In the bullet cluster, the dark matter did not collide or heat up, as it is collisionless. It passed right through the collision. The baryonic gases did however collide, heat up and get stuck in the middle. This is how the hot gases became separated from the dark matter.

Dark matter does not collide with itself or normal matter. It does not interact electromagnetically. It only interacts gravitationally.

Light is redshifted as it climbs out of a gravity well, but blueshifted as it enters. You seem to be speculating that light is only redshifted by gravity as it passes through space. Unless I'm misunderstanding something, in order for what we observe to be due to gravitational redshift as you propose, the universe must consist of a giant gravity well with the Earth sitting atop a rather large "gravity hump". In other words, Earth must for some reason be at a higher gravitational potential than pretty much everything else we observe in the distant universe. That seems unlikely, to say the least.

Thanks Tassel, I think I'm beginning to understand this. I watched the video referenced in article on Dark Matter:
http://chandra.harvard.edu/photo/200...dia/bullet.mpg And also read Wiki's at: http://en.wikipedia.org/wiki/Bullet_cluster , but I must admit I am stumped, if what they say is correct. Not everyone agrees with this, as mentioned here: Bullet Cluster Shoots Down Big Bang
http://www.thunderbolts.info/tpod/20...letcluster.htm , where it says: "According to the authors of the Chandra X-Ray Observatory website, the galactic cluster imaged above "was formed after the collision of two large clusters of galaxies, the most energetic event known in the universe since the Big Bang." Though the announcement by the Chandra team never uses the words "theory," "hypothesis," or "interpretation," its every sentence rests on a jumble of assumptions, from supposed galactic "collisions" to wildly conjectural "gravitational lensing," all wrapped around the discredited notion that redshift is a reliable measure of velocity and distance. The capper is the announcement appearing in numerous scientific media that the image "proves the existence of dark matter." Also, per NASA: http://www.thunderbolts.info/webnews/dark_matter.htm , the argument for non-baryonic dark matter seems pretty solid. So for now, I'm stumped.

I agree that light will blueshift into a gravity well, no matter how small it is, i.e. HI gas in deep space, and redshift out of the well, so it would appear a null result, that gravitational redshift is not the right term for why light would redshift while traveling through very large distance in cold deep intergalactic space, at least not per this explanation. The question that remains in my mind is whether it may be possible for light to 'lose' something of its energy to the (hypothetically) high G of HI gas in deep space? Of course, this starts to look an awful lot like 'tired light' theory, which I believe had been soundly disproven. Alas, no new ideas from this end, so remain stumped for now, and must accept non-baryonic dark matter, at least in principle.

__________________
Credibility is simply incredible... sometimes even to me.
disclaimer

Oh boy... that was quite a read. Their analysis relies on quasars not being at cosmological distances. It also requires that the galaxy clusters are not actually galaxy clusters, but "fragments of a quasar".

I agree this observational analysis is suspicious, since it requires discounting much of what is observed as being something other than what astronomers had come to accept. My assumption is that original observations are correct, that gravtiational lensing is working as it is supposed to, so no need to look behind the props for what's holding up the dark matter theory.

I do not want to drift too far from Peter Wilson's OP, but there is something in NASA's press release, http://www.nasa.gov/home/hqnews/2006...rk_Matter.html , that did catch my attention: BTW, the "some scientists" is not me! But it seems somebody out there is at least considering the possibility. It also says: So there is real evidence this dark matter does exist, whether or not we understand what it is, it must be there in some form. What I find difficult to understand, and why I am stumped here, is this: However, I do not see "hot gas was the most massive component in the cluster" if by 'hot' is means 'lighter' mass; rather, it would be cold gas that would be 'heaviest' in terms of mass, dark matter gravitationally speaking. So there's still stuff to be considered here, as to why 'dark matter' interacts gravitationally only, but not electromagnetically/thermally with other matter. If only interacting gravitationally, why is dark matter NOT stopped dead in its tracks? If they interact gravitationally, there should be some sort of measurable motion and thermal followthrough, I would think. But I can't claim to really know much about this at this point.

Some puzzling things (to me) are as follows:

1. Why luminous baryonic matter can pass by each other in galactic collisions without at least some stars blowing up in the process?

2. Why 'dark matter', warm or cold, not swept up gravitationally by passing galactic masses?

3. Does non-baryonic dark matter interact with other matter in any way besides gravitationally; why does it not respond to normal baryonic matter, except gravitationally? No thermal residue from interactions?

