Is the Cosmological Principle Limited to Space Only?

[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.

 

[AWA]

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.

What was the name of the model before it was called LCDM? I call it CDM since we didn't know about dark energy yet, in that thread two fish argues that the dscovery of the accelerated expansion was a pretty radical thing at the time and that it took some effort for cosmologists to make it fit in the previous model at first, he specifically compares the breakthru with what it would mean to find out the Equivalen principle was wrong.
But he contradicts me here saying "Not true" when I say that accelerated expansion came as a big surprise in 1998 for the previous model followers.

Anyway I'm sure he can speak for himself, can't he?

 

[Chalnoth]

But he contradicts me here saying "Not true" when I say that accelerated expansion came as a big surprise in 1998 for the previous model followers.

He made it pretty clear what he was responding to, I thought. Accelerated expansion really didn't have anything to say about cold dark matter. They're very different concepts. He clearly wasn't disagreeing that dark energy was surprising (it was), but that it had anything to say about cold dark matter.

You may not have meant that, but it seems pretty clear to me that that's what he read.

Anyway I'm sure he can speak for himself, can't he?

Of course. If I'm wrong, I'm wrong. But in this case I'm pretty sure it's a miscommunication due to your use of very non-standard language. In the mean time, I have no reluctance to respond until he chooses to do so.

 

[twofish-quant]

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 the standard models of cosmology and stellar evolution are right then it should be impossible. If we see one then we know that something is seriously wrong. Since the universe is supposed to be 13 billion years, if you see something that looks like it is 12 trillion years old, you have some explaining to do.

We've done enough searches to be able to confidently say if there were any non-main sequence red dwarfs within 1000 l.y., we would have seen them. The fact that we've not seen a single one is pretty significant.

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

It's not ludicrous. If you have a good physics model then it should be robust and easily to show that it is obviously wrong. It's better to build physics theories from *obvious* facts, because the subtle ones may be incorrect because of instrument issues.

We know that the universe appears more or less isotropic and more or less homogenous, just like the Earth is more or less round.

We also know that the universe is not completely homogenous. I'm looking at a bottle of water in front of me. That's different from the mouse on my right hand. The standard model of cosmology *assumes* that these small differences don't affect the general expansion of the universe, and it's easy to do some quick calculations to show that they don't.

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

But I think he is right. In 1450, you could argue that there was this giant island in middle of the Atlantic ocean. By 1550, you really couldn't because people have sailed back and forth across the Atlantic, and if there were giant islands, we would have seen them. In 1950, you could have argued that Venus was this vast sea of oil, but by 1970 you couldn't because we've sent space probes there.

In cosmology, the fact that we sent out space probes since the mid-1990's and because our telescopes have gotten a lot better because of computer technology is like sailing across the Atlantic. Once you've figured out the shape of the North American coast, it's not likely to change radically.

 

[twofish-quant]

What was the name of the model before it was called LCDM? I call it CDM since we didn't know about dark energy yet, in that thread two fish argues that the dscovery of the accelerated expansion was a pretty radical thing at the time and that it took some effort for cosmologists to make it fit in the previous model at first.

There were strong experimental reasons to believe that CDM existed before the accelerating universe. The point that I'm trying to get across is that people didn't believe that CDM existed because of any cosmological model. People believed that CDM existed because

1) you had funny galaxy rotation curves
2) without dark matter, your deuterium calculations go way off

Now the dark matter had to be *cold*

3) you have large scale structures which would have gotten washed out by any hot matter. Think of taking an snowflake and dropping it into hot water. Same physics as taking a galaxy cluster and dropping it into a vat of hot dark matter.

Also it didn't take that much effort to fit in the model. The lambda parameter is something that you need to make the universe inflate. The "standard assumption" in 1995, was that at anything after inflation, the parameter would be zero. Guess not.

It *was* surprising, even shocking. The reason this was surprising was that if you assuming that lambda is zero and the critical density is one, then you could argue that there was some basic symmetry in the universe. Guess not.

The point that I'm trying to get across is that the circularity you are complaining about just doesn't exist. Also there isn't a grand model that's some holy writ. You can think of the "standard model" as something like wikipedia where people are constantly adding and removing stuff as new data comes in.

