What if the universe was much older?
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Well youngsters, let me tell you. When I was your age gravity was faster. Now I don't mean stuff fell faster. Not a bit. It was just that everybody got on with the business at hand. Something pulled on you and you fell down. None of this shilly-shallying we got nowadays. Back then, a whole solar system would collapse down in a week, 10 days tops. But then all the baryons decided we needed "organization" and "processes". Every few billion years some bright spark would come up with a sure-fire way to "avoid all the chaos". So everybody would spend a million years arguing about what was now the best way to fall down. Meanwhile all the dark matter would mill around in confusion, going to meeting after meeting and not getting anything done, before finally giving up and just stop any interaction. 'Cept for gravity, of course. There's always gravity. It's just slower now.
Low probability, as in 10^-43%: we know pretty much the story that happened after the first 10^-43 seconds [1], and we know the universe became transparent after circa 300,000 years post Big Bang. The oldest known galaxy was GN-z11, ~400 milion years post Big Bang, by JWST the oldest is JADES-GS-z13-0, ~325 million years post Big Bang.
[1] https://en.wikipedia.org/wiki/Timeline_of_the_early_universe...
Seems incredible hubris to think we can reverse-calculate to 14 billion years ago with sub-second accuracy when science can't even predict if it'll rain tomorrow.
Seems incredible hubris we can write 4 equations [1] and control some kind of switches at 10^-8 meters to multiply matrices and handle decision-making with sub-second (10^-9) accuracy in the "real world" of objects beyond 10^-3 meters.
Science can predict if it will rain tomorrow: "[a] seven-day forecast can accurately predict the weather about 80 percent of the time and a five-day forecast can accurately predict the weather approximately 90 percent of the time." [2]
Feel free to hubristically read articles such as Cosmic Evolution of Viscous QCD Epoch in Causal Eckart Frame [3] and tell us all where they are wrong. An introductory article into cosmology and inflationary theory [4].
[1] https://en.wikipedia.org/wiki/Maxwell%27s_equations#Macrosco...
[2] https://scijinks.gov/forecast-reliability
[3] https://inspirehep.net/files/b54851f57716f3e8fb650bc1c057ba1...
Then we'd have a different set of observations that conflict. We know the age of the universe from several different directions: the rate at which distant galaxies move, the temperature of the cosmic microwave background, the temperature of white dwarfs, etc.
These agree to within a relatively small range. If the number were substantially different, it would imply that something deeply fundamental (and probably several deeply fundamental things) was wrong.
It's much more likely that our understanding of galaxy formation is wrong. That's much less fundamental, and much harder to observe.
It's just like debugging code. You start with the obvious stuff. It's much more likely that the error is in your program, for example, and not in the compiler. That's not proof, but you'd be foolish to start anywhere else.
I often wonder - what if "something" happened 13.8 billion years ago that generated the CMB but it wasn't the beginning of the universe? If we keep detecting galaxies all the way back to that point...