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Dark matter


Kevin_Hall
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30 minutes ago, Kevin_Hall said:

What is dark matter and what we know about it?

There is not much point people here reciting basic  information that is widely available on-line. Suggest reading, say, the Wiki article and then asking any questions you have  arising from that.

But I must say this is a very odd question for someone who teaches astronomy to be asking.  

Edited by exchemist
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DM is of course what is needed along with the observed matter, that explains the gravitational rotation of galaxies and large scale structure of the universe. It is termed "dark"not because it is non luminous but because we are ignorant at this time, as to its nature. 

It is thought by many to be some unknown form of non baryonic matter, that interacts only via gravity, and that is evidenced by the bullet cluster anomaly....

Chandra :: Photo Album :: MACS J0025.4-1222 :: August 27, 2008

https://en.wikipedia.org/wiki/Bullet_Cluster

Other thoughts are microscopic or primordial BH's that may have formed just after the BB.  https://www.scienceforums.net/topic/126284-primordial-black-holes/

 

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On 12/21/2021 at 5:21 PM, exchemist said:

There is not much point people here reciting basic  information that is widely available on-line. Suggest reading, say, the Wiki article and then asking any questions you have  arising from that.

But I must say this is a very odd question for someone who teaches astronomy to be asking.  


In my opinion, SCIENCE forum is not about to express your emotions, but facts, exchange knowledge and objective thoughts.
Thanks, gentlemen. 

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We do know what it is not: Not hot, not atomic, not electrically charged. There are some other nots.

Some candidates to explain what it is are:

1) Swarms of little black holes

2) Exotic particles coming from super-symmetric extensions of the standard model of elementary particles

3) Really a signal that Einstein's equations must be modified at long distances

(ordered according to my personal preferences)

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On 12/21/2021 at 2:56 PM, beecee said:

It is termed "dark"not because it is non luminous but because we are ignorant at this time, as to its nature. 

Part of the reason it’s called dark is because it doesn’t interact electromagnetically, and is thus not visible 

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2 hours ago, joigus said:

1) Swarms of little black holes.

(ordered according to my personal preferences)

There is another thread ( oddly, by the same name ) where I voice my objections to your preference 🙂 .

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30 minutes ago, swansont said:

Part of the reason it’s called dark is because it doesn’t interact electromagnetically, and is thus not visible 

I don't know why it's called dark, but not interacting electromagnetically makes it rather transparent than dark.

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19 minutes ago, Genady said:

I don't know why it's called dark, but not interacting electromagnetically makes it rather transparent than dark.

If it could interact electromagnetically it would emit thermal EM radiation. The ability to emit light has implications about how it behaves - easy dissipation of energy would allow it to “clump” more readily. And “dark” is also an acknowledgement that we don’t know what it is, as beecee has noted.

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23 minutes ago, MigL said:

There is another thread ( oddly, by the same name ) where I voice my objections to your preference 🙂 .

 

17 minutes ago, beecee said:

Thanks for reminding me. I've changed my mind at least a couple of times about what a plausible explanation might be.

@MigL, if it's not too much trouble, could you lead me to that thread? The one @beecee linked to --though very interesting and certainly related-- is not the one you're referring to, I think.

 

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On 12/21/2021 at 3:56 PM, beecee said:

It is thought by many to be some unknown form of non baryonic matter, that interacts only via gravity, and that is evidenced by the bullet cluster anomaly....

Many think that it interacts not only via gravity but also weakly, i.e. made of WIMPs.

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1 hour ago, Genady said:

Many think that it interacts not only via gravity but also weakly, i.e. made of WIMPs.

Careful - they are not thought to interact via the weak interaction, but via some new method on the scale of, or weaker than, the weak interaction

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3 hours ago, MigL said:

There is another thread ( oddly, by the same name ) where I voice my objections to your preference 🙂 .

OK. I changed my mind:

1) Really a signal that Einstein's equations must be modified at long distances

2) Exotic particles coming from super-symmetric extensions of the standard model of elementary particles

3) Swarms of little black holes (that for some reason do not evaporate)

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19 hours ago, swansont said:

If it could interact electromagnetically it would emit thermal EM radiation.

Is it necessarily so? If it could interact electromagnetically by completely reflecting an EM radiation it wouldn't emit a thermal EM radiation, would it?

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2 minutes ago, Genady said:

Is it necessarily so? If it could interact electromagnetically by completely reflecting an EM radiation it wouldn't emit a thermal EM radiation, would it?

Reflection certainly implies the matter in question must interact, i.e. be polarisable by, EM radiation. If it can be polarised by it, there will be some wavelength at which it will  absorb and emit, won't there? And, if it reflects, wouldn't we observe reflected radiation.   

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1 hour ago, Genady said:

Is it necessarily so? If it could interact electromagnetically by completely reflecting an EM radiation it wouldn't emit a thermal EM radiation, would it?

I don’t think complete reflection is physically possible. Reflection requires a momentum transfer, since the momentum of the light changes, and thus energy transfer. So if energy is lost from the incoming light, the reflection can’t be complete.

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18 hours ago, joigus said:

OK. I changed my mind:

1) Really a signal that Einstein's equations must be modified at long distances

2) Exotic particles coming from super-symmetric extensions of the standard model of elementary particles

3) Swarms of little black holes (that for some reason do not evaporate)

(1) A possibility sure, but are we not also seeing the need for "extra matter" over smaller scales? Plus GR is our gravitational explanation/theory for the very large.

(2) Yep a possibility.

(3) Little BH's in the early denser universe, could grow into bigger SMBH's before they had time to evaporate.

My thoughts are (1) Primordial BH's MACHO's (2) WIMPS, and (3) GR

Edited by beecee
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