Getting grips on the pre-big-bang era with a universe/anti-universe pair

One can generally speculate anything about what happened before the big bang. This is due to the simple reason that our understanding of Nature breaks down at the exact time of the big bang.

However, this has never prevented physicists from trying to build new theories (coffee helps).

I have very recently read an article, which has appeared on the arxiv a couple of days ago, depicting such an attempt.

In a few words, the authors use the CPT symmetry (I will come back to this concept further in this post) to explain that the universe after the big bang and the universe before the big bang are just symmetric images of each other.

In terms of particles, the model only requires the Standard Model (see all the posts referenced here for an introduction), with extra neutrinos. I will of course also come back to that further in this post.

The interesting feature to keep in mind for the moment is that those extra neutrinos allow for explaining the problematics of dark matter. This model hence seems to cure several issues in one go! It is a 1+1 free somehow!

_{As usual, something stupid is hidden in this post. Can you find it? Please leave a comment.}

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THE CPT SYMMETRY

Let us start by providing a definition for the CPT symmetry.

First, this has nothing to do with the Committee for the Prevention of Torture (and this is not the stupidity to find in this post). In physics, this corresponds to a threefold operation: charge, parity and time-reversal, or the C, P and T operations.

The P transformation reverses the spatial configuration of the system. This is equivalent to look into a mirror. Everything is flipped (left becomes right, for instance).

The C transformation implies to replace all particles by the corresponding antiparticles, and vice versa.

The T transformation reverses the course of time to make it running backwards.

Quantum field theory tells us that there are fundamental reasons that nature is CPT symmetric. Equivalently, the laws governing nature are invariant under a joint C, P and T transformation.

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THE CPT SYMMETRY AND THE BIG BANG

Taking all cosmological observations, it turns out that the structure of the universe is described, a few seconds after the big bang, by a spatially-flat Friedmann-Robertson-Walker metric.

This means that we have a standard three-dimensional space (cool, we are still 3D objects that can thus be printed), a time dimension plus a so-called scaling factor describing the expansion of the universe.

At the very first moment of the universe, the scaling factor is proportional to time. The expansion of the universe is thus more and more violent with time passing during the first fractions of a second of the life of the universe.

Inversely, this also means that the scaling factor vanishes at the exact big bang moment. This is the initial singularity that many mention.

In the research paper mentioned earlier in this post, it is shown that the initial singularity can be removed by an appropriate change of units. Furthermore, this also allows time to be negative.

The authors moreover show that once negative times are included, we have an apparent T symmetry between the post-big-bang and pre-big-bang moments. What is going on before the big bang is the image of what is going on after it.

As the CPT symmetry is fundamental in quantum field theory, we can move one step further and make the universe CPT-symmetric.

This is an interesting idea, and this corresponds to see our universe as a pair of universe anti-universe merging from nothing. In the same way that an antiparticle is a particle running backwards in time, the anti-universe would correspond to what happened prior to the big bang.

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AND DARK MATTER IN ALL OF THIS?

In terms of particles, the theory assumes the Standard Model of particle physics together with a weird species of neutrinos known as right-handed neutrinos.

This has nothing to do with how neutrinos are writing, but how their spin is aligned or anti-aligned with their momentum.

In more details, the spin is an intrinsic form of angular moment carried by each particle. This for instance tells us how particles react to magnetic fields. On the other hand, the momentum describes roughly how the particles moves. The handedness of the neutrinos is then the way these two quantities are aligned.

So far, there is not any experimental sign of a right-handed neutrino, i.e. a neutrino whose spin is aligned on its momentum. But it is very easy to embed those guys in the Standard Model.

And once they are there and assuming the CPT universe hypothesis, one can calculate their properties.

It has been found that right-handed neutrinos can be very good dark matter candidates as they are stable, super heavy (200 billion times heavier than the heaviest known particle) and super-weakly interacting.

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TAKE-HOME MESSAGE

The very first moments of the universe are fascinating, especially when considering big bang cosmology. The latter however suffers from an initial singularity that makes the theory breaking down at that moment. In a recent paper whose longer version is available here, it has been shown that this singularity can be removed.

As a result, our universe has to be seen as a universe/anti-universe pair and satisfies the CPT symmetry appearing in quantum field theory.

A nice feature of this theory is that it can naturally accommodate dark matter within the Standard Model through the presence of right-handed neutrinos, i.e. neutrinos with specific properties that have not been observed so far.

The problem is that these right-handed neutrinos are almost impossible to detect… This scenario has however other ways to be observed and potentially confirmed experimentally, using the left-handed neutrinos and gravitational waves.

Therefore, please stay tuned to see how this interesting model survives (or not) future data!

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