Dark Matter Real: Studies Confirm, Modifying Gravity Can't Work

[RUTA]

And I'm asking you to explain, for any or all of those ansatzes, how the ansatz gives rise to a correction to either $M_{RC}$ (the mass we infer from the rotation curve) or $M_L$ (the mass we infer from luminosity data), or both, that could make them come out the same, without using your terminology. How should the process of obtaining $M_{RC}$ from the observed rotation curve (which we assume is fixed by observation), or the process of obtaining $M_L$ from the observed luminosity data (which we assume is fixed by observation), or both, be changed? I understand that you don't have an exact solution or a numerical model that accurately captures an actual galaxy; but I'm not asking the above question with regard to an exact solution for an actual galaxy. I'm asking it for any one, or all, of your ansatzes, and you should be able to answer the question for those since you picked them precisely in order to illustrate how such corrections could arise.

Equations 12, 13 and 14 with $dM_p \rightarrow dM_{RC}$ and $dM \rightarrow dM_L$. Arguments for all three are presented in the surrounding paragraphs.

 

[RUTA]

This doesn't prove your claim, because the two FLRW geometries are different, as I've already pointed out. Saying "the FLRW solution is time symmetric" doesn't change that, because we're not talking about a single FLRW solution and its time reverse, we're talking about two different FLRW solutions, with different spatial geometries, so they aren't time reverses of each other.

What stops you from doing this? You agree that you can attach either an expanding or collapsing dust to the interior or exterior of the Schwarzschild vacuum right? Just do both.

 

[PeterDonis]

What stops you from doing this?

I'm not saying you can't have a model with an inner collapsing FLRW region and an outer expanding FLRW region separated by a Schwarzschild vacuum region. Of course you can.

What I'm saying is that the two FLRW regions are not time reverses of each other. More precisely, while you can construct a solution where the two regions are time reverses of each other, such a solution is highly fine-tuned. If we extend the solution into the past, we reach a spacelike surface where the Schwarzschild region vanishes and the two FLRW regions meet. If the two FLRW regions are to be time reverses of each other, at the meeting, the two FLRW regions must have exactly the same density, which means that the time from the initial singularity of the expanding FLRW region to the meeting point, for an expanding comoving observer, must be equal to the time from the meeting point to the final singularity of the collapsing FLRW region, for a collapsing FLRW observer. Put more colloquially, if the meeting point happens a billion years after the Big Bang of the expanding region, the collapsing region would have to take a billion years to collapse; while if the meeting point happens two billion years after the Big Bang, the collapsing region would have to take two billion years to collapse. Any solution that does not satisfy this constraint will not satisfy your claim that the two FLRW regions "behave the same" either--the $M_p$ of the collapsing region will not be the same as the $M_p$ of the "piece" of the expanding region that was removed to make room for the collapsing region (plus the Schwarzschild vacuum separating them). And the solutions that satisfy the constraint are a set of measure zero in the space of all solutions with the same basic structure, which is why I say solutions satisfying the constraint are highly fine-tuned.

 

[PeterDonis]

Equations 12, 13 and 14 with $dM_p \rightarrow dM_{RC}$ and $dM \rightarrow dM_L$.

Is this referring to the AJP paper or the arxiv paper?

 

[RUTA]

I'm not saying you can't have a model with an inner collapsing FLRW region and an outer expanding FLRW region separated by a Schwarzschild vacuum region. Of course you can.

What I'm saying is that the two FLRW regions are not time reverses of each other. More precisely, while you can construct a solution where the two regions are time reverses of each other, such a solution is highly fine-tuned. If we extend the solution into the past, we reach a spacelike surface where the Schwarzschild region vanishes and the two FLRW regions meet. If the two FLRW regions are to be time reverses of each other, at the meeting, the two FLRW regions must have exactly the same density, which means that the time from the initial singularity of the expanding FLRW region to the meeting point, for an expanding comoving observer, must be equal to the time from the meeting point to the final singularity of the collapsing FLRW region, for a collapsing FLRW observer. Put more colloquially, if the meeting point happens a billion years after the Big Bang of the expanding region, the collapsing region would have to take a billion years to collapse; while if the meeting point happens two billion years after the Big Bang, the collapsing region would have to take two billion years to collapse. Any solution that does not satisfy this constraint will not satisfy your claim that the two FLRW regions "behave the same" either--the $M_p$ of the collapsing region will not be the same as the $M_p$ of the "piece" of the expanding region that was removed to make room for the collapsing region (plus the Schwarzschild vacuum separating them). And the solutions that satisfy the constraint are a set of measure zero in the space of all solutions with the same basic structure, which is why I say solutions satisfying the constraint are highly fine-tuned.

