Dark matter hides, physicists seek (Forwarded)

News Service
Stanford University
Stanford, California

Contact:
Dawn Levy, News Service
(650) 725-1944

Comment:
Blas Cabrera, Physics
(650) 723-3395

November 28, 2006

Dark matter hides, physicists seek
By Clara Moskowitz

Scientists don't know what dark matter is, but they know it's all over the
universe. Everything humans observe in the heavens -- galaxies, stars,
planets and the rest -- makes up only 4 percent of the universe,
scientists say. The remaining 96 percent is composed of dark matter and
its even more mysterious sibling, dark energy. Scientists recently found
direct evidence that dark matter exists by studying a distant galaxy
cluster and observing different types of motion in luminous versus dark
matter. Still, no one knows what dark matter is made of. Now, a pioneering
international project co-led by Stanford physicist Blas Cabrera may
finally crack the case and pin down the elusive particles that form dark
matter.

"It's harder and harder to get away from the fact that there is a
substance out there that's making up most of the universe that we can't
see," says Cabrera. "The stars and galaxies themselves are like Christmas
tree lights on this huge ship that's dark and neither absorbs nor emits
light."

Buried deep underground in a mineshaft in Minnesota lies Cabrera's
project, called the Cryogenic Dark Matter Search II (CDMS II). University
of California-Berkeley physicist Bernard Sadoulet serves as spokesperson
for the effort. Fermilab's Dan Bauer is its project manager, and Dan
Akerib from Case Western Reserve University is the deputy project manager.
A team of 46 scientists at 13 institutions collaborates on the project.

To catch a WIMP

The experiment is the most sensitive in the world aiming to detect exotic
particles called WIMPS (Weakly Interacting Massive Particles), which are
one of scientists' best guesses at what makes up dark matter. Other
options include neutrinos, theorized particles called axions or even
normal matter like black holes and brown dwarf stars that are just too
faint to see.

WIMPS are thought to be neutral in charge and weigh more than 100 times
the mass of a proton. At the moment these elementary particles exist only
in theory and have never been observed. Scientists think they haven't
found them yet because they're excruciatingly difficult to capture. WIMPS
don't interact with most matter -- the shy particles pass right through
our bodies -- but CDMS II aims to catch them in a rare collision with the
atoms in the project's special-made detectors.

"These particles mostly pass through the Earth without scattering,"
Cabrera says. "The only reason we even have a chance of seeing events is
because [there are] so many of the particles that very rarely one will
come [into the detector] and scatter."

The detectors are hidden under layers of earth in Minnesota's Soudan mine
to protect them from cosmic rays and other particles that might collide
with the detectors and be mistaken for dark matter. In fact, half the
battle for the scientists working on CDMS II is to shield their
instruments as much as possible from everything but WIMPS and to develop
elaborate systems to tell the difference between dark matter and more
mundane particles.

"Our detector is this hockey-puck-shaped thing that needs to live at 50
thousandths of a degree above absolute zero," says Walter Ogburn, a
graduate student at Stanford who works on the project. "It's hard to make
things that cold."

To that end, the instruments are nestled in a canister called an icebox,
lined with six layers of insulation, from room temperature on the outside
to coldest on the inside. This keeps the detectors so cold that even atoms
can't shiver.

The detectors are made of crystals of solid silicon and solid germanium.
The silicon or germanium atoms sit still in a perfect lattice. If WIMPS
crash into them, they will wiggle and give off tiny packets of heat called
phonons. When phonons rise to the surface of the detectors, they create a
change in a very sensitive layer of tungsten, which the researchers can
record. A second circuit on the other side of the detector measures ions,
charged particles that would be released from a collision of a WIMP and an
atom in the detector.

"Those two channels let us discriminate between different kinds of
interactions," says Ogburn. "Some things make more ionization and some
things make less, so you can tell the difference that way."

It takes a squad of scientists at multiple facilities to build the
detectors. The team buys the crystals from an outside company, and
researchers at Stanford's Center for Integrated Systems make measuring
instruments on the surfaces of the detectors. "We use the same things to
make these that people use to make microprocessors because those are also
super tiny," says Matt Pyle, another graduate student in Cabrera's lab.

Clumps of clues

A subset of WIMPS, called neutralinos, are the lightest particles expected
by supersymmetry, a theory that predicts a mate for every particle we've
already observed. If CDMS II is successful in finding neutralinos, this
would be the first evidence for supersymmetry. "Supersymmetry suggests
there's a whole other sector out there of particles that are the partners
to our existing particles," Cabrera says. "There are many ways in which
supersymmetry looks very likely. But there's no direct evidence yet for
any matching [supersymmetric] particle pair."

