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| Sitav |
 Posted: Mon Mar 12, 2007 2:16 pm |
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what exactly is dark matter? i've heard about it but cant get a clear
idea of what it is. |
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| Brian Tung |
| Posted: Mon Mar 12, 2007 2:16 pm |
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Ed wrote:
Quote: Could Dark Matter be Anti Matter?
No. The whole point is that dark matter interacts weakly (if at all)
with electromagnetic radiation, and anti-matter doesn't fit the bill,
as it interacts with EM radiation the same way that normal matter does.
An example of a known constituent for dark matter is neutrinos. At
least some of the are known to possess some mass (and my intuitive guess
is that they all do), so they exert some gravitational pull, but they
essentially don't interact electromagnetically.
However, the current best estimate for neutrino counts suggests that
either there aren't enough neutrinos to make up all the dark matter, or
the dark matter theory is inaccurate. (Or, it could be both.)
--
Brian Tung <brian@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html |
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| Chris L Peterson |
| Posted: Mon Mar 12, 2007 2:44 pm |
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On 12 Mar 2007 12:16:17 -0700, "Sitav" <sitav_nabi@yahoo.com> wrote:
Quote: what exactly is dark matter? i've heard about it but cant get a clear
idea of what it is.
Dark matter is a form of matter that interacts very weakly (or not at
all) with electromagnetic radiation. This means it doesn't emit, absorb,
scatter, or reflect light (or x-rays, radio, etc). Basically, it's
invisible. But it interacts gravitationally just like ordinary matter,
so we can detect its presence by how it affects visible matter around
it.
Nobody knows for sure what dark matter is. Most evidence (and theory)
suggests it is composed of a different set of elementary particles,
which are called non-baryonic (baryons include ordinary matter such as
protons and neutrons).
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com |
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| Sam Wormley |
| Posted: Mon Mar 12, 2007 4:04 pm |
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Sitav wrote:
Quote: what exactly is dark matter? i've heard about it but cant get a clear
idea of what it is.
Hi Sitav.
Dark matter maps reveal cosmic scaffolding
http://xxx.lanl.gov/abs/astro-ph/0701594
http://msnbcmedia4.msn.com/j/msnbc/Components/Photos/070107/070107_cosmos_hmed3p.h2.jpg
Ordinary baryonic particles (such as protons and neutrons) account
for only one-sixth of the total matter in the Universe. The remainder
is a mysterious "dark matter" component, which does not interact via
electromagnetism and thus neither emits nor reflects light. As dark
matter cannot be seen directly using traditional observations, very
little is currently known about its properties. It does interact via
gravity, and is most effectively probed through gravitational
lensing: the deflection of light from distant galaxies by the
gravitational attraction of foreground mass concentrations. This is a
purely geometrical effect that is free of astrophysical assumptions
and sensitive to all matter -- whether baryonic or dark. Here we show
high fidelity maps of the large-scale distribution of dark matter,
resolved in both angle and depth. We find a loose network of
filaments, growing over time, which intersect in massive structures
at the locations of clusters of galaxies. Our results are consistent
with predictions of gravitationally induced structure formation, in
which the initial, smooth distribution of dark matter collapses into
filaments then into clusters, forming a gravitational scaffold into
which gas can accumulate, and stars can be built.
Limits set on size of dark matter clumps (Feb 27)
http://physicsweb.org/article/news/11/2/22
If dark matter really does reside only in large clumps, these cannot be
any bigger than one-tenth the mass of the Earth. This is the claim of
physicists from Germany and the UK, who have studied gravitational
lensing data from almost 300 distant supernovae. The findings could
require the notions of dark matter -- a mysterious substance thought to
constitute the vast majority of matter in the universe -- to be revised
by cosmologists, some of whom have assumed that dark matter clumps could
have masses many times that of the Sun (Phys. Rev. Lett. 98 071302).
This galactic cluster collision is ideally suited to study differences
in dark matter and baryonic matter.
http://antwrp.gsfc.nasa.gov/apod/ap060824.html
http://antwrp.gsfc.nasa.gov/apod/image/0608/bulletcluster_comp_f2048.jpg
Explanation: The matter in galaxy cluster 1E 0657-56, fondly known as
the "bullet cluster", is shown in this composite image. A mere 3.4
billion light-years away, the bullet cluster's individual galaxies
are seen in the optical image data, but their total mass adds up to
far less than the mass of the cluster's two clouds of hot x-ray
emitting gas shown in red. Representing even more mass than the
optical galaxies and x-ray gas combined, the blue hues show the
distribution of dark matter in the cluster. Otherwise invisible to
telescopic views, the dark matter was mapped by observations of
gravitational lensing of background galaxies. In a text book example
of a shock front, the bullet-shaped cloud of gas at the right was
distorted during the titanic collision between two galaxy clusters
that created the larger bullet cluster itself. But the dark matter
present has not interacted with the cluster gas except by gravity.
The clear separation of dark matter and gas clouds is considered
direct evidence that dark matter exists.
