

Dark matter scaffolding of universe detected for the first time - fiaz
http://ns.umich.edu/new/releases/20623-dark-matter-scaffolding-of-universe-detected-for-the-first-time

======
syncerr
It is worth noting that there was no direct detection of dark matter. It comes
from a measurement of gravitational lensing, which has been going on for
awhile˟˟.

Critical response: [http://www.scilogs.eu/en/blog/the-dark-matter-
crisis/2012-07...](http://www.scilogs.eu/en/blog/the-dark-matter-
crisis/2012-07-05/a-filament-of-dark-matter-between-two-clusters-of-galaxies)

Source:
[http://www.nature.com/nature/journal/v487/n7406/full/nature1...](http://www.nature.com/nature/journal/v487/n7406/full/nature11224.html)

˟˟
[http://physicsworld.com/cws/article/news/2006/aug/25/gravity...](http://physicsworld.com/cws/article/news/2006/aug/25/gravity-
lens-reveals-dark-matter)

~~~
lloeki
Indeed, it's more like observing a magnetic field by dropping iron dust on a
sheet of paper, and gently hit/shake it until the field lines "appear".

It makes me wonder why we can't observe dark matter (which seems to emit only
pure gravitation, and no light nor electromagnetic radiation). Could it be
because there's no actual matter (i.e pure gravitational waves, like the
magnet+iron+paper experiment)? Or are they massive clouds overloaded with
Higgs bosons?

~~~
syncerr
[http://en.wikipedia.org/wiki/Dark_matter#Direct_detection_ex...](http://en.wikipedia.org/wiki/Dark_matter#Direct_detection_experiments)

------
ok_craig
There's one thing I don't understand about the mystery of dark matter. I don't
understand why the simple explanation for it isn't just that it's regular
matter that is not stars. Maybe there are just bajillions of planets and dust
clouds out there. Matter that isn't directly circling stars, thus not
reflecting light. Why is the popular assumption that if the mass isn't stars
or things in orbit of stars, that it must be a mystery substance? I assume
there's scientific reasoning behind this, but I've never heard it explained
before. If someone could fill me in, that would be awesome.

~~~
InclinedPlane
Science!

We can tell how much dark matter is out there because we can "weigh" it
through indirect measures. And then we can take different theories of dark
matter (such as, say, the theory that it's all just a bunch of interstellar
orphaned planets and "black dwarfs" and what-have-you made up of ordinary
matter) and figure out what sorts of implications that would have, make
predictions on observable effects of those different models and then test
those predictions. And that is precisely what happened about 20-30 years ago.
A lot of work was done to pin down what type of dark matter makes up the
majority of it out there.

For example, you can point a telescope at a set of neighboring galaxies and
look for brightening effects due to gravitational micro-lensing from a chance
alignment of a "macho" (e.g. orphaned gas giant planet) along the line of
sight. Surveys were set up and indeed found that there were orphaned "macho"
objects in our galaxy, but the statistics showed that they were orders of
magnitude too rare to make up the bulk of dark matter we know about from other
studies. Another line of evidence involves studying the large-scale structure
of the Universe (e.g. the layout of galaxies, galaxy clusters, etc.) and
comparing it with various computer simulations of models with different
assumptions on the composition of the mass of the Universe (e.g. 100%
"ordinary" baryonic matter, various percentages of "special" dark matter such
as cold and hot dark matter, WIMPs, etc.)

From this and many other lines of evidence we came up with very strong
evidence that the vast majority of the mass budget of the Universe is in the
form of so-called "cold dark matter" which is composed of weekly interacting
massive particles other than neutrinos (neutrinos are dark matter, but we've
been able to place an upper limit on how much they contribute to the dark
matter budget of the Universe, because they are detectable to a degree, and
it's only a fraction).

So that's it, just a simple matter of comparing the predictions of different
theories with observations and eliminating the theories that do not predict
what we actually see out there in the Universe.

~~~
mkup
(A bit off-topic question to physics expert on HN)

I've read that supermassive black hole accretion is the most energy-effective
process of mass to energy conversion in the Universe (50% efficiency or so).

I'm just curious: Where does all that energy go? Extremely powerful jets of
radiation are emitted into the intergalaxy space and then what? Does it just
disappear? Isn't this energy responsible for Universe expansion? It must push
galaxies away from each other, right?

~~~
Tloewald
Same place sunlight goes.

~~~
jeggers5
It would be amazing if somebody could answer this too. If I turn a lamp on and
off in a sealed room of mirrors, why doesn't the light just keep bouncing off
the walls and illuminate the room?

~~~
Retric
Some of that energy turns into heat every time the photons hit something, but
heat is a mix of kinetic energy and light so you do get a fraction of that
light bouncing around indefinitely, it's just not in the visible spectrum.

Think of it like dropping a ball an a hard vs soft surface. In both cases
things bounce. Even on a hard floor the ball stops bouncing after a while, and
in both cases the ball / floor / air get's get's warmer from the balls energy.

