Lambda-CDM theory - Woo or not?

[Michael Mozina]

Unfortunately magic expansion isn't consistent with GR.

Sure it is. I can take any paper on "dark energy" and do a search and replace with "magic energy" or "God energy" and believe in anything I want to believe in. Anything I stuff into that constant become instantly consistent with GR. Big deal.

 

[Reality Check]

Relative to what? ONE SIDE OF THE PLATE?

That is the problem with your ignorance of physics and mathematics - you have to ask trivial questions. Relative to nothing! It is a general relationship.
Pressure is -dE/dV
dE is a chnage in energy.
dV is a change in volume.

When dE is the opposite sign as dV then pressure is positive, e.g. for a gas.
When dE is the same sign as dV then pressure is negative, e.g. for

• The Casimir effect
• A positive cosomological constant in GR.
• The inflationary period of the universe.

 

[Tubbythin]

And it still hasn't been either.

Your denial of the fact that I provided evidence to the contrary does not stop the fact that I provided evidence to the contrary. Nor does it mean that you didn't shift the goal posts.

No! I bitched because Guth tried to claim his vacuum contained "negative pressure".

I gave an example, you shifted the goal posts. It is as plain as day for anybody to read Michael.

Your "bang" theory starts with *ONE* and ONLY ONE "mass thingy" and a vacuum.

Does it? What is a mass thingy? Why do you think there was a vacuum at the start?

You're the one now adding caveats to your claim by adding a SECOND OBJECT to the vacuum. There's no "second blog" in Guth's claim! That's a blatant shift of the goalposts.

No I'm not. I haven't claimed that the Casimir effect is the same as Guth's inflationary hypothesis or a Universe with non-zero cc. That is a confusion entirely of your own.

FYI, the whole debate about the Casmir effect began when I bitched about Guth's evocation of a magic negative pressure vacuum.

No it didn't. The debate about the Casimir effect began (this time around at least) when you asked for an example of a negative pressure in a vacuum in a lab environment.

Sure as long as you accept the fact that the plate has 6 sites, not just one.

Pardon?

Actually, we're talking so far.... You mean the "relative pressure" between the plates is "less than" the pressure outside of the plates.

No. I mean the pressure in the vacuum between the plates is negative.

Relative to what? ONE SIDE OF THE PLATE?

Relative to nothing. If -dE/dV is negative then so is the pressure... by definition!

 

[Tubbythin]

Sure it is. I can take any paper on "dark energy" and do a search and replace with "magic energy" or "God energy" and believe in anything I want to believe in.

And? I can replace electromagnetic force with thingumywhatsit force. It makes no difference to the physics. Do you actually think you have a meaningful argument?

Anything I stuff into that constant become instantly consistent with GR. Big deal.

You can't stuff anything in to the constant. It must be (in the right units) a real scalar value. Just like G or e or hbar. It is also constrained by observation.

 

[Reality Check]

Sure it is. I can take any paper on "dark energy" and do a search and replace with "magic energy" or "God energy" and believe in anything I want to believe in. Anything I stuff into that constant become instantly consistent with GR. Big deal.

Big deal, MM - You have admitted that you can use a text editor !

I would be an idiot if I said:

I can take any paper on XXXX and do a search and replace with "magic energy" or "God energy" and believe in anything I want to believe in.
Anything I stuff into that constant become instantly consistent with XXXX.
​

Don't make yourself an idiot by doing this simple mistake.

As an example: it would be totally delusional to take any paper on classical mechanics, look for "energy" and do a search and replace with "magic energy" or "God energy" and think that this invalidated classical mechanics !

Changing terminology does not change meaning. "A rose by any other name would smell as sweet"!

 

[ben m]

The only reason "dark energy" could do anything to cosmology is because GR cares about energy densities. If you have a hypothesis about the vacuum energy density, plug it into the GR energy-density term, and there you go---you have a hypothesis about spacetime in the presence of vacuum energy.

The only reason "quintessence" could do anything to cosmology is because GR cares about energy densities. If you have a hypothesis about the quintessence energy density, plug it into the GR energy-density term, and there you go---you have a hypothesis about spacetime in the presence of quintessence.

The only reason "God magic energy" could do anything to cosmology is because GR cares about energy densities. If you have a hypothesis about the God magic energy density, plug it into the GR energy-density term, and there you go---you have a hypothesis about spacetime in the presence of God magic energy.

But you don't have a hypothesis about god magic energy; you don't want to plug it into the GR energy-density term; you just made up a funny name so you could have a laugh about it. Good for you! You must be so proud.