4. Why galactic gasses colliding produce thermal energy, but 'dark matter' colliding produces none?

So I really don't know if we know everything there is to know about the Bullet Cluster to come to a full conclusion. So I remain stumped, but will look into it some more, since this galactic collision is interesting. If 'cold dark matter' and 'warm dark matter' can interact with ordinary matter only gravitationally, gravitational considerations (such as "some scientists" mentioned above) is not entirely off the table, even MOND may still have merrit of sorts. Greater gravity in the vast cold reaches of space may or may not cause light redshift, but it might be responsible for flat rotation curves, so hot gases and galaxies do not fly apart, an effect identical to our illusive 'dark matter'. Perhaps we still do not know well enough what we saw in the 100 hours of Chandra X-ray telescopy to really understand what we saw?

__________________
Credibility is simply incredible... sometimes even to me.
disclaimer

It's far simpler then that!

The universe is a 5d anti de Sitter space with a boundary of 4d de Sitter space.

That's the universe in a nut shell!! It could easily fit in your pocket.

(acc. to S. Hawking)

http://online.itp.ucsb.edu/online/co...ng2/oh/19.html

Yes. Basically there are observations that suggest that in galaxies and galaxy clusters there is either more matter there that we can't see (i.e. "dark matter") or gravity does not work the same on scales of galaxies and galaxy clusters. Both ideas are being pursued by professional astronomers, though "dark matter" has more proponents.

No. By "hot" they mean higher excitation. For the gas to be emitting in the X-rays it has to be hot indeed!


Yes. That is a postulated characteristic of "dark matter". In order to match the observations this dark matter must have this property. That puts constraints on what the dark matter could be. I.e. it can't be cold molecular gas for example.

The cross-section for interaction between stars in a "collision" like this is very very small. Imagine two groups of people shooting bullets at each other. The bullet swarms would pass through each other with very very few bullets colliding. Or think of two swarms of flies flying past each other.

The dark matter passes through just like the stars do. Without collisions.

The gas is not a collisionless system, and is subject to things like drag and ram pressure. That's why the gas is left in the middle of the two clusters and heated up.

I have half a notion to ask you all to not turn this into a dark matter thread...but all it is very interesting I have certain soft-spot in my heart for DM, because I used to be a DM skeptic, but came to believe in it after joining BAUT

Nonetheless...returning to the OP:

Conceptually there is a vast gulf between Newtonian gravity and GR. In Newtonian G, space is fixed-and-absolute; in GR, the velocity of light is fixed-and-absolute

Also, in Newtonian G, a body’s mass is fixed-and-absolute, whereas in GR its mass depends on its total energy, potential + kinetic, divided by c^2.

Numerically, however, there is only a slight difference: Newton’s theory of gravity predicts that a dark, 2-body system is stable; GR predicts a dark, 2-body system will slowly decay (contract) due to the radiation of gravitational waves/energy. Since for “typical” 2-body systems the decay under GR will take “billions and billions” of years, the practical difference between the predictions of GR and those of Newtonian are nil.

For a thought experiment, let us imagine a 1 kg. test-mass, at sea-level on earth, coupled to a lifting device driven by solar energy. After x amount of time, solar energy has lifted the 1 kg. by an elevation of y-meters. It has gained potential energy proportional to g*y (earth’s gravitational field times height). From the mass-energy equivalence, we conclude the 1.0 kg test-mass has increased its rest-mass by an amount equal to g*y/c^2, which we will call z. Z is a small fraction of kilogram…very small.

There is a problem with this reckoning, however. We have assumed that all the energy added to the system (the solar energy used to lift the mass) is to be found in the test-mass that was raised. We have done the problem as-if the earth was fixed, and the mass was raised. But equivalently, we could, in principle, fix the test-mass and move the earth. Work is force-times-distance, so it doesn’t matter which way we figure it, we get the same answer.

This creates a conundrum: where does the increase in energy appear? In the test-mass, or the earth? The “system” gains energy, but which part of it?

GR gets around this by saying the energy is neither in the earth, nor the test-mass, but in the space-between them. Since this energy is associated with force, it takes on the name of stress-energy tensor. Returning to the “real world,” we have the earth, the moon, and the potential energy associated with their separation. This potential-energy is stored in the stress-energy tensor. So the space between the earth and the moon is not a complete vacuum…the stress-energy tensor has a tiny, non-zero value there.

