A question relating to expansion rate of the Universe

[Tanelorn]

1. Forgive me for this novice question, but can variation in the expansion rate of the universe, be partially explained by an increasing number of stars turning mass into energy, hence reducing gravitational attraction between galaxies, and at the same time applying radiation pressure (a small pushing effect) to matter outside of galaxies? Or are these effects, if any, negligible?

2. Another related question about the voids between galaxies clusters: Are they observed to be expanding in size at a rate which confirms that the cosmological constant is truly constant everywhere, including within the threads of galaxies, after taking into account gravitational attraction?

3. Another related question about dark matter: Since dark matter is estimated at 5.5x the mass of the ordinary matter in the Universe, is this sum of all of this gravitational attraction still consistent with the expansion rate of the Universe? I believe that dark matter is thought to accumulate near the rim of spiral galaxies, but I am not sure about this. If so, is this situation also accounted for in question 2?

http://en.wikipedia.org/wiki/File:080998_Universe_Content_240_after_Planck.jpg

4. I just saw this chart above for the first time, why is the percentage of dark matter and ordinary matter changing?! Is this solely the result of fusion in stars? Do we consider the matter inside Black Holes to be ordinary matter?

Quite a lot covered there, thanks for any replies.

 

[Mordred]

In the first question, is the number of stars increasing? I doubt it. The earliest stars were far larger and burned off their fuel quickly compared to say our sun. This is due to a higher percentage of heavier elements than was available in the earlier stages of the universe. Where only hydrogen was available.
The burning off of matter to energy has miniscule effects on the expansion rate if any.

Question 2. Yes expansion outside of gravitational bound regions is increasing. Gravity is sufficient in energy to overcome expansion locally. The energy density of the cosmological constant per cubic metre is quite small so locally easily overpowered. So the regions between galaxies in a local group isn't affected as they are gravitationally bound.
Outside the graviationally bound regions however it is homogeneous and isotropic.
The dark matter effects is accounted for. As DM has a larger influence on the gravity side this is more important to be accurate on than baryonic matters small percentage.

Question 4. During the development of the universe. The DM and matter percentage does change. This has a large part to do with the background temperature. When the universe first formed the temperature was too hot for matter to form. When inflation expanded to universe that temperature cooled enough that matter and DM was able to form.
The major stages on expansion/contraction rates are in the following stages.

1) Radiation dominant
2) matter dominant
3) lamnda dominant.

 

[mfb]

1. Forgive me for this novice question, but can variation in the expansion rate of the universe, be partially explained by an increasing number of stars turning mass into energy, hence reducing gravitational attraction between galaxies, and at the same time applying radiation pressure (a small pushing effect) to matter outside of galaxies?

No. First, gravity acts on the total energy (density), and stars do not change that. Second, the amount of matter converted to energy in stars is negligible. And finally, radiation (and all other pressure from regular matter) slows down expansion as well, as it has a positive energy and pressure density.

2. Another related question about the voids between galaxies clusters: Are they observed to be expanding in size at a rate which confirms that the cosmological constant is truly constant everywhere, including within the threads of galaxies, after taking into account gravitational attraction?

We cannot watch the expansion in real-time, but the velocities of galaxies at the borders of voids agree with expanding voids (together with everything else). While the acceleration of the expansion is well-measured, it is hard to see if its rate is different in different regions of space, as that needs much more precise data.

3. Another related question about dark matter: Since dark matter is estimated at 5.5x the mass of the ordinary matter in the Universe, is this sum of all of this gravitational attraction still consistent with the expansion rate of the Universe?

Sure. And it is inconsistent with the gravitational attraction without dark matter.

4. I just saw this chart above for the first time, why is the percentage of dark matter and ordinary matter changing?!

63%/12%=5.25 and 26.8%/4.9%=5.47 are nearly the same, I guess the small difference comes from rounding errors.

Do we consider the matter inside Black Holes to be ordinary matter?

Yes (but it is negligible).

 

[marcus]

Quite a lot of interesting things to discuss here! Mfb covered all the points adequately but there is still room for conversation just going over some of it again. Particularly that NASA pie chart!

