# Dark Matter or Dark Mass?

## Main Question or Discussion Point

We know particles have mass. Thusfar we don't know of anything that has mass which is not a particle so we assume the likeliest explanation for apparent missing mass must be missing particles. This makes some good sense. But what if this assumption is wrong?

Has anyone looked into the idea of dark mass instead of dark matter?

Let me illustrate one possible idea that shows the difference:

1) We have some instances where fields themselves can contain energy, like the EM field, if I am not missing something. So a charged particle can transfer its kinetic energy to an EM field thereby slowing down the charged particle and increasing the energy of the field.

2) Could there not be an equivalent ability for a field defining mass (Higgs? or something else) to contain mass itself?

3) We know that as Wheeler said: mass tells spacetime how to curve and spacetime tells mass how to move.

4) What if spacetime was springy and wanted to be straight so that the curvature itself implied some stored energy that shows up in our observations as missing mass at very low accelerations?

That's just one possible idea, but I'm sure there could be many other similar ones I've overlooked. The basic idea is that spacetime itself or a mass field might be able to store/contain mass.

Are there any papers discussing this idea or refuting it? Or are there obvious problems with it that I'm not seeing that make it such a silly idea that no one would have even bothered?

Ich
What if spacetime was springy and wanted to be straight so that the curvature itself implied some stored energy
Spacetime is springy and wants to be straight, and so curvature itself implies stored energy. That is what makes GR nonlinear. But it does contribute almost nothing at the low densities we're talking about.

The problem with DM is that it's cold. That means that its constituents should be stable, massive particles - unlike the quanta of all the fields we know.
Maybe it is some exotic field, but that's far more speculative than simply assuming there's another stable particle.

That means that its constituents should be stable, massive particles - unlike the quanta of all the fields we know.
What about Higgs? We don't know if it is stable. As Higgs will be the only scalar boson known (if it exists) it may have some "scalar" conservation factor, making it stable.

Spacetime is springy and wants to be straight, and so curvature itself implies stored energy. That is what makes GR nonlinear. But it does contribute almost nothing at the low densities we're talking about.
Thanks Ich, it sounds like I need to keep working my way through Tensor Calculus and Riemann Geometry so I can really understand GR.

Chalnoth
What about Higgs? We don't know if it is stable. As Higgs will be the only scalar boson known (if it exists) it may have some "scalar" conservation factor, making it stable.
Seems unlikely to me. From what I understand, scalar particles act as a mediator of a fifth force that acts very much like gravity, except without interacting with radiation. I suspect this means that if there was any significant amount of the stuff around, we would have seen its effects by now in gravitational lensing observations. I could be wrong, however.

That said, any specific dark matter hypothesis is unlikely at this point, because we have so many, and none that are obviously more likely than any others.

Spacetime is springy and wants to be straight[...]
get out of town. How do yo figure this? When I think of a very dense chunk of matter, I think if infalling spirals of more matter that will increase the curvature. What did you have in mind that would tend to flatten things out?

Seems unlikely to me. From what I understand, scalar particles act as a mediator of a fifth force that acts very much like gravity, except without interacting with radiation. I suspect this means that if there was any significant amount of the stuff around, we would have seen its effects by now in gravitational lensing observations. I could be wrong, however.

That said, any specific dark matter hypothesis is unlikely at this point, because we have so many, and none that are obviously more likely than any others.
Since we have not observed any scalar particles, it is safe to say we don't know how they act. If you don't expect an interaction between them and radiation (photons), what lensing are you talking about?

Chalnoth
Since we have not observed any scalar particles, it is safe to say we don't know how they act.
No, it's not, really. The mathematics of a scalar field determine quite a lot about their behavior.

If you don't expect an interaction between them and radiation (photons), what lensing are you talking about?
Well, as I said, scalar particles act as an intermediary force very much like gravity, except that they couple to the trace of the stress-energy tensor instead. Since the stress-energy tensor for radiation is traceless, this force doesn't interact with radiation, just slower-moving particles.

Though I do have to admit that I'm not entirely clear on whether or not having a gas of scalar particles around would actually affect the strength of the force felt.

get out of town. How do yo figure this? When I think of a very dense chunk of matter, I think if infalling spirals of more matter that will increase the curvature. What did you have in mind that would tend to flatten things out?
I'm interested to see Ich's response but in thinking about it, it's obvious that spacetime moves back to straight/flat if you take the matter away, so in that sense, it wants to be straight/flat.

