Split Thread Michael Mozina's thread on Dark Matter, Inflation and Cosmology

[Tim Thompson]

Dark Energy is a Cosmological Constant

Dark energy is not any manifestation of classical electromagnetism, ...

Dark energy isn't even "real" Tim. It can't "be" anything. Electromagnetism is real, and it has a real affect on real things, including the ability to accelerate plasma.

Don't make us laugh too hard. Of course dark energy is "real", how could it not be "real"? We all know that the words "dark energy" are just the arbitrary name given to "that which causes the acceleration of the expansion of the universe", whatever "that" may be. So dark energy is real by definition, which is a pretty hard "real" to get around. Now, as I recall ...

Although I personally favor an acceleration "interpretation" of the available data just like the mainstream, ...

So this Mozina fellow does agree with the interpretation of the data that the acceleration is real. And then he said ...

There's only one known force of nature that can generate a continuous acceleration of plasma, and that is the EM field.

So this Mozina fellow says the acceleration is real, and he says the cause of the acceleration must be electromagnetism. Therefore, by definition, whether you like it or not, you do in fact think that "dark energy" is in fact electromagnetism. Now, I provided a little calculation to show that dark energy cannot be an electric field (post 677 in thread) and ben_m provided a little calculation to show what kind of magnetic field is required to push stars around (post 721 in thread). I don't recall seeing any equally quantitative & objective response from you, so I can only conclude that you now know as well as we do that the acceleration of the expansion of the universe cannot be a result of anything done by classical electromagnetic fields (but see my post 655 in thread which shows that quantum electrodynamic effects are being considered by some as a plausible candidate for "dark energy").

If you cannot provide a quantitative physical argument in opposition to the quantitative physical arguments already presented, then the discussion is over, isn't it?

Likewise if you can't produce a physical empirical demonstration that "dark energy" isn't simply a figment of your overactive imagination, and an ad hoc gap filler of epic proportions, the discussion is over isn't it? When did "dark energy" ever cause even a single atom to "accelerate" in a lab Tim?

Whether or not dark energy accelerates anything in a lab is not at all relevant to the question of its reality. We have been down this road before, and one of the many weaknesses you demonstrate, is the ability to redefine common words & concepts to suit your own arbitrary purpose, without bothering to mention it. For instance, you can look up the word "empirical" and see that it does not require a controlled laboratory experiment. You can also look up the word "science", and see that it too can get along without controlled laboratory experiments. Indeed, I am sure there are plenty of archaeologists and zoologists and astronomers (and others) out there who would be thrilled to hear that Michael Mozina has evicted them from the pantheon of "science" to some lesser state of mythologists.

There is a great deal of valid scientific evidence from which the valid scientific conclusion is drawn that the expansion of the universe has accelerated over cosmological time, and that the driving force for this acceleration is not one of the known classical forces, gravity or electromagnetism (I think we can all agree that the strong & weak nuclear forces with such microscopic effective range are out of the running). Therefore, the cause of the acceleration must be some other, previously unknown agent. The observational data are most consistent with a cosmological constant term in Einstein's equations. See, for instance, Riess, et al., 1998 & Perlmutter, et al., 1999 (these were the first papers to report observational evidence for an accelerated expansion); Wood-Vasey, et al., 2007 constrains the nature of dark energy, and Davis, et al., 2007 demonstrate the superiority of the cosmological constant over other exotic models of dark energy.

And an interesting recent abstract deserves attention (emphasis mine):

Analytical Considerations about the Cosmological Constant and Dark Energy
Aberu, de Assis & Dos Reis; International Journal of Modern Physics A, Volume 24, Issue 28-29, pp. 5427-5444 (2009).
Abstract: The accelerated expansion of the universe has now been confirmed by several independent observations including those of high redshift type Ia supernovae, and the cosmic microwave background combined with the large scale structure of the universe. Another way of presenting this kinematic property of the universe is to postulate the existence of a new and exotic entity, with negative pressure, the dark energy (DE). In spite of observationally well established, no single theoretical model provides an entirely compelling framework within which cosmic acceleration or DE can be understood. At present all existing observational data are in agreement with the simplest possibility that the cosmological constant be a candidate for DE. This case is internally self-consistent and noncontradictory. The extreme smallness of the cosmological constant expressed in either Planck, or even atomic units means only that its origin is not related to strong, electromagnetic, and weak interactions. Although in this case DE reduces to only a single fundamental constant we still have no derivation from any underlying quantum field theory for its small value. From the principles of quantum cosmologies, for example, it is possible to obtain the reason for an inverse-square law for the cosmological constant with no conflict with observations. Despite the fact that this general expression is well known, in this work we introduce families of analytical solutions for the scale factor different from the current literature. The knowledge of the scale factor behavior might shed some light on these questions mentioned above since the entire evolution of a homogeneous isotropic universe is contained in the scale factor. We use different parameters for these solutions and with these parameters we establish a connection with the equation of state for different DE scenarios.
​

