**Ask a monkey a physics question thread**

[MadMojoMonkey]

Do I understand correctly that the Universe is expanding in all directions?

Yes.
It is also interesting to note that it is accelerating in the expansion, which is, as of yet, not explained by physics.

If so, how do we reconcile that with the existence of super massive black holes?

These 2 things are unrelated.
A) SMBH's were formed in the very early universe. For some unknown reason, the cosmic soup that was the big bang was not homogeneous in its density distribution. The variations in density gave rise to the coalescence of matter into some regions and not so much others. Black holes have been present since basically forever.

B) The universal expansion happens at all points in space. The space in between the black holes is expanding. If there is enough distance between a pair of black holes, then the universal acceleration driving them apart is greater than the gravitational acceleration pulling them together.

The expansion of the universe happens over vast distance scales. The Milky Way, our Local Group of galaxies and the greater supercluster of galaxies in which the Milky Way exists are more strongly gravitationally bound than the current rate of acceleration of the universe will affect.

In this respect, the SMBH's are counteracting the universal acceleration within this domain.

The milky way is getting sucked into one right now, is it not?

It is not. The galaxy is in orbit about Sagittarius A* (the SMBH in the Milky Way), much the same that the planets are in orbit about the sun.

Black holes are a gravity source like any other unless you're "near" the event horizon.

Left alone, black holes are theorized to slowly evaporate in a process known as Hawking Radiation, named after Stephen Hawking, who discovered the physical-mathematical framework which implies this.

Black holes only suck things up when those things get flung into one of their event horizons. The swirling mass of stars that form the Milky Way occasionally leads to near collisions of stars. When stars pass too close to each other, there is a transfer of gravitational energy. This can leave one of the objects with an orbital velocity that is too slow to avoid the galactic nucleus. If so, that object is heading toward the black hole and will get consumed by it.

Merely getting flung "close" to the black hole may be enough, since most black holes have an accretion disk, or a disk of swirling, in-falling material. If a body enters that stuff, there is plenty of friction there to further reduce its orbital velocity and send it into the black hole... well... it'll likely get torn to bits and remain in the accretion disk for quite a while as it is consumed.

Eventually this galaxy, and all the galaxies around us will be sucked into this thing , correct?

ALL the galaxies? No. (see above)
All the galaxies in our supercluster of galaxies? ... maybe... probably, even... given obscenely long time scales.

Well... either sucked into a black hole or ejected into inter-galactic space... perhaps to be captured by another galaxy, but the odds are against it. The amount of "empty space" out there is truly a rude amount to think about.

So how can the universe be expanding if there are black holes floating out there condensing entire galaxies into super dense masses?

Both of these processes are happening simultaneously. On "local" scales, gravitational sources win out and bind systems together. On the grandest length scales, though, the space between the isolated pockets of stuff is expanding, driving those pockets ever-further apart.

[BananaStand]

It is not. The galaxy is in orbit about Sagittarius A* (the SMBH in the Milky Way), much the same that the planets are in orbit about the sun.

All the galaxies in our supercluster of galaxies? ... maybe... probably, even... given obscenely long time scales.

Make up your mind dude. Is the milky way circling the drain or not.

[MadMojoMonkey]

Make up your mind dude. Is the milky way circling the drain or not.

Both. (see above)

[BananaStand]

Both. (see above)

No. Does not compute. Either the milky way is on its way into that black hole, or it isn't.

Assuming that it is, at some point, going to be consumed by the black hole, then wouldn't that make the black hole bigger, denser, and with a stronger gravitational pull? In other words, it will be able to consume things from further away. And so on, and so on.

So what's stopping these black holes from eventually consuming the entire universe?

[CoccoBill]

No. Does not compute. Either the milky way is on its way into that black hole, or it isn't.

Even though your car is probably heading to the junkyard some day, it isn't heading there now.

[BananaStand]

Even though your car is probably heading to the junkyard some day, it isn't heading there now.

Not an apt analogy. Not even close.

[CoccoBill]

Not an apt analogy. Not even close.

