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The Search for Dark Matter and Dark Energy
mlimber writes "The New York Times Magazine has a lengthy article on dark matter and dark energy, discussing the past, present, and future. 'Astronomers now realize that dark matter probably involves matter that is nonbaryonic ["meaning that it doesn't consist of the protons and neutrons of 'normal' matter"]. And whatever it is that dark energy involves, we know it's not 'normal,' either. In that case, maybe this next round of evidence will have to be not only beyond anything we know but also beyond anything we know how to know.'"
The 'size' of the universe is an ill-defined question. We can only observe what's in our past light cone, and it is *that* universe which suffers from a budget shortfall of matter/energy.
To follow knowledge like a sinking star, / Beyond the utmost bound of human thought. ("Ulysses", Tennyson)
"Since the invention of the telescope four centuries ago"
I didn't know telecopes were that old. Is this a typo, and didn't they mean decades instead? If not, what did ancient telescopes do?
FooBarWidget, meet Galileo: Widely credited as the inventor of the modern telescope, in 1609.
Though, as with all major developments in human history, some accounts have him as merely improving on preexisting tech, whether copying the work of Lippershey from 40 years before, or even the possibly MUCH older designs of the ancient Persians.
So no, not a typo.
This could all be accounted for by dark matter save for the observations of Type 1A supernovae which indicate accelerating expansion, and this requires domination by a state of matter with negative pressure, and this is what's been coined dark energy.
To follow knowledge like a sinking star, / Beyond the utmost bound of human thought. ("Ulysses", Tennyson)
There is some possibility that dark matter could be non-luminous dust, but there are some limits placed on observations of the comsic microwave background, which would have had to travel over 13 billion light years through such dust without being significantly attenuated.
Galactic dark matter, which is required to explain the rotation curves of spiral galaxies, can be completely explained by baryonic dark matter, which would be at least partially dust.
Extra-galactic dark matter cannot be primarily baryonic. The baryon density of the universe is known from big bang nucleo-synthesis and the primordial H/He ratio, and is too small to account for extra-galactic dark matter. Therefore extra-galactic dark matter has no relation at all to galactic dark matter, as it cannot be made of the same stuff as galactic dark matter.
So there are at least two completely different, totally unrelated dark matter problems. One can and probably is solved by baryons. The other requires exotic particles or possibly exotic physics.
Blasphemy is a human right. Blasphemophobia kills.
http://en.wikipedia.org/wiki/Black_hole#Black_hol
Here is an excellent article by Sean Carroll of the California institute of Technology that explains why all the suggestions of the parent post may not be correct.
Basically, what it says is that if two large clusters of galaxies went right through each other, and dark matter was really like the normal matter in the way the parent post suggests, we would get a different result from what would happen if dark matter was for real. Astronomers have discovered one such system and this provides conclusive evidence for the existence of dark matter.
It's true that there are multiple scales to the dark matter problem, and that our arguments for exotic dark matter apply on the extra-galactic scale. I don't think theorists seriously argue that baryons solve the galactic dark matter problem, however. The Bullet cluster result (Google for Sean Carroll's excellent piece on this) tells us that the dark matter in galaxy clusters can't be baryonic either (it interacts too weakly with ordinary matter). The numbers we have from various experiments add up best if even galaxies are dominated by dark matter halos.
GR does not work at Galactic Levels, so there is no question of it working at the Cosmic levels.
r g%3Aastro-ph%2F0610298 (A New Force in the Dark Sector?)r g%3Aastro-ph%2F0609125 (Fits the Bullet Cluster with TeVeS, the MOND relativistic theory)
The real problem is MOND. If it did not exist then Dark Matter would be free to exist wherever it wanted. But with MOND the picture becomes more complex, now DM must fit MOND. It is quite easily provable that DM cannot fit MOND, just apply it to small cluster of stars at the outer edge of Milky Way which show Dark Matter. The problem is that for DM to fit Milky Way, it cannot be present in the Clusters. But some clusters do require DM. Now MOND fits both reasonably.
If you talk about Bullet Cluster, then don't because it proves that DM should have a new type of interaction because the DM itself is experiencing higher gravity than relativity predicts. Now this can be due to MOND. MOND can be fit to the Cluster by using hot and massive neutrinos. See the following papers.
http://www.citebase.org/abstract?id=oai%3AarXiv.o
http://www.citebase.org/abstract?id=oai%3AarXiv.o
The additional benefit is that MOND supplies the higher than gravity force required to fit the velocity of DM in Bullet Cluster.