Of course, the interior solution has to match the exterior solution or you can't get M for the metric of the vacuum annulus to match both. I don't know why you keep trying to use this heuristic example to model galactic matter distributions. That's not reasonable as we state explicitly in the paper. The REAL compound GR solution would be impossibly complex. Thus, the ansatzes and discussion pertaining thereto.

 

[RUTA]

Is this referring to the AJP paper or the arxiv paper?

ArXiv paper

 

[PeterDonis]

Of course, the interior solution has to match the exterior solution or you can't get M for the metric of the vacuum annulus to match both.

The $M$ for the Schwarzschild vacuum region doesn't have to match both. It doesn't have to have any relationship at all to the $M$ for the exterior FLRW region. (In fact, if the exterior FLRW region is spatially infinite, as it is in our best current model of our actual universe, its $M$ is infinite, so it obviously can't be the same as the finite $M$ of the Schwarzschild vacuum region surrounding an isolated system like a galaxy.)

I don't know why you keep trying to use this heuristic example to model galactic matter distributions. That's not reasonable as we state explicitly in the paper.

Then you shouldn't be basing any arguments on it. But you are. If you agree to retract all those arguments, I'll gladly drop this line of discussion. But then you would have to retract a substantial portion of your papers. The only reason I'm discussing this type of model at all is that you did.

 

[PeterDonis]

ArXiv paper

Ok. Then I'm afraid I don't find equations 12, 13, and 14 and the discussion surrounding them at all convincing. I just see curve fitting and handwaving.

Perhaps I'd better explain in more detail what I'm looking for by considering another idealized example: a static, spherically symmetric matter region of constant density surrounded by Schwarzschild vacuum. Of course this example is highly unrealistic because of the constant density assumption, but it's a common one used for pedagogy in textbooks because it has a known exact solution. The metric for this solution in Schwarzschild coordinates is

$$
ds^2 = - J(r) dt^2 + \frac{1}{1 - \frac{2m(r)}{r}} dr^2 + r^2 d\Omega^2
$$

where $d\Omega^2$ is the standard metric on a 2-sphere. There are two regions, the matter region and the vacuum region, with a boundary between them at $r = R_0$. In the vacuum region, we have $m(r) = M$ and $J(r) = 1 - \frac{2M}{r}$. In the matter region, we have

$$
m(r) = 4 \pi \rho \int_0^r r^2 dr = \frac{4}{3} \pi \rho r^3
$$

and

$$
J(r) = \left( \frac{3}{2} \sqrt{1 - \frac{2M}{R_0}} - \frac{1}{2} \sqrt{1 - \frac{2M r^2}{R_0^3}} \right)^2
$$

Note that the above equation for $m(r)$, when we plug in $r = R_0$, gives

$$
M = \frac{4}{3} \pi \rho R_0^3
$$

We can compute the orbital frequency $\omega$ of a free-fall circular orbit at radius $r$ using the methods in my Insights article on Fermi-Walker transport [1], which gives a simple way to compute the proper acceleration as a function of $\omega$ (and we can then set the proper acceleration to zero to find the free-fall orbit value of $\omega$):

$$
A = \frac{1}{2} g^{rr} \left[ \left( \partial_r g_{tt} \right) u^t u^t + \left( \partial_r g_{\phi \phi} \right) u^\phi u^\phi \right] = \frac{1}{2} \left( 1 - \frac{2m(r)}{r} \right) \left( \frac{1}{J(r) - \omega^2 r^2} \partial_r J(r) + \frac{\omega^2}{J(r) - \omega^2 r^2} 2 r \right)
$$

The condition for $A = 0$ is then

$$
\partial_r J(r) = 2 \omega^2 r
$$

which gives

$$
\omega = \sqrt{\frac{1}{2r} \partial_r J(r)}
$$

For the Schwarzschild vacuum region, we have $\partial_r J(r) = \frac{2 M}{r^2}$, so we get the familiar formula:

$$
\omega = \sqrt{\frac{M}{r^3}}
$$

For the interior matter region, we have

$$
\partial_r J(r) = 2 \left( \frac{3}{2} \sqrt{1 - \frac{2M}{R_0}} - \frac{1}{2} \sqrt{1 - \frac{2M r^2}{R_0^3}} \right) \frac{1}{4} \left( 1 - \frac{2Mr^2}{R_0^3} \right)^{-\frac{1}{2}} \frac{4Mr}{R_0^3}
$$

which gives

$$
\omega = \sqrt{ \left( \frac{3}{2} \sqrt{\frac{R_0^3 - 2M R_0^2}{R_0^3 - 2M r^2}} - \frac{1}{2} \right) \frac{M}{R_0^3} }
$$

