I sent this question in to the SD:
I recieved this as part of their reply:
I realize that substances can be detected and analyzed via light/radiation passing through them, but basically anything that’s not burning brightly can’t be seen - dark matter, no? So, what else does “dark matter” mean, and why is it so suprising that it’s there?
What’s surprising is not that there is dark matter, but that the calculations that led scientists to hypothesize its existence indicate that there’s a hell of a lot of it.
You could take all the non-radiating mass of the solar system as a whole, and it would make up a tiny percentage of the mass of the sun. So in observing the Universe, Scientists look at the shiny things, and reasonably assume that any planets, asteroids, etc. are negligible.
Given the knowledge we have about luminous objects (it’s extensive) we can tell just be observation and calculation how much mass there is in the Milky Way, for example, and there for how much gravitational force all these masses exert on each other.
But this total gravitational force is not enough to hold the galaxy together by a long shot. Even if we assume a super-giant black hole at the center of the galaxy, there’s still not enough gravity to make the galaxy look the way it does.
So there seems to be some unaccounted-for source of gravity, and the only source of gravity that we know of is a body with mass. The amount of mass needed to make the universe lok the way it does from a gravity perspective indicates that all the luminous bodies we see can only make up about 10% of the mass there must be.
Planets don’t add enough mass, in our experience, so there is some other type of “dark matter” that makes up most of the mass of the universe that we have not detected yet. So the hypothesis goes. No one, to my knowledge has developed a way to find this dark matter yet.
BUT, there is recent research that suggest that neutrinos have mass, so maybe that would account for it.
Was this from an SDMB thread? Maybe you could post a link to it so I can read it in context. I don’t think it’s right - as far as I know, dark matter refers to any matter that does not radiate observable radiation. That includes planets.
No, this was from an e-mail reply, the rest of which was “auto-reply boilerplate”, if you know what I mean.
OK, well I wonder if the kind of “normal” heavier elements we’re used to could account for the gravitational effect we see, or does it have to be something else like neutrinos, quantom foam, or such like. Given the probable age of the universe, could enough what we consider normal matter have been created by stars, or is this out of the question? Has anyone tried to calculate this? Thanks.
:smack: I just noticed that you said luminous matter would only account for ten percent of of the gravitational effect. That seems incredible. You might say “suprising”.
But now I’ve heard that, since the universe seems to be expanding faster with time, they want to postulate an anti-gravitational substance to explain this. I saw this on PBS’ Nova, so you might be able to find this on their site:
Wouldn’t this lessen the need for a great deal of matter to explain what we observe?
The Anti- Gravity Substance-(if it exists)
which is sometimes called Negative Energy-
acts independently to the Dark Matter- (if it exists)
which acts inside galaxies in order to make them rotate in the way that they are observed to do-
galaxies rotate as if they were many times heavier than the stars we can observe, while the universe expands as if it were accelerating and contains some sort of Negative Energy wich releps individual galaxies.
Got that?
me neither.
Candidates for Dark Matter include
Black holes
Dyson Spheres
Neutrinos
Axions
Photinos
Wimps
monopoles
cosmic string
domain walls
textures
the gravity from adjacent branes
mirror matter
and some others I’ve forgotten…
Sci-fi worldbuilding at
http://www.orionsarm.com/main.html
-contains some sort of Negative Energy wich repels individual galaxies.-
is what I meant to say.
Sci-fi worldbuilding at
http://www.orionsarm.com/main.html
Yikes! I have some reading to do. I’ve never even heard of some of these “candidates”. Dyson Spheres, Wimps, mirror matter, textures, domain walls… Any recommendations for the layman? Thanks!
Actually, astrophysicists are quite sure that the dark matter can’t all be planets, sub-stars, and the like. In fact, it can’t be anything made of protons, neutrons, and electrons.
The problem is, the abundance of helium and lithium post-BB are very sensitive to the density of protons and neutrons in the early universe. If there were 10 times more of them, BB predictions for the helium and lithium abundances would be way off from the observed abundances.
The discovery of dark energy doesn’t change the amount of dark matter needed, because there are enough different kinds of observation to separate the dark matter and dark energy components.
