Slashdot Mirror
Galaxy Sans Dark Matter
ChromaticDragon writes "Astronomers have crunched some numbers on a galaxy to discover that its rotation can be fully explained by the gravity of the observable matter — in effect, this galaxy seems to lack dark matter. This shouldn't come as a total surprise given that one of the stronger observations of Dark Matter was the
Bullet Cluster where supposedly a good deal of Dark Matter and good old fashion regular matter had separated."
I'm not sure if it is the story the submitter was trying to link to, but this article seems to cover the subject.
Tomato wedge sperm darts that are Republican.
I'm not the GP but can answer your question.
...which leads to how do you measure doppler shifts.
You measure the dopler shift of the stars on each side of the galaxy. Waves from stars travelling towards you compress, waves travelling away from you so it helps if galaxy is seen more edge on than top or bottom towards us.
(Of course it won't be perfectly edge on so you have to calculate the component that is edge on to work out actual speeds around the galactic center. The less edge on the more accurate you can be because the component that's edge on is larger).
One way is to look at the spectral lines of light in a star (ie split the light through a prism or diffraction grating). Chemicals that make up the star's surface absorb at precisely known wavelengths. It's actually really easy to do some calculation once you know what wavelength these lines have shifted to. (I did it when I did my astronomy masters. It's basic algebra andsimple equations). The difficult part is building equipment that can measure spectra so accurately. In the early days they'd be literally measuring the difference between wavelengths on glass plates.
http://aether.lbl.gov/www/science/galrotcurve.html
"To make a rotation curve one calculates the rotational velocity of stars along the length of a galaxy by measuring their Doppler shifts, and then plots this quantity versus their respective distance away from the galactic center."
These posts express my own personal views, not those of my employer
When we say "Galaxy Rotation Speed" we mean a measure of velocity as a function of distance from the center. To get this, you just measure the speed of stars and measure how far they are from the center. As to how you get a stars velocity, well you look at the red shift relative to the galaxy, that if the whole galaxy is moving away it will have a redshift, but stars rotating away from us in the galaxy will have a higher redshift on average, and stars rotating towards us have a lower redshift.
At least that's how I imagine they do it... IANAABIAAC (I am not an astronomer but I am a cosmologist), so I'm a little rusty on all this nearby stuff.
Umm, no.
There are some good reasons to believe it isn't normal matter that isn't making light. For starters one would still expect it to absorb light and thus be observable. Additionally our models of galaxy formation would suggest it should have a certain distribution which doesn't conform with what is necessery to explain the rotation behavior. In fact it may even need to be relatively free from interactions to be as spread out as needed. Most relevantly the observations that suggest that dark matter doesn't collide with itself or normal gas when galaxies collide suggests it isn't normal matter.
Of course your general sentiment is right. There are reasons to believe dark matter isn't made up of neutrinos but it isn't any more mysterious than they are. It is probably just some weakly interacting particle much like those we have already discovered.
If you liked this thought maybe you would find my blog nice too:
Note, by the way, that you could apply your same argument to the people who claimed that an unseen planet was perturbing the orbit of Uranus. "You're just adding new garbage to explain away the failures of Newtonian gravity!" But there really was "dark matter" detectable through its gravitational effects: Neptune. On the other hand, Mercury's perihelion precession was not explained by new unseen matter ("Vulcan"), but by a new theory of gravity (general relativity). The lesson: you can explain gravitational anomalies by either dark matter or modified gravity, and both can be right: it's up to Nature to decide. The original calculations of galactic rotation used _Newton's_ equations! That is why they came up with answers at odds with observation. They completely ignored General Relativity, the accepted law of gravitation. This has been pointed out many times. It's been pointed out many times by people who obviously have never calculated anything using general relativity.
Someone who has calculated orbits in general relativity would tell you that GR is utterly incapable of producing observed galactic rotation curves — GR is only a second order correction to Newton, i.e., it's too similar to Newtonian gravity on those scales to produce such a large discrepancy as what is observed. Explaining galactic rotation curves using modified gravity requires a first order correction, i.e., not just a small addition to the Newtonian acceleration GM/r^2, but rather a replacement of it. That's why the main competitor to dark matter has been MOND, which is Modified NEWTONIAN Dynamics. (They use a gravitational acceleration of sqrt(GMa0)/r, where a0 is a new physical constant.) Unfortunately MOND fails observational tests such as the Bullet Cluster dynamics. (Well, it's still possible that MOND is true if there is also dark matter to account for things like the Bullet Cluster, but that kind of defeats the purpose: MOND alone is no replacement for dark matter.)
It turns out that GR can produce some measurable deviations from Newtonian dynamics at the galactic scale, with the right matter distribution, but it doesn't come close to getting rid of the need for dark matter, and using it to fit galactic rotation curves screws up galactic cluster observations, so you can't win. As I said, dark matter is unique in that it's compatible with observed phenomena on many different scales.
Now, GR is relevant to other gravitational phenomena, such as cosmology, but the point is that you can't use GR as a replacement for dark matter. Believe it or not, astrophysicists do know about GR. The reason why they don't use it instead of dark matter is not because they are mind-bogglingly stupid and somehow managed to forget that GR exists. It's because it doesn't explain our observations.
Incidentally, if you're thinking of the preprint by Cooperstock and Tieu which was reported on Slashdot 3 years ago, which claimed that galactic rotation curves are explained by GR, that was disproved a couple weeks after the preprint was posted. Sadly, they keep posting the same arguments in the form of unpublished Internet preprints every year