
Decades-old scientific paper may hold clues to dark matter - ohjeez
http://news.sciencemag.org/physics/2014/10/decades-old-scientific-paper-may-hold-clues-dark-matter
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inclemnet
> If they exist, WIMPs would have a mass between one and 1000 times that of a
> proton. They would interact only through the feeble weak nuclear force

 _And gravity_. The article even mentions this earlier, but seems to have
forgotten about it by the second paragraph. Honestly, I think that a lot of
dark matter's popular science reputation as misguided and made up is that
articles like this seem so eager to ignore the relatively large amount of
information we have about its gravitational behaviour.

(Not that it is necessarily WIMPs, but I think people would be a lot less
eager to dismiss this explanation as 'aether-like' if they had a better idea
of why WIMPs are proposed in the first place)

~~~
lisper
So this is what I don't understand: there is supposedly 5x as much dark matter
as matter. With that much of it floating around, how is it possible that we
ever convinced ourselves that general relativity is true? Dark matter is
supposedly embedded inside galaxies to the point where stars within those
galaxies move faster than they otherwise would. Well, shouldn't the same be
true of _our_ galaxy? In fact, shouldn't it be true of our solar system? I
just don't see how it's possible that we ever collected data in support of GR
only to be surprised later by an anomaly nearly an order of magnitude bigger
than the base data. If someone who understands cosmology could please explain
this to me I would be most grateful.

~~~
aroberge
Space is largely empty; this is a crucial point I will come back to.

Assume a spherical distribution of (dark) matter, with density small enough
that we can ignore relativistic effects (which is almost always the case). If
you are inside the spherical distribution at a distance R from the center of
that distribution, the only effect you will feel is that of the mass inside a
sphere of radius R, centered on that distribution. (This is well known for any
inverse square law; I can explain more if required.)

The _average_ density of normal/dark matter is very, very small. Inside our
solar system, the dominant gravitational effect is that of the Sun: any dark
matter having the expected density (from other measurements) would contribute
only a tiny amount, so as to be essentially undetectable.

On much, much larger scales, such as the scale of galaxies, far enough from
the center (and assuming the dark matter is essentially a spherical
distribution coinciding with the galaxy), the cumulative amount of the
additional gravitational pull is enough to be detected. Remember that the
distance to the nearest stars is _huge_. Yet, our Sun is within a galaxy. So,
as I wrote above, the _average_ density of normal/dark matter is very, very
small. However, over such large distances as those of the size of galaxies,
the cumulative effects can be observed. However, distances involved within our
solar system are such that no additional pull from dark matter (comparing the
motion of say Mercury to that of Jupiter) is expected to have a visible
effect.

~~~
netcraft
is it possible that dark matter is regular matter, just so diffuse that we
don't notice/see it? could it be hydrogen atoms just floating in empty space,
at such low densities we would never notice but in the grand scheme of things
add up? Edit: I dont mean to propose a potential solution here, it was more a
question of why it must be unknown matter, why can't it be normal matter.

~~~
aroberge
No, it is not possible. Cosmological models are actually tightly constrained
by data. Even in the late 80s, when we did not have such good data, I remember
being amazed at the fact that primordial nucleosynthesis calculations together
with observational data were enough to conclude that the number of light
"normal" neutrino species was 3, while there were still speculations in
Particle Physics that we could have more than 3 generations of particles. The
data accumulated over the last 30 years has constrained things even more.

If some other explanation than invoking some new type of matter that can not
be seen was possible to explain the observational data, don't you think that
Physicists would use that instead of coming up with postulating that the
Universe is dominated by "dark matter" and "dark energy" which we have not
seen yet?

~~~
netcraft
Looking back at what I wrote I can understand your reaction - im wasnt trying
to propose a new solution that someone hadn't considered. It was more a
question of why couldn't it be that, which you answered quite well, thank you.