But he contradicts me here saying "Not true" when I say that accelerated expansion came as a big surprise in 1998 for the previous model followers.

Your statement was "accelerated expansion was pretty radical and could have killed CDM". Which is very different statement. The thing is that it could be our equations for how we think the universe expands could be quite wrong, but the fact that we think that there is cold dark matter is based on observational puzzles.

 

[twofish-quant]

What was the name of the model before it was called LCDM? I call it CDM since we didn't know about dark energy yet,

You have to be careful here, because you are using the term CDM in two different ways, which may be confusing. The "CDM equation" is definitely wrong. The evidence for "cold dark matter" which is the thing that the equation describes, is independent of the equation.

Also it turns out that if you assume that gravity has certain mathematical properties, there are only a few ways of writing the equation, and LCDM just uses an extra term that Einstein proposed in the 1920's.

What happened when Einstein worked out general relativity is that it become obvious that you couldn't have a static universe. The universe had to keep expanding or contracting. This really bothered him, so Einstein put in an extra energy term in his equations to keep the universe fixed. Unfortunately, that doesn't work. The trouble is that it's unstable. If you have a static universe and expands a bit, the extra energy comes out and makes it expand even more. Once Hubble noted that the universe was expanding, Einstein threw away this dark energy term, and it sat in the attic for 70 years until someone figured that it would work to model dark energy.

This points out an important point. Theorists create models not to be write. You can figure out things just from theory. The point of a theorist to come up with interesting ideas and arguments, and you can have brilliant and interesting ideas that make progress because they are wrong.

 

[Ich]

It *was* surprising, even shocking. The reason this was surprising was that if you assuming that lambda is zero and the critical density is one, then you could argue that there was some basic symmetry in the universe. Guess not.

It was not that bad, because in 98 they had some problems emerging: the universe was too young, and inflation predicted Omega = 1. They couldn't imagine where the 70% missing matter should be as the numbers more and more firmly said that it's probably 30% and not very much more.
With Lambda, you can have both: the universe suddenly is about the right age, and at critical density. The data already started to point at Lambda even before the SN measurements confirmed it. For example read the first paragraph of this http://iopscience.iop.org/0004-637X/501/2/461/pdf/0004-637X_501_2_461.pdf".
So it was surprising, but this observation did not raise additional prolems (except that such a low vacuum energy is a rather nutty idea), instead it made life considerably easier.

 

[AWA]

If the standard models of cosmology and stellar evolution are right then it should be impossible. If we see one then we know that something is seriously wrong. Since the universe is supposed to be 13 billion years, if you see something that looks like it is 12 trillion years old, you have some explaining to do.

Sure, if they are right, then it should be impossible. The problem to use this as support of the standard model is that it might be highly improbable to find any with our current technology in a reasonable amount of time if the models weren't in fact right.

We've done enough searches to be able to confidently say if there were any non-main sequence red dwarfs within 1000 l.y., we would have seen them. The fact that we've not seen a single one is pretty significant.

How would you calculate the probability to find one in a radius of 1kly in a few decades considering the fact that it might take a mean of trillions of years for any of them to exit the main sequence? I'd say it's pretty low.

The point that I'm trying to get across is that the circularity you are complaining about just doesn't exist. Also there isn't a grand model that's some holy writ. You can think of the "standard model" as something like wikipedia where people are constantly adding and removing stuff as new data comes in.

I like that analogy, but given the bad fame of wikipedia in terms of consistency and reliability, I'm not sure many people will agree to it.

Your statement was "accelerated expansion was pretty radical and could have killed CDM". Which is very different statement. The thing is that it could be our equations for how we think the universe expands could be quite wrong, but the fact that we think that there is cold dark matter is based on observational puzzles.

If in fact the confusion arose from my bad choice of words ,(I guess I should have said the standard model in 1996) I admit my rant at you wasn't justified. I must say though, to be honest that after reading some more of your posts in the forum your dialectic style still seems to me to be more of a lobbyist or salesman than of a scientist.

 

[Chalnoth]

I like that analogy, but given the bad fame of wikipedia in terms of consistency and reliability, I'm not sure many people will agree to it.

