Cold Dark Matter vs Hot Dark Matter

[cp05]

Hey guys, I'm a bit confused about how hot/cold dark matter plays into our theory of structure formation.

Hot Dark matter is relativistic then it decouples in the early universe, and these particles are light and fast and do not form small structures since there is a large mass needed to keep them gravitationally bound, so this is a Top Down approach to structure formation since this forms large structure that fragments into small.
On the other hand, CDM is non relativistic when it decouples, so it moves slowly and clumps on all scales and is a bottom up approach.

The reason I am confused is because our favored model is the CDM model...but I thought structure formation was top down, since smaller scales are wiped out by the time we get to recombination (completely damped by silk mass, or if their scales are less than the particle horizon, perturbations will not collapse and will simply oscillate). Where is my thinking wrong? Because this seems really contradictory to me.

Perhaps I should add another question. Since Dark Matter is non-interacting...after the era of recombination, how is baryonic matter able to fall into the dark matter potential wells to enlargen the parturbations?? Because they should not be interacting with each other...

I apologize if my thinking is completely wrong haha This stuff if not making any sense to me right now.

 

[Chronos]

The gravitational properties of dark matter are believed to be identical to baryonic matter. The major difference being that dark matter particles are collisionless [i.e., cannot form stars, etc.]. They pass right through each other [and baryonic particles[ and exit the opposite direction in yo-yo like fashion. Clouds of dark matter function as gravity traps for baryonic particles. One of the reasons hot dark matter is disfavored is because it inhibits formation of the massive dark matter bubbles believed necessary for large scale matter structures.

 

[Chalnoth]

Hey guys, I'm a bit confused about how hot/cold dark matter plays into our theory of structure formation.

Hot Dark matter is relativistic then it decouples in the early universe, and these particles are light and fast and do not form small structures since there is a large mass needed to keep them gravitationally bound, so this is a Top Down approach to structure formation since this forms large structure that fragments into small.
On the other hand, CDM is non relativistic when it decouples, so it moves slowly and clumps on all scales and is a bottom up approach.

Well, it's not the temperature at decoupling that is important. Instead, it's the temperature at around the time structure starts to form.

Anyway, I think the main problem here is that top-down structure formation simply doesn't work. So what you would get in reality with a hot dark matter model is just a lot of big things, and not many small things. Instead, the models which seem to describe our universe rather well are ones where the big things tend to be made up of a much larger number of small things, and the smaller structures tend to always be much more numerous. There are currently some observational difficulties in observing dwarf galaxies, though, so it's difficult at present to nail this down.

 

[mersecske]

Hi everybody, are there any good figure showing all restrictions on any kind of dark matter?

 

[scvblwxq]

Monash student finds Universe’s missing mass
23 May 2011
Dr Jasmina Lazendic-Galloway, Amelia Fraser-McKelvie and Dr Kevin Pimbblet
A Monash student has made a breakthrough in the field of astrophysics, discovering what has until now been described as the Universe’s ‘missing mass’. Amelia Fraser-McKelvie, working within a team at the Monash School of Physics, conducted a targeted X-ray search for the matter and within just three months found it – or at least some of it...

http://www.monash.edu.au/news/show/monash-student-finds-universes-missing-mass
Her work has been published in one of the world’s oldest and most prestigious scientific journals, Monthly Notices of the Royal Astronomical Society.
paper: http://arxiv.org/abs/1104.0711

 

[Chalnoth]

Monash student finds Universe’s missing mass
23 May 2011
Dr Jasmina Lazendic-Galloway, Amelia Fraser-McKelvie and Dr Kevin Pimbblet
A Monash student has made a breakthrough in the field of astrophysics, discovering what has until now been described as the Universe’s ‘missing mass’. Amelia Fraser-McKelvie, working within a team at the Monash School of Physics, conducted a targeted X-ray search for the matter and within just three months found it – or at least some of it...

http://www.monash.edu.au/news/show/monash-student-finds-universes-missing-mass
Her work has been published in one of the world’s oldest and most prestigious scientific journals, Monthly Notices of the Royal Astronomical Society.
paper: http://arxiv.org/abs/1104.0711

This has nothing whatsoever to do with dark matter, the usual thing people talk about when they say "missing mass". In the paper, they don't even examine the wider cosmological implications of their observations. It's purely a statement that with their method, they are able to detect more normal matter in the specific galaxy clusters they've examined than has previously been the case.