Inverting this formula gives us a way to infer $M$ from observations of $\omega$ as a function of $r$ for objects in free-fall orbits in the interior matter region; in other words, it gives us a way to infer $M_{RC}$ from an observed rotation curve. And we will find that $M_{RC}$ inferred in this way is given by the formula for $M$ above, i.e.,

$$
M_{RC} = \frac{4}{3} \pi \rho R_0^3
$$

Now, suppose all the matter in the interior matter region is luminous, and obeys some known mass-luminosity relation. Then we can infer a mass $M_L$ from the observed luminosity. Since the mass-luminosity relation will be derived, as you note, from observations of stars in our own galaxy, we expect that $M_L$ for the interior matter region will be something like "integrate the density over the proper volume". But we know what that integral is:

$$
M_L = 4 \pi \rho \int_0^{R_0} r^2 \sqrt{g_{rr}} dr
$$

where the extra factor of $\sqrt{g_{rr}}$ is the correction to make the integral over the proper volume. We don't even need to evaluate this to see that, since $g_{rr} > 1$ for the entire range of integration, we must end up with $M_L > M_{RC}$.

As I said, this model is obviously unrealistic; but just on heuristic grounds, I would expect a similar relationship $M_L > M_{RC}$ to hold for any stationary bound system (and a galaxy is such a system, certainly to a good enough approximation for our purposes here) in which all of the matter is luminous, for the simple reason that I've already given in a prior post, and which is obvious from comparing the integrals above: for any stationary bound system, the proper volume, which determines $M_L$, will be larger than the "Euclidean volume" we infer based on the area of the system's boundary (which is what the $r$ coordinate in Schwarzschild coordinates is measuring), which determines $M_{RC}$, and all of the other factors involved are the same. Therefore, if we see a stationary bound system, like a galaxy, where we have $M_L < M_{RC}$, and by a large margin, we should infer that there is missing mass that is not luminous.

What I'm looking for is an argument from you, based on some kind of ansatz for a solution describing a galaxy (that will obviously be different from the ansatz I adopted above), for why the simple heuristic argument I gave above should not apply to the actual galaxies we observe. Or, alternatively, you could make an argument that I'm somehow misinterpreting the ansatz I gave and how $M_{RC}$ and $M_L$ would be determined for that idealized case, and that when the correct method of determining them is used, we find $M_L < M_{RC}$. Just saying "mass is contextual" won't do it. And just pointing to equations 12, 13, and 14 in the arxiv paper, and their surrounding discussions, won't do it, because I don't see any ansatz in there that is based on any kind of physical property that the actual galaxies we observe have (whereas my ansatz above is based on an obvious property they all have, that they are stationary bound systems); as I said at the start of this post, I just see curve fitting and handwaving.

[1] https://www.physicsforums.com/insights/fermi-walker-transport-in-schwarzschild-spacetime/

 

[RUTA]

The $M$ for the Schwarzschild vacuum region doesn't have to match both. It doesn't have to have any relationship at all to the $M$ for the exterior FLRW region. (In fact, if the exterior FLRW region is spatially infinite, as it is in our best current model of our actual universe, its $M$ is infinite, so it obviously can't be the same as the finite $M$ of the Schwarzschild vacuum region surrounding an isolated system like a galaxy.)

When you marry up the solutions, since M is part of the Schwarzschild metric, it does become determined by the constant, co-moving radial coordinate of the FLRW geometry. So, it is necessarily related to the proper mass of the interior FLRW ball as I show in the AJP paper. Now, you have to again marry up the Schwarzschild metric at the exterior FLRW solution which has the same constant, co-moving radial coordinate but subtends a larger area. That difference in subtended area is matched at the appropriate Schwarzschild radial coordinate per radial free fall. The mass exterior to the radially free falling material with fixed radial FLRW coordinate is irrelevant. So, the proper mass interior to that radial coordinate (which is fixed per constant $\rho a^3$), as far as the geometry of the FLRW exterior is concerned, is that of the FLRW dust ball.

Then you shouldn't be basing any arguments on it. But you are. If you agree to retract all those arguments, I'll gladly drop this line of discussion. But then you would have to retract a substantial portion of your papers. The only reason I'm discussing this type of model at all is that you did.

First, concerning two different masses for the same matter in GR you said (post #41):

How is this possible in GR? I'm not aware of any solution of the Einstein Field Equation that has this property.

Which I then explained. Then you thought our argument relating proper and dynamic masses gave $M_{RC} < M_{L}$, as if all our fits would work in such a situation and not be noticed by any referee or reader to date. I corrected your misconception. Then when I described how the proper mass of the interior FLRW dust ball mass would be that inside the disconnected exterior FLRW solution as if there was no vacuum between them, you said (post #67):

This can't be right, because the two FLRW solutions have different geometries.

