# Dark matter, dark energy

1. Aug 11, 2004

### salsero

Could anybody list (and write a few explanatory words about) the various theories which explain the origin of the dark matter and the dark energy?

Thanks!

2. Aug 11, 2004

### Mike2

I suppose (not a claim, yet) that one explanation for accelerated expansion might be because in general relativity, mass may decrease with distance from other mass. If so, then lighter particles would travel faster.

3. Aug 11, 2004

### mathman

The origins of dark matter and dark energy are still in the guessing stage, especially since physicists don't really know what either of them are, although there are several theories about dark matter and at least two about dark energy. Presumably everything started with the big bang.

4. Aug 11, 2004

### Chronos

Dark matter is matter that we cannot see, but, is required to explain the gravitational force that holds galactic clusters together as well as the rotational speed of individual galaxies. We know some of the dark matter is ordinary matter, such as planets and other stuff not hot enough to glow [emit detectable EM radiation]. However, there does not appear to be nearly enough of these type objects to account for the unseen matter needed [which is much more than the stuff we can see]. Scientists have therefore decided much, if not most of the missing material is not ordinary, baryonic, matter, but another form called non-barionic matter. Non-barionic matter has gravity, just like the regular stuff, but is barely noticeable because it otherwise almost completely ignores ordinary matter. A neutrino is an example of a non-baryonic particle [although we aren't yet sure if it has 'mass', hence gravitaty] we know exists. In fact we knew they must exist long before we actually saw one since they can pass through millions of miles of lead without hitting anything [you can't see them unless they hit something]. But, even if they do have mass, there are not nearly enough of them to account for the missing dark matter. So, scientists think there is some yet undiscovered type or types of non-baryonic particles out there still waiting to be discovered.

Dark energy is the force causing the universe to expand. Einstein realize long ago that something was preventing gravity from causing the universe to simply collapse into a gigantic big black hole. He called it the 'cosmological constant' and added just enough of it to his equations to make the universe stand still. Later on, scientists discovered the universe was actually expanding [well, most of them think it is]. Einstein decided his idea was all wrong and called it his biggest mistake. It now looks like he was right all along, he just didn't know how much 'cosmological constant' was needed to make the universe look the way we now know it looks.

For more detail from reputable authorities on these matters, see

http://map.gsfc.nasa.gov/m_uni/uni_101matter.html
http://www.nasa.gov/missions/science/f_dkenergy.html

5. Aug 11, 2004

### meteor

Supersymmetric theories propose the existance of a particle called the Lightest Supersymmetric Particle. This is a WIMP that is a candidate to dark matter, and has the property of self-annihilation (two LSP in contact would annihilate). This paper proposes that part of the flux of gamma rays coming from the center of the galaxy is due to the annihilation of LSPs
http://xxx.lanl.gov/abs/astro-ph/0408192
"TeV $\gamma$-radiation from Dark Matter annihilation in the Galactic center"

Last edited: Aug 11, 2004
6. Aug 11, 2004

I read somewhere that the cause of dark matter might be the k-meson. Not shure if it is true, but it was a theory.

7. Aug 11, 2004

### Mike2

If mass were lighter in less dense surroundings, e.g. the edges of a galaxy, this would be the same as the gravitational constant of the universe being smaller and particles would orbit faster at the edges and explain the strange spin of galaxies, right?

8. Aug 12, 2004

### loke137

Chronos is wrong on what he said. Einstein postulated the cosmological constant as a repair to his theory that preddicted a expanding universe (not a contracting one, although the theory thus predict that also).

9. Aug 12, 2004

### Chronos

This is a quote taken from an article by Dr. Sten Odenwald in the April 1991 issue of 'Sky and Telescope'

"In 1917, Albert Einstein tried to use his newly developed theory of general relativity to describe the shape and evolution of the universe. The prevailing idea at the time was that the universe was static and unchanging. Einstein had fully expected general relativity to support this view, but, surprisingly, it did not. The inexorable force of gravity pulling on every speck of matter demanded that the universe collapse under its own weight.

