# Centre of mass of Universe

1. Mar 8, 2005

### thomate1

What we humans know about the center of mass of Universe?

2. Mar 9, 2005

### blue_sky

I guess not enough

3. Mar 9, 2005

### SpaceTiger

Staff Emeritus
This is partially philosophical prejudice, but I'm going to go out on a limb and say that there is no center of the universe, in any sense of the word.

4. Mar 9, 2005

### ohwilleke

We're not sure if it exists, and definitely don't know where it is if it does.

Note, however, that asking about a "center of mass of the universe" does at least potentially make more sense in the context of GR than asking "where did the Big Bang happen" which clearly doesn't make sense, because Big Bang theory suggest that you can map every point in the universe back to the Big Bang.

Among the big unknown and unknowable parts of answering the question are the question "what exists beyond observed space?" because we can't observe on Earth anything that happened more than the age of the universe time the speed of light away from Earth. When you know you don't know and can't know information that would appear to be necessary to give a correct answer, you are stuck.

5. Mar 9, 2005

### Chronos

The center of mass of the universe has always been everywhere in the universe at all times. The only alternative is to assume we are at the center of the universe. And that is highly improbably. If the universe is spatially finite, it curves around upon itself - a hall of mirrors effect - making it mostly indistinguishable from an infinite universe. We do know this much, if it is finite, it's mighty big - >24 Gpc:
http://arxiv.org/abs/astro-ph/0310233

Last edited: Mar 9, 2005
6. Mar 10, 2005

### SpaceTiger

Staff Emeritus
That's not necessarily true. The center could be somewhere else.

7. Mar 10, 2005

### Chronos

If there was a center, it would have observational consequences. For example, objects located in opposite directions at equal distances [relative to us] would have different redshifts.

8. Mar 15, 2005

### thomate1

Does it means that I can say that

9. Mar 15, 2005

### SpaceTiger

Staff Emeritus
You are the center of your observable universe...but then so am I.

10. Nov 19, 2010

### FifthEngineer

Just as the engineer that I am, this is hard to comprehend.

Could it be that we are just that far away from a presumably centre that differences in redshift between the objects even farther and the objects closer are not measurable?

And does anybody know, since Dr. Hubble, how many objects have been checked for this redshift consistency? I would presume in the order of 10^5 out of the estimated 10^11 galaxies (my bad, I rely on Discovery channel figures). And not a single one of these escapes the statistics?

Yet again based on Discovery, about the Deep Space Hubble Space Telescope experiment. Is there any ideea if they looked towards the universe edge or on a "diagonal" within? Is there any ideea whether a tenfold increase in the experiment resolution would yield,e.g., 10^15 galaxies?

These are just questions. Any enlightment will be highly appreciated, thank in advance.

11. Nov 19, 2010

### bcrowell

Staff Emeritus
Hi, FifthEngineer,

Welcome to Physics Forums!

This thread dates back to 2005. Usually it's better to start a new thread if you want to continue a discussion from this far back. Anyway, modern cosmological models do not have a center. Redshifts only depend on the velocity of one galaxy relative to another, so they can't be used to find a center to the universe.

-Ben

12. Dec 16, 2011

### tom.stoer

In order to discuss the 'center of mass' one has to find a mathematical definition; the first proposal would be

$$\langle\vec{r}\rangle = \frac{1}{M}\int_\text{Universe}d^3r\,\vec{r}\,\rho(\vec{r})$$

which is mathematically nonsense in the context of GR and especially for infinite universes or expanding universes.

Please not that not even the mass itself

$$M = \int_\text{Universe}d^3r\,\rho(\vec{r})$$

is a reasonable concept!

13. Dec 16, 2011

### petm1

Each massive particle is a center connection to the visible universe, after all the connection was started when the universe was local to one clock, relative in time to my present.

14. Dec 16, 2011

### phinds

I have no idea what you are talking about, do you?

15. Dec 17, 2011

### petm1

Right "now" in my present it is 1212 pm 17 December 13,700,002,011, give or take. At the beginning of this "time" our visible universe was local to one clock. Why do you think that one Hydrogen atom looks just like another, why can we use any point as a center of a frame of reference? Isn’t everything connected in time? Thanks to relativity we can plot our connection, using massive points, back to one point in time that appears as a singularity in space.

16. Dec 17, 2011

### phinds

Well, at least this time I understood the first sentence. The rest of it is just words strung together as far as I can make out. I don't know you, don't want to be rude to you, but I really don't get that you're making any sense.

17. Dec 17, 2011

### SpecialKM

I would assume: if the universe started out as a single point with infinite mass and infinite energy and expanded from that single point, that every point in space and time is 'the center'. This assumption, however, is just an assumption. Someone correct me if I am wrong.

18. Dec 17, 2011

### phinds

You are wrong on both counts. First, there was no point and second if there had been a point then NOT every point would be the center.

Last edited: Dec 17, 2011
19. Dec 17, 2011

### SpecialKM

Well that's embarrassing, how did it occur then?

20. Dec 17, 2011

### phinds

Well, the nature of the singularity itself is unknown. Basically "singularity" translates to "we don't know WHAT the heck that was all about", but the current cosomological model says that there was the unbelievably dense matter in an area that may or may not have been infinite but was definitely not a point and it was all of space and time.

Starting at 10E-43 seconds later (one Plank Time) it all started expanding and now there's us.