
Is there dark matter at the center of the Milky Way which also decays? - dnetesn
https://phys.org/news/2019-12-dark-center-milky.html
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knzhou
This is a good article because it gives insight into how the scientific
process works: nobody is "for" or "against" various dark matter hypotheses, an
analysis that definitively supports _either_ direction can be very
interesting, and people who think a lot about dark matter often publish papers
showing that various effects _can 't_ be due to it.

Actually, this is the default situation in our field, and it's a good one. The
only issue is that typically only the "hottest" stuff get popular exposure,
which usually means the most blaring declarations of discovery, like the
hundreds of fifth force articles we just got hit with.

(My one minor complaint is that the headline by itself is misleading. The
question is whether there is dark matter at the center of the Milky Way _which
also decays_ , leading to the signals mentioned in the article.)

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raxxorrax
Isn't it basically just matter that isn't detectable but has to exist because
the known laws of nature told us it does? Instead some mystery goo that hides
from our sight?

Yes, I am sure the mystery goo theories would get much more exposure. The
pitfalls of pop-sci probably. I mean wich kid will get out of bed for a boring
pulsar these days?

> "I was pretty excited, because I knew the implications were very big—it
> meant that the dark matter explanation was back on the table"

I don't understand this. Isn't this basically feeding? Doesn't this just say
we know less than we thought? Were we in danger of knowing everything?

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noselasd
>Isn't it basically just matter that isn't detectable but has to exist because
the known laws of nature told us it does?

> Instead some mystery goo that hides from our sight?

Maybe. We assume "dark matter" has to exists because it interacts with/creates
gravity. The only thing we know that can do so is matter.

However this besides detecting gravity interactions with normal matter, this
dark matter cannot be seen, not in any wavelenghts, nor does it block any
light or radiation from normal matter that's behind it.

It's not like it's just a bunch of normal matter that hides in a corner we
cannot see by normal means.

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amluto
> Maybe. We assume "dark matter" has to exists because it interacts
> with/creates gravity. The only thing we know that can do so is matter.

This isn’t really accurate. _Everything_ interacts via gravity, notably
radiation and gravitational waves. (The latter hasn’t been directly
experimentally tested AFAIK.)

A major reason to think of dark matter as _matter_ is its equation of state,
i.e. how it behaves when stretched or diluted. If you put 10g of light in one
box (this is possible in principle — it’s a _lot_ of light, and you need a box
with impossibly shiny walls) and put 10g of cool gas in another, they
gravitate identically. But, if you expand the boxes the same amount, the light
gets diluted far more than the gas. That is, if you double the box’s volume by
moving one of the walls, you still have about 10g of gas but much less than
10g of light. If you had a third box full of 10g of dark energy and you double
the box’s volume, you end up with 20g of dark energy. Yes, this is really
weird.

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david-gpu
Wow, really? Isn't energy conserved? I would expect that expanding the volume
of the box would have a negligible effect on the mass of the contents, whether
they are light or matter.

And I say negligible only because my understanding is that even vacuum has a
tiny amount of energy in it due to Heisenberg's uncertainty principle, so a
box twice as large would contain a tiny bit more energy even if it was equally
empty in both cases.

What's the name of the phenomenon you describe? I'd like to learn more.

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amluto
Energy is indeed conserved [0], but you have to take pressure into account.
Suppose you build a cylinder with a piston, just like in a car engine or a
bike pump. Start with the piston in a position that puts the interior of the
cylinder at half its maximum volume. For simplicity, imagine that the outside
of the cylinder is in a vacuum [1].

Now put some gas, light, or dark energy in the cylinder and then pull the
piston out to its full extension. You've doubled the volume, but you're also
done some work by pulling out the piston. If the cylinder is full of gas or
light, the pressure is positive, so you did negative work (received energy) by
pulling out the piston, and the energy in the cylinder decreases. For dark
energy, the pressure is negative (crazy, I know), and you had to pull hard to
extend the piston, so you put energy into the cylinder and, in fact, doubled
the total energy in the cylinder.

[0] In general relativity, on large scales, conservation of energy is
complicated.

[1] With a real bike pump or car engine, nothing fundamental changes if you
have air outside the cylinder, but when you're talking about cosmology, you
start considering that the whole universe is full of more or less the same
stuff and it gets a bit more complicated. It all works out in the end, though.

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donmatito
This looks like a very healthy process, of people questionning their own
results ! The scientific method, as it should be. beautiful

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acqq
It really seems to be very interesting and relevant work:

One of the authors vas a coauthor of the 2015 paper that was now disproven
about its “it can’t be dank matter” claim:

"My hope was that this would be just the first of many studies of the galactic
center region using similar techniques. But by 2018, the main cross-checks of
the method were still the ones we'd done in 2015, which made me pretty nervous
that we might have missed something."

And the whole article about this new paper is informative.

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tsukurimashou
> dank matter

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AnimalMuppet
Well, if the dark matter is also damp...

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raverbashing
It's an interesting approach.

More "debunking the debunking" studies like this are needed.

It seems there is a bias and "debunking with a weak method" is much more
likely to be accepted than "proving with a moderate method". Ok, the null
hypothesis might be more likely, but some methods are just inappropriate.

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mnw21cam
[https://www.youtube.com/watch?v=avCGl2_6MQc](https://www.youtube.com/watch?v=avCGl2_6MQc)

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girishso
Slightly tangential. One thing I always wonder about, is there no dark matter
around us, I mean here on earth?

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maxnoe
There should be and we are looking for it. Several experiments are looking for
WIMPs or other proposed particles on Earth or in the solar system.

But the amount us tiny, current estimates of the dark matter density in the
Milky Way result in roughly 700 grams of DM in the Earths volume

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DougWebb
I've read (elsewhere, I guess) that some 95% of the Milky Way's mass is
estimated to be dark matter. If there are only 700g of it nearby, it can't be
distributed the same way normal matter is. How can that be, since it interacts
with normal matter gravitationally? Does the solar wind blow it away, or
something like that?

I've also seen an article not long ago showing 'visualizations' of dark matter
around a spiral galaxy, and it was mostly in teardrop-shaped lobes above and
below the galaxy's center and aligned on the axis, rather than distributed
through the disk. I didn't think of it at the time but now that I do, it seems
like that distribution also indicates that light from the stars is pushing the
dark matter away.

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cdumler
The reason why normal matter forms disks is because of electromagnetic
repulsion: electromagnetic atoms naturally resist being in the same place as
another atom. There becomes a tension between the repulsion to not stack up
and gravity to compress things together. Since angular momentum is conserved,
the matter will spin and flatten out: ie, a galaxy.

If dark mater is a particle, it rarely interacts with anything, including
itself; so, it cannot collapse. Most of the particles will be in a constant
state of free fall throughout the galaxy. Only if the particle just happens to
have a nearly matching velocity would it get trapped in a star or planet. Even
then, it would still be wobbling around with nothing to touch.

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ianamartin
It's kind of impressive to me how much of modern science is really just people
misapplying statistical models they don't understand to data that doesn't fit
assumptions.

Snark aside, what's really interesting is the way that different fields
respond to it when the model errors are discovered. Physics and Chemistry seem
to be fairly (or at least comparatively) professional and receptive when they
find out about mistakes. But mention a model error or data mismatch to someone
in Neuroscience or Climatology? You're suddenly an anti-science heretic and
probably murder kittens for fun.

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Tagbert
For Climatology, there are a lot of crazy or disingenuous people who try to
weigh in on topics. That makes everyone gun-shy and likely to lash out at
criticism because they are so used to getting attacked by unsupported
arguments.

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ianamartin
This is excellent. Thank you.

