Dark Galaxy Discovered: Unbelievable Findings

[ubavontuba]

That equation is the energy of a mass starting with zero velocity at $r=r_0$. It doesn't matter what the form of the potential is, as long as it's spherically symmetric (or, alternatively, one-dimensional), that equation will be valid. If it were true that $V_r(r_0) > V_g(r_0)$, then the orbit would be unbound and the particle would be able to escape.

Think about it this way. A mass that is "free" will have only kinetic energy, which must be positive. If its total energy is negative at any point in the orbit, then conservation of energy demands that it never be "free".

Hmm... Is it a one-dimensional consideration? Did you consider that in this model at $r=r_0$ the DE potential is zero but at any other given distance it is no longer zero?

 

[SpaceTiger]

Hmm... Is it a one-dimensional consideration? Did you consider that in this model at $r=r_0$ the DE potential is zero but at any other given distance it is no longer zero?

I'm allowing the DE potential to have any value at any radius, as long as its magnitude is much less than that of the gravitational potential. This approximation will break down eventually, but only at distances much larger than the separation of the objects (as stated in my assumptions).

 

[ubavontuba]

I'm allowing the DE potential to have any value at any radius, as long as its magnitude is much less than that of the gravitational potential. This approximation will break down eventually, but only at distances much larger than the separation of the objects (as stated in my assumptions).

Okay, let's go with this and I'll ask you these questions. Supposing you apply a reasonable figure for DE to galactic sized orbits, what happens? How big must the orbit be, to be adversely affected?

 

[SpaceTiger]

Supposing you apply a reasonable figure for DE to galactic sized orbits, what happens?

You mean a value for the cosmological constant? Reasonable values for the current cosmological model will have very little effect on galactic orbits. There be an extremely undetectable shift, but that's all.

How big must the orbit be, to be adversely affected?

About as big as a galaxy cluster (>~10 Mpc). We still can't measure the effect directly (yet), but it would have a non-negligible impact.

 

[ubavontuba]

Supposing you apply a reasonable figure for DE to galactic sized orbits, what happens?

You mean a value for the cosmological constant? Reasonable values for the current cosmological model will have very little effect on galactic orbits. There be an extremely undetectable shift, but that's all.

Aren't you essentially saying the facts fit the model without examining the facts?

How big must the orbit be, to be adversely affected?

About as big as a galaxy cluster (>~10 Mpc). We still can't measure the effect directly (yet), but it would have a non-negligible impact.

If there is this fine line in regards to galaxy cluster formation that essentially creates a demarcation line between gravity and DE, why do galaxy clusters come in so many different sizes and densities?

I feel I understand your DE orbital model, but have problems with it in accordance with dark matter (DM). As DE pushes orbits apart, angular momentum decreases faster than gravity decreases, therefore stability is achieved (albeit in a slightly higher orbit).

Modelling this on a large scale, it would seem that DE would tend to make orbits higher and slower. However, we observe galactic orbits to be too low and fast due to DM.

This just seems to be a kind of paradox to me. Since DM supposedly reacts with gravity, it should have this same high speed galactic rotation and therefore it should easily be pushed out by even the weakest DE.

The only way I see this working is if DM doesn't react with DE at all, but only with gravity. But like I said earlier, logic rules this out since it is obvious that DE reacts with gravitational mass and DM is purportedly a gravitational mass, therefore DE should react with DM. (I forget the name of this logic form).

See? It's not dark matter or dark energy individually as hypothesis that I have a problem with. It's that when you combine them in accordance with NM, they tend to look rather messy. Has anyone else noticed this? How do they explain these inconsistencies... MOND? (Where is Occam's razor when you need it?)

 

[Garth]

(Where is Occam's razor when you need it?)

Here - read my signature!

View attachment William of Ockham small.bmp
(Taken from a stained glass window in All Saints Church, Ockham, near where I live in Surrey. William was a fourteenth-century Scholastic philosopher, born at or near the village of Ockham)

In the standard model DE produces a small 'antigravity' force that is overwhelmed by normal gravitaiton at normal ranges but only becomes dominant at the largest scales when the average density, and hence normal gravitational forces, are small enough.

Garth

 

[ubavontuba]

Occam sock 'em robots!