The weak interactions of WIMPS are why, even though dark matter particles
have mass and obey the laws of gravity, they do not clump into galaxies
and stars like normal matter. In order to clump, particles must crash and
stick together. But WIMPS most often would fly right by each other. Plus,
because WIMPS are neutral, they do not form atoms, which require the
attraction of positively charged protons to negatively charged electrons.

"Dark matter permeates everything," Cabrera says. "It just never collapsed
the way atoms did."

Since dark matter never formed stars and other familiar heavenly objects,
for a long time scientists never knew it was there. The earliest
indication of its existence came in the 1930s when Fritz Zwicky, a
Swiss-American astronomer, observed clusters of galaxies. He added up the
masses of galaxies and noticed that there was not enough mass to account
for the gravity that must exist to hold the clusters together. Something
else must provide the missing mass, he deduced.

Later in the 1970s, Vera Rubin, an American astronomer, measured the
speeds of stars in the Milky Way and other nearby galaxies. As she looked
farther out toward the edges of these galaxies, she found that the stars
do not rotate more slowly as scientists expected. "That didn't make any
sense," Cabrera says. "The only way you could understand it is if there
was a lot more mass there than what you saw in the starlight."

Over the years, more and more evidence for dark matter has piled up.
Although scientists don't yet know what it is, they have a better idea of
where it is and how much of it there should be. "There's very little
wiggle room left for having different quantities," Cabrera says.

"We've not seen anything that looks like an interesting signal to date,"
he says. But the CDMS II researchers continue the search. So, too, do
other groups. ZEPLIN, an experiment run by physicists at the University of
California-Los Angeles and the United Kingdom Dark Matter Collaboration,
aims to catch WIMPs in liquid vats of xenon in a mine near Sheffield,
England. And at the South Pole, a University of Wisconsin-Madison project
called IceCube is under construction that will use optical sensors buried
deep in the ice to look for neutrinos, high-energy particles that are
signatures of WIMP annihilations.

Meanwhile, CDMS II continues to evolve. Its researchers are building
bigger and bigger detectors to increase their chances of finding WIMPS. In
the future, the team hopes to build a 1-ton detector that should be able
to discover many of the most probable types of WIMPS, if they exist.
"We're taking data now with more than twice as much target mass of
germanium than we had before, so we're definitely exploring new territory
right now," says Ogburn. "But there's a lot more to cover."

[Clara Moskowitz is a science-writing intern with Stanford News Service.]

-30-

Editor Note: Science-writing intern Clara Moskowitz wrote this release. A
photo of the detector is available on the web at
http://newsphotos.stanford.edu/CDMS/

Relevant Web URLs:

* Cryogenic Dark Matter Search II Website
http://cdms.berkeley.edu/

"Andrew Yee" schreef in bericht
...

Dark matter hides, physicists seek
By Clara Moskowitz

Scientists don't know what dark matter is, but they know it's all over the
universe. Everything humans observe in the heavens -- galaxies, stars,
planets and the rest -- makes up only 4 percent of the universe,
scientists say. The remaining 96 percent is composed of dark matter and
its even more mysterious sibling, dark energy. Scientists recently found
direct evidence that dark matter exists by studying a distant galaxy
cluster and observing different types of motion in luminous versus dark
matter. Still, no one knows what dark matter is made of.


"Dark matter permeates everything," Cabrera says. "It just never collapsed
the way atoms did."

Since dark matter never formed stars and other familiar heavenly objects,
for a long time scientists never knew it was there. The earliest
indication of its existence came in the 1930s when Fritz Zwicky, a
Swiss-American astronomer, observed clusters of galaxies. He added up the
masses of galaxies and noticed that there was not enough mass to account
for the gravity that must exist to hold the clusters together. Something
else must provide the missing mass, he deduced.

Later in the 1970s, Vera Rubin, an American astronomer, measured the
speeds of stars in the Milky Way and other nearby galaxies. As she looked
farther out toward the edges of these galaxies, she found that the stars
do not rotate more slowly as scientists expected. "That didn't make any
sense," Cabrera says. "The only way you could understand it is if there
was a lot more mass there than what you saw in the starlight."