...
3. Another related question about dark matter: Since dark matter is estimated at 5.5x the mass of the ordinary matter in the Universe, is this sum of all of this gravitational attraction still consistent with the expansion rate of the Universe? I believe that dark matter is thought to accumulate near the rim of spiral galaxies, but I am not sure about this. If so, is this situation also accounted for in question 2?

http://en.wikipedia.org/wiki/File:080998_Universe_Content_240_after_Planck.jpg
4. I just saw this chart above for the first time, why is the percentage of dark matter and ordinary matter changing?! Is this solely the result of fusion in stars? Do we consider the matter inside Black Holes to be ordinary matter?...

I can't make sense of Mordred's response in the given context of the pie chart question 4. It's true there was a radiation dominated era, as M. says. But here the early universe pie is dated year 380,000 which would have been origin of CMB at around redshift 1100. this is AFTER radiation dominated era. So it needs further clarification.

...
Question 4. During the development of the universe. The DM and matter percentage does change. This has a large part to do with the background temperature. When the universe first formed the temperature was too hot for matter to form. When inflation expanded to universe that temperature cooled enough that matter and DM was able to form.
The major stages on expansion/contraction rates are in the following stages.

1) Radiation dominant
2) matter dominant
3) lamnda dominant.

Tanelorn, I think from your perspective you might want to ask what happened to the neutrino and photon energy in the year 380,000 pie chart? You see, as Mfb points out, that the ratio of dark matter to ordinary baryonic matter ("atoms" in the chart's simplified language) stays the same. It is about 5.5, as you said in your initial post.

...

63%/12%=5.25 and 26.8%/4.9%=5.47 are nearly the same, I guess the small difference comes from rounding errors...

The problem Tanelorn may be experiencing is "what happens to the OTHER stuff?" Like the "dark energy" component is too small to even count in year 380,000 so it does not even show up in the pie chart. And neutrinos and photons are too small a share to bother depicting in the present-day chart.

So DM and ordinary remain in balance, but their combined share of the total pie changes. Why?

Well the answer is in the physics of the neutrinos and photons and the "dark energy" number corresponding to the cosmological constant. How those things change as distances shrink as you go back to like roughly redshift 1000. (Really more like 1100 but for round numbers say 1000)

Distances shrink by a factor of 1000 so matter density increases by factor of 1000³ = a billion. But "dark energy density" stays the SAME. So it becomes minuscule! Negligible by comparison with other things. Too skinny a sector to show on the chart.

Also as you go back in time not only distances but photon wavelengths shrink, so the photons become more energetic BY A FACTOR OF A THOUSAND. So on the year 380,000 chart the photons account for roughly the same as the atoms (15% and 12%, roughly similar) but by now, expansion of wavelengths will have taken its toll on the photons and their energy will be like a THOUSANDTH of the total for the atoms. So too skinny to be shown on the presentday chart.
Something similar happens to the NEUTRINO share of energy. Neutrinos energy is mainly kinetic, their rest mass is very small but they start out traveling at near light speed with a lot of kinetic energy. Expansion drains energy from them too. So their share is too tiny to show on the presentday chart.

There is plenty to talk about regarding charts like this. Also the information on these particular charts is old. One chart is based on WMAP5 data and the other seems based on WMAP7 data, and neither data set agrees exactly with the latest PLANCK results, plus the two charts are not quite consistent because WMAP5 ≠ WMAP7, plus as Mfb mentioned there is round-off. But the inconsistency and imprecision does not really detract from the overall usefulness. It gives a good impression and a lot to think about. thanks for bringing this stuff up for discussion!

 

[Mordred]

Yeah I thought of that after I posted glad to see you caught that too early in the am hadn't had my first cup of coffee lol

 

[marcus]

Yeah I thought of that after I posted glad to see you caught that too early in the am hadn't had my first cup of coffee lol

Well what you said was, I thought, good information. Just didn't directly address why those two pie charts were so different. I wish we had more quantitative questions (involving standard cosmic model) like that one just now by Tanelorn, don't you? Like with some actual numbers to explain?
A quantitative element, rather than purely verbal, makes it more fun and easier to get traction. Words are slippery, numbers sometimes add a bit of grit.

