I Question regarding Evaporation of galaxies

[Buzz Bloom]

I recently looked at

https://www.physicsforums.com/threads/will-temperature-go-down.104791/​

and then looked at the link

http://math.ucr.edu/home/baez/end.html .​

My question is related to a quote from this link.

(It may seem odd that first the galaxies form by gravitational attraction of matter and then fall apart again by "boiling off", but the point is, intergalactic matter is less dense now than it was when galaxies first formed, thanks to the expansion of the universe. When the galaxies first formed, there was lots of gas around. Now the galaxies are essentially isolated — intergalactic space is almost a vacuum. And you can show in the really long run, any isolated system consisting of sufficiently many point particles interacting gravitationally — even an apparently "gravitationally bound" system — will "boil off" as individual particles randomly happen to acquire enough kinetic energy to reach escape velocity. Computer calculations already suggest that the solar system will fall apart this way, barring other interventions. With the galaxies it's even more certain to happen, since there are more particles involved, so things are more chaotic.)​

I am assuming that by "point particles" the author meant large particles like planets, stars, and black holes. (Molecule sized particles would interact by EM, and radiate particle velocity away by photons.) If the mechanism for "boiling off" involves "[it] will 'boil off' as individual particles randomly happen to acquire enough kinetic energy to reach escape velocity," then isn't it the case that for each hunk of mass that obtains escape velocity (relative to the galaxy's center of mass (CoM) ) other hunks of mass will lose velocity and become more tightly bound to the CoM and fall towards the CoM where the escape velocity is greater. This concept makes me have doubts about the computer simulations mentioned. For example, did the simulation take into account the capturing of chunks of mass into the large black hole at or near the CoG? Did it take into account that velocity is also lost by Gravitational wave effects.

 

[mathman]

Reaching "escape velocity" seems (to me) very difficult. Stars in galaxies usually follow elliptical orbits, which would be hard to alter.

 

[PeterDonis]

isn't it the case that for each hunk of mass that obtains escape velocity (relative to the galaxy's center of mass (CoM) ) other hunks of mass will lose velocity and become more tightly bound to the CoM

Yes. And eventually what remains of the central mass will become a black hole (or will fall into the black hole that's already at the center). So at some point what remains of the original system will be just a black hole--everything that didn't end up in the hole will have "boiled off" and escaped.

Reaching "escape velocity" seems (to me) very difficult. Stars in galaxies usually follow elliptical orbits, which would be hard to alter.

The alteration is done by collisions--or more generally close encounters--between the objects. These are rare, but over a long enough time span, you will get encounters that give one of the two objects enough kinetic energy to escape.

 

[kimbyd]

I am assuming that by "point particles" the author meant large particles like planets, stars, and black holes. (Molecule sized particles would interact by EM, and radiate particle velocity away by photons.) If the mechanism for "boiling off" involves "[it] will 'boil off' as individual particles randomly happen to acquire enough kinetic energy to reach escape velocity," then isn't it the case that for each hunk of mass that obtains escape velocity (relative to the galaxy's center of mass (CoM) ) other hunks of mass will lose velocity and become more tightly bound to the CoM and fall towards the CoM where the escape velocity is greater. This concept makes me have doubts about the computer simulations mentioned. For example, did the simulation take into account the capturing of chunks of mass into the large black hole at or near the CoG? Did it take into account that velocity is also lost by Gravitational wave effects.

Yes. As parts of it boil off, other parts collapse inward.

Gravitaitonal radiation only has a significant impact very near exceptionally dense objects like neutron stars and black holes. It has very little impact on the overall dynamics discussed here.

Reaching "escape velocity" seems (to me) very difficult. Stars in galaxies usually follow elliptical orbits, which would be hard to alter.

Elliptical orbits are the Newtonian result for two-body systems. Systems with more objects can have far more complicated orbits, and over very large spans of time many of them will come close enough to one another to exchange momentum.

 

[Buzz Bloom]

The alteration is done by collisions--or more generally close encounters--between the objects. These are rare, but over a long enough time span, you will get encounters that give one of the two objects enough kinetic energy to escape.

Hi Peter:

What seems to be missing in Baez'a paper is a calculation that shows what fraction of the mass in a galaxy (or group of galaxies) ends up in one (or more) black hole(s), and what fraction "evaporates". Can someone post a link to a source that provides this information based on some simulation study? It would also be interesting to see for a typical a galaxy the energy per trillion years rates of evaporation, black hole capture, and gravitation wave radiation.

ADDED
It has occurred to me that since there is about five times as much dark as baryonic matter, the behavior of dark matter with respect to evaporation and capture by a black hole might have different results to the questions I pose above, and might significantly alter the estimates for the total mass.

Regards,
Buzz