A Space & astronomy forum. SpaceBanter.com

Go Back   Home » SpaceBanter.com forum » Astronomy and Astrophysics » Research
Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

Hubble makes 3D dark matter map



 
 
Thread Tools Display Modes
  #11  
Old January 15th 07, 08:06 PM posted to sci.astro.research
Kent Paul Dolan
external usenet poster
 
Posts: 225
Default Hubble makes 3D dark matter map

Hans Aberg wrote:

And would the planetary friction be equally
negligible at this low density?


Well, there shouldn't be any "planetary friction",
as usually described, dark matter's _only_
interaction with normal matter is via gravity:

http://www.nasa.gov/home/hqnews/2006...rk_Matter.html

Can the dark matter within the solar system be
observed somehow?


Observation of dark matter is "challenging":

There are also several experiments claiming
positive evidence for dark matter detection,
such as DAMA/NaI, PVLAS, and EGRET, but
these are so far unconfirmed and difficult
to reconcile with the negative results of
other experiments. Other experiments
searching for dark matter include the
Cryogenic Dark Matter Search in the Soudan
mine or the ArDM experiment.

http://en.wikipedia.org/wiki/Dark_matter

HTH

xanthian.
  #12  
Old January 16th 07, 10:09 AM posted to sci.astro.research
[email protected]
external usenet poster
 
Posts: 41
Default Hubble makes 3D dark matter map

In article ,
Hans Aberg wrote:

So, if I got it right, the large distance between the stars admits the
dark matter to exist at such a low density that it does no affect the
planetary orbits*within the star systems.


Yes, that's right.

And would the planetary friction
be equally*negligible at this low density?


Yes. Even assuming that the dark matter were made of strongly-interacting
particles that bounced off the planets' surfaces whenever they hit them,
I think that the time scale for a drag force to change the planets' orbits
significantly is much longer than the age of the solar system. I just did
that calculation pretty quickly and roughly, so I could have messed it up.
If anyone thinks I have, let me know.

But if the dark matter were composed of particles that interacted relatively
easily with ordinary matter, we'd have detected them by now, by noticing
scattering events with particles in the atmosphere or something. That's one
of the reasons that the leading hypothesis these days is that the dark
matter consists of weakly interacting particles. Such particles would pass
right through solid matter most of the time with only a low probability of
interacting. In that case, any friction-like force due to planets passing
through these clouds of dark matter would be even smaller.


Can the dark matter within the
solar system be observed somehow?


We certainly hope so, but it hasn't been done yet. There are several
experiments underway to search for dark matter particles passing
through our neighborhood. They're similar in concept to experiments
to detect solar neutrinos: they involve looking for the effects of
particles bouncing off of atoms in detectors in deep underground labs.
There was one controversial claim of a detection a few years ago, but
it's generally believed that there was something wrong with that
experiment.

-Ted


--
[E-mail me at , as opposed to .]
  #13  
Old January 16th 07, 10:10 AM posted to sci.astro.research
[email protected]
external usenet poster
 
Posts: 41
Default Hubble makes 3D dark matter map

In article ,
Richard Saam wrote:
An interesting calculation.

Here is another for Pioneer Spacecraft deceleration 'a'

a ~ 2*Area*rho*c2 / M
~ 2*(58,965 cm2)*(6.38E-30 g/cm3)*(3E10 cm/sec)^2 /(241,000 g)
~ 2.8E10-9 cm/sec2 for Pioneer spacecraft


I haven't paid attention to the Pioneer anomaly stuff lately, so
I apologize if I'm just being stupid, but I don't see the physics
behind this relation. In particular, I don't see how c gets involved.
If you assume that the spacecraft is moving at velocity v through a cloud
of stationary particles that bounce elastically off of it whenever they hit
it, then you get a relation something like the above, but with v instead
of c. That makes a big difference, of course.

In addition, the dark matter particles are thought to be weakly interacting,
so most of them pass right through Pioneer without exerting any force
on it at all.


Your average calculated local density of 7 x 10^{-25} g/cm^3
is quite a bit higher than 30 x 6.38E-30 g/cm^3 or 2E-28 g/cm^3
but there still remains a conceptual mechanism
on how dark matter influences solar objects
according to their area/mass
(Pioneer some and planets negligible).
Perhaps the local dark matter density is on the order of
30 x 6.38E-30 g/cm3 or 2E-28 g/cm^3.


The calculation I did was based on well-measured Galactic dynamics. If
you want to get a different answer, you'll have to come up with a different
explanation for the motion of stars in the Galaxy.