~~~
Tloewald
Yeah... Warmer is kind of mixing metaphors since warmth is just a statistical
aggregation of stuff moving around. The point is that photons result from
electrons changing energy levels and disappear when they hit an electron and
change its energy level. A big change makes a high energy photon which might
bump into an electron, resulting in a higher energy electron and a new, less
energetic photon. This keeps on happening.

Seen purely as electrons -- one electron had a lot of energy, now a lot of
electrons have a little. Seen purely as photos, one photon had a lot of
energy, now there are lots on very low energy photons.

Overall, the collective term for this is entropy -- over time we get fewer
opportunities for big photons to get created, until it's all small changes in
energy and small photons -- total entropy -- and everything is background
radiation.

------
Xcelerate
This is really cool. If you look at this graph of the Standard Model
interactions
[http://upload.wikimedia.org/wikipedia/commons/4/4c/Elementar...](http://upload.wikimedia.org/wikipedia/commons/4/4c/Elementary_particle_interactions.svg)
you will see the ways in which all of the particles we currently know about
interact.

If you'll notice though, there's one interaction between ALL of the particles
that is missing: gravity. Gravity affects anything with _energy_. Photons,
leptons, quarks -- they are all attracted to each other because they possess
energy (negligible, unmeasurable attractions, but still extant).

Wouldn't it be interesting if the only way that dark matter interacted with
the other particles was through the gravitational force? Maybe from some
alien's perspective it would constitute the matter of everyday life, but
because it didn't interact with any of our particles except through gravity we
would be missing out on a large aspect of our universe!

Furthermore, is it that far-fetched to think there might exist particles that
do not interact at all with the ones we have discovered? Gluons, for example,
only interact with themselves and with quarks. Some other particle may
interact with nothing we are familiar with -- and thus we could never study
it. Is it even "real" then?

(Any particle physicists on here, please feel free to educate me further!)

~~~
tb
> Some other particle may interact with nothing we are familiar with -- and
> thus we could never study it. Is it even "real" then?

Philosophically, this is equivalent to the question of whether other
universes, which do not interact with ours and therefore we cannot study,
exist or are "real". It is not a question that Science can answer.

~~~
kamaal
Oh well,

That question really has far reaching implications. Because if we say Science
is what we observe and describe as per our interpretations of logic(And the
language of logic - 'Math') then our science is really broken. Because what we
can observe doesn't often turn out to be true and what is true is not often
observed.

Because look at it this way. We are now saying Dark Matter doesn't interact
anyway with light nor something else. Hence observing, detecting or modeling
them out through conjectures manufactured through thin air is nothing more
than what religion was some centuries ago.

Anything unexplainable was attributed to some form of divinity in times
before.

We know it exists, but we can't show you, can't explain you what it is, how it
looks is the text book definition of god throughout centuries.

~~~
drostie
It is only "broken" if you assume incorrectly that the goal of science is to
Discover Truth. Moreover it is broken in a more direct way: there exist
certain models which are _mathematically equivalent_ but which describe
_contradictory states of being_. A reasonably good example of this is
heliocentrism vs. geocentrism: classical mechanics allows you to say "the
Earth is at the center of the Solar System, there are gravitational, Coriolis
and centrifugal forces around it affecting all of the stuff in space", but it
also allows you to say "The Sun-Jupiter barycentre is at the center of the
Solar System, and the only force we need is gravity." There is no experiment
which can distinguish between those two; they are mathematically equivalent.

(A slightly better example comes from quantum mechanics. In the "Schrodinger
picture" there is a "wavefunction of the universe" which changes from moment
to moment, while the definitions of space and momentum stay the same. In the
"Heisenberg picture" the wavefunction stays the same while the definitions of
space and momentum change. You would think there would be an ontological
difference to the question, "is the state of the universe different from the
state of the big bang?" but, in fact, on this description there is no
observable difference, and science could never settle the question.)

This does not reduce science to a religion; science simply studies the
observable differences and must be content with not knowing everything --
which most scientists are already content with, since they have to deal with
matters of uncertainty and the distinctions between correlations and
causations.

Dark Matter does interact with other things, but it does so indirectly,
because it has mass and therefore warps spacetime. This is not actually the
first use of gravitational lensing to observe dark matter; in fact, earlier it
had been used to settle the question of whether dark matter felt any
electromagnetic force at all, by looking at galaxy collisions. The prediction
would be that the dark matter clouds of two galaxies would more or less "go
through each other" in a collision while the luminous stuff would "bump into
each other". This was observed as early as 6 years ago, see
<http://chandra.harvard.edu/photo/2006/1e0657/> .

We certainly can show you, and we can explain to you what it is. The only
problem is the same problem that neutrinos have: it's just very hard to detect
these particles because they don't have an electric charge and therefore don't
care about the electrons which make all the rest of chemistry happen. Our best
tool for understanding dark matter is still gravity; its a force which we know
the dark matter feels.

