A Dark Question

Dr Becky Smethurst put a video out last week about a possible resolution to the “Hubble Tension”/“Crisis in Cosmology”.  The work has not yet been published, but instead is covered in a talk by Wendy Freedman, but it is interesting to note that the result that the JWST people arrived at is H₀=69.1±1.3km/s/Mpc (which corresponds with a Hubble Time of T_H=14.15+0.27 billion years).

It was quite timely because I was already thinking about expanding on something I was talking with someone about in the past week.

Imagine that soon after Erwin Hubble had identified the redshift of distant objects (in the 1920s), sufficiently advanced telescopes were developed and used to determine the value of the Hubble parameter to be close to 70km/s/Mpc (didn’t happen until the 1990s).  Say then that someone had quickly worked out that ~70km/s/Mpc is the inverse of ~14 billion years (fitting excellently with the age of the oldest known star, although its age was only determined to fit nicely after revision to models in 2015 and 2021).  Then, a short time later, someone else was fiendishly clever enough to use the technology available at the time to measure the geometry of the universe and determine that it is flat, meaning that the density of the universe is critical (this wasn’t really determined until 2000 with analysis of the BOOMERanG experiment results from 1997 and 1998).

So, in this hypothetical world we would have had, in about 1930, all the details necessary to conclude that our universe is a FUGE universe.  A FUGE universe starts out as an “instanton”, effectively a Planck black hole of half a unit of Planck mass-energy with a radius of one unit of Planck length, adding half a unit of Planck mass-energy and expanding its radius by one unit of Planck length every unit of Planck time.  Such a universe has a Hubble parameter that is the inverse of its age and has critical density throughout its life (meaning that it is, has always been and will always be flat).

Now say that in this hypothetical world, about 30 years after the FUGE universe model was established, someone discovers the cosmic microwave background (CMB).  Analysis of this raises bit of a mystery because the CMB has an unexpectedly high level of isotropy.

Under these conditions, would it be reasonable to posit inflation (about 15 years after the discovery of the CMB)?  Note that one of the motivations for inflationary theory would be missing in our hypothetical world, because the flatness problem would not exist – critical density (and thus flatness) of the universe is perfectly explained by the FUGE model.  The other motivations also have other potential explanations: gravity may suffice to explain the homogeneity of the horizon problem and the magnetic-monopole problem only relates to the absence of hypothetical particles (the standard approach, when finding that your hypothesis predicts the existence of some non-existent thing, is to reassess your hypothesis rather than engage in a form of special pleading – especially after 90 years have passed with no observational evidence).

Note also that in a hypothetical world which has accepted the FUGE model, we have a very simple chronology – with smooth expansion of the universe over ~14 billion years to arrive at a Hubble parameter value that is the inverse of ~14 billion years and a density that matches the observed (critical) density.  In order to arrive at the value of the Hubble parameter, after having introduced inflation, we have to posit  a much more complex chronology at least three phases: smooth FUGE-like expansion for a fraction of a second (grand unification epoch), inflationary expansion for a fraction of a second (during which mass-energy would have had to have been added at a much higher rate if critical density were to be maintained) and an approximately 14 billion year-long phase in which the expansion was precisely that necessary to make the universe today look like it had only undergone FUGE-like expansion.

Personally, I don’t think it would be reasonable.

Our situation is actually worse than described above because, in the Standard Model, there are five phases: FUGE-like expansion (grand unification epoch), inflation, two periods of reduced expansion (less than FUGE-like: radiation dominated and matter dominated) and a current period of accelerated expansion (greater than FUGE-like) at a rate necessary to make the universe today look precisely like it had only undergone FUGE-like expansion – a situation that would not have been the case since a fraction of a second after the instanton arose and won’t be the case ever again (because the explanation for observed accelerated expansion is that we are in a dark-energy-dominated era [other explanations are available] and such domination by dark energy is unlikely to suddenly dissipate in order for us to return to FUGE-like expansion on an on-going basis and we are unlikely to return to the conditions of earlier putative eras of reduced expansion [the radiation dominated and matter dominated eras]).

Is it truly reasonable to have such outrageous fiddling of the universe, given the option of the FUGE model (or something like it)?