Galaxies receding from us and the Cosmological Principle

[DanRozenfarb]

My understanding is that Lemaître and Hubble discovered that faraway galaxies are receding from us with speed proportional to their distance. From this observations one might assume that we are at the center of the Big Bang, but the Cosmological Principle states we live in no special place and thus everything is (mostly) receding from everything.
My question is: the fact that we are not at the center of the universe, is evidence based or just a philosophical aftermath?

[antoniseb]

... My question is: the fact that we are not at the center of the universe, is evidence based or just a philosophical aftermath?

It is not a fact. It is based on our current model.

In a sense, by analogy, I could say: "I live in Patagonia, and everywhere I look the Earth curves away at just about the same rate... so Patagonia must be the center of the Earth." But that measure applies to every place on the Earth. So which one is really the center?

[DanRozenfarb]

"I live in Patagonia, and everywhere I look the Earth curves away at just about the same rate... so Patagonia must be the center of the Earth." But that measure applies to every place on the Earth. So which one is really the center?

I see but I think the analogy breaks when we have an actual chance to go to Siberia, see the same thing happening and THEN conclude Patagonia is not the center of anything. But so far we were unable to do that with the Universe, so we are not sure, are we?
I think the Cosmological Principle stems from history lessons (like the Ptolemaic Geocentric Model) but perhaps we might be mistaken this time. This model broke at some point because we could't explain the obervations and I'm not sure this is happening not. I believe we are not the center, but it might be unscientific to think this way when hard evidence seems to point to us being a center.

[WayneFrancis]

I see but I think the analogy breaks when we have an actual chance to go to Siberia, see the same thing happening and THEN conclude Patagonia is not the center of anything. But so far we were unable to do that with the Universe, so we are not sure, are we?
I think the Cosmological Principle stems from history lessons (like the Ptolemaic Geocentric Model) but perhaps we might be mistaken this time. This model broke at some point because we could't explain the obervations and I'm not sure this is happening not. I believe we are not the center, but it might be unscientific to think this way when hard evidence seems to point to us being a center.

The problem is if you think we are at some real centre point and everything else is actually moving away from us, through space, instead of space just expanding, which leads to all points in space having the same observations is:

We see many object receding at a rate faster then light. If this is real movement through space and not space expanding then we've got a problem with special relativity being drastically wrong.

So if you believe in all the evidence for SR then you have to rule the recession of galaxies as real movement through space.

[pzkpfw]

... faraway galaxies are receding from us with speed proportional to their distance. ...

We are receeding from those distant objects at a speed proportional to our distance from them. What would an observer on that distant object see?

[DanRozenfarb]

What would an observer on that distant object see?

Will see the same with respect to us, but different with respect to any other galaxies.
If you are at the "center" we see what we see today, but a galaxy in the "periphery" (if such thing exists) would see otherwise

[pzkpfw]

Will see the same with respect to us, but different with respect to any other galaxies.
If you are at the "center" we see what we see today, but a galaxy in the "periphery" (if such thing exists) would see otherwise

As noted above, the math works out that every observer would see the same thing.

If we can figure out the "speed" of all those things (relative to us) then we can also work out the "speed" of those things - as one of them would see it. We don't have to actually go there (as with your Siberia comment) to figure out that math.

And it turns out they all see the same thing we do, the Universe is expanding away from them.

So our view is not "special".

Take A through F. They start with one space between each thing.
Each time unit one space unit is added between each thing.
From the point of view of "C", "B" recedes at 1 space unit per time unit and "A" recedes at 2 space units per time unit.

      A B C D E F
    A  B  C  D  E  F
  A   B   C   D   E   F
A    B    C    D    E    F

But "D" has a point of view too: "C" recedes at 1 space unit per time unit and "B" recedes at 2 space units per time unit.

         A B C D E F
      A  B  C  D  E  F
   A   B   C   D   E   F
A    B    C    D    E    F

[caveman1917]

As noted above, the math works out that every observer would see the same thing.

And it turns out they all see the same thing we do, the Universe is expanding away from them.

So our view is not "special".

That argument is somewhat circular, since the math you refer to is derived from the assumption that every observer sees the same thing (the FLRW metric drops out of the field equations if you assume isotropy and homogeneity).

It is possible to construct models that actually have us at a specific "center" of the universe that are also (mostly) consistent with observations.

I think Jeff Root has it more to the right end here, the cosmological principle is an assumption we make. We have good reasons to make it, such as Occam's razor (no need for an extra parameter that would put us in a special place), and because for as far as we can see the universe is indeed quite homogeneous.

[pzkpfw]

That argument is somewhat circular, since the math you refer to is derived from the assumption that every observer sees the same thing (the FLRW metric drops out of the field equations if you assume isotropy and homogeneity).

