Re: Dark Matter

In article <gud3kl$lop$5@xxxxxxxxxxxxxxxxx>, Paul J Gans wrote:
Robert Carnegie <rja.carnegie@xxxxxxxxxx> wrote:
On May 12, 12:15 pm, Klaus Hellnick <khellSPAMn...@xxxxxxxxxxxxx>
wrote:
Garamond Lethe wrote:
On Mon, 11 May 2009 20:46:26 -0700, Andre Lieven wrote:

On May 11, 11:12 pm, "Suzanne" <shil...@xxxxxxxxx> wrote:
Everyone has heard of dark matter by now, surely. But what exactly is
it? Scientists want to know. What things do you know about dark matter
that you can explain about it? Is it magnetically attracted to matter?
When it passes through matter, is it just going in a random way, and
the matter objects happen to be in it's path? Or...is it traveling to
some unkown destination? Does it go every which way, or in a certain
direction? How can it pass through matter? What can the Hubble
Telescope do to help us better understand dark matter? How can a
telescope picture assist in any kind of understanding of what dark
matter is?
http://en.wikipedia.org/wiki/Dark_matter

Duh. Creationists are really, really DUMB...

Which part of that post was dumb?  

Suzanne may not get a lot of things right, but she's unfailingly civil
and manages to express herself in standard English without resorting to
profanity.  You could learn a lot from her.

Andre

Calling a dumb person dumb is not profanity.
Many of her questions, in her post here, were dumb.
Furthermore, she could have answers to her other questions with 30
seconds of searching using Google or Dogpile.
Klaus

But there's an awful lot of "maybe" and "don't know" in there.

For instance, quite recently, we were told that the mass of the
universe isn't mainly "dark matter", it is mainly "dark energy". Bait
and switch scam??? :-)

No.

As I pointed out in my post above, you are seeing the development
of a theory up close and personal.

We need to explain certain characteristics of the rotation of
galaxies. Dark matter, intereacting gravitationally but not
electromagnetically with ordinary matter could explain this.

However, it can be shown that the "missing energy" of the
universe can't be totally made up of dark matter, at least
according to present ideas. Thus we need to also invent
"dark energy" to explain the difference.

We postulate properties for dark matter based on our understanding
of what it could be. Those properties can be tested. Folks are
looking. That's where things stand right now.

Dark energy, on the other hand, can't gravitationally affect
ordinary matter, because if it did the rotation of galaxies would
be quite different. So it is something else.

Don't forget, ALL of this could be quite wrong and new ideas may
supplant all of it.

This is science developing theories as we watch.

Whee!

And Paul from another post:
Sometimes collisions must occur. The results of such collisions
have been postulated and astronomers are looking for the resultant
debris. Finding such debris would go a long way toward proving
that dark matter is real.

I'll throw in a couple more cents, and recommend the book
_Einstein's Telescope_, which I've recently finished.

Our current cosmological entertainment comes from two
observations (yecs may now recoil in horror).

The first is, as Paul has described, from observing how fast stars
are moving about the center of their galaxy as a function of their
distance from the center. Also galaxy speeds w.r.t. centers of
galaxy clusters. If observable matter (stuff that emits, absorbs,
or scatters, light -- i.e., interacts electromagnetically) were
everything, we'd expect the stars far from the center to be moving
much more slowly than stars only half way out, and so on. Such a
thing is what we observe in our solar system, and is called Keplerian
motion. In fact, what we observe is that the farther and far out stars
are moving just as fast as the closer in ones. Either there's more
mass than we see (dark matter), or there's something different
with how gravity works than we think (MOND -- MOdified Newtonian
Dynamics seems to be the leading guess there). My read is that
although MOND is not rigorously ruled out, it also doesn't have
much support.

By way of gravitational lensing, Einstein's Telescope per the book,
it is possible to map out where mass is, independantly of where
the light-emitting matter is. Recent work shows a galaxy collision
where the light emitting material is on one side of the frame, but
the mass is on the other. Dark matter caught in the act. Given the
rotation curves, we expect far more matter to be dark matter, and
that, too, is what is seen.

The second observation line comes from observing how fast the
universe is expanding as a function of its age. The original
thought was to establish limits on how much matter there was
and thence what the universe's future looked like. Full expectation
that the expansion rate would be slowing down as you came towards
the present since, regardless of how much mass there was, the
mass would be pulling on other mass and slowing down the expansion.
The observation (10 years old now for the original) was, instead,
that the rate of expansion has been increasing. That was seriously
weird, and took more confirming observational experiments to be
believed. They came out, and here we are.

For these time and space scales, we turn to the General Theory of
Relativity. One can consider the equation involved to be:
Geometry effects = Mass/energy effects.
So we have two options for matching the observations. We can add a
term to the Geometry (a cosmological constant sort of term), which
would say, in effect, that on very large space-time scales the existence
of that much space-time drives expansion. As most of the people
involved are physical scientists rather than geometers, they favor
the other way of writing it. Namely, they add a term to the Mass/energy
side of the equation, which gives us a 'dark energy', whose behavior
is to _repel_ things with mass (and itself), rather than to attract.


We're at an exciting time in the science here. Every few months
something significant on one or both of these observational lines
comes out. Even though I've had an eye on at least Science and
Nature for 20+ years, and had seen most of the work that _Einstein's
Telescope_ discussed, the summary provided in the book was amazing.
While I'd seen the originals, I'm not a professional in the area
so hadn't been stitching them together into a coherent picture.
Just a lot of pretty pebbles. The real assemblage is magnificant
and only getting more interesting.

--
Robert Grumbine http://moregrumbinescience.blogspot.com/ Science blog
Sagredo (Galileo Galilei) "You present these recondite matters with too much
evidence and ease; this great facility makes them less appreciated than they
would be had they been presented in a more abstruse manner." Two New Sciences

.

Relevant Pages

  • Re: Curved space
    ... not caused by gravity in the same sense of scale from Newtonian models. ... force by knowing the mass of the star and the mass of the photon and the ... Dark Matter came about from the discovery that stars at the edges of spiral ...
    (talk.origins)
  • From The office of the Magizian
    ... It would actually take somthing like jettisoning mass from the earth ... I'm beginning to believe they ONLY purchase FROM THEMSELVES, no matter ... I may very well be showing the masses how to create gold from ... The material refered to as dark matter, ...
    (sci.math.num-analysis)
  • From The office of the Magizian
    ... It would actually take somthing like jettisoning mass from the earth ... I'm beginning to believe they ONLY purchase FROM THEMSELVES, no matter ... I may very well be showing the masses how to create gold from ... The material refered to as dark matter, ...
    (sci.astro)
  • From The office of the Magizian
    ... It would actually take somthing like jettisoning mass from the earth ... I'm beginning to believe they ONLY purchase FROM THEMSELVES, no matter ... I may very well be showing the masses how to create gold from ... The material refered to as dark matter, ...
    (sci.lang)
  • Re: Astronomers claim first dark galaxy find
    ... 23% invisible matter ... 73% invisible energy ... My model of the universe predicts that dark matter and energy comprise ... Intense visible-light ...
    (sci.physics)