Einstein wondered what the stress-energy tensor would be if there was no mass present. He assumed it would be zero, but realized it did not necessarily have to be. Hence lambda was put into the equations…in the event the stress-energy tensor of “space,” in-and-of-itself, turns out to have a non-zero value.

While all of this is quite mind-bending, and provides fodder for endless Q&A and lots of confusion, when you crunch the numbers, the difference between Newtonian & GR is very slight. Whether you calculate the energy involved using simplistic Newtonian force-times-distance, or you calculate it using the stress-energy tensor of GR, the numbers are almost the same.

Finally, let us look at the earth-moon system again, carefully. The system is expanding…but it is not gaining energy. What is happening is that earth’s rotational energy is being converted into gravitational energy…but the loss of one is equal to the gain of the other. The distance to the moon is increasing, but the total energy of the earth-moon system is not, because the earth is losing rotational energy as fast as the moon is gaining gravitational energy.

What I am suggesting is the same sort of thing is happening on cosmic scale: yes, the distance between everything (remote) is increasing, but not the total energy of the system. This is because local gravitational energy is being lost faster than remote gravitational energy is being gained.

__________________
PW -- Plant Whisperer

How often do I have to repeat the word "collisionless"?

If the dark matter were simply cold hydrogen, we should see at least its 21 cm line. Also, it would not be collisionless.


What the word says: the stuff does not collide. Neither with itself nor with other matter. You would be hard pressed to explain why hydrogen atoms should not collide with each other or with other matter!


But the point is that they are not colliding! That's what e. g. the bullet cluster shows!



Err, no. The thread is about the question if expansion on large scale and contraction at small scales is possible simultaneously. And for that to be possible, you need neither dark matter nor dark energy. Ordinary, old-fashioned BBT will do quite well (not quantitatively - for that you'd need dark matter and dark energy - but qualitatively, it works nicely).


Try explaining the results of Tolman tests. Try explaining the CMBR. Try explaining why the ages of the oldest stars we know match quite nicely with the calculations for the age of the universe. etc.

Additionally, try explaining how all redshift could be caused by gravitation, although the studies of the integrated Sachs-Wolfe effect show that we are able to distinguish between gravitational redshift and redshift caused by expansion.


So General Relativity is wrong, too?


Why don't these "hot energy sources" radiate, i. e. why are they "dark"?



Oh, thanks for admitting that your idea has a fatal flaw.


Cosmologists don't attribute the redshift to a Doppler effect usually.


Feel free to explain both the SN data and the WMAP observations without dark energy.


What you ignore that the observational results in this study were predicted quantitatively by the dark matter hypothesis. Feel free to present such a quantitative prediction from your "higher G" hypothesis...


It is even more meaningless, since so far, you have not even shown that your idea can actually explain the already available data quantitatively. As long as you don't show that, I don't bother to look at your web page.

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

These "some scientists" would e. g. be Milgrom and Bekenstein; see e. g. here:
http://www.talkorigins.org/faqs/astr...bang.html#mond


No, "hot" does not mean "lighter mass", but simply "hot": i. e. at higher temperature.


But we see the hot gas directly: by its X-ray emissions.


Interacting thermally only works if it can collide with the other matter, or give of electromagnetic radiation. And collisions are also only possible essentially if it interacts electromagnetically with other matter.

This lack of interaction is easily explained if dark matter consists, as has been hypothesized for at least a decade now, of "neutralinos" (the lightest supersymmetric partner particles).


Why should dark matter be "stopped" then? Both its inertia and the gravity of its host galaxy "carries" it along.

Have you ever compared the size of stars to the distances between them?
A good analogy is marbles with distances of some hundred kilometers between them! A collision between them is so improbable, it simply won't happen in most cases.

And even if it happened, I don't know if that would be visible over such distances. (would that lead to a large explosion like a supernova? I don't know)

Quote:

Originally Posted by nutant gene 71 2. Why 'dark matter', warm or cold, not swept up gravitationally by passing galactic masses?

I'd say that partly, it is. But the bulk of the matter remains with the host galaxy. Compare to the luminous matter in other pictures of colliding galaxies: some matter is usually torn from the galaxies, but most remains within.

Quote:

Originally Posted by nutant gene 71 3. Does non-baryonic dark matter interact with other matter in any way besides gravitationally; why does it not respond to normal baryonic matter, except gravitationally? No thermal residue from interactions?

