A general comment first, Michael: at a sufficiently high level, the approach I have used, in the post of mine you are quoting, follows the modern (astronomical) scientific method: if a theory can reproduce all the observed phenomena within (the theory's) domain, to within the uncertainties of the good, relevant observations and experiments, well, that's pretty much all you can ask of any such theory.
As we have already established, in your own, personal, view of the nature of astronomy, this may be different.
However, this Cafe is not an appropriate place for you to try to reconcile your own personal views of the nature of astronomy, as a science, with what astronomers actually do, when they do scientific research.
Now for some specifics ...
Michael Mozina wrote:Nereid wrote:Let's make some simplifying assumptions. When we're done, we can go back and see how making the assumptions less simple might change our conclusions*.
Let's consider just 'DM' and 'atoms'. Let's assume that they both interact gravitationally, according to GR (actually, according to just Newtonian gravity will do), but that DM is not affected by the electromagnetic force (EM)^. Atoms are obviously strongly affected by EM.
FYI, I'll save my comments for DE in the other thread.
When you make proclamation statements such as these about DM, and assign properties to it (in this case you claim it is unaffected by EM fields), I am at a loss to understand how to test such a concept. I am already having to rely on "faith" here, and simply take your word for it, otherwise you are incapable of demonstrating such a trait in any controlled condition. It is simply "assumed" from the start that this is true, and I have no way to validate or falsify that statement.
This is as good an example as any ...
The modern approach starts with postulates (or assumptions), as I have said in other, earlier, posts. The postulates themselves have no particular meaning, or importance. From the postulates, you can derive - logically, consistently, objectively - conclusions about phenomenology (what you should expect to see, with telescopes, etc). If you see those things, then your theory (and its postulates) have been (provisionally) validated; if not, then not.
A general, non-specific, requirement for 'controlled conditions' is a red herring - should your theory lead to potentially 'in the lab, controlled' testable phenomena, then good ... you can get a grant, buy the equipment (etc), set up the experiment, perform it, write up the observations/results, publish them, and you're done.
If not, you can't ... and that's what astronomy is, to a great extent, about ... you can't make a Sun in your lab, so anything to do with 'Suns' under controlled, lab, conditions is unrealistic (today).
I am still unclear how you arrived at the percentage of DM compared to the percentage of normal matter in the universe based on the WMAP data. Could you explain that part?
There's a separate thread on that.
Let's take a large number of DM particles (which may be of any mass), distributed uniformly over a sphere in otherwise empty space. After a while, they will be no longer distributed uniformly, but in a spherically symmetric way, with a density profile that peaks at the centre of the sphere. The detailed profile depends upon a number of factors, such as how 'hot' the DM was to start with, and how long is 'a while'.
You just assigned several more "properties" to DM that cannot be tested in a controlled manner as far as I know. I now have to 'assume" a density profile of DM as well as several "properties'' that are unique to DM.
We would still expect DM to be attracted to normal matter would we not? We would expect it to affect normal matter (gravitationally) would we not? Wouldn't they tend to attract one another like all forms of mass?
Indeed ... and I covered that later in the same post that you're quoting from!
If we run the experiment with atoms, instead of DM, we get a very different result. First, the behaviour is a great deal more complicated - atoms can ionise when they collide; the gravitational potential energy of the system can be 'lost' as photons carry away energy; and so on. The most dramatic result (depending on the part of parameter space we're exploring) is the formation of 'stars' - the process of gravitation collapse (due to the combination of gravity plus loss of energy due to radiation) is halted when gas pressure (etc) produces a stable configuration. Another interesting result is the formation of disks - gravitationally collapsing clouds of atoms produce disks, due (fundamentally) to the ability of atoms to transfer energy and angular momentum in (inelastic) collisions.
Ok, suppose I assume with you that there is an EM difference between normal matter and DM. Even still, the gravitational fields of matter are going to affect the movements of dark matter and visa versa, correct? Even if only in an indirect way, DM is going to be influence by the effect of the EM fields isn't it?
That doesn't really make sense from my perspective. If stars do all sorts of cluster dances when they get close to one another, based on GR influence alone, why wouldn't dark matter do similar things as well? In other words even without any EM field present, there isn't really any reason to believe that dark matter won't "clump" into objects of various sizes, or is there some "property" of DM I'm still missing?
In one sense, it's incredibly simple: a bunch (hundred, billion, ...) of objects which interact only via (Newtonian) gravity will evolve into a (relatively) stable (equilibrium) configuration ... 'relaxed' is the technical term. The stars in a globular cluster behave like DM particles - they (effectively) never collide, and their motions are determined entirely by gravity .... and they are relaxed. To a first order of approximation, systems of DM objects will 'look like' globular clusters - nearly spherical distributions, with a distinctive radial profile (and central peak).
