
Unprecedentedly wide and sharp dark matter map - dnetesn
https://phys.org/news/2018-03-unprecedentedly-wide-sharp-dark.html
======
lisper
I'm confused. This article says it's about a dark matter map, but it spends
most of its time talking about accelerated expansion, the cause of which is
labeled dark _energy_ , not dark matter. Dark matter (I am given to understand
-- IANACosmologist) acts on a much smaller scale to hold galaxies together
which would come apart under the gravitational influence of visible matter
alone. What am I missing?

~~~
dnautics
Dark matter and dark energy are both fudge factors to explain gravitational
observations that don't make sense. Dark matter corrects for the speed
distribution of stars around galaxies and dark energy for cosmological
expansion. The observations here collect indirect observations of dark matter
via a different method and postulate that the discrepancies from what was
predicted may obviate the need for dark energy. (Disclaimer: I am not an
astrophysicist)

~~~
oldandtired
Fudge factors - lovely. As one physicist put in a public lecture, if
scientists use the term "dark" then they are discussing something that they
have no clue about.

~~~
ssivark
I don't see why there is such an agitated response to the term "fudge factor"
(I see downvoted comments, several knee-jerk defensive replies, etc)

It's important to embrace one's ignorance -- especially when we're at the
absolute frontier of human knowledge. If you've seen some of the calculations
involving dark matter's effects, calling it a "fudge factor" is a fair
characterization, and one that is acceptable among physicists. We postulate a
component with certain properties that fudges our calculations in just the
right way to match some indirect observations (circumstantial evidence).

We still don't know what is the thing which produces the behavior we expect,
and what its properties might be. We're still seeking evidence that would seal
the deal, so to speak. It's true that several such fudges historically turned
out to be profound stepping stones, but till we get there, "fudge factor" is
absolutely honest.

I'll leave some Richard Feynman quotes here:

* "The first principle is that you must not fool yourself — and you are the easiest person to fool."

* "I can live with doubt and uncertainty and not knowing. I think it is much more interesting to live not knowing than to have answers that might be wrong. If we will only allow that, as we progress, we remain unsure, we will leave opportunities for alternatives. We will not become enthusiastic for the fact, the knowledge, the absolute truth of the day, but remain always uncertain … In order to make progress, one must leave the door to the unknown ajar."

~~~
raattgift
> We postulate a component with certain properties

versus

> We still don't know ... what its properties might be.

Which is it?

> stepping stones

Observation of missing momentum-energy in Lithium-6 decays (Pauli, 1930 [1])
until direct detection of the (electron anti-)neutrino (Reines and Cowan,
1956) took 26 years and a number of false starts including failures to detect
neutrinos in detectors at nuclear weapon tests and nuclear reactors, where the
production of neutrinos was expected. Observation of solar neutrinos took a
further thirteen years (Davis, HOMESTAKE, 1969).

Neutrinos are in the strictest sense dark matter: they do not feel or
participate in electromagnetism. However, they also have a tiny rest mass, so
it is hard to keep them from carrying their momentum-energy away at
relativistic speeds. Their fast motion makes them too "hot" to avoid smearing
out visible matter during structure formation, so they are not a candidate for
the momentum-energy missing in galaxy cluster interactions. On the other hand
their fast motion makes it relatively easy to spot atomic recoils (or really
the latter's ionization energy) when neutrinos interact with the matter in a
detector. That's why detectors were placed at extremely violent neutrino-
producing events: the hotter the neutrino, the easier to detect it.

The standard cosmology predicts a relic neutrino field comparable to the relic
photon field called the Cosmic Microwave Background. These relic neutrinos
were relativistic ("hot") in the early universe, but have cooled down
adiabatically to about 2 kelvins, just as the hot photons in the CMB have
cooled down to a bit under 3 kelvins. The Cosmic Neutrino Background is vital
at high redshift (z > 3000) and there is good indirect evidence for it (BBN
element abundance, suppressed early small-scale structure formation, damped
acoustic oscillations of the CMB). However, it is extremely difficult to
detect relic neutrinos with our current level of technology (although
experiments like KATRIN will try:
[https://www.katrin.kit.edu/](https://www.katrin.kit.edu/) and
[https://arxiv.org/abs/1304.5632](https://arxiv.org/abs/1304.5632) ), because
the recoils from slow-moving neutrinos will be smaller than those from
relativistic neutrinos. These relic neutrinos are _literally_ Cold Dark
Matter.