No, it's not, really. The mathematics of a scalar field determine quite a lot about their behavior.
And what specific scalar field do you talk about? As far as I know we have advanced the science by stretching the known theories and then finding they don't work there. My idea that Higgs can be stable is a shot in the dark, I agree. However shooting it down with a theory that have never been confirmed will not help either.

Ich
How do yo figure this?
it's obvious that spacetime moves back to straight/flat if you take the matter away
Yes.
I was referring to the common analogy of those "dents in a rubbersheet". You have this flat, undisturbed background, and whenever you go away with your matter and things, it gets back to this state. Like something springy.
And if you bend it quickly, you really have to expend energy, and the deformation you caused will propagate away, like phonons in some springy crystal.
It's an analogy which guides intuition rather than hamper it, IMHO.

That said, I'm aware of what happens in the long term if you let both spacetime and matter interact. The result is quite the opposite of the flatness that I said is spacetime's "natural" state. And indeed, that's where you get entropy from.
So I won't argue if you don't like the analogy.

Chalnoth
And what specific scalar field do you talk about? As far as I know we have advanced the science by stretching the known theories and then finding they don't work there. My idea that Higgs can be stable is a shot in the dark, I agree. However shooting it down with a theory that have never been confirmed will not help either.
You can't separate a hypothetical explanation from its other implications. However, after a bit more thought, the connection between Higgs = dark matter and a fifth force may be less of a consideration. At any rate, it seems unlikely to me, as any Higgs particle must interact with normal matter in order to give it mass, which would seem to undercut the fact that dark matter is largely non-interacting.

Yes.
I was referring to the common analogy of those "dents in a rubbersheet". You have this flat, undisturbed background, and whenever you go away with your matter and things, it gets back to this state. Like something springy.
And if you bend it quickly, you really have to expend energy, and the deformation you caused will propagate away, like phonons in some springy crystal.
It's an analogy which guides intuition rather than hamper it, IMHO.

That said, I'm aware of what happens in the long term if you let both spacetime and matter interact. The result is quite the opposite of the flatness that I said is spacetime's "natural" state. And indeed, that's where you get entropy from.
So I won't argue if you don't like the analogy.
Thanks. That's what I was interested in hearing. Like inflector, I imagine, I was more interested in the idea behind your statement rather than if it were arguably right or wrong.

zonde
Gold Member
4) What if spacetime was springy and wanted to be straight so that the curvature itself implied some stored energy that shows up in our observations as missing mass at very low accelerations?

That's just one possible idea, but I'm sure there could be many other similar ones I've overlooked. The basic idea is that spacetime itself or a mass field might be able to store/contain mass.
I believe that idea about spacetime being springy is mainstream viewpoint.

I fancy a bit different idea. What if gravitation of matter is not property of matter but it's state. Meaning that spacetime by itself is completely passive and it requires matter in it's opposite state (antigravitating) to uncurl spacetime.

Concentration of matter in this opposite state at certain places would alter gravity quite radically. I believe that you would require far less "dark matter" if you put it at the opposite side of the "fence" i.e. outside (disk-like) gravitating system of matter in ring-like formation.

Isn't simple ionized hydrogen (protons) a candidate for dark matter?

Chalnoth
Isn't simple ionized hydrogen (protons) a candidate for dark matter?
Nope. First, ionized hydrogen interacts rather strongly with photons, so we can actually see the stuff, as long as we look for it in the right frequency range. But the real clincher is the CMB evidence.

Before the CMB was emitted, the normal matter in our universe was a plasma: everything was ionized. Such a plasma interacts rather strongly with light, so when it fell into a gravitational well, it would bounce back out again. In the CMB, we can see evidence of this bounce, and it turns out that the bounce is much smaller than would be explained by a purely matter-dominated universe: we need roughly five times as much dark matter (which doesn't bounce) as normal matter.

Chalnoth, I had not come across the term bounce before, or that dark matter doesn't bounce. Are these effects and properties speculative or strongly certain? Do you have a list of all known properties of dark matter including any speculative properties?

Chalnoth
Chalnoth, I had not come across the term bounce before, or that dark matter doesn't bounce. Are these effects and properties speculative or strongly certain? Do you have a list of all known properties of dark matter including any speculative properties?
The proper name for the physics at work here is called "Baryon Acoustic Oscillations", and yes, it is highly certain because the only physics we're talking about here is simple electromagnetic interactions. The WMAP data show beyond a reasonable doubt that around 5/6ths of the matter distribution doesn't interact electromagnetically.

