What implications to cosmology would it have if Webb Telescope probing the far end of our observable Universe finds out a large number of galaxies?

Question

Since it is predicted that Cosmos is no less than 250 times that of the observable Universe (13.8 Blyrs) in size:

MIT Technology Review

I wondering how you can calculate any age of matter creation (13.8 Byrs) in a small pocket of the total Universe thus cosmos?

Would the above not mean that in cosmological scale our observable Universe could be already there long before the 13.8 Byrs life span and there was no matter creation or any Big Bang but only inflation of the already existed matter in that region of space? This could be just an effect of Dark Energy expansion of space.

Of course that would also mean that matter in our observable Universe is much more older.

It is believed according to BB theory that the more to the far end of the observable Universe you go the number of galaxies will drop and galaxies will be much more elementary in their morphology.

But what if the new Webb space telescope finds out that this is not the case as predicted?

Would that mean that the Big Bang theory is wrong? Or needs to be revised?

What if the Webb's new more powerful than Hubble, Ultra-Deep Field image reveals that the number of galaxies is much more than predicted?

Answer 1

The current "bottom-up" idea of structure formation suggests that galaxies are built up over cosmic time from the mergers of lots of smaller galaxies. There is now plenty of chemical and dynamical evidence that this happened for the Milky Way galaxy (see Helmi 2020 for a review of that evidence) and we can see the process of galaxy mergers taking place all around us (e.g. the Antenna galaxies). It is also well established that more distant galaxies tend to be smaller and to resemble less-and-less the grand-design spirals and giant ellipticals that are seen in the low redshift universe.

Therefore it is not altogether correct to say that as we go back in time, the number (per unit comoving volume) of galaxies should decrease. Conselice et al. (2016) show that it is true that the comoving number density of large galaxies (stellar mass $>10^{10}M_\odot$) decreases monotonically from now back to $z=8$. However, the density of smaller galaxies (stellar mass $<10^7 M_\odot$) continues to increase back to this epoch.

Nevertheless, it is true that as you go back even further, one should encounter the "dark ages", where the first galaxies and stars were yet to form, probably at $z \geq 15$. At present, the most distant galaxies observed are at $z \sim 10$ and are unlikely to be "typical" examples - they are likely luminous quasars or galaxies undergoing powerful bursts of star formation. It is to be hoped that JWST will provide a much more detailed picture of what is going on at these redshifts with much less observational selection bias.

If JWST revealed a higher number density of galaxies at high redshifts than predicted by structure formation models then the implications would depend on what kind of galaxies they were. Basically though it would mean that structure formation needs to happen earlier. One thing to bear in mind is that the relationship between redshift and time since the big bang is highly non-linear. The difference between redshifts of 10 and 11 is equivalent to only a small age difference between 470 and 420 million years after the big bang (i.e. 50 million years). The difference between redshifts of 14 and 15 is only 300 to 270 million years after the big bang.

But you are right - for example finding strong evidence for a quasars at $z>15$ would be a massive challenge to current models.

I don't follow your argument that our observable universe might be somehow older than the universe as a whole. The region outside our observable universe would have been created in the same big bang and would have the same age. There are multiverse models where entirely separated universes can be created in different pockets of inflation but I don't think that is what is being referred to by the statement that the universe much be $>250$ times the size of the observable universe. I suspect that is an extrapolation based on limits to the flatness of the observable universe. In any case, JWST can only measure the observable universe and it is the observable universe that is estimated to have an age of 13.8 Gyr.