-Ted

--
[E-mail me at , as opposed to .]
  #14  
Old January 16th 07, 02:52 PM posted to sci.astro.research
Hans Aberg
external usenet poster
 
Posts: 49
Default Hubble makes 3D dark matter map

In article ,
wrote:

So, if I got it right, the large distance between the stars admits the
dark matter to exist at such a low density that it does no affect the
planetary orbits*within the star systems.


Yes, that's right.

And would the planetary friction
be equally*negligible at this low density?


Yes.* Even assuming that the dark matter were made of strongly-interacting
particles that bounced off the planets' surfaces whenever they hit them,
I think that the time scale for a drag force to change the planets' orbits
significantly is much longer than the age of the solar system.*


But it might still be measurable (see below, though).

I just did
that calculation pretty quickly and roughly, so I could have messed it up.
If anyone thinks I have, let me know.

But if the dark matter were composed of particles that interacted relatively
easily with ordinary matter, we'd have detected them by now, by noticing
scattering events with particles in the atmosphere or something.*


What would happen if the matter is very thin infancy matter, of the kind
known to form very young stars? Would that be easily detectable, or
possible to rule out?

That's one
of the reasons that the leading hypothesis these days is that the dark
matter consists of weakly interacting particles.* Such particles would pass
right through solid matter most of the time with only a low probability of
interacting.* In that case, any friction-like force due to planets passing
through these clouds of dark matter would be even smaller.


Like neutrinos, but I*recall there were reasons*against that explanation,
though I do not recall what. :-)

Can the dark matter within the
solar system be observed somehow?


We certainly hope so, but it hasn't been done yet.* There are several
experiments underway to search for dark matter particles passing
through our neighborhood.* They're similar in concept to experiments
to detect solar neutrinos: they involve looking for the effects of
particles bouncing off of atoms in detectors in deep underground labs.
There was one controversial claim of a detection a few years ago, but
it's generally believed that there was something wrong with that
experiment.


If the matter is, as I suggested above, by infancy matter, would the solar
wind blow it away from the inner parts of the solar system? If so, one
might have to look further out, where the solar wind is weaker.

--
Hans Aberg
  #15  
Old January 16th 07, 04:49 PM posted to sci.astro.research
Hans Aberg
external usenet poster
 
Posts: 49
Default Hubble makes 3D dark matter map

In article ,
wrote:

So, if I got it right, the large distance between the stars admits the
dark matter to exist at such a low density that it does no affect the
planetary orbits*within the star systems.


Yes, that's right.

And would the planetary friction
be equally*negligible at this low density?


Yes.* Even assuming that the dark matter were made of strongly-interacting
particles that bounced off the planets' surfaces whenever they hit them,
I think that the time scale for a drag force to change the planets' orbits
significantly is much longer than the age of the solar system.*


But it might still be measurable (see below, though).

I just did
that calculation pretty quickly and roughly, so I could have messed it up.
If anyone thinks I have, let me know.

But if the dark matter were composed of particles that interacted relatively
easily with ordinary matter, we'd have detected them by now, by noticing
scattering events with particles in the atmosphere or something.*


What would happen if the matter is very thin infancy matter, of the kind
known to form very young stars? Would that be easily detectable, or
possible to rule out?

That's one
of the reasons that the leading hypothesis these days is that the dark
matter consists of weakly interacting particles.* Such particles would pass
right through solid matter most of the time with only a low probability of
interacting.* In that case, any friction-like force due to planets passing
through these clouds of dark matter would be even smaller.


Like neutrinos, but I*recall there were reasons*against that explanation,
though I do not recall what. :-)

Can the dark matter within the
solar system be observed somehow?


We certainly hope so, but it hasn't been done yet.* There are several
experiments underway to search for dark matter particles passing
through our neighborhood.* They're similar in concept to experiments
to detect solar neutrinos: they involve looking for the effects of
particles bouncing off of atoms in detectors in deep underground labs.
There was one controversial claim of a detection a few years ago, but
it's generally believed that there was something wrong with that
experiment.


If the matter is, as I suggested above, by infancy matter, would the solar
wind blow it away from the inner parts of the solar system? If so, one
might have to look further out, where the solar wind is weaker.

--
Hans Aberg
  #16  
Old January 16th 07, 04:50 PM posted to sci.astro.research
Hans Aberg
external usenet poster
 
Posts: 49
Default Hubble makes 3D dark matter map

In article , "Kent Paul
Dolan" wrote:

And would the planetary friction be equally
negligible at this low density?


Well, there shouldn't be any "planetary friction",
as usually described, dark matter's _only_
interaction with normal matter is via gravity:

http://www.nasa.gov/home/hqnews/2006...rk_Matter.html


Not really, that is how it is discovered, not a description of all its
properties. See the post by .