Which I then explained. Now you're saying the FLRW-Schwarzschild adjoined solution must be mapped to galactic matter distributions for the argument of the paper to follow. As a coauthor of the paper, I can tell you that is not true either. The FLRW-Schwarzschild adjoined solution is used only to show that GR contextuality is a big enough effect to account for the astrophysical missing mass. That's it.

 

[RUTA]

Ok. Then I'm afraid I don't find equations 12, 13, and 14 and the discussion surrounding them at all convincing. I just see curve fitting and handwaving.

Perhaps I'd better explain in more detail what I'm looking for by considering another idealized example: a static, spherically symmetric matter region of constant density surrounded by Schwarzschild vacuum. Of course this example is highly unrealistic because of the constant density assumption, but it's a common one used for pedagogy in textbooks because it has a known exact solution. The metric for this solution in Schwarzschild coordinates is

$$
ds^2 = - J(r) dt^2 + \frac{1}{1 - \frac{2m(r)}{r}} dr^2 + r^2 d\Omega^2
$$

where $d\Omega^2$ is the standard metric on a 2-sphere. There are two regions, the matter region and the vacuum region, with a boundary between them at $r = R_0$. In the vacuum region, we have $m(r) = M$ and $J(r) = 1 - \frac{2M}{r}$. In the matter region, we have

$$
m(r) = 4 \pi \rho \int_0^r r^2 dr = \frac{4}{3} \pi \rho r^3
$$

and

$$
J(r) = \left( \frac{3}{2} \sqrt{1 - \frac{2M}{R_0}} - \frac{1}{2} \sqrt{1 - \frac{2M r^2}{R_0^3}} \right)^2
$$

Note that the above equation for $m(r)$, when we plug in $r = R_0$, gives

$$
M = \frac{4}{3} \pi \rho R_0^3
$$

We can compute the orbital frequency $\omega$ of a free-fall circular orbit at radius $r$ using the methods in my Insights article on Fermi-Walker transport [1], which gives a simple way to compute the proper acceleration as a function of $\omega$ (and we can then set the proper acceleration to zero to find the free-fall orbit value of $\omega$):

$$
A = \frac{1}{2} g^{rr} \left[ \left( \partial_r g_{tt} \right) u^t u^t + \left( \partial_r g_{\phi \phi} \right) u^\phi u^\phi \right] = \frac{1}{2} \left( 1 - \frac{2m(r)}{r} \right) \left( \frac{1}{J(r) - \omega^2 r^2} \partial_r J(r) + \frac{\omega^2}{J(r) - \omega^2 r^2} 2 r \right)
$$

The condition for $A = 0$ is then

$$
\partial_r J(r) = 2 \omega^2 r
$$

which gives

$$
\omega = \sqrt{\frac{1}{2r} \partial_r J(r)}
$$

For the Schwarzschild vacuum region, we have $\partial_r J(r) = \frac{2 M}{r^2}$, so we get the familiar formula:

$$
\omega = \sqrt{\frac{M}{r^3}}
$$

For the interior matter region, we have

$$
\partial_r J(r) = 2 \left( \frac{3}{2} \sqrt{1 - \frac{2M}{R_0}} - \frac{1}{2} \sqrt{1 - \frac{2M r^2}{R_0^3}} \right) \frac{1}{4} \left( 1 - \frac{2Mr^2}{R_0^3} \right)^{-\frac{1}{2}} \frac{4Mr}{R_0^3}
$$

which gives

$$
\omega = \sqrt{ \left( \frac{3}{2} \sqrt{\frac{R_0^3 - 2M R_0^2}{R_0^3 - 2M r^2}} - \frac{1}{2} \right) \frac{M}{R_0^3} }
$$

Inverting this formula gives us a way to infer $M$ from observations of $\omega$ as a function of $r$ for objects in free-fall orbits in the interior matter region; in other words, it gives us a way to infer $M_{RC}$ from an observed rotation curve. And we will find that $M_{RC}$ inferred in this way is given by the formula for $M$ above, i.e.,

$$
M_{RC} = \frac{4}{3} \pi \rho R_0^3
$$

Now, suppose all the matter in the interior matter region is luminous, and obeys some known mass-luminosity relation. Then we can infer a mass $M_L$ from the observed luminosity. Since the mass-luminosity relation will be derived, as you note, from observations of stars in our own galaxy, we expect that $M_L$ for the interior matter region will be something like "integrate the density over the proper volume". But we know what that integral is:

$$
M_L = 4 \pi \rho \int_0^{R_0} r^2 \sqrt{g_{rr}} dr
$$

where the extra factor of $\sqrt{g_{rr}}$ is the correction to make the integral over the proper volume. We don't even need to evaluate this to see that, since $g_{rr} > 1$ for the entire range of integration, we must end up with $M_L > M_{RC}$.

