As hinted last week, the Chandra X-Ray Observatory has some new results conclusively showing that dark matter is a real, physical thing. This is big news, because the previous evidence for dark matter was all indirect, and based on inferring the mass distribution of galaxies and clusters of galaxies from looking at their motion. These results could indicate the presence of dark matter, or they could point to a flaw in our understanding of gravity at extremely long ranges.
The new observation shows fairly conclusively that dark matter and ordinary matter are different things, by combining two different observations: graviational lensing of distant objects by a cluster of galaxies, to show where the mass of the cluster is concentrated, and X-ray observations of the hot gas associated with the cluster, to show the mass distribution of ordinary matter in the cluster. These turn out to be in two very different places, showing that the gas cannot account for the mass distribution, even if we fiddle with the mechanics of gravity.
Sean Carroll, who was on the panel NASA put together to announce the result, has a very nice explanation of the whole thing. With spiffy pictures!
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This is pretty exciting news. Now the real work is likely to be combining this with similar observations to pin down properties of the
dark matter. I don't think it rules out the dark matter being composed of nonluminous ordinary matter, such as brown dwarfs, although it sure would seem to be a stretch for any theory to come up with enough of them.
I don't think it rules out the dark matter being composed of nonluminous ordinary matter, such as brown dwarfs, although it sure would seem to be a stretch for any theory to come up with enough of them.
This study by itself doesn't, no -- what it shows is that the mass producing the gravitational lensing (most of which is dark) is not associated with the hot gas, which is the majority of the luminous matter.
But local studies, such as number counts of nearby brown dwarfs and gravitational microlensing studies of the Milky Way's halo, do tend to rule out brown dwarfs and similar candidates. Plus, if there's that much ordinary (baryonic) matter in the universe -- that is, enough for brown dwarfs to make up the dark matter -- then the Big Bang nucleosynthesis would have produced different ratios of hydrogen, helium, and lithium from what we actually see.