Cosmological principle and quasar distribution

  • #26
Jonathan Scott
Gold Member
2,294
991
Besides you can't assert that a frame is arbitrary and globally valid for all points in the universe at the same time. An arbitrary choice of frame is perfectly valid for any location, but if all locations share the CMB as a valid rest frame, then you are imposing an absolute frame in GR, I'm not sure this is possible.
No-one is trying to define a relativistically synchronized time coordinate across the universe. What we are saying is that there is a sufficiently consistent global concept of "proper time since the big bang for a slow-moving observer", for example relative to the CMB or local galaxies.

Expansion of the universe means that the CMB or local galaxy rest frame is different at different locations, and local motions could make a little difference too, but remember that the fractional effect of speed on time rate is about [itex](1/2) (v^2/c^2)[/itex] so even 1% of the speed of light would only have an effect of 0.005% on the time rate.
 
Last edited:
  • #27
3,507
26
The overall expansion tends to dampen motions relative to that expansion. Basically, if you are moving away from the Earth (for instance) in some direction at high velocity, you will be moving towards other stuff that is moving away from the Earth. Over time, you'll slowly catch up to stuff that is moving faster and faster, which, in the end, means that your relative speed compared to the expansion goes down.

This overall expansion, then, sets a global rest frame that everything is drawn towards. It is with respect to this global rest frame that we can sensibly talk about a global time.
Frankly , this makes little sense to me because it seems a totally circular argument, you are using expansion to justify that all galaxies are stationary, so what is expansion then? expansion either has something to do with the galaxies proper velocities or it's something so ethereal that is useless, I know some people say expansion has nothing to do with the motion of galaxies, that is just "expanding space", but that needs to be based on the reference of motion of the galaxies to have any meaning at all. You can't have just "expanding space" because noone can say if space is expanding or not without matter as reference of that expansion.
So you basically are saying that galaxies motion is defined relative to "overall expansion", and at the same time we define expansion relative to galactic motion. Nice.
If you don't glimpse some circular reasoning here we must be talking about different universes.
But the funny thing is I don't see the need to invoke expansion, we are dealing with a possible incompatibility between the cosmological principle and the observed distribution of quasars and I haven't seen any sound argument that shows this not to be the case

No-one is trying to define a relativistically synchronized time coordinate across the universe.
That seems to be exactly what you are defining. Maybe is not what you are trying to do, but yu are doing it in practice.

What we are saying is that there is a sufficiently consistent global concept of "proper time since the big bang for a slow-moving observer", for example relative to the CMB or local galaxies.

Expansion of the universe means that the CMB or local galaxy rest frame is different at different locations, and local motions could make a little difference too, but remember that the fractional effect of speed on time rate is about [itex](1/2) (v^2/c^2)[/itex] so even 1% of the speed of light would only have an effect of 0.005% on the time rate.
Here not only you are defining a global frame but you are apparently claiming like Chalnoth that the universe is static.
When you say "sufficiently consistent global concept of proper time" even though "expansion means that the CMB is different at different locations" you are either contradicting yourself or assuming the CMB or local galaxy rest frame is the same for every location, and then you go on to add the motion of galaxies wrt this common frame is basically stationary, how do you define expansion then, what is expanding here? You are defining a purely coordinate expansion, not based on redshift that is coordinate independent and must be related to motion of galaxies to indicate expansion, but based on the spatial homogeneity assumption only. Bt this is the very assumption that we are debating wrt quasar distribution.
 
  • #28
Chalnoth
Science Advisor
6,195
443
Okay, TrickyDicky, you really need to take a step back here, because you really aren't understanding what we're saying.

When you have an expanding universe, you can describe it as having an overall, average expansion, along with local motions relative to that expansion. If you imagine that those motions are constant (i.e. no local interactions), then those motions gradually catch up to the expansion. Over time, unless you have local interactions, the expansion becomes more and more uniform.

