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I was thinking about the big bang, and I remembered that neutrinos not only travel at the speed of light, but they were also created in massive quantities very early in the universe (Hadron epoch).

Since the universe around that time was about the size of our solar system now, and neutrinos don't interact with matter very much, wouldn't those neutrinos be the 'edge' of the universe? I'll call this edge a 'neutrino shell'.

If I remember correctly, the Shell Theorem should apply here - except for one thing: if the speed of gravity is the speed of light, wouldn't matter feel some slight more gravitational power closer to the 'side' of the universe it started in?

That's because the other 'side' of the universe (assuming a sphere) is moving away from us faster than the speed of light - implying the force of gravity exerted by those neutrinos at some point disappeared.

And this would also mean that at some point the entirety of this 'neutrino shell' of the universe would be so far away from every piece of 'physical matter' in the universe, moving at speeds faster than the speed of light, that it would stop exerting gravity upon all matter?

And wouldn't that mean that after that time, the universe should stop expanding and start deflating (because galaxies inside the shell now have no 'external' gravity force exerted by the neutrino shell)?

I don't know if this makes sense or am I missing a fundamental piece of physics here?

Eduardo Sahione
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The Big Bang didn't happen at a point then expand outwards.

The model we use to describe the universe, the FLRW metric, is based on the assumption that the universe is homogeneous i.e. it is everywhere the same. That means the neutrino density is constant everywhere and has been constant for as long as the neutrinos have existed. There is no shell of neutrinos at the edge of the universe.

Neutrinos are relativistic and as a result their energy density falls as $a^{-4}$, where $a$ is the expansion scale factor. By comparison the energy density of non-relativistic matter falls as $a^{-3}$ so the ratio of neutrino energy density to normal matter energy density falls as $\tfrac{1}{a}$. This means that for very small $a$, i.e. close to the Big Bang, the gravitational influence of neutrinos dominates over normal matter. However for large $a$ (small $\tfrac{1}{a}$) the gravitational influence of normal matter dominates over neutrinos.

I'm not sure when the switchover occurred, but neutrinos are currently gravitationally insignificant. Relativistic matter, including photons as well as neutrinos, ceased to dominate around 30,000 years after the Big Bang. Just as well really, as if neutrinos were still gravitationally significant they would have stopped matter clumping to form stars then galaxies and we wouldn't here to debate the point.

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John Rennie
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  • It's more complicated for neutrinos. Although there is no doubt they were relativistic in the early universe, they are probably not now. – ProfRob Dec 15 '15 at 12:24
  • One thing I don't understand about the big bang: how come you can say that the entire universe was the size of our solar system, yet that it doesn't have a center? I mean, it must, no? Even if the 'grid' as per the answer is infinite, we know, for instance, that 0 is at the center of the real line - because if we take a point and subtract that point to itself we get to 0. – Eduardo Sahione Dec 15 '15 at 15:54
  • Also, isn't the assumption that the universe is infinite fairly strong? I mean, we have no idea. It may be practically infinite, since it expands at speeds that matter can't travel at, but it could be finite at every point in time. – Eduardo Sahione Dec 15 '15 at 16:02
  • @EduardoSahione: The real line has no centre. You can pick any number you like, and the amount of the real line to the left and right of your chosen number will be the same. That's the wonder of infinity :-) An infinite universe has no centre for the same reason. – John Rennie Dec 15 '15 at 16:08
  • @EduardoSahione: a flat homogenous universe must be either infinite or topologically closed. Infinite seems simpler to me than topologically closed, though your mileage may vary. I suppose you could argue that the universe is not homogenous, but then the FLRW metric no longer applies so all bets are off. – John Rennie Dec 15 '15 at 16:12
  • @JohnRennie But zero is the unique additive identity element. Meaning we take any point and add to it it's additive inverse we get to the additive identity element. I guess the universe is not considered a field? – Eduardo Sahione Dec 15 '15 at 16:19
  • @JohnRennie Sorry if I sound like a newbie, just trying to wrap my head around this and this is my way of doing so. So, assuming that there is no center and the metric has expanded from the beginning of time until now, does that mean that our atoms are only together due to nuclear interactions and that if they didn't exist all atoms would be moving apart from each-other? And is there a force that causes this expansion? And if so, does that mean that at some point in time in the future we could have that force bigger than the strong force? – Eduardo Sahione Dec 15 '15 at 16:28
  • @EduardoSahione: look at the results from this search for lots on info about why the expansion of spacetime doesn't affect matter. – John Rennie Dec 15 '15 at 16:32
  • @JohnRennie Thanks man! Highly appreciate the help. :) – Eduardo Sahione Dec 15 '15 at 16:32