His remedy for this dilemma was to add a new 'antigravity' term to his original equations. It enabled his mathematical universe to appear as permanent and invariable as the real one. This term, usually written as an uppercase Greek lambda, is called the 'cosmological constant'. It has exactly the same value everywhere in the universe, delicately chosen to offset the tendency toward gravitational collapse at every point in space."

10. Aug 12, 2004

### Garth

Consider: GR is a gravitational theory that accurately predicts solar system orbits (geodesics) and laboratory experiments. When solved for the cosmological case, when the universe is assumed to be homogeneous and isotropic, then it predicted: the expanding universe, the primordial relative abundance of the elements, 3/4 hydrogen 1/4 helium and very little of anything else, and the microwave background. So it seems well established.
However there were three cosmological problems, the density problem, the horizon problem and the smoothness problem. These all arose because the universe was predicted by GR to be decelerating in its expansion.
So it needed a fix. That fix was provided by the theory of inflation. At a very stage of its history the universe was said to suffer a phase of enormous acceleration in its expansion, which solved the three problems above.
However inflation required the universe to be virtually 'flat' and have a specific density, the critical density.
The next problem was the density of observed matter, and the density of ordinary baryonic matter allowed by the Big Bang nucleo-synthesis reactions, only came to about 4% of this critical density. So there was a lot (96% of the entire universe) of 'missing matter'.
Studies of the rotation rates of galaxies, the orbital velocities of galactic clusters, and the gravitational lensing (by nearer galaxies) of distant quasars, indicated that the universe had a density of about 30% of the critical density. So Dark Matter of unknown composition - not ordinary baryonic matter - was invented to fill the gap 4 - 30 %.
Next, the observation of supernovae in distant galaxies indicated the universe must have accelerated, at least in recent history, rather than decelerate.
Finally, analysis, under GR, of the WMAP data of the microwave background theory indicated the universe was flat after all. Therefore roughly 70% of the universe had to be of some further unknown substance. Dark energy was invented to fill this gap and it could conveniently, perhaps, explain also why the universe is accelerating. Hence we arrive at the present concordance model of 4% ordinary matter, 23% dark matter and 73% dark energy.
So the orginal theory, GR, only fits the facts with the introduction, or invention, of inflation, dark matter and dark energy. Indeed most (96%) of the universe is of unknown composition. These three constructs are all undiscovered by laboratory physics even after several decades of intensive laboratory research!

Remember the Ptolemaic theory? When Galileo confronted it it was a successful theory, successful because every time a problem had arisen with the basic paradigm they added another epicycle to make the theory fit the data.
Perhaps Inflation, dark matter and dark energy are just modern examples of 'adding extra epicycles' and, just as in Galileo's time, maybe, we are going to see a paradigm shift?

Last edited: Aug 12, 2004
11. Aug 13, 2004

### Mike2

Could Dark Energy and Dark Matter both be due to gravitational effects on rest mass predicted by General Relativity that we have not accounted for yet?

There is a gravitational redshift for photons coming out of a gravitational well. This is due to time being stretched as the gravitational field weakens. Now if matter is the result of vibrational modes of strings or membranes, then even the rest mass of particles would be affected by gravitational effects as well.

I haven't actually done the calculations, but it seems in principle that if mass gets lighter as the universe becomes less dense with expansion, then this would account for accelerated expansion - the particles are getting lighter so their velocity increase as required by conservation of energy.

Also, if mass gets lighter at the edges of galaxies, then photons would be less redshifted towards the edges. This would make it appear as if they were moving faster than otherwise. So the faster velocities at the edges may not be due to dark matter, but due to less gravitational redshifting of photons.

And so, dark energy and dark matter may be just an as yet unaccounted for affect of gravity on rest mass.

