If you haven’t heard, the various galaxies in the universe are moving away from each other- makes sense, right? There’s a big bang, everything flies off in a different direction, and things just keep going like that for billions of years.
It makes sense that the speed with which these objects are moving away from each other would be slowing down, since the force of gravity is a drag on everything’s movement. But it turns out that isn’t the case- every galaxy is moving away from each other, but the speed with which they do it is increasing, not decreasing. So we get theories of ‘dark matter’, some mysterious force in the universe that is pushing everything apart. Apparently what we can see is only a small fraction of the matter in the universe, and the rest must be this ‘dark matter’ for the galaxies’ behavior to make sense.
I was thinking about this while going for a run the other day, and the answer seemed obvious: what if there is an even-more colossal universe beyond what we are able to observe? An object can be so far away that its light never reaches us at all. So, if the universe is far, far larger than what we can see, then the galaxies we can see are being accelerated by plain ol’ gravity, from the outside.
I suspect there is a very good explanation for why this isn’t a good answer, and that’s what I’m really asking- do we know for sure there isn’t a huge universe outside what we can observe?
IANACosmologist, but if the cosmos did start from some Big Bang, then there are only three scenarios of how the expansion can play out due to our current understanding of gravity:
**1) Expansion slows down because there’s enough mass/energy in the universe to bring about a Big Crunch.
There’s the perfect amount of mass/energy to halt the expansion.
There’s not enough mass/energy in the universe to halt or reverse expansion, and matter slows down/flies away indefinitely, but would not accelerate.**
The key here, is in any of these scenarios, mass/energy would already have to exist elsewhere in the universe for the expansion to accelerate, which the Big Bang doesn’t currently hypothesize.
Someone more knowledgeable can make any corrections and take it from here.
We can only observe up to the Cosmic Microwave Background, because the universe was hot, dense and opaque with photons at that point in time (~300,000 years after the Big Bang). Yet, we’re pretty certain there’s light and gravity beyond that observable line in the sand.
I believe light and gravity has a decay that approaches, yet never quite reaches, infinity. So, given enough time, gravity could get its hooks in something mind-blowingly far away.
You’re confusing dark matter and dark energy. Dark matter is postulated to explain why galaxies act like they’re more massive than they seem to be if we add up the masses of all their stars. We don’t know what type of particles dark matter is, but there’s pretty strong evidence for its existence.
Dark energy is more mysterious. It’s really just a placeholder term for “whatever is making the expansion of the universe speed up”.
Such an acceleration wouldn’t make everything expand. It would pull everything in a particular direction. And, in fact, there is some evidence that something massive outside the visible universe is affecting the velocity of some distant galaxies near Centaurus and Hydra. But such localized effects can’t explain the overall acceleration of the expansion.
The speed of expansion accelerates, leading eventually to the Big Rip.
Remember, distant galaxies are receding from us not because they’re moving, but because space itself is expanding. We don’t know the mechanism that causes space to expand and what determines if it slows down or speeds up.
Don’t think of the Big Bang as an explosion. The universe popped into existence with everything more or less stationary to everything else. (Except for minor, local eddies). Over time the fabric of space has expanded at varying rates, sometimes faster, sometimes slower. We don’t know what determines its rate of expansion, but right now it is increasing.
Not dark matter, dark *energy *- two entirely different things.
Dark simply means “unknown” to scientists. It’s use in two similar astronomical terms is unfortunate because it confuses lots of people. Dark matter is a type of matter that does not interact electromagnetically but makes its effects known only through gravity. Dark energy is a type of energy that counteracts gravity and its effect of slowing down the expansion of space.
Yes, it is correct. We see an observable universe with a radius of about 13.7 billion light years. Our calculations from what we know about the big bang tell us that there is more universe beyond that. That part of our own universe does not and cannot effect us in any way known to science because it’s retreating at a relative speed faster than light and gravity is also thought to move with the speed of light.
Additionally, the expansion of the universe is equal in every direction. Any point inside the universe can be thought of as the center of a perfect expanding sphere. Obviously, nothing off to one “side,” whatever that might mean, could cause a perfect sphere to expand equally.
That Wikipedia link also cites several scientists and studies who refute this claim. It’s the type of claim that should be discounted until and unless the evidence goes way past “some” to “overwhelming.”
Although the existence of the Dark Flow is not proven, there’s nothing about it that violates the current laws of physics. The hypothetical massive attractor would still be within our light cone, just beyond the observational wall of the CMB. If we had some way to see through the hot plasma of the early universe, it would be directly observable (assuming it does exist).
I get that the claim is not in the same ballpark as the outside universe attractor. The difference is that the effect would have happened while the universe was small enough for areas to directly effect one another, even is one piece of that is now beyond visual range.
I need to make this explicit because the way you stated it probably makes it seem to those who aren’t getting the whole outside universe impossibility in the first place like you are equating the two.
