Is the Cosmological Principle Limited to Space Only?

[Chalnoth]

The metric tensor is invariant under Lorentz transformations, but, even in SR, is not invariant under general coordinate transformations.

Yes, sorry I didn't make this clear. For example, the metric (and therefore the stress-energy tensor for vacuum energy) looks rather different in Cartesian coordinates vs. spherical coordinates. Changed my post to clarify this point.

 

[AWA]

Yes, sorry I didn't make this clear. For example, the metric (and therefore the stress-energy tensor for vacuum energy) looks rather different in Cartesian coordinates vs. spherical coordinates. Changed my post to clarify this point.

Well, this is actually a deviation or distraction from the main discussion.
It is quite obvious that General covariance affects the GR field equations, not individual tensors, therefore the specific covariance or invariance (or lack of) of the matter stress-energy tensor is totally irrelevant. This tensor gives us the quantities of the components of the source of the gravitational field, nothing to do directly with the specific distribution of the universe.
Actually general covariance is not concerned directly with the specific distribution of matter of the universe, either, which is more of an empirical issue.
We are the ones that introduce from outside in the GR field equations assumptions concerning this distribution, this assumptions(spatial isotropy and homogeneity) are in part due to philosophical-historical reasons and in part due to empirical observations that are also constrained by a certain interpretation of these observations (redshift, statistical treatment of galaxy surveys like SDSS, 2MASS, etc).

 

[Chalnoth]

Well, this is actually a deviation or distraction from the main discussion.

No, it's not really. The point is that under any transformations in the Poincaré group, which includes Lorentz transformations, rotations, and translations, the stress-energy tensor for vacuum energy doesn't change. This is a very non-trivial statement, even in General Relativity, because you can reduce any space-time point to Minkowski space in some local region around that point.

This is relevant to the discussion because previously you expressed concern that the FRW metric was establishing a universal time coordinate. The demonstration that only the stress-energy tensor for vacuum energy can be invariant under Lorentz transformations indicates that there is no way to formulate a metric for the universe that includes any matter but which also doesn't include some sort of universal time coordinate.

Now, the particular choice for this universal time coordinate will be somewhat arbitrary, but if the system at hand has any symmetries to exploit, then some choices of the time coordinate will be much more convenient than others. In this situation, there is a specific sort of translational symmetry, where at every location in space, there is a particular time at which the space looks the same as some specific time at every other location in space. In other words, the universe is homogeneous in space for a specific choice of time coordinate. Exploiting this symmetry leads to much simpler equations.

 

[AWA]

This is relevant to the discussion because previously you expressed concern that the FRW metric was establishing a universal time coordinate. The demonstration that only the stress-energy tensor for vacuum energy can be invariant under Lorentz transformations indicates that there is no way to formulate a metric for the universe that includes any matter but which also doesn't include some sort of universal time coordinate.

I can't quite follow the logic from the invariant vacuum tensor to the necesity of including some sort of universal time coordinate in the presence of matter. Precisely GR is about the possibility to formulate any metric. Obviously this metric will include a time coordinate that you can consider "universal" for a number of practical reasons, but that doesn't make it really "universal", unless you want to go back to the concept of absolute time from Newton, but wasn't that what Einstein tried to change?

Now, the particular choice for this universal time coordinate will be somewhat arbitrary, but if the system at hand has any symmetries to exploit, then some choices of the time coordinate will be much more convenient than others.

If the system has them, sure.

In this situation, there is a specific sort of translational symmetry, where at every location in space, there is a particular time at which the space looks the same as some specific time at every other location in space. In other words, the universe is homogeneous in space for a specific choice of time coordinate. Exploiting this symmetry leads to much simpler equations.

That is right, if the assumptions are well founded, and I'm not saying there are no good reasons to believe it. As I said there is empirical observations that seem to indicate it, and philosophical reasons to expect it. But, we must also be prepared for surprises.

 

[Chalnoth]

That is right, if the assumptions are well founded, and I'm not saying there are no good reasons to believe it. As I said there is empirical observations that seem to indicate it, and philosophical reasons to expect it. But, we must also be prepared for surprises.