More
http://en.wikipedia.org/wiki/Dark_matter |
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| Ed |
| Posted: Mon Mar 12, 2007 5:11 pm |
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| Could Dark Matter be Anti Matter? |
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| Sam Wormley |
| Posted: Mon Mar 12, 2007 5:28 pm |
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Brian Tung wrote:
Quote: Ed wrote:
Could Dark Matter be Anti Matter?
No. The whole point is that dark matter interacts weakly (if at all)
with electromagnetic radiation, and anti-matter doesn't fit the bill,
as it interacts with EM radiation the same way that normal matter does.
An example of a known constituent for dark matter is neutrinos. At
least some of the are known to possess some mass (and my intuitive guess
is that they all do), so they exert some gravitational pull, but they
essentially don't interact electromagnetically.
However, the current best estimate for neutrino counts suggests that
either there aren't enough neutrinos to make up all the dark matter, or
the dark matter theory is inaccurate. (Or, it could be both.)
The distribution of Dark Matter, requires something "Colder" than
neutrinos. However, it is possible that they are a minor player in
the totality of Dark Matter. |
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| Llanzlan Klazmon the 15th |
| Posted: Mon Mar 12, 2007 6:56 pm |
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brian@isi.edu (Brian Tung) wrote in news:et4jbt$sbc$1@praesepe.isi.edu:
Quote: Ed wrote:
Could Dark Matter be Anti Matter?
No. The whole point is that dark matter interacts weakly (if at all)
with electromagnetic radiation, and anti-matter doesn't fit the bill,
as it interacts with EM radiation the same way that normal matter does.
An example of a known constituent for dark matter is neutrinos. At
least some of the are known to possess some mass (and my intuitive guess
is that they all do), so they exert some gravitational pull, but they
essentially don't interact electromagnetically.
However, the current best estimate for neutrino counts suggests that
either there aren't enough neutrinos to make up all the dark matter, or
the dark matter theory is inaccurate. (Or, it could be both.)
The main problem with neutrinos is that the mass they do have is so low,
that thermal neutrinos produced well before the CMB was released would
still have far too high a speed to show any gravitional clumping. If the
cosmological dark matter is an elementary particle, then it must be a
reasonably massive one.
Klazmon. |
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| Condort |
| Posted: Tue Mar 13, 2007 2:21 am |
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no because the only known affect is gravitatinal - anti matter
would interact w matter causing an immediate large expression in
energy
Ed wrote:
> Could Dark Matter be Anti Matter? |
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| Condort |
| Posted: Tue Mar 13, 2007 3:18 am |
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Sitav wrote:
Quote: what exactly is dark matter? i've heard about it but cant get a clear
idea of what it is.
Dark Matter and Dark Energy are both "theoretical concepts"
the behavior of matter for which there is no other explanation.
The provocative aspect of these theories is they hypothesise
the actual existence of elementary particles and profound
fundamental forces (and relationships) which so far have never
been seen or detected in any lab, or in Nature for that matter.
It is a little like the hypothesis of the gravitron to explain Gravity,
where gravitrons have never been detected either.
Several things make these theories palatable and attractive,
First, these theories are basically simple. Either dark matter and
dark energy exist or they dont. This suggests something which
ultimately will be testable. Second, the theories rise off the back
of well established tested theories which are failing to predict
concrete behaviors which they should be predicting, but arent.
It is very unlikely the known theories like the theory of Gravitation
are wrong. Something else is going on. It's a little like muddy human
footprints in your house without any plausable explanation and the police
saying: "it was your cougar"!
Three, the time for Dark Matter and Dark energy may have arrived. The
catalog of basic elementary particles and forces has been fairly well
known and understood for many years without
any trully new significant breakthroughs. To my mind that even
includes the discovery of background radiation in the universe
which I classify as a very modest discovery and no surprise at all,
surely not worth a Nobel Prize! What has not been revealed
however is how the basic constructs and forces came to exist
in the first place, or what the very earliest particles in Nature
actually looked like, or what all of this came from and how?
Dark Matter and Dark energy might have something to say
about this.
Four, Dark Matter and Dark Energy are theories of some substance vs.
earlier theories like the theory of an Aether which
quickly proved to be unecessary, or an even earlier theory of
Epicycles which proved plained wrong. If these theories had come
before Relativity they probably would be thrown out. But they
arrive with Realtivity, a bit late, but ultimately they have Relativity
to back them up (they also have a better Quantum theory behind
them!), and that alone makes anyone serious about science take
fairly seriously, the hypotheses of a Dark Matter and Dark Energy.
There are very specific gravitational and even structural facts
now observed for which there is no explanation, of a serious
kind. The non electromagnetically interactive status of Dark
Matter where there seems to be Gravitational affects, is something
brand new! This is being observed on the macro scale. Can it
also be demonstrated on a smaller scale or even on the micro
level? Is there some quantum dynamic explanation which could
explain the behaviors being observed on the macro scale?
These kinds of questions are important because they go to the
heart of what elementary particles and forces are, or are not,
such that some larger scale affect could even be observed.