In the standard model DE produces a small 'antigravity' force that is overwhelmed by normal gravitaiton at normal ranges but only becomes dominant at the largest scales when the average density, and hence normal gravitational forces, are small enough.

Right, but what about this matter of scale? Are you saying that DE which can push whole, gazillion-ton-mass galaxies around at a separation distance of milions of light years can't push a couple of virtually massless particles around at a distance of a 100,000 light years? Why?

 

[Garth]

Right, but what about this matter of scale? Are you saying that DE which can push whole, gazillion-ton-mass galaxies around at a separation distance of milions of light years can't push a couple of virtually massless particles around at a distance of a 100,000 light years? Why?

Basic Newton: The gravitational force acting on a body is proportional to its mass, therefore it can "push" "gazillion-ton-mass galaxies" as easy as "a couple of virtually massless particles". Cannon balls and feathers fall at the same rate in vacuo.

In GR the Equivalence Principle says the same thing: "gazillion-ton-mass galaxies" and "virtually massless particles" all 'travel' along their geodesics of curved space-time (although this is a simplification for an extended mass such as a galaxy).

DE modifies that space-time either because of the presence of a cosmological constant, or the presence of some exotic 'quintessence' with density & pressure that meets the DE equation of state. It is, however, negligible except at the largest (galactic cluster) scales where the average density drops to within a few OOM of the DE energy-density.

I hope this helps.

Garth

 

[ubavontuba]

Round and round we go...

Basic Newton: The gravitational force acting on a body is proportional to its mass, therefore it can "push" "gazillion-ton-mass galaxies" as easy as "a couple of virtually massless particles".

Sure, but this would mean the DE would have to be acting in concert on the whole universe, rather than arising out of individual low density areas (that is it couldn't be "vacuum energy"). This is because the acceleration from galaxy to galaxy must be relatively identical, or mass would tend to be pushed together by larger voids into smaller voids and small, "Big Crunches" would be the result. Also, since it has been noted that DE appears to be elongating certain mass accumulations (preventing star formation), we can tentatively say it has direction, or a vector/vectors. In other words, it must be a directional (radial?) pulling force/expansion and not a pushing force from between mass concentrations. Essentially, this leads into my question in the thread "Are we in a black hole?"

Cannon balls and feathers fall at the same rate in vacuo.

This isn't entirely true. This only happens in the gravitational field of overwhelmingly large mass as opposed to the two dropped objects. For instance, two cannon balls 1mm apart in space will fall nearly twice as fast together as a cannonball and a feather will.

In GR the Equivalence Principle says the same thing: "gazillion-ton-mass galaxies" and "virtually massless particles" all 'travel' along their geodesics of curved space-time (although this is a simplification for an extended mass such as a galaxy).

Actually, since the gazillion-ton-mass mass of a galaxy creates its own space-time geodesic, it will not fall nor accelerate the same as a particle point mass in DE (if DE behaves like an 'antigravity' force). In fact, the point masses should shoot out way ahead of galaxies from similar starting points under the influence of this hypothetical version of DE, as they will essentially not be attracted to one another by gravity like galaxies would (that is, if DE was a pushing force originating from voids, rather than an expansion force acting on the universe as a whole).

DE modifies that space-time either because of the presence of a cosmological constant, or the presence of some exotic 'quintessence' with density & pressure that meets the DE equation of state. It is, however, negligible except at the largest (galactic cluster) scales where the average density drops to within a few OOM of the DE energy-density.

As I said above, it must then act on the whole universe at once and not be localized in any way, or local crunches would result.

Therefore, instead of looking for a mysterious "dark energy" acting between the galaxies, wouldn't Occam's razor tell us to look instead to a universal gravitational attraction to the cosmological event horizon (CEH) by all mass in the universe? That is, wouldn't the act of everything falling toward the CEH explain all of the observed phenomena associated with DE?

Or, isn't it easier to say the universe is accelerating toward the CEH as if it was falling outward, rather than confusing the issue by invoking a new force to explain it?

See? Dark matter, conservation, and gravity (local and universal) make sense to me. Dark matter, gravity and a separate "dark energy force" just doesn't seem to work so well.

I hope this helps.

It helps to explain the current thinking, but it still doesn't do much to my original questions regarding the validity of two entities, dark matter and dark energy, working together in one universe.