 

[Naty1]

I believe that dark matter is thought to accumulate near the rim of spiral galaxies, but I am not sure about this.

It turns out more likely the reverse is true...galaxies formed where dark matter has accumulated.

In the early universe, normal matter and dark matter tended to attract one another, so that an overabundance of dark matter generally also coincided with an overabundance of normal matter as galaxies formed. [All part of primordial quantum perturbations.] So it's not so much that dark matter collects around the edges of galaxies, but that galaxies form in the centers of large blobs of dark matter. But things don't always turn out quite so simply: when stars are born amid violent explosions and galaxies form, a lot of matter may be ejected as stars form leaving a galaxy with mostly dark matter.

From Marcus, above:

Like the "dark energy" component is too small to even count in year 380,000 so it does not even show up in the pie chart. ...Well the answer is in the physics of the ... "dark energy" number corresponding to the cosmological constant. How those things change as distances shrink as you go back to like roughly redshift 1000. (Really more like 1100 but for round numbers say 1000)... But "dark energy density" stays the SAME. So it becomes minuscule!

all ok... turning this description around, matter density drops as the cosmos expands; but dark energy density remains constant [the 'cosmological constant'] and so the total dark energy [depicted in the charts] gets bigger as the cosmos gets bigger. As it became a more and more significant proportion of the total matter/energy some 6 billion years ago, we became an 'energy dominated universe' and expansion began accelerting and continues to do so.

Up until the late 1990's [ yes, just that recently] it was assumed that gravity was slowing down the rate of expansion. Actual measurements showed to everyone's surprise not only is the rate of expansion NOT slowing down, it is accelerating... So "dark energy" has been around all along and about 6 billion years ago, it started to overcome gravititational attraction and cause the expansion to start accelerating.

At the same time, as Marcus explained, matter density and hence gravitational attraction is decreasing. So factors constraining accelerated expansion are losing influence while factors favoring expansion are gaining influence. We appear heading toward an ultimate 'cold, empty, dead' ever more rapidly expanding universe many billions of years from now.

It is also worth noting a 'fun fact' from Leonard Susskind: He claims some 99% of everything resides [hidden] within black holes! So what you see in those charts are merely 'remnants'...leftovers...stuff that hasn't been swallowed yet.

So not only do we not understand the 95% or so of matter/energy we can 'sort' of observe [dark matter, dark energy in your charts] everything in the universe may be nothing more than 1% 'leftovers' from what was originally there. Cosmic crumbs...like stuff left over on a counter after a bake fest.

And this doesn't even address where most of the matter/antimatter may have gone at the moment of the initial big bang...annihilation...leaving a relatively small preponderance of matter that started with our universe. [Anybody remember THAT number...I recall like 1 extra matter particle for every billion or trillion total articles ...]

I am feeling especially insignificant at the moment...time to walk my Yorkshire terriers and feel important...

 

[Mordred]

. I wish we had more quantitative questions (involving standard cosmic model) like that one just now by Tanelorn, don't you? Like with some actual numbers to explain?
A quantitative element, rather than purely verbal, makes it more fun and easier to get traction. Words are slippery, numbers sometimes add a bit of grit.

I fully agree I also enjoy questions that make you think as well as research to answer properly

 

[mfb]

It is also worth noting a 'fun fact' from Leonard Susskind: He claims some 99% of everything resides [hidden] within black holes! So what you see in those charts are merely 'remnants'...leftovers...stuff that hasn't been swallowed yet.

Where are all those black holes? Their mass would be significant for the evolution of the observable universe, if they are somewhere.

And this doesn't even address where most of the matter/antimatter may have gone at the moment of the initial big bang...annihilation...leaving a relatively small preponderance of matter that started with our universe. [Anybody remember THAT number...I recall like 1 extra matter particle for every billion or trillion total articles ...]

To radiation. That's why our universe was radiation dominated initially. It is just unclear how this happened.

 

[Tanelorn]

Thanks for replies everyone. I haven't read it all yet but I shall and will ask again if I see something I don't get.

 

[Tanelorn]

First response to mfb: "And finally, radiation (and all other pressure from regular matter) slows down expansion as well, as it has a positive energy and pressure density."

