Game, David. The use of “dark matter” in cosmological speculation is simply used to refer to massive quantities of matter which do not shine by their own light (as one might expect that amount of concentrated matter to do, through fusion, gravitational energy, degeneracy pressure, or whatever it is that causes neutron stars to put out gamma rays). Whether it is a massive concentration of asteroids, planets, black dwarfs, black holes, or pink fluffy bunnies or purple fairies (that’s where they went to, Gaudere) is immaterial. There are those who assert that they are either concentrations of neutrinos (obvious arguments against that notwithstanding) or “strange” matter (i.e., with non-cancelled strange quarks) which can only be detected by its gravitational pull. I am skeptical of either explanation. But I don’t have anything better to offer.
There is an article in the Jan. issue of “Sky & Telescope” to the effect that much so-called “dark matter” may be isotopic hydrogen. I don’t have it at hand, and haven’t read it yet, but when I do, I’ll post something about it.
"I prefer shows of the genre, “World’s Blankiest Blank.”
At the risk of de-railing this thread completely…
I’m sure you all saw the recent announcement that astronomers have now confirmed the existence of 28 extra-solar planets. Maybe there are enough planets to make up this “dark matter”?
(Well, maybe not. As David pointed out, if there are enough subatomic particles with mass, that would be sufficient. The best candidate so far is the neutrino, which, if it has mass, would account for it all. If someone wants a link, I’ll go look for it.)
Fighting my own ignorance since 1957.
From the same bed of 530 year old rock mentioned in my post above, this week, reports are coming out of better preserved specimens of a Chordate life form, (named Haikouella, after a nearby village.) While there is the usual bickering about the precise interpretation of the fossils there seems little doubt that the history of vertebrate life is far longer than we had suspected only a few years ago.
Interestingly enough, the eyes and muscles mentioned are the primary points of contention in the arguments about the appropriate placement of this species taxonomically. The neural development described by the discoverers is surprising for this early date.
See this weeks Science Magazine, for more on the discovery.
<P ALIGN=“CENTER”>Tris</P>
I doubt very much that the neutrino is the best candidate for all of the supposed ‘dark matter’ in the Universe. I remember reading somewhere that neutrinos are constantly passing right through the Earth, and travelling at (or very close to) the speed of light.
IF neutinos have a mass, it would have to be almost infinitely small, as any object moving at the speed of light would attain infinite mass, and since anything with infinite mass would fill the entire Universe, this is very unlikely from my point of view.
If someone can show me a clear, scientific, unbiased and independant website which says that neutrinos have (even an infinitesimal) amount of mass, then I MIGHT JUST be inclined to believe that dark matter could be accounted for by neutrinos. This is an explanation that I would like to believe, because to me, dark matter cannot be simply a load of rocks floating around out there. Or black holes (unless something miraculously enlightening turns up in one of the ‘Black holes’-related debates).
“Now be quiet before I rather clumsily knight you with this meat cleaver” - Edmund Blackadder
Matt said:
I don’t know that anybody said it was the “best candidate for all” dark matter. In fact, I’ve repeatedly pointed out that you are standing on “dark matter” but you seem to keep missing that point. It is, however, a candidate for some of the dark matter.
Er, where do you get the idea that anything of infinite mass would fill the entire universe? Some of the heaviest things we know of are also amongst the smallest (the singularity inside a black hole, for example, or a neutron star, which is estimated to weigh about a billion tons per cubic centimeter). And, yes, they are thought to have a very small mass. But you know, if you have lots and lots of things with very small mass, they can eventually add up. And there are lots and lots of neutrinos around. To give you an idea of how many: “about 70 billion neutrinos produced by nuclear reactions inside the Sun pass through every square centimeter of Earth (and you) every second.” - Almost Everyone’s Guide to Science, by John Gribbin.
Unbiased and independent from what? Is there a neutrino conpsiracy out there?
I don’t have a website handy, but I think pretty much all particle physicists agree that neutrinos have a very small mass. I’ll see what I can find if I have time.
[/quote]
This is an explanation that I would like to believe, because to me, dark matter cannot be simply a load of rocks floating around out there.
[/quote]
And we all know that physics is based on what Matt can believe…
Scientists Discover That Neutrinos Have Mass
<P ALIGN=“CENTER”>Tris</P>
Thanks, Tris.
I see they announced this result at “Neutrino '98.” Wow, do these guys know how to party or what? 
Actually, I said that neutrinos were the best candidate for “dark matter”, or, “missing matter,” which may be a better term: It’s the matter we can’t find to explain the fact that galaxies behave as if they weigh more than they appear to.
Something on that article tris cited that caught my eye: A detector in Ohio caught some neutrinos from that supernova that was seen in 1987, and they did so more than ten years before that 1998 article was written, which means the neutrinos were detected in 1988 or before, which means the neutrinos got here shortly after the light of the supernova did.
Which means neutrinos travel at near the speed of light, right?
(But how did they know those particular neutrinos came from that supernova and not somewhere else?)