As I said, this model is obviously unrealistic; but just on heuristic grounds, I would expect a similar relationship $M_L > M_{RC}$ to hold for any stationary bound system (and a galaxy is such a system, certainly to a good enough approximation for our purposes here) in which all of the matter is luminous, for the simple reason that I've already given in a prior post, and which is obvious from comparing the integrals above: for any stationary bound system, the proper volume, which determines $M_L$, will be larger than the "Euclidean volume" we infer based on the area of the system's boundary (which is what the $r$ coordinate in Schwarzschild coordinates is measuring), which determines $M_{RC}$, and all of the other factors involved are the same. Therefore, if we see a stationary bound system, like a galaxy, where we have $M_L < M_{RC}$, and by a large margin, we should infer that there is missing mass that is not luminous.

What I'm looking for is an argument from you, based on some kind of ansatz for a solution describing a galaxy (that will obviously be different from the ansatz I adopted above), for why the simple heuristic argument I gave above should not apply to the actual galaxies we observe. Or, alternatively, you could make an argument that I'm somehow misinterpreting the ansatz I gave and how $M_{RC}$ and $M_L$ would be determined for that idealized case, and that when the correct method of determining them is used, we find $M_L < M_{RC}$. Just saying "mass is contextual" won't do it. And just pointing to equations 12, 13, and 14 in the arxiv paper, and their surrounding discussions, won't do it, because I don't see any ansatz in there that is based on any kind of physical property that the actual galaxies we observe have (whereas my ansatz above is based on an obvious property they all have, that they are stationary bound systems); as I said at the start of this post, I just see curve fitting and handwaving.

[1] https://www.physicsforums.com/insights/fermi-walker-transport-in-schwarzschild-spacetime/

You could have skipped all your erroneous complaints thus far and made this legitimate complaint (but then you wouldn't have learned anything new). It is the only valid complaint you have registered. You are saying that it is incumbent upon me to justify the precise functional form of an ansatz from the general properties of adjoined GR solutions a la your argument above that doesn't work. You're not buying any of the reasons for the ansatzes in the paper because they do not come from GR directly. That's a legitimate complaint and that's what I meant had to be done when I said I intend to return to this problem. That being said, there are a number of interesting/non-trivial results from our ansatz fits, so it's not an entirely worthless exercise per empirical science regardless of the motivation for the ansatz per se (see Discussion and Conclusions).

Keep in mind the alternatives are not pretty, i.e., exotic new matter with properties that are incredible (see Carroll) or changing one of our most accurate theories to date (GR).

 

[PeterDonis]

You could have skipped all your erroneous complaints thus far and made this legitimate complaint (but then you wouldn't have learned anything new).

I agree that I was able to give a much more precise statement of the issue in my latest post, yes. But as you say, I would not have been able to do that without all the prior discussion, which has helped me to understand the general problem and to focus in on the key issues that I see.

That's a legitimate complaint and that's what I meant had to be done when I said I intend to return to this problem.

Fair enough.

 

[PeterDonis]

When you marry up the solutions, since M is part of the Schwarzschild metric, it does become determined by the constant, co-moving radial coordinate of the FLRW geometry.

The interior FLRW geometry, yes.

you have to again marry up the Schwarzschild metric at the exterior FLRW solution

Yes.

which has the same constant, co-moving radial coordinate

This I don't understand. If we take the interior FLRW coordinate chart and extend it outward through the Schwarzschild vacuum region (or, for that matter, if we take the exterior FLRW coordinate chart and extend it inward through the Schwarzschild vacuum region), the comoving radial coordinate at the boundary with the interior FLRW region should be smaller than the comoving radial coordinate at the boundary with the exterior FLRW region.

but subtends a larger area

The spatial surface area of the Schwarzschild-exterior FLRW boundary will be larger than the spatial surface area of the Schwarzschild-interior FLRW boundary, yes.

 

[PeterDonis]

Then you thought our argument relating proper and dynamic masses gave $M_{RC} < M_{L}$, as if all our fits would work in such a situation and not be noticed by any referee or reader to date.

Just to be clear, this was an early attempt of mine to get at the issue that I described in post #78. In other words, I was not arguing that the ansatzes you actually gave in the paper should give $M_{RC} < M_{L}$; obviously the ansatzes you actually gave in the paper don't do that, as is clear just from looking at the formulas, much less from looking at the fits to the data. I was arguing that I don't think the ansatzes you actually gave in the paper are justified, for reasons that I didn't articulate clearly until post #78.

