From the 'Dark side', do we need Dark Energy?

[nutant gene 71]

From the ‘Dark side’, why do we need Dark Energy?

I really don’t understand this: Greatest Mysteries: Where is the Rest of the Universe?

The say about ‘dark matter’ in one breath “that light doesn’t interact with it”, then three paragraphs later “Another hallmark of dark matter is gravitational lensing, similar to the effect of light passing through a piece of polished glass.” What gives? Either dark matter and light interact or they do not, no?

Here’s the problem: If light interacts with ‘dark matter’ supposedly comprising 22% of the universe energy budget, then why not simply accept that light is gravitationally redshifted as it passes through these large parts of the intergalactic cosmos? If this redshift is gravitational, either because light is passing through dark matter, with which it ONLY interacts gravitationally (whether from a Machian totality of ‘dark matter’ gravity well in line of sight, or from dark space-vacuum encounters), then why need ‘dark energy’ at all? Wouldn’t this disallow for Doppler space expansion, if redshift is ‘dark matter’ gravitational instead? That’s where I’m stuck, as to either or: we either accept ‘dark matter’ interacts gravitationally with light, or we reject it, meaning we cannot use it for gravitational lensing. But we know the latter is false, because we can observe gravitational lensing. So we are left with only one conclusion, logically, that space gravity of dark matter must interact to cause redshifting gravitationally. All that gravitatinally massive 'dark matter' in the universe, therefore, is giving us the illusion, line of sight, as if it were expanding Doppler like, which may only be the ‘Dark side’ fooling us.

The corollary to this conundrum is then, what do we do with the balance of the matter/energy budget, the remaining 74% now ascribed to ‘dark energy’? If E = mc^2 is correct, then where is that energy hiding, if dark matter lensing and gravitational redshifting for intergalactic light is accounted for? Could that energy budget be locked up inside the dark matter itself somehow? Could this other missing energy be 'eaten up' by the process of redshift itself, if due to dark matter BIG gravity?

I don’t know, but I can’t go over to the Dark side yet, not unless I can understand where the rest of that energy went in the universe. Please help me see the light!

[Tim Thompson]

Dark matter does not interact directly with light. However, dark matter has mass, mass has gravity, and that deflects light via space-time curvature. So, microlensing is just what happens when light (or any other electromagnetic radiation) passes through the space curved by the gravity of dark matter.

As for "dark energy", that may be a poor choice of words. If it is in fact a manifestation of a cosmological constant, as seems to be the current consensus, then it it not energy at all, and one need not worry about E = mc².

[triclon]

Gravitational redshift works both ways. As the light approached a massive body, it will be blue shifted, and as it recedes away it will be red shifted. The net shift in spectral lines we then observe is zero so we see no gravitational red or blue shift as the light passes through galaxies, dark matter, or whatever.

I think the only time gravitational redshift comes into play is when the source of the light has a strong gravitational field itself, so the light has to "climb" out of the gravity well, and in the process is redshifted. This happens with light from neutron stars, white dwarfs, matter falling into black holes, ect.

[nutant gene 71]

Gravitational redshift works both ways. As the light approached a massive body, it will be blue shifted, and as it recedes away it will be red shifted. The net shift in spectral lines we then observe is zero so we see no gravitational red or blue shift as the light passes through galaxies, dark matter, or whatever.

I think the only time gravitational redshift comes into play is when the source of the light has a strong gravitational field itself, so the light has to "climb" out of the gravity well, and in the process is redshifted. This happens with light from neutron stars, white dwarfs, matter falling into black holes, ect.

I understand, both yours triclon, and (I think) Tim's response. I kind of suspected this might be the case, which is why I had written this clause (in parenthesis) in my above:

"If this redshift is gravitational, either because light is passing through dark matter, with which it ONLY interacts gravitationally (whether from a Machian totality of ‘dark matter’ gravity well in line of sight, or from dark space-vacuum encounters), then why need ‘dark energy’ at all? Wouldn’t this disallow for Doppler space expansion, if redshift is ‘dark matter’ gravitational instead?"

What I'm trying to say is that if we view the whole line of sight as a long 'gravity well' filled with dark matter, this may be causing light to redshift at the Hubble constant, or close to it, from our point of view. And if so, this may cause the energy budget to shift over enough to wipe out, in effect, the dark energy needed to balance out the matter/energy totality that is our universe. Wouldn't that obviate a Doppler space-expansion? That's the question. Therefore, if dark matter is gravitationally interactive with light, could this line of sight gravity well (much like coming from a very long white dwarf or neutron star syndrome) give us redshift at Hubble constant that is distance dependent?

