homogeneity and isotropy), there is still no precise physico-mathematical framework to describe it. And according to a Canadian theorist, it could never exist because of certain censorship parameters intrinsic to the Universe, even when one wishes to describe the most microscopic scales.
A physicist thinks he knows why models fail to provide a detailed physical description of the phenomenon called inflation:the Universe won't let us. More precisely, the author describes a new conjecture according to which, concerning the young universe, "the observer should be protected" from the direct observation of the smallest structures of the cosmos, those located at the Planck scale.
In other words, by definition, physicists may never be able to build an inflation model using the usual tools, and they will have to find a better way. This new conjecture pinpoints a particular feature of inflation models. These patterns take very, very small fluctuations in space-time and magnify them. But we don't have a complete physical theory of these small fluctuations, and so inflation models that have this feature (almost all of them) will probably never work.
Observations of the large-scale structure of the Universe and the remnant light of the Big Bang have revealed that in the early Universe, our cosmos likely experienced a period of incredibly rapid expansion. This remarkable event, known as inflation, caused the universe to become trillions of times larger in a tiny fraction of a second.
In this process, inflation has also made our cosmos a little chaotic. As inflation grew, the smallest random quantum fluctuations—fluctuations embedded in the very fabric of spacetime—became much, much larger, meaning that some regions were more densely filled with matter than others.
Eventually, these microscopic differences grew to become macroscopic, in some cases extending across the Universe. Millions and billions of years later, these tiny differences in density became the seeds of stars, galaxies and the largest structures in the cosmos.
Astronomers strongly suspect that the inflation occurred in the earliest moments of the Universe, when it was less than a second old; even so, they don't know what started the inflation, what fueled it, how long it lasted, or what cut it. In other words, physicists lack a complete physical description of this momentous event.
Adding to the mix of mysteries, in most inflation models, fluctuations on tiny scales inflate to become macroscopic differences. Initially, these fluctuations are the Planck length, or about 1.6 x 10 -35 metre. This is the scale where the force of gravity rivals that of the other fundamental forces of nature. On this scale, we need a unified theory of physics to describe reality. But we have no such theory.
So we have a problem. Most (if not all) inflation models require the Universe to grow so large that the Planckian differences become macroscopic. But we don't understand Planckian physics. So how could we build a theoretical model of inflation if we don't understand the underlying physics?
Maybe the answer is:we can't. Never… This concept is called the Trans-Planckian Censorship Conjecture, or TCC. Robert Brandenberger, a Swiss-Canadian theoretical cosmologist and professor at McGill University in Montreal, recently wrote about CBT. According to Brandenberger, "CBT is a new principle that constrains viable cosmologies".
According to him, the CBT implies that any observer in our large-scale world can never observe what is happening on the very small trans-Planckian scale. Even if we had a theory of quantum gravity, the TCC states that anything living under the sub-Planckian regime will never "cross" the macroscopic world. As for what the CBT might mean for inflation models, that's unfortunately not good news.
Most inflation theories rely on a technique known as effective field theory. Since we don't have a theory that unifies high-energy and small-scale physics (conditions like inflation), physicists try to build lower-energy versions to make progress. But under CBT, this kind of strategy doesn't work, because when we use it to build inflation models, the inflation process happens so quickly that it "exposes" the sub-Planckian regime to observation. macroscopic.
String gas cosmology is a possible approach to modeling the early universe under string theory, which itself is a promising candidate for a unified theory of classical and quantum physics. In the cordist gas model, the Universe never experiences a period of rapid inflation. Instead, the period of inflation is much smoother and slower, and fluctuations below the Planck length are never "exposed" to the macroscopic universe.
Physics below the Planck scale never grows to become observable, and so the CBT is satisfied. However, cordist gas models are not yet detailed enough to be tested against observable evidence of inflation in the Universe. CBT relates to another sticking point between inflation and unified physics theories 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) is only one part. It seems that most (if not all) inflation models are incompatible with string theory. Instead, they belong to what string theorists have called the “swamp” — the region of possible universes that are simply not physically realistic. The TCC could be an expression of the rejection of inflation in the swamp.
It may still be possible to construct a traditional inflation model that satisfies CBT (and outside the string theory swamp); but if the CBT is true, it severely limits the kinds of models physicists can build. If inflation manages to continue for a short enough period, while planting the seeds that will one day become massive structures, the theory of inflation might work.
Currently, CBT is unproven; it is only a conjecture. This aligns with other string theory lines of thought, but string theory itself is not experimentally proven. But ideas like this are helpful, because physicists fundamentally don't understand inflation, and anything that can help sharpen that thinking is welcome.
