Worldbuilding an Expanding Universe

What if the universe had expanded much more slowly, so that Big Bang Nucleosynthesis lasted not a mere 20 minutes but rather, say, a trillion years? The age of stars would begin far later and start with a lot more heavy elements, completely reshaping the chemical landscape for when life as we know it arises.

Tweaking the expansion of space, what one might call alternate cosmology, in the spirit of alternate history, is very underrated as a worldbuilding tool, especially for harder worldbuilders who wish to hew closer to scientific rather than magical cosmogonies. Many such universes can be constructed, but for the nonce I’ll offer some thoughts primarily on the idea that inspired me to write this: the one with a much longer Big Bang Nucleosynthesis (or what I might call the cosmoreactor) stage.

A much longer Big Bang Nucleosynthesis

Nucleosynthesis as far as I understand it ended because the universe expanded enough to make the temperature too cold, the density too low, for nuclear fusion to continue. Thus instead of getting a bunch of heavy elements primordially, we had to wait for stars to produce them, in far lower quantities than could have been made in a cosmic-scale reactor. Indeed, nucleosynthesis was a bit insipid: hydrogen nuclei, i.e. lonely protons, predated the epoch, so the primary effect was to turn a quarter of the universe’s ordinary mass into helium. Big whoop.

A longer nucleosynthesis would have given enough time for the alpha processes to produce significant quantities of heavier elements. A time scale similar to stellar cores, millions of years, would presumably have converted significant quantities of primordial matter to heavier elements. Contrary to popular belief, all the elements, even the heaviest ones, can and do form inside stellar cores, so a cosmic reactor should be able to produce the more neutron-rich heavy elements too. I can only assume that the longer suitable conditions lasted the more nuclei would be converted.

How much longer? Well, if you’re changing around the expansion of space you can make it however long you like. Millions of years, billions of years, or even longer. I’m thinking a trillion years for my little universe, since that’s a super-long stretch of time, and for once avoids the “billions and billions” cliche. Now there’s a length of time that could really shift the elemental abundances to the heavy side!

A trillion years is 9.7×1026 longer than 20 minutes, which means that it takes almost 27 orders of magnitude longer for space to expand by a given amount in this universe I’m building. The problem with this comes with the necessity for the universe as we know it to be after the time of recombination, when the universe became transparent to light. In our universe that happened 370,000 years after the Big Bang; if that took 27 orders of magnitude longer, then that time when the universe was no longer fusing elements but rather was a dark fog would have instead been 3.7×1032 years. That amounts to 8.277×1022 half-lives of uranium. Meaning that all the juicy primordial radioactive elements will have decayed away! Oops.

A Universe as we know it defined by an incipient Big Rip

So what this world needs is for the expansion of space to expand rapidly after the trillion-year mark. Dark energy is accelerating the expansion of space in our universe right now, so positing a much more rapid onset of the acceleration at a different date isn’t much of a stretch. Indeed, as I understand it the idea that the expansion of space can and does vary with time is a fundamental feature of the quintessence theory of dark energy. A universe with such a field is compatible with the laws of physics as we know them, so let’s roll with it in this world!

The expansion of space may be as rapid as one desires. How rapid? In real life the expansion decelerated over time, until about 5 billion years ago when it started to become exponential. From the end of the reactor stage to the end of recombination the universe expanded from 300 light-years to 42 million light-years in radius. 5 orders of magnitude in 370,000 years. By contrast, in the 13.8 billion years since recombination the universe expanded up to 46 billion light-years in radius, just 3 orders of magnitude, and that’s with a bit of acceleration recently courtesy of dark energy.

Varying the expansion rate so the dark fog era lasts far shorter than real life (like a second instead of a hundred thousand years) but the age of stars lasts at least a billion years or so is the only solution, which also conveniently leaves a lot of those juicy primordial radioisotopes in great abundance: less time to decay before they find their way to planetary surfaces. The expansion rate decelerates enough to give plenty of time for galaxies as we know them to form, perhaps a billion years or so, then speeds up again, building toward a rip singularity where distances expand to infinity. I’m thinking this should happen much less than 13 billion years after recombination, so the universe as we know it lasts shorter in this world than it has in real life. That would be cool.

Or maybe such a universe could end up in a Big Crunch again; that’s another cool idea. The quintessence concept not only allows us to vary dark energy’s effect over time, it also permits it to become attractive rather than repulsive; so again it’s not all that far-fetched to suppose the universe after a couple billion years of this expansion closes back in on itself, ending in a Big Crunch singularity in another couple billion years.

Conclusion

By our standards the universe would have been young throughout its existence in both these scenarios, so that’s cool. Many other concepts are possible; maybe you have an idea that flips the ones I have on its head, or maybe something else entirely. Maybe I’ll get an idea different and interesting enough to share on this blog again. In any case, i encourage worldbuilders to give more thought to manipulating the expansion of space: it might unlock interesting and unexpected possibilities for your setting!

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