What this means is that in the end, when you have an old universe, there ends up being hardly any motion relative to the overall expansion. The most significant motions come from the fact that there are local interactions: galaxies attract one another. But in the end, these local motions are still small compared to the overall expansion, topping out at maybe 1% of the speed of light in the largest galaxy clusters.

Because all motion different from the expansion rapidly approaches the expansion, we can define a coordinate system that is stationary with respect to the expansion. In this coordinate system, it is sensible to talk about things like global time, because this is the time coordinate that anybody will experience sitting in a solar system in any galaxy anywhere in the universe.
 
  • #29
3,507
26
Okay, TrickyDicky, you really need to take a step back here, because you really aren't understanding what we're saying.
Might perfectly be the case, I'm not particularly smart. I'll try harder.

When you have an expanding universe, you can describe it as having an overall, average expansion, along with local motions relative to that expansion. If you imagine that those motions are constant (i.e. no local interactions), then those motions gradually catch up to the expansion. Over time, unless you have local interactions, the expansion becomes more and more uniform.

What this means is that in the end, when you have an old universe, there ends up being hardly any motion relative to the overall expansion. The most significant motions come from the fact that there are local interactions: galaxies attract one another. But in the end, these local motions are still small compared to the overall expansion, topping out at maybe 1% of the speed of light in the largest galaxy clusters.

Because all motion different from the expansion rapidly approaches the expansion, we can define a coordinate system that is stationary with respect to the expansion. In this coordinate system, it is sensible to talk about things like global time, because this is the time coordinate that anybody will experience sitting in a solar system in any galaxy anywhere in the universe.
But then, how exactly is expansion ascertained? I thought expansion had something to do with motion. But if motion of cosmic objects is referred to overall expansion, what do we refer overall expansion to? The CMB? And I'm still not sure if you are saying that the CMB is a global or just a local frame?, if I understand you correctly then by the overall expansion link to the CMB, it must be a global frame. I've seen some pictures of the observable universe as a sphere with us in the center and the CMB as the boundary of the sphere, with the microwave radiation reaching us isotropically from every point of the boundary, this would be compatible with the CMB as global frame and with the coordinate system defined respect to the expansion. The problem with this picture and your corrected coordinate system is that in itself it is static, and if you don't use the objects in the universe to define the expansion, how do you do it? You end up just with spatial homogeneity to justify the expansion, but you have used the slow motion of galaxies and the CMB global frame to justify that.
Cosmological redshift being coordinate-independent wouldn't help us here according to your stationary coordinate system if you separate the motion of the galaxies from the overall expansion.
I mean if anybody in any galaxy anywhere in the universe can use this global reference frame and has the same preferred choice of coordinates you defined, with no motion relative to overall expansion it could lead to think that expansion is a coordinate property and it can be dispensed with in practice.
Since I don't think you mean that , there must be something I'm misunderstanding. Maybe if you or someone answered directly any of my questions that'd help.
 
  • #30
Chalnoth
Science Advisor
6,195
443
Might perfectly be the case, I'm not particularly smart. I'll try harder.



But then, how exactly is expansion ascertained? I thought expansion had something to do with motion. But if motion of cosmic objects is referred to overall expansion, what do we refer overall expansion to? The CMB? And I'm still not sure if you are saying that the CMB is a global or just a local frame?, if I understand you correctly then by the overall expansion link to the CMB, it must be a global frame. I've seen some pictures of the observable universe as a sphere with us in the center and the CMB as the boundary of the sphere, with the microwave radiation reaching us isotropically from every point of the boundary, this would be compatible with the CMB as global frame and with the coordinate system defined respect to the expansion. The problem with this picture and your corrected coordinate system is that in itself it is static, and if you don't use the objects in the universe to define the expansion, how do you do it? You end up just with spatial homogeneity to justify the expansion, but you have used the slow motion of galaxies and the CMB global frame to justify that.
Cosmological redshift being coordinate-independent wouldn't help us here according to your stationary coordinate system if you separate the motion of the galaxies from the overall expansion.
I mean if anybody in any galaxy anywhere in the universe can use this global reference frame and has the same preferred choice of coordinates you defined, with no motion relative to overall expansion it could lead to think that expansion is a coordinate property and it can be dispensed with in practice.
Since I don't think you mean that , there must be something I'm misunderstanding. Maybe if you or someone answered directly any of my questions that'd help.
Okay, so, maybe the issue you're having here is that your argument here seems to be based upon some idea of global motion: that you can sensibly say, in some absolute sense, that we are stationary, but some far-away object is moving. Or, at the very least, that you can say that some far-away object is moving relative to us.

