A Hypothetical Pre-Big Bang Universe And More Conjectures

There are at least three plausible solutions to the question of why we live in a matter dominated universe when almost all processes experimentally observed conserve the number of matter particles minus the number of antimatter particles. 

One is that the initial Big Bang conditions were matter dominated (as our post-Big Bang universe almost surely was a mere fraction of a second after the Big Bang). There is no scientific requirement that the universe had any particular initial conditions.

A second is that there are new, non-equilibrium physics beyond the Standard Model that do not conserve baryon and lepton number and are strongly CP violating at extreme high energies. No new physics is necessary for the Standard Model to continue to make mathematical sense to more than 10^16 GeV, known as the grand unified theory (GUT) scale. And, there is no evidence of such physics yet. But the most powerful particle colliders and natural experiments that function as particle colliders have interaction energies far below the 10^16 GeV. The most powerful man made collider, the Large Hadron Collider (LHC), is probing energies on the order of 10^4 GeV, about a trillion times lower than those of the immediate vicinity in time of the Big Bang.

A third is that matter, which can be conceived of as particles moving forward in time, dominates our post-Big Bang universe, while there is a parallel pre-Big Bang mirror universe dominated by antimatter, which can be conceived of as particles moving backward in time. To the extent that this calls for beyond the Standard Model physics, the extensions requires are very subtle and apply only at the Big Bang singularity itself. 

I tend to favor this quite elegant approach, although the evidence is hardly unequivocal in favoring it over the alternatives, and it may never be possible to definitively resolve the question.

The introduction to a new paper and its conclusion, below, explain the features and virtues of this third scenario. 

The paper argues that the primary arrow of time (since fundamental physics observes CPT symmetry to high precision) is entropy as one gets more distant in time from the Big Bang, that cosmological inflation and primordial gravitational waves is not necessary in this scenario, and in this scenario, it makes sense that the strong force would not violate CP symmetry, despite the fact that there is an obvious way to insert strong force CP violation into the Standard Model Lagrangian. 

In contrast, cosmological inflation is quite an ugly theory, with hundreds of variants, many of which can't be distinguished from each other with existing observations.

In a series of recent papers, we have argued that the Big Bang can be described as a mirror separating two sheets of spacetime. Let us briefly recap some of the observational and theoretical motivations for this idea. 

Observations indicate that the early Universe was strikingly simple: a fraction of a second after the Big Bang, the Universe was radiation-dominated, almost perfectly homogeneous, isotropic, and spatially flat; with tiny (around 10^−5 ) deviations from perfect symmetry also taking a highly economical form: random, statistically gaussian, nearly scale-invariant, adiabatic, growing mode density perturbations. Although we cannot see all the way back to the bang, we have this essential observational hint: the further back we look (all the way back to a fraction of a second), the simpler and more regular the Universe gets. This is the central clue in early Universe cosmology: the question is what it is trying to tell us. 

In the standard (inflationary) theory of the early Universe one regards this observed trend as illusory: one imagines that, if one could look back even further, one would find a messy, disordered state, requiring a period of inflation to transform it into the cosmos we observe. 

An alternative approach is to take the fundamental clue at face value and imagine that, as we follow it back to the bang, the Universe really does approach the ultra-simple radiation-dominated state described above (as all observations so far seem to indicate). Then, although we have a singularity in our past, it is extremely special. Denoting the conformal time by τ , the scale factor a(τ) is ∝ τ at small τ so the metric gµν ∼ a(τ)^2ηµν has an analytic, conformal zero through which it may be extended to a “mirror-reflected” universe at negative τ. 

[W]e point out that, by taking seriously the symmetries and complex analytic properties of this extended two-sheeted spacetime, we are led to elegant and testable new explanations for many of the observed features of our Universe including: (i) the dark matter; (ii) the absence of primordial gravitational waves, vorticity, or decaying mode density perturbations; (iii) the thermodynamic arrow of time (i.e. the fact that entropy increases away from the bang); and (iv) the homogeneity, isotropy and flatness of the Universe, among others. 

