There seems to be not enough matter to account for the fact that the speed of some stars located far from the Galaxy center is almost the same speed as those stars found nearer to the center of the Galaxy, therefore it seems there should be even more matter: so this missing matter is termed dark matter. There is a kind of stellar object that isn't massive enough to produce its own light but is many times more massive than Jupiter, a.k.a brown dwarf. Since it isn't bright enough to be detected by our deep space telescope nor the earth bound counterparts, they should be numerous in galaxies as I believe these objects sits in the middle range between planets and stars. However it seems the scientific community had ruled out this potential dark matter candidate without scanning the unobservable Milky Way galaxy as well as billions other with different light spectrums. Why aren't brown dwarfs considered candidates for the mysterious dark matter?
2 Answers
Two reasons.
We know from looking at galaxy rotation curves and the motion of galaxies in clusters and from gravitational lensing, that the amount of "dark matter" is some 30% of the density of the universe. But on the other hand, estimates of the abundances of deuterium, helium, tritium and lithium produced in the big bang indicate that only 5% of the density of the universe can be in the form of normal matter (e.g atoms, protons, neutrons etc). As brown dwarfs are made of these things, they can only contribute to this small percentage and cannot be responsible for most of the dark matter (the same is true for anything made of normal, "baryonic matter").
Astronomers have counted how many brown dwarfs there are, both in the local neighbourhood and in star clusters (see for example Andersen et al. 2008; Kirkpatrick et al. 2011; Burningham et al. 2013; . In other words we can see brown dwarfs - they emit most of their light in the infrared part of the spectrum, and surveys like SDSS, 2MASS and WISE have uncovered them in their thousands. It turns out that there is about 1 brown dwarf for every 4 more massive stars. So, although numerous, they contribute a very tiny fraction of the normal matter in our Galaxy. In addition, microlensing experiments towards the Galactic bulge and Magellanic clouds suggest that although there are lots of brown dwarfs elsewhere in our Galaxy (e.g. Alcock et al. 2000; Novati et al. 2008), there are not enough to make much of a contribution to dark matter, and so the results from our solar neighbourhood appear representative of the Galaxy as a whole.
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$\begingroup$ I had thought he only reason we know about exo-gas giants was the wobble they induce in the stars they orbit. But how about brown dwarfs not orbiting a star? If we can't see brown dwarfs, how were they counted? Some cites would be helpful. $\endgroup$– HopDavidCommented Apr 5, 2015 at 14:08
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$\begingroup$ @HopDavid We can see brown dwarfs. I'll put some references in. $\endgroup$– ProfRobCommented Apr 5, 2015 at 14:12
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$\begingroup$ IIRC, you can also estimate brown dwarf density by checking to see how often the objects eclipse more distant stars. $\endgroup$ Commented Apr 5, 2015 at 15:51
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$\begingroup$ @WayfaringStranger That's news to me - are you sure you don't mean microlensing, which is what I am talking about. Transits occur for brown dwarfs in orbit around stars, but as their radii are indistinguishable from giant planets, then you would not be able to tell a brown dwarf from a planetary mass object. Could you point out a project that finds free-floating brown dwarfs via the eclipse of background stars? $\endgroup$– ProfRobCommented Apr 5, 2015 at 16:53
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$\begingroup$ @RobJeffries Can't point one out. Read this some years ago when people were claiming dark matter might amount to nothing more than rogue planets and small stars; normal matter in condensed form, that was simply too dark to see. By knowing the needed density to match galactic rotation curves, you could predict how often stars should be eclipsed by such large, dark objects. Photometry didn't find as many single-time events as needed. This was before WMAP data, so the world has moved on. Microlensing may have been involved, but as I said, I'm not finding any current info. $\endgroup$ Commented Apr 5, 2015 at 21:10
Brown Dwarfs are bright in radio. They emit plenty of light, just not much visible light.
One of the properties of dark matter is that it also does not obscure light. Interstellar dust (which can be comprised of many larger planet-sized particles) tends to obscure light even if it doesn't emit any. So due to it neither emitting nor obscuring light, we can rule out it being brown dwarfs, dust, or pretty much anything in between.
It could be a field of lightweight black holes, but that seems unlikely as black holes are both rare and generally surrounded by evidence of their formation. It is unreasonable to assume that there are huge swarms of lightweight black holes making up a large fraction of the mass in the universe.
Exotic matter seems strange to us, but it's really not so uncommon. We already know that neutrinos exist and in fact are very abundant. Neutrinos pass through normal matter almost completely uninhibited. They can't be what dark matter is made of, because they travel at the speed of light like photons, while plenty of dark matter is relatively stationary. It seems reasonable to believe that it is some form of exotic matter that doesn't interact with light. It's strange to us, but it seems very consistent with what we know of particle physics.
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5$\begingroup$ Neutrinos have mass and do not travel at the speed of light. It is generally thought that neutrinos in the cosmic neutrino background are now travelling at considerably less than the speed of light because their rest mass energy exceeds their kinetic energy. $\endgroup$– ProfRobCommented Sep 26, 2016 at 19:47
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$\begingroup$ Cold brown dwarfs wouldn't necessarily be bright in radio. $\endgroup$– VikkiCommented Jul 4, 2021 at 17:17