This is the reverse of what I was expecting. Is this perhaps because most of the Universe is receding faster than the speed of light?
Response to Marcus: "I wish we had more quantitative questions (involving standard cosmic model) like that one just now by Tanelorn, don't you? Like with some actual numbers to explain?"

I am very glad to be able ask questions. I didn't think I used many numbers though except the 5.5 one and the pie chart. Most of my questions are conceptual because the math is too far beyond me to be able to question it. Even in my younger days I would build a mental picture of situations and then use the math for an exact answer of what I was thinking, if I got them both right!

So what did happen to the neutrino and photon energy in the year 380,000 pie chart? :)


"Distances shrink by a factor of 1000 so matter density increases by factor of 1000^3 = a billion. But "dark energy density" stays the SAME. So it becomes minuscule! Negligible by comparison with other things. Too skinny a sector to show on the chart."

So dark energy is about the same but the gravitational force is a billion times more powerful so DE is missing on the pie chart. So expansion back then has to be as a result of something else? Remains of Inflation? Is there any difference between the two?
Response to mfb: "63%/12%=5.25 and 26.8%/4.9%=5.47 are nearly the same, I guess the small difference comes from rounding errors."

Of course! I didn't think my math had got that bad! Must have been the number of thoughts happening at once before first coffee!

Response to mfb: "It is also worth noting a 'fun fact' from Leonard Susskind: He claims some 99% of everything resides [hidden] within black holes! So what you see in those charts are merely 'remnants'...leftovers...stuff that hasn't been swallowed yet."

Including anti-matter from the BB? Does anti-matter have negative mass and positive gravitation attraction?
http://en.wikipedia.org/wiki/Gravitational_interaction_of_antimatter"Where are all those black holes? Their mass would be significant for the evolution of the observable universe, if they are somewhere."

How about at the largest scales (eg. 10^32 of observable universe) the universe is a Dyson sphere of expanding black holes created in the BB containing all the things that we want to go away like anti matter? I don't think so, too convenient :)

 

[Naty1]

Quote by Naty1

It is also worth noting a 'fun fact' from Leonard Susskind: He claims some 99% of everything resides [hidden] within black holes! So what you see in those charts are merely 'remnants'...leftovers...stuff that hasn't been swallowed yet.

mfb: Where are all those black holes? Their mass would be significant for the evolution of the observable universe, if they are somewhere.

Found it: Pg 434 of Susskind's book, THE BLACK HOLE WAR, Chapter 24, 'HUMILITY'

The most notable inhabitants of our universe - the galaxies- are built around giant black holes that are continually gobbling up stars and planets. Out of every 10,000,000,000 bits of information in the universe 9,999,999,999 are associated with the horizons of black holes. It should be evident that our naive ideas about space,time, and information are wholly inadequate to understand most of nature.

That's all he has, no further details of which I am aware...He notes elsewhere an infalling observer to a black hole sees such information 'inside' the black hole, a stationary exterior observer sees it smeared and scrambled with Hawking radiation on the event horizon...I assume that's his reference to 'naive' in the last sentence I quoted.

And this doesn't even address where most of the matter/antimatter may have gone at the moment of the initial big bang...annihilation...]

To radiation. That's why our universe was radiation dominated initially. It is just unclear how this happened.

Oh, good point. My explanation did NOT illustrate what I was addressing very well.



__________________

 

[Naty1]

First response to mfb: "And finally, radiation (and all other pressure from regular matter) slows down expansion as well, as it has a positive energy and pressure density."

This is the reverse of what I was expecting. Is this perhaps because most of the Universe is receding faster than the speed of light?

An easy way to think about this kind of thing is to start from 'regular matter'...it all exhibits
gravitational attraction, right??. Since 'normal energy' is equivalent to 'normal matter', via E = mc², then energy is also attractive like matter...radiation, heat, kinetic, in all forms.


On the other hand, cosmological expansion is caused by negative pressure...negative pressure is repulsive...it is not 'regular' gravity...traditional gravity is a separate entity. Empty space has no traditional gravity, but does have negative pressure! A negative pressure dark energy, which seems to permeate all of spacetime, is the same thing as a cosmological constant and causes expansion of space.