Neither is the case in General Relativity.

I figure you are probably aware of these facts, but you don't seem to be applying them.

The first point can be summed up with this simple statement: everything is stationary with respect to itself. Furthermore, if you have two objects separated by some distance which, according to one coordinate system are moving with respect to one another, you can come up with an alternative coordinate system where they are stationary with respect to one another. For example, if you really wanted to, you could write down some coordinates where the Sun and the Earth are both stationary. In those coordinates, anything we currently see as evidence of our motion around the Sun would still happen, but would be a result of some other physical effect in the new coordinate system. For example, the seasons would be caused not by the Earth's axis pointing in more or less the same direction but the Earth going around the Sun, instead the twisted space-time around the Sun in this new coordinate system would be applying a torque to the Earth that would cause its axis to precess. All observational effects would still work out the same (General Relativity guarantees this), but they would appear to come from different parts of the equations.

Of course, we don't make use of a coordinate system where the Earth and the Sun are both stationary, because such a coordinate system would be horribly complicated, difficult to use, and lead to all sorts of strange effects (such as the entire universe rotating around us).

But in the case of the expanding universe, it turns out that a coordinate system where everything is (nearly) stationary is incredibly useful. The redshift, instead of being due to other galaxies' motions with respect to us, instead comes about just due to the expansion of space in the interim from when the photon left to when it arrived. In this view, photons are expanded by the same effect that is causing the distances between galaxies to increase: new space is being created. So the expansion doesn't go away simply because we've used a coordinate system where everything is stationary, but the observational effect (redshifts) has a different physical explanation in the different coordinate system.

This view, where everything in the universe is nearly stationary but the space between things is increasing, is exactly the same as the view that we are stationary but everything is moving away from us, and the things further away are moving faster. The two are just different ways of looking at the exact same universe.

However, it turns out that the former view, with everything stationary and space expanding, is one heck of a lot easier to write down mathematically. And it is easier to understand some physical effects in this situation as well, such as the point that brought about this entire discussion: the fact that quasars were once common, but are now rare. In this coordinate system where everything is stationary, the time coordinate for everything is the proper time since the big bang for everything. So if, in this coordinate system, we talk about some event happening 3 billion years ago, then no matter where that event happened, we know it happened 10.7 billion years after the start of our universe from the perspective of an observer stationed at the event. And when we note that quasars were once common but are now rare, we can understand this as being due to quasar only having enough fuel to be really bright in the early universe, and that fuel later being used up.
 
  • #31
29
0
The Standard Model depends on the CP (and GR) and although the CP isn't "unfalsifiable" it's also unprovable.

The quasar issue isn't the only problem for the CP. The quadrupole and octupole modes of the WMAP that seem to orient along the ecliptic plane is a problem. The Pioneer anomaly is a problem. Inflation being in tatters doesn't help.

I know of one paper that provides data regarding concentric rings at intervals radiating out... from here, home.

A bounded, finite Universe with a gravitational center would explain a lot of things, and it doesn't require an unprovable CP. But nobody is brave enough to touch it, because of the implications.
What are the implications?

The data from wmap showing that alignment on the elliptic plane has concerned me, and I never seem to get a good explanation for it.

Could you give a brief explanation of a theory that explains it or direct me to source which can preferably put it in simpler terms.