 

[Michael Mozina]

And? I can replace electromagnetic force with thingumywhatsit force. It makes no difference to the physics. Do you actually think you have a meaningful argument?

The empirical difference is that the EM field by any name shows up in a lab regardless of what you call it.

You can't stuff anything in to the constant. It must be (in the right units) a real scalar value. Just like G or e or hbar. It is also constrained by observation.

FYI, you aren't actually even constraining it by observation as you seem to think. You're technically constraining it by SUBJECTIVE INTERPRETATION of redshift information and some supernova brightness observations.

 

[Tubbythin]

The empirical difference is that the EM field by any name shows up in a lab regardless of what you call it.

And the reality or otherwise of dark energy is unaffected by what you call it. Giving it a stupid is thus only indicative of your inability to make a scientific objection.

FYI, you aren't actually even constraining it by observation as you seem to think. You're technically constraining it by SUBJECTIVE INTERPRETATION of redshift information and some supernova brightness observations.

If by subjective you mean "by multiple cross checking using different experimental methodologies and independent experiments" then you'd be correct. But that would be a very odd definition of subjective.

 

[ben m]

The empirical difference is that the EM field by any name shows up in a lab regardless of what you call it.

Yep. That's how we know that EM fields do not explain the long-range behavior of the supernova redshift/magnitude observations, nor of the CMB/CMBpol/LSS etc.

Understand?

 

[DeiRenDopa]

FYI, you aren't actually even constraining it by observation as you seem to think. You're technically constraining it by SUBJECTIVE INTERPRETATION of redshift information and some supernova brightness observations.

If by subjective you mean "by multiple cross checking using different experimental methodologies and independent experiments" then you'd be correct. But that would be a very odd definition of subjective.

In Mozphysics, nearly every term has a very odd definition!

For example, "empirical", "lab", "maths", "pressure", "force", "energy", "General Relativity", "electromagnetism", "particle", ...

 

[DeiRenDopa]

Let's see now ...

No word from MM on this, so:
a) MM did not, in fact, send an email
b) MM sent an email, but Ari did not reply

b) would in fact be the correct answer.

Thanks.

Why don't you folks try contacting yourself this time with a more detailed explanation of your criticism and see what happens?

I can't, of course, speak for anyone other than myself ...

MM, there are thousands, possibly tens of thousands of crackpot ideas concerning physics (astrophysics, cosmology, etc). Many of them are easily found, on the internet, with but a minute or two's searching. A significant fraction of these crackpot ideas are backed up (if that's an appropriate Mozphysics term) by arXiv preprints.

Unless and until any such, um, alternative idea gets published in a relevant, peer-reviewed journal, I have little interest in spending my time even reading it.

However, if you think this idea has legs, by all means spend your time digging into it, and preparing valid physics-based responses* to the explicit criticisms of it.

I'm actually quite curious to hear his answer.

Well, when you get it, do be sure to post it, here, in full ...

* not, of course, Mozphysics-based responses; as everyone (other than you) who's taken part in the 'discussion' in this thread has attested, Mozphysics is, at best, incoherent (at worst it's complete gibberish).

 

[ben m]

However, if you think this idea has legs, by all means spend your time digging into it, and preparing valid physics-based responses* to the explicit criticisms of it.

The double-standard is pretty funny. An anti-LCDM paper that apparently doesn't even use Maxwell's Equations correctly by a factor of 10^20, and still doesn't match the data? Mozina wants to contact the author and ask him for his clarification.

Ten thousand mainstream astronomers assemble a fairly complete theory of spacetime, including both E&M and GR, with extremely powerful comparisons to a vast suite of cosmological data? In that case, Mozina launches into well I read that these other astronomers have apparently needed to change a factor of 2 in a star-formation-rate estimate so it's all wrong all wrong all wrong and no i don't need to read misner's book to find out what you cultists think about it or whatever the usual spiel is.

 

[sol invictus]

Anything I stuff into that constant become instantly consistent with GR. Big deal.

Michael, I'm pretty sick of trying to discuss this with you, mainly because it never seems to go anywhere. But when I read comments like that, all I can think is that you've just totally misunderstood something somewhere.

There's only one constant. It's one number. It's only allowed to have one value. And as a result, the theory really is very predictive, because (at least in principle) you can determine that number with just one observation, and then every other observation you make is a test of the theory.

For my own reasons I just read a paper about an alternative to lambda-CDM, where you set lambda to zero but assume we're in the middle of a giant void (i.e. a spherical region with fewer galaxies in it than the average). It turns out you can get away with setting lambda to zero in such models - but the price you pay is that you have to assume that for some reason the earth is almost at the exact center of this big spherical void. In other words, that the earth really is at the center of the universe.