So the current consensus, based on empirical scientific evidence is that the expansion of the universe is real, its acceleration is real, the cause of the acceleration is real, the cause of the acceleration is most likely a cosmological constant, and the cause of the acceleration (which is by definition "dark energy") is not strong, weak or electromagnetic in nature.

Now, Mr. Mozina endlessly declares that its all just some kind of "fudge factor" or "myth", but other than his own highly subjective opinion, Mr. Mozina has not come forth with a single atom of objective reason to reject all of the observational data, in favor of his own electro-mythical whatever that even he can't describe or explain. One gets the idea that the Mozina Motto is "damn the science, full speed ahead!".

 

[Dancing David]

Thanks to all for the E in JREF.

 

[edd]

Don't make us laugh too hard. Of course dark energy is "real", how could it not be "real"? We all know that the words "dark energy" are just the arbitrary name given to "that which causes the acceleration of the expansion of the universe", whatever "that" may be. So dark energy is real by definition, which is a pretty hard "real" to get around. Now, as I recall ...

Again, I have to point out this isn't right. There are models that have acceleration that don't have dark energy. They are generally thought to have something at least as bad though.
As an example, http://arxiv.org/abs/0709.2044 details one such - the LTB models which do away with the Copernican Principle.

That said...

So this Mozina fellow says the acceleration is real, and he says the cause of the acceleration must be electromagnetism.

I'm not aware of any that reproduce acceleration using EM or anything like it. It's always some gravitational effect, whether produced by dark energy, large scale inhomogeneity or a change in the physics of gravity.

I also agree it's nuts to expect to be able to measure dark energy in the laboratory. The amount of dark energy is a free parameter which we fit with astronomical observation. When we do so, we get a quantity that is about 10 milligrams for the total amount in the volume of the Earth. I don't know how you would ever be expected to measure that in a laboratory context against the mass of the Earth - the ratio is about 1 part in 10^30.

Just because it is not measurable in the laboratory (by any means I can think of) does not mean it is not real or not measurable elsewhere - in fact its the very measurements we can make outside the lab that show it can't be measured in the lab.

 

[Perpetual Student]

Again, I have to point out this isn't right. There are models that have acceleration that don't have dark energy. They are generally thought to have something at least as bad though.
As an example, http://arxiv.org/abs/0709.2044 details one such - the LTB models which do away with the Copernican Principle.

But doesn't acceleration inevitably involve energy? From your link: " a simple spatial variation in the Hubble rate can account for the distant supernova data in a dust universe without any dark energy." How does a "simple variation in the Hubble rate" happen without energy? BTW, I'm not a physicist.

 

[Michael Mozina]

But doesn't acceleration inevitably involve energy?

 

[edd]

But doesn't acceleration inevitably involve energy? From your link: " a simple spatial variation in the Hubble rate can account for the distant supernova data in a dust universe without any dark energy." How does a "simple variation in the Hubble rate" happen without energy? BTW, I'm not a physicist.

Usually you build a cosmology by assuming we're nowhere special in the universe, and everywhere is much the same as everywhere else. You have, in other words, on sufficiently large scales a uniform density.

In a LTB model you put yourself at, or very close to, the centre of an enormous underdensity. This means the Hubble parameter is not the same everywhere, and evolves differently over time. It also means it's not the same for every observer.

It's perfectly ordinary gravity working on perfectly ordinary matter - we've just contrived things and put the matter in the right place and ourselves in the right place to make it look like the universe is expanding and accelerating, when in fact it's only doing that for a certain subset of the stuff in the universe, and only when you look at it from where we are. You've basically just put a whole bunch of stuff just out of view and have it pulling on the matter we see accelerating away from us.