How does a universe with a gravitational centre 'work'
 
  • #32
Chalnoth
Science Advisor
6,195
443
What are the implications?

The data from wmap showing that alignment on the elliptic plane has concerned me, and I never seem to get a good explanation for it.

Could you give a brief explanation of a theory that explains it or direct me to source which can preferably put it in simpler terms.

How does a universe with a gravitational centre 'work'
Basically, when you do the statistics right, there isn't anything there.
http://arxiv.org/abs/1001.4758

One way to think about it is that the WMAP satellite gives constraints on around a million unique harmonic modes of the CMB. The fact that some tiny fraction of those modes seems, on the surface, a little bit weird, is completely expected.
 
  • #33
29
0
Thanks, i did read that paper. And understood parts of it other parts were way over my head. I do understand that some things which we might consider unlikely are completely expected. Like the initials 'sh' Stephen Hawkings showing up aligned etc.

But reading that paper the conclusion does state the alignment at the quad octopole are very tight and there may still be a theory to explain it, although it could just be coincidence that is expected.

I'm interested in knowing the gravitational centre theory that dougal is suggesting and the implications of such. Because like he said, maybe the implications whatever they are, are so outstretched people aren't willing to contemplate them. But we might just miss the obvious if just dismiss it as a coincidence or expected pattern/anomaly.

When will the planck data be published? Perhaps that would confirm or negate this data
 
  • #34
Chalnoth
Science Advisor
6,195
443
Thanks, i did read that paper. And understood parts of it other parts were way over my head. I do understand that some things which we might consider unlikely are completely expected. Like the initials 'sh' Stephen Hawkings showing up aligned etc.

But reading that paper the conclusion does state the alignment at the quad octopole are very tight and there may still be a theory to explain it, although it could just be coincidence that is expected.

I'm interested in knowing the gravitational centre theory that dougal is suggesting and the implications of such.
Since it's aligned with the scanning strategy of the telescope, a systematic error would not at all be unlikely.
 
  • #35
3,507
26
Okay, so, maybe the issue you're having here...
Nope, that's not my issue at all AFAIK. I was the first to remark that in previous posts.
I asked some questions related to what had been discussed before, but nevermind, in some other universe of our multiverse surely you can see what I mean :wink:
 
  • #36
Chalnoth
Science Advisor
6,195
443
Nope, that's not my issue at all AFAIK. I was the first to remark that in previous posts.
I asked some questions related to what had been discussed before, but nevermind, in some other universe of our multiverse surely you can see what I mean :wink:
Well, I know you've said it, but it doesn't seem to me that you've understood the implications.
 
  • #37
Jonathan Scott
Gold Member
2,294
991
How about reverting to the balloon model to illustrate this?

The balloon radius is the analogy of the global time. If things have moved around a little on the surface during history, then their path as the balloon expanded may have been a little different from a straight radius, so their proper time may vary a little, but not very much, as the ratio of sideways to radial motion is like the ratio of the speed to c, so it is really tiny.

What we see when we look at far away objects is what was happening when the balloon was smaller. Everyone sees the same thing at a given distance, but the properties at that distance reflect the state of the surface of the balloon at that past time.

The global time is not like a synchronized local SR flat space coordinate system. That would be like sticking a flat piece of card to the balloon, which obviously only works for a small area. When we want to discuss the evolution of distant objects, we don't mean in terms of a synchronized local SR coordinate system, but rather in terms of an approximate age in terms of their own proper time since the big bang.
 
Last edited:

Related Threads on Cosmological principle and quasar distribution

  • Last Post
5
Replies
114
Views
14K
  • Last Post
Replies
6
Views
3K
  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
3
Views
784
Replies
3
Views
469
  • Last Post
6
Replies
136
Views
18K
Replies
82
Views
10K
Replies
5
Views
675
Replies
4
Views
3K
Replies
50
Views
3K
Top