Wikipedia has been demonstrated to be about as reliable as more traditional encyclopedias.

 

[AWA]

Wikipedia has been demonstrated to be about as reliable as more traditional encyclopedias.

I like wikipedia and often use it, but you must be aware of its limitations, and contrast info with other sources, the problem with traditional encyclopedias is different, they are dated very fast, many by the time they get published.

To keep on topic, I'd like to ask you if in your opinion the distribution of matter in the universe, as defined by such properties like isotropy and homogeneity (or their lack of, depending of the specific formulation according to observation), obeys a fundamental physical law.

 

[Chalnoth]

I like wikipedia and often use it, but you must be aware of its limitations, and contrast info with other sources, the problem with traditional encyclopedias is different, they are dated very fast, many by the time they get published.

Not really. A recent article in Nature showed that Encyclopedias have a similar number of major errors, not just due to being outdated. The nice thing about Wikipedia is that it provides hotlinks to its sources right there on the webpage, which allows the user who is interested in checking to actually do that easily and efficiently.

For myself, I have browsed a number of webpages in areas I am intimately familiar with, and found them to be of high quality overall. There have been a couple of instances where I've fixed a problem here or there, but it's almost always been very minor. At least for the majority of issues, Wikipedia really is highly reliable. It only makes sense to go beyond Wikipedia and examine the sources provided there, if you really want to delve into a subject, but Wikipedia is generally a very good place to start on a subject.

To keep on topic, I'd like to ask you if in your opinion the distribution of matter in the universe, as defined by such properties like isotropy and homogeneity (or their lack of, depending of the specific formulation according to observation), obeys a fundamental physical law.

No. I think the homogeneity and isotropy of our universe is a consequence of its particular history, not a fundamental law. One might argue that the nature of fundamental law makes an observation of homogeneity/isotropy likely, but that's another discussion.

 

[AWA]

No. I think the homogeneity and isotropy of our universe is a consequence of its particular history, not a fundamental law.

If the homogeneity and isotropy were there from the very first moment of existence of matter, I can't figure out what particular history you refer to here.

 

[Chalnoth]

If the homogeneity and isotropy were there from the very first moment of existence of matter, I can't figure out what particular history you refer to here.

Well, the thing is, with inflation, if you start out with a rather inhomogeneous universe, inflation itself makes it more and more homogeneous with time (while, at the same time, causing quantum vacuum fluctuations which make very small deviations from homogeneity, seeding the growth of structure).

Inflation itself has some problems we haven't worked out, leaving some things unexplained. But it does provide a nice dynamical explanation for the degree of homogeneity that we can see in our region of space-time.

 

[AWA]

Well, the thing is, with inflation, if you start out with a rather inhomogeneous universe, inflation itself makes it more and more homogeneous with time (while, at the same time, causing quantum vacuum fluctuations which make very small deviations from homogeneity, seeding the growth of structure).

Inflation itself has some problems we haven't worked out, leaving some things unexplained. But it does provide a nice dynamical explanation for the degree of homogeneity that we can see in our region of space-time.

Ah, ok, you mean a history of 10^-32 seconds. A bit short to call it hystory I'd say, but time is relative or so they say.
If inflation were true, let's imagine it is, there you have your fundamental law for the structure of the distribution of matter at large-scale, don't you think?

 

[Chalnoth]

Ah, ok, you mean a history of 10^-32 seconds. A bit short to call it hystory I'd say, but time is relative or so they say.
If inflation were true, let's imagine it is, there you have your fundamental law for the structure of the distribution of matter at large-scale, don't you think?

But inflation isn't some sort of fundamental law. It's just something that can happen if you have enough energy in a quantum field with the right properties. In other words, inflation is a model of a specific type of event, not a fundamental law. How often inflation occurs and how big of a universe it tends to produce depends upon fundamental laws, but isn't, in and of itself, a fundamental law.

 

[twofish-quant]

To keep on topic, I'd like to ask you if in your opinion the distribution of matter in the universe, as defined by such properties like isotropy and homogeneity (or their lack of, depending of the specific formulation according to observation), obeys a fundamental physical law.

I don't understand the question.

