alt Hacker NewsWhy Does the Universe Expand?
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Whew, really fantastic article!
> “In physics, description is explanation” (author quoting his PhD advisor)
Yes, i have always hated this about physics. It is the only scientific field that apparently feels no responsibility to explain “why”, deciding that “what” is sufficient.
I wish the author all success so that i can someday read a satisfying answer to some of the “whys” of physics.
Edward Arthur Milne's proposal was that the universe started out expanding. That is Milne cosmology, and oddly enough it's equivalent to Standard cosmology when you take out the gravitational effects of mass-energy. What's odd about that is that in Milne cosmology you have a big bang at one point with the edge moving at arbitrarily close to c and with uniform distribution of stuff and velocities, while in Standard cosmology we speak of the big bang happening everywhere all at once. This oddity is resolved when you realize that Milne is hyperbolic, so you if you "unfurl" a Milne universe it then looks the same as Standard excluding gravity.
Because our universe is actually the inside of a blackhole.
I kind of like the idea that as the edge of the universe is observed, it expands to fit that observation. "Observed" being in the same sense as the uncertainty principle. But there's also the idea of the universe or space as a kind of mesh that's stretching, not really growing.
When I think about things at this scale, it really makes me feel minuscule.
Attempts to tackle why in science are actually, underneath, just a how-it-really-happens, or what-happens answers.
No scientist can explain the why, and most can't even explain the how (Heisenberg uncertainty, for one accessible example).
My favorite idea is that the universe expands because mass-energy in our 4d parent universe is falling into our universe, which is a black hole. We exist as information upon the 4d hole’s 3d event horizon, and experience its inflation as our time dimension (as well as space inflation).
Inflation is accelerating because as the black hole grows it eats more.
Don’t know how crackpot this is. From what I’ve read it actually can be coaxed into fitting observation but there’s just no possible way to test it.
> The standard cosmological model affords no such explanation
Einstein GR → Friedmann eqns → universe can be expanding or contracting
Rest of this amounts to "what about initial conditions? things become divergent at t→0"
Yes, this is called the Big Bang singularity, GR breaks down there. You are not the first person to notice this. It is considered one of the biggest open problems in the field.
The stuff about accelerating expansion is confused (AI?) slop which eventually gets around to admitting that there's no problem at t>0 and no change to the existing problem at t=0, so I don't know why you didn't just cut it.
This is some real "unsolicited snail mail to physics departments"-tier content, even for HN I think it's embarrassing to have this on the front page.
I guess because every code monkey in Silicon Valley has wet dreams about overthrowing the hidebound physics establishment for some reason
I understand that, for a stationary black hole without spin (which does not exist), an object that falls into it will freeze, then the image will redshift until it disappears.
https://en.wikipedia.org/wiki/Event_horizon
I also understand that there is a cosmological (event) horizon, where galaxies freeze, then rapidly redshift in the same way. This happens because these distant objects are accelerated away from us faster than light.
It appears to me that we are seeing black hole event horizons from the outside, and the cosmological event horizon is seen from the inside.
The similarities do give one pause.
The mechanisms are different; one is gravity, and the other is dark energy.
https://en.wikipedia.org/wiki/Cosmological_horizon