(And if you wanna know “What supernova? I didn’t see any supernova in 1987!” it’s because that supernova is in one of the Magellanic Clouds, two small satellite galaxies which are visible south of the Equator only, and which are named after Ferdinand Magellan.)
Personally, I’m guessing that most dark matter will be in the form of brown dwarfs. We’ve only detected one so far (Gliese 229B), but I’ll betcha dollars to donut-holes that these things are more common than Fundamentalists on the Kansas schoolboard.
“Love 'em, fear 'em, and leave 'em alone.” – Dr. Spockiavelli
The detectors for neutrinos that are in operation in several locations in the world are directional. (Yeah, right, a bathtub full of fluid is directional. Uh, huh.) Computer evaluations of the cascade of particulate interactions are examined to determine the direction from which the neutrino came. Analysis of directional information from disparate locations is a fairly accurate triangulation.
Neutrinos have only very small masses, and the forces in a Supernova are extraordinary. The combination does accelerate them to very close to relativistic velocities. It may be the case that there are other neutrinos that are not moving so fast. Detecting them would be very difficult, with our current technology. If the high-energy neutrinos we are able to detect are only a small portion of the total, the combined mass could be a very significant percentage of the mass of the universe.
Or not.
<P ALIGN=“CENTER”>Tris</P>
CABBAGE, n. A familiar kitchen-garden vegetable about as large and wise as a man’s head.
**Ambrose Bierce **(1842-1914?)
I quote David B -
“And we all know that physics is based on what Matt can believe…”
Well well well, we DO have a moody little one here don’t we? Did I ever even IMPLY that I was basing all of physics on my beliefs? Did I ONCE say that I would not accept any other viewpoints because ‘I am always right’? Oh look… NO!
David - since ‘infinite’ refers to a concept which means ‘without end’ and yes, I accept that a black hole may HYPOTHETICALLY have ‘infinite’ density, it does not have INFINITE mass. If anything had infinite mass, it would have mass ‘without end’ - eg enough mass to fill up the entire Universe. Do you understand now?
By the way, I didn’t ‘miss the point’ when you said the earth and other planets were ‘dark matter’, I just didn’t respond to it because I thought (mistakenly, obviously) you were trying to contribute something constructive to the debate (which, at the time, I thought you did).
Enough of that, I don’t enjoy defending myself from verbal (textal?) abuse.
I don’t personally believe based on the available scientific evidence and astronomical observations (good enough, David?) that there are enough planets/asteroids etc. to make up 80% of the matter in the Universe (as the astrophysicists say dark matter must). I accept that they may contribute to this ‘hidden’ matter, but when you consider the mass of a single star (for example - our Sun) to even the biggest planet in our solar system - Jupiter - there is no doubt that it would take a hell of a lot of planets to make up that amount of matter.
Just one small request - David - if you disagree with me in the future (or vice versa) could you please put something constructive in the debate and reserve flaming me to email - you’re more than welcome to put as much abuse as you want in that, should you feel the need!
On another note - thanks for informing me, Tris, that neutrinos have mass. At last - I can see the light… (fades into distance).
“Now be quiet before I rather clumsily knight you with this meat cleaver” - Edmund Blackadder
Good show, Matt. Ignore everything I’ve said to you except a couple of sarcastic lines I threw in when I got tired of you ignoring all the things I posted to you or you making untrue blanket statements! Wow. You sure showed me.
Hmmm.
I don’t see why we have to limit dark matter to only one species.
The missing matter may well be a combo of neutrinos, brown dwarfs, singularities and something we haven’t even heard of.
Could be. Maybe. Possibly.
It’s immoral to allow a sucker to keep his money.
If we could get back to the OP, I thought you’d might like to see what a Darwin Fish may have looked like: www.personal.u-net.com/~paleomod/p97/aca-intr.htm It’s the analysis of the fossils of Acanthostega gunnari, an amphibian that lived in the Devonian period. It looks for all the world like the “Darwin Fish” logo you may have seen and which I have adopted.
And you could look at paintings of feathered dinosaurs here: www.ndirect.co.uk/~luisrey/
>< DARWIN >
__L___L
I dunno, Jab, I don’t see the letters “DARWIN” on that picture anywhere!
Now they are postulating “brown dwarfs”?
That sounds racist to me. 
AND, making funny of the height-challenged!
Heck, where’ve you been Phaedrus? Brown dwarfs have been the subject of astrophysical speculation for decades. Gliese 229B was first detected 'way back in 1995. They’re called that because they emit a little bit of light, so they’re not black dwarfs, but they give off a heck of a lot less light than white dwarfs do.
I’d personally like to see a brown dwarf survey undertaken. The sun might have neighbors a lot closer than Proxima Centauri!
The truth, as always, is more complicated than that.
Oh, and despite having “dwarf” in their name, brown dwarfs are not made of super-dense degenerate matter (like white dwarfs and the hypothetical black dwarfs are).
Unfortunately, stellar astronomers also use the term “dwarf” to mean “main-sequence star”, so tiny Proxima Centauri, the sun, and even the massive star Sirius all qualify as “dwarfs”. Calling an average-sized specimen a “dwarf” sounds like penis envy to me.
The truth, as always, is more complicated than that.