Finished that popcorn now Michael?

 

[ben m]

But doesn't acceleration inevitably involve energy? From your link: " a simple spatial variation in the Hubble rate can account for the distant supernova data in a dust universe without any dark energy." How does a "simple variation in the Hubble rate" happen without energy? BTW, I'm not a physicist.

(very quick glance at paper). The model Edd links to is of an inhomogeneous Universe; it's expanding at one rate over here and at another rate over there---that's fine from a GR perspective, since the expansion rate is a "local" property of spacetime determined by initial conditions at some early point in the Big Bang. If the Big Bang had been inhomogenous, then the FRW equations we use to calculate its evolution don't apply, and so our ability to extrapolate redshifts and distances into "trajectories" would be wrong.

But this hypothesis can only look like the data IF the "inhomogeneity" is a bunch of spherical shells centered on Earth. It requires a sort of conspiracy theory in which the Earth is located at the one, unique place in the Hubble curve "accidentally" is isotropic in all directions and "accidentally" has the acceleration-like turnover.

ETA: edd understands it better and (as usual) explained it better. I want to emphasize, though, that the "underdensity" of the LTB model isn't just any random underdensity---those would not be too surprising---it has to be a perfectly spherical one to a precision of 10^-5, with the Earth located within 10^-5 of the center.

 

[edd]

Yeah, it's hideously contrived, which is why we generally go for the one-parameter addition of a cosmological constant instead. There are people checking into this sort of thing in more detail to build tests that can tell the two circumstances apart, but I've never met anyone that thinks it's very likely given how well lambda-CDM works.

But it does demonstrate that dark energy is not the only conceivable cause of acceleration.

 

[ben m]

But it does demonstrate that dark energy is not the only conceivable cause of acceleration.

... which is one of the ways a casual observer can tell that Dark Energy is a scientific hypothesis and not ... well, a cult or a dogma or whatever else MM thinks it is.

Also: the LTB paper is a nice example of a competently-presented alternative cosmology theory. It does not contain any epistemological rants against dark energy, dark matter, inflation, or magnetic reconnection. It presents a detailed quantitative theory which any physics-educated reader can compare, on their own, to any data. It does not avoid doing any math, nor argue that math is irrelevant or constructed or whatever. It straightforwardly reviews the strengths and weaknesses of the new hypothesis as well as the strengths and weaknesses of the standard dark-energy hypothesis.

 

[Tim Thompson]

Energy and the Accelerated Expansion

But doesn't acceleration inevitably involve energy?

Actually, from the point of view of classical physics (quantum mechanics not allowed), not necessarily (as I have already pointed out elsewhere). One need only look at Einstein's equations to see that the "cosmological constant" is pure geometry. So, if the cosmological constant is the simplest explanation for the accelerating expansion, then it does not involve "energy" at all in any classical sense, but only the pure geometry of space. Hence, "dark energy" may have been an unfortunate choice of words for the name, since energy need not be involved.

Of course, once we try to establish a quantum field theory for the cosmological constant, like vacuum energy or some such thing, then we have energy back in then game as one might expect. It's just not there as an entity of classical physics. We can make the accelerated expansion due to classical energy by shying away from the cosmological constant and asserting an additional scalar field into the stress-energy tensor (i.e., "quintessence" theory; Wetterich, 2002; Chongchitnan & Efstathiou, 2007).

It's easy to get hung up on the words and miss the physics. "Dark energy" may not be any form of classical energy, just as "magnetic reconnection" may not be a literal physical description of what's happening. But in both cases there is a unique physical system behind the name that obeys the laws of physics just as rigorously as anything else. It's just that, at least in the case of dark energy, we are willing to set aside the momentary arrogance required to demand that we already know what all of the laws of physics really are and there is nothing left to discover in the universe. Some of the people who work these threads would do well to set that arrogance aside.

 

[sol invictus]

Actually, from the point of view of classical physics (quantum mechanics not allowed), not necessarily (as I have already pointed out elsewhere). One need only look at Einstein's equations to see that the "cosmological constant" is pure geometry. So, if the cosmological constant is the simplest explanation for the accelerating expansion, then it does not involve "energy" at all in any classical sense, but only the pure geometry of space. Hence, "dark energy" may have been an unfortunate choice of words for the name, since energy need not be involved.