The Dark Flow (again, “Dark” here just means unknown cause) is legitimate physics, but it’s a pretty big claim. Lots of equivalently-possible claims have been set forth over the years and few, if any, of them ever get confirmed. They make better headlines in the popular science magazines than they do cosmology. The ones that get confirmed are big news, admittedly.
Maybe it’s just semantics, but as The Hamster King linked to, there seems to be a “Dark Flow” or a “Great Attractor” somewhere beyond the observable universe, of which the OU has a radius of about 46.5 billion LYs (not 13.7, that’s the age of the universe), due to the inflationary period.
I get there are regions that are expanding away from us faster than the speed of light, but isn’t that more of a localized effect of the vastness of the universe, conventionally using our solar system at the “center”.
If you placed this observational “center” just beyond the edges of our observational reach (say 47 billion LYs away), you’d never be able to observe our solar system, as space is expanding faster than our light and gravity can reach over there.
So, right, in that sense, with expansion doing what we currently see, there’d be a limit to the affect of gravity/light, but because of the expansion of space, not as some limitation of distance in and of itself.
Interesting. So, is it impossible that other big bangs could somehow exist far outside our visible universe? A big bang creates its own space, but is there any possibility multiple ones could yet exist in the same context?
The picture I was forming musing about this was of a, say, trillion year old universe with the remnants of big crunches scattered around at amazing distances, like the stars in relation to our solar system. Any visible light from them would have long since passed by, but their gravity would pull everything in this part of the universe apart. Gravity can stretch space as well as bend it? Yeah I realize I don’t much know what I’m talking about.
Where does Einstein lay out gravity’s effect on space? I took 1 (one) modern physics class, so maybe if I beat my head against it enough it will click.
I agree that we can only detect things that were close enough at one point for their radiation to reach us within 13.7 billion years, but they may be farther away than that today.
As The Hamster King said, there is nothing within our physics that prevents distant parts of our universe having localized effects on one another. The word “localized” is key. No outside universe could ever explain the isometric expansion of the universe that we do see. Well, unless we make the absurd stipulation that our spherical universe is encased within a much larger, perfectly spherical hollow universe with perfect uniform density throughout. That’s awesome in its dottiness, but even less likely than Hollow Earth theories.
This may seem like a semantic point, but dark energy does not counteract gravity. Gravity acts on dark energy using the exact same rules that it acts on everything else. It’s just that, because of the peculiar properties of dark energy, those rules result in outward acceleration instead of inward.
Well, as it stands there is no explanation for it at all. What I am musing about in my amateur way is a kind of multiverse in which ancient universes much, much older than our own have run their courses at all points from where we are. If this multiverse is truly infinite, hyper-massive Big Crunches at astonishing distances have had enough time to exert their gravitational pull on this part of space from all directions, inexorably pulling it apart. Since we’re talking about infinite distance and mass, and near-infinite time, it could account for the ‘isometric expansion of the universe that we do see’, because the sheer scale of it would tend to smooth out any irregularities.
The major problem with this hypothesis is its possibility- can Big Bangs occupy the same “space-time context” in this way? Can a multiverse of multiple Big Bangs exist in such a way that, given enough time, they can gravitationally interact with each other at great distance, rather than being separated in entirely different dimensions? I just don’t know.
The alternative, to me at this time anyway, is my imagination clawing at the air, unable to come up with even an implausible explanation for why space is expanding as observed, which, needless to say, is a little uncomfortable. I have my doubts about the plausibility of my suggestion, but I literally can’t imagine any other possible explanation
The ‘shell of stuff around our universe’ explanation doesn’t work, though, as the net gravitational field inside would be zero, thus not producing any acceleration at all. This is known as the shell theorem: intuitively, every part of the shell exerts forces on every part of the inside such that they cancel each other out. This is easiest to see for the exact center. So the uniformity of the surrounding matter we would have to postulate in order to explain the isotropy of the observed expansion ensures that the explanation must fail. (Provided, of course, we can validly use Newtonian gravity on the relevant scales.)
Besides, even if the explanation worked, it would have the undesirable character of putting us necessarily at the center of the universe—otherwise, we would not observe an isotropic expansion. But this, of course, just introduces a new fact as much in need of explanation as the existence of dark energy (if you believe that’s in need of explanation, that is—as an explanation of the expansion of the universe it’s perfectly adequate, just as e.g. the neutrino postulated by Pauli was perfectly adequate to account for energy conservation in the case of beta decay; this we typically think as being a sufficient explanation, although strictly speaking we don’t yet know ‘why neutrino’).
I’d like to thank you for that link, The Hamster King.
From the Wiki page on the Big Rip, I proceeded to the pages on the Big Bounce and the Anthropic Principal, and wound up at the exhaustive and fascinating page on the Fermi Paradox, with side trips to Seyfert galaxy, Radio source SHGb02+14a, Adapa, and Apkallu.
Not really relevant to the discussion here after a certain point, but an enormous amount of fun.
