Well guys I have a couple of curiosities that I would appreciate if you shed some photons of understanding on, here goes :Can you have a dark matter black hole? Does anything like anti-darkmatter exist? Does anti matter have anti-gravity or the normal of the self thingy? Can you even have anti-gravity?

Antimatter does not have negative gravity. The entire Earth could be made out of anti-matter and we wouldn't have a clue in everyday living. Antimatter is just particles with opposite charge. So an electron with a positive charge is called a positron. So no anti-gravity, no negative mass.

Could there be a dark matter black hole? Who knows. We don't know much about dark matter. There might be something about dark matter that doesn't let enough of it occupy a small enough volume to form a black hole.

Can we have anti-gravity? So far my bet is no unless you consider dark energy as anti-gravity. I think we have to understand gravity a lot better before we can start commenting if there is or can be a anti-gravity.

Building on what WayneFrancis said: if we assume that cold dark matter is largely made of Neutralinos (the Supersymmetric Neutrino), the only things preventing a black hole from forming being made mostly of these particles is the unlikelihood of that many of them being close enough at one time to create an event horizon. It is a necessary property of dark matter (to explain what we observe) that the particles cannot strongly bind to each other.

Also, if Neutralinos are the main dark matter particle, they are their own anti-particle, so anti-darkmatter would not exist.

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Right now, dark matter is in the same category as Einstein's cosmological constant: totally unproven, and nothing more than a convenient way for the theorists to add weight to equations that don't balance.

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1. isn't dark matter thought to be composed of subatomic particles currently?

2. if so, isn't it likely that the density of dark matter would be too low to form black holes?

Dark Matter is not totally unproven. It has been observed and measured from its effects in galaxy clusters and galaxies. Dark Energy is a little more like your description above.

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I guess I missed the pictures.

I reckon they measured an unexplained effect out there, and invented DM as a theoretical construct to attempt to explain those measurements. If they have made new observations that match the old ones, all they've proved is that the universe still doesn't fit the old model? Dark Matter is still the Cosmological Constant of the 21st Century, IMO. It hasn't been disproved, but it's a long way from proved.

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Where mainstream ideas can't be found,
some novel theories can be had.
But keep your feet both on the ground;
Sometimes the astronomy's bad.

You may see the cosmos through the noise,
But take care which path you choose.
Remember when reading conspiracy ploys,
A mind's a terrible thing to lose.

So keep your spider sense on high
And before they drive you to booze,
Take time to roll up your pant legs, boys,
It's too late to save your shoes.

Apparently so.

On what do you base this reckoning? Have you done any sort of literature review?

In fact, the existence of cold dark matter was recently independently supported using a technique involving weak gravitational lensing and the apparent weakly interacting characteristic of DM, while observing the collision of two large galaxy clusters. There have probably been additional such observations by now....

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Aren't these observations just inferring the presence of dark matter? Again, dark matter is a convenient hypothetical construct that makes the equations balance. Not seen. NOT proved.

It's one thing to say observations are in agreement with the theory of dark matter. It's quite another thing to claim it has been proved. I remain skeptical.

__________________
Where mainstream ideas can't be found,
some novel theories can be had.
But keep your feet both on the ground;
Sometimes the astronomy's bad.

You may see the cosmos through the noise,
But take care which path you choose.
Remember when reading conspiracy ploys,
A mind's a terrible thing to lose.

So keep your spider sense on high
And before they drive you to booze,
Take time to roll up your pant legs, boys,
It's too late to save your shoes.

But not ruled out, either. The best explanation could turn out to be dark matter, MOND, or something we have not thought of yet.

(bold added)

Sceptical is good.

I'm curious: a great many 'presences', in astronomy, are 'inferred' - magnetic fields, starspots, exoplanets (most of them anyway), to name just a few - yet I doubt that you'd be sceptical of those ... if so, why is CDM different?

Also, what, for you, would constitute 'proof' of the existence of CDM?

Thanks. That must mean I am very, very good indeed...

Those few you name are all pretty well documented. I think we've all experienced magnetic fields (though we may not understand them.) Possibly in the future we will have a more detailed understanding of some other force that is behind what we call magnetism; that won't change the reality that we now experience. Starspots -- do you mean like sunspots? Well I've seen those myself, and while again we may not fully understand the mechanisms involved, their existence is apparent. Exoplanets are now pretty well established, though I wonder if we really know as much about them as some reports indicate.