At the next level of approximation, various details start to be important ... leading to a number of different profiles (but the
Navarro-Frenk-White profile (NFW) is the most widely used).
The Orion nebula serves as a good illustration of what distributions of atoms look like. Note that if the Orion nebula contained no 'metals' (astronomers refer to all elements other than H and He as 'metals'), it would have a very different distribution. At another level,
Beta Pictoris illustrates what distributions of atoms look like. Note that no DM distribution could naturally look like either the Orion nebula or Beta Pictoris.
I'm a bit confused now. In theory, DM "could" do anything, but in one moment you were claiming that normal matter and DM can't be compared because of the effect of EM fields on matter, and in the next minute you seem to be suggesting that DM can be compared to material items which I would expect are strongly influenced by local EM fields, particularly the Beta Pictoris option you cited. It seems like you wish to have your cake and eat it too.
If all DM particles (no matter how small, or large; no matter how light, or massive) can do is interact gravitationally, and if there's nothing but DM particles, then no Orion nebulae or Beta Pic discs could ever form.
What about if you have a mixture of atoms and (cold) DM? This gets very complicated! However, the good news is that quite a number of just such mixtures have been simulated, in great detail. One the biggest and most extensive simulations is
the Millenium simulation.
As a computer programmer by trade, I appreciate the power of computers to model things. I also know that a computer program can be useless if it is built upon false assumptions. I'd be a lot more impressed by a controlled scientific tests that demonstrated DM existed and had some effect on normal matter that I could replicate here on earth.
Indeed.
One, of many, wonderful things about these kinds of simulations is the stimulus it provides for observational tests.
One serendipitous discovery of SDSS is the richness of stellar streams in the Milky Way's halo ... the PIs got funding to do an extension of SDSS, specifically to look more closely into the nature of the stellar populations in the MW halo.
And what has this got to do with DM? Cosmological-scale simulations, using realistic inputs (DM, 'atoms', etc), produced some interesting patterns at the galaxy and galaxy halo scale. To the extent that our MW galaxy is representative, we should expect to find those patterns in the halo ... let's go look!
Back to the solar system.
Here's a method for estimating the mass of DM distributed in a spherically symmetric fashion, centred on the Sun:
-> observe the motions of (massive) solar system bodies - especially planets, their moons, asteroids (and their moons), and KBOs (Kuiper Belt Objects)
-> estimate the mass of the planets, using observations of the motions of their moons (for Venus and Mercury, use the observed motions of the relevant space probes; for asteroids and KBOs without moons, extrapolate from mass estimates of those with moons)
-> analyse the observational data, apply Newton, estimate the mass distributed in a spherically symmetrical fashion, interior to the orbit of each planet, asteroid, and KBO; make sure to take out the effect of the planets (etc), using the masses estimated in the step above
-> search for a trend in the 'mass interior to orbit' estimates, where that mass increases with orbital radius
-> use any such statistically significant trend to estimate the distribution of DM within the solar system.
AFAIK, there is no statistically significant trend; ergo, there is no observably detectable DM in the solar system, at least beyond the orbit of Mercury.
While the bounds are not as tight, you can use the observed motions of sun-grazing comets to constrain the mass of DM between (say) 1 million km above the photosphere and Mercury's orbit ... same result, no detectable DM.
I'm not convinced that is a valid argument. If we can't see DM, then it could be liberally dispersed throughout the solar system with enough mass on the outside of the solar system to "offset" much of it's effects inside the solar system. In other words, you are excluding the option that DM exists inside *and* outside out solar system of planets. Furthermore you do seem to be excluding the possibility that the sun itself and every planet is composed of a certain amount of dark matter, meaning you could actually be *overestimating* the amount of normal mass in the sun and planets, or you could be underestimating that mass depending on the distribution of DM in and around the solar system.
[snip]
Not really.
Remember that I've only sketched, in the briefest possible way, just one test.
And most of your (good) questions are beyond the scope of the specific hypothesis I set out to test.
Let's take "lots of DM outside the solar system": unless it were distributed in a particularly non-uniform way, or unless there were a very great deal of it, the test I described won't show it. However, there are plenty of other tests ...
Or "lots of DM inside each and every planet, moon, asteroid, the Sun ...": the test I described in my post is mute about this; to put strong constraints on the amount of DM within the solid, liquid, or (some weaker constraint) gaseous boundary of any (large) solar system body, you need to do different tests.
Would you like me to sketch some such tests?
That doesn't mean that you can provide it, or that it cannot be provided to me in other scientific theories, even scientific theories related to the same observations.