See above. (it's not clear to me what you mean with "thermal residue" here)


[QUOTE=nutant gene 71;908355]
4. Why galactic gasses colliding produce thermal energy, but 'dark matter' colliding produces none?

Dark matter doesn't collide.

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

Please point out where exactly GR does say that. Quote, please.


Energy is essentially always associated with force, hence I doubt that this is the source of that term. Again: quote, please.


The stress-energy tensor is simply a mathematical object which describes how energy and pressure are distributed in space and time. Saying that energy is "stored" in it makes little sense.


For your simple example above, right. But obviously wrong if one considers larger region of the universe!


As I've said already several times: feel free to address the actual data based on that idea.

And, what I also have pointed out already several times: in GR, the term "gravitational energy" isn't even well defined.

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

No, the misunderstanding is on your side. When I talked about quantifying your ideas, I meant that you should address the actual data quantitatively. I. e.: explain the large-scale structures observed in the universe (the distribution of galaxy (super)clusters, filaments and voids, their masses etc.), the SN data, etc. The usual BBT can do all that - with great success.


It is quantitative, but it has little to nothing to do with actually quantitatively addressing the data. It is merely calculating some numbers without comparing how they can explain the actual observations.

Or again in other words: the scientific method usually works in this way:
(1) Make some observations.
(2) Develop a hypothesis based on these observations. (this hypothesis will usually include already some quantitative calculations).
(3) Make some quantitative, testable predictions based on this hypothesis.
(4) Test these predictions by comparing them to other observations; if necessary, do new observations.

As far as I can see, you are at step (2) now. What I am asking for are the steps (3) and (4).


"rather sloppily" is not enough. There are many alternative models out there who can match at least part of the data far better than "rather sloppily".


Thanks for admitting that.

Well, if you don't know that, then look up the data and compare!


So far, you have not presented a "reasonable fit". Merely some numbers thrown around which look vaguely similar to some other numbers.


No, it lies exactly in your failure to address the data quantitatively so far. In your hand-wavings: "well, it looks about right to me...."

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

Thanks Bjoern, I can only address a couple of your comments at this time, just for clarification, but don’t know enough to comment on all. In effect, I agree ‘dark matter’ as you describe, and what is currently acceptable, that it is “collisionless” as now assumed. But there are a couple of posts that leave open other possibilities to consider. These are:

Could possibly ‘cold’ hydrogen molecules in space, say at cold background temperatures of about 2.3K, still exist there without registering in our instruments at the 21 cm line? I don’t know for certain this is possible, and if impossible, then end of conversation. However, if this is possible, then we may be calling ‘dark matter’ what is simply very cold hydrogen molecules in deep space. In effect, is there some ‘cut off’ temperature where deep space hydrogen no longer reads at 21 cm?

Which leads to this question:
Why could this “dark collisionless matter” be of necessity collisionless? If the hypothesis that it is cold hydrogen (not exotic dark matter), then the two interacting galaxies would also interact thermally, which would then be the ‘hot gases’ registered, what was left behind this interaction. We now postulate “collisionless” except for gravitationally, so see the results of Bullet Cluster one way; but if their interaction was not collisionless, but the cold hydrogen continues along with their host galaxies, while the newly heated up hydrogen (what Chandra X-ray reads) remains behind, like a puff of dust, then it would mean something other than our postulated collisionless dark matter. Remember we never saw ‘dark matter’ up close anywhere, so only surmise its existence at a distance, from weak gravitational lensing et al. However, for this latter scenario to be true, something else must be happening in deep space from what we observe on Earth. The hypothesis presented, as an alternative to current theory, is that ‘heated matter’ acts differently from ‘cold matter’ regardless of whether it is a gas cloud of hydrogen or any other matter. The masses respond differently: hot and near stars masses are ‘lighter’, while cold and far from stars, the masses are ‘heavier’ gravitationally, which mimics ‘dark matter’ now postulated. In effect, this is a case of momentum, p = mv, becoming the dominant theme, that momentum behavior for masses of different (per Equivalence) G environments behave differently.

I know this is WAAAAY OFF topic, and in truth, way off from current thinking in cosmology in general. But could this not be a parallel theory that shows the same observational results, if from radically different causes, where the effects are the same, but cause is not? Viz.: two galaxies interact, the very far apart hot stars miss each other, but the extremely vast stretches of cold space hydrogen between these stars collide, somewhat, to form 'hot gases' that lose momentum and stay behind, while the remaining unheated hydrogen gases continue on their old momentum, and keep up with the galaxy as a whole. Can this be possible, or is it simply impossible?