Anyway, not a suggestion that this is so, merely a query to see if it may be possible. Any possibility this could be so? Thanks.

[Ken G]

Therefore, if dark matter is gravitationally interactive with light, could this line of sight gravity well (much like coming from a very long white dwarf or neutron star syndrome) give us redshift at Hubble constant that is distance dependent?

A point that is rarely appreciated is that although the observed redshift is an invariant and physically real quantity, the explanations for why things redshift is neither, and generally depend on the chosen reference frame or coordinate system. It is perfectly appropriate, for example, to explain the cosmological redshift in just the way you are doing-- as primarily a gravitational redshift due to dark matter. Does that "obviate a Doppler-space expansion"? No, they are just different ways of looking at the same thing. The standard meaning of "gravitational redshift" is local to the special objects the light path encounters, so the cosmological redshift is not conventionally attributed to gravitational redshifting. In other words, one normally talks about gravitational redshifting due to spatial changes in gravity, not changes with time. But when gravity changes with time and redshifting results, it still seems like a perfectly natural form of gravitational redshift to me, especially since temporal changes can be transformed into spatial changes by simply choosing a nonstandard coordinate system. Still, it is natural that there are standard ways of talking about things, but these standard coordinate systems are too often mistaken for absolute statements about reality. If relativity is right about anything, then what is real is what is invariant.

[nutant gene 71]

Thanks Ken, what I’m hearing is that both possibilities can exist side by side, where on the one hand we have redshift due to Doppler space-expansion, while on the other ‘dark matter’ gravitational redshift may also be at work. This raises perhaps another issue, which follows naturally from these two possibilities, but especially from the ‘dark matter’ related redshift. The issue would be: Can we then estimate the amount of this ‘dark matter’ since we know the redhift Hubble constant and distance involved? It would seem that if two of the three are known, the third, the amount of ‘dark matter’ (which is now estimated at 22% as the total matter/energy budget of the universe, with the balance of 4% baryonic and 74% 'dark energy') should be subject to confirmation by the other two known factors. However, I am the first to admit this is a very new idea for me, and I am sure I have no way of making such calculations. Anybody think it may be possible to confirm the amount and density of ‘dark matter’, in line of sight, by this gravitational redshift method? Any idea of how?

Though I realize I now am asking a new question, it might open new possibilities in astronomical observations, if it were possible. Thanks.

[Tim Thompson]

... where on the one hand we have redshift due to Doppler space-expansion, ...

I think it is pointworthy to mention that the cosmological redshift, though commonly called a Doppler shift, is in fact nothing of the kind, at least in standard cosmology. The cosmological redshift arises as a result of space "stretching", as the universe expands, and that "stretches" the photons riding along with the expanding space, so that they appear to be redshifted. It is all model dependent, as is nearly everything in cosmology, but in the models one can distinguish between a true Doppler shift and a "stretching" redshift. And so I think the point of Ken G's remark is that, since gravity in GR is simply a geometric effect of curved spacetime, then the geometric effect of an expanding space is "gravitational" in some sense, since it is also an effect of geometry.

So, you can construct a model where expansion causes the redshift, or a model in which dark matter causes the redshift, as both will affect the geometry of spacetime, and both models are in any case constrained to be consistent with observation. For me, it is easier to conceive of the redshift being due to expansion, rather than dark matter, partly because it is more in keeping with general relativity (where the dark matter resides on a spacetime background), and partly because the universe evolves. If the redshift is due to expansion alone, then one need only worry about the photon's local spacetime, and global considerations can be ignored (spacetime not associated with the photon at any point on its trajactory will not affect the redshift). But if one goes the dark matter redshift route, then one has to worry about the propagation of gravity from non-local sources of dark matter.

[Ken G]

And so I think the point of Ken G's remark is that, since gravity in GR is simply a geometric effect of curved spacetime, then the geometric effect of an expanding space is "gravitational" in some sense, since it is also an effect of geometry.