Yes, but one of the nice things is that there's a very simple way to check: continue improving the precision and accuracy of our measurements. If something is wrong about one of our assumptions, then it is highly likely to show up as a set of observations that do not agree with one another.

There are, since the advent of the Lambda-CDM model, no clear indications of this to date, with all apparent discrepancies in areas where the systematic errors are not under adequate control.

 

[AWA]

Yes, but one of the nice things is that there's a very simple way to check: continue improving the precision and accuracy of our measurements. If something is wrong about one of our assumptions, then it is highly likely to show up as a set of observations that do not agree with one another.

Unless the model is built in such a way that whenever an observation that doesn't agree with the assumptions does show up (say, like faint SNaeIa) it can be integrated by changing the parameters of the model. So it's not so simple, the assumptions are alway right it seems, or would you give me an example of an observation that would make us reconsider some fundamental assumption?

 

[Chronos]

Unless the model is built in such a way that whenever an observation that doesn't agree with the assumptions does show up (say, like faint SNaeIa) it can be integrated by changing the parameters of the model. So it's not so simple, the assumptions are alway right it seems, or would you give me an example of an observation that would make us reconsider some fundamental assumption?

A high redshift object superimposed over a lower redshift object, an impossible orbital velocity in a system whose distance has been determined by parallax . . .

 

[Chalnoth]

Unless the model is built in such a way that whenever an observation that doesn't agree with the assumptions does show up (say, like faint SNaeIa) it can be integrated by changing the parameters of the model. So it's not so simple, the assumptions are alway right it seems, or would you give me an example of an observation that would make us reconsider some fundamental assumption?

Well, the difficulty is that if the observations we have really are consistent with the standard model anyway, then merely contemplating other models won't help you discover that any other model is actually better.

Basically, the only time that it becomes really useful for the progress of science to propose a new idea is if that new idea leads to the development of new experiments/observations that would not have been performed otherwise. The problem right now in Cosmology is that there isn't any clear direction for deviations from the current model, so the most reasonable course of action is to just continue to test the current standard model more and more accurately (well, that and engage in low-cost experiments that test alternative hypotheses, but unfortunately in cosmology that's a bit challenging).

As a final point, it isn't just a matter of fitting the supernova data, but instead of fitting combinations of data, from supernovae to galaxy cluster counts to CMB data to baryon acoustic oscillations. They all have to add together and point in the same direction, or something is wrong. Most often that turns out to be some sort of systematic error, but if the same discrepancy keeps popping up again and again, that will eventually become a clear indication of where we should move from the standard model.

That, right now, be our best bet for progress in cosmology. Our second best bet is for new discoveries at the LHC to provide us with new ideas about the nature of dark matter (don't hold your breath, though: the LHC isn't very good at making dark matter particles, even if those particles have the right sort of properties).

 

[AWA]

A high redshift object superimposed over a lower redshift object

Well that observation seems to have been made but discredited on grounds of impossibility according to our model, and statistical irrelevance. So it's a good example of what I am saying, discordant observations are either integrated, dismissed as irrelevant for statistical reasons, or completely ignored, I'm just wondering if such a systematic approach to observations discordant with the standard model can be called science, since it provides us with a perfect excuse for never questioning the initial assumptions.
But perhaps this is not about science at all.

 

[Chalnoth]

Well that observation seems to have been made

Uh, what? No, this observation certainly has not been made. What we have seen are high redshift objects that closely align with low redshift objects. But we have not seen any that show indications of these high redshift objects actually being in the foreground.

The way you test this, by the way, is by looking at absorption lines. Intervening gas blocks light preferentially at specific wavelengths, and so we can see both intervening gas, and its redshift, by looking for such absorption lines.

So what you see is the background object emits at some frequencies and absorbs in others, while the foreground object emits at other frequencies (emitting light at some wavelengths the background object did not emit light), while absorbing light at other frequencies where the background object did emit light. Because the foreground object basically erases the background object's absorption lines, absorption lines tell us primarily about the foreground object.

And those absorption lines, whenever there is such an alignment, come from the low-redshift object, not the high-redshift one.