Oh. I didn't think that was an assumption, I thought that was a specific result of figuring out what the "view would be like" for those distant observers, based on what we calculate their "motion" to be. Happy to have been told I'm wrong.

[Jeff Root]

There is a complete lack of evidence as to whether we
are at the center of the Universe or not, except for the
philosophical observation that it would be really, really
weird to find that we *are* at the center despite the
fact that there is no evidence that we are.

Or to put it another way: There is no evidence suggesting
that we are at the center, so why would anyone think we
are at the center?

-- Jeff, in Minneapolis

[Jens]

My question is: the fact that we are not at the center of the universe, is evidence based or just a philosophical aftermath?

As Jeff Root said, it's based on the fact that it would be a fantastic coincidence if we were to be at exactly the center. Why would we be at the center and not anybody else?

[glappkaeft]

Also the math works out that every place sees all (nonlocal) galaxies receding. Every place can't be the center.

[danscope]

Surely..... the ego must be at work here .

[Aethelwulf]

My understanding is that Lemaître and Hubble discovered that faraway galaxies are receding from us with speed proportional to their distance. From this observations one might assume that we are at the center of the Big Bang, but the Cosmological Principle states we live in no special place and thus everything is (mostly) receding from everything.
My question is: the fact that we are not at the center of the universe, is evidence based or just a philosophical aftermath?

oh no --- it certainly does not imply an arrow... In fact, there is no such an arrow.

For an arrow of time, one needs strongly a linear flow of time. A flow of times does not exist within the framework of GR where a spactime exists. Spacetime exists purely as an artifact of geometry - there is nothing linear or flow-like about it. you can't sit in any direction of space and call it the center, unless you are willing to describe every point in space as the center. Linear arrows of time, time reversibility and flows or time are all highly abused terminologies.

[Jeff Root]

Wayne,

That argument depends on there being a difference between
"galaxies moving through space" and "space expanding".
It has not been made clear that there is such a difference,
or what that difference is.

If you observe two galaxies which are getting farther and
farther apart from each other, is there any way that you
can distinguish their motion due to the expansion of space
from their proper motions? I don't think so.

In this animation, the dots get farther and farther apart
from each other:

http://www.freemars.org/jeff2/expand5c.htm

The view from any dot is the same as from any other dot.
Can you say that the increasing distance between dots is
due to the expansion of space rather than proper motion?
No, you can't. There is nothing to distinguish expansion
of space from proper motion in that universe. Is the real
Universe different in that regard?

-- Jeff, in Minneapolis

[pzkpfw]

Can you say that the increasing distance between dots is
due to the expansion of space rather than proper motion?

Surely you can if that alleged proper motion turns out to exceed c?

[Jeff Root]

Please explain how!

You might need to define "local" when you are describing
the relative speed of the dots.

Also -- or perhaps *alternatively* -- it isn't clear to me
whether the dots are physical things or are just imaginary
things. We imagine the array extending forever in all
directions. Does special relativity apply in that case?
It *would* apply if we required that the dots be visible
on an infinite computer monitor screen. There is no way
that the motion of the dots could be coordinated on it.
But since we can just imagine the infinite array, I think
we can do without the imagined infinite screen.

In that case, would special relativity be violated by very
widely separated dots moving apart faster than c?
Can you show that it would be?

-- Jeff, in Minneapolis

[caveman1917]

In that case, would special relativity be violated by very
widely separated dots moving apart faster than c?

It would, but that's not really the problem. What we see is redshift, not velocity. We put that redshift in terms of a recessional velocity through the use of a certain model, which gets us a superluminal velocity. If however you use a different model (ie stick with an explosion in minkowski space), you'd interpret the redshift differently (as purely a doppler effect), such that the velocity is still below c. You do get some other problems at the very far edges of the observable universe though, putting it in those terms you'll get weird acceleration curves for galaxies (because you interpret their velocities differently), which would require an explanation.

[antoniseb]

... What we see is redshift, not velocity. ...

We also see a relationship between redshift and SN1a lightcurve width. If I understand correctly this relationship is different depending on whether you interpret the redshift as resulting from cosmological expansion as opposed to special relativity in non-expanding space (BTW, it completely falsifies any tired light model). Sorry, I have no paper handy to back that up. Can someone more involved with that aspect of things help me out on this (or refute me)? Thanks in advance.

[caveman1917]

We also see a relationship between redshift and SN1a lightcurve width.

Do you mean by "lightcurve width" the time interval between "start" and "end"?
If yes, i don't quite see how it would be different, since both models take place in a non-interacting vacuum[*], and thus redshift, change in wavelength and change in frequency (ie time dilation) are related the same way (ie , so the same redshift and thus will produce the same or lightcurve width). But i haven't thought this completely through, nor do i either have a paper handy at the moment. We could calculate it out though, it's just that i don't see how it could be different.

* this is what falsifies the tired light model, since by having light interact with the vacuum it alters the redshift-wavelength change-frequency change relation.

