"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.

Comments

Galactic vs. extragalactic microlensing

[KjetilK]

Yep, these are really interesting observations! Galactic microlensing, which is discussed in this article, is a field which is growing rapidly and has attracted a lot of interest. I look forward to seeing the lightcurves of this event.

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... :-)

Re:Galactic vs. extragalactic microlensing

[KjetilK]

Well, the term isn't really in use. Most probably, most people would think about Einstein's speculations around gravitational lensing. Einstein considered gravitational lensing, but only deflection by stellar masses, and concluded therefore that the phenomenon would most probably remain unobserved. Since "galactic microlensing" refers to unresolved images of an object lensed by things in our galaxy, one could argue "galactic macrolensing" should refer to resolved images of objects lensed by things in our galaxy, but no such object has been seen, and Einstein was probably right in that we won't see it for a long time.

"Macrolensing", by itself, usually refers many different situations, but characterized by that several images of the object is resolved. There are a few known objects. This database includes only multiply imaged quasars, mostly lensed by a single galaxy, but you can have lensing by galaxy clusters as well.

Actually, the question arised some controversy here among my fellow students as to whether what is known as "weak lensing" should be considered a part of macrolensing, but after consulting The Book, we figured it probably shouldn't.

Re:Um, if it's a star it can't be dark matter....

[nerdlyone]

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).

Re:damn it...

[kaisyain]

.all this time, physicists have assumed that "dark matter" - the matter that provides a great deal of the gravitational force that holds the universe together - is "invisible" or "unobservable" or in some extreme cases "existing in a separate yet intertwined reality".

No they haven't. Let me quote from a Scientific American article on dark matter.

Astronomers and physicists offer a variety of explanations for this dark matter. On the one hand, it could merely be ordinary material, such as ultrafaint stars, large or small black holes, cold gas, or dust scattered around the universe--all of which emit or reflect too little radiation for our instruments to detect.

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?)

Re:Um, if it's a star it can't be dark matter....

[Captn Pepe]

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".