 

[JDoolin]

Unless the model is built in such a way that whenever an observation that doesn't agree with the assumptions does show up (say, like faint SNaeIa) it can be integrated by changing the parameters of the model. So it's not so simple, the assumptions are alway right it seems, or would you give me an example of an observation that would make us reconsider some fundamental assumption?

Lessons learned from physics 101. If the data doesn't fit your equations, change the data.

Is that true? It seems to me that faint SN would imply a longer distance--and hence a smaller Hubble Constant, hence an earlier event.

 

[AWA]

Well that observation seems to have been made but discredited on grounds of impossibility according to our model, and statistical irrelevance. So it's a good example of what I am saying, discordant observations are either integrated, dismissed as irrelevant for statistical reasons, or completely ignored, I'm just wondering if such a systematic approach to observations discordant with the standard model can be called science, since it provides us with a perfect excuse for never questioning the initial assumptions.
But perhaps this is not about science at all.

Uh, what? No, this observation certainly has not been made.

I meant that the observation has been claimed, not entering on whether the claim is right, I was using it just as an example since Chronos brought it up.

The core of my reflection is more general, and it's been generally responded by saying that noone's come up with anything better than what we have (the standard model), and that is actually true, and that everybody would be delighted and thrilled to find something that solved the standard model problems or misteries, even if it meant changing some apparently obvious assumptions ,and that is probably true too (but I'm less sure about this).

But IMO we must get rid of some circular reasonings that are often used here and that don't do any good to the standard model nor to science as a whole, like justifying expansion because there is spatial homogeneity, and spatial homogeneity because of expansion, this alone explains nothing, some other ingredient is needed, for instance redshift.

 

[Chalnoth]

But IMO we must get rid of some circular reasonings that are often used here and that don't do any good to the standard model nor to science as a whole, like justifying expansion because there is spatial homogeneity, and spatial homogeneity because of expansion, this alone explains nothing, some other ingredient is needed, for instance redshift.

Except that the reasoning isn't circular at all when you combine the results of multiple, independent observations that rely differently upon these assumptions. As I've shown previously, such detailed measurements can and do rule out inhomogeneous cosmologies:
http://arxiv.org/abs/1007.3725

 

[twofish-quant]

Unless the model is built in such a way that whenever an observation that doesn't agree with the assumptions does show up (say, like faint SNaeIa) it can be integrated by changing the parameters of the model. So it's not so simple, the assumptions are alway right it seems, or would you give me an example of an observation that would make us reconsider some fundamental assumption?

Just off the top of my head...

1) If we find any star or galaxy with less that 20% of helium.
2) If we find evidence that the H, He, D, Li abundances change in any radical way by direction
3) If we find a highly evolved red dwarf or any black dwarf or anything else that is obviously more than 13 billion years old
4) If we find any evidence of heavy elements in the era of the CMB
5) If we find any reason to suspect that GR is wrong from any local experiment
6) Any new particles at CERN may cause reconsideration of LCDM. If we find another generation of quarks that would cause a rethink
7) If any of particle parameter goes out of certain bounds we'd have something to figure out. For example if it turns out that neutrinos are heavier than we think they are then this could cause a rethink
8) If we go for another decade and we can't pin down exactly what dark matter is, then we should probably rethink what is going on
9) Any sort of systematic asymmetry or anisotropy in the CMB or galaxy counts. For example, if someone points to a direction in space and finds five times as many galaxies in that direction, then we got some explaining

Also those are the things we could find now. There are about another dozen things that we could have found that would have killed LCDM, but we didn't find them.

 

[twofish-quant]

But IMO we must get rid of some circular reasonings that are often used here and that don't do any good to the standard model nor to science as a whole, like justifying expansion because there is spatial homogeneity, and spatial homogeneity because of expansion, this alone explains nothing, some other ingredient is needed, for instance redshift.

Except that's not what is going on.

Spatial homogeneity of galaxies is an observation. There's nothing to justify. You point your telescope and that's what you see. If we find any sort of direction in space in which there are more galaxies than in other directions, then the universe is not homogenous.