Finally, if I might possibly touch on this related post as well:
This relates back to my 21 cm question above, that perhaps ‘cold hydrogen’ does not radiate enough energy for our instruments to pick up, but it might actually really still be there.

The point here is that though we have interpreted observational data assuming Dark Matter exists, it may not exist, but be nothing other than ordinary baryonic matter at very cold temperatures, so we cannot read it. If this were so (and I don’t know it is so only pose it as a possibility to expand on the idea of ‘duality’), then here is a dual parallel mechanism that acts like Dark Matter, even MOND like, but is not what we concluded, by assuming that it is non-radiating and collisionless exotic matter. Now, this leads into then the next idea that this cold stuff is causing light redshift (in some still undetermined manner) which gives us the SN readings et al, all of which we interpreted as ‘expanding space’, which it may not be.

Personally, I like a simpler universe rather than one made up of untested assumptions about what is happening at great distances from us, so we should work with what we do know what is observed here, except for one thing: we maybe do not have the “universal constant” gravity part of it right. However, until such time that we can test falsifiably for this, this idea is still off the table as theory, but merely speculative as hypothesis, for now. Can these two dual worlds of what we observe at great distances coexist? If yes, then the universe is simpler, and we look for cause without Dark Matter; but if not, then we are stuck with what we’ve got. Okay with me, either way, I'm game.

[Ps: I did read Bjoern's article on the Big Bang: http://www.talkorigins.org/faqs/astr...g.html#bigbang , where he describes in laymnan's language how BBT is understood today, as an extension of GR. It is the math, or geometry of space-time as it affects expanding space and cold matter, that defines how we understand the Big Bang, especially based upon an assumption of the 'Cosmological Principle'. The supporting evidence is observed redshift, observed gravitational bending of light near large mass, and the 'homogenous and isotropic' nature of the observed universe, at any distance. My alternate hypothetical universe is the same, except that nature has an additional factor of variable gravitational 'constant' G, where it is not homogenous and isotropic, which seems to invalidate, of necessity, what the geometry and mathematics of GR describes. In effect, the universal space, those very vast distances in between observed matter is 'isotropic and homogenous' at a very different G from what we know form Earth's observations. There lies the great difference of how we think of the observed universe, and where the possibility (only a possibility sans actual evidence of differing G) yields a radically different interpretation of the same data observed. Can a variable G disprove GR? Perhaps, but that is not the main issue. Rather, the issue is that if a parallel interpretation of the universe exists, meaning we see the same observational data from two different perspectives, then there must also be some physical proof that G is not a universal constant. If such proof is found, even if at close quarters within our solar system (ala Pioneer Anomaly), then the necessary component of GR, that G is a universal constant, becomes a contested idea. However, what such a hypothesis would yield, if G is found variable, is at this point still unknown, and unknowable, except to say that the Cosmological Principle would probably have to be reconsidered. That is potentially very exciting, or it is sheer nonesense. I'm only looking, not yet ready to buy anything. ]

__________________
Credibility is simply incredible... sometimes even to me.
disclaimer

Although it gets into quantum physics (and liable to be corrected by someone who knows more than me), the 21cm is from the radiation of the hydrogen ground state. There are two possible states for the electron in hydrogen. Spin up and spin down. If the electron spin state is the same as the proton spin state the combination has a slightly higher energy level than when spins are not in the same state (it's called parallel for the same states and anti parallel for the opposite states). When the spin states are parallel, there is a chance for the electron to change to an antiparallel configuration, emitting a photon with a 21cm wavelength. When that change happens, you get the radiation line. You wondered if the temperature was low enough, if the hydrogen wouldn't change and thus be invisible. If the temperature is low enough, there would be a absorption line (instead of the radiation line. Which we could detect because of the different energy)where any energy (photons from distant galaxies, the CMB etc) would be absorbed at the 21 CM line. This absorbtion line would be from the hydrogen absorbing energy to kick a antiparallel spin up to parallel). So, either way, the hydrogen would be visible. In actuallity, the CMB is warm enough (even at 2.3 K) to keep the neutral Hydrogen radiating by kicking the hydrogen into the higher, parallel spin configuration.

