[Mad Mark (OP)]

I often read about the possibility that our Universe theoretically could at some time undergo a kind of phase transition from its currant state and therefore undo all that there is in our universe.
If this is possible long before a heat death to our universe and the evaporation of everything how would this transition effect supermassive black holes that exist?
As their matter is already gone would they not then remain for the next phase transition in a new universe?

[evan_au]

As (black hole) matter is already gone

It is true that black holes radiate a percentage of their mass/energy as gravitational waves when they collide.
In the first discovery of black holes, two black holes with a total mass 65 times greater than the Sun radiated about 3 solar masses as gravitational waves, ie around 5% of their mass is gone.

But most of their mass still exists - 95% is just crunched into the singularity at the center.
See: https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves

would (black holes) not then remain for the next phase transition in a new universe?

This is getting into more speculative areas, but some theoreticians think that the interior of a black hole could in fact be another universe, or the opening into another big bang.

Indeed, according to this theory, we could be inside a black hole now.
See: https://en.wikipedia.org/wiki/Black-hole_cosmology

[GoC]

As their matter is already gone would they not then remain for the next phase transition in a new universe?

If there matter was gone in a BH they would not have gravity. The distance an electron travels in normal matter is about one football field to the proton being a marble. So normal matter is mostly space. In a BH mass is marbles right next to each other for density affecting the fabric of energy c. BH's are completely kinetic with no motion in the electrons we can measure as time. It might be radiation is separate from mass with kinetic energy just being a created wave and not a loss of mass. Your model limits your understanding and while your current understanding might be correct thee is always the chance it might not be.

[evan_au]

In a BH mass is marbles right next to each other

I think you are imagining a situation where matter has similar density to the nucleus of an atom.

Such a thing does exist - it is called a neutron star
- They have been known since the 1960s as pulsars
- in 2017, the merger of 2 neutron stars was detected by the emitted gravitational waves, and also the cloud of heavy radioactive elements sprayed into space.

In a neutron star up to around 2 times the mass of the Sun, the strong nuclear force is sufficient to keep the neutrons and protons apart.
- However, beyond this mass, it is thought that the gravitational force will crush it to such a high density that the escape velocity exceeds the speed of light
- And all matter is crushed into a singularity at the center
- We call this a black hole, and it is distinctly different from a neutron star.

See: https://en.wikipedia.org/wiki/Neutron_star

with no motion in the electrons we can measure as time

It is a useful simplification to imagine that electrons are like little planets orbiting a star (the nucleus).

It is an oversimplification to think that you could measure time by the rotation of these electrons
- just like we measure years by the orbit of the Earth around the Sun
- In fact, electrons are smeared out around the nucleus - you can never identify exactly where the electron is, and how fast it is moving
- All you can hope is to determine the probability that it will be in some particular location

To make it even more complex, in the supernova which forms a neutron star, the density is so high that (most of) the electrons are crushed together with protons, forming a neutron (and releasing a neutrino). The electrons have disappeared, apart from a very thin atmosphere of "normal" matter around the outside.

See: https://en.wikipedia.org/wiki/Neutron_star#Structure