 

[Chalnoth]

8) If we go for another decade and we can't pin down exactly what dark matter is, then we should probably rethink what is going on

I think that may be a bit optimistic, personally. We currently only have some tentative hints that maybe we're seeing something. If these hints are real, then yes, we expect to have dark matter nailed in a few years. But if these hints are not real, then it could be a few decades yet.

I don't think that many of our theories of dark matter will really be ruled out in the next ten years, though these first tentative hints of it that we have seen so far will likely be either confirmed or ruled out by then.

 

[twofish-quant]

Yeah, right. Do you mean that matter here is special, that the physics here is different than in other points of the universe, that our instruments are special and follow special laws of physics? That is not a very popular opinion in modern cosmology. You yourself have said many times that isotropy is ubiquitous in our universe, that otherwise it would go against the Copernican principe. ( the no special place principle)

It's not a popular opinion because the observations we have seem to indicate that the basic physics doesn't change over time. However, we could come up with different observations that shows that this is wrong.

Do you mean then that the matter here on Earth is different than in the rest of the universe? that our instruments have something special that wouldn't work outside the earth?

The experiments that you mention would not rule that out. Now you can think of other experiments that might, but the one's that you listed won't.

If there is isotropy it is the same here and everywhere, think about it.

No it isn't. I'm in a cloud of dust. Things look isotropic to me. I move outside the cloud, things aren't.

 

[twofish-quant]

This is all understood and fine, I'm just taking that assumption to its last logical consequences if we take relativity seriously, and if we agree that if you observe long distances spaces you are also observing the past, one cannot be homogenous if the other isn't too, and viceversa. As they say, you can't have one without the other.

It means that in the big bang model, the universe will look different for people at different times, and that the universe itself is quite different over time. If that's what you mean.

This leads to some contradiction with standard cosmology, so when in doubt, of course we choose standard cosmology, right?

When in doubt, you do an experiment and try to resolve things with observations.

I should point out that the fact that the big bang model does create a universe that changes over time was a big argument against it. From a mathematical elegance point of view, the steady state model is far, far more elegant than big bang. It just doesn't find observations.

 

[twofish-quant]

I guess the moral of the story is that one must not take GR to seriously because that is considered naive at best and against standard cosmology at worse.

That's not true. You need to take GR seriously because a lot of experiment data happens to agree with GR.

What you shouldn't take seriously is the idea that some of the mathematical ideas that led Einstein to formulate GR are fundamental principles of the universe, because they aren't. I should point out that Einstein really hated the cosmological implications of his theory.

Now you tell me that spatial homogeneity, even though it is a property as physical as it can be, only appears with a determinate choice of coordinates that produce a certain privileged slicing of spacelike hypersurfaces, and that this homogeneity disapears if we try to make it coordinate invariant when we change the coordinates, appearing instead a sort of statistical homogeneity wrt both space and time (spacetime) and inhomogeneity or radial density dependence in the purely spatial hypersurface, and both of this things are forbidden by standard cosmology and astronomical observations and I have to take your word on this, no matter what GR says because you know more than me and standard cosmology says so and I'm a responsible citizen.

One should point out that "standard cosmology" is shorthand for "what people believe at a given time." The standard cosmology of 2010, is different from the standard cosmology of 1995, which is different from the standard cosmology of 1970, which is different from the standard cosmology of 1935.

There are parts of the standard cosmology-1995 which are considered to be dead wrong in 2010, and there are also parts of the standard cosmology-2010 that will be considered dead wrong in 2025.

Also its perfectly possible to create a universe that is both isotropic in space *and* time. All you really have to do is to assume that some matter is being created over time. That's the steady-state model.

The way that I think about this is that symmetry and beauty can be useful as "poetic inspiration." For example, I can trying to see if I can create a universe that is homogenous in space and time with the big bang being something of a "local" event. Then I work through the consequences, and I'll come up with something interesting for the observationists to think about.

 

[Chalnoth]

One should point out that "standard cosmology" is shorthand for "what people believe at a given time." The standard cosmology of 2010, is different from the standard cosmology of 1995, which is different from the standard cosmology of 1970, which is different from the standard cosmology of 1935.

