# Does everything orbit around some universal "center of mass"?

I have read many questions about the center of the Universe. I even have some favorite ones.

I understand the fact that every point in the universe experiences expansion of space the same way. Also that it doesn't matter where you are, it will seem like you are in the center.

I have been thinking about a universal center of mass, though.

The moon orbits around a center of mass it shares with the Earth. The Earth-Moon system does the same with regards to the Sun. I believe the Sun may orbit a center of mass shared with other stars (I have a book from the 80's mentioning the hypothesis that this center might be close to Geminga, though I think this idea has been dropped since). And these stars orbit the galactic center. The Milky Way in its turn revolves around the gravity center of the Local Group which includes Andromeda and the Magellan Clouds. The Local Group in turn orbits some point within the Virgo Supercluster.

If we follow this progression... What does the Virgo Supercluster orbit around? And what does the Supercluster's "parent object" (if I may call it so - sorry, I don't know if this is the proper term) orbit around? And so on...

Would that lead us to a universal center of mass?

• You might have many almost equal masses orbiting around each other. Though to human mind, this seems like the less likely posibility. Jul 9, 2014 at 22:44
• Careful with the word "inflation", which means something very specific in the cosmology jargon, and not the Hubble law-type of space expansion. Use "expansion of space" instead if this is what you want to refer to. Jun 4, 2015 at 22:25
• @KyleOman I didn't know about that. Thanks! Jun 7, 2015 at 4:55

## 2 Answers

The centre of mass of a system is simply the weighted average position of the mass distribution in that system. Since the universe is thought to be homogeneous and isotropic, any observer should roughly observe themselves as being at the centre of mass for their observable universe. However, I do not think that is quite the answer you were looking for.

From the context, it seems as though you are wondering if there is perhaps a central point around which everything in the observable universe "orbits". In short, the answer is no.

Bear with me, this might get complicated and I am no Richard Feynman; complicated explanations are not my forte. But let me start by saying that gravitationally bound systems (meaning objects orbiting a central point) are sometimes referred to as structures. What you are suggesting is a structure the size of the observable universe. Ok, definition done, now the explanation.

...

In the beginning (epic orchestral flare), or at least just slightly after the beginning, the universe was homogeneous and the size of the observable universe was essentially the entire universe. Everything was in causal contact and there was virtually no limit to the size of large scale structures. Then inflation began. During the time of inflation, the universe expanded so rapidly that the regions in contact with each other vastly shrunk. The causal horizon of the universe grew much smaller relative to the size of the universe itself. As a result, most structures ended up becoming much larger than the size of the observable universe. Even scales the size of the solar system were outside the range of causal contact. As a result, these structures became frozen in time. One part of a structure could not feel the influence of the other parts and so they could not continue to change or evolve.

When inflation ended, the universe went through periods where radiation and then matter were the dominant forms of energy present. During these periods, the gravitational attraction of matter and radiation caused the rate of expansion of the universe to gradually slow down. This allowed the causal horizon of the universe to expand; information could finally travel between objects in some of the smaller structures. As this continued, first the small scale structures like star systems entered into causal contact. This allowed stars to form and planets around them. Soon, the size of the observable universe was large enough to allow galaxies to form. And later clusters, superclusters, and cosmic filaments.

But then something happened. Dark energy recently became the dominant form of "energy" in the universe. This has made the rate of expansion begin to accelerate again. And so, the relative size of the universe, in which things can causally communicate, is again starting to shrink. The problem? Although the ancient structures from before inflation that are the size of our observable universe can now communicate with some common centre, there has not been enough time for them to evolve and develop fully into a structure with orbits or a common centre of rotation. It is still true that one side of such a structure cannot communicate with the other side.

The largest scale for a gravitationally bound structure is determined by whether enough time has passed, since the cosmic horizon became larger than that scale, for primordial fluctuations (structures) to evolve in a non-linear way (meaning have the complexity to form gravitational structures). The largest scale that can exist today is slightly larger than super-clusters. Then cosmic filaments are the largest scale but even they are a much smaller scale than the size of the observable universe.

Ok, I think I have sufficiently forgotten where I was going with this. The point I was trying to make was that there simply is not enough of a causal connection between objects at distances the size of the observable universe for there to be a common centre of orbit. And because of the new period where dark energy dominates, there is not likely to be any gravitationally bound structures larger than the ones we have today. Cosmic filaments are the largest out there. So no, there is no universal centre of mass as you would be looking for.

• Let me just say that I love your answer :) I have a much better understanding now. I have to ask, though: does this mean that if expansion stopped completely, and given enough time, the Universe could eventually develop a center for a universe sized structure? I am slighly confused because I think this should be impossible due its homogeny and isotropy - I don't think any single point whould be "eligible" for a center - but it should hypothetically be possible for all structures to causally communicate among them with the right conditions. Jul 10, 2014 at 16:46
• If the universe stopped expanding and didn't start contracting, everything would eventually be in causal contact. If there is a geometric centre, then homogeneity would have everything start orbiting around it. If not, then you would end up with very large structures the size of our observable universe and larger. An infinite universe could have infinitely large structures
– Jim
Jul 10, 2014 at 17:45
• I'm not sure about your claim that the causal horizon of the Universe is shrinking (or will be during $\Lambda$ domination). Maybe true in comoving coordinates? Think you could check/clarify/specify coordinates? Still +1, nice answer :) Jun 4, 2015 at 22:29
• @KyleOman Yes, it is the comoving hubble horizon that is shrinking at the moment. But the size of perturbations that eventually turn into large-scale structures are also comoving in size, which means that current scales the size of the observable universe will never be within the proper hubble horizon.
– Jim
Jun 8, 2015 at 15:11

It seems like a lot of your problem, aside from the fact that small structures have causal contact so much a part of them that it is difficult to imagine objects so far apart that they do not even have gravitational contact, is that space is bent. And it is bent to a very high degree. Going in any direction in as straight a line as you can go (wtih any possible measuring instruments to determine your "straightness") brings you full circle to your starting point. What any type of "center" could mean in such a situation is hard to even conceptualize. But the universe is not just a big ball. New replacements for General Relativity that address some of its lacks may answer some of these questions. But they imply that there can be no such thing as empty space without mass since space is a set of relations between masses. Matter and space become 2 sides of the same coin and it becomes impossible to see space existing without mass. Check out "relational" theories based on affine geometries that require no origin in the coordinate systems. That is a major weakness of General Relativity.

You might find Mach's Principle interesting, too. Einstein never really could wrap his mind around it. But the simple Foucault pendulum that we all know shows the problem. The pendulum makes a path in the sand as the Earth rotates beneath it. So far, so good. But what does that mean? Does it mean that the pendulum is remaining in the same orientation relative to something else while the Earth moves? If it is, what is the inertial frame of the pendulum at rest relative to? If it is a moving frame, what is it moving relative to? The distant stars is a common answer, but that clearly does not answer the question very well.