That is correct.
If you compare the rotation curve of our planets around the Sun
with the stars around the centre of our galaxy
then in the first case the speed almost decreases lineair with distance
(1/r)
while in the second case first there is an increase with distance
and after a certain distance the speed is constant and the rotation
curve becomes flat.
To explain the first is easy because the planets behave like point masses.
To explain the second requires that the 3D shape of the galaxy
including all the stars and planets have to be taken into account.
Such an explanation starts by taking all the visible stars into account.
Current observations reveal (Hubble) that galaxies are much larger
and consists of much more visible stars (made of ordinary matter)
than original thought of.
The result is that if you compare a simulated rotation curve of
a galaxy in the past (less stars) with the one now (more
stars) they become flatter and longer.
In short the difference with observation becomes less
and what is important less missing mass has to be included
to completely match observation. (of rotation curve)
In short less and less dark matter is required.

My prediction is that in the future no dark matter is required
to explain the rotations curves of our galaxy and all galaxies.

Nicolaas Vroom
http://users.pandora.be/nicvroom/

In article ,
"Nicolaas Vroom" writes:
Current observations reveal (Hubble) that galaxies are much larger
and consists of much more visible stars (made of ordinary matter)
than original thought of.

Reference, please? I'm not aware of any Hubble observations
suggesting nearby galaxies are either larger or have more stars than
previously thought.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

I speculate that dark energy is not a repelling force as scientists
speculate, but a propelling force that arises in regards to rotating
spiral galaxies. The expansion of the Universe may have a different
explanation than dark energy, say if spiral galaxies compact and gain
rotating speeds, perhaps they gain speed. The result is that spiral
galaxies are like trains that are gaining speed everywhere, and the
result is the appearance of an accelerated expansion (this should not
apply to elliptic galaxies, but they might be dragged along).

gmb

In article . com,
wrote:

I speculate that dark energy is not a repelling force as scientists
speculate, but a propelling force that arises in regards to rotating
spiral galaxies. The expansion of the Universe may have a different
explanation than dark energy, say if spiral galaxies compact and gain
rotating speeds, perhaps they gain speed. The result is that spiral
galaxies are like trains that are gaining speed everywhere, and the
result is the appearance of an accelerated expansion (this should not
apply to elliptic galaxies, but they might be dragged along).

Galactic flat rotation curves....

--
This space reserved for Jeff Relf's 5-dimensional metric.

--
Posted via a free Usenet account from http://www.teranews.com

"Steve Willner" schreef in bericht
...
In article ,
"Nicolaas Vroom" writes:
Current observations reveal (Hubble) that galaxies are much larger
and consists of much more visible stars (made of ordinary matter)
than original thought of.

Reference, please? I'm not aware of any Hubble observations
suggesting nearby galaxies are either larger or have more stars than
previously thought.

I'am wrong with respect to Hubble,
but the following URL clearly makes my point:
http://www.gemini.edu/index.php?opti...sk=view&id=144
or
http://www.gemini.edu/index.php?option=com_gem_releases
Select 2005 August 10: Gemini Uncovers 'Lost City' of Stars
The text states:
"The finding also implies that our own Milky Way Galaxy could
be much larger than current textbooks say. "

But this link is also interesting:
http://space.newscientist.com/article/dn8746
The text states:
The team estimates there may be more than a million cataclysmic variables
and about one billion active stars in the galaxy.
These figures are about 100 times higher than some recent estimates,
but Mukai thinks the numbers may in fact be much higher again.

The following link is also interesting:
http://www.sdss.org/news/releases/20...ndromeda9.html
They also speak about dark matter in relation to those
faint galaxies. However my conclusion is different.

IMO all those pictures show that there are much more stars
in galaxies than original thought of / observed.
My conclusion is that this leads to less dark matter
(not more) in order to explain the rotation curves.

Nicolaas Vroom
http://users.pandora.be/nicvroom/

"Nicolaas Vroom" schreef in bericht
...

"Steve Willner" schreef in bericht
...
In article ,
"Nicolaas Vroom" writes:
Current observations reveal (Hubble) that galaxies are much larger
and consists of much more visible stars (made of ordinary matter)
than original thought of.

Reference, please? I'm not aware of any Hubble observations
suggesting nearby galaxies are either larger or have more stars than
previously thought.

I'am wrong with respect to Hubble,
but the following URL clearly makes my point:
http://www.gemini.edu/index.php?opti...sk=view&id=144
or
http://www.gemini.edu/index.php?option=com_gem_releases
Select 2005 August 10: Gemini Uncovers 'Lost City' of Stars
The text states:
"The finding also implies that our own Milky Way Galaxy could
be much larger than current textbooks say. "

But this link is also interesting:
http://space.newscientist.com/article/dn8746
The text states:
The team estimates there may be more than a million cataclysmic variables
and about one billion active stars in the galaxy.
These figures are about 100 times higher than some recent estimates,
but Mukai thinks the numbers may in fact be much higher again.