There are parts of the standard cosmology-1995 which are considered to be dead wrong in 2010, and there are also parts of the standard cosmology-2010 that will be considered dead wrong in 2025.

I wouldn't go that far. It is only in the past ~10 years or so that we've really had detailed cosmological observations. Before that time, there were huge problems with the observations that really prevented most sorts of detailed study. The anisotropies of the CMB measured by COBE and solidified by WMAP were a tremendous step, for instance.

What remains in cosmology is mostly tweaking of the models. A significant revision as we've seen since 1995 is highly unlikely now. We can expect significant changes to, for instance, inflation, because as of right now we don't have a good handle on inflation in the first place. But we are not going to see significant changes in the makeup of the universe today (i.e. the amount of dark matter, dark energy, normal matter, and the spatial curvature).

 

[Chronos]

I meant that the observation has been claimed, not entering on whether the claim is right, I was using it just as an example since Chronos brought it up.

Huh? How do unsound claims support anything? There is no credible evidence of any high redshift galaxy superimposed over a low redshift galaxy. Give citations, if you disagree.

The core of my reflection is more general, and it's been generally responded by saying that noone's come up with anything better than what we have (the standard model), and that is actually true, and that everybody would be delighted and thrilled to find something that solved the standard model problems or misteries, even if it meant changing some apparently obvious assumptions ,and that is probably true too (but I'm less sure about this).

But IMO we must get rid of some circular reasonings that are often used here and that don't do any good to the standard model nor to science as a whole, like justifying expansion because there is spatial homogeneity, and spatial homogeneity because of expansion, this alone explains nothing, some other ingredient is needed, for instance redshift.

Huh? It appears you are using circular assumptions to support your claim of circular reasoning.

 

[twofish-quant]

What remains in cosmology is mostly tweaking of the models. A significant revision as we've seen since 1995 is highly unlikely now.

I don't think that we are likely to find the things that we've observed since 1995 to be totally wrong, but I do think it's likely that we'll find something that will make us rethink the data, in much the same way that relativity didn't contradict Newtonian physics but can hardly be thought of as a minor tweak.

The big change that I'm thinking in terms of is figuring out what happened before t=0. Everything goes dark right in the inflationary period. Every proposal that I've heard of for trying to figure out what happened before t=0 are not minor tweaks.

Also the standard cosmology does not have a good model for galaxy formation and that's not going to be a minor tweak.

One of the big equations that I think will be resolved over the next twenty or thirty years is "are we alone?" Is the universe that we see, the only universe or are we part of something bigger. The other big question is whether or not it is possible to know what happened pre-inflation or not. If it turns out that someone shows that it is *impossible* to know what happened before t=0 in much the same way we can't know what happened inside a black hole, that would be interesting.

We can expect significant changes to, for instance, inflation, because as of right now we don't have a good handle on inflation in the first place.

Or pre-inflation. It's really interesting that you have some of the constants in LCDM add up to almost one. But my guess is that whatever inflation is is going to be part of the standard model in 2050.

 

[Chalnoth]

I don't think that we are likely to find the things that we've observed since 1995 to be totally wrong, but I do think it's likely that we'll find something that will make us rethink the data, in much the same way that relativity didn't contradict Newtonian physics but can hardly be thought of as a minor tweak.

The big change that I'm thinking in terms of is figuring out what happened before t=0. Everything goes dark right in the inflationary period. Every proposal that I've heard of for trying to figure out what happened before t=0 are not minor tweaks.

Well, they're minor in the sense that these sorts of things will likely have no measurable effect on the physics significantly after t=0 (except in setting up initial conditions). New discoveries may well lead to new ideas about the fundamental nature of reality, but these discoveries aren't going to have any impact upon our understanding of, say, the amount of dark matter out there.

Also the standard cosmology does not have a good model for galaxy formation and that's not going to be a minor tweak.

Well, most of that is down to known physics, with some minor inputs from the precise initial conditions and the precise nature of dark matter/dark energy. The primary difficulty here is not fundamental but instead computational (we don't know how to calculate the formation of a galaxy properly given currently-accepted physics).