One might possibly surmise from this study that if it can be extended to other galaxy clusters, we might have an idea of where some of the missing normal matter is. You see, the cosmic microwave background provides our most sensitive and least error-prone estimate of the total amount of normal matter in our universe. But when we look at the galaxies around us and try to count up the normal matter, we come up short (by a large amount). So there's clearly quite a bit of normal matter out there that we just haven't found yet, and this work may have found some of it.

So this particular bit of research is interesting in its own right, but it has nothing whatsoever to do with the topic of this thread.

 

[mersecske]

Hi, I don't understand something about dark matter.

As I can imagine, dark matter can be a particle which is here in our solar system also, but we cannat see.
Or can be larger objects which is not here in our solar system,
but we can found on larger scale, for example not shining stars or remnants like black holes, or planets and rocks, etc.

But how it is possible, that we are still thinking on the first one?
How it is possible to not see the dark matter in the solar system,
since its mass is much more larger that the mass of the Sun
(because there are more dark matter than normal matter on average)?
We can see the anomalities in the rotation curve of the galaxy,
but cannot detect the anomalities in the "rotation curve" of our solar system?

My other question is about the clastering of matte and gravitation.
On large scale the gravitation is the main force.
How it is possible that the dark matter is not clastered such as normal matter?
Maybe the clusterization time scale differs from normal matter, but why?
I think this time cannot depend on the properties of non-gravitational interactions.

 

[Chalnoth]

Hi, I don't understand something about dark matter.

As I can imagine, dark matter can be a particle which is here in our solar system also, but we cannat see.
Or can be larger objects which is not here in our solar system,
but we can found on larger scale, for example not shining stars or remnants like black holes, or planets and rocks, etc.

But how it is possible, that we are still thinking on the first one?
How it is possible to not see the dark matter in the solar system,
since its mass is much more larger that the mass of the Sun
(because there are more dark matter than normal matter on average)?
We can see the anomalities in the rotation curve of the galaxy,
but cannot detect the anomalities in the "rotation curve" of our solar system?

The dark matter is distributed much more evenly. While the total amount of dark matter is very large, it's spread so thinly that if you do the simple, back-of-the-envelope calculations, the total mass in our solar system is far, far too small for us to detect (so far) in solar system gravity experiments (our solar system is almost all normal matter, with almost no dark matter).

The dark matter, however, adds up on longer distance scales. Just to give you a sense of scale, the typical distance between stars in our galaxy is around a million times larger than the distance from the Earth to the Sun. And the distances where we can start to measure the effects of dark matter are somewhere in the regime of ten to a hundred times further still.

My other question is about the clastering of matte and gravitation.
On large scale the gravitation is the main force.
How it is possible that the dark matter is not clastered such as normal matter?
Maybe the clusterization time scale differs from normal matter, but why?
I think this time cannot depend on the properties of non-gravitational interactions.

The thing is, if you only have gravity, things don't cluster very tightly. They will move into orbits, but once they are in some orbit, they still stay in that orbit pretty much until the end of time. In order to move into lower orbits, matter has to experience friction. Normal matter does this by interacting with the electromagnetic force: it is able to emit photons, thereby losing energy and moving to lower orbits.

So dark matter will move into a gravitational potential well, but still stay spread-out. Normal matter, which can emit energy efficiently, collapses rather rapidly into much smaller objects.