 

[mfb]

First response to mfb: "And finally, radiation (and all other pressure from regular matter) slows down expansion as well, as it has a positive energy and pressure density."

This is the reverse of what I was expecting.

It is unintuitive, indeed.

Is this perhaps because most of the Universe is receding faster than the speed of light?

No.

So what did happen to the neutrino and photon energy in the year 380,000 pie chart? :)

The photons got redshifted (-> lost energy) and the neutrinos lost kinetic energy due to the expansion of space, and at the same time the density went down (as the volume increased).

So dark energy is about the same but the gravitational force is a billion times more powerful so DE is missing on the pie chart. So expansion back then has to be as a result of something else? Remains of Inflation? Is there any difference between the two?

Expansion came from the initial expansion (inflation, or whatever mechanism came before that) - it was fast enough to keep the universe expanding even with the high energy density.

Including anti-matter from the BB? Does anti-matter have negative mass and positive gravitation attraction?
http://en.wikipedia.org/wiki/Gravitational_interaction_of_antimatter

It is known that antimatter has positive (inertial) mass and it is expected that it behaves exactly like matter in terms of gravity. Everything else would be a huge violation of General Relativity.

Response to mfb: "It is also worth noting a 'fun fact' from Leonard Susskind: He claims some 99% of everything resides [hidden] within black holes! So what you see in those charts are merely 'remnants'...leftovers...stuff that hasn't been swallowed yet."

Ignore that point, see below.

The most notable inhabitants of our universe - the galaxies- are built around giant black holes that are continually gobbling up stars and planets. Out of every 10,000,000,000 bits of information in the universe 9,999,999,999 are associated with the horizons of black holes. It should be evident that our naive ideas about space,time, and information are wholly inadequate to understand most of nature.

@Naty1: That is a completely different statement compared to your original claim. That is entropy, not mass. Yes, black holes have a huge associated entropy. So what? The entropy of the air in my room has more bits than all binary data stored in computers in the whole world. That does not make this air more interesting.

 

[Tanelorn]

Ever get the feeling that ordinary matter is a just an unimportant side product of something much more Important?

 

[mfb]

Ordinary matter can form structures like stars, planets, and life, while there is absolutely no hint that dark matter or even dark energy could do that (as it cannot clump on those scales).
In terms of gravity, your impression is certainly true.

 

[Naty1]

from post #14:

So dark energy is about the same...

I'm not sure what you mean here, but dark energy is vastly more significant now than in the distant past...see the charts you reference...its a 'remnant' from inflation. As the universe expands [for 13.7 b years or so] dark energy increases with it...because the energy density of space is believed to be constant...

So expansion back then has to be as a result of something else?

related, but different: inflation. Inflation likely started via a quantum like, low energy, statistical, vacuum fluctuation...Brian Greene explains it this way:

Since during inflation the energy of the inflation field grew by at least 10 ¹⁰ the inflation field didn’t need to have much energy at the onset of inflation. All that was required was a uniform inflation field, one getting stuck for the briefest moment on a high level potential energy bowl...Matter and radiation were produced at the end of the inflationary phase as the inflation field released its pent up energy...

Quote by Susskind

The most notable inhabitants of our universe - the galaxies- are built around giant black holes that are continually gobbling up stars and planets. Out of every 10,000,000,000 bits of information in the universe 9,999,999,999 are associated with the horizons of black holes. It should be evident that our naive ideas about space,time, and information are wholly inadequate to understand most of nature.

mfb:

@Naty1: That is a completely different statement compared to your original claim. That is entropy, not mass. Yes, black holes have a huge associated entropy. So what? The entropy of the air in my room has more bits than all binary data stored in computers in the whole world. That does not make this air more interesting.

I believe my comments consistent although I never mentioned 'mass'.

If only 1 of every 10¹⁰ bits really does remain outside black holes, and I have no idea where Susskind got that from, I'd call that more than just 'interesting' ...fascinating, in fact. But I AM rather easily amused!

Tanelorn

Ever get the feeling that ordinary matter is a just an unimportant side product of something much more Important?

Important to us, but still just leftover cosmic 'crumbs'...

 

[mfb]

I believe my comments consistent although I never mentioned 'mass'.

