[extropy-chat] re: Dark matter galaxy discovered

Amara Lynn Graps Amara.Graps at ifsi.rm.cnr.it
Thu Feb 24 11:51:06 UTC 2005

I wrote this to Robert and Damien one year ago in answers to
some questions they had. Dark matter is about as far from 
my astronomy field as is string theory (in which I am still 
ignorant), so I cannot really answer detailed questions,
especially since I am too sleep-deprived to be coherent.

Let's start with Krauss' table, in order to distinguish between
kinds of dark matter:


            CONTENTS of the UNIVERSE
Type       Likely Composition       Main Evidence     Omega Contrib

Visible      Ordinary matter (protons,     telescopic         0.01
Matter       neutrons) that forms          observations     
             stars, dust and gas
Baryonic     Ordinary matter that is       Big Bang           0.05
Dark         too dim to see (brown or      nucleosynthesis    
Matter       black dwarfs, massive         and observed
             compact halo objects:         deuterium abundance
Nonbaryonic  Very light "exotic"           Gravity of visible  0.3
Dark Matter  particles such as             matter is insuffi-  
             axions, neutrinos w/mass      cient to account
             or weakly interacting         for orbital speeds 
             massive particles:            within galaxies
             WIMPS                         and galaxies within

Cosmological Cosmological Constant         Microwave back-     0.?
Dark Energy  (energy of empty space)       ground suggests
                                           cosmos is flat but
                                           there is not enough
                                           matter to make it so.
from [1] 

Re: the long-running debate that galaxies rotate too fast for the
matter that is observed in them, and that they should have flung
themselves apart.

It is an old story. In 1932, Oort found evidence for extra matter
within our galaxy, and then one year later Zwicky inferred a
large density of matter within clusters of galaxies. [2]

The conceptual idea is to look at the motions of various kinds of
astronomical objects, and assess whether the visible material is
sufficient to provide the inferred gravitational force. If it is
not, the excess attraction must be due to extra invisible

Since the 1970s there has been a discrepancy between the observed
rotational velocities of stars in the outer regions of spiral
galaxies and the orbit velocities that one would expect according
to Newton's Laws from the distribution of visible stars in the
galaxy. This discrepancy indicates that there should be much more
matter in the outer parts of the spiral galaxies. [3]

In particular, mass is widely distributed in a galaxy, so then
the rotation rates of gas and stars should increase with distance
from the center until most of the galaxy's mass is inside their
orbit, then slow further out. From Kepler:
                             v     G M(R)
                            ---- = ------
                             R        2

                                    G M(R)
                          v  = sqrt(------)

At large distances, enclosing most of the visible part of the
galaxy, we expect that the rotational velocity to drop off as the
square root of R. It doesn't. Instead, galactic rotation rates
never drop, the velocity stays roughly constant. This is evidence
that unseen matter well beyond the visible disk controls the
stars' velocities.  The figure that people seem to like that
clearly shows this is the rotation curve for spiral galaxy NGC
3198, in an article published by Albada and Sancisi, 1986.

Re: the suggestion that gravity from massive black holes may be
holding things together.

Two problems with that:

1) If you mean black holes in the center, like those found in the
centers of a number of galaxies -

No, because the unseen mass must be at, or outside, of the
galactic disk edges to influence the motion of the visible
objects in the outer edge of the disk. Astronomers think that the
unseen mass is distributed in a gigantic sphere or ellipsoid
("halos of dark matter") around the spiral galaxy disk.

2) black holes are the wrong kind of 'matter'.

Before the 1980s, the dark matter was thought to be of the
baryonic kind, but now cosmologists don't think that because the
massive object must lack detectable radiation, yet possess enough
mass to have an observed effect on the galactic rotation. Low
mass stars or black holes cannot be accounted for in large enough
numbers. This leaves the possibilities: exotic fundamental
particles: axions, neutrinos, photinos?, magnetic monopoles?....
Most of these are so-far-undetected particles coming from
theories in particle physics. For example, the supersymmetry
model describes a symmetry between fermions (particles with
half-integer spin) and bosons (particles with integer spin),
where every boson has a fermionic partner called a superpartner
and vice versa: squarks for quarks and photoinos for photons,
etc.. And the axion is a very light particle that is postulated
to solve a puzzle associated with charge and parity symmetry.