Dark matter, OTOH, remains undetected despite some tantalizing observations. Just as an unexplained loud noise in the forest is not proof of Bigfoot, observations which are consistent with the existence of dark matter don't prove it. In fact, as some observations are improved, it turns out we may not need dark matter to explain some of the phenomena (see the Universe Today link in my previous post.)

Something more than a noise in the forest.

I've read that even those models that have most of the dark matter as non-baryonic matter, would still require 3 times as much baryonic DM as all the known baryonic matter in the universe. If it's out there, it seems like someone will spot it.

Note that I've not said dark matter doesn't exist. I've merely tried to suggest that slavishly jumping on the bandwagon of the latest popular theory does not guarantee a win in the general election of the universe. An open mind must also be open to the idea that an idea is just a wild guess. It might be right, but then again......

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Where mainstream ideas can't be found,
some novel theories can be had.
But keep your feet both on the ground;
Sometimes the astronomy's bad.

You may see the cosmos through the noise,
But take care which path you choose.
Remember when reading conspiracy ploys,
A mind's a terrible thing to lose.

So keep your spider sense on high
And before they drive you to booze,
Take time to roll up your pant legs, boys,
It's too late to save your shoes.

Is that what it takes? Good documentation? And how do you justify your excessive skepticism if you are, for the most part, unaware of the "documentation", i.e., the scientific literature, or you don't have the background involving years of study to understand that literature?

I don't think you are aware of or understand recent observations in this area of investigation. In other words, it appears you just don't know what you're talking about.

And it probably should have been mentioned earlier, but your demand for "proof" evidences a misunderstanding about science in general. There is no "proof" in science. In mathematics, yes, but not in science. If something was "proved" in science, that would mean the explanation was fixed forever, with no possibility for modification. Science is an opponent of such dogmatic thinking. The more independent lines of observational support a theory has, the stronger that theory is. But you'll never hear a scientist claim any theory represents "absolute truth."


Well, if you're claiming that dark matter is just a wild guess, then, again, you really don't know what you're talking about. I'm not saying dark matter must be the answer, either; however, the observational support for dark matter has fairly recently taken a leap forward, and the DM explanation has become quite compelling. And there is no competing theory that explains all the observations. As DM continues to gain observational support, your excessive skepticism of it becomes less and less reasonable.

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Everyone is entitled to his own opinion, but not his own facts.

Perhaps I should link to what I'm talking about with respect to recent findings wrt DM. The authors of this paper thought it was such a tight argument that they titled the paper A DIRECT EMPIRICAL PROOF OF THE EXISTENCE OF DARK MATTER [pdf]. Empirical proof? I doubt they used the term without giving it a lot of thought. A repeatable observation can be termed a "fact." I still say theories are contingent on future observations or better future explanations.

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Everyone is entitled to his own opinion, but not his own facts.

I'm quite interested in how a person - such as yourself - becomes so apparently certain, and the extent to which scepticism is perceived, and acknowledged, as being inconsistently applied.
From your posts I surmise that you may be unfamiliar with the techniques used to infer the presence of magnetic fields in the ISM (interstellar medium) and in some stars, the presence of spots on stars other than the Sun, and of exoplanets (other than by direct observation); if so, perhaps if I walk you through one it might help to highlight why I'm curious about an apparently inconsistent application of scepticism.

The method that has been used to infer the presence of the majority of exoplanets reported to date involves taking high resolution spectra of some stars, repeatedly, over a timeframe of up to several years. The lines in these spectra are carefully analysed to derive an estimate of the relative line-of-sight velocity of the star and observatory. Then the estimated motion of the observatory, at the time of each observation, relative to the solar system barycentre is subtracted, and the data analysed to see if there is a particular pattern that is consistent with the motion of the star under the gravitational attraction of an exoplanet.

Or, saying this another way, these exoplanets* are convenient hypothetical constructs that makes the equations balance; they are not seen; they are NOT proved.

(I could go through a couple of other techniques used to infer the presence of some exoplanets if you're interested, and also magnetic fields in the ISM, starspots, and/or any of the dozens of other 'presences inferred, not observed' in astronomy).

