Is the Cosmological Principle Limited to Space Only?

[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.