 

[Dr. Courtney]

Great discussion. I'm not an expert in dark matter, just an interested physicist trying to follow. A few observations:

The absence of evidence is not (always) the evidence of absence. Failing to observe dark matter in these two galaxies is only compelling that they do not contain dark matter if the error bars are small enough. It has been disappointing for me that the uncertainty in the quantity of dark matter in these galaxies has been treated in such a qualitative manner - more rigorous quantitative treatments of the uncertainties would make the fundamental observational question much clearer and also set a better example for young scientists.

I'm always a bit skeptical when Occam's razor (or some similar idea) is used to favor one hypothesis (or theory or model) over another, especially when the error bars on the essential data are large. Occam's razor is NOT an arbiter among competing ideas in science - experimental data is the ultimate arbiter. (See: https://arxiv.org/ftp/arxiv/papers/0812/0812.4932.pdf ) In this case, it seems clear that more data is needed. I'm not even sure Occam has a solid track record of picking eventual winners in cases where the available data is as sparse as it is now regarding dark matter. Though I do think more solid evidence for galaxies without dark matter IS (or will be) compelling support for dark matter being real in galaxies where dark matter is the simplest explanation for observations.

 

[Jarvis323]

As an outsider, based on what I've been reading, it seams that the missing matter problem might be due to at least these things (not considering religion or simulation hypothesis):

1. Our cosmological observations are wrong and it is either currently out of reach or impossible to make correct ones.
2. Our calculations are wrong and it may be currently out of reach or impossible to make correct ones.
3. Our theory of gravity is inaccurate.
4. There is non-baryonic dark matter out there.

Is this accurate, and are there others?

I wonder to what extent should we be trying to pick any particular small set of theories as 'bests ones', as a basis for narrowing our minds? And I wonder what is the main point of cosmology, and whether those goals align well with the current scientific approach? Is it for intellectual satisfaction, or sport? Currently one main contribution seams to have been to convince a large portion of the human population (that don't know any better) into believing things that we aren't sure of.

To me, it's more interesting to enumerate the possibilities than it is to try believing in one. Even if the truth is impossible to prove, or the would be correct theory is not-falsifiable, I would still like to think on it.

 

[sector99]

Newbie DM person.

DM spherically collects in galaxies (Not all). Does it concentrate near black holes. If not ... why?

If not...is this not a large clue? Do black holes discriminate against DM?

 

[zonde]

Newbie DM person.

DM spherically collects in galaxies (Not all). Does it concentrate near black holes. If not ... why?

If not...is this not a large clue? Do black holes discriminate against DM?

Interesting question. It made me think.
If DM is interacting only gravitationally how it can form a gravitationally bound state? It has to give up some energy in order to become gravitationally bound, but it has no means to give up that energy. So is the idea of DM sound at the very basic level?

 

[sector99]

Interesting question. It made me think.
If DM is interacting only gravitationally how it can form a gravitationally bound state? It has to give up some energy in order to become gravitationally bound, but it has no means to give up that energy. So is the idea of DM sound at the very basic level?

Investigators see (so far) that DM doesn't aggregate towards galactic centers (where black holes reside (mostly). Would a two-part gravitational interaction fit?

Protons repel excepting very large breaching forces to fuse them. This seems to be a two-part force-Strong when forced close...repel under common states.

DM thus might not be drawn into black holes and might not contribute to universe entropy totals.

It's so complicated. Does E=Mc^2 apply for DM?...the "M" isn't the same M.

 

[RUTA]

Interesting question. It made me think.
If DM is interacting only gravitationally how it can form a gravitationally bound state? It has to give up some energy in order to become gravitationally bound, but it has no means to give up that energy. So is the idea of DM sound at the very basic level?

This is one of the reasons Carroll gives for being surprised by the existence of dark matter.

 

[sector99]

This is one of the reasons Carroll gives for being surprised by the existence of dark matter.

A major constituent of the universe that (1) collects around galaxies but (2) doesn't aggregate or "clump" towards their centers is a big red flag.

It's a "can you top this one" moment in cosmology.

Maybe a rescue observation will emerge...

 

[Nik_2213]

"If DM is interacting only gravitationally how it can form a gravitationally bound state?"

Three-body interaction ??

 

[strangerep]

If DM is interacting only gravitationally how it can form a gravitationally bound state? [...]

Huh? I'm pretty sure there are bodies in stable orbit around other bodies where only their gravitational interaction is significant.

Or did I misunderstand you?

 

[PeterDonis]

I'm pretty sure there are bodies in stable orbit around other bodies where only their gravitational interaction is significant.