"99% of everything"
Mass, energy and particles are certainly "something".

If only 1 of every 10¹⁰ bits really does remain outside black holes, and I have no idea where Susskind got that from, I'd call that more than just 'interesting' ...fascinating, in fact. But I AM rather easily amused!

As I posted, you can assign huge numbers of entropy to black holes. That does not make those bits more interesting than the exact distribution of gas molecules in my room.

 

[Naty1]

mfb:

"99% of everything"
Mass, energy and particles are certainly "something".

we agree.

mfb:

As I posted, you can assign huge numbers of entropy to black holes. That does not make those bits more interesting than the exact distribution of gas molecules in my room.

I was highlighting that the HUGE proportion of information/entropy inside black holes is unavailable for current observation ...And I had not thought of what are likely vastly greater amounts hidden from us beyond the cosmological horizon...so it turns out the measly 1 bit out of every 10¹⁰ that Susskind claims is a vast exaggeration. Cut that by another 10¹⁰ or even more, maybe...

Maybe we should be amazed how much we have learned when so little is available for inspection/observation; rather remarkable...I never thought of it that way.

PS: MFB..Are you trying to get me in more trouble here? I have already been censored for 'hijacking a thread' when I in fact was responding to an OP...so I am worried this diversion may again 'cost me'...[I don't mind getting censored all that much , but when I tell my wife she claims "You can't even get along with 'those' people"...]

 

[mfb]

this diversion

Good point. Let's stop this side-discussion.

 

[Tanelorn]

As OP I am ok with the discussion if that is all that matters. I thought simple structures had very little information?

 

[mfb]

Thermodynamics would allow something that we do not understand yet. So it is still unclear if all those available bits (calculated with thermodynamics) are "used" in any way.

 

[Naty1]

As OP I am ok with the discussion if that is all that matters.

I have seen critiques leveled to the effect that if you change the subject, start a new thread...I have never figured out the 'rules'...

I thought simple structures had very little information?

not so. A complicated subject without satisfying resolution that I have found anywhere.
You can search 'information' or 'entropy' in these forums or Wikipedia for lots more viewpoints.


Here is a flavor from other posts I have made and posts of others I liked:

The 'holographic principle' [thanks to Leonard Susskind, Gerard t'Hooft] is a property of quantum gravity and string theories which says that the description of a volume of space can be thought of as encoded on a boundary, like a sphere,say, to the region—preferably a light-like boundary like a gravitational horizon.

adapted from Wikipedia

So information appears to be related to enclosed surface area, NOT volume! What a crazy situation. Information in a black hole is FINITE. [for more on this, try 'Bekenstein Bound']


Leonard Susskind in his book THE BLACK HOLE WAR (his controversy with Stephen Hawking) has some really interesting insights on information and horizons...like the horizon of a black hole is "stringy"...it can be described in terms of quantum strings...and so hidden information is proportional to the total LENGTH of a string!...and Hawking radiation can be viewed as string bits breaking loose from just outside the horizon...due to quantum fluctuations...a perspective akin to virtual particles causing the Hawking radiation.

Also: consider acceleration...that also causes an event horizon to form...which leads to a temperature (called the Unruh effect)...that means different particle counts [different information] are observed under acceleration...this is the same as sitting stationary outside a black hole horizon... so just what information is available locally?? not as obvious as once thought. A stationary and accelerating observer passing each other locally read different temperatures!

As another perspective: someone posted this elsewhere and I liked it:

So the rule for objectivity is not that everyone lives in the same reality, it is that no two observers' realities can be inconsistent with each other. This also means that "complete" information does not imply a unique description of the reality, it merely implies access to all the information that is locally available to that observer in principle. The locality of the information is what preserves causality…”

I think it is Brian Greene in one of his books that has interesting observations on entropy/information...can't find my quotes/notes...but a rough description is that in the absence of gravity, say mfb's room of air he mentioned as an example, the maximum entropy would be a completely random distribution of air molecules...air molecules in every corner, so to speak, neglecting gravity, would contain the most information..be the stable 'maximum entropy'.