12. Aug 13, 2004

### Mike2

Wouldn't this also have a tendency to prove Stringtheory - that all particles, including massive ones, are extended objects that vibrate with frequency? How else could time dialations affect the mass of a particle?

13. Aug 13, 2004

### marcus

this is a great condensed history and summary of the concordance model.
I cant even find a nit here to disagree with if I wanted to, which I dont.
the comparison between epicycles and the postulated DM and DE is apt.
and and as a small mini-essay it's efficiently written.

BTW Smolin's 3 talks at the WS-2004 symposium seem to verge on considering Lambda to be something else besides the effect of a postulated dark energy density----something more like a new invariant length scale L
on the order of 9.5 billion lightyears.

After all Lambda is a curvature so it is an inverse length squared. So the square root of 1/Lambda is a length L. And that length scale could have something to do with what spacetime is, might be intrinsic to its geometry, and yet not correspond to some real form of energy uniformly distributed in space and time. I guess it is just a difference in nuance (you could still associate an energy density rho-sub-Lambda with it so nothing changes, more of an attitudinal shift.)

I will get a link to the WS-2004 symposium, it has the lecture slides available for download
http://ws2004.ift.uni.wroc.pl/html.html

Last edited: Aug 13, 2004
14. Aug 13, 2004

### marcus

Hi Mike, you have a chain of 4 posts here---#2, #7, #11, and #12.
Each subsequent one of your posts quotes the one preceding it. so
it is like a chain of reasoning.
The initial premises (here in post #2) are in error.
In GR mass does not decrease with distance from other mass.
Lighter particles do not necessarily travel faster.
If you know of some GR essay on the web which says these things

The next post (#7) is likewise in error, you say
you have it backwards. If either the gravitational constant G or particle mass declined with distance from center, then the particles near the edges would orbit more slowly than one would expect using a strait Newtonian model---but in fact we observe the opposite: circular orbit speeds are faster than what one would reckon naively.

So your declining mass or declining G hypothesis predicts the opposite of what is observed.

either one must modify the Newtonian law so that circular orbit speed (and centripetal acceleration) decline less abruptly with distance from center
so that there is more acceleration towards center than Newtons law predicts (see also the pioneer anomaly)

or one must postulate the presence of additional mass in the galaxy which we do not see----to be the cause of the stronger acceleration towards center than would otherwise be expected at such great distances.

Mike I dont believe it would be useful to comment more on your posts specifically, or to respond to further ones along these lines.
But since you raise the issue of galactic rotation curves, which are currently a puzzle, I will try to focus on the issue of and see if I can find out anything of general interest

Last edited: Aug 13, 2004
15. Aug 13, 2004

### Mike2

Thanks for your reply, marcus. Yes, this is just a theory that I seem to have stumbled upon. And I'm still feeling my way through it. I plan to specifically study SR and GR soon in more detail. So maybe I'll be able to put it in the language of mathematics soon. Though I would think that this would be easy for those skilled in the art.

Yes, I've argued with others about the "invariance of mass" in GR. Mass is invariant in SR but not in GR. From Rober M. Wald's book General Relativity, page 61, "In special relativity, energy is recongnized to be the "time component" of the 4-vector pu... Since the spacetime metric, Nab, is flat and thus parallel transport is path independent, we may define the energy of the particle as measure by an observer who is not present at the site of the particle to be the energy measured by the observer who is at the site of the particle and has 4-velocity parallel to that of the distant observer."

However, on page 68 concerning gravitational fields, Wald writes, "Because spacetime is curved, there is no well defined notion of vectors at different points being parallel; parallel transport is curve dependent. Thus, there is no natural "global family" of inertial observers, and a given observer cannot, in general, define the energy of a distant particle." So the question is how do we calculate the mass of distant objects.