Nah it was great, as car analogies always are.

[MadMojoMonkey]

No. Does not compute. Either the milky way is on its way into that black hole, or it isn't.

Black holes don't "suck things up" any differently than any other massive body. I blame Hollywood for this one.

If something is in orbit, it will remain in orbit unless / until an outside influence affects that relation.
The pull exerted by gravity doesn't necessarily pull things into collision courses.

A galaxy isn't a rigid object. It's more like a gas, if we think of the stars as particles.

In order for something which is in orbit about the black hole to alter its course, it must change its energy.
Conservation of energy states that in order to change its energy, it must cause an equal and opposite change in energy to something else (the system around it).
If it is not interacting with anything nearby (the prevalent case for most stars not near the galactic nucleus), then it cannot change its energy, and the black hole can't "suck it in."
In order for something to be "sucked in" it has to deliver some of its energy to another body or bodies in the system.
So in order for something to be sent on a "sucked up" collision course, something else had to be sent the other direction.

So your question is incorrect. It assumes that ALL of the Milky Way will be consumed by the black hole or NONE of it. The actual answer is in between. Some of the Milky way will end up as Sagittarius A*, some of it will be flung into intergalactic space.

Assuming that it is, at some point, going to be consumed by the black hole, then wouldn't that make the black hole bigger, denser, and with a stronger gravitational pull? In other words, it will be able to consume things from further away. And so on, and so on.

As black holes consume matter, they grow in size and the gravitational effect on spacetime increases.

Yes, but this rate of expansion is slow, because most of space does not contain anything to consume... hence the name, I suppose.

So what's stopping these black holes from eventually consuming the entire universe?

Some of the stuff in the universe is moving away from the black holes at a rate faster than their escape velocity. The growing gravitational influence of the black holes is not fast enough to counter this.

When we look at the longest time scales and play the guessing game about what will happen, there is an extreme time scale at which all the stars have burned out, all the stellar remnants have burned out, and the only things left are black holes and a smattering of hydrogen atoms in the diffuse "vacuum" of space.

Eventually, even the black holes evaporate and the universe is left a diffuse gas of particles, each separated by their closest neighbor by distances too vast to overcome their relative velocities to ever stop them and pull them back into a bound state.

[BananaStand]

None of that sounds right.

Some of the stuff in the universe is moving away from the black holes .....

OR...they're moving toward other black holes that you don't know about.

I think it's pretty obvious that everything in the universe is getting pulled into black holes, and eventually they will start to converge, and coalesce until the entire universe is compressed into a singularity.

Then the big bang will happen again.

Just need to reconcile that with this whole "expanding universe" narrative. Maybe our observation point (earth) is being moved away from stuff so it just looks like the universe is expanding. I wonder if that has anything to do with the massive black hole sucking us in right now.

[MadMojoMonkey]

None of that sounds right.

Irrelevant.

QM sounds like loony talk. GR sounds like loony talk.
This is not relevant so long as these models produce accurate predictions of observable phenomena.

OR...they're moving toward other black holes that you don't know about.

Which either means
A) we already discussed that in the prior discussion and that object is NOT unbound, as it merely swapped its binding state from one SMBH to another.
or
B) the other black hole it's moving toward is receding away from it fast enough that they will never be bound to each other

I think it's pretty obvious that everything in the universe is getting pulled into black holes, and eventually they will start to converge, and coalesce until the entire universe is compressed into a singularity.

I'd love to see the evidence which makes this so obvious to you.
I can't wait to read your publication which clearly shows that decades of data have been misinterpreted.

Then the big bang will happen again.

Our current data suggests that the dark energy acceleration is prevalent over gravitational acceleration over the longest length scales.

Just need to reconcile that with this whole "expanding universe" narrative. Maybe our observation point (earth) is being moved away from stuff so it just looks like the universe is expanding. I wonder if that has anything to do with the massive black hole sucking us in right now.

The problem with your assertions is that there is a wealth of evidence which indicates that those hypotheses do not form or fit into a consistent model which makes accurate predictions about observable phenomena.