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.

http://www.talkorigins.org/faqs/astronomy/bigbang.html

21. Dec 18, 2011

tom.stoer

As I already said in post #13

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!

Please note that the integrals above are not well-defined mathematically even if you avoid the singularity at 't=0'

http://relativity.livingreviews.org/Articles/lrr-2009-4/ [Broken]

Last edited by a moderator: May 5, 2017
22. Dec 18, 2011

SpecialKM

Ahh, It seems I've fallen into the common misconceptions category. Thanks for this!

23. Jan 20, 2012

FifthEngineer

Now us engineers know of and like to refer to a mass center.
Actually there is no need to bother with integrals in this respect. Refer to a center of the space if you feel more comfortable with it.

Put it another way, the universe is said to have been some X ligth years "accross" some Y years after its birth (i do not recolect reasonable figures right now for the ensemble X,Y). One would suppose that it was a sphere of radius X/2.
The big question would be where is the center of that sphere now?

I know you will say that it was not necessarily a sphere, but then what was it?
And then again which is its current size? "Accross" or whatever formulation you prefer...

Of what I read this year after my last post I understood these - or at least i hope i do. There is little idea about the shape of the Universe; there is a large acception that it is flat (and hence it is not a sphere) but it is not unanimously.
There is no evidence about the size of the Universe.

But look here: we are definitey sure it does not have any kind of center.

As i put forward in the start, let alone the mass (which is sparse) and focus on the space itself (which is continuosly created by the expansion of the universe, right?). Whenever physicists integrate the Einstein general relativity equations they do some assumptions, one of which is that the space-time is a continuum.

Then there are only two more options:
Either the space is unbounded, or otherways put infinite; but then what is expanding, the infinite? And when did it got there in those mere 14 bny?

or

The space is bounded. Then certain integrals can be computed theoretically, and a center can be put out.

We simply know too little about it. Based solely on some common sense I think that the human will have the knowledge of a "center of universe" in the future. Based on the current knowledge, i would say this may come as late as 1000 years ahead.

Not that we could not wait that long as a specie.

PS I formulated all the above in the simple engineering friendly 3D understanding of reality. Do not add time in an asnwer - it would not do it for me, as long as no one proves there is no "instant state" of the whole universe. Do not add extra dimensions by no mean unless you have ALL the equations ready.

24. Jan 20, 2012

FifthEngineer

And another simple dumb question from a simple engineer. We see a galaxy say 10 billions LY from us, and another one say 2 or 5 billions LY from us.

By redshifts we can calculate that the farher one is speeding away from us much faster the the nearest one.

But when? Why is this a prove that the Universe is accelerating its expansion now, and not the other way around - it could be seen that galaxies farther away from us in time recede quicker.

Not to mention that we have no ideea about the other relative velocities than radial, not even for galaxies as near as Andromeda.

Honestly, these are just questions for a passionate theoretical physicist that could spare some time here. No debate here, and a human readable answer would be highly appreciated.

Last edited: Jan 20, 2012
25. Jan 20, 2012

phinds

Indeed we do, and that knowledge applies very nicely to finite things. It does not apply to an infinite universe.

That is NOT said, at least not by any reputable physicists. It is unknown whether the universe was finite or infinite at its birth. I realize that the "birth" of something infinite doesn't seem to make sense. Welcome to cosmology. Whether it was infinite or finite it does NOT have to have a center and all bets currently are that it did not.

Unknown and presently unknowable.

Good. We can agree on something.

Again we agree.

unbounded and infinite are not the same thing. I'm not clear whether you understand that or not. There are unbounded finite models and unbounded infinite models. Infinite IS unbounded but unbounded is not necessarily infinite.

14 Billion Years is the time since the singulariy as measure by a clock that is comoving with the CMB

nonsense. Bounded, such as the surface of a sphere, most emphatically does NOT imply a center.

If you are going to rely on "common sense" I suggest you stick to engineering. Cosmology and quantum mechanics will just frustrate you, since neither has anything much to do with common sense.