In a forthcoming paper, we show that, with our new mechanism for ensuring conformal symmetry at the bang, this picture can also explain the observed primordial density perturbations. 

In this Letter, we show that: (i) there is a crucial distinction, for spinors, between spatial and temporal mirrors; (ii) the reflecting boundary conditions (b.c.’s) at the bang for spinors and higher spin fields are fixed by local Lorentz invariance and gauge invariance; (iii) they explain an observed pattern in the Standard Model (SM) relating left- and right-handed spinors; and (iv) they provide a new solution of the strong CP problem. . . . 

In this paper, we have seen how the requirement that the Big Bang is a surface of quantum CT symmetry yields a new solution to the strong CP problem. It also gives rise to classical solutions that are symmetric under time reversal, and satisfy appropriate reflecting boundary conditions at the bang. 

The classical solutions we describe are stationary points of the action and are analytic in the conformal time τ. Hence they are natural saddle points to a path integral over fields and four-geometries. The full quantum theory is presumably based on a path integral between boundary conditions at future and past infinity that are related by CT-symmetry. The cosmologically relevant classical saddles inherit their analytic, time-reversal symmetry from this path integral, although the individual paths are not required to be time-symmetric in the same sense (and, moreover may, in general, be highly jagged and non-analytic). 

We will describe in more detail the quantum CT-symmetric ensemble which implements (12), including the question of whether all of the analytic saddles are necessarily time-symmetric, and the calculation of the associated gravitational entanglement entropy, elsewhere.

The paper and its abstract are as follows:

We argue that the Big Bang can be understood as a type of mirror. We show how reflecting boundary conditions for spinors and higher spin fields are fixed by local Lorentz and gauge symmetry, and how a temporal mirror (like the Bang) differs from a spatial mirror (like the AdS boundary), providing a possible explanation for the observed pattern of left- and right-handed fermions. By regarding the Standard Model as the limit of a minimal left-right symmetric theory, we obtain a new, cosmological solution of the strong CP problem, without an axion.

Latham Boyle, Martin Teuscher, Neil Turok, "The Big Bang as a Mirror: a Solution of the Strong CP Problem" arXiv:2208.10396 (August 22, 2022).

Some of their key earlier papers by some of these authors (which I haven't yet read and don't necessarily endorse) are: "Gravitational entropy and the flatness, homogeneity and isotropy puzzles" arXiv:2201.07279, "Cancelling the vacuum energy and Weyl anomaly in the standard model with dimension-zero scalar fields" arXiv:2110.06258, "Two-Sheeted Universe, Analyticity and the Arrow of Time" arXiv:2109.06204, "The Big Bang, CPT, and neutrino dark matter" arXiv:1803.08930, and "CPT-Symmetric Universe" arXiv:1803.08928.

Moreover, if Deur's evaluation of gravitational field self-interactions (which is most intuitive from a quantum gravity perspective but he claims can be derived from purely classical general relativity) is correct, then observations attributed to dark matter and dark energy (or equivalently a cosmological constant) in the LambdaCDM Standard Model of Cosmology, can be explained with these non-Newtonian general relativity effects in weak gravitational fields, predominantly involving galaxy and galaxy cluster scale agglomerations of matter. 

This would dispense with the need for any new particle content in a theory of everything beyond the almost universally predicted, standard, plain vanilla, massless, spin-2 graviton giving rise to a quantum gravity theory that could be theoretically consistent with the Standard Model. 

It would also imply that there are no new high energy physics that need to be discovered apart from one at the very Big Bang singularity itself where matter and antimatter pairs created according to Standard Model physics rules segregate between the post-Big Bang and pre-Big Bang universe at this point of minimum entropy, to explain all of our current observations. 

We would need no dark matter particles, no quintessence, no inflatons, no axions, no supersymmetric particles, no sterile neutrinos, no additional Higgs bosons, and no new forces.