Does this sound like a reasonable approach?

 

[Garth]

. For instance, two cannon balls 1mm apart in space will fall nearly twice as fast together as a cannonball and a feather will.

You need to think about that one, ever heard of Galileo?

Garth

 

[ubavontuba]

two cannon balls 1mm apart in space will fall nearly twice as fast together as a cannonball and a feather will.

You need to think about that one, ever heard of Galileo?

Sure. The cannon balls each fall .5mm. The feather falls nearly a full 1mm. If the acceleration is the same, the shorter distance is quicker, right?

 

[SpaceTiger]

Aren't you essentially saying the facts fit the model without examining the facts?

"Examining the facts" requires one to run large simulations -- the model we explored is only good for understanding the concept. As we've been telling you repeatedly, these simulations have been done and the results are as we're saying.

If there is this fine line in regards to galaxy cluster formation that essentially creates a demarcation line between gravity and DE, why do galaxy clusters come in so many different sizes and densities?

Nothing we've discussed prevents the formation of galaxy clusters over a large dynamic range in size and density. The more overdense a cluster is, the larger it can be before dark energy shows its effects.

I feel I understand your DE orbital model, but have problems with it in accordance with dark matter (DM). As DE pushes orbits apart, angular momentum decreases faster than gravity decreases, therefore stability is achieved (albeit in a slightly higher orbit).

No, angular momentum is conserved. That, along with energy conservation, is what prevents the orbit from being "pushed" too far.

Modelling this on a large scale, it would seem that DE would tend to make orbits higher and slower. However, we observe galactic orbits to be too low and fast due to DM.

The observations of "fast" orbits due to DM are typically on scales that are too small for DE to have a noticable influence.

The only way I see this working is if DM doesn't react with DE at all, but only with gravity. But like I said earlier, logic rules this out since it is obvious that DE reacts with gravitational mass and DM is purportedly a gravitational mass, therefore DE should react with DM. (I forget the name of this logic form).

The theory does not have the DE and DM directly interacting, only indirectly by their gravitational influence. For practical purposes, DM leads to attractive gravity, DE to repulsive gravity.

See? It's not dark matter or dark energy individually as hypothesis that I have a problem with. It's that when you combine them in accordance with NM, they tend to look rather messy. Has anyone else noticed this?

DM and DE do interact gravitationally, of course, but there are no paradoxes or "radiating galaxies". It is exactly as you have been told and it seems clear that your misunderstanding of Newtonian Mechanics is holding you back here, so I suggest we make sure you understand that before proceeding any further.

 

[SpaceTiger]

Sure, but this would mean the DE would have to be acting in concert on the whole universe, rather than arising out of individual low density areas (that is it couldn't be "vacuum energy").

You're overinterpreting "vacuum energy". In standard theory, DE does exist everywhere.

Actually, since the gazillion-ton-mass mass of a galaxy creates its own space-time geodesic, it will not fall nor accelerate the same as a particle point mass in DE (if DE behaves like an 'antigravity' force). In fact, the point masses should shoot out way ahead of galaxies from similar starting points under the influence of this hypothetical version of DE, as they will essentially not be attracted to one another by gravity like galaxies would (that is, if DE was a pushing force originating from voids, rather than an expansion force acting on the universe as a whole).

You're giving the appearance of a person trying to lift the Chrysler building with their bare hands. If you want to understand cosmology at this level, you should start simple and gradually build the machinery necessary to tackle advanced problems. It's not a good use of time for either one of us to try to unravel your misunderstandings at the advanced level when they're based on a flawed intuition (and lack of quantitative skill) in basic physics.

 

[ubavontuba]

Aren't you essentially saying the facts fit the model without examining the facts?

"Examining the facts" requires one to run large simulations -- the model we explored is only good for understanding the concept. As we've been telling you repeatedly, these simulations have been done and the results are as we're saying.

Right, but I'm questioning whether DE and DM have been properly applied in these models together? That is, have the rammifications of their interactions been considered? So far you've asserted that models have been run, but not that the potential interactions between these two forces has been run.

Nothing we've discussed prevents the formation of galaxy clusters over a large dynamic range in size and density. The more overdense a cluster is, the larger it can be before dark energy shows its effects.

