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"Dark Matter" Observed
An anonymous submitter writes: "The space news site Space Flight Now has an article about the first direct "observation" of so called dark matter. Galaxies appear to have more gravitation (mass) than we can currently observe. The theory of dark matter tries to explain this missing mass by the existence of massive bodies too faint to detect. These bodies include everything from dim stars to exotic particles called WIMPs. The previously dark matter, a dwarf star, was detected when it passed in front of a brighter blue star, creating a gravitational lens. It is thought that there are many more like it out there creating all that extra gravity, we just can't see them." Wired has another story; or see the European Space Agency's original article.
It was indeed Bohdan Paczynski who wrote the first paper about that specific phenomenon, if I recall correctly, the paper was titled "Microlensing on small optical depths". And indeed, he was the one who invented the term "microlensing".
However, I'm more concerned with "extragalactic" microlensing. The funny thing is that stars in remote galaxy can cause microlensing of even more remote quasars. This was first discussed by Chang and Refsdal in an article in Nature, December 6 1979.
The great thing about this is that in galactic microlensing, there are very few MACHOs between us and the stars, so you would have to watch a lot of stars (millions), whereas in extragalactic microlensing, there will be lots of stars, so microlensing events happen all the time. You only need to separate it from the intrinsic variations of quasar...
Now, galactic microlensing has been a so much bigger field of study than extragalctic microlensing, we haven't really got that much attention. In part, it can be becuase galactic microlensing gives so much more solid results, but then, it is just addressing what's going on in our backyard, while the extragalactic microlensing really deals with the universe... :-)
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I, too, used to think that 'dark matter' was some powerful, mystic thing that sucked in light like a black hole.
:)
My current understanding is that dark matter is just normal matter that doesn't emit light. For reference, all matter does 'suck in' light (meaning the energy is absorbed, usually given off as heat).
So, I'm gonna go soon, and eat my dark-matter lunch
Moderation: Put your hand inside the puppet head!
I think the term "dark matter" does not necessarily apply only to non-luminous matter. I think it is used to refer to any unobserved matter that can account for the apparent gravity we see in galaxies. MACHOs have been a candidate for dark matter for a while, because they are mostly failed stars that do not emit light (at least not enough for us to see), though they do interact with the EM field. Other candidates for dark matter are indeed non-luminous, even non EM interacting (WIMPS-weakly interacting massive particles--that only interact with the weak nuclear force and gravity, but not EM so they can't be "seen" using light).
No they haven't. Let me quote from a Scientific American article on dark matter.
Hey, notice that part where they say a variety of explanations are offered?
(BTW, what do you mean by "invisible" other than it doesn't have light bouncing off of it?)
The term "dark matter" has wound up being overloaded in astrophysical discussions, because it has been used to name the solution to a number of different problems.
First, people noticed that we cannot observe enough luminous matter to either produce a flat universe, or account for the dymanical behavior of large-scale systems. This was long assumed to consist of halos of cold gas, dust, brown dwarfs, etc.
However, cosmological considerations (especially primordial nucleosynthesis) rules out this scenerio, because we can use the deuterium mass fraction to calculate the ratio of photons to baryons in the early universe. We know how many photons there are (per comoving volume, as usual), and it turns out that there are only enough baryons to account for about 4% of the density needed to produce a flat universe. Since the universe is not noticably non-flat, we can assume there is "a lot" of non-baryonic matter out there, in axions, massive neutrinos, or something more exotic. This stuff is called non-baryonic dark matter, unsurprisingly, and often gets confused with the other kind.
Finally, in the last five years or so we have received a couple of cool new data points: the angular size of the first harmonic mode of perturbations in the cosmic microwave background, and the distance scale to various redshifts, as seen using type Ia supernovae. The CMB data tells us that the universe really is flat, to high accuracy; otherwise, the perturbations -- we know how big they should be after all -- would be "lensed" by the curvature of spacetime. The supernovae data tells us that -- BIG surprise! -- the universe's expansion is accelerating, not slowing down at all. This implies that there is actually more vacuum energy than matter and energy combined. Best guess, the universe is roughly 70% vacuum energy, 30% matter. For some bizarre reason, people have been calling this the "dark energy" lately. Thus, even more confusion about what you mean when you say "dark matter".
Quantum mechanics: the dreams that stuff is made of.
> My biggest problem with modern science (physics
> and astrophysics in particular) is this truly
> inane method of making "conjectural"
> observations...that is, assuming that and
> unobservable activity has been proven simply
> because something observable has occurred.
OK. We know from the distribution of light and the measured rotational velocities of most galaxies we can see that they seem to be embedded in a large halo of gravitating mass. This has been measured and confirmed many, many times over that past 40 years.
When you add up the total amount of emitted light from a galaxy, you can get an estimate of it's mass and that turns out to be about 10**12 solar masses, say.
Looking and the dynamic motion of the galaxy using the Doppler shift of spectral lines from stars in the galaxy, you calculate that the required amount of mass for the galactic motion is roughly 100 TIMES the amount you count up by counting stars/gas/glowing stuff alone.
1) Maybe this 'dark' matter is not being illuminated by stars? No - do the calculations and it turns out that this stuff would be detectable. Instead, we see nothing. So, we can rule out baryonic (protons, electrons, photons) matter. Therefore, it has mass but it doesn't interact with baryonic matter - it is only gravitationally coupled with baryonic matter.
2) Maybe it is condensed into cool stars that we can't see? Again, no luck there. Really dim stars are hard to detect, but over the past 5 years, enough have been detected to make a guess as to whether dark matter is this form. There isn't.
So, we still have no clear idea what dark matter is made up of, but a lot of ideas that we can test. I'll admit that it's incredible, but believe me, there's a lot of evidence for dark matter. Alternative hypotheses, such as modified long-range forces have been tried out and don't work (and no, it's a separate issue from non-zero lambda cosmologies!) so we are back into the 'small, energetic, low mass subatomic particle' game.
What we are NOT doing is inventing dark matter, as you imply. We tend to leave it to the mystics.
If you're interested in the more detailed reasons why, please feel free to contact me.
mak at as arizona edu