But they didn't get into those stable orbits with only gravitational interactions. A typical gravitationally bound system of ordinary matter, like our solar system, had to emit a lot of electromagnetic radiation to infinity in order to get into its current bound state. Dark matter can't do that (if it could emit EM radiation, it wouldn't be dark), so its ability to form bound systems ought to be much reduced compared to ordinary matter.

 

[Janus]

But they didn't get into those stable orbits with only gravitational interactions. A typical gravitationally bound system of ordinary matter, like our solar system, had to emit a lot of electromagnetic radiation to infinity in order to get into its current bound state. Dark matter can't do that (if it could emit EM radiation, it wouldn't be dark), so its ability to form bound systems ought to be much reduced compared to ordinary matter.

While DM doesn't emit EMR it does emit gravitational waves due to its gravitational interaction with both itself and other matter. This does give it a means of losing energy in order to become bound. Since gravitational radiation is many orders weaker than EMR, this is going to be a much slower process, The type of structures you would expect to see in dark matter would be much less compact than what we see with baryonic matter, and this is just what we see.

 

[PeterDonis]

Since gravitational radiation is many orders weaker than EMR, this is going to be a much slower process

Yes, this is why I said DM's ability to form bound systems is expected to be much reduced compared to ordinary matter.

 

[sector99]

On the point of "seeing DM", (1) what exactly are observers "seeing"? and (2) are galactic baryons orbiting through DM or orbiting with it?

Here I assume DM somehow avoids galaxy centric black holes.

 

[Janus]

On the point of "seeing DM", (1) what exactly are observers "seeing"? and (2) are galactic baryons orbiting through DM or orbiting with it?

1. They are observing galaxy rotation curves that do not match what you would get if you had baryonic matter alone, they are also observing gravitational lensing.
2. DM is orbiting, whether or not it has a net angular motion wit respect to the galactic center is an open question.

Here I assume DM somehow avoids galaxy centric black holes.

Why? Sure, only a small fraction of DM would end up crossing the event horizon, but we are talking about a pretty small target to hit (at 45 AU), compared to the size of the DM halo.

 

[sector99]

For her discovery, Vera should have received more.

https://www.nasa.gov/vision/universe/starsgalaxies/dark_matter_proven.html
A bit of digging for query (1) suggests that x-ray photons, hugely lensed by a galaxy cluster, from a massive, irregular "halo" cloud are what the obsrvers are defining as "proof" of DM.

Hot gas away from the parent galaxies, emitting x-ray photons. This is an indirect photonic measurement attributed as evidence for DM.

These pictures suggest the photonic source gas (attributed to DM) completely envelope a vast galactic cluster, including the space between the galaxies. Perhaps this is Rubin's "missing mass"?

The link present a model image of a sphere of putative DM - gravitationally bound to the cluster. my second query (2) remains: Is DM sensitive to the gravity of a black hole?

Offered in another way: Why would black holes discriminate against DM?

 

[zonde]

my second query (2) remains: Is DM sensitive to the gravity of a black hole?

Offered in another way: Why would black holes discriminate against DM?

For DM to aggregate around black hole it would have to lose energy while in vicinity of black hole. As DM is not interacting with anything (except gravitationally) it can't do that. So as it falls down in gravitational well it just as quickly climbs out of it except if it hits BH directly, but as Janus pointed out it's very small target to hit.

 

[Nik_2213]

There's also the vast accretion disk, a massive, very rapidly rotating cloud. Its 'regular matter' exhibits multiple 'internal dissipative processes', but DM, as yet, would seem to notice only gravity. If so, DM may only 'see' the rapid rotation, with some flung 'wide and wild' as if from a spinning lawn sprinkler...

On a smaller scale, this may account for the apparent lack of DM here-abouts. Perhaps local DM was swept up, blown away by our Local Bubble's supernova shock fronts ??

==
OT: Can any-one identify a recent 'Where Are They ?' tale in which 'our' region of space is analogous to 'Sargasso Sea' due to lack of DM for 'fuel' ??

 

[sector99]

For DM to aggregate around black hole it would have to lose energy while in vicinity of black hole. As DM is not interacting with anything (except gravitationally) it can't do that. So as it falls down in gravitational well it just as quickly climbs out of it except if it hits BH directly, but as Janus pointed out it's very small target to hit.

DM near a BH "Having to lose energy" due to baryonic non-interaction. To my newbie status, this is a puzzle. DM is mass of an unknown type. It may, for whimsey's sake, be a simple (as yet under-appreciated) field that is somehow modified by baryons & gravity wells.