In Liddle's book (pg. 69-70), he mentions MACHOs as a candidate
to solve this problem, and they were apparently detected with
microlensing experiments of stars in the Large Magellanic Cloud.
[4] However, even though MACHOs seem to be present, they appear
to have insufficient density to completely explain the galactic
halo, and that the best hope is if the dark matter particles
interact not only gravitationally, but also through the weak
nuclear force (hence the name Weakly Interacting Massive Particle
or WIMP), the reason being that a only-gravitation-interacting
particle provides too tiny a force (the gravitational force of an
individual particle with say a proton mass is minuscule).
Supersymmetric particles in particular are thought to be
potentially detectable if they indeed make up dark matter.

Re: The evidence pointing to galaxy clusters having more matter
than is observed.

Analogous halos to those around individual galaxies are thought
to keep clusters of galaxies bound together. [5]

Groupings of galaxies account for almost all visible matter. [1,
page 36] Most of their luminous content takes the form of hot
intergalactic gas, which emits x-rays.

Re: If there consensus on how much of the dark matter is within
galaxies vs. between galaxies within clusters?

I don't know if there is a consensus, because I'm not in this
field (cosmology) to know all of the discussions and players, but
I'll tell you what I read.

In Krauss' article [1], he described some studies of galaxies by
Simon White at (MPA, Garching), where White and his colleagues
compiled information about several different clusters, to argue
that luminous matter accounted for between 10 and 20 percent of
the total mass of the objects.

When combined with the measurements of deuterium, these results
imply that the total density of clustered matter -- including
protons and neutrons as well as more exotic particles such as
certain nonbaryonic dark matter candidates -- is at most 60
percent of that required to flatten the universe.

So then either the universe is open, or it is made flat by some
additional form of energy that is not associated with ordinary

Another set of information, from the simulation theorists, show a
slow growth of the number of rich galaxy clusters over the recent
history of the universe, suggesting that the density of matter is
less than 50 percent of that required for a flat universe. [See
"The Evolution of Galaxy Clusters," by J. Patrick Henry, Ulrich
G. Briel and Hans Boehringer, Scientific American, December

Note that when one talks about clusters of galaxies (and clusters
of clusters), then you enter the cosmological realm of structures
in the universe, and how those structures evolve (gravitational
instability), inflation theory, enlargement of quantum
fluctuations to macroscopic size, the COBE data and the CMB,
general relativity, the curvature of space, acceleration and
deceleration, dark energy, etc.

"Matter tells spacetime how to curve, and spacetime tells matter
how to move." (John A. Wheeler).

Here is a final table from Krauss' article (pg. 39) that you
might find interesting:



Measurements of the contribution to Omega from matter are in
rough concordance. Most astronomers now accept that matter alone
cannot make Omega equal to 1 (*). But other forms of energy, such
as the cosmological constant, may also pitch in.

Observation                               Omega_matter

Age of universe                           <1
Density of protons and neutrons         0.3 - 0.6
Galaxy clustering                       0.3 - 0.4
Galaxy evolution                        0.3 - 0.5
Cosmic microwave background radiation   <=1
Supernovae Type Ia                      0.2-0.5

(*) Omega = 1 is predicted by inflation, which is equivalent to:
k = 0 (space is flat), n = 1.0 +/- 0.2 (the primordial lump
spectrum from satellites such as COBE). [2/99 Sky&Tel, pg. 38]


[1] "Cosmological Antigravity" by Lawrence M. Krauss in
Scientific American, Special Edition: The Once and Future Cosmos,
Volume 12, Number 2, 2002, pg. 33.

[2] Liddle, Andrew, _An Introduction to Modern Cosmology_, 1999,
Wiley, pg. 62-3.

[3] Hawking, Stephen, _The Universe in a Nutshell_ Bantam, 2001,
pg. 186-7.

[4] K.H. Cook et al Phys Rev Lett April 10, 1995.

[5] "The Life Cycle of Galaxies" by Guinevere Kauffann and Frank
van den Bosch  in Scientific American, Special Edition: The Once
and Future Cosmos, Volume 12, Number 2, 2002, pg 16.


Amara Graps, PhD        www.amara.com
Istituto di Fisica dello Spazio Interplanetario, CNR - ARTOV, 
Via del Fosso del Cavaliere, 100, I-00133 Roma, ITALIA
tel: +39-06-4993-4375      fax:  +39-06-4993-4383
Amara.Graps at ifsi.rm.cnr.it http://www.mpi-hd.mpg.de/dustgroup/~graps/ 
"We came whirling out of Nothingness scattering stars like dust." 

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