Of course, once we try to establish a quantum field theory for the cosmological constant, like vacuum energy or some such thing, then we have energy back in then game as one might expect. It's just not there as an entity of classical physics.

That's not correct. The cosmological constant appears in the Einstein-Hilbert Lagrange density as, well, a constant. If you carry out the integral it you get the 4-volume of spacetime times that constant. In other words including the CC is equivalent to including a constant energy density (note that what I've said has nothing to do with quantum mechanics, it's purely classical field theory).

In non-gravitational physics such an energy density wouldn't have any effect (since it's only energy differences that matter, and so a universal additive constant doesn't do anything), but gravity turns out to act on all forms of energy, even constants, and so the CC affects the geometry and can drive acceleration. Without such a term there is no way to achieve an accelerated expansion, because all ordinary forms of matter and energy act to decelerate it.

 

[Michael Mozina]

But doesn't acceleration inevitably involve energy?

So how's a layman supposed to comprehend this stuff, when "experts" like Tim and Sol can't even agree? FYI, Sol is correct.

 

[Reality Check]

So how's a layman supposed to comprehend this stuff, when "experts" like Tim and Sol can't even agree? FYI, Sol is correct.

Actually Tim and Sol are both correct. The cosmological constant is a funny wee beasty.

The Einstein field equations are usually quoted as:

In this form the cosmological constant appears with the Ricci curvature tensor and the scalar curvature. It is thus part of the geometry of spacetime (as per Tim Thompson's post).

But it is just as valid to subtract the cosmological constant term from both sides of the equation. In that form the CC appears with the stress-energy tensor. It is thus part of the stress-energy that is being considered as the source of the gravitational field (as per sol invictus's post).

What Tim and others have been pointing out in recent posts is that the CC is not the only candidate for dark energy. It is however the candidate that requires the least departure from known physics.

 

[sol invictus]

Actually Tim and Sol are both correct. The cosmological constant is a funny wee beasty.

The Einstein field equations are usually quoted as:

[qimg]http://upload.wikimedia.org/math/3/f/5/3f50fd206f2fe543a6a8a3e687cf74c3.png[/qimg]

In this form the cosmological constant appears with the Ricci curvature tensor and the scalar curvature. It is thus part of the geometry of spacetime (as per Tim Thompson's post).

You can't change the interpretation of a term by subtracting it from both sides of an equation. If you want to know whether it corresponds to an energy density or not, look at the action principle from which that equation comes. The CC appears as an additive term (with no derivatives) in the Einstein-Hilbert Lagrangian; therefore it's an energy density. It has units of energy/volume. It can arise due to quantum vacuum energy, or due to a constant piece in a classical scalar potential energy function, or in several other ways, but it's always an energy density.

Granted, energy in general relativity is a very subtle topic (actually energy is always a surprisingly subtle thing). But in this case I don't think there's much ambiguity.

 

[Reality Check]

You can't change the interpretation of a term by subtracting it from both sides of an equation. If you want to know whether it corresponds to an energy density or not, look at the action principle from which that equation comes. The CC appears as an additive term (with no derivatives) in the Einstein-Hilbert Lagrangian; therefore it's an energy density. It has units of energy/volume. It can arise due to quantum vacuum energy, or due to a constant piece in a classical scalar potential energy function, or in several other ways, but it's always an energy density.

Granted, energy in general relativity is a very subtle topic (actually energy is always a surprisingly subtle thing). But in this case I don't think there's much ambiguity.

My impression was that the inclusion of an additive term in a Lagrangian implicitly defines that additive term as an energy density. But it seems that I am wrong.

It is many, many years since I did my GR courses but I recently viewed Leonard Susskind's lectures on GR via YouTube. He did seem to imply that the CC was a geometrical term when it appeared on one side of the EFE and an energy term on the other side but he may have been simplifying things for the students.

ETA:
I got that impression from his addition of the CC to EFE about 14 minutes into lecture 10. Starting from EFE without the CC term:

• We expect/hope that energy and momentum will be conserved on the right hand side of the EFE, i.e. the covariant derivative of the stress-tenergy tensor will be zero.
• This means that the covariant derivative of the left hand side will also be zero, i.e. the covariant derivative of the Einstein tensor will be zero ("covariant conservation").
• You can add a constant times the metric to the left hand side and still have covariant conservation.

