I suppose this question could have two interpretations:
Where is the center of mass of the visible matter, that is, of all the galaxies and whatever visible intergalactic matter there is?
Same thing but including dark matter.
We’ve never seen dark matter, so it might be quite difficult to determine the center of mass if we include it, so let’s just try to answer the first interpretation.
As far as I know, the universe, while immense, is still finite. So there must be a center out there somewhere among that alphanumeric soup of galaxies. Have astrophysicists come up with an estimate? If so, in what direction in the night sky would one turn his gaze (realizing, of course, that one might have to be gazing at the night sky in the southern hemisphere)?
There is no center to the universe of any kind. If there were one it would be a privileged reference frame and Einstein says we can’t have that sort of goings on.
Instead, any point in the universe can look around and consider itself to be the center, with an equal distribution of mass in all directions. That’s contrary to our everyday expectations, but the universe is not just like home.
The universe is finite in extent in the same way that the surface of a balloon is finite in extent. There is a finite area, but no edge or center to it.
The center of the visible Universe is us. This, of course, says nothing at all about the Universe itself, but only about the limitations of what we can see of it.
So the age-old astronomers who considered Earth to be the center of the Universe were as right as anybody. Gives us a whole new perspective on the case of Church vs Galileo.
However, the Earth is not the centre of the Solar System, and it’s not the centre of the Milky Way Galaxy. It’s only the centre of the universe in the sense that everywhere is the centre of the universe.
If we lived in a closed universe, the question would actually be perfectly meaningful, and the answer would be the Big Crunch. It does not, however, appear to be the case that we live in a closed universe.
I don’t believe Einstein ever said that. We can certainly imagine a universe with a center that obeys all the laws of Special and General Relativity. We could have a flat, open universe, infinite in extent, with one tiny solar system. The center of mass would be in that solar system. We could probably even imagine a closed universe, finite in extent, but with a lumpy distribution of matter that would have a center of mass at some specific point.
Cosmologists like to consider the simplest possible models, homogeneous and isotropic; but they know that those are just approximate models.
How does that follow? Note that we’re talking about mass, not just theoretical concepts (of which there are clearly an infinite number). The total mass could be finite or infinite.
Furthermore, if we define the universe as “that which is” including all the quantum foam that could spawn new um, “sub-universes”, if I understand it correctly, each would have (or not have) its own center of mass, but if there would be zero interaction between different sub-universes, then it doesn’t make any sense to talk about the center of mass, for the whole.
True, if it’s infinite, which it might not be.
The consensus best guess seems to change every generation or so. I get the impression that lately, infinite is the favored answer.
Not really. As already noted by **Giles **above, they were wrong about the Earth being the center of anything smaller than the Universe. Or it being in any way a privileged position.
As well, what is the correct way to think about an opinion which is coincidentally partially right but entirely based on wrong reasons? Philosophy would tell us that’s a total error, though I forget the term of art they prefer.
“Center of mass” is a classical concept and I’m not sure how it’s useful within the context of general relativity. It’s probably possible to derive a local estimate of where the “center of mass” is based on the metric tensor. For example, an object in Earth orbit will know the direction of the “force” of gravity and how that direction changes with the object’s motion (the radius of curvature) will give an effective position for a locally observed “center of mass”. The same process could be done at finer levels, giving a “center of mass” for the solar system, the galaxy, larger scale structures, and potentially the observable universe. I don’t expect we can make measurements with enough precision to extract that last one (or even for several of the latter ones).
Your logic is incorrect. Our best understanding is that the universe is infinite, but that is based on measurements, not logic. If the measurements had been different, we could’ve concluded the universe is finite.
If space is curved, should the centre of mass then not lie outside the universe, like the centre of mass of a torus lies outside of the physical entity of the torus? Maybe the universe is like a torus, wrapped around a bubble of fourth-dimensional nothingness, which contains the universe’s centre of mass (and maybe all those single socks that mysteriously disappear from everyone’s households).
That could maybe even explain the accelerating expansion: as the universe expands, everything gets closer to that centre, and gets pulled increasingly stronger. Like the earth pulls us stronger when we’re at sea level.
I don’t see anyone specifically claiming that the universe has infinite mass or an infinite number of particles. So do we agree on finite mass (excluding dark matter for the moment)?
It seems to me that if we were dealing with a 3D Cartesian space with a finite amount of mass distributed in some random fashion within it, we could determine the center of mass.
So we are apparently dealing with some other kind of space, such that we on earth see a more or less uniform distribution of galaxies around us no matter which way we look. And an observer in the farthest galaxy would also see the same distribution. Presumably that observer could see our galaxy if we can see his. Then if he turns around an looks in the opposite direction he sees the galaxies that we see if we turn around and look in the opposite direction. Otherwise we would have a string of galaxies streaming out forever.
No, we do not. The simplest models of the Universe consistent with our observations are infinite, and even if it’s finite, it’s definitely larger than the portion which we can see, which makes it impossible to determine anything about the mass distribution in the rest of it. And even if it’s finite, that still doesn’t imply that it has a definable center, not even an undeterminable one.