Sure, but what about large but relatively thin clusters? How do they maintain their size under the same conditions?

No, angular momentum is conserved. That, along with energy conservation, is what prevents the orbit from being "pushed" too far.

Okay then, I didn't attend a class on orbital dynamics. So what you are saying is that higher orbits must normally have a higher corresponding angular momentum, right? So, what I really mean is that the requirement for the angular momentum at a particular distance is increased, not that the actual angular momentum decreases.

Since the angular momentum is not increased with the application of DE, a balance is struck where DE butresses the height of the orbit for an orbit where the angular momentum would normally be insufficient to maintain it. Is that better?

The observations of "fast" orbits due to DM are typically on scales that are too small for DE to have a noticable influence.

Yes. I will agree with this in principle. I'm only contending that under Newtonian considerations DE should nibble away at the edges of this effect. It might be a slow process (undectable?) or not. But, under simple Newtonian conditions these two supposed forces contradict each other, possibly in ways that haven't been properly modeled.

The theory does not have the DE and DM directly interacting, only indirectly by their gravitational influence. For practical purposes, DM leads to attractive gravity, DE to repulsive gravity.

Sure, that's how it's modeled. My question is; is this model correct? Simply put, if you add a quantity (gravity) and counter it with a negative quantity (antigravity) then you are simply left with less gravity, not two distinct entities.

DM and DE do interact gravitationally, of course, but there are no paradoxes or "radiating galaxies". It is exactly as you have been told and it seems clear that your misunderstanding of Newtonian Mechanics is holding you back here, so I suggest we make sure you understand that before proceeding any further.

Are you proposing a test?

You're overinterpreting "vacuum energy". In standard theory, DE does exist everywhere.

Right. Isn't that what I said? I'm simply stating that it cannot be density dependent (locally) since it must act on the whole universe in concert. Perhaps it is density dependent in regards to the whole universe, but must this be so? Is there no other possible considerations?

You're giving the appearance of a person trying to lift the Chrysler building with their bare hands. If you want to understand cosmology at this level, you should start simple and gradually build the machinery necessary to tackle advanced problems. It's not a good use of time for either one of us to try to unravel your misunderstandings at the advanced level when they're based on a flawed intuition (and lack of quantitative skill) in basic physics.

Well, now you've gone and outright insulted me. Is this the best discourse you can provide?

I'm merely posing questions. Questions that may or may not be important. All I'm asking for is answers. If you don't want to provide answers, that's okay with me. You needn't be rude.

 

[SpaceTiger]

Right, but I'm questioning whether DE and DM have been properly applied in these models together? That is, have the rammifications of their interactions been considered? So far you've asserted that models have been run, but not that the potential interactions between these two forces has been run.

No, in fact, that's exactly what I'm saying. Their gravitational interaction is considered in simulations, just as we considered it in the toy model.

Sure, but what about large but relatively thin clusters? How do they maintain their size under the same conditions?

Cluster density profiles are usually pretty similar from one to the next. I'm not aware of any that are too "thin" to be held together in the presence of dark energy.

Okay then, I didn't attend a class on orbital dynamics. So what you are saying is that higher orbits must normally have a higher corresponding angular momentum, right? So, what I really mean is that the requirement for the angualr momentum at a particular distance is increased, not that the actual angular momentum decreases. Is that better?

Yes.

Yes. I will agree with this in principle. I'm only contending that under Newtonian considerations DE should nibble away at the edges of this effect. It might be a slow process (undectable?) or not. But, under simple Newtonian conditions these two supposed forces contradict each other, possibly in ways that haven't been properly modeled.

No, in a universe with a cosmological constant, this is not true, as I've already demonstrated.

Sure, that's how it's modeled. My question is; is this model correct? Simply put, if you add a quantity (gravity) and counter it with a negative (antigravity) then you are simply left with less gravity, not two distinct entities.

Whether or not they are distinct entities or just illusions created by an alternative gravity model is something that is yet to be determined. The properties of the DM and DE, as well as the theoretical guesses as to their identities, suggest that they are in fact distinct entities.

However, there is nothing inherently wrong with the DM+DE theory, at least nothing like what you're suggesting.

Are you proposing a test?

I'm proposing that you start your questions at a more basic level. I will elaborate via PM and please refrain from posting here until we've talked things out privately.