~~~
lutusp
> ... there exist certain models which are mathematically equivalent but which
> describe contradictory states of being. A reasonably good example of this is
> heliocentrism vs. geocentrism: classical mechanics allows you to say "the
> Earth is at the center of the Solar System, there are gravitational,
> Coriolis and centrifugal forces around it affecting all of the stuff in
> space", but it also allows you to say "The Sun-Jupiter barycentre is at the
> center of the Solar System, and the only force we need is gravity." There is
> no experiment which can distinguish between those two; they are
> mathematically equivalent.

How do heliocentrism and geocentrism represent "contradictory states of
being"? They are trivially related to one another, and are mathematically
equivalent as you point out. They represent a simple example of geometric
relativity.

Consider a gravitational slingshot maneuver, a way to harvest some of a
planet's orbital momentum to accelerate a passing spacecraft. If you make the
planet the frame of reference, or the sun, or the spacecraft, the math and
physics come out the same. No "contradictory states of being".

> A slightly better example comes from quantum mechanics. In the "Schrodinger
> picture" there is a "wavefunction of the universe" which changes from moment
> to moment, while the definitions of space and momentum stay the same. In the
> "Heisenberg picture" the wavefunction stays the same while the definitions
> of space and momentum change.

No,. this isn't really a "better example" -- Dirac demonstrated the
mathematical equivalence of Schrodinger's wave mechanics and Heisenberg's
matrix mechanics. Again, the difference is only apparent and superficial.

~~~
drostie
What's different in both cases is _ontology_ , which is a fancy way of saying
_how things actually are_. It may help to think of how you would program a
computer to simulate either universe, or for that matter it may help to reduce
the computation to something much simpler, like the following two functions:

    
    
        def odd_sum(n):
            return sum(range(1, 2*int(n), 2))
    
        def square(n):
            m = int(n)
            return 0 if m <= 0 else m ** 2
    

Those two functions are mathematically indistinguishable if all you are doing
is putting in various values. There happens to be a vast mathematical identity
that the sum of the first n odd numbers is the n'th square number, allowing
for that indistinguishability. But to claim that they're exactly the same
realization of this function is obviously mistaken.

Similarly, there is a qualitative difference between a wavefunction of the
universe which changes and a wavefunction of the universe which does not
change. It does not matter that the two positions can be made mathematically
equivalent; one is X and one is not-X, and science is permanently incapable of
figuring out which it really is.

------
carterac
How do these filaments stay stable and not collape under their own gravity due
to instabilities? Or, if there is 0 net force causing them to collapse, why
doesn't the dark matter drift apart naturally and become less dense and more
diffuse over time? Either way, filaments of high density don't seem to be a
natural stable state. Can someone explain this?

~~~
archgoon
Well, the particles also have no way to radiate away energy, so the dark
matter particles just whisk around each other under the gravitational force.

You can check out something like this on your computer

<http://http.developer.nvidia.com/GPUGems3/gpugems3_ch31.html>

To see what pure 1/r^2 interactions look like. You can download the
simulations from here:

<http://developer.nvidia.com/cuda/cuda-toolkit-40>

------
dragonbonheur
Just as predicted by Mr. Jean-Pierre Petit: <http://jp-
petit.org/science/colloque2001/Colloque_2001_1.htm>

------
InclinedPlane
So, interestingly enough the leading theory for the identity of the bulk of
dark matter is the "weekly interacting massive particle" (or WIMP) the
neutralino, and there has been recent evidence from the Fermi gamma-ray
telescope that supports the theory that the neutralino is the primary
component of dark matter.

<http://arxiv.org/abs/1201.1003>

<http://arxiv.org/abs/1205.1045>

~~~
kyberias
Since this is Hacker News I have to point out that the "weekly interacting
massive particle" could be, for example, my over-weight boss whom I have to
meet every monday. ;)

------
Jarihd
This is something i thought about Dark Matter: I believe Dark Matter to be the
resultant force(and/or field) generated due to the interaction of the
forces(and/or fields) of individual moving objects(matter).

consider a magnet(refer here as object) - something which has the property to
attract(gravity like) and repel(field like): Now if you were to have 2
magnets(moving objects) come close enough such that they repel(or attract);
but due to forces(and/or fields) of other moving objects in their vicinity(or
far enough[1]); they get locked or entangled such that their movement(and
other properties) is now dependent on the strongest forces or fields of nearby
objects. Over time; these other objects also get entangled and tend to form
clusters and keep moving(exhibiting other properties like radiation etc). But
now their movement(and other properties) seem to be the resultant effect of
forces (and/or fields) of all the objects now entangled - giving an illusion
of some matter that exists - now known as Dark matter.

I have used magnets as just as an example - one could think of matter having
both these properties to attract and repel - such that the area affected by
them could vary depending on various properties of the objects(matter).

[1](far enough) - such that their observation is neglected; but these objects
tend to have forces(and/or fields) that they affect a particular system under
observation.

