# Question regarding movement of universe

Gold Member
I'm no physicist, but I just have some questions...

1) The universe is expanding, correct? This means that its centre of mass is constantly changing, and the centre of the universe is also constantly changing (well... if the universe is spherical, and was increasing in proportion to a sphere). Is this why our Solar System is moving?

(going to post the other questions about neutronium in other thread, since I was given a warning for asking more than 1 question in a thread)

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mathman
You assume that there is a center of mass. That is open to question. Using the well known balloon analogy, where is the center of the SURFACE of a balloon?

Gold Member
The surface of the universe? I don't know... that is confusing. Does the universe have a gravitational force? And does it move?

I was just thinking that, if it *did* move, then that would imply that there would be an object having a greater mass than the universe due to gravitational force.

...Wait, how can something not have a centre of mass?

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The surface of the universe? I don't know... that is confusing. Does the universe have a gravitational force? And does it move?

I was just thinking that, if it *did* move, then that would imply that there would be an object having a greater mass than the universe due to gravitational force.

...Wait, how can something not have a centre of mass?
Think of the balloon analogy for a 2d universe. The universe is on the surface of the balloon, which is expanding. Points on the balloon move away from each other as it expands. Even though the universe is expanding, it doesn't make sense to talk about motion of a centre of mass.

Torquil

1) The universe is expanding, correct? This means that its centre of mass is constantly changing, and the centre of the universe is also constantly changing (well... if the universe is spherical, and was increasing in proportion to a sphere). Is this why our Solar System is moving?
The universe IS expanding but that says nothing about possible movement of it's "center of mass". Every distance point moves away from evry other distant point; nearby points are usually held firm by gravitational attraction.

A "center of mass" for the universe depends on what model of the universe you are considering...a practical problem is that 99.9999.....% of the universe is probably not observable..meaning we have no causal contact, no means to detect it.

Each increment of space not bound by gravitational attraction apparently gives rise to new space...due to the cosmological constant, maybe dark matter. That new space is "empty" and has only zero average energy, quantum fluctuations, and so has no inherent mass nor energy nor I would guess pressure...all gravitational components in general relativity. That means insofar as we know that the total mass and energy in the universe is constant...conservation of mass energy....

This question should be moved to the cosmology forum...evolution of the universe..