--
Hans Aberg
  #17  
Old January 16th 07, 04:55 PM posted to sci.astro.research
Richard Saam Richard Saam is offline
Member
 
First recorded activity by SpaceBanter: Jan 2005
Posts: 83
Default Hubble makes 3D dark matter map

wrote:
In article ,
Richard Saam wrote:

An interesting calculation.

Here is another for Pioneer Spacecraft deceleration 'a'

a ~ 2*Area*rho*c2 / M
~ 2*(58,965 cm2)*(6.38E-30 g/cm3)*(3E10 cm/sec)^2 /(241,000 g)
~ 2.8E10-9 cm/sec2 for Pioneer spacecraft



I haven't paid attention to the Pioneer anomaly stuff lately, so
I apologize if I'm just being stupid, but I don't see the physics
behind this relation. In particular, I don't see how c gets involved.
If you assume that the spacecraft is moving at velocity v through a cloud
of stationary particles that bounce elastically off of it whenever they hit
it, then you get a relation something like the above, but with v instead
of c. That makes a big difference, of course.


The conceptual physics is very simple.
Assume that space is a continuous medium
with mass critical density of 6.38E-30 g/cm^3 .
Then think of an object passing through it
defined something like a ram jet engine
(analogous to atmospheric oxygen intake)
which takes in this space medium with neglible effect (m v^2)
but which expells it as m c^2
with resultant thrust in the same direction as intake
resulting in object deceleration.

Objects of different areas and masses could be designed to pass
through this space and would be deceleration probes of this medium.


In addition, the dark matter particles are thought to be weakly interacting,
so most of them pass right through Pioneer without exerting any force
on it at all.



Your average calculated local density of 7 x 10^{-25} g/cm^3
is quite a bit higher than 30 x 6.38E-30 g/cm^3 or 2E-28 g/cm^3
but there still remains a conceptual mechanism
on how dark matter influences solar objects
according to their area/mass
(Pioneer some and planets negligible).
Perhaps the local dark matter density is on the order of
30 x 6.38E-30 g/cm3 or 2E-28 g/cm^3.



The calculation I did was based on well-measured Galactic dynamics.
If you want to get a different answer, you'll have to come up with a different
explanation for the motion of stars in the Galaxy.


I have replicated your calculations and they are right.
I think you will agree
there are potentially many components to Galactic dynamics.

Richard
  #18  
Old January 17th 07, 09:58 AM posted to sci.astro.research
[email protected]
external usenet poster
 
Posts: 41
Default Hubble makes 3D dark matter map

In article ,
Hans Aberg wrote:

What would happen if the matter is very thin infancy matter, of the kind
known to form very young stars? Would that be easily detectable, or
possible to rule out?


I don't know the term "infancy matter," but it sounds like it means
stuff that includes a lot of hydrogen gas. If you try to put
a large amount of diffuse gas in the Sun's neighborhood of our Galaxy,
I think it'd be pretty easy to spot. In particular, it'd produce whopping
great absorption lines in the light from nearby stars.

I think you're right that it'd be blown out of the inner solar system
by the solar wind, by the way. That's why I think that the main way
to look for it would be on slightly larger scales.

So if you want to put enough hydrogen- and helium-rich stuff in our
neighborhood to make a significant contribution to the dark matter,
you can't make it diffuse. You might try to stick it in
gravitationally-bound (Jupiter-ish) lumps. That gets around the
absorption line problem. You can detect such lumps in other ways,
especially by the technique known as gravitational microlensing. The
limits set by this technique show that some such objects exist, but
that there can't be enough of these lumps to make up all of the dark
matter.

-Ted


--
[E-mail me at , as opposed to .]
  #19  
Old January 17th 07, 09:59 AM posted to sci.astro.research
[email protected]
external usenet poster
 
Posts: 41
Default Hubble makes 3D dark matter map

In article ,
Richard Saam wrote:
wrote:
In article ,
Richard Saam wrote:

An interesting calculation.

Here is another for Pioneer Spacecraft deceleration 'a'

a ~ 2*Area*rho*c2 / M
~ 2*(58,965 cm2)*(6.38E-30 g/cm3)*(3E10 cm/sec)^2 /(241,000 g)
~ 2.8E10-9 cm/sec2 for Pioneer spacecraft



I haven't paid attention to the Pioneer anomaly stuff lately, so
I apologize if I'm just being stupid, but I don't see the physics
behind this relation. In particular, I don't see how c gets involved.
If you assume that the spacecraft is moving at velocity v through a cloud
of stationary particles that bounce elastically off of it whenever they hit
it, then you get a relation something like the above, but with v instead
of c. That makes a big difference, of course.