Right, it is gravitational in a sense, but even more, just where this redshift actually comes from is entirely coordinate-dependent. The same is true in special relativity, it's just even more profoundly true in GR. In SR, you can see some spectral line in space as being blueshifted, let's say. Did the blueshift "happen" as the light was being emitted, or just as you observed it, or was it continuously occuring all during propagation? The answer is, it entirely depends on the coordinates you use to analyze the question. So even in cosmology, the statement that "space is expanding" is not an invariant physical reality, it is just a coordinate choice. A very useful one, no doubt, and often the preferred way to explain cosmological redshifts, but the only "reason" space expands is because we have chosen it to do so, it's an arbitrary though intelligent coordinate choice.

So, you can construct a model where expansion causes the redshift, or a model in which dark matter causes the redshift, as both will affect the geometry of spacetime, and both models are in any case constrained to be consistent with observation.

Yes, whatever you do with the coordinates you have to connect the physics of the emission to the physics of the detection, and relativity will keep track of how to make that connection in an invariant way. Both the dynamics of the universe and the presence of dark matter will affect the invariant result, but "expansion of space" need not necessarily be there (if it isn't, the redshift will show up some other less reasonable sounding way).

For me, it is easier to conceive of the redshift being due to expansion, rather than dark matter, partly because it is more in keeping with general relativity (where the dark matter resides on a spacetime background), and partly because the universe evolves.

No doubt, the presence of dark matter alone would not yield redshift-- it could even yield blueshift (in a Big Crunch scenario). It is the combination of the dark matter and the overall dynamical history of the universe that yields the redshift, but it is customary to include the effects of the former entirely in the latter by using comoving frame coordinates (the "expanding space" coordinates). That is an elegant way to embed what dark matter is doing (and dark energy too) entirely into what "space" is doing, even though there is no need for "space" to exist as a physical entity.

[nutant gene 71]

Thanks Tim, I think I understand the subtle difference you make between Doppler space expansion, which is a physical-motion idea, and space “stretching” which is a more cosmological-expansion concept. But I’m not sure I fully appreciate how “the propagation of gravity from a non-local sources of dark matter” works. If dark matter is prevalent throughout intergalactic space, regardless of whether or not it too is being “stretched”, it should still sum up to be the same thing, in toto for line of sight. So the geometry of GR gravity, at least over the distance in line of sight, should be different from what Ken refers to in his on gravitational lensing, where in line of sight there would be no "bending" of light. GR geometry should be expected to work around massive bodies, at any angle, but where the viewing angle is line of sight, zero degrees, then GR geometry should self cancel over that distance into a line. Am I missing something here?

Getting back to original question on “why Dark energy?” - if redshift were Dark-matter related in line of sight, there seems to be another conundrum with which physics is struggling. In Space.com’s recent article: Greatest Mysteries: Is There a Theory of Everything? , they allude to how gravity, and hence likewise ‘Dark matter’ gravity, is odd man out when putting all the known forces of nature together in the Standard Model. This could also mean ‘Dark energy’ is somehow not part of the model, since it is beyond normal matter and energy. My question could resolve this issue, in a manner of speaking, if Dark-energy is really not driving space “stretch” apart but merely an optical effect from Dark-matter “absorbing” that energy due to its high gravity redshift (line of sight only), so that space may be static but "appears" visually to be stretching apart. However, that is not the main issue here. What is interesting in this article is that what happens in the space-vacuum on a point, when expanded, may show up as Strings of energy, which I believe is the main point of String Theory. If so, the universe’s energy budget now gets more complicated, unless symmetry is found to reconcile Strings into the energy left behind by redshifted light. I know this sounds confusing, but what part of universal cosmology isn’t? I havent gone over to the Dark-side yet.

On another thought, could Dark-matter, and maybe or maybe not Dark-energy, be part of the original primordial universe, such as calculated in the first seconds of existence, but later all got diluted when baryonic matter and electromagnetic energy got introduced? So we're left over today with ordinary gravity, at least as far as we know. What this would mean, in effect, is that Dark-matter which is gravitational higher than normal baryonic matter is the original stuff, while everything we see now is new. Could the Standard Model, which reconciles okay in the first few seconds into expansion, be reconciled better if Dark-matter were allowed into the equations? But now I’m way in the deep end and over my head… Thanks.

[Kwalish Kid]

There is a significant problem with assigning gravitational redshift due to dark matter as a cause for anomalous redshift in distant objects. That problem is that this redshift works opposite to the way that dark energy works. If we have enough dark matter that it's mass introduces a significant redshift, then it's cosmological redshift will be even greater. Both of these redshifts will indicate that objects that are more distant will tend to have a greater redshift than simply the Hubble relation on its own supposes. However, we see the opposite: objects at a great distance have less redshift than we expect given the Hubble relation on its own.