__________________
Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

Keep in mind that the 21cm radiation is from atomic hydrogen. Molecular hydrogen is difficult, but not impossible to detect. I think molecular hydrogen has been suggested as a form of "dark" matter, but I don't think it can explain the amount of mass needed to account for all "dark matter".

Thanks for reply, Tensor, Amber, but if what you're saying is correct, my HI-DM idea pretty much runs out of gas. I'm foldin'.

Cheers, until something new shows up.

__________________
Credibility is simply incredible... sometimes even to me.
disclaimer

Are you (any and all mainstreamers) satisfied that DM and DE both have the same units, i.e. density?

__________________
PW -- Plant Whisperer

Peter, density is nothing more than the amount of energy in a given volume. Thus, if you measure the density of both, they will be measured in the same units. However, they do have different properites and enter into the GR equations in different places. DM enters into the equations in the Stress-energy tensor and DE enters in the equations through the cosmological constant.

__________________
Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

There's also the small matter of all that x-ray emission, from rich clusters ... how do you get several trillion (and more) sols of very cold H - either atomic or molecular - to be in equilibrium with several trillion (and more) sols of very hot gas ...?

Foldin'? Good move.

First, the background temperature is more like 2.7 K - but that's a minor quibble.

Even background radiation at a temperature of only 2.3 K would still have a peak wavelength of about 1 mm, and hence an energy more than enough to excitate the 21 cm line. Look up this:
http://en.wikipedia.org/wiki/Wien%27s_displacement_law


Use again the law mentioned above. That gives a temperature of about 0.01 K.


Well, observations show that it is, and hypotheses about its nature (the most popular is "neutralinos") also say so.
http://en.wikipedia.org/wiki/Neutralino


But if it was cold hydrogen, it should all have been left between the galaxies. But what we observe is that there is still lots of dark matter in the galaxies - not between them.

I. e.: galaxies contain both hydrogen gas and other stuff. The hydrogen gas is left between the galaxies, the dark matter is dragged along.


There is very hot hydrogen gas (I think at about the same temperatures - at least both are in the range of millions of Kelvins) right here in the solar system, in the solar corona. No different gravitational behaviour has been observed for that gas.

Also, I don't see how this hypothesis could explain why even after the galaxy collision, most of the dark matter is still in the galaxies, not betwen them.


I don't see how hydrogen gases could remain unheated and stay with the galaxy in such a collision.



But there are also other observations beside redshift which show that space is expanding. See my article.



And Tolan tests etc. Are you sure you read the whole article?


No, certainly. If G is variable, the equations of GR are no longer covariant!

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

Huh? DM and DE are concepts, not physical quantities. Hence they don't have units.

However, both for DM and DE, one can give the energy (or mass) densities. And those obviously have the same units!

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

One should add that DE can also be included in the stress-energy tensor by a slight mathematical rewriting.

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

Yeah, I know. I was trying to keep it simple and put the emphasis on the differences.

__________________
Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

1. Observation: All finite gravitational systems behave the same way: over time, their dense, central regions tend to become more dense (contract); their more rareified, outer-regions tend to become even more rareified (expand). All finite gravitational systems exhibit duality.

2. Hypothesis: The universe over-all is dualistic as well.

3. Prediction: Energy released by contraction is greater than energy taken up by expansion.

4. Test: Contraction energy is about 3 times greater than expansion energy.

QED: No hypothetical energy field(s) is needed to explain observations.

__________________
PW -- Plant Whisperer

Thanks for the lengthy response above, Bjoern, though I do not feel deserving, being a mere layman. I'll have to read more of what you referenced, but one thing jumped out:

There is very hot hydrogen gas (I think at about the same temperatures - at least both are in the range of millions of Kelvins) right here in the solar system, in the solar corona. No different gravitational behaviour has been observed for that gas.

From what I read elsewhere, and Wiki, there seems to be a little understood phenomenon regarding the solar wind near the very hot corona. For example, Wiki's http://en.wikipedia.org/wiki/Solar_wind says:

It consists mostly of high-energy electrons and protons (about 1 keV) that are able to escape the star's gravity in part because of the high temperature of the corona and the high kinetic energy particles gain through a process that is not well understood at this time.

Not this proves my point, but there are still some 'unknowns' when it comes to very high temperatures in a gravitational field, where these solar generated particles accelerate at about the very hot corona, what becomes the solar wind; so perhaps this might be a clue to the scenario where high electromagnetic energy, perhaps of some specific wavelength or range, has an effect on ordinary matter, such as protons carried by the solar wind, that behave in ways we do not yet understand, given our models of what happens there. But'll read up on the rest. Thanks.