We observe that the universe is more or less isotropy and homogeneous just like we observe that the Earth is more or less round.

Once we have some observations, then the theorists come in. What I do as a theorist would be to *assume* something about the universe and then figure out the consequences. I *assume* the universe is completely homogeneous and see what happens. Or I *assume* that the universe is non-homogeneous and see what happens.

It gets more interesting. For example, for a lot of things, I can get good calculations if I *assume* the Earth is a perfect sphere. For some things, that just doesn't work, and it's perfectly obvious that the Earth isn't a perfect sphere, and it's not even a perfect sphereoid. One thing that theorists work out is the limit of models. How much does the Earth need to deviate from a perfect sphere before calculation X becomes non-sense.

Physical laws are just assumptions about how the universe works. Sometimes they are strong assumptions. Somethings they are weak assumptions.

I should point out that personally, I think that the standard model of cosmology is fundamentally broken and that we are missing something basic (and I'm not the only one that thinks that). The problem is that you can't write a paper based on "gut feeling" and I can't think of anything better.

 

[twofish-quant]

If inflation were true, let's imagine it is, there you have your fundamental law for the structure of the distribution of matter at large-scale, don't you think?

Part of the problem here is that I don't understand what you mean here by fundamental law.

Lots of cosmology involves fudge factors that are intended to deal with our ignorance. Someone (Alan Guth) pointed out that if you *assume* the universe expanded very rapidly at one point, a lot of annoying problems disappear, and no one has come up with a better "magic wand."

A lot of cosmology involves "minimizing magic wands." Inflation, dark matter, and dark energy are stupid assumptions that we are just putting into make our models fit what we are seeing in the telescopes. The thing is that by making only three stupid assumptions, you end up explaining a lot, and getting things down to the point that you have to make *only* three stupid assumptions, is quite amazing.

Also they aren't vague assumptions. The nice thing about LCDM is that you just can't say "we need dark energy." You have to say "we need exactly this much dark energy and it has to behave in this way." For example, with dark matter. We know it's cold. We know that it can't react in certain ways with ordinary matter.

 

[twofish-quant]

Sure, if they are right, then it should be impossible. The problem to use this as support of the standard model is that it might be highly improbable to find any with our current technology in a reasonable amount of time if the models weren't in fact right.

Not true. We can see red dwarfs out to several thousand light years, and catalogued hundreds of thousands of them. A red dwarf that is off main sequence would be *easier* to see than a main sequence one.

Part of doing physics involves knowing the limits of your technology. The fact that we haven't seen any population III stars isn't that fatal because we would not be expected to see them. If there were any highly evolved red dwarves in this corner of the galaxy, we'd see them.

Now you can argue that maybe there are highly evolved red dwarfs in some other galaxy, and you'd be right. But not seeing highly evolved red dwarfs nearby puts constraints on what is possible,

You can't prove any theory right, but you work with process of elimination. Any cosmology that requires the Milky Way to be 10 trillion years old is dead.

How would you calculate the probability to find one in a radius of 1kly in a few decades considering the fact that it might take a mean of trillions of years for any of them to exit the main sequence?

It's an experimental thing and not a probability thing. You figure out what an evolved red dwarf would look like, and then you ask your telescope friend whether he'd see an object like X if it existed.

I don't know if the Loch Ness monster exists or not. I do know that it's not hiding my bed since I just looked for it.

I like that analogy, but given the bad fame of wikipedia in terms of consistency and reliability, I'm not sure many people will agree to it.

Wikipedia has been more consistent and reliable than main stream encyclopedia and the reason for that is that it works more like science does than main stream encyclopedias. Science works like wikipedia and not like Encyclopedia Britannica.

Also, one reason wikipedia works well, is that it's a lot easier to get an expert to work on Wikipedia than on Encyclopedia Britannica.

If in fact the confusion arose from my bad choice of words ,(I guess I should have said the standard model in 1996) I admit my rant at you wasn't justified. I must say though, to be honest that after reading some more of your posts in the forum your dialectic style still seems to me to be more of a lobbyist or salesman than of a scientist.

How many scientists outside this forum have you met?

Most people have never been taught science and don't really know what scientists do and how they argue.