Dark matter (split from Dark matter does not exist ?! What do you think ?)

[MSC]

Could dark matter be accounted for in colossal black holes hidden in the voids between galaxy clusters along the cosmic web?

What exactly is in those void areas? What are the theories? 

[MigL]

Wouldn't account for the anomalies in galactic rotations

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

[MSC]

20 minutes ago, MigL said:

Wouldn't account for the anomalies in galactic rotations

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

If my hypothesis were correct, wouldn't that account for the anomalies in terms of closer proximity to SMBHs at the center of galaxies vs collosal BHs in the voids outside of them? 

Let me ask another way; assuming a black hole from the early universe was able to keep being supplied with more mass, how big could it hypothetically be now? I thought black hole evaporation takes an extraordinarily long time and outlasts the lifespans of stars and galaxies? Is it even possible for any but the tiniest and most isolated to have evaporated yet? Considering how everything used to be much closer together the further back in time you go. 

There is also a more anecdotal and emotional reason I ask. Everytime I look at the Bootes void or some other void I get the strangest feeling my eyes are being tricked, my hair stands on end and I get a shiver running down my spine. 

Doesn't the Schwarzschild radius of a BH increase as it gobbles up more mass?

 

[MigL]

1 hour ago, MSC said:

If my hypothesis were correct, wouldn't that account for the anomalies in terms of closer proximity to SMBHs at the center of galaxies vs collosal BHs in the voids outside of them? 

How exactly ???
I suggest reading the link provided ...

"The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre."

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

[MSC]

7 minutes ago, MigL said:

How exactly ???
I suggest reading the link provided ...

"The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre."

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

I did, but I'm probably just not understanding it, even after my second read through just now it's still a little head crunchy. The MOND thing being non relativistic sounds confusing to me and I'll probably need to follow up on those relativistic ones. 

I guess what I'm asking now is, Why don't they follow the rules?

[MigL]

11 hours ago, MSC said:

I did, but I'm probably just not understanding it, even after my second read through just now it's still a little head crunchy.

If you look at the rotation of a typical galaxy and take into consideration the visible mass ( estimated star, gas and dust mass) distribution ( flattened spiral ), you would expect the outer spiral arms to be 'winding up'. IOW, just as the outer planets of the solar system take much longer than the inner planets to orbit, the outer parts of the spiral arms should be 'lagging' in their rotation about the center, but, what we actually see, is that galaxies rotate like a giant pinwheel, there is no lagging rotation at the outer extremities.

This implies one of two possible things …

1 - Could the laws of Gravity ( Newton, GR ) be wrong at very large scales ?
This is addressed by MOND ( Modified Newtonian Dynamics ), but MOND seems to work for galactic rotation and then reverts to good old gravity at even larger scales ( between galaxies and clusters ). It also has other issues which make it a non-starter ( Google MOND ).

2 - Could the rotation be affected by other factors of which we are currently unaware ?
It turns out that a spherical mass distribution ( not just a flattened spiral mass ) would result in the observed rotation rates.
But we don't detect any Electromagnetic radiation from this 'dark' mass, so it must mean that whatever mass is there doesn't interact electromagnetically, only gravitationally.

Since option 2 is the more plausible, the search for this Dark Matter was begun, and is still ongoing.
The indirect evidence for its existence is piling up ( Google Dark Matter ), from gravitational lensing to galaxy 'seeding', but still no viable model or direct evidence for it.

Edited October 12, 2020 by MigL

[Janus]

2 minutes ago, MigL said:

If you look at the rotation of a typical galaxy and take into consideration the visible mass ( estimated star, gas and dust mass) distribution ( flattened spiral ), you would expect the outer spiral arms to be 'winding up'. IOW, just as the outer planets of the solar system take much longer than the inner planets to orbit, the outer parts of the spiral arms should be 'lagging' in their rotation about the center, but, what we actually see, is that galaxies rotate like a giant pinwheel, there is no lagging rotation at the outer extremities.

This implies one of two possible things …

1 - Could the laws of Gravity ( Newton, GR ) are wrong at very large scales ?
This is addressed by MOND ( Modified Newtonian Dynamics ), but MOND seems to work for galactic rotation and then reverts to good old gravity at even larger scales ( Between galaxies and clusters ). It also ahs other issues which make it a non-starter ( Google MOND ).

2 - Could the rotation be affected by other factors of which we are currently unaware ?
It turns out that a spherical mass distribution ( not just a flattened spiral mass ) would result in the observed rotation rates.
But we don't detect any Electromagnetic radiation from this 'dark' mass, so it must mean that whatever mass is there doesn't interact electromagnetically, only gravitationally.

Since option 2 is the more plausible, the search for this Dark Matter was begun, and is still ongoing.
The indirect evidence for its existence is piling up ( Google Dark Matter ), from gravitational lensing to galaxy 'seeding', but still no viable model or direct evidence for it.

A couple of points:

Galactic rotation curves show that orbital velocity remains nearly flat as you move outward.  For the galaxy to appear to rotate as one piece, they would need to increase proportionally with R.    At the same velocity, a star closer in still takes less time to complete an orbit than one further out. 

This image plots the various curves. note that the vertical axis is in km/sec.

 

 

 

So, why don't we see the spiral arms "wind up"?

It is because the spiral arm formation is due to a different mechanism than the orbital motion of the stars.  They are produced by density waves moving through the Galaxy. Since these waves do not travel at the same rate as the rotation, stars drift in and out of them.   A bright spiral arm is where a wave has the highest density. As a result, you get a higher rate of stars being born there.  This includes massive, bright stars.  But these stars have short lives, and burn out fairly quickly.  By the time the stars in a spiral arm drift out, these brighter stars have spent themselves and you are left with the dimmer longer life stars.  The spiral arms look bright not because they contain that much more stellar material, but because they contain a larger percentage of bright stars.

This bright star vs. dim star paradigm happens in our own night sky.   If you go out on a clear night away from city lights, you will see a sky full of stars.  But you are really only seeing, for the most part, brighter than average stars.   Our own Sun would be only visible by naked eye over a distance of a few 10's of light years.  The vast majority of stars we see are much further than that, they are just much brighter than our Sun.

Within 17 light years of the Earth, there are ~50 stars, only 10 of which are visible by the naked eye.

5 minutes ago, Tunnel said:

Dark matter exist if the universe and gravity are exactly what they are thought to be.  A better question is where is the 85 percent of the universe believed to be dark, hiding from view?

Some of it might be objects like black holes, etc ( MACHO's, or MAssive Compact Halo objects)  Though there are limits as to how much these can contribute due to how they would have effected the early formation of the universe.

Some of it could be made of particles which don't interact via electromagnetism (WIMPs, or Weakly Interacting Massive particles)  There is nothing that rules out such particles (In fact, the neutrino has these characteristics.)

And don't let the fact that it makes up so much the universe mislead you.  It is also spread out over much larger volumes than the more compact structures like galaxies.  It is estimated that the density of dark matter within our solar system is so low that the total of its mass would only equal that of a small asteroid.

[MigL]

Thanks for the fuller explanation Janus.
I was trying to keep it as simple as possible since all the above is explained in the Wiki link, and MSC didn't seem to fully understand it.
And if yo look at the video in the link, it shows what I meant by 'wind up'.