Examining the Expected Effects of Dark Matter On the Solar System 190
First time accepted submitter LiavK writes "Ethan Siegel recently wrote a great post for ScienceBlogs discussing the expected total mass of dark matter in the solar system. As far as we can tell, dark matter only interacts weakly, via gravity, both with itself and normal matter. So, it can't collide with itself, meaning that it has no way of getting hotter and radiating away energy and momentum. This means that it remains a diffuse mess, with a density that is ridiculously low, to the point where detecting its local effects is likely to remain... challenging for the foreseeable future."
Relativistic space under tension? (Score:3, Interesting)
Or you could say space has a property of localized time. Which means time doesn't scale or progress uniformly throughout the universe. If you've got enough gravity, it's going to make things appear even more massive then they are because of time dilation. The relationship of gravity vs. time also means c should be treated as a coefficient rather than a constant. (The effective value of c still remains fixed, but that's because relationship of distance vs. time has both parts as variables. Time effectively rescales itself at higher energies to maintain c for a given distance traveled by a particle, but if you don't account for that, the extra momentum approaching or exceeding c looks like a gain in mass.)
Somebody with better math skills than myself could probably re-jigger Special Relativity in this regard and account for missing mass. It may even show a cumulative effect with gravitational time dilation when you have a system of multiple orbiting objects. But you might also have to toss the idea of a "Big Bang" out the window. (Makes "age" of things in the universe fairly irrelevant when a localized second is defined by the gravitational or acceleration field it's being measured under. Not to mention under certain conditions the typical light-year measuring stick astronomers like to use will also look about as uniform as a funhouse mirror. The funny-sounding Dr. Who sci-fi explanation of time being "Wibbly wobbly" may have some real logic to it.)
Of course it sounds nutty, because it opens up a lot of loopholes. Probably explains why Einstein was uncomfortable with some things, even if it provided the template for a more accurate model than some later revisions.
Dark matter, dark energy, and M-theory (Score:5, Interesting)
This is probably a dumb question, but I've been wondering about it for something like a decade, and I never see it referenced (even to debunk it) in legitimate science discussions.
A mysterious effect which looks like matter, but is invisible except for its gravitational effect. A second mysterious effect which causes the rate-of-expansion of the universe to increase.
I grow more and more skeptical of string theory and its relations every year, but the first of those definitely sounds to me like matter that's in another brane. The second one seems (to my non-physicist mind) like it could also be explained by the same thing, just a different set of matter in a different position relative to the first.
If our universe really is a 3D brane in a hyperdimensional space with others, isn't this exactly the sort of thing we'd expect to see? Further, wouldn't we see related effects like neutron stars unexpectedly flashing into black holes when they come into close-enough contact with dense clumps of matter in adjacent branes (IOW, when there's not enough observed mass in our own to explain the change to a black hole)?
Re: Dark Matter (Score:4, Interesting)
Don't make the mistake of thinking there was just one ether theory. There were lots of them, many quite compatible with special relativity. Quite a few that sound like 1890s versions of quantum electrodynamics.
Re:The problem with dark matter (Score:1, Interesting)
Re:The problem with dark matter (Score:2, Interesting)
Buy it or not but people smarter than I have spent decades modelling dust -- I know a good few people with PhDs and postdocs in the matter -- and what we expect to see from it. What we expect:
1) Radiation. Dust is heated, dust radiates. If nothing else, dust is bathed by the CMB, and therefore will still radiate.
2) Dust also scatters radiation. If nothing else, this is obvious in the CMB, but there *are* other things -- light from stars in our own galaxy, other galaxies, quasars, etc.
3) Dust is typically charged, since it is typically made from metals (in astronomer jargon where anything above helium is "metal", water also counts as a metal by the way). This leaves extremely obvious signatures on any light that happens anywhere near it. The presence of the magnetic fields this charge implies also leads to other types of radiative emission through interactions with electrons, such as synchotron.
4) Dust is by definition baryonic (in the cosmological sense; ie matter described by the standard model of particle physics). We have extraordinarily tight constraints on how much baryonic matter there can be in the universe, and that is somewhere around 5% of the critical density. We simply *cannot* introduce more than that because we would seriously fuck up big bang nucleosynthesis; the proportions of, say, deuterium, lithium, sodium etc. are exquisitely sensitive to the proportions of baryonic matter and radiation in the early universe. While it is possible to criticise the cosmological model - and I built a career on doing so - no-one who isn't a crackpot would argue that the model *wasn't* valid during BBN.
5) If it were "rocks" as you put it we would see vastly more microlensing events than we actually have. Point (4) (which is a trump point here) also applies: these are baryonic.
6) The cosmological model is more than "speculation" with "no empirical evidence". It's certainly true that the cosmological model runs into severe difficulties in the late universe -- difficulties that the majority of professional cosmologists seem blissfully unaware of, and which call into question any attempts to ascribe physical meaning to "dark energy" in particular, but also to some degree "dark matter". However, at earlier times the evidence in support of the model is so vast as to be practically incontrovertible. We can change our model of gravity and doing so may or may not change our model of cosmology, but if we assume general relativity is even approximately valid across cosmological scales we get this model of cosmology, valid up to a redshift of, to pick a wild and extremely conservative number, 100. (In reality I'd trust it -- by which I mean a Robertson-Walker metric plus perturbations to second-order -- up to a redshift of about 2 or 3 so long as I was careful about the scales of validity at later times.) That evidence comes from the CMB more than anything. Its theoretical underpinnings are remarkably shaky, yes, but the objections to the underpinnings are valid only at late times. At early times they're fine... and those early times include the epochs of BBN and the formation of the CMB, on which I would tend to base my conclusions about the "existence" of dark matter.
Basically dark matter simply cannot be dust. That doesn't say that dark matter is absolutely an unknown, weakly interacting massive particle. Sure, some of it is (neutrinos are very definitely a warm dark matter, and the presence of axions still seems at least plausible and even neutralinos or gravitinos are no less likely now than they were a year or two back), and maybe even a lot of it is. Other contributions can simply be that we don't understand gravity -- neither its nature, nor how to apply it. It seems likely that any successful theory will be metric-based, but even there the successes of MOND on galactic scales may (or may not -- absolutely nobody pretends MOND is anything other than phenomenology) suggest that we have to drop the idea of a metric on such scales. I doubt that, to be honest, but it's possible. But knowing it'