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Powerman 5000
2004-Feb-19, 10:18 PM
Can Dark matter be cold and hot from what i read activity (atoms) in dark matter can change would this effect its size, shape, or movement?

zrice03
2004-Feb-20, 02:43 AM
Well, cold and hot are arbitrary measurements, but I think you mean can dark matter heat up or cool down. I actually don't think that's likely. Dark matter is "dark" because it doesn't absorb or emit any light. Since it doesn't absorb light, it can't heat up, and since it can't emit light it can't cool down.

It is affected by gravity, that's how we know it's there, so it can be heated and cooled by tidal forces, by this effect is likely very small.

To answer your question, based on this known information, I would think that ordinary dark matter would be similar in temperature to the cosmic background, which is about 3 Kelvin, -270 Celsius, or -455 Farenheit.

Algenon the mouse
2004-Feb-20, 04:52 AM
I think it sort of depends on what dark matter is made out of. One theory is that it may be just ordinary (baryonic) matter that does not emit light. (white dwarfs, neutron stars, black holes etc.).

Others think that it might consist of neutrinos, though no one really has any idea if they have any mass and they appear to have no charge.

And still others think they might be some yet unknown kind of particle(s).

GOURDHEAD
2004-Feb-20, 12:37 PM
It is affected by gravity, that's how we know it's there, so it can be heated and cooled by tidal forces, by this effect is likely very small.

Let's hope it possesses sufficient elasticity to avoid heating from gravitationally interacting with its neighbors. Since it can't lose heat, any provided by gravity will accumulate until something unimaginable happens. Maybe a "large" bang. :unsure:

NasaBoy
2004-Feb-21, 02:11 AM
I remember hearing that if there is too much Dark Matter that the Universe will start to shrink, and that if there isn't that much, the Universe won't shrink or something like that. Its some weird stuff.............................

stevo_jimmy
2004-Feb-23, 02:18 PM
I think you're referring to critical density, if the Universe exceeds this then it will eventually begin to collapse under it's own gravity, if below it will continue to expand forever and if exactly at the critical density it will forever approach a specific limit without ever getting there. Someone gave a better expalnation in the forum about what's at the edge of Universe I think. As no one is sure how much dark energy there is it the amount of dark energy that is crucial to the fate of the Universe. As far as I'm aware the current thinking is that the Universe will expand forever.

VanderL
2004-Feb-23, 05:04 PM
I think it sort of depends on what dark matter is made out of. One theory is that it may be just ordinary (baryonic) matter that does not emit light. (white dwarfs, neutron stars, black holes etc.).

Others think that it might consist of neutrinos, though no one really has any idea if they have any mass and they appear to have no charge.

And still others think they might be some yet unknown kind of particle(s)

And still others think that there is no Dark Matter at all. It started out as missing matter and because we couln't find this missing matter, it is now named Dark Matter.
Cheers.

Tiny
2004-Feb-23, 05:41 PM
Hye guys mind tell me where did the dark matter itself come from?


Can Dark matter be cold and hot from what i read activity (atoms) in dark matter can change would this effect its size, shape, or movement?

Hot dark matter - high velocity particle(e.g a massive neutrino)

Cod dark matter - highly massive, slow moving particle (baryon - I think I spell wrong :ph34r: )

Both H + C Dark matter makes up 90% of the universe not visible, even with our best telescope on Earth still can't even see...

GOURDHEAD
2004-Feb-26, 11:10 PM
Let's examine this problem from the most basic observations we know about.

The emotional factor that makes dark matter appealing is that we would like the universe to neither collapse nor cool (thin out) to an extremely small density (i.e., discover an omega equal to one). Lay this aside; the universe can't care at all.

The observed phenomenon that we are trying to explain with dark matter is that stars in orbit about galaxies and galaxies in orbit about their groups and clusters of groups have a line of sight velocity component as observed on earth as manifested by the blue shift of the atomic spectra that seems too high for the gravitational field associated with the presumed mass of the objects. Also, I assume the tangential velocity of the peripheral stars/galaxies is somehow not commensurate with that of the stars/galaxies orbiting nearer to the respective mass centers; although I have not seen this concluded/stated by an expert in the field.

Where can one find definitive quantitative analyses that present the details of this dilemma such as: galaxy XXXX with mass YYYY megasols has peripheral stars with tangential velocity VVVV where velocity V'V'V'V' should have been observed? Are the observers certain that gravitationally coupled stars in various star cluster sizes in the periphery have not added their cluster orbiting velocities to that of the orbiting star cluster center of mass. In the galactic periphery their mutual gravitational attraction could be more overwhelming than that of stars closer to the mass center (and therefore subject to cluster disruption by the gravitational effects of the galactic mass center) due to their distance from the galactic center of mass thus generating more star clusters in the periphery. A similar argument applies to galaxies with respect to their groups and groups with respect to their clusters etc., Emission spectral line broadening should be an indication of peripheral star clustering since a number of stars in the cluster will have receding velocities within the star cluster while others will be traveling such that their star cluster orbiting velocity will have no line of sight to earth component.

