On 7/4/15 3:44 PM, wlandsman wrote:
We have no confirmed non-gravitational detection of dark matter
either astrophysically or in the laboratory. But that does not
mean that we cannot learn anything about dark matter. Recently,
Massey et al. a claimed possible detection of dark matter interacting
with itself in a galaxy in the cluster Abell 3827.

1. http://arstechnica.com/science/2015/...f-interacting/
2. http://www.eso.org/public/usa/news/eso1514/

3. http://arxiv.org/abs/1504.06576
4. http://arxiv.org/abs/1503.07675

The ref 3 complements Massey et al. work ref 1,2

"Further studies of such systems are imperative to establish
if the indication from A3827 for a non-zero self-interaction
cross section (of order 1 cm^2/g) is indeed correct."

"It should therefore be clear that A3827 is no
more sensitive to DM self-interactions than other systems
considered in this context and can certainly not be used to
probe cross-sections as small as 10^(-4) cm^2 g^(-1)"

This conclusion from ref 3
would indicated that the 0.47 cm^2/g cross-section ref 4
may be weighted more towards .47 cm^2/g

Ref 1 indicates "it 'on the order 1 cm^2/g'
would be just right to solve the observational issues with the leading
dark matter model, which calls for a cross-section between 0.1 and one
square centimeter per gram

My question- What is the leading dark matter model that predicts DM
cross-section on the order of 1 cm^2/g ?

The authors use gravitational lensing to map out the dark matter
distribution in the galaxy cluster. They highlight a galaxy falling
into the (dark matter) cluster core, in which the dark matter halo
of the galaxy appears to lag behind its visible contents. They
argue that the dark matter halo is slowed down by its interaction
with the dark matter cluster core (whereas the baryonic contents
experience no interaction)
Dark matter slowing down at a rate other than observed baryonic contents
does not in itself exclude a baryonic DM character.
Give this evidence alone,
DM could be a different baryonic size/mass distribution
providing a different lag character.

I have two questions about this work.
1. How likely is it that this tentative detection will be confirmed
(or disproved)?
see ref 4 "Chandra and Hubble Space Telescopes we have now observed 72
collisions, including both `major' and `minor' mergers"
for concluded aggregate 0.47 cm^2/g cross-section
2. How much does a measurement of self-interaction actually tell
us about dark matter.
Ref1 provides the DM lag mechanism "The more they collide (or the higher
the cross-section), the more a blob of dark matter should lag, due to
the friction created by the colliding particles"
Another mechanism may be DM drag or lag
due to vacuum (momentum transfer) viscosity
Vacuum viscosity is physically discussed in
arXiv:0806.3165v3 [hep-th] 14 Nov 2008
Hydrodynamics of spacetime and vacuum viscosity

For the first question, I am astonished by the rapid progress in
the use of gravitational lensing to map out the distribution of
non-baryonic dark matter. (I was previously impressed with the
rapid progress in microlensing to rule out significant baryonic
dark matter.) But Massey et al. point out the fortuitous circumstances
that made their observation possible. In particular they require
a nearby cluster (so that small lag can be measured) and a complex
background galaxy (so the the mass distribution of the lensing
galaxy can be measured). I should think we need measurements
of three or four such systems giving consistent results before we
can fully believe we have detected self-interacting dark matter.

see ref 4 for Chandra and Hubble Space Telescopes we have now observed
72 collisions, including both `major' and `minor' mergers
As for what this measurement can tell us about dark matter, the
fact that we *don't* see self-interaction at the galaxy cluster
level (e.g. the bullet cluster)
Ref 1:"In the Bullet Cluster observations, no lag was found, but the
study was not precise enough to rule it out entirely.
But the study did constrain it;
if dark matter self-interactions were taking place,
they weren't doing so that often
(the cross-section had to be 1.25 square centimeters per gram or less)."

Also pointing to other work
The Bullet Cluster provides further evidence of DM decceleration
http://arxiv.org/abs/astro-ph/0608408
http://en.wikipedia.org/wiki/Bullet_Cluster

Bullet Cluster E0657-56 length
7.714E+23 cm 250 kpc
Bullet Cluster E0657-56 time after collision
4.730E+15 sec 1.50E+08 years
Bullet Cluster E0657-56 acceleration
2*length/time^2 = 6.9E-08 cm/sec^2
This deceleration is of the same order
as the Pioneer deceleration.
A Pioneer 10 and 11 cross_section value 0.24 cm^2/g
(within range of Ref 1,3,4)
may be computed from the NASA published Pioneer dimensions
(pi/4)*pioneer_diameter^2/pioneer_mass
= (pi/4)*274^2/241,000 = 0.24 cm^2/g
with universe critical density (3/8pi)*H^2/G ~ 1x10^-29 g/cm^3
6.9E-08 = Cd * .24 * 1x10^-29 * c^2 = 2x10^-9 cm/sec^2
where a reasonable unitless hydrodynamic drag coefficient Cd = 32

The case for Pioneers' deceleration
predominately due to thermal affects
http://arxiv.org/pdf/1204.2507v1
may have been premature
considering reported decelerations
did not include the residual constant values
as these spacecraft entered intergalactic space
with concurrent diminishing thermal affects.
https://groups.google.com/forum/#!se...E/FJy5LOLrSOEJ
Further data analysis is required
particularly with Pioneers' spin deceleration
as a vacuum viscosity (momentum transfer) probe
that was entirely excluded from
http://arxiv.org/pdf/1204.2507v1.

but do see at the galaxy level
suggests that it might solve one of the very few problems with
Lambda CDM. Lambda CDM works extremely well with large-scale
structure but less well at the galaxy level, perhaps because because
current models neglect dark matter self-interaction.
and the viscosity of space
arXiv:0806.3165v3 [hep-th] 14 Nov 2008
Hydrodynamics of spacetime and vacuum viscosity
Lambda CDM appears to be too smooth & too perfect
But the Massey et al measurements only return a single interesting
parameter-- the self-interaction rate -- that doesn't satisfy the
question of what dark matter is. In the July 2015 issue of
Scientific American (quarantined) Dobrescu and Lincoln suggest
that there are a variety of dark matter particles and "dark forces"
analogous to electromagnetism which mediate only between dark
particles.
Seems too complicated.
If true, then we may have a new tool for studying
dark matter but it is an extremely crude one at best.
Given the logarithmic nature of the universe,
having a value within one order of magnitude should be very useful.
Richard D Saam for comments

--Wayne