[pzkpfw]

Please explain how!

You might need to define "local" when you are describing
the relative speed of the dots.

Also -- or perhaps *alternatively* -- it isn't clear to me
whether the dots are physical things or are just imaginary
things. We imagine the array extending forever in all
directions. Does special relativity apply in that case?
It *would* apply if we required that the dots be visible
on an infinite computer monitor screen. There is no way
that the motion of the dots could be coordinated on it.
But since we can just imagine the infinite array, I think
we can do without the imagined infinite screen.

In that case, would special relativity be violated by very
widely separated dots moving apart faster than c?
Can you show that it would be?

-- Jeff, in Minneapolis

At my simplistic "this is just the Q&A forum" level, I thought it was like this:

If two ants are standing on something, and the distance between the ants is seen to be increasing faster than an ant could possibly walk, it might be found that what they are standing on is in fact a rubber band that is being stretched.

... Can you say that the increasing distance between dots is
due to the expansion of space rather than proper motion?
No, you can't. There is nothing to distinguish expansion
of space from proper motion in that universe. Is the real
Universe different in that regard?

That is, I thought that if we don't think of the Universe as expanding, then those distant galaxies that appear to be moving away from us faster than c would actually have to have proper motion faster than light. I thought the expansion of the Universe is something that avoids this need for faster than light motion.

[Jeff Root]

We do not actually observe any galaxies moving faster than
light relative to us. If the relative motion between distant
galaxies were attributed entirely to "proper motion", we still
would never see any galaxy moving faster than light. So it
isn't clear that special relativity would be violated.

I think that measurement of relative speed is necessarily a
local measurement. We can't measure the speeds of distant
galaxies -- only calculate what their speeds must be from
local observations of light which left them long, long ago.

-- Jeff, in Minneapolis

[WayneFrancis]

We do not actually observe any galaxies moving faster than
light relative to us. If the relative motion between distant
galaxies were attributed entirely to "proper motion", we still
would never see any galaxy moving faster than light. So it
isn't clear that special relativity would be violated.

I think that measurement of relative speed is necessarily a
local measurement. We can't measure the speeds of distant
galaxies -- only calculate what their speeds must be from
local observations of light which left them long, long ago.

-- Jeff, in Minneapolis

Jeff, even if you take the raw z value and figure out that a z value of 4.5 = 0.936c you still have to explain how, given expansion rates are seeming to speed up not slow down, where did all the energy come from to push one galaxy to that speed let alone all galaxies at those distances.

The >c come in when you factor in the comoving distance. Another problem of just saying these z values red shifts are due to proper motion is that proper motion does not explain the angular size issues unless you can come up with an explanation why the more distant galaxies have a ever increasing size to match the angular distance measurements. This problem is solved very nicely by expansion.

So while you can say that you'll never get a z value that produces a value that gives you a speed >c it is because of how it is defined as that it approaches c. The point where it exceeds c is the point that object is no longer in our Hubble volume. So sure, we can't see z values that give a value > c but that is because if an object hits that point, in expansion, then there is no z value at all because you don't have . It is just becomes a divide by zero problem and isn't defined. Its like asking how red shifted light from inside an event horizon of a black hole is to an external observer. It isn't that it is infinite. It just isn't.

[RandyD123]

the Cosmological Principle states we live in no special place and thus everything is (mostly) receding from everything.

Interesting in the sense that I never thought of like that but it makes me look at "our" space much differently. If space is expanding faster than light, then the space we live in must also be expanding faster than light. So if the earth is spinning at 1,000mph and rotating around the sun at 67,000mph , and so on.....see here"http://www.thelivingmoon.com/41pegasus/02files/Speed_of_Earth.html" It is amazing we can even see anything out in space!!

[WayneFrancis]

Interesting in the sense that I never thought of like that but it makes me look at "our" space much differently. If space is expanding faster than light, then the space we live in must also be expanding faster than light. So if the earth is spinning at 1,000mph and rotating around the sun at 67,000mph , and so on.....see here"http://www.thelivingmoon.com/41pegasus/02files/Speed_of_Earth.html" It is amazing we can even see anything out in space!!

Personally I'm not a fan of the comment "space is expanding faster then light". It is a very misleading term and here is why. The rate space is expanding is VERY slow. 74km/s might sound fast but that is only ~0.00025 the speed of light and that is over a 3 million light year distance. The reason you can get 2 points in space that are receding at a rate > c is that space is HUGE and you can add a lot of those 3 million light year distances up, over 4,000 to be precise, to obtain a velocity >c

So Sure the Earth is spinning fast, It is orbiting the sun faster, our star is orbiting the centre of our galaxy, our galaxy is moving within its cluster and the cluster may be moving at a nice pace with respect to the CMBR. All of this is relative motion and we can see blue and red shifting of various objects do to this movement. But it is no where near c.