__________________
Credibility is simply incredible... sometimes even to me.
disclaimer

Well, if you want to continue with your idea that gases at high temperature behave differently in gravitational fields, then I'd suggest that you don't look into cosmology for "proof" of that, but try to analyze the corona first. It's right here in the solar system, can be observed quite well, and there is lots of data available about it.

I don't think that your idea works - but nevertheless, I wish you good luck!

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

Err, I also provided a counterexample to this: two bodies (stars or whatever) simply moving away from each other.


According to what you wrote above in (1), this means: the universe contracts in its "central regions" (what's that supposed to mean?) and expands in its "outer regions" (what's that supposed to mean?).


How does that follow from your hypothesis, as outlined above?

As already pointed out (and admitted by you!): your calculations are rather crude and invoke several assumptions. Hence you haven't done a real test so far - only a "back-of-the-envelope" first estimate.


You haven't "proved" anything, so the "QED" makes no sense here.

First, even a lot of confirmed predictions do not ever "prove" a scientific theory.

Second, you have presented only one confirmed prediction.

Third, even that confirmation is highly questionable, since the calculations done to support it are, as you yourself admitted, essentially only crude estimates.

Fourth, it is not even clear how that prediction follows from your hypothesis.

Fifth, even if you had shown that the fact that the universe expands can be explained by your hypothesis, you still would not have addressed the fact that this expansion accelerates. Hint: dark energy (I suspect that's what you meant with "hypothetical energy field"?) was "invented" in order to explain the acceleration of the expansion, not the expansion itself. If you know of an example of a gravitational system where the contraction of the "center regions" leads to an accelerated expansion of the "outer regions", feel free to provide it! As long as you don't do that, your "duality" idea evidently does not explain the observed acceleration of the expansion, and hence dark energy is still needed.

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

10% mass-to-energy black-hole efficiency

emphasis added

Source

The BH in the galaxy's center weighs about 4 million solar-masses source. At 10%, this means over the course of its existance, it has radiated away 400,000 solar masses of pure radiant energy!!

That is a lot of energy.

Yeah, yeah...it doesn't prove anything. Just a little "wow" factor

4.4 million solar masses of baryons fell in, but it has an inertial mass of only 4 million suns, because the rest of that potential energy was radiated away.

That energy is now "in space."

__________________
PW -- Plant Whisperer

Sure, but such counter-examples are exceedingly rare.
Bjoern, you’re trying too hard to not not understand me

I do not have to tell you: what I’m trying to describe is impossible to picture. You are familiar with the clumsy attempts to describe what cannot be pictured: the 2D analogies of the trampoline or expanding balloon. We can picture the 2D surface of a trampoline being curved in 3 dimensions (by a mass), or the “2D” surface of a balloon expanding into pre-existing 3D space. But we cannot picture 3D space curving in 3D. We can only talk about it.

Same with duality: you cannot picture it. But there is a clear pattern, and this pattern is easily discernible.

In (virtually all) N-body finite systems, this pattern repeats: the dense regions grow denser; the less-dense regions grow even less denser. This pattern is fractal in nature: it repeats on every scale. Extend this pattern to the cosmos, and the conclusion is all-but inescapable: we live in a region that is denser than the average, therefore its average density should continue to increase. Beyond the Local Group of galaxies, however, the density of matter is below average, so we can expect the density “out there” to grow even lower.

This observation is also Copernican: most observers in the universe will find themselves in regions that are more-dense than average. Thus, there is nothing special about our perspective. We are in a region that is more-dense than average, and we find it growing ever more-so, while the universe at-large is less dense than it is here, “locally,” so we can expect the universe over-all to be growing less dense. And it is!

What I am saying is: if contraction supplies the energy for expansion, then no additional energy source is required.

Consider the down-elevator. Say the car weighs 1 ton (a metric ton, ), and is falling at 1 m/s. At the same time, the counterweight is rising at the same 1 m/s. If the counterweight weighs less than the car—say 750 kg—then no hypothetical source of energy is needed to explain it. The 1,000 kg car is losing so-many j/s, and the 750 kg counterweight is gaining so-many j/s. The latter is smaller than the former; the difference is dissipated as heat (radiation).

