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Gravity causes homogeneity of the universe:
From the Big Bang to the present
Gravity can accelerate the homogenization of space-time as the universe evolves. This insight is based on theoretical studies of the physicist David Fajman of the University of Vienna. The mathematical methods developed within the research project allow to investigate fundamental open questions of cosmology such as why the universe today appears so homogeneous.
The temporal evolution of the universe, from the Big Bang to the present, is described by Einstein's field equations of general relativity. However, there are still a number of open questions about cosmological dynamics, whose origins lie in supposed discrepancies between theory and observation. One of these open questions is: Why is the universe in its present state so homogeneous on large scales?
[...] Up to now it was not clear whether the homogenization of the universe can be explained completely by Einstein's equations. The reason for this is the complexity of the equations and the associated difficulty to analyze their solutions -- models for the universe -- and to predict their behavior.
In the concrete problem, the time evolution of the originally strong deviations from the homogeneous state as cosmological gravitational waves has to be analyzed mathematically. It has to be shown that they decay in the course of the expansion thus allowing the universe to get its homogeneous structure.
Such analyses are based on modern mathematical methods in the field of geometric analysis. Until now, these methods could only achieve such results for small deviations from the homogeneous space-time geometry. David Fajman from the University of Vienna has now succeeded for the first time to transfer these methods to the case of arbitrarily large deviations.
Journal Reference:
David Fajman. Future Attractors in 2+1 Dimensional Λ Gravity, Physical Review Letters (DOI: 10.1103/PhysRevLett.125.121102)
(Score: 0) by Anonymous Coward on Monday September 28 2020, @10:05AM (3 children)
Two different descriptions are used in TFS:
1) the homogeneity of the universe.
2) the homogenization of space-time.
Now, if you talk to me about the homogeneity of the universe, I immediately think that you are telling me there is stuff everywhere and it's distributed fairly evenly.
However, if you talk to me about the homogenization of space-time, I begin to think you are saying that space-time is evenly distributed in the universe, but wonder what the universe would look like if there was an odd distribution of blobs of space-time here and there.
Hence I don't know WTF this guy is on about with his math for deviants, but then cosmology is not my field. Still you'd think there would be a clearer way to word things so that mere mortals could follow your drift.
And the question they are trying to answer seems pretty easy - since after any sort big bang you typically find stuff everywhere.
Forgive me for being an idiot.
(Score: 0) by Anonymous Coward on Monday September 28 2020, @11:34AM (1 child)
good comment. went to read the abstract. it didn't help.
to be honest, I'm going with the gut instinct now, and just saying this is "interesting but probably irrelevant math".
it's explicitly said that this is about gravity in 2+1 spacetime, but the universe is 3+1 spacetime.
I'm certain that if the results applied to 3+1 spacetime, they would have said so.
(Score: 2) by HiThere on Monday September 28 2020, @12:34PM
That was the thing I noticed from the title. Many things that work in 2 dimensions + time don't work in 3 dimensions + time, and that's the minimum number required to match what we see around us. (Plausibly there' should be more dimensions to track things like electromagnetism, though that's not really independent of spatial location.)
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(Score: 2) by inertnet on Monday September 28 2020, @11:50AM
As far as I understand it, he's trying to reason away the ugly 'inflation' part of the big bang theory. I'm too mere mortal to go check his math as well, but in my layman's view of the big bang, everything started as a single point (or without dimensions), including us. We've been inside of this ongoing explosion from the start, so we don't have an outside view. I simply reason away the inflation part because at the very beginning, gravity and speed made time go extremely slow. What scientists call the first nanoseconds, may have lasted an eternity as seen from the inside. So what now looks like hyper inflation probably really stayed within the laws of nature so in my uneducated opinion you don't need separate laws of nature for inflation.
What he's also trying to explain is why the background radiation is nearly the same in all directions. What I'm wondering is if the background radiation image is the same no matter where you observe it (do you get the same image halfway across the universe), or is it unique for our vantage point? I can't even explain why I think this is relevant here.
(Score: 2) by FatPhil on Monday September 28 2020, @11:51AM (2 children)
Not that it's a theory I particularly subscribe to, it seems too much of a just-so story to me. Sure, some mathematical physicists created some formulae that would explain certain quirks that have been observed, but that doesn't mean the world actually worked that way.
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 2) by HiThere on Monday September 28 2020, @12:50PM
Yes, I agree. The problem is we don't have more than one working model to choose between. Inflation "works", pretty much, with a few Finagle factors. We don't have an alternative that's even approximately as good. (AFAIKT, quantum theory can't be made to apply in this area, except for a few tweaks.)
The thing is, "TRUTH" is not determinable. The *best* we can do is to have a model that matches every observed value. Calling that "true" just because it works everywhere we have looked it a bit of an overreach, but it's the best that it's even theoretically possible to do. The current state is that we've got two theories that "should" explain everything, but they disagree in places that we can't check, and the math on both is too complicated to make predictions in a lot of places we *can* check. Still, General Relativity and Quantum physics currently match in all the places we can look. Neither one, AFAIKT, predicts inflation...but maybe they do, and we just haven't properly understood the math. (And each one has several Finagle factors, where things are tuned to match observations. Like the "Cosmic Constant".
FWIW, to me inflation looks as if it's incompatible with BOTH Quantum Theory and General Relativity...in their simple forms. So for Quantum Theory they propose an unobserved particle. I'm not sure how General Relativity solves the problem. (I get my information from popular science stories, so this doesn't imply they don't "solve" it.
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(Score: 3, Interesting) by Immerman on Monday September 28 2020, @03:51PM
For homogeneity, it seems to me another explanation could be decay of the false vacuum in a cold dead universe.
Consider - in the distant future of our universe all the mass and energy will be spread so thin as to be functonally noneistent, while the energy level of the quantum vacuum will remain pretty much uniform everywhere, with a density somewhere between "very large" and "infinite". If somewhere that quantum vacuum decayed to a lower energy state, that decay would propagate outward at the speed of light, leaving behind it a wake of newly created particles with no net momentum and an almost perfectly uniform mass distribution.
I'm sure there's some reason cosmologists don't consider that as an alternative to the big bang, but I haven't yet found an explanation.