And the other price you pay is that the theory is much less predictive, because it has many numbers in it (the density is a function of radius) that need to be fixed by data, and so it's a lot easier to adjust it to match whatever observations get made. But it's still possible to distinguish from lambda CDM, and it seems that within a few years one or the other will be ruled out by data.

 

[Tubbythin]

For my own reasons I just read a paper about an alternative to lambda-CDM, where you set lambda to zero but assume we're in the middle of a giant void (i.e. a spherical region with fewer galaxies in it than the average). It turns out you can get away with setting lambda to zero in such models - but the price you pay is that you have to assume that for some reason the earth is almost at the exact center of this big spherical void. In other words, that the earth really is at the center of the universe.

It's not obvious to me why being at the centre of a void would also lead to us being near the centre of the Universe. Is there a simple answer for that or do I have to read the paper?

 

[Vorpal]

FLRW is isotropic and homogeneous (on average), so everything is spherically symmetric about every point. Take isotropy but not homogeneity instead, so the distribution of matter is spherically symmetric about a particular point rather than all of them. Since we observe the universe to be closely isotropic, we must be at or very near that special point. That's the kind of situation that's considered.

 

[sol invictus]

It's not obvious to me why being at the centre of a void would also lead to us being near the centre of the Universe. Is there a simple answer for that or do I have to read the paper?

FLRW is isotropic and homogeneous (on average), so everything is spherically symmetric about every point. Take isotropy but not homogeneity instead, so the distribution of matter is spherically symmetric about a particular point rather than all of them. Since we observe the universe to be closely isotropic, we must be at or very near that special point. That's the kind of situation that's considered.

Exactly. We have very good constraints on isotropy from the cosmic microwave background (among other observables). If we weren't at the center of the void there would be a large dipole, and probably a large l=3 mode too. But the constraints on isotropy without homogeneity are much weaker.

 

[edd]

It's not obvious to me why being at the centre of a void would also lead to us being near the centre of the Universe. Is there a simple answer for that or do I have to read the paper?

The voids in these cases need to be big - about a Gpc across and we need to be in the middle to something like 1% of that I think (probably depends on exactly which paper on the subject sol read). It's handwavingly the same sort of thing as saying you're in the centre of the universe - you need to be really precisely in the middle of some structure that's not way off the size of the horizon, although strictly speaking there's nothing preventing you having more universe with similar voids and corresponding over densities on yet larger scales (a kind of swiss cheese universe - yes we really call it that).

To put it another way: it might not put us literally at the centre of the universe, but it gives cosmologists the same kind of heebie-jeebies that doing so would - it still puts us in a special place in the universe.

 

[sol invictus]

That all sounds quite logical. The problem from my perspective is that you folks aren't actually deriving that number from direct observation, rather that number comes from a subjective "interpretation" of data, typically of redshift data.

http://arxiv.org/PS_cache/arxiv/pdf/1109/1109.6571v1.pdf

There are typically multiple ways to interpret that very same data. What makes a "dark energy" interpretation preferable in your mind, particularly since you can't even tell me where it comes from?

By the way Michael, I forgot to point this out before, but f(R) gravity is a form of dark energy. It's mathematically and physically identical to gravity plus a scalar with potential energy - i.e., "dark energy", and dark energy that can evolve with time, no less.

 

[Tubbythin]

The voids in these cases need to be big - about a Gpc across and we need to be in the middle to something like 1% of that I think (probably depends on exactly which paper on the subject sol read). It's handwavingly the same sort of thing as saying you're in the centre of the universe - you need to be really precisely in the middle of some structure that's not way off the size of the horizon, although strictly speaking there's nothing preventing you having more universe with similar voids and corresponding over densities on yet larger scales (a kind of swiss cheese universe - yes we really call it that).

To put it another way: it might not put us literally at the centre of the universe, but it gives cosmologists the same kind of heebie-jeebies that doing so would - it still puts us in a special place in the universe.

Ok. That was kind of my main thought I think: can we not be near the centre of one of many large voids? I guess that's still pretty improbable unless there would be some reason why intelligent life could only evolve in such a place. I can't think why there would be.

 

[edd]

Yeah, it's that it's still requiring something quite improbable. If we take it that we need to be in 10Mpc of a 1Gpc void - that's the central 1 millionth of the volume of the void. Even if you have more voids you are still in a 1-in-a-million position.