WMAP data meaning the magnitude of fluctuation of the temperatures in different directions is lower than expected for more ordinary matter?

Since you are familiar with the WMAP, I have a question regarding the first year map of the CMB. I see a single blue patch and three patches of red. What is causing this, are they statistically relevant?

http://en.wikipedia.org/wiki/WMAP

Chalnoth
WMAP data meaning the magnitude of fluctuation of the temperatures in different directions is lower than expected for more ordinary matter?
It's a bit more complicated than that. Basically, this information can be gleaned from the power spectrum, which is a measure of the typical amplitude of fluctuations of a certain size on the sky. Here is a plot of this power spectrum from the seven-year WMAP data:
http://lambda.gsfc.nasa.gov/product...nyear/powspectra/images/med/dl7_f01_PPT_M.png

The plot goes from large-angle fluctuations on the left, to small-angle fluctuations on the right. The first peak is at about one degree on the sky, the second peak at about half a degree, and so on. This first peak represents potential wells that were large enough that matter had just enough time to fall into them. The second peak represents potential wells that matter had enough time to fall into, then bounce back out of. Because the dark matter doesn't bounce, this second peak is suppressed by the existence of dark matter. In fact, every even-numbered peak is reduced by the existence of dark matter.

Since you are familiar with the WMAP, I have a question regarding the first year map of the CMB. I see a single blue patch and three patches of red. What is causing this, are they statistically relevant?

http://en.wikipedia.org/wiki/WMAP
Well, there has been some discussion about the large angular scale correlations in the WMAP data, but so far none of these have been demonstrated to be anything significant. So basically, WMAP is, so far, fully-consistent with standard models of cosmology. This is the paper where the WMAP team went through these issues for the 7-year data release:
http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/anomalies/wmap_7yr_anomalies.pdf

zonde
Gold Member
The WMAP data show beyond a reasonable doubt that around 5/6ths of the matter distribution doesn't interact electromagnetically.
You can't be serious. How long is chain of untested or poorly tested assumptions behind this interpretation of observations?

I believe that search for dark matter relies on observations with much more direct interpretations than this. Like galaxy rotation curves and anomalies in gravitation lensing around colliding galaxy clusters.

Just wrote a really long reply and it got lost..

Chalnoth
You can't be serious. How long is chain of untested or poorly tested assumptions behind this interpretation of observations?
Um, the basic assumptions are:
1. General Relativity is valid.
2. There is some amount of normal matter.
3. There is some matter that doesn't interact electromagnetically.

That's about it. While there are other parameters that effect our interpretation of the CMB, they don't have much of any impact upon the measured ratio of normal matter to dark matter, because they have extremely different signatures in the CMB power spectrum.

I believe that search for dark matter relies on observations with much more direct interpretations than this. Like galaxy rotation curves and anomalies in gravitation lensing around colliding galaxy clusters.
The CMB evidence is actually some of the strongest evidence there is for the existence of dark matter. Galaxy rotation curves, for instance, can somewhat easily be explained by modified gravity. But it is extremely difficult for modified gravity to explain the CMB results without dark matter.

Chalnoth, thanks for taking the time to discuss this, I really appreciate it.

So this plot is showing power/temperature variation with angle which is not random, but which is a repeatable pattern every degree of so. This repeatable pattern shows the effect that dark matter is having on suppressing "bounce" of the ordinary matter in the plasma.

Wouldn't the plasma dark matter mixture be so finely and randomly mixed as to prevent any such patterns? Also wouldn't noise, galactic lensing, variations in space time expansion, or other radio sources cause any such pattern to be lost in noise?

The WMAP data is obviously a major cornerstone of modern cosmology and I would to be certain of the measurements and assumptions.

As an Radio engineer myself I would be concerned about the instrumentation. If the sky was a couple of orders of magnitude smoother than measured would the instrumentation still see this? Also if the angular variation in the spectrum were a couple of orders of magnitude smaller than measured would the instrumentation also see this?

Regarding other interferer RF sources I see that the galactic plane is very strong in some of the plots. Is this the effects of the galactic plane removed from other plots? However, it shows how much radio noise a single galaxy contributes to the picture. Now if you add in there a quasi infinite number of galaxy sources from every area of the sky with lensing and dispersion from dust does this not also add a measure of uncertainty to the WMAP data?