Same idea in the cosmos. Local matter represents the down-moving car, and distant, remote matter represents the counter-weight. Local matter is falling faster than remote matter is rising, so there is no energy shortage. The difference, as in the earthly elevator, shows up as dissipated heat (radiation).

In the worst-possible scenario, my estimates are so far off, essentially the reverse occurs: in my analogy, I have to make the elevator car weigh 750 kg, and the counter-weight weighs 1,000 kg. Now there is an energy shortage, because 750 kg falling at 1 m/s cannot generate enough power to lift 1,000 kg at 1 m/s.

If such is the case: fine. I have absolutely no problem with that. If pre-existing gravitational energy is not enough to explain the expansion, then—but only then—does it make sense to postulate some other unknown-to-science source of energy.

We are not there yet.

My back-of-the-envelope calculation shows there is no energy shortage. Until someone can show it is wrong—and merely being an approximation does not make it wrong—the dark-energy postulate is unnecessary.

I’m not trying to prove anything; I’m only trying to show: what we see happening on the largest scale is the same thing we see happening on smaller scales.

You are right: I have not proved there is a forest out there. But you can see for yourself all the trees. I do not know what is stopping you from seeing the forest

__________________
PW -- Plant Whisperer

[QUOTE=Peter Wilson;913894]
On the scale of stars and galaxies, yes. But I'd say that on the scale of galaxy clusters, this is the rule, not the exception. Simply look up the velocities of galaxy clusters and their masses. You can the use the virial theorem to see that they are not gravitationally bound to each other. And that means that no energy is required for them to fly away from each other.

That's one of the biggest problems with your idea: you have so far not shown that the expansion on the scale in which it actually happens (i. e. several Mpc), energy is actually needed!


No, I'm simply pointing out that what you wrote was internally inconsistent.



What you entirely ignore here is that there is a clear mathematical description of the concept of expanding space: the Robertson-Walker metric. That's what is missing for your idea! And no, simply calculating two numbers and saying that one measures the rate of contraction and the other the rate of expansion does not in any way match this.



The less-dense regions usually only become less dense because all the material from them is "sucked" to the more dense regions. But that has nothing to do with expansion!

If you think otherwise, please provide a specific astronomical example where it is clearly visible that the contraction of a central region leads to an expansion of the outer region (a SN doesn't count, since there we have an explosion of the outer layers, not merely an expansion).


The only thing that your idea predicts then is that the Local Group attracts the surrounding matter gravitationally and thereby these become less dense. But your idea does not in any way predict expansion on large scales.



But the problem is that you haven't provided any evidence so far that
(1) the cosmological expansion even needs energy
(2) the local contraction can even lead to an accelerated expansion
(3) your idea can quantitatively match actual cosmological data, e. g. the SN data
And, as far as I can see, you have not provided any mechanism how the energy "released" by contraction can "power" the expansion.


But you haven't "explained" the expansion. You have merely pointed to an approximate agreement between two numbers. You can only claim that you have "explained" the expansion when you have actually quantitatively addressed the available cosmological data (for starters, the SN data). What's so hard to understand about that?


You wrote "QED". That's usually used to show that a proof is completed.


The simple fact that a back-of-the-envelope calculation is not even showing the trees. All you have shown so far is one tiny sapling, and based on that, you claim that there is a forest.

__________________
The universe is big. Really big. It may seem like a long way to the corner chemist, but compared to the universe, that's peanuts. (Douglas Adams, Hitchhiker's Guide to the Galaxy)

Now you are using duality to argue against it. As you point out, there are two realms to consider: the near and the far. Nearby, everything is in orbit around a local center; far away, everything is moving apart.

There is a saying, You cannot wake somebody pretending to be asleep. And I have learned, You cannot teach somebody what they already know.

For example, I already know what powers the sun: nuclear energy. It is impossible to teach me that it is EM, or anything else, because I already understand it.

I have been greatly mistaken in assuming the BAUT community is mystified by Dark Energy. I have explained what is going on in terms so simple a child could understand it, but of course, mainstream supporters are not children.

You are highly intelligent, educated adults, with a clear understanding of many complex subjects, like the Hubble expansion. It was caused by Inflation in the past, and is caused by Dark Energy in the present.

I apologize for the misunderstanding.

Nereid, you may lock this thread, as I hereby concede defeat. The observed contraction does not explain the observed expansion.

It has already been explained!!!

Oh well, it was fun while it lasted

__________________
PW -- Plant Whisperer