Examining the Expected Effects of Dark Matter On the Solar System 190
First time accepted submitter LiavK writes "Ethan Siegel recently wrote a great post for ScienceBlogs discussing the expected total mass of dark matter in the solar system. As far as we can tell, dark matter only interacts weakly, via gravity, both with itself and normal matter. So, it can't collide with itself, meaning that it has no way of getting hotter and radiating away energy and momentum. This means that it remains a diffuse mess, with a density that is ridiculously low, to the point where detecting its local effects is likely to remain... challenging for the foreseeable future."
Re:The problem with dark matter (Score:5, Informative)
The problem with dark matter observation in this case is that science is based on empirical observation. If you can't see it, can't measure it, and can't even draw inferences from what you can see and measure to detect something indirectly... it's not science. What this is saying is that the effects are so miniscule that there is no equipment presently capable of separating an actual effect or observation from systemic inaccuracy in the equipment itself. That is, you can't tell whether it's just random 'noise' or an actual signal.
But we do find it empirically. There is extra mass there, affecting other objects. We can detect it through it's gravitation, just not through light. It's a very strong signal, for example in the rotation velocity of galaxies. A lot of other science is, too, done without directly detecting the object of study, but through indirect effects and inference.
Everyone would like to get rid of Dark Matter. But its effects are clearly there. And we need to explain it. It does not have to be particles, or a kind of matter we know. You can call it something else than Dark Matter if you don't like the name. Anyone is welcome to come up with explanations. But they have to be in agreement with the observations.
Re:General relativity (Score:2, Informative)
Rebuttal: Bullet cluster.
Re:Just the opposite (Score:5, Informative)
Michelson and Morley found nothing, they were full of shit.
They found nothing, and that was their great discovery. http://en.wikipedia.org/wiki/Null_hypothesis [wikipedia.org]
Re:The problem with dark matter (Score:5, Informative)
Why invent exotic matter when the right combination of dust could be the answer?
Simply put, because baryonic matter (ie. dust) radiates. This article would be titled, "Why our instruments are sensitive enough to detect all that dust that's affecting galaxies and superclusters rotation" if it was dust.
Here's a recent summary paper [arxiv.org] on the evidence for nonbaryonic dark matter. Dust has, alas, been hypothesized, tested, and rejected.
Re:The problem with dark matter (Score:2, Informative)
"Extremely good reasons" is not the same as proof. And there are other possibilities, like large numbers of rogue planets, or multiple different mechanisms explaining different phenomena.
Don't get me wrong: weakly interacting dark matter is plausible, but until there is independent, direct evidence and observation, it remains just plausible speculation.
Re:The problem with dark matter (Score:3, Informative)
[sarc]Naturally, everything you see on teevee is true and accurate, and all distinguished presenters are to be trusted, and all science program scripts are written for maximum accuracy and conveyance of relevant information. Why would we ever question something we saw on a tv program? Tune in for next week's "Ancient Aliens" for proof that the anti-TV conspiracy started in ancient Egypt![/sarc]
People interested in real science don't get their science from TV. People interested in real science learn from books written by scientists, from papers written by scientists, and by talking to real scientists doing real work in the actual field. Everything else is just the pop culture treatment.
-dentin
Re:The problem with dark matter (Score:4, Informative)
Dark matter, in various forms, is the hypothesis(ses) that explain empirical observations. For the last couple of decades we've been at the stage of hypothesizing various kinds of dark matter and testing them to see if they fit. The one that fits best so far, and is thus the leading contender, is a new kind of subatomic particle that interacts weakly and is fairly heavy. The dark matter story is an excellent example of how science is supposed to work.
Re:The problem with dark matter (Score:5, Informative)
Planets are just clumps of dust.
But clumps of dust with a really low surface area for the mass involved. For example, Jupiter has a density of 1,330 kg per square meter and an average radius of almost 70,000 km (7*10^7 meters), a third more than water at STP. If instead, Jupiter were broken up into many equally sized balls of a smaller radius, then the mass stays the same, but the increase in surface area is inversely proportional to the decrease in radius.
For example a Jupiter-mass cloud of micron sized spheres, each with the density of Jupiter, would have a surface area 7*10^13 larger than Jupiter. That surface area incidentally happens to be roughly a twentieth of a square light year (roughly 4*10^30 square meters by my calculation) meaning at the right densities, such a cloud could intercept and radiate a lot more energy than Jupiter could, perhaps even be visible in small amateur telescopes at a few lightyears.
My point here is that some baryonic matter is a lot more visible, many orders of magnitude more visible, than other baryonic matter. And planet-sized objects are going to interact mostly by gravity as well meeting most of the desired characteristics of dark matter.
