# The Naked Scientists Forum

### Author Topic: Is there a centre of the Universe?  (Read 2685 times)

#### percepts

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##### Is there a centre of the Universe?
« on: 01/06/2014 07:43:44 »
I don't get this "no center of the center of the universe" thing.

If I take an elastic band and mark it at one third along its length and call the nearest end the origin. I then stretch pulling the other end until its twice its length. The mark on the band should still be one third along its length. But the mark would not have travelled as far the end furthest from it. Therefore the end and the mark are travelling at different speeds when its stretched. Knowing that and assuming you can measure the different rates of expnasion at different points, then why can't the center of the universe be calculated?

Or is the rate of expansion the same everywhere? I don't think so since we're told expansion is speeding up so unless its speeding up everywhere equally then it must be possible to calculate the center of the universe. Have I got this wrong?

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« Last Edit: 02/06/2014 10:05:36 by chris »

#### evan_au

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##### Re: center of the universe
« Reply #1 on: 01/06/2014 08:38:45 »
The original analogy proposed seems to be a linear elastic, stretched at a constant rate(?)

1. Expanding universe
If we mark the elastic band at the 1/2 & 2/3 points:

An observer at the 1/3 point would see:
• The beginning & 2/3 points are equidistant, and moving away at the same speed.
• The end point is twice as far away as the 2/3 point, and is moving away twice as fast.
• This is the main observation of the Hubble Law
• This same law would be deduced by every observer at every point on the elastic.
So you cannot locate the center of the universe by Hubble's Law.

2. Accelerating Universe
Let's extend the analogy: Instead of expanding at a steady rate, let's assume that the rate of expansion is accelerating, and that it's rate is faster than the speed of light (so a photon could never reach the end from the beginning).

An observer will see that more distant points are moving away more slowly than predicted by the Hubble Law. (But they will need a yardstick which does not depend on redshift.)

3. Unbounded Universe
A limitation of the original analogy is that observers at the start and end will see nothing beyond their position, so they will know their position is "special" (ie the universe has a "boundary").

So let's extend the analogy again to a circular elastic band, marked at 4 equidistant points, and expanding. Now every observer will deduce the same law of expansion, and no point can deduce that it is at the center or the edge.

This is often imagined as inflating a spherical balloon.

#### percepts

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##### Re: center of the universe
« Reply #2 on: 01/06/2014 08:57:05 »
I see. Thanks.

#### Bill S

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##### Re: Is there a centre of the Universe?
« Reply #3 on: 02/06/2014 18:30:54 »
This puzzled me for quite some time.  The following is an extract from the notes I made when I was trying to sort it out in my mind.

Having worked through this, I was still left with the feeling that all we could say was that we could not identify, physically or theoretically, a centre for the Universe, but that there should be one – somewhere.

#### evan_au

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##### Re: Is there a centre of the Universe?
« Reply #4 on: 02/06/2014 21:44:01 »
Quote
Obviously your original dot has expanded, but has it spread across the balloon?
Modern Big Bang theories include a period of "inflation", where the universe expanded from a size much less than the size of a proton to the size of a grapefruit.

Assuming you cannot draw a dot smaller than an atom, then your initial dot would cover the entire universe.

#### percepts

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##### Re: Is there a centre of the Universe?
« Reply #5 on: 04/06/2014 06:47:26 »
Since sound doesn't travel through space there could not have been a big bang

I think the problem arises when mathematicians try to define a model and if there is no maths to calculate where the center is, then by default the theoretical model is going to conflict with our intuitive ideas of a center.

So I still think that somewhere in space there was a point of origin. The question really should be whether that point of origin, if we could calculate it, would be of any use to us.
« Last Edit: 04/06/2014 06:55:38 by percepts »

#### dlorde

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##### Re: Is there a centre of the Universe?
« Reply #6 on: 04/06/2014 10:30:59 »
Quote
Obviously your original dot has expanded, but has it spread across the balloon?
Modern Big Bang theories include a period of "inflation", where the universe expanded from a size much less than the size of a proton to the size of a grapefruit.

Assuming you cannot draw a dot smaller than an atom, then your initial dot would cover the entire universe.
The entire observable universe. It is not known how much there is beyond what we can observe, so the descriptions of the pre and post inflationary size of the universe can only refer to the history of what we currently observe. There's no good reason to suppose the universe just happens to be the size of what we can observe at the present time. As far as the greater universe is concerned, we can only say that at one time it was all hot and dense; we don't know its extent - it might have been (and could still be) infinite.

#### Bill S

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##### Re: Is there a centre of the Universe?
« Reply #7 on: 04/06/2014 15:17:21 »
Quote from: percepts
Since sound doesn't travel through space there could not have been a big bang ;)

Which space were you thinking of?

Sound waves need an appropriate medium in which to be propagated.  As far as we are aware, there was no such medium surrounding the nascent Universe.  The Universe itself would have been dense enough to have permitted the passage of sound waves – there would have been nowhere else for them to be.

It is difficult to imagine the pitch of any sound produced by a wave that could fit within a space so small that even our most sophisticated modern instruments would be unable to detect it, not to mention a space that might be “infinitely small”.

I understand that high resolution mapping of the CMB radiation has lead to some interesting observations in this regard.  “Translating the observed frequency spectrum directly to sound yields tones far too low for ears to hear – some 50 octaves below middle A – but transpose the score up all those octaves and you can listen to it”.  (Mark Whittle).

#### PmbPhy

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##### Re: Is there a centre of the Universe?
« Reply #8 on: 04/06/2014 17:58:13 »
Quote from: percepts
I don't get this "no center of the center of the universe" thing.

If I take an elastic band ….
You used an elastic band that has bounds to it. However when it is said that there is no center of the universe it is in reference to the theory that the universe has no bounds. In order to make your analogy more accurate try instead of using a finite elastic band/strip use a band that is closed, i.e. that forms a loop/circle instead. I worked something out here
http://home.comcast.net/~peter.m.brown/gr/circle_universe.htm

Quote from: percepts
Since sound doesn't travel through space there could not have been a big bang
The “bang” in “Big Bang” is only meant as an analogy to our experiences with the rapid release of energy and matter. It was never meant to mean that there was actually something like a sound. In fact if there was an instant where some sort of “big bang” when our universe started expanding then it’s not in the standard model of cosmology as it stands today.

Quote from: percepts
I think the problem arises when mathematicians try to define a model and if there is no maths to calculate where the center is, then by default the theoretical model is going to conflict with our intuitive ideas of a center.
If the standard model is correct in that the geometry of the universe is one of the three possible ones that are commonly discussed in cosmology. There is simply no center to the surface or anything that can be thought of as a center just as there is no point on the surface of a sphere, which is so special that it stands out from the rest, unlike, for example, the vertex of a cone.

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##### Re: Is there a centre of the Universe?
« Reply #8 on: 04/06/2014 17:58:13 »