
Dark Matter Sighting Gets a Reprieve in New Analysis - dnetesn
http://nautil.us/blog/dark-matter-gets-a-reprieve-in-new-analysis
======
User23
Can someone please explain how dark matter, which can’t be directly observed,
is mathematically any different than adding an arbitrary parameter to the
equation and tuning it to fit? I can’t help but think of von Neumann’s
elephant.

This isn’t me trying to be clever, I’d really appreciate some clarity on this.

~~~
XorNot
It's probably worth considering this question from the perspective of how we
know most subatomic particles are real. They can't be directly observed either
- the LHC and other particle accelerators only ever observe electrical
disturbances in their sensors following a pattern of results from decay
product interactions. No one sits down with a microscope and says "yep that's
a gluon".

EDIT: Consider that the Higgs boson wasn't "directly" observed till very
recently but was assumed to exist for decades prior for the exact same reasons
- you needed to add an additional term to the equations to break the symmetry
and cause particles to have mass, which in turn implied a pervasive field
throughout the universe we hadn't observed yet. Which in turn implied it
should exist discreetly under high energy conditions as a particle, which
became the Higgs.

~~~
nonbel
I thought the extra term was added so the equations didn't result in
probabilities outside the 0-1 range.

> "The Higgs boson gives mass to other elementary particles such as quarks and
> electrons, and its existence is necessary for the theory to give sensible
> results (i.e., the probability of processes to occur cannot be greater than
> 100 percent)."
> [https://www.albany.edu/news/87798.php](https://www.albany.edu/news/87798.php)

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joe_the_user
It seems like the title should be something like "Dark Matter Sighting Get
Reprieve". Dark matter is pretty well established, it's just a supposed direct
sighting that now seems to be getting a second look.

~~~
guelo
Right. The article doesn't explain why an electromagnetic sighting is better
than the gravitational one.

~~~
webmaven
That's because it isn't "better", it's just different.

The unspoken assumption is that two different types of evidence are generally
better than just one type, not that one type is better than the other.

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zw123456
wouldn't it be crazy if dark matter turned out to be Dyson Spheres ?

~~~
cletus
This is basically impossible unless someone has figured out a way to break the
laws of thermodynamics. And if that's true a whole bunch of stuff goes out the
window.

Basically a Dyson Sphere is about the least subtle thing a civilization could
do and would be detectable from hundreds of millions of light years away. All
energy ends as heat. The only way to get rid of heat in space is to radiate it
away. It radiates away in a predictable way based on the temperature of the
object. At any sort of temperature we're talking about this would be a massive
glowing infrared cloud with the energy output of a star. It'd be hard to miss
that.

Think about this: a Dyson sphere here would be absorbing the energy output of
a star. That energy has to go somewhere.

Another point against it is the Elapsed Time Argument, which can be used to
effectively disprove many such hypotheses. The argument goes like this: if a
given phenomenon is artificial it should be less common the more distant you
look. At the edge of the observable universe we're looking at a universe
significantly younger than ours. Because of this something artificial should
be less common further away because less time has elapsed there. If that's not
the case it's almost certainly not artificial.

It's also worth pointing out that dark matter out-masses regular matter by
10-to-1 or more. That's a lot to attribute to Dyson Spheres. That would make
the universe an extremely populated place where we somehow still can't detect
anyone.