(bold added)

Maybe it's time to talk about what 'detection' means? As in what criteria should be used, by someone wearing their sceptical hat, to assess whether something in the sky has been detected (or not).

I feel it's very important to stay focused on astronomy, and also to leave aside in situ observations (such those of the solar wind by spacecraft in it, or of Titan's surface by the Huygens probe).

(You see that I can re-write part of your post as "Exoplanet {insert name}, OTOH, remains undetected despite some tantalizing observations.")

Which, I'm sure you'll agree, isn't a terribly practical answer.

Perhaps we could start with asking whether you consider exoplanets inferred solely by the method I described above have been proven to exist or not?

But how?

Surely, wearing your sceptical hat, you should have, ahead of time, a pretty clear set of ideas on what would, for you, constitute 'spotting', shouldn't you?

(bold added)

May I ask how much of the history of (cold, non-baryonic) dark matter, in astronomy, you are familiar with?

Also, you seem to think that a scientific theory is essentially no different than a wild guess; do you?

* one caveat to add: a small subset of exoplanets inferred using this technique have also been inferred using an independent technique (transits); let's leave these aside for now.

When did I say I was "certain" about anything?

I'd guess sometimes that's the case.

__________________
Where mainstream ideas can't be found,
some novel theories can be had.
But keep your feet both on the ground;
Sometimes the astronomy's bad.

You may see the cosmos through the noise,
But take care which path you choose.
Remember when reading conspiracy ploys,
A mind's a terrible thing to lose.

So keep your spider sense on high
And before they drive you to booze,
Take time to roll up your pant legs, boys,
It's too late to save your shoes.

Here are some direct quotes, from posts by you in this thread:

"Right now, dark matter is in the same category as Einstein's cosmological constant: totally unproven, and nothing more than a convenient way for the theorists to add weight to equations that don't balance."

"Again, dark matter is a convenient hypothetical construct that makes the equations balance. Not seen. NOT proved."

"Exoplanets are now pretty well established, though I wonder if we really know as much about them as some reports indicate."

"Dark matter, OTOH, remains undetected despite some tantalizing observations."

Of course, there are plenty of other things you wrote in this thread that certainly indicate a lack of certainty (well, certainly apparently indicate).

Anyway, what I'm interested in is the extent to which you yourself perceive that you are applying scepticism inconsistently, and if not then can you please help me understand what the consistent, sceptical, basis is of your views wrt CDM (the existence, or presence, thereof) and at least some exoplanets?

[jk]And is your guess a wild one? Theoretically speaking, of course ... [/jk]

If you want pretty pictures of dark matter, here's a good one:

http://esamultimedia.esa.int/images/.../1e0657_H1.jpg

In that image, the pink is where most of the normal matter (primarily gases) has been detected through normal (electromagnetic radiation) methods, while the blue is where the majority of the matter has been detected through gravitational lensing. This shows that the bulk of the matter in the pair of colliding clusters is in a separate place compared to the bulk of the baryonic matter. It's pretty hard to find any good explanation for this other than dark matter.

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Ptronic asked about dark matter black holes. Given that dark matter, whatever it is or isn't, does not interact much with anything including itself, except through gravity, it probably won't form a black hole by itself. In order for a cloud of something to coalesce, the cloud must have a means of dissipating alot of its gravitational potential energy. Normal matter can collide, heat up, and radiate away energy. Dark matter doesn't seem to do this.
Given that we already have alot of normal matter black holes, dark matter ought to get captured by these black holes just like any other type of matter. I think it probably doesn't get captured too easily though, since most of the matter captured by black holes is funneled in via dissipative processes. Dark matter would have to directly collide with a black hole before it could be captured (unless maybe GR lends a hand, and something like frame dragging could remove some energy from the DM). My speculative guess:-) black holes start out with all of their mass contributed by normal matter, but as they get older, an increasing fraction of their mass is from DM. Old black holes that have captured or ejected all of the normal matter around them will continue to capture DM.

It's not quite the "latest popular theory." The concept of Dark Matter goes back to 1933:

"The first person to provide evidence and infer the existence of a phenomenon that has come to be called "dark matter" was Swiss astrophysicist Fritz Zwicky, of the California Institute of Technology in 1933."

http://en.wikipedia.org/wiki/Dark_matter

Rob