There certainly is a gravitational effect on the frequency of photons. This is well established. It is considered in cosmology when determining the velocities of distant galaxy cluster from measured redshifts. However, it does not seem to be common to consider the effect of the gravitational field of the entire universe when looking at distant redshifts. The redshift seems to be attributed entirely to velocity and none due to the fact that the universe was in a deeper gravitational well when those photons were released. However, I see no reason not to consider the gravity of the entire universe. And I don't seem to be the only one concerned about this.

http://www.astronomycafe.net/anthol/expan.html
"It is tempting to refer to cosmological redshifts as Doppler shifts. This choice of interpretation has in the years since Hubble's work led to an unfortunate misunderstanding of big bang cosmology, obscurring one of its most mysterious beauties. As noted with a hint of frustration by cosmologists such as Steven Weinberg and Jaylant Narlikar and John Wheeler, "The frequency of light is also affected by the gravitational field of the universe, and it is neither useful nor strictly correct to interpret the frequency shifts of light...in terms of the special relativistic Doppler effect."

It's been explained to me that the gravitational redshift is due to time distortions caused by gravity. Fine, then it would seem that all objects that vibrate with a given frequency would also be affected by this gravitational effect. I've seen how in Superstring theory, the mass of strings are dependent on frequency of quantum oscillators. So it would seem that mass would have to be affected by this gravitational effect as well.

The possiblity of gravitational fields affecting mass as well as photons is intriquing. And valid or not, it seems worthy of some mathematical proof before rejecting or accepting in full. But just on an intuitive level, it seems to at least point in a direction that might explain dark matter and dark energy. Though I don't know about exact values at this point.

If may very well explain the acceleration of expansion. If matter becomes lighter with expansion, then even in its local frame of reference, it would seem that galaxies would have to speed up to conserve energy. This would be just like a rocket ship able to move faster because it looses the mass of fuel.

It may very well explain the early deceleration of the universe if the stronger redshift of the early expansion is due more to gravity then than now. Could it be that the universe has only been accelerating since its birth? In other words, could the change in gravitational redshift be more dramatic in the early stages than in the latter stages. This might make it appear as if the early universe were decelerating when it was actually accelerating.

It may explain the strange spin of galaxies where the outer regions seem to go faster than they should. Could this be due to matter being lighter in the less dense outer regions? Or perhaps this can be explained by the fact that galaxies were heavier in the early universe. I'm not sure how that would affect orbital speeds. I suppose a computer simulation is in order. Is this strange orbital spin just as profound in Andromida as for very distant/early galaxies?

It might also explain the greater gravitational lensing affect than what can be attributed by accountable matter. The mass of early objects was greater then than now. Is gravitational lensing more profound in the earlier, more dense universe?

I do appreciate your input, thanks.

16. Aug 16, 2004

### Mike2

At this point, I don't know if I'm talking about changes in G. But I do seem to be talking about mass decreasing with less density. And the Newtonian equations have mass as linear on both sides of the equations. So it would seem the change on one side equals the change on the other so in Newtonian physics it doesn't seem to matter if orbital mass changes. But I'm not sure that Newtonian physics is valid on a galactic scale.

17. Aug 17, 2004

### Chronos

In a sense, you are assigning a preferred reference frame to gravitational force over distance. If this were true, it would be easily observed within the solar system [proportionately less gravity effects from more distant planets]. But it is not true. Were that the case, Pioneer and the Cassini probe would have been way off course [not to mention missions to the moon]. The same effect would be also be obvious in particle accelerators.

18. Aug 20, 2004

### Garth

But they are! The Pioneer anomaly is that both spacecraft exhibit an unexplained sunwards acceleration roughly equal to cH.

19. Aug 21, 2004

### Chronos

While of theoretical importance, I resist calling one part in a billion significant compared to measurement error.

Last edited: Aug 21, 2004
20. Aug 21, 2004

### Mike2

I don't think there is any choice, really. If mass is due to vibrations, then the frequency of those vibrations are just as susceptable to gravitational redshifting as are photons.

Consider MOND (for modified dynamics). See:
http://www.astro.umd.edu/~ssm/mond/faq.html