~~~
zw123456
Hmmm, your response is amazing. I was just sort of pondering a far out idea.
But the amount of time and energy you put into your response is awesome. So
cool that you thought about my dumb idea that much. Not trolling just thinking
about far out ideas. I like your response a lot.

~~~
raattgift
You'd need an awful lot of Dyson Spheres, and they'd all be in places you
wouldn't expect suitable stars at all, and then you'd have to hide them from
searches for brown dwarfs and black holes as components of Dark Matter. See
[https://en.wikipedia.org/wiki/Massive_compact_halo_object](https://en.wikipedia.org/wiki/Massive_compact_halo_object)
for a brief overview.

The killer problem is that stars bright enough to gather up energy via Dyson
Sphere mechanisms aren't seen around any of the many thousands of galaxies
we've looked at. Moreover, in order to account for an appreciable fraction of
the halo masses, there would have to be at least hundreds of millions of Dyson
Spheres for every galaxy in the sky. The Dyson Spheres would also have to be
very very old because, thanks to gravitational lensing by foreground galaxy
clusters, we have decent enough views of galaxies billions of light-years away
that we would see large numbers of not-yet-Dyson-Sphered stars well outside
galaxies. The most distant spiral-type galaxies in particular would look very
different than much closer ones, and that is not the case. Have a look for
yourself:
[http://legacysurvey.org/viewer#IC%203556](http://legacysurvey.org/viewer#IC%203556)

Bright stars only form in regions where the dust and gas is sufficiently dense
to collapse gravitationally; those regions are generally found only within
galaxies or in "low points" of gravitational potential within clusters of
galaxies (the dust and gas gathering in such points may form a new galaxy).
Dust that doesn't fall into galaxies or into cluster "low points" becomes too
diffuse to form even the dimmest stars.

By comparison, dark matter tends not to fall into galaxies or cluster low
points because collisions between dark matter particles, or between them and
ordinary matter, does not produce radiation. Radiation from particle
collisions carries off the angular momentum that keeps the particles on higher
orbits. Ordinary dust and gas collisions release light (and radio and so on)
which enables gravitational collapse into stars. We can see the light (and
radio and microwaves and so on) with telescopes.

This tendency of dark matter to stay on high orbits well above the luminous
matter in galaxies leads to a greater angular momentum than expected in the
outer reaches of the luminous matter. That angular momentum excess is how dark
matter was discovered. Why don't the fast-moving stars and dust fly away from
their galaxies? Because dark matter throughout the galaxy adds more mass than
is seen electromagnetically, so there is a sufficient tendency to fall inwards
counteracting the excess momentum. Why doesn't the fast-moving luminous matter
collide more often, throw off more radiation, and collapse into the centres of
galaxies more rapidly? Because the shell of dark matter well outside the
bright parts of galaxies holds it up, somewhat like a scaffold. It's a
balance, but not an especially fine one; it seems to be pretty generic for a
collisionless, cold, non-self-interacting (except by gravitation), non-
radiating, microscopic dust in which the visible components of galaxies are
immersed.

Let's ignore the other responders' concerns about not being able to keep the
Dyson Spheres as cool as the background to all observers (how would a Dyson
Sphere builder in a galaxy 100 million light years away know that _we_ might
be looking from our galaxy, and so decide to cloak themselves _only_ from
us?). Let's also ignore whether such things would be transparent, or would
occlude optical emissions from the galaxies they surround. We can't however
ignore their gravitational influence: these Dyson Spheres have to "hold up"
the non-Dyson-Sphered stars they surround, to stop their host galaxies from
collapsing. You can't do that with only a few Dyson Spheres, and as you add
more, you'd expect to see more small gravitational lensing distortions.

Although our telescopes aren't good enough to probe many galaxies, there are
no such (gravitational lensing) distortions around our own galaxy, or
Andromeda, and yet both galaxies' luminous matter's behaviour essentially
requires an abundance of dark matter.

(There also isn't a dust eclipsing distant sources of light, which we would
expect for a very dim but not exactly invisible Dyson Sphere would; there also
isn't a large number of pointlike sources of deep infrared or radio in our or
Andromeda's galactic halos, which one would expect if a Dyson Sphere builder
were extremely energy-efficient. We also don't see excited dusts and gas
around galaxies suggesting accidental "painting" with lasers or the like being
shot out of Dyson Spheres if their operators decided not to radiate unused
energy in every direction).

Finally, there is a hierarchical problem here: not only does dark matter
surround galaxies and affect their structure gravitationally, it also
surrounds whole clusters of galaxies. Without lots of dark matter hanging
around waaaaay outside galaxies within a galaxy-cluster, clustering would be
very different from what we see in the sky. Moreover, we know from Einstein
lensing that the bulk of the magnification done by clusters is not traceable
to the luminous matter in the galaxy cluster.

So you'd need to have lots and lots and lots of invisible Dyson Spheres many
tens of thousands of light years outside galaxies. By lots, for a cluster
similar to ours, this would mean a few hundred trillion Dyson Spheres, all
very carefully engineered _and positioned_ to mimic cold dark matter in areas
where stars are not naturally seen. Also all evidence of towing stars, or
generating new stars from scratch, has to be hidden from view in tens of
thousands of galaxies over a span of a billion or more years, and somehow no
galaxies known are older than the completion of the Dyson Sphere project, or
somehow undisturbed early galaxies look identical to much more recent
galaxies.

The engineering challenges would be outright cosmological, not just
astronomical.

Wanna scare yourself with sci-fi thoughts? If this type of engineering is all
done by a single culture, what would make them want to _hide_ so carefully?

~~~
raattgift
PS: I wrote the long reply above because it's refreshing in these comments to
see someone asking "crazy questions" honestly, rather than advancing some
crazy idea as if it were fact.