Not so in the presence of strong gravity: In strong gravity, 'clumpiness' is the norm, the state of maximum entropy with the most information, the final state. Those air molecules would tend to pull together to the gravitational source. This has fascinating [to me] implications.

You may be aware the entropy of the cosmos increases as the universe evolves... ...planets, stars, galaxies, black holes in the presence of gravity serve as 'clumpiness galore'. It is not at all obvious that such clumpiness in cosmic conditions results in increasing entropy. Some think everything will eventually be swallowed by black holes...the 'ultimate clumpiness' perhaps.

[Note that even though total entropy increases, total energy is not believed to do so. That is another subject without a firm consensus. ]

good night. pre-season football awaits!

 

[Tanelorn]

In the early universe, normal matter and dark matter tended to attract one another, so that an overabundance of dark matter generally also coincided with an overabundance of normal matter as galaxies formed. [All part of primordial quantum perturbations.] So it's not so much that dark matter collects around the edges of galaxies, but that galaxies form in the centers of large blobs of dark matter. But things don't always turn out quite so simply: when stars are born amid violent explosions and galaxies form, a lot of matter may be ejected as stars form leaving a galaxy with mostly dark matter.

Thanks Naty, Do you have a good link for this description of DM? What stops DM from building up inside Stars and planets or even creating its own similar bodies?

 

[mfb]

Dark matter has no strong short-range interaction, it cannot "collide", or radiate away energy. There is no way to lose energy to clump together on a scale smaller than galaxies.

 

[Tanelorn]

Thanks mfb, Does DM have mass and therefore the same gravitational force proportional to inverse distance squared as normal matter? If so, what could stop it clumping? Is it because it is very small, something like a heavy proton?

 

[Drakkith]

Thanks mfb, Does DM have mass and therefore the same gravitational force proportional to inverse distance squared as normal matter?

As far as we know.

If so, what could stop it clumping? Is it because it is very small, something like a heavy proton?

No, it literally doesn't interact. Imagine a particle that doesn't respond to gravity. It could go right past our Sun and not have its path altered. DM is like that but with the other forces. It just goes through stuff completely.

 

[Tanelorn]

Thanks drakkith, so could DM be something like a very! heavy neutrino?

IF DM is distributed equally in the region of spiral galaxies and not at their rims, is this still consistent with the need to explain spiral galaxy rotational speed at the rim?

 

[Drakkith]

Thanks drakkith, so could DM be something like a very! heavy neutrino?

The possibility exists, but there are limitations on what it can be.
http://en.wikipedia.org/wiki/Hot_dark_matter
http://en.wikipedia.org/wiki/Warm_dark_matter
http://en.wikipedia.org/wiki/Cold_dark_matter

IF DM is distributed equally in the region of spiral galaxies and not at their rims, is this still consistent with the need to explain spiral galaxy rotational speed at the rim?

To my knowledge dark matter must be concentrated in a "halo" around the galaxy in order to account for this discrepancy.

 

[Tanelorn]

What stops DM from accumulating inside the rims and possibly near the centers of galaxies or is this unknown?

 

[Naty1]

Answer is still the same as when posted in #25.

edit; Maybe I should say that's about as much as we think we know...
there seems to be a lot more we don't know than we know..."Do you have a good link for this description of DM?"

not so much...search these forums, and Wikipedia has a good outline under DARK MATTER...

and I think also discusses MACHOS and WIMPS...

and I see Drakkith has posted links to hot,warm, cold dark matter...

lots of candidates, lots of questions, not so many detailed answers yet.

 

[Drakkith]

What stops DM from accumulating inside the rims and possibly near the centers of galaxies or is this unknown?

The issue is that DM can't interact through anything other than gravity. What happens is that DM follows an orbit around our galaxy just like everything else. In order for it to fall in towards the center of our galaxy and stay it must give up energy, otherwise it just swings back around all the way out to the halo region again. (or doesn't fall in at all)

When you have a dust cloud that is collapsing under its own gravity, the particles get rid of energy by colliding with each other and giving off EM radiation. This allows it to collapse into dense regions and eventually form stars, planets, etc.

But dark matter can't do this. It simply passes through itself without absorbing or giving off any energy. So it can't collapse into dense regions very easily, if at all.