Thanks again!

Chalnoth
So this plot is showing power/temperature variation with angle which is not random, but which is a repeatable pattern every degree of so. This repeatable pattern shows the effect that dark matter is having on suppressing "bounce" of the ordinary matter in the plasma.
Actually, it is very random. It's just that the plot I posted above, the power spectrum, is the correlation between pixels separated by some angle on the sky. I don't think it would be useful to go into the mathematical details here (they're pretty simple to write out, but can be difficult to understand if you're not familiar with spherical harmonics).

But the basic idea is that each $C_\ell$ is the variance of the amplitude of a wave on the sky of a particular wavelength. The larger the variance, the greater the typical amplitude of the waves. To our knowledge, the CMB is comprised of such waves drawn with a completely random distribution, but with different mean amplitudes at different length scales.

Wouldn't the plasma dark matter mixture be so finely and randomly mixed as to prevent any such patterns? Also wouldn't noise, galactic lensing, variations in space time expansion, or other radio sources cause any such pattern to be lost in noise?
Well, the plasma itself tends to suppress the small angular scale fluctuations, because the phase transition from an opaque plasma to the transparent CMB took some finite amount of time, so when we look at the CMB we're actually looking through some finite distance of semi-transparent material. This causes the power spectrum to rather rapidly die away at $\ell$ values above a thousand or so (visible with many ground and balloon-based experiments, as well as Planck).

Instrument noise itself tends to increase the error at the higher-frequency end as well. In the WMAP data, you can see this effect blowing up the noise around $\ell=1000$ or so. Planck is expected to be able to measure the power spectrum at least out to $\ell=2000$, and many ground-based and balloon-borne experiments go to even higher resolutions, but the ground-based and balloon-borne experiments only see a fraction of the sky, and as a result aren't able to as effectively measure the variance of the fluctuations, because they aren't looking at as many fluctuations as WMAP or Planck.

Galactic lensing and radio sources also provide significant effects, but again mostly at small angular scales. These effects need to be corrected for to get accurate estimates of the CMB at small angular scales.

Variations in space-time expansion basically have zero effect on the CMB signal, as for the most part we only see the integrated amount of expansion, not how much it has varied over time. Still, there is a weak signal at the large angular scale range due to the space-time expansion effecting the growth of structure. This is known as the Integrated Sachs-Wolfe Effect.

As an Radio engineer myself I would be concerned about the instrumentation. If the sky was a couple of orders of magnitude smoother than measured would the instrumentation still see this? Also if the angular variation in the spectrum were a couple of orders of magnitude smaller than measured would the instrumentation also see this?
Well, the WMAP instrument is a differencing instrument. It is actually two telescopes sitting back-to-back looking at two different directions of the sky at any given time. It then takes the difference between the signal from corresponding radiometers in each telescope. The differencing allows WMAP to very accurately measure small differences in temperature on the sky without any active cooling, though it is completely insensitive to the absolute temperature. Obviously if the level of variations was substantially lower, WMAP wouldn't have been able to measure them nearly as well. But we knew by the time WMAP launched what the level of variations was, because they were measured by COBE.

Planck, by the way, uses active cooling. This allows it to get much better signal-to-noise for its radiometers, and also allows it to have bolometers to capture higher-frequency signals.

Regarding other interferer RF sources I see that the galactic plane is very strong in some of the plots. Is this the effects of the galactic plane removed from other plots? However, it shows how much radio noise a single galaxy contributes to the picture. Now if you add in there a quasi infinite number of galaxy sources from every area of the sky with lensing and dispersion from dust does this not also add a measure of uncertainty to the WMAP data?

Thanks again!
I believe the majority of the signal along the galactic plane on the WMAP data is from synchrotron emission from our galaxy. This stems from electrons emitting radiation as they are accelerated by the magnetic fields around the galaxy. I don't think the stars themselves emit very much radiation at these wavelengths, so they aren't much of a concern. The cold dust in the galaxy also emits radiation, but most of that is at slightly higher frequencies than WMAP is sensitive to.

However, Planck is sensitive to these frequencies, and gets a very nice picture of the dust, as you can see in this image. This image was composed from multiple Planck channels, with red coming from the lower-frequency channels, and blue coming from the higher-frequency channels. So the dust you can see as blue emission, and the synchrotron looks more red (note that due to the particular frequency scaling chosen, the CMB itself looks very red in this image). There are a number of point sources visible as well, but not as many as you might think.