3 posts • joined Thursday 25th August 2011 05:29 GMT
Technical article reference too?
Often with science articles posted here, I've wished for a reference to a corresponding technical article by the authors. Here... well... I'd *really* love to read what they themselves have to say, if it's published (though from some googling (hint: vixra.org), maybe the answer is, Not Yet, but wait for the CERN seminar on Friday 9/23...)
MACHOs! and, BBN.
Could much mass be in lumps of cool matter (dark rocky basketballs)? Nice question - was asked just this on an astrophysics test years ago.
One kind of test made since: looking for "MACHO"s, massive compact halo objects. If much of our galaxy's missing mass were in brown-dwarf or other planet-to-star-sized lumps, they'd be detectable by gravitational lensing when one passed between us and a distant star. MACHO searches stared at huge fields of stars for the right kind of variability - brightening then fading over a day or so, with characteristic wavelength-independence that grav. lensing would do. Some have turned up, but not enough to be a large fraction of our Milky Way's gravitating matter. (But, lensing searches wouldn't see basketballs.)
The more fundamental limit on the amount of ordinary matter (protons &c.) comes from "big-bang nucleosynthesis". As I (vaguely) understand, the density of ordinary (subject-to-nuclear-reactions, "baryonic") matter in the very early hot universe determines the ratios of helium/deuterium/hydrogen/lithium/photons that were left over once things cooled off. Those ratios - 'primordial abundances' - are more or less measurable and set limits on those early-time densities.
Result: even if lots of today's ordinary matter were in the form of cold dark invisible basketballs, that matter would have had to be present during Big Bang times too. Observed primordial abundances and known nuclear physics say there was 'way too little of that, no matter what form it takes today, to explain the amounts of 'dark matter' which is detected by its gravity in holding together galaxies and galaxy clusters today.
(Presumably the mysterious dark matter, *whatever* it is, went through the Big Bang too, but doesn't participate much in nuclear reactions. And, whatever it is, there seems to be about 5x more of it than of all the ordinary matter (visible and in-) put together. Exciting times to do physics, hence the thumbs up...)