My take is having a significantly higher than expected fraction of the mass of your galaxies in rogue planets and similar things would be a way to account for dark matter.
But then there's the early universe observations. For example, the most damning evidence against dark matter hiding in planets and such, is observations of the cosmic microwave background [learner.org] (CMB), which is effectively the study of the period of the universe in which it started to become transparent to photons (about 400k years after the big bang according to the above link). That period of time is not a lot of time in which to create massive objects. And the fluctuations of the CMB yield dark to visible mass of roughly 5 to 1 (again according to claims in the above link).
So that indicates to me that there probably some sort of exotic matter out there which we haven't discovered yet.
Re:The problem with dark matter (Score:5, Informative)
Yes, we know, 100% that is is transparent. There are HUGE spots in the sky where there is gravitational lensing affecting background galaxies, but no obstructions in front of the galaxies. Something is causing the gravity, but it is letting the background light through perfectly clearly, minus the lensing.
We're not talking about small amounts of gravity either, whole galaxy masses worth. If you had a galaxy worth of gravitational lensing, you'd hope to find something causing it. Instead the background light comes through crystal clear, like nothing is there.. hmmmm...
Re:The problem with dark matter (Score:4, Informative)
The reason for this was that the MACHO theory made very specific predictions that could be tested using sensitive instruments, such as gravitational lensing (remember, there is supposed to be enough to dramatically effect the amount of gravity acting on a galaxy) and others (which I won't get in to). This was one of the first and most strongly believed in theories when dark matter was detected, so you can be sure that astronomers fought for it until the evidence against became too overwhelming.
That being said, there are still some astronomers researching MACHOs, since they have been detected, just not in the amount that accounts for the unexplained gravitational effects.
Re:Dark Matter, the Aether of the 21st century. (Score:5, Informative)
Let me know when they find supporting evidence [of dark matter]
You mean like the rotational curves of galaxies, the velocity dispersion of stars in galaxies (including observations of globular clusters with very little dark matter, leaving MOND with even more problems), gravitational lensing (including the bullet cluster), fluctuations in the cosmic microwave background etc. [wikipedia.org]?
Mind you, I can't forward a better theory to explain why things have mass
Dark matter has nothing to do with why things have mass. That would be the Higgs field (or, rather, why fundamental particles have mass. Most of the mass of normal matter has another explanation).
Re:The problem with dark matter (Score:2, Informative)
What's all this bullshit about paywalls? Could you provide me with a link to the papers that are being paywalled that you're bitching about? I'm on a university network -- I'll post the fucking things up myself if you want. Though since this is astronomy I'd be shocked if those papers aren't available through the arXiv or, if they're old enough, through adsabs. Have a hunt before you start accusing people who have devoted years of their lives to educating themselves on the topic of being religious zealots involved in some farcical conspiracy theory.
"Frankly, I am still convinced that the root of this whole problem lies in incorrectly estimating galactic mass density."
Bully for you. Write it up and try and publish it - the referee will tell you to fuck off if it even gets that far and, on a challenge from you, will provide you with references as to why this doesn't work.
"I have not seen any raw data on this subject; do you know where I might get it?"
Gee, let me fucking Google that for you shall I? First result, follow a link through to the updated page, and there is raw data here: http://astroweb.case.edu/ssm/data/ There are literally millions of pages with the raw fucking data. Do your own research in the future.
"Just looking at a picture of a galaxy gives the impression of a more or less flat disk with density not too far from uniform."
Ohhh, stop the science! 'Chemisor' can solve everything by just looking at a picture of a galaxy! Which galaxy? You are aware that there are plenty of galaxies that aren't spirals, right? There are low surface brightness galaxies with no discernible shape, and giant ellipticals that are, as the name suggests, elliptical, and that *every single one of these* has a rotation curve problem?
"A flat uniformly dense disk will have a flat velocity curve, so my observation can't be too far off the mark."
No it won't, so yes it would, and the ignorance on display is pretty impressive.
"I would visually estimate that maybe half of the galactic disk would be dark matter, far less than the typical predictionsI am seeing."
Now, why have we spent so much money on telescopes and mainframes when we could simply send you some grainy pictures and you could visually estimate the amount of dark matter?
"Something is really fishy in those calculations and I would really like to check it."
Knock yourself out.
"Where is the data? Not released by researchers."
Yes it is.
"Where are the papers? All hidden behind a paywall nobody can afford."
Not the case. Almost all astronomical papers are available. In the unlikely event they're not available then email the author directly; he'll either mark you down as a crackpot (no idea where he'd get that idea) or will email you a copy of the fucking paper. We retain the right to distribute preprints; we can do that.
"Where is the science?"
Not in your post, that's for sure.