[Ken G]

If dark matter is prevalent throughout intergalactic space, regardless of whether or not it too is being “stretched”, it should still sum up to be the same thing, in toto for line of sight.

No, because there are two types of "bending". One is a net bend to the left or right, and that's what you won't get with dark matter everywhere, but another type of bending simply affects the rate of spreading of the rays, and that type you do get-- that's what is known as "spacetime curvature", and that's what the cosmological dark matter is all about. Perhaps more important to picture is the associated redshifting that goes along with that ray bending, which, as I said above, I very much do think of as a gravitational effect (even though thinking of it in terms of expanding space is an intelligent way to go with that).

GR geometry should be expected to work around massive bodies, at any angle, but where the viewing angle is line of sight, zero degrees, then GR geometry should self cancel over that distance into a line. Am I missing something here?

Yes-- the effect I refer to above does not cancel.

My question could resolve this issue, in a manner of speaking, if Dark-energy is really not driving space “stretch” apart...

This kind of statement is exactly why I make such a point of distinguishing the explanations we choose from what can be properly said to be "really happening"-- that which is invariant. Dark energy is not "really driving space"! It is pure coordinate choice to say that this is something that is happening to space. It's a very useful way to say it, but other descriptions are entirely possible in other coordinates that will sound totally different yet not actually be any different at all.

In other words, there is little point in debating the verbiage of the explanations, you must focus on the invariants-- the impact of dark energy on what we measure when we look at very distant galaxies. Questions like "could the real reason be due to some particular other property of space" generally have the answer-- "certainly-- but if you produce the same invariants, you actually have just the same theory in different clothes". And if you do not produce the same invariants, then you have a problem agreeing with observations. In short, all this will be resolved by new observations, not by new explanations.

What this would mean, in effect, is that Dark-matter which is gravitational higher than normal baryonic matter is the original stuff, while everything we see now is new.

Every type of matter has been there all along, in the Big Bang model, but the higher rest masses "froze out" sooner, so we say they "came first". We know that dark matter should be "cold", so have a relatively higher rest mass, but I'm not sure how high it would need to be compared to, say, protons and electrons.

Could the Standard Model, which reconciles okay in the first few seconds into expansion, be reconciled better if Dark-matter were allowed into the equations?

You may be assured that dark matter is already in those equations of the early Big Bang-- that is indeed one of the reasons it is expected that this matter must be nonbaryonic, not just "dark". People don't realize that the "cold dark matter" model is not just a gravitational cluge to explain galaxy dynamics, it's a fully self-consistent part of modern cosmological modeling.

[Ken G]

There is a significant problem with assigning gravitational redshift due to dark matter as a cause for anomalous redshift in distant objects.

By "anomalous redshift", I presume you mean redshift that implies that the expansion is accelerating? So you are saying that we need dark energy, we can't do it with dark matter alone, because fitting the redshifts with dark matter by itself won't give universal dynamics that are consistent with general relativity. I agree-- that's a concise summary of the need for dark energy, in a nutshell-- if the observational interpretations are correct.

[nutant gene 71]

There is a significant problem with assigning gravitational redshift due to dark matter as a cause for anomalous redshift in distant objects. That problem is that this redshift works opposite to the way that dark energy works. If we have enough dark matter that it's mass introduces a significant redshift, then it's cosmological redshift will be even greater. Both of these redshifts will indicate that objects that are more distant will tend to have a greater redshift than simply the Hubble relation on its own supposes. However, we see the opposite: objects at a great distance have less redshift than we expect given the Hubble relation on its own.

Right Kid, I can see where the "added" gravitational redshift would mean it is even more distant than observed. I didn't have that in mind, though, but merely the idea that the observed Hubble already has Dark-matter gravity in it, not added. However, as Ken also points out, all these things had already been factored into the Standard Model, so the likelihood of Dark-matter-gravity affecting the end results for redshift may already all be figured in. If it weren't figured in, the energy budget of the universe would then be off, I suspect.

Anyway, the question in OP was whether or not we still needed Dark-energy if Dark-matter-gravity was responsible for Hubble constant, and it appears to me that we still need it, at least for now. Still, I am drawn to the temptations of the Dark-side... sort of.