"Dark Matter" Observed 209
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.
Galactic vs. extragalactic microlensing (Score:5, Informative)
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... :-)
Misconceptions (Score:2, Informative)
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
Re:Um, if it's a star it can't be dark matter.... (Score:3, Informative)
Re:Misconceptions (Score:2, Informative)
Nucleosynthesis (Score:0, Informative)
The relative abundance of the light elements is supposed to have been determined by the Big Bang, and that sets a limit on ordinary matter. I also understand that Guth's inflation theory predicts that the Universe has the critical mass density, which is many times both the visible mass as well as the allowed ordinary-matter mass, hence the search for exotic forms of matter (WIMP's -- weakly-interacting massive particals).
Does anyone have good mass census numbers for M1 (fraction-of-critical-mass in galactic stars), M2 (FOCM seeming to hold galaxies together), and M3 (FOCM seeming to hold galaxy cluster together)? I thought M1 = 0.001, M2 = 0.01, M3 = 0.1 is roughly the current numbers. That means that those lumps of dirt (M2 - M1) is less than 1 percent of critical mass, although M2-M1 needs to be some kind of dirt lumps (or mini black holes) not to go flying off.
Does anyone know the FOCM for mass of stuff (baryonic particles) from the Big Bang nucleosynthesis limit?
Re:Fate of the Universe . . . (Score:1, Informative)
Re:damn it... (Score:3, Informative)
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?)
Re:Um, if it's a star it can't be dark matter.... (Score:1, Informative)
Re:Galactic vs. extragalactic microlensing (Score:4, Informative)
"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 [harvard.edu]. 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:Fate of the Universe . . . (Score:1, Informative)
That statement is completely false.
> Without dark matter, there is insufficient gravity to bind all matter together forever.
That is true - but there isn't enough dark matter to do that.
> If there is enough dark matter, with its attendant gravity, then eventually the universe will collapse back onto itself. Probably the end result of that would be another Big Bang.
We know that cannot be the case. It is a simple game of adding up the energy in the universe, and using the information: 'we know the universe is flat'.
Since the universe is flat (to very high precision - see the 'boomerang' experiment), we know that the total energy density of the universe is 1. If it is less than one, then the universe has negative curvature. If it is greater than one, then the universe has positive curvature, and is finite.
Now, it used to be the case that cosmology was about finding how much matter there was, and seeing which one of the three cases is the one we have. Unfortunately, over the past two to three years, things have gotten much more complicated.
It turns out that there exists a cosmological constant. The cosmological constant acts like matter with a negative pressure, yet with a positive energy density. (Strange stuff - it has earned the nick-name 'dark energy') The cosmological constant causes the universe to experience a force causing it to expand. This force is proportional to the size of the universe.
Using several experiments, we now know that the energy density of the cosmological constant is 0.7, and since the universe is flat - this leaves 0.3 for the rest of the matter. Since about 5% of the matter required for a flat universe was previously known (it is the luminous stuff), there needed to be a third category - dark matter - making up 25% of the total. If more of these low-mass dim stars are found, then a greater fraction of the 25% will be known to be normal baryonic matter.
So what does this mean for the fate of the universe?
The answer is 'absolutely nothing'.
Since we know the value of the cosmological constant - and we know how much matter is in the universe the answer is already known. The universe will expand exponentially - and we'll end up with the heat-death scenario. Finding that the fraction of dark matter which is baryonic is greater does not change this fact.
Note: The exponentially expanding universe is not an 'abnormal' state. It has done this before. There is good evidence for the 'inflation' era near the big bang, when the visible universe grew from something the size of a proton - to something the size of an orange in an extremely short time.
Re:damn it... (Score:1, Informative)
Nobody is assuming that anything is proven! Dark matter is still considered to be one of the largest unsolved questions in astrophysics.
Yeah, but then, that's your own strawman misconception.
It's invisible to us, because we haven't seen it!
Not unobservable in principle. You are dark matter, according to an astrophysicist's definition, because if you're far enough away, astronomers won't be able to see you, yet you will exert gravitational influence upon things. It's just that dark matter in that form of people, or even planets, can't make up much of the universe. Burnt-out or failed stars (MACHOs) might be.
You're describing shadow matter, a particularly far-fetched proposal coming from certain high-energy theories. It's hardly accepted.
Yes, that's one proposal: that's exactly what MACHOs are! Didn't you read the article?
What the article didn't say, however, is that microlensing observations have already put constraints on how many events we should have seen by now. And we should have seen much more for all of the dark matter to be MACHOs, so some of it is
likely something else. WIMPs are the main alternative candidate.
As if you know what physicists do and don't believe. We don't sit around all day trying to think up the most wacked-out explanations possible. The vast majority of astrophysicists would be very happy if all the dark matter turned out to be normal baryonic matter, because it doesn't require any exotic new speculative physics. (The particle physicists would be disappointed, of course.) The problem is that there are actually reasons why observations suggest that all of the dark matter is not normal baryonic matter. These ideas aren't made up for the fun of it, you know.
Thank you, Mr. Expert. Perhaps if you would like to read the past several decades of literature on the subject, you would find all the papers in which people tested the "very simple explanations" and found them wanting.
Nobody has come up with any unprovable theories.
"Irrational" phenomena? There are very many rational reasons to believe that dark matter exists -- the only question is what it is.
They are unexplained, you nitwit. There are tons of theories floating around, but nobody knows what all of the dark matter is. Anybody who found out would have the Nobel prize by now.
Alternate Explanation: MOND (Score:1, Informative)
MOND stands for MOdified Newtonian Dynamics. It is a modification of the usual Newtonian force law hypothesized in 1983 by Moti Milgrom of the Weizmann Institute as an alternative to Dark Matter.
MOND can be interpreted as either a modification of gravity through a change to the Poisson equation, or as a modification of inertia through a breaking of the equivalence of inertial and gravitational mass.
The modification occurs at very small accelerations. Above a critical acceleration a0 (the one parameter of the theory), everything is normal. Below a0, the effective acceleration approaches a = (gN a0)1/2, where gN is the normal Newtonian acceleration. The two regimes are joined smoothly by an interpolation function mu(x) with the asymptotic property mu(x) -> 1 for x >> 1 and mu(x) -> x for x 1, where x = a/a0.
Much more, including links to literature, experimental results, IAS proceedungs, etc. call all be found though:
http://www.astro.umd.edu/~ssm/mond/
Re:Um, if it's a star it can't be dark matter.... (Score:3, Informative)
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".
Re:damn it... (Score:2, Informative)
> 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