The conceptual physics is very simple.
Assume that space is a continuous medium
with mass critical density of 6.38E-30 g/cm^3 .
Then think of an object passing through it
defined something like a ram jet engine
(analogous to atmospheric oxygen intake)
which takes in this space medium with neglible effect (m v^2)
but which expells it as m c^2
with resultant thrust in the same direction as intake
resulting in object deceleration.


It's always embarrassing when something is described as "very simple"
and I still don't get it! I don't understand the model you're describing
in about half a dozen different ways. What does "expels it as m c^2" mean?
Why doesn't the final answer depend on the speed of the object through
the medium? Surely the faster the object is moving, the more of this
stuff it'll sweep up.

We're hypothesizing something like the Pioneer moving through a cloud
of particles, right? When a single particle of mass m strikes the
Pioneer, the *largest* impulse that can be imparted is surely 2mv,
where v is the relative speed of the particle and Pioneer. The total
mass of all such particles striking per second is something like rho A
v, so the total force would be 2 rho A v^2. So I get the same formula
as you have above, but with v instead of c. And that's bound to be an
overestimate, because not every particle imparts the maximum possible
impulse.

Your average calculated local density of 7 x 10^{-25} g/cm^3
is quite a bit higher than 30 x 6.38E-30 g/cm^3 or 2E-28 g/cm^3
but there still remains a conceptual mechanism
on how dark matter influences solar objects
according to their area/mass
(Pioneer some and planets negligible).
Perhaps the local dark matter density is on the order of
30 x 6.38E-30 g/cm3 or 2E-28 g/cm^3.



The calculation I did was based on well-measured Galactic dynamics.
If you want to get a different answer, you'll have to come up with a different
explanation for the motion of stars in the Galaxy.


I have replicated your calculations and they are right.
I think you will agree
there are potentially many components to Galactic dynamics.


I'm sorry; I'm missing your point yet again. The question I thought
we were talking about was, more or less, "what's the dark matter?" If
we're now talking about some component whose density is several orders
of magnitude less than the density implied from Galactic dynamics,
then more or less by definition we're not talking about the dark
matter.

I'm not sure I see the point of postulating that the solar system
contains some component of some unknown material with a density equal
to the critical density. We know that the density in the solar
neighborhood is orders of magnitude larger than that, so what problem
are we trying to solve by assuming such a component?

-Ted

--
[E-mail me at , as opposed to .]
  #20  
Old January 18th 07, 09:55 AM posted to sci.astro.research
Hans Aberg
external usenet poster
 
Posts: 49
Default Hubble makes 3D dark matter map

In article ,
wrote:

What would happen if the matter is very thin infancy matter, of the kind
known to form very young stars? Would that be easily detectable, or
possible to rule out?


I don't know the term "infancy matter," but it sounds like it means
stuff that includes a lot of hydrogen gas.*


It was discussed in this newsgroup before. It would just be the matter
that the youngest stars are formed of, so it would be close to their
compositions, taking some fusion into account. It will contain the so
called astronomic metals in specific proportions, and one knows what
these*proportions are (I don't recall details). In addition, one might
surmise there be higher fundamentals, from exploding stars, to as it is
possible to form planets containing those. I do not know why
these*compounds are sorted out in planet forming. Perhaps it*depends on
the distance from the star; Saturn and Jupiter are gaseous.

If you try to put
a large amount of diffuse gas in the Sun's neighborhood of our Galaxy,
I think it'd be pretty easy to spot.* In particular, it'd produce whopping
great absorption lines in the light from nearby stars.

I think you're right that it'd be blown out of the inner solar system
by the solar wind, by the way.* That's why I think that the main way
to look for it would be on slightly larger scales.

So if you want to put enough hydrogen- and helium-rich stuff in our
neighborhood to make a significant contribution to the dark matter,
you can't make it diffuse.* You might try to stick it in
gravitationally-bound (Jupiter-ish) lumps.* That gets around the
absorption line problem.* You can detect such lumps in other ways,
especially by the technique known as gravitational microlensing.* The
limits set by this technique show that some such objects exist, but
that there can't be enough of these lumps to make up all of the dark
matter.


One idea that comes to my mind is that very young, nearby*galaxies are
very hard to observe for two reasons: they are faint, and quickly gets
absorbed into larger galaxies. Perhaps there is something similar going on
here, too, that might explain why observation is so difficult.

--
Hans Aberg
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Hubble tracks down a galaxy cluster's dark matter (Forwarded) Andrew Yee Astronomy Misc 0 July 17th 03 01:42 PM


All times are GMT +1. The time now is 10:20 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 SpaceBanter.com.
The comments are property of their posters.