Is Dark matter homogeneous in Universe?

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Discussion Overview

The discussion centers around the nature of dark matter in the universe, specifically whether it is homogeneous and isotropic on very large scales. Participants explore concepts related to the distribution of dark matter compared to baryonic matter and the implications for cosmic structure.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants argue that dark matter is not homogeneous, as it clusters similarly to normal matter, albeit in a more diffuse manner.
  • Others suggest that on very large scales, dark matter is considered isotropic and may be treated as homogeneous, particularly in the context of the observable universe.
  • A participant notes that dark matter structures likely influenced the large-scale structure of baryonic matter through gravitational wells formed in the early universe.
  • There is a discussion about the definitions of "homogeneous" and "isotropic," with some participants emphasizing the need for clarity in these terms.
  • One participant presents a visual representation of a mapped region of the universe, arguing that it shows an approximately homogeneous distribution of both dark and visible matter on large scales.
  • Another participant questions the meaning of "large" in the context of the universe's scale, leading to further clarification about the observable universe's radius.

Areas of Agreement / Disagreement

Participants express differing views on the homogeneity of dark matter, with some asserting it is isotropic and homogeneous on large scales, while others challenge this notion and call for clearer definitions. The discussion remains unresolved regarding the precise nature of dark matter's distribution.

Contextual Notes

There are ongoing discussions about the definitions of homogeneity and isotropy, as well as the scale at which these properties apply. Participants highlight the importance of specifying what is meant by "large scales" and the granularity acceptable for considering a distribution homogeneous.

RyanH42
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Is Dark matter homogenenius in Universe ? I don't think so but I don't know any idea about it.
Thank you
 
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No, it isn't. It clusters like normal matter, but in more diffuse distribution due to the lack of dissipative interactions. You get, for example, halo-like structures around galaxies.

Try googling 'dark matter distribution', and you'll get plenty of hits to scientific papers, popular articles and images mapping the distribution. IIRC, some of the free planetarium software out there also allows viewing DM distribution as it's known today (Digital Universe should have that functionality, I think).
 
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The funny thing is that dark matter structure was likely responsible for the large scale structure of baryonic matter through the formation of gravitational wells in the early Universe. Without dark matter, there would be significantly less structure on large scales.
 
Just to add to what has already been said, on VERY large scales, dark matter, like regular matter, is taken to be homogeneous, or more technically appropriate, "isotropic" but not in scales such as galactic clusters (which can be REALLY REALLY big by human standards)
 
I am talking about very very large scales.Not small distances.Is phinds idea true ?
 
RyanH42 said:
I am talking about very very large scales.Not small distances.Is phinds idea true ?
Do you think I"m making it up?

How large is "large" to you?
 
Like universe radius distance.
 
RyanH42 said:
Like universe radius distance.
The universe HAS no "radius" so that doesn't make any sense.

If you mean the radius of the observable universe (as opposed to "the universe") then yes it absolutely is isotropic.
 
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Here's a 1 billion ly across mapped region of the universe.
Sideshear.jpg

(taken from: http://www.cfht.hawaii.edu/News/Lensing/)
Brighter regions are more dense concentrations.
You can see that while a filament-like structure is still visible, it is approximately homogenous - there are no clearly defined voids and overdense regions. You can easily extrapolate this to an even large scale.
The bottom line is: both dark and visible matter are approximately homogeneous on large scales.

And once again, google is your friend. That picture took maybe 5 seconds to retrieve.
 
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  • #10
Isotrophic ok What about homogeneius ?.Or Isotrophy universe makes also homogeneius ?
 
  • #11
RyanH42 said:
Isotrophic ok What about homogeneius ?.Or Isotrophy universe makes also homogeneius ?
You'll have to define your terms. The sand on a beach seems VERY homogenous looked at from a distance but if you hold a few grains in your had it doesn't look homogeneous at all.
 
  • #12
I am talking about large scales.

In the the universe ,dark matter distrubition homogeneius and isotrophic at the scale of observable universe radius ?
 
  • #13
RyanH42 said:
I am talking about large scales.

In the the universe dark matter distrubition homogeneius and isotrophic at the scale of observable universe radius ?
You STILL have to define what YOU mean by "homogeneous". How much granularity is OK to still call it homogeneous?
 
  • #14
At least Homogeneius like the baryonic matter distrubition
 
  • #15
RyanH42 said:
At least Homogeneius like the baryonic matter distrubition
Yes, it is as isotropic as normal matter. They tend to go together at roughly the same ratio (about 5:1) in clumps we call galaxies.
 
  • #16
@phinds: be mindful of the terms you use. Homogeneity and isotropy are not synonymous, even though the large-scale distribution of dark as well as baryonic matter satisfies both.
 
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  • #17
Bandersnatch said:
@phinds: be mindful of the terms you use. Homogeneity and isotropy are not synonymous, even though the large-scale distribution of dark as well as baryonic matter satisfies both.
Right ... that's why I've tried to stick with isotropic and have encouraged the OP to define and understand his terms and I have been specific about how I am using "homogeneous" (see post #11)
 
  • #18
Thank you again.
 
  • #19
Bandersnatch said:
And once again, google is your friend. That picture took maybe 5 seconds to retrieve.
Could you help me here? I am not interested in an image. I would like to have access to concrete data. Thanks!
 
  • #20
Omega0 said:
Could you help me here? I am not interested in an image. I would like to have access to concrete data. Thanks!
Follow the link below the picture.
 

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