We aren't quite there. We have some final details about neutrino physics to pin down. Our measurements of the fundamental particle masses, CKM matrix elements, and PMNS matrix elements need greater precision to really decisively support a theory behind the source of these physical constants. We have QCD to explain hadrons but can't really do calculations sufficient to derive the spectrum of all possible hadrons and all of their properties yet, even though it is theoretically possible to do so. And, of course, there are lots of non-fundamental physics questions in both atomic and larger scale lab physics and in the formation of the universe that are almost certainly emergent from these basic laws of physics in complex circumstances, which we haven't yet fully explained.

There would also be room for further "within the Standard Model" physics to derive its three forces plus gravity, and couple dozen physical constants from a more reductionist core, but that is all. And, there is even some room in the form of conjectures about variants on an extended Koide's rule and the relationship between the Higgs vacuum expectation value and the Standard Model fundamental particles to take that further.

It is also worth noting that even if Deur's treatment of gravitational field self-interactions is not, as claimed, possible to derive from ordinary classical General Relativity, either because it is a subtle modification of Einstein's field equations, or because it is actually a quantum gravity effect, there is still every reason to prefer his gravitational approach, that explains all dark matter and dark energy phenomena and is consistent with astronomy observations pertinent to cosmology (for example, explaining the CMB peaks and resolving the impossible early galaxy problem) with a simple and elegant theory, neatly paralleling QCD, that has at least two fewer degrees of freedom than the LambdaCDM Standard Model of Cosmology despite fitting the observational data better at the galaxy and galaxy cluster scales.

And, Deur's approach is pretty much the only one that can explain the data attributed to dark energy in a manner the does not violate conservation of mass-energy (a nice compliment to a mirror universe cosmology that is time symmetric since conservation of mass-energy is deeply related to time symmetry).

Deur's paradigm has the potential to blow away completely the Standard Model of Cosmology, and the half century or so of astronomy work driven by it and modest variation upon it, in addition to depriving lots of beyond the Standard Model particle physics concepts of any strong motivation.

Milgrom's Modified Newtonian Dynamics (MOND) has actually done a lot of the heavy lifting in showing that observational data for galaxies can be explained, for observations within this toy model theory's limited domain of applicability, without dark matter. 

But Deur's theory, by providing a deeper theoretical justification for the MOND effects that it reproduces, by extending these observations of galaxy clusters and cosmology scale phenomena, by making the theory naturally relativistic in a manner fully consistent with all experimental confirmations of General Relativity, and by providing an elegant solution to observations seemingly consistent with dark energy or a cosmological constant, unifies and glows up MOND's conclusions in a way that makes a gravitational explanation of dark matter far more digestible and attractive to astrophysicists who have so far clung to the increasingly ruled out dark matter particle hypotheses.

A mirror universe cosmology, likewise, has the potential to stamp out the theoretical motivation for all sorts of new physics proposals that simply aren't necessary to explain what we observe as part of a new paradigm of the immediate Big Bang era cosmology.

We are now in a position where physicists can see fairly clearly what the metaphorically promised land of a world where the laws of physics are completely known, even if the scientific consensus hasn't yet caught up with this vision.

It turns out that many of the dominant topics of theoretical physics discussion over the last half-century, from dark matter, to dark energy, to cosmological inflation, to supersymmetry, to string theory, to the multiverse, to cyclic cosmologies, to the anthropic principle, to technicolor, to multiple Higgs doublets, to a grand unified theory or theory of everything uniting physics into a single master Lie group or Lie algebra, do not play an important role in that future vision. Likewise, this would dispense with the need for the many heavily analyzed, but less subtle than Deur's variations on Einstein's Field Equations as conventionally applied, which are the subject of regular research.

If the scientific method manages to prevail over scientific community sociology, in a generation or two, all of these speculative beyond the Standard Model physics proposals will be discarded, and we will be left with a moderately complicated explanation for the universe that nonetheless explains pretty much everything. 

I may not live to see that day come, but I have great hope that my grandchildren or great-grandchildren might live in this not so distant future when humanity has grandly figured out all of the laws of physics in a metaphysically naturalist world.