Since he was talking about geometry at that point I thought that the CC term (on the left hand side of the EFE) was a geometrical term. However from then on CC is refereed to as a energy density.

 

[Michael Mozina]

Keep in mind PS that the original "prediction" (say 25 years ago) of "big bang" theories in general was that the expansion process was slowing down over time. The ad hoc insertion of "dark energy" 15 years ago was a direct result of the observation of apparent acceleration. That acceleration process was *NOT* a prediction of earlier BB theories, in fact it's exactly the opposite of earlier predictions. "Dark energy" was specifically and intentionally stuffed in there about 15 years ago or so to "explain" that observation of acceleration. That observed acceleration of the physical universe absolutely requires additional "energy". As Sol points out, all gravitational forces act to 'slow down' the expansion process. Whatever speeds it up must absolutely add energy to the system.

 

[Reality Check]

Keep in mind PS that the original "prediction" (say 25 years ago) of "big bang" theories in general was that the expansion process was slowing down over time. The ad hoc insertion of "dark energy" 15 years ago was a direct result of the observation of apparent acceleration. That acceleration process was *NOT* a prediction of earlier BB theories, in fact it's exactly the opposite of earlier predictions. "Dark energy" was specifically and intentionally stuffed in there about 15 years ago or so to "explain" that observation of acceleration. That observed acceleration of the physical universe absolutely requires additional "energy". As Sol points out, all gravitational forces act to 'slow down' the expansion process. Whatever speeds it up must absolutely add energy to the system.

First asked 29 January 2010
MM,
Can you give a citation for your assertion that BBT 25 years ago only predicted that the expansion process was slowing down over time?

This may be a case of your ignorance of science yet again.

1. BBT has always predicteded that the expansion process was either slowing down, speeding up or constant. Generally people went with the constant expansion option.
2. Dark energy is an observation not a theory. The theories are about the composition of dark energy, e.g. the cosmological constant.
3. As sol invictus and oithers have pointed out dark energy is not the gravitational force. The effects of dark energy are the same as a repulsive gravitational force but a cosmological constant results in a force law that varies as r not 1/r^2.
4. We are talking about the universe. There is no energy to be added to the system.

 

[ben m]

Keep in mind PS that the original "prediction" (say 25 years ago) of "big bang" theories in general was that the expansion process was slowing down over time.

So what? Those theories have been shown to be incorrect. The "original Big Bang theory", if by that you mean "the EFE with lambda=0", does not agree with cosmology data.

Are you ever going to comment on the fact that electromagnetic forces cannot be contributing to this or any other large-scale acceleration?

 

[sol invictus]

Keep in mind PS that the original "prediction" (say 25 years ago) of "big bang" theories in general was that the expansion process was slowing down over time. The ad hoc insertion of "dark energy" 15 years ago was a direct result of the observation of apparent acceleration.

There's a name for that process. It's "science".

That acceleration process was *NOT* a prediction of earlier BB theories, in fact it's exactly the opposite of earlier predictions.

No, not really. First of all there isn't really such a thing as "BB theory". The only theory is general relativity, but that just tells you the dynamics, not what the universe is composed of or whether it has a CC. All of that has to be fit using data. The current data, which are much more precise and plentiful than the data 25 years ago, show that a small positive CC fits well.

It's very much like using Newtonian gravity and mechanics to describe the solar system. You have to make observations, use your observation to fit some parameters (like positions and velocities of planets), and then test your model against more observations. If it doesn't fit, it might be because there's another body that's hard to see that is perturbing the others.

"Dark energy" was specifically and intentionally stuffed in there about 15 years ago or so to "explain" that observation of acceleration.

Again, that's called "science".

That observed acceleration of the physical universe absolutely requires additional "energy". As Sol points out, all gravitational forces act to 'slow down' the expansion process. Whatever speeds it up must absolutely add energy to the system.

Gravitational forces due to ordinary matter and radiation act to slow down the expansion. By the way, that includes all forms of classical electromagnetic energy.

Gravitational forces due to dark energy or a positive CC act to speed it up.

 

[Michael Mozina]

Without such a term there is no way to achieve an accelerated expansion, because all ordinary forms of matter and energy act to decelerate it.

I think it should be noted that your statement is true only in a closed system. For instance, a greater concentration of matter beyond our visible sliver of the universe might act to accelerate matter. Likewise other forces of nature might work as well, they just need to be acting from the outside.