------
The_rationalist
MOND is epistemologically far superior to the hack of Dark matter. What does
Dark matter explain that MOND can't?

~~~
raattgift
> What does Dark matter explain that MOND can't

MOND _describes_ , it doesn't explain. By adding a simple function to Newton's
law of universal gravitation, it does a very good job of describing the
momentum-deficit in the bulk motion of large optical and radio sources in a
variety of galaxies compared to the predictions from Newton's unmodified
theory. That it does so, and with such a simple modification, is interesting.
Milgrom's function in MOND is almost certainly the simplest post-Newtonian
gravitational term extending Newton's theory consistent with observations.
(There are other adaptations of Newton's theory to add post-Newtonian
elements, for example to describe motions within the solar system. These are
generally much more complicated than MOND [see [1] for a quick overview].)

In practically any post-Newtonian term expanding Newton's law of universal
gravitation, one will discover a greater mismatch with observation as the
relative speeds of material participating in Newtonian gravitation increase.
One has to "patch" with additional terms and counter-terms, resulting in
something comparable to a Taylor Series. One generally experiences obvious
failure with this approach as the relative speeds of the masses become
comparable to tenths of the speed of light.

MOND doesn't describe the momentum-deficit in the motion of large clusters of
galaxies, nor the momentum-anisotropies in the cosmic microwave background. It
doesn't describe large mass-flows, or especially relativistic mass-flows as
seen in Seyfert galaxies and a number of galaxy clusters. It doesn't describe
fast binary objects. It doesn't accurately describe the _fine_ details of the
bulk sources that it describes very well. It also doesn't accurately describe
our solar system; one has to "patch" MOND with additional terms, and it
quickly becomes easier just to discard MOND as a contribution to solar system
dynamics altogether. MOND also doesn't abolish the need to use General
Relativity to describe the angle-brightness-redshift relationship of the
reddest galaxies (and the supernovae within them) visible in the sky. MOND
does not provide sufficient post-Newtonian correction for GPS and other
orbiting satellites with very precise frequency generators on board, nor for
the results of experiments in MESSENGER [3] and numerous other spacecraft.

Let's return to what MOND does do very well: describing the relative angular
momentum of slow-moving inner and outer bulk bits of galaxies.

Attempts to adapt MOND so that it accurately describes relativistic
galaxy/cluster components have generally resulted in adding a field to
standard General Relativity [3], and it is mostly cultural history that keeps
MOND from being properly promoted as a source contributing to the stress-
energy tensor. That would put it on identical footing to dark matter _in the
bulk_ , focusing minds on the question: "what are the microscopic details of
the beyond-the-Standard-Model-of-Particle-Physics contributions to the stress-
energy tensor?".

> epistemologically far superior

What is Newton's explanation for his law of universal gravitation, which MOND
slightly modifies in a very specific way? What is the modern explanation of
gravitation, which can encode MONDian behaviour in various ways?

\- --

[1] [https://www.researchgate.net/publication/327607544_Post-
Newt...](https://www.researchgate.net/publication/327607544_Post-
Newtonian_satellite_orbits)

[2]
[https://pgda.gsfc.nasa.gov/products/66](https://pgda.gsfc.nasa.gov/products/66)

[3] Chapter 7 of
[http://adsabs.harvard.edu/abs/2012LRR....15...10F](http://adsabs.harvard.edu/abs/2012LRR....15...10F)
(arxiv version:
[https://arxiv.org/abs/1112.3960](https://arxiv.org/abs/1112.3960))

------
nonbel
>"The galactic center shines too brightly, like the glow of a metropolis at
night where maps show only a town. [...] The new study, along with two others
that came out in March, reopens the possibility that space-based instruments
have found the first direct evidence of the elusive “dark matter” thought to
pervade the universe."

Please give an example of what dark matter cannot explain.

~~~
nonbel
I am serious. This is what I have seen:

    
    
      Galactic rotation curves exactly as predicted: explained by dark matter
      Galactic rotation curves not as predicted: explained by dark matter
    
      Area of sky brighter than predicted: explained by dark matter
      Area of sky dimmer than predicted: explained by dark matter.

~~~
Avshalom
Not entirely sure what you'r referring to by area of the sky but as to
rotation curves it's:

-Galactic rotation curve not as predicted by relativity: explained by dark matter

-Galactic rotation curve exactly as predicted by relativity: _allowed_ by dark matter, not explained by theories that propose a different uniform law of gravity that also explains case A.

~~~
nonbel
This is not my point here, but the galaxy you refer to is consistent with
MOND: [https://tritonstation.wordpress.com/2018/04/04/the-dwarf-
gal...](https://tritonstation.wordpress.com/2018/04/04/the-dwarf-galaxy-
ngc1052-df2/)