So there have been stepping stones on the road towards direct detection of a
form of cold dark matter. It is not the cold dark matter working cosmologists
or galaxy astrophysicists are looking for to explain galaxy-and-higher-scale
momentum-energy anomalies (notably because its rest mass is too low), but it's
suggestive of something more than "fudge".

There are options other than a heavy neutrino-like particle or even particle
dark matter for galaxy-scale and even cluster-scale problems; indeed, a
relativistic "left hand side" term in the Einstein Field Equations is
plausible -- one would tend to treat that as a modification of gravity.
However cold dark matter is needed to explain several fine features in the
cosmic microwave background, and those features are ever sharper in the
results of each generation of CMB observatory since BOOMERanG.

Finally, returning to the ~ 40 years between observation of anomalous results
to direct detection for neutrinos: Vera Rubin et al.'s galaxy rotation curve
paper was published in The Astrophysical Journal in 1980. So the long gap
between the apparent need for dark matter and today does not seem so long
compared to the long gap between the apparent need for a light spin-1/2
particle [1] and direct detection.

> "fudge factor" is absolutely honest

How exactly does this honesty scale work?

Apropos honesty and fudging things, who exactly is in each of the six
instances of "we" (excluding the ones in the Feynman quotes) in your comment?

\- --

[1] I think Pauli's letter is interesting in itself. An English translation
follows on the second page: [http://microboone-docdb.fnal.gov/cgi-
bin/RetrieveFile?docid=...](http://microboone-docdb.fnal.gov/cgi-
bin/RetrieveFile?docid=953;filename=pauli%20letter1930.pdf)

~~~
ssivark
IMHO, we seek models explaining certain observed cosmological behavior. As
long as plausible models for dark matter range from WIMPs to primordial black
holes or axions and what not, I consider it unreasonable to claim that we know
what dark matter is, and all it’s properties.

Eg: if all dark matter experiments (direct and indirect detection) returned
null results over the next several decades, then dark matter would still be an
open puzzle.

With regards to “we”, I think it applies equally well to any group as small as
physicists working on DM, to all scientists, or everyone discussing the
subject. I expect most particle/astro physicists I’ve interacted with to find
those statements very reasonable.

------
thyrsus
Suppose the gravitational attraction of particles, like spin, was signed, such
that like attracts like, and unlike repels unlike? Because of the weakness of
gravity, every other force would dominate at the local scale. But the
prevalence of one or the other sign in a region (at the galactic scale) would
vary - as there would have been a (very gentle, as gravity is weak) "sorting"
in the early universe, as like signs attracted and unlike signs repelled.
"Dark matter" would be variation in the prevalence of one sign or the other.

How would one test such a model?

~~~
dwaltrip
On the galactic scale, my hunch is that the sorting dynamic would be very
strong, and you would basically have g+ and g- galaxies. Which would be very
interesting, but doesn't solve the dark matter issue.

I think that this would also create a lot of other complex behavior that we
don't see. For example, I think we would expect to see many cases of two
galaxies or even two galaxy clusters pushing away from each other, because one
is g+ and the other g-.

~~~
thyrsus
Expansion does have the appearance of "pushing away". But, as noted, to
explain "dark matter", one would observe "anti-gravitational" separations in
inert gases at local scale, which has never been reported.

~~~
dwaltrip
Ah yeah nice idea. However, the expansion we see is stronger when things are
farther away. Gravity gets weaker at a quadratic rate as distance increases,
so I don't think opposite gravity signs could be contributing significantly to
the observed expansion.