I'm not expecting any fundamental change to physics that has anything but a very minor impact on our universe after t=0. The ways in which known physics play out still need to be worked out, which will lead to a new understanding of things like galaxy formation, but in all of the most significant, observational ways, the fundamental physics for cosmology are figured out.

 

[AWA]

Except that the reasoning isn't circular at all when you combine the results of multiple, independent observations that rely differently upon these assumptions.

I specified it is circular when they are used without other aditional observations.

Just off the top of my head...

1) If we find any star or galaxy with less that 20% of helium.
2) If we find evidence that the H, He, D, Li abundances change in any radical way by direction
3) If we find a highly evolved red dwarf or any black dwarf or anything else that is obviously more than 13 billion years old
4) If we find any evidence of heavy elements in the era of the CMB
5) If we find any reason to suspect that GR is wrong from any local experiment
6) Any new particles at CERN may cause reconsideration of LCDM. If we find another generation of quarks that would cause a rethink
7) If any of particle parameter goes out of certain bounds we'd have something to figure out. For example if it turns out that neutrinos are heavier than we think they are then this could cause a rethink
8) If we go for another decade and we can't pin down exactly what dark matter is, then we should probably rethink what is going on
9) Any sort of systematic asymmetry or anisotropy in the CMB or galaxy counts. For example, if someone points to a direction in space and finds five times as many galaxies in that direction, then we got some explaining

Also those are the things we could find now. There are about another dozen things that we could have found that would have killed LCDM, but we didn't find them.

Certainly is hard to find something "really big" like many of the things you list because we would have already found out, accelerated expansion was pretty radical and could have killed CDM , but as Chalnoth says it was tweaked instead to fit it.
2)Well the Li7 problem discussed in other thread is close to what I'm thinking of, but I would not call it radical, again, don't think anything radical is going to be found anywhere soon.
3) tell me a way to obviously determine the age of a red dwarf if it remains in the main sequence, depending on its mass the light ones coul remain there many billions of years, how old is Proxima Centauri?

Except that's not what is going on.

Spatial homogeneity of galaxies is an observation. There's nothing to justify. You point your telescope and that's what you see. If we find any sort of direction in space in which there are more galaxies than in other directions, then the universe is not homogenous.

I't,s not so evident nor so easy as you make it appear, first of all the statistical analysis are built with the assumption that there is going to be spatial homogeneity so they are biased. http://arxiv.org/abs/0910.3833 and there is a lot of hints woth checking that cast some shadows on spatial homogeneity(dark flow, voids, galaxy counts in voids with less galaxies than predicted by LCDM-Peebles,Nature 2010, etc)

The experiments that you mention would not rule that out. Now you can think of other experiments that might, but the one's that you listed won't.

I didn't say that they would rule that out, I'm assuming matter here behaves according to the same physical laws than in any other part. It's not an emperical assumption but I tend to think I'm not the only one that holds it, wouldn't you?

No it isn't. I'm in a cloud of dust. Things look isotropic to me. I move outside the cloud, things aren't.

Of course but in large scales if you look further enough this shouldn't happen to you.

What you shouldn't take seriously is the idea that some of the mathematical ideas that led Einstein to formulate GR are fundamental principles of the universe, because they aren't.

That is your opinion but is highly debatable.

The way that I think about this is that symmetry and beauty can be useful as "poetic inspiration." For example, I can trying to see if I can create a universe that is homogenous in space and time with the big bang being something of a "local" event. Then I work through the consequences, and I'll come up with something interesting for the observationists to think about.

Great, let's see that.

But we are not going to see significant changes in the makeup of the universe today (i.e. the amount of dark matter, dark energy, normal matter, and the spatial curvature).

I guess you have a crystal ball, otherwise I don't know how can you predict the future with such assuredeness.

Huh? It appears you are using circular assumptions to support your claim of circular reasoning.

What circular reasoning am I using?

 

[twofish-quant]

ICertainly is hard to find something "really big" like many of the things you list because we would have already found out

Not necessarily. We haven't looked at all of the stars in the universe, and all we have to do is find one that is made of up 99% hydrogen and people will say HUH?

accelerated expansion was pretty radical and could have killed CDM

Not true. CDM and accelerated expansion are pretty orthogonal.

