
Dark matter becomes less 'ghostly' - ghosh
http://www.bbc.com/news/science-environment-32303622?ocid=socialflow_twitter
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mherrmann
As someone with little background in physics I've always found dark matter to
be a very inelegant concept. To introduce a new kind of "matter" which can
hardly be observed sounds very arcane to me. It reminds me of the Luminiferous
aether which was postulated as the medium in which light travels in the late
19th century. I'm looking forward to new theories that can explain what we now
observe as "dark matter" without having to invent a new kind of matter
specifically for it!

~~~
inclemnet
An important fact about the luminiferous aether is that it was abandoned as an
idea because it was inconsistent with experiments. In contrast, dark matter
has continued as an idea and become a popular explanation because new
experiments have always fit very well with what it would predict.

We don't know what dark matter is, or if it even is a kind of matter rather
than e.g. modified gravity (though modified gravity has many problems of its
own), but I think it is a common mistake that people are so eager to associate
it with aether on the grounds only of what they find 'elegant'.

Edit: Also, the notion of a kind of matter that can hardly be observed is not
particularly weird or 'arcane'. For instance, neutrinos have many of the right
properties. I think they are ruled out as dark matter candidates mainly
because their mass is too low to fit with observations.

~~~
JackFr
I am not a physics expert, but isn't dark matter simply a plug because mass of
galaxies and large scale astronomical structures calculated from light and
other electromagnetic radiation, doesn't agree with mass calculated by
observing they're gravitational interaction? That is, we assume that both
models (radiation & gravity) are correct, and that 90+% of the universe is
made up of matter which has never been directly observed and is radically
different than any matter we observe directly.

The whole thing seems ludicrous -- but for the problem that no better
explanation has yet been put forth. One gets the feeling though that one day
in the future 'dark matter' is gonna look a lot like 'epicycles'.

~~~
JonnieCache
It's not just galaxy rotation. That's how people first noticed it. Since then
there's stuff like this:
[http://en.wikipedia.org/wiki/Bullet_Cluster](http://en.wikipedia.org/wiki/Bullet_Cluster)

Also there's apparently information about the universe's composition to be
gleaned from the CMB anisotropies; its all somewhat beyond me, but that curve
certainty seems to fit extremely well, I'll give them that.

[http://www.astro.ucla.edu/~wright/CMB-
DT.html](http://www.astro.ucla.edu/~wright/CMB-DT.html)

The combination of all this stuff is apparently pretty convincing.

EDIT: I see that your comment is actually referring to the bullet cluster more
than the rotation curves. It's all pretty tenuous in the grand scheme of
things, I agree, but then so much of cosmology is. I guess they have the
benefit of the string people next door making them look reasonable by
comparison.

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guelo
> Dr Thomas Kitching, a co-author of the study from University College London,
> said ... "It's too early to say if this is a dark matter effect, or caused
> by normal astrophysical processes. What we need to do now, is make
> simulations of these collisions to distinguish these possibilities."

------
thom
There was a recent episode of BBC radio's In Our Time covering dark matter,
which was useful to someone uninitiated like me:

[http://www.bbc.co.uk/programmes/b054t3s2](http://www.bbc.co.uk/programmes/b054t3s2)

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phkahler
What is this:

According to widely accepted ideas, the visible matter in galaxies exists
inside clumps of dark matter. Without dark matter's gravity to stabilise them,
galaxies like the Milky Way would tear themselves apart as they spin.

Is this the tired old "galactic rotation curves don't fit kepler's law" still
kicking around?

~~~
gamegoblin
Even in pretty accurate simulations using just the law of gravity as we know
it, if you simulate the galaxy, it'll fall apart. If you add in some
mysterious dark matter, it stays together.

Seems like there are two things which could follow from that:

1) We don't completely understand gravity at scale

2) Dark matter in some form or another exists

It just so happens that the huge majority of evidence points to #2.

I'd be hesitant to accuse a huge body of research collected hundreds of people
over decades of making some sort of trivial error like that.

~~~
phkahler
But if you throw is dark matter and have it influence the other matter...
Shouldn't the regular matter influence the dark matter too? At that point
doesn't it end up in the same distribution as the regular matter? And isn't it
then just unaccounted for? And wouldn't it have zero effect on the shape of
the rotation curve?

I've seen reports of "fixing" the problem by assuming a galaxy is enclosed in
spherical ball of dark matter. But of course no explanation of why it would
maintain that shape. In other words, the model of dark matter isn't even
complete. In some cases I've seen people misapply the divergence theorem as
well.

It would not be the first time people used a hack to "fix" what ultimately
boils down to numerical problems or inadequate integration methods.

Somewhere down my bucket list is to do galaxy simulation. So far all I've done
is plot rotation curves that don't look like keplers laws, but no dynamic
simulation yet.

~~~
dm_throwaway
The evolution of a galaxy is affected by collisions -- the interactions of
regular matter. Dark matter doesn't interact with itself in the same way.
That's exactly what this article is about!

(I don't know what's wrong with people that they think that, somehow, the
entire astrophysics/cosmology community has been too stupid to think these
fairly simple things through. If there was an obvious flaw in these models, a
researcher would have a large incentive to point these out and get the
citations!)

~~~
phkahler
>> I don't know what's wrong with people that they think that, somehow, the
entire astrophysics/cosmology community has been too stupid to think these
fairly simple things through.

Because they are: [http://www.csmonitor.com/Science/2015/0412/Universe-s-
expans...](http://www.csmonitor.com/Science/2015/0412/Universe-s-expansion-
may-not-be-accelerating-as-much-as-thought-say-scientists)

So the whole dark energy thing hinges on measurements of distances to
galaxies, which in turn depend on this notion that supernovas are all the
same. The entire community would rather embrace an exotic hard to prove
concept than go back and look at older assumptions.

And I kid you not about Keplers Laws being used too often when they don't
apply. But that's my usual story.