Speaking of wells, "climbing back out [of a BH well] also is a puzzle to me. Either DM falls into a gravity well-or it doesn't. What would make DM change its inward trajectory?
**************
Protons repel each other unless a certain closeness is forced-then they attract and stick.

Is the general DM/gravity attraction (but not aggregation) as you imply-reversal back up away from BH wells similar?

Also if DM doesn't aggregate to galactic centers does this imply that DM's "normal" state is that it can't be "compressed" ... except perhaps by DE?

 

[Janus]

DM near a BH "Having to lose energy" due to baryonic non-interaction. To my newbie status, this is a puzzle. DM is mass of an unknown type. It may, for whimsey's sake, be a simple (as yet under-appreciated) field that is somehow modified by baryons & gravity wells.

Speaking of wells, "climbing back out [of a BH well] also is a puzzle to me. Either DM falls into a gravity well-or it doesn't. What would make DM change its inward trajectory?
**************
Protons repel each other unless a certain closeness is forced-then they attract and stick.

Is the general DM/gravity attraction (but not aggregation) as you imply-reversal back up away from BH wells similar?

Also if DM doesn't aggregate to galactic centers does this imply that DM's "normal" state is that it can't be "compressed" ... except perhaps by DE?

DM falling in towards a BH is in principle no different then trying to hit the Sun with an in-falling object from, let's say, Pluto orbit distance. It doesn't take much for you to miss. You don't need much of a "sideways" component for the object to whip around the Sun rather than hit it. A sideways component of just over 114 meters/sec would put it into a trajectory that just skims above the surface of the Sun and heads back out into space, eventually returning to where it started. Shrink the Sun to a BH with a an event horizon just a few km across, and you make it even harder. Now you have to get any sideways component down to under 0.237 m/sec ( under 1/2 a mile per hr).
And this is starting with no inward component. Adding a inward starting component actually decreases the allowed sideways component (An object with an initial inwards velocity component reaches the Sun's vicinity faster, giving the Sun's gravity less time to curve the path in towards the Sun.)

In terms of gravity, BHs are no different than anything else, other than the fact that their small radius allows objects to get much closer to their center of mass without hitting a surface (in this case, the event horizon) which produces a region of extreme gravity near the BH.

 

[sector99]

DM falling in towards a BH is in principle no different then trying to hit the Sun with an in-falling object from, let's say, Pluto orbit distance. It doesn't take much for you to miss. You don't need much of a "sideways" component for the object to whip around the Sun rather than hit it. A sideways component of just over 114 meters/sec would put it into a trajectory that just skims above the surface of the Sun and heads back out into space, eventually returning to where it started. Shrink the Sun to a BH with a an event horizon just a few km across, and you make it even harder. Now you have to get any sideways component down to under 0.237 m/sec ( under 1/2 a mile per hr).
And this is starting with no inward component. Adding a inward starting component actually decreases the allowed sideways component (An object with an initial inwards velocity component reaches the Sun's vicinity faster, giving the Sun's gravity less time to curve the path in towards the Sun.)

In terms of gravity, BHs are no different than anything else, other than the fact that their small radius allows objects to get much closer to their center of mass without hitting a surface (in this case, the event horizon) which produces a region of extreme gravity near the BH.

Thanks for this. So...the DM/Gravity "force" is very weak leading to a highly likely BH "miss" at perigee and a conventional (if loose) "orbit" around the averaged gravitational galactic centers–all assuming DM can't be "compressed".

Whether DM enters a BH is thus a very low probability ie. inconsequential event. However, it seems of interest insofar as concerns DM & interaction with the BH Conversion Zone–BHCZ (Event Horizon–where baryons are converted into whatever is in BH). Is DM even "convertable"? I see other questions occupy much higher priority.

As for DM and DE: Are they like oil and water?

 

[Ibix]

So...the DM/Gravity "force" is very weak leading to a highly likely BH "miss"

You are missing the point. Normal matter would normally swing past a black hole and escape. Dark matter is not special in this regard. It's just that space around a black hole can be quite crowded and normal matter collides with other normal matter and loses energy to friction. So it gets trapped. Dark matter doesn't interact this way - it just passes through other matter (normal and dark) so it doesn't experience friction, doesn't slow down, and doesn't get trapped.

 

[sector99]

You are missing the point. Normal matter would normally swing past a black hole and escape. Dark matter is not special in this regard. It's just that space around a black hole can be quite crowded and normal matter collides with other normal matter and loses energy to friction. So it gets trapped. Dark matter doesn't interact this way - it just passes through other matter (normal and dark) so it doesn't experience friction, doesn't slow down, and doesn't get trapped.

I now appreciate the baryonic frictional drag effect that DM doesn't feel. Thanks.