Also important to note that the process of stellar collapse and then black hole accretion takes absolutely enormous amounts of time to collate a large amount of mass together. It's also an extremely energetic process, you would expect to see very bright black holes if millions of solar masses of matter were infalling creating very large and bright accretion disks. We do see some active galactic nuclei but not that many. There's just no way there was enough time for this to happen in the early universe, or really even after a measly 14 billion years (i.e. seeing these young supermassive black holes is challenging for the stellar collapse theory, but the theory was already pretty challenged).
Not to mention if supermassive black holes were being formed by accretion, you would expect to see many intermediate mass black holes (1000-1000000 solar masses) everywhere, but we see almost none.
There's just no way there was enough time for this to happen in the early universe, or really even after a measly 14 billion years
Why do we assume mass distribution in the early universe followed a regular pattern? We can't explain why the universe isn't isotropic and we can't explain why there's more matter than antimatter so why couldn't there have been clumps of very dense matter ready-made to collapse into a black hole?
Because we can see the distribution from the CMB, 380,000 years after the big bang. It's almost perfectly homogenous and isotropic. The images you see of the CMB are amplified a lot, the deviations are on the order of 10^-5 or something.
This in turn puts constraints on the primordial quantum fluctuations that were inflated during the inflation phase, and backtracking through simulations it puts constraints on the entire dark matter and matter history from now back to age 380,000 years.
Just my 2 cents :
The universe was smaller, and maybe a lot of stars created in the same region early after the big bang were unstable, transforming into stellar black holes and just merged to create these super massive black holes (SMBH).
I wonder the number of stellar black holes it takes to merge to create a SMBH.
Either what we know about black hole formation is basically complete (it goes gas -> star -> black hole -> accretion + collisions) but the environment in the early universe was sufficiently different/dense that parameters which rule out the formation of supermassive black holes now were different. Maybe there were many intermediate black holes just in the millions of years after the big bang and things were still close enough together that accretion could happen and collisions were "likely" at the rate needed to form SMBHs after just a billion years. If that is true we might expect to see many many active galactic nuclei as we get better telescopes and look further back, depending on how quickly such black holes formed.
The other option is there is a mechanism of black hole formation that bypasses the above chain which we understand. People talk about supermassive stars, gas clouds collapsing directly into black holes, or primordial black holes that existed due to essentially random distributions of density moments after the big bang causing some regions of space to collapse into massive black holes which then persisted. Such things are far more difficult to observe, but could be inferred if we don't see many many active nuclei as we get better telescopes but all other indications of the accuracy of the big bang + inflationary theory hold true.
I wonder if it's possible that the laws of physics were simply different in the early universe. Perhaps the universe didn't spring into being with the laws being exactly the same as they are now, leading to things happening differently than they do now, causing our models to fail because we're trying to extrapolate backwards with the assumption that the laws of physics are static.
Direct gas collapse would work if there was little angular momentum in the region compared to the overdensity that starts to collapse. I'm sure this has been simulated, how probable is that?
Quasi stars are one of the theories for the existense of the super massive black holes. Here is a nice video https://www.youtube.com/watch?v=aeWyp2vXxqA by Kurzgesagt on this topic.
Not to mention if supermassive black holes were being formed by accretion, you would expect to see many intermediate mass black holes (1000-1000000 solar masses) everywhere, but we see almost none.