The following link is also interesting:
http://www.sdss.org/news/releases/20...ndromeda9.html
They also speak about dark matter in relation to those
faint galaxies. However my conclusion is different.

An other one:
http://www.msnbc.msn.com/id/16521828/
about M31 Andromeda Galaxy

IMO all those pictures show that there are much more stars
in galaxies than original thought of / observed.
My conclusion is that this leads to less dark matter
(not more) in order to explain the rotation curves.


Nicolaas Vroom
http://users.pandora.be/nicvroom/

In article ,
"Nicolaas Vroom" writes:
http://www.gemini.edu/index.php?option=com_gem_releases
Select 2005 August 10: Gemini Uncovers 'Lost City' of Stars

As often happens, the press release is misleading. The abstract of
the published paper is at
http://www.journals.uchicago.edu/ApJ....abstract.html
and states "The luminosity profile is well described by a nucleus
plus a simple exponential profile out to 10 optical scale lengths."
In other words, the star density is about what is expected; the
new result is that there is no tidal truncation or other effect on
the outer disk.

http://space.newscientist.com/article/dn8746

This suggests more cataclysmic variables than expected. That could
be caused either by more white dwarfs than expected or a greater
fraction of them being cv's. Either way, it doesn't suggest greater
numbers of stars overall. (In spite of the note above, I haven't
bothered to read the paper itself.)

http://www.sdss.org/news/releases/20...ndromeda9.html

Not sure what your point is here. It wouldn't surprise anyone if
there are lots more dwarf satellites of M31 and the Milky Way.

http://www.msnbc.msn.com/id/16521828/

I can't find the paper or preprint, but if these stars are so
difficult to find, they represent a trivial amount of mass.

IMO all those pictures show that there are much more stars
in galaxies than original thought of / observed.

You need to quantify "much." How does it compare to the known mass?
And what form is this purported matter supposed to be in? In
particular, what mass to light ratio does it have at, say, R or 3.6
microns?

My conclusion is that this leads to less dark matter
(not more) in order to explain the rotation curves.

The sense of the argument is right: if the luminous mass is higher,
there's less dark matter needed. However, the _amount_ of change
suggested by the references looks trivial to me. Moreover, the
rotation curves require a matter _distribution_ different from the
luminosity distribution, not only an increase in quantity.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

"Steve Willner" schreef in bericht
...
In article ,
"Nicolaas Vroom" writes:
http://www.gemini.edu/index.php?option=com_gem_releases
Select 2005 August 10: Gemini Uncovers 'Lost City' of Stars

As often happens, the press release is misleading. The abstract of
the published paper is at
http://www.journals.uchicago.edu/ApJ....abstract.html
and states "The luminosity profile is well described by a nucleus
plus a simple exponential profile out to 10 optical scale lengths."
In other words, the star density is about what is expected; the
new result is that there is no tidal truncation or other effect on
the outer disk.

That same abstract shows:
"which doubles the known radial extent of the optical disk."
and:
"We find no evidence for truncation of the stellar disk"

http://space.newscientist.com/article/dn8746

This suggests more cataclysmic variables than expected. That could
be caused either by more white dwarfs than expected or a greater
fraction of them being cv's. Either way, it doesn't suggest greater
numbers of stars overall. (In spite of the note above, I haven't
bothered to read the paper itself.)

http://www.sdss.org/news/releases/20...ndromeda9.html

Not sure what your point is here. It wouldn't surprise anyone if
there are lots more dwarf satellites of M31 and the Milky Way.

The same as above: M31 contains (much ?) more stars as previously
known.

http://www.msnbc.msn.com/id/16521828/

I can't find the paper or preprint, but if these stars are so
difficult to find, they represent a trivial amount of mass.

See below.

IMO all those pictures show that there are much more stars
in galaxies than original thought of / observed.

You need to quantify "much." How does it compare to the known mass?

I cannot correctly quantify this.
I have no idea what the size/mass is of the newly discovered individual
stars is in NGC300 relative to Our Sun.
(Any idea ?)
I agree the total mass is small relative to the total mass.

However (and that is important) the size of the rotation curves
should be shown much larger as previously known.
The question is does the rotation curve stays flat.
I assume it (more or less) does.

My simulations of a galaxy with a visible mass M and disc size l
compared with an extended galaxy of vis. mass M+dm and
with size 2*l (that means outer disc has vis mass dm) show
that if you compare the two rotation curves the speed v
of the second rot. curve will increase at distance l.
That means the rotation curve becomes more flat
(specific the old part before distance l)
and compares better with observation.

