I am researching about the Deuterium Bottleneck, the concept that the early universe (<20 min old) would have, at some point, been cold enough for regular matter particles to exist, the right temperature for fusion, but too high for stable deuterium to exist (a necessary intermediate step to fusing Helium-4). Therefore, it slowed down the conversion of hydrogen to helium to a smaller window of temperatures, which then quickly ended anyway in less than 20 minutes, leaving us with the end result of only about a quarter of the normal matter being converted to helium.

Now imagine a hypothetical situation where deuterium was made quickly and easily, and was stable as soon as fusion started, and therefore hydrogen started fusing to helium quickly and almost all matter converted to helium within the first 20 minutes. Nothing but helium and some insignificant trace elements… What would be different about our universe?

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Not being a scientist, I can think of several major changes, just correct me if I’m wrong in any of these assumptions, and I would love to hear more major differences also.

No hydrogen means no free protons available for chemical and nuclear reactions, which seems like a huge deal. But I know some reactions also emit/create protons, so there may be a small number of free ones around after a while I guess?

Also, no hydrogen means no water, along with no hydrocarbons and all the complex chemistry they bring, so forget anything resembling life as we know it. And since helium reacts chemically with almost nothing, basically you can forget all chemistry, really, for all practical purposes, so forget all life completely, at least until stars could make new elements. Then that life would need to not require hydrogen. Could most complex chemistry even work well without free protons (hydrogen)?

I’m assuming stars could still form and burn, but they would have to be massive enough to fuse helium, so no light stars, no red dwarfs. No main sequence stars at all. Star formation would be different, fusion would be delayed until the gas was much more compressed and hot (to fuse helium). How would going straight to helium fusion change a star’s life? And since almost all stars are smaller stars, our universe would would be much darker. No proton-proton chain reactions, no CNO cycle which also uses protons. So that means fusion energy production would be governed by the triple alpha process I’m assuming?

Most objects that would normally be a star would be brown dwarfs if not massive enough to fuse helium, all this matter trapped in a deep gravity well, never to be available to produce any fusion energy. Nebulae would be helium. Most of the interstellar medium would be helium. How might this affect other major changes in the universe?