Is the distribution configuration of the dark matter such that it is additive to that of the center of mass of the visible matter? Does guessing at a larger mass for the galactic central black hole assist in the resolution of this problem? How does assuming that the dark matter hovers just outside the galactic halo in either a toroidal shape or a spherical shape help? Would the axiomatic constancy of the speed of light result in light coming from a star inside the dark matter shell cause it to be blue-shifted as it traveled toward the shell and subsequently red-shifted after it has passed through the shell and is on its way to earthbound observers?
Is the shifting required to be symmetrically distributed?

Do we have substantive evidence for dark matter or do we just hope it may be there to support a favorite "set of epicyclic musings". :unsure:

devilmech
2004-Feb-27, 05:39 AM
Originally posted by zrice03@Feb 20 2004, 02:43 AM
Dark matter is "dark" because it doesn't absorb or emit any light. Since it doesn't absorb light, it can't heat up, and since it can't emit light it can't cool down.

That's a bit erronous. Light may produce heat, but light is not required for the production of heat itself. Go into a completely dark room and rub your fingers together for a minute. They should heat up, unless you somehow have frictionless fingers. In addition to friction, many other natural forces can produce heat as a direct result or a by-product.


One interesting thing that "dark" matter brings to mind is absorption. White objects appear white because they relfect all of the visible spectrum of light. Black objects absorb all the visible spectrum. One would think that if "dark" matter did not absorb any light, then it would be "white" matter instead of "dark" matter, in which case it could be proven that current theories are wrong because if a significant portion of the universe is made up of it, we would be able to observe it.

Anyone know of an explanation as to how something can be "dark" and not absorb light? Is there perhaps a 5th dimension in which dark matter and gravity reside?

Matthew
2004-Feb-27, 08:43 AM
Ahh, but dark matter refers to dark in the electromagnetic spectrum. Not just in the tiny visable section.

VanderL
2004-Feb-27, 02:49 PM
Ahh, but dark matter refers to dark in the electromagnetic spectrum. Not just in the tiny visable section.

Which makes the dark matter problem even bigger. The only thing that can be said is that it does interact gravitationally, so whatever the make-up of dark matter it should at least respond to gravity. After 70 years of searching there isn't much to show.
Cheers.

Littlemews
2004-Feb-27, 06:34 PM
Dark Matter sometime we call it dust instead some kinda energy or dark light, due to the dust absorb all the lights, which block everything in the universe...

ebbixx
2004-Feb-28, 12:25 PM
Originally posted by Algenon the mouse@Feb 20 2004, 04:52 AM
Others think that it might consist of neutrinos, though no one really has any idea if they have any mass and they appear to have no charge.

I've read references to "neutralinos." Possibly this is just a variant name for neutrinos, or perhaps this is a subparticle distinct from the neutrino. . . As stated elsewhere, I'm not enough of a physicist to say one way or the other. I was tempted recently to pick up a book on the subject, prompted in part by this thread, that's as far as I've gone in giving myself some current background in the subject.

I've read of theories regarding both "hot" and "cold" forms of dark matter. My sense is that we are not yet in a position to determine through direct observation which of any such theories might be correct, and likely will not be so for some time to come. Until a point comes where physical verification can be made, the best astrophysicists can do is propose various hypotheses, and challenge each one in mathematical terms or with reference to observations in the non-visible spectra that might eliminate candidates, or add evidence on which to posit new hypothetical forms of dark matter (and/or energy).

ebbixx
2004-Feb-28, 12:39 PM
Originally posted by GOURDHEAD@Feb 26 2004, 11:10 PM
The emotional factor that makes dark matter appealing is that we would like the universe to neither collapse nor cool (thin out) to an extremely small density (i.e., discover an omega equal to one). Lay this aside; the universe can't care at all.

I don't follow this. My understanding is that dark matter is used merely to explain observed levels of expansion that cannot be explained by current estimates based on the presumed sum of matter in the universe, based on the most current collected data and inferences made from that data.

At present there seems to be a fairly universal acceptance within the astrophysics establishment that the universe will keep expanding ad infinitum until entropy renders it non-existent -- or at least so cold and diffuse, that to speak of its existence is to speak of a non-issue -- there would be no life, no observers, no stars, perhaps no solid bodies? (If observers existed they would have to exist somehow "outside" of the universe).

So I don't understand an "emotional factor" here. I'm sure there have been those romantics who hoped that the "oscillating universe" model would be the one deemed most probable. But at least for now, my understanding is there is no credible theory that allows for an oscillating universe or for any other sort of "encore" performance of the Big Bang -- at least not in this universe, the potential for multiple, parallel universes still remains (at least in theory) a possibility, if I understand Hawking and others correctly.

GOURDHEAD
2004-Mar-02, 02:21 PM
So I don't understand an "emotional factor" here. I'm sure there have been those romantics who hoped that the "oscillating universe" model would be the one deemed most probable.

I mentioned it because each of us is likely more affected by it than we are willing to admit (to ourselves) or even become aware, and it affects the enthusiasm with which we embrace or discard various concepts, hence it is an ever present detriment to truth assessment. I am so affected, so it is easy for me to believe that each of you are also.

My purpose was to note that we must be aware of what we would like to see and not let that color our perception of what we are observing. I had hoped the thrust of that post would have been to focus on the quantification of the evidence for the presence of dark matter and dark energy.

As for speculation, not really a theory, about causes for an oscillating universe; how about a very low frequency compression wave traveling through the Higgs field or postulating a maximum limit to the size a black hole can have which, when exceeded, causes it to explode thereby maintaining the Higgs field oscillations.