(There is even reason to assume that M can now be larger)

All in all there is less darkmatter required to make the disc flat.

And what form is this purported matter supposed to be in? In
particular, what mass to light ratio does it have at, say, R or 3.6
microns?

microns ? I do not understand

My conclusion is that this leads to less dark matter
(not more) in order to explain the rotation curves.

The sense of the argument is right: if the luminous mass is higher,
there's less dark matter needed.
I agree

GOTO: http://www.aao.gov.au/local/www/jbh/A1/
and listen to D: How do discs form .mp3

However, the _amount_ of change
suggested by the references looks trivial to me.
I expect you mean small.

The question is what is the smallest size individual stars that are
currently discovered. If this are sun like stars than you can expect
that there are also many more smaller ones still to be discovered,
decreasing the amount of darkmatter required to make the rotation
curve flat.

Nicolaas Vroom
http://users.pandora.be/nicvroom/

Moreover, the
rotation curves require a matter _distribution_ different from the
luminosity distribution, not only an increase in quantity.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

http://www.journals.uchicago.edu/ApJ....abstract.html

In article ,
"Nicolaas Vroom" writes:
That same abstract shows: "which doubles the known radial extent of
the optical disk." and: "We find no evidence for truncation of the
stellar disk"

Let's back up a little. Galaxy disks are described by an exponential
law, where density d = d0*exp(-r/l). In this formula, d0 is the
central density of the disk, r is distance from the galaxy center,
and l is a constant called the "scale length." The scale length for
M31, for example, is about 6 kpc; other galaxies have their own
values. In order to find the total stellar light, observers measure
the surface brightness out as far as they can, then fit the
observations to the above functional form. (There's a complication
from the galaxy's bulge, but it is fit and subtracted away.) Then
one calculates the _total_ light at all radii (in other words,
integrate the function from zero to infinity). This is accurate
enough even if the measurements only extend to a few scale lengths;
the light outside 3 scale lengths is only 20% of the total, and the
light outside 5 scale lengths is 4% of the total.

What the paper above reports are _observations_ of stars far from the
center of NGC 300, specifically at 10 scale lengths. The
observations confirm that the density is as expected from the
exponential law. That's a bit surprising, given the possibility of
tidal interactions, but it doesn't increase the mass or radius of
NGC 300 over what was already expected from the exponential law. The
distant stars were already taken into account even though they had
not yet been observed. If the exponential law continues beyond 10
scale lengths, there will be another 0.05% of the galaxy's mass
beyond that radius, but it will hardly make any difference to the
dynamics whether it's there or not.

The paper above does not give any evidence that NGC 300 was larger in
size or more massive than thought before.

http://www.sdss.org/news/releases/20...ndromeda9.html

M31 contains (much ?) more stars as previously known.

The reference is to a dwarf _companion_ galaxy of M31. There are
lots of those around, and they don't change the mass of M31.

You need to quantify "much." How does it compare to the known mass?

I cannot correctly quantify this.

Even if some observation shows more stars than previously known, you
have to ask whether the new stars have enough mass to affect the
rotation. That's a _quantitative_ question of the sort seldom if
ever addressed in press releases.

And what form is this purported matter supposed to be in? In
particular, what mass to light ratio does it have at, say, R or 3.6
microns?

microns ? I do not understand

I was referring to the wavelength of observation. 3.6 microns is a
good wavelength for measuring luminous mass of galaxies because the
mass is fairly insensitive to the exact types of stars, and sensitive
observations are possible with the Spitzer Space Telescope.

The question is what is the smallest size individual stars that are
currently discovered. If this are sun like stars than you can expect
that there are also many more smaller ones still to be discovered,
decreasing the amount of darkmatter required to make the rotation
curve flat.

Here you are asking about the stellar mass function: the relative
numbers of stars of different masses. As you say, the least massive
stars are not measured directly, and one has to assume a mass
function and thus a mass to light ratio. There are a number of
constraints on M/L (not least local observations in the Sun's
neighborhood), but it could be systematically wrong or different in
different galaxies. People work on this question in various ways,
for example by observing at many different wavelengths and by using
spectroscopy, but there is still some uncertainty. However, none of
the observations you have cited changes our knowledge of the mass
function, nor will any _simple_ change in the mass function by itself
get rid of the need for dark matter. Of course you can always "fix
up" a mass function that changes with radius in just such a way as to
reproduce the rotation curve, but there's no separate evidence for
such a change.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)