3) tell me a way to obviously determine the age of a red dwarf if it remains in the main sequence, depending on its mass the light ones coul remain there many billions of years, how old is Proxima Centauri?

What you are looking for is a red dwarf that is off the main sequence. We know Proxima Centarui is less than some number X, because if it was older, it would have moved off the main sequence.

I't,s not so evident nor so easy as you make it appear, first of all the statistical analysis are built with the assumption that there is going to be spatial homogeneity so they are biased.

If you have twice as many galaxies on one half of the sky than the other then spatial homogeneity won't work. It so happens that we see don't see this. It we get to the point where we have to start doing statistical tests to see if it's homogeneous, then this puts some limits on inhomogeneity.

I didn't say that they would rule that out, I'm assuming matter here behaves according to the same physical laws than in any other part. It's not an emperical assumption but I tend to think I'm not the only one that holds it, wouldn't you?

But it's an assumption that is subject to experimental tests, and could in fact be wrong.

Of course but in large scales if you look further enough this shouldn't happen to you.

Except that you can't look further out because the dust is in the way.

 

[Chalnoth]

I specified it is circular when they are used without other aditional observations.

That's a completely pointless argument, because we do have a large number of independent observations to draw upon.

I guess you have a crystal ball, otherwise I don't know how can you predict the future with such assuredeness.

It's just a matter of having an understanding of how precise our measurements today are.

Of course, it does require a couple of caveats, but it is exciting to realize that we actually do have the basics figured out.

 

[AWA]

Not true. CDM and accelerated expansion are pretty orthogonal.

In this thread you say exactly the opposite to Old Smuggler https://www.physicsforums.com/showthread.php?t=425163&page=2
I guess you are the argumentative type without real solid positions on anything, that changes arguments on convenience. Like you are trying to sell arguments and you can sell one argument and its contrary without any problem.
Many of the answers you give me are on that vein so I don't consider them seriously.

What you are looking for is a red dwarf that is off the main sequence. We know Proxima Centarui is less than some number X, because if it was older, it would have moved off the main sequence.

Exactly, we are looking for a red dwarf off the main sequence, but given their low mass they could remain on the main sequence for a much larger time than 13bly, the problem is that it is not easy to find a reddwarf off themain sequence, as you say Proxima should be less than x, x depending on its mass.

If you have twice as many galaxies on one half of the sky than the other then spatial homogeneity won't work. It so happens that we see don't see this. It we get to the point where we have to start doing statistical tests to see if it's homogeneous, then this puts some limits on inhomogeneity.

We are talking abut much more subtle statistical variations than your ludicrous example.

That's a completely pointless argument, because we do have a large number of independent observations to draw upon.

You tell them. I'm not the one who makes that argument, I'm denouncing cosmologists that do.

It's just a matter of having an understanding of how precise our measurements today are.
Of course, it does require a couple of caveats, but it is exciting to realize that we actually do have the basics figured out.

Oh, boy, I guess you do really live in cloud cuckoo land. Be happy then.

 

[Chalnoth]

In this thread you say exactly the opposite to Old Smuggler https://www.physicsforums.com/showthread.php?t=425163&page=2

What? Where? I'm definitely not seeing it.

 

[AWA]

What? Where? I'm definitely not seeing it.

Have you read the posts? Starting with posts 19 and 22 and subsequents.

 

[Chalnoth]

Have you read the posts? Starting with posts 19 and 22 and subsequents.

Oh, I see where your problem is here. CDM is cold dark matter. CDM is not a cosmological model, it's merely a particular parameter within a cosmological model. Dark energy and cold dark matter don't have much of any impact on one another.

What he was saying before was that the acceleration of the universe was, at the time, an extremely surprising claim (though I think that perhaps it shouldn't have been, had we done our math right). And because it was so surprising, it really needed extraordinary evidence to become supported. That evidence was presented, so now it's accepted.

These two positions of twofish-quant's are perfectly consistent and quite accurate.