The books by Timothy Ferris, Coming of Age in the Milky Way and The Whole Shebang, are a good place to start. Not sure if he gets to domain walls and mirror matter, tho…
BTW there was a recent report that some galaxies have ** no dark matter**! How to lose 90% of your mass? That’s quite a trick.
Since I’ve never seen anyone describe the alternative–what SHOULD our galaxy be doing with its paltry 10% of necessary matter (pretend dark matter is a fiction)? Should it be rotating much more slowly/quickly? Flying apart at the seams? Forming itself into a simple glob rather than a disk with spiral arms?
I imagine that our galaxy would need to be far more compact to counteract the centripetal momentum of masses.
There’s been at least one popular science book devoted entirely to dark matter: Riordan and Schramm’s Shadows of Creation. But I can’t really recommend it since a) I haven’t read it and b) it was written back in 1990, so it won’t be particularly up-to-date, though it may still be a useful introduction.
Some of the candidates on the list were never serious one; I suspect Dyson spheres were suggested in much the same vein as were bound copies of Astrophysical Journal. (The point of the latter old joke was that since the critical density is so low, dark matter can be pretty thinly spread and hence hard to detect even if in mundane form.) Most of these topics, even the ones that were no more than off-the-wall suggestions, have however probably rated at least one article in New Scientist over the last decade. Especially the off-the-wall ones.
It rather depends what sort of dark matter you’re talking about. The original source of the problem was to explain the rotation speed of the outer parts of galaxies. Looking at different parts of a galaxy and using the Doppler effect, astronomers could work out how fast different parts were rotating around its core. The surprise was that the outer reaches were rotating faster than anticipated. It looks like the mass distribution of a galaxy as you move away from its centre falls off slower than the distribution of stars does. In this case, the answer’s straightforward: the stars on the fringes wouldn’t be rotating about the centre quite as fast as they actually are.
However, this is only part of the issue. There had always been a school of thought that hoped that the density of mass in the universe would be the critical density. This answer tended to be preferred by theorists, since the arguments in favour were largely aesthetic, even philosophical, ones. Observational astronomers were more “we see only enough for 10% or so of the critical density, so why can’t it just be that ?” Realising that there was lots of unseens stuff out there obviously strengthened the first school’s position. However, you can handily explain galactic rotation curves without being forced to have the critical density.
At more or less this point, galaxy formation got dragged into the question. People were modelling how galaxies formed after the Big Bang and they’d run into a problem with their models. Galaxies didn’t form. At all. Being able to invent odd new forms of matter gave them a whole new game to play. For a start, the higher the density the easier it is to form the clumps of stuff that become galaxies. Different solutions for dark matter give different scenarios, some of which work and some of which don’t. For instance, it was realised early on that neutrino masses can’t be the answer by themselves. Why ? Even with a mass, the neutrinos would still be terribly zippy and wouldn’t form clumps due to gravity. Hence they didn’t help form galaxies. Over the last two decades, this sort of simulation has become a field in itself. Because how galaxies are scattered about statistically will depend on how they formed, observations of the universe at large have helped eliminate some of the possibilities.
All messy stuff. But it does lead to an even simpler answer to your question. There wouldn’t be galaxies in the first place.
The “dark energy” (whatever it is) is not “negative” in any meaningful sense. And it does allieviate some (but not all) of the need for more conventional dark matter, depending on whom you ask. You see, there are two different reasons to suppose the existence of dark matter. First, galaxies apparently contain a lot more mass than the amount that’s glowing. That’s been well-established for decades, leading to the conclusion that there’s “galactic dark matter” out there. The second problem is that there’s theoretical reason[sup]*[/sup] to believe that the Universe is flat (that is to say, Euclidean geometry works on large scales), and you need a certain amount of density to cause that flatness. Even if you include all of the galactic dark matter, you still don’t have enough, again by about a factor of ten. So the rest of it needs to be “cosmic dark matter”.
Now, this “dark energy” stuff does act as “cosmic dark matter”, and in fact, appears to account for about 70% of the density of the Universe. But it would not account for what we observe in galaxies, and there’s not enough of it in any single galaxy anyway (it’s distributed uniformly, and most of space is between galaxies).
[sup]*[/sup]Actually, we now have experimental evidence, as well, that the Universe is flat, or very nearly so. But at the time that the cosmic dark matter problem was first discussed, it was just theoretical.