The observations could be erroneous, but that is rather doubtful.
It then falls back on theory to explain and it is this inability of
theory to explain that kicks back the question into the realm of
fundamental theory, potentially.
We have been used to thinking about our Universe as a rather
closed system, Big Bang, Inflation, and all, not because this was
some bias particularly but because the facts seemed to suggest
a closed system. I for one have never been satisfied with this.
Our universe could be more open than we know it is, in several
different directions. Or it could be as closed as it currently seems.
Dark Matter and Dark Energy may be wrong or they may find
an explanation in some extension of theory which already exists.
The observations may be modified to change the need for dark
matter etc.
Right now I opt on the side that this will all disappear with better
more precise observations. I think its more wish than reality.
klm |
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| Ed |
| Posted: Thu Mar 15, 2007 6:16 am |
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Sam,
I remember John Dobson saying that he thought Dark Matter was in the
form of planets or could it be asteroids & comets and or dust? |
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| Chris L Peterson |
| Posted: Thu Mar 15, 2007 8:33 am |
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On 15 Mar 2007 04:16:42 -0700, "Ed" <ed1ward2@verizon.net> wrote:
Quote: I remember John Dobson saying that he thought Dark Matter was in the
form of planets or could it be asteroids & comets and or dust?
John Dobson should stick with making telescopes in his garage. When it
comes to cosmology and astrophysics, he's fully in the crank camp.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com |
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| Sam Wormley |
| Posted: Thu Mar 15, 2007 8:46 am |
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Ed wrote:
Quote: Sam,
I remember John Dobson saying that he thought Dark Matter was in the
form of planets or could it be asteroids & comets and or dust?
This distribution is wrong. Planets, asteroids, comets and dust all
interact with photons. Dark Matter does not appear to do so, Ed.
-Sam |
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| AustinMN |
| Posted: Thu Mar 15, 2007 1:43 pm |
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On Mar 15, 8:33 am, Chris L Peterson <c...@alumni.caltech.edu> wrote:
Quote: On 15 Mar 2007 04:16:42 -0700, "Ed" <ed1wa...@verizon.net> wrote:
I remember John Dobson saying that he thought Dark Matter was in the
form of planets or could it be asteroids & comets and or dust?
John Dobson should stick with making telescopes in his garage. When it
comes to cosmology and astrophysics, he's fully in the crank camp.
If the solar system had the same ratio of visible/dark matter as
galaxies appear to need to hold together, and that matter was in the
form of planets, then the solar system would have to have about a
thousand planets.
Could you imagine they sky filled with 125 Jupiter-sized planets?
Near-Earth asteroids would be irrelevant. It would be Near-Earth
planets that we would be searching for. ;-)
Austin |
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| Sam Wormley |
| Posted: Thu Mar 15, 2007 6:26 pm |
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AustinMN wrote:
Quote: On Mar 15, 8:33 am, Chris L Peterson <c...@alumni.caltech.edu> wrote:
On 15 Mar 2007 04:16:42 -0700, "Ed" <ed1wa...@verizon.net> wrote:
I remember John Dobson saying that he thought Dark Matter was in the
form of planets or could it be asteroids & comets and or dust?
John Dobson should stick with making telescopes in his garage. When it
comes to cosmology and astrophysics, he's fully in the crank camp.
If the solar system had the same ratio of visible/dark matter as
galaxies appear to need to hold together, and that matter was in the
form of planets, then the solar system would have to have about a
thousand planets.
Could you imagine they sky filled with 125 Jupiter-sized planets?
Near-Earth asteroids would be irrelevant. It would be Near-Earth
planets that we would be searching for. ;-)
Austin
The ratio of Dark to Baryonic is about 5:1 and the distribution
is such that it's influence at our solar system level is so weak
that it has yet to be detected.
-Sam Wormley
http://edu-observatory.org/eo/cosmology.html |
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| Erik |
| Posted: Fri Mar 16, 2007 11:43 am |
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On Mar 13, 1:18 am, Condort <jam...@hondo.org> wrote:
[snip]
Quote: These kinds of questions are important because they go to the
heart of what elementary particles and forces are, or are not,
such that some larger scale affect could even be observed.
The observations could be erroneous, but that is rather doubtful.
It then falls back on theory to explain and it is this inability of
theory to explain that kicks back the question into the realm of
fundamental theory, potentially.
We have been used to thinking about our Universe as a rather
closed system, Big Bang, Inflation, and all, not because this was
some bias particularly but because the facts seemed to suggest
a closed system. I for one have never been satisfied with this.
Our universe could be more open than we know it is, in several
different directions. Or it could be as closed as it currently seems.
Dark Matter and Dark Energy may be wrong or they may find
an explanation in some extension of theory which already exists.
The observations may be modified to change the need for dark
matter etc.
Right now I opt on the side that this will all disappear with better
more precise observations. I think its more wish than reality.
klm
I am curious, then. If you reject the Dark Matter hypothesis, is
there an alternate model that you find more convincing?
Erik
socalsw@yahoo.com |
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