Magueijo paper -- Faster than c light vs inflation

[kodama]

The critical geometry of a thermal big bang
Niayesh Afshordi, Joao Magueijo
(Submitted on 9 Mar 2016 (v1), last revised 8 Nov 2016 (this version, v2))
We explore the space of scalar-tensor theories containing two non-conformalmetrics, and find a discontinuity pointing to a "critical" cosmological solution. Due to the different maximal speeds of propagation for matter and gravity,the cosmological fluctuations start off inside the horizon even without inflation, and will more naturally have a thermal origin (since there is never vacuum domination). The critical model makes an unambiguous, non-tuned prediction for the spectral index of the scalar fluctuations: nS=0.96478(64). Considering also that no gravitational waves are produced, we have unveiled the most predictive model on offer. The model has a simple geometrical interpretation as a probe 3-brane embedded in an EAdS2×E3 geometry.
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)
Journal reference: Phys. Rev. D 94, 101301 (2016)
DOI: 10.1103/PhysRevD.94.101301
Cite as: arXiv:1603.03312 [gr-qc]

proposes that light can travel faster than c can be tested via cosmic microwave background spectral index value of 0.96478. so instead of inflation, light can exceed c in the earliest moments of the big bang

credibility of this theory and how would it affect the rest of fundamental physics if this theory is verified?

i.e instead of inflation, light was faster than c. so no need for an inflaton.
how does it effect the structure of QFT and SM if light can exceed c during big bang, and this and not inflation explains universe?

ramifications for BSM including string/lqg?

 

[bapowell]

I don't see how faster than light travel in the early universe could replace inflation. Two very different phenomena.

 

[haushofer]

Just wondering: would a higher speed of Light imply a universe which behaves more non-relativistically?

 

[no-ir]

I don't see how faster than light travel in the early universe could replace inflation. Two very different phenomena.

Well, according to the article, inflation was introduced to alleviate the horizon problem: why do cosmological structures in regions which should have been out of causal contact in the early universe (=outside their respective "horizons" which shrink as you roll back time towards the Big Bang) appear so similar today? Inflation offers one explanation: the regions actually were in causal contact before inflation (and so could thermalize, for instance, and thus evolve similarly at later times), but then a period of rapid growth of space-time distance between them (inflation) separated them into the apparently causally-disconnected regions we observe today.

The article cited by the OP offers an alternative explanation: perhaps the space-time distance between regions was not actually smaller in the very early universe (as inflation would have it), but instead light was simply faster and so could bring structures that would have been causally disconnected (with today's value of c) into causal contact (again, allowing the regions to thermalize, thus producing similar evolution of those regions at later cosmological times).

Even though I am no expert in cosmology, the precise prediction of the values of both the spectral index and the amplitude of primordial density fluctuations (two separate parameters obtained from precise microwave background radiation measurements) from a theory with only one free parameter (if you fix the amplitude you can uniquely calculate the spectral index) seems quite remarkable to me. This also makes the theory eminently testable (which is Good): if future measurements show that the values of the amplitude and the spectral index of primordial density fluctuations are not connected in the way that this theory predicts, then the theory is disproven.

I would appreciate any insight from an expert, though:

1. What predictions does inflation make, that could not be easily reproduced by this varying-speed-of-light theory instead?
2. Does inflation also link the values of the spectral index and the amplitude of primordial density fluctuations, or is this feature unique to this theory? (I'm guessing, no?)
3. Any other misgivings with the new theory that would make you still stick to the inflationary scenario instead (aside from the fact that inflation had been studied for longer)?

 

[bapowell]

1. What predictions does inflation make, that could not be easily reproduced by this varying-speed-of-light theory instead?

Flatness maybe?

2. Does inflation also link the values of the spectral index and the amplitude of primordial density fluctuations, or is this feature unique to this theory? (I'm guessing, no?)

Only in specific models.

 

[no-ir]

Flatness maybe?

Flatness is supposedly solved in the varying-speed-of-light model as well. From the concluding paragraph of the article:

One may wonder about the status in our model of the other cosmological problems, such as the flatness, homogeneity and isotropy problems.
...
Furthermore, we find that at least the flatness problem can be solved, in a single package, during the phase transition. The conformal symmetry of the theory in the UV [20, 21] not only fixes the potential but requires exact flatness (Eqs.(5) lead to a contradiction in the presence of spatial curvature).