Rebecca Smethurst aka Dr. Becky is an astrophysicist specializing in
research on Supermassive Black Holes aka SMBHs, arbitrarily defined as
having at least 10^5 solar masses.

The following is a 16-minute Youtube video that describes an
interesting conundrum about SMBHs:

<https://www.youtube.com/watch?v=6tXIoViA_0g>

Short version: There is good evidence that almost all galaxies have
SMBHs near their centers, and their masses are strongly correlated to
the size of their associated galaxies, which suggests they co-evolved.
There is also good evidence that about 10% of galaxies have merged
with at least one other of about the same size in their lifetime, as
Andromeda and Milky Way are expected to do in the next few billion
years. This suggests these merged galaxies have either:

1. multiple SMBHs, or
2. the SMBHs merged into one.

The conundrum is the math implies the SMBH's can't get closer to each
other than about a parsec within the age of the universe. In the
process of orbiting ever closer to each other, the SMBHs should kick
out from the merged galaxies objects that would otherwise reduce their
orbital velocity and allow them to continue to move closer to each
other. Without these objects, the SMBHs can't slow down and so can't
get closer. OTOH merging black holes are the only known way to create
SMBHs.

I have previously posted links which discuss Pulsar Timing Arrays,
which can detect gravitational waves created by merging SMBHs. The
Laser Interferometer Space Antenna aka LISA is also designed to
directly detect the extremely long wavelength gravitational waves of
merging SMBHs. However, this is an extremely complex and expensive
system, and is scheduled to be launched in 2037. Considering how many
times JWST was delayed, I would be surprised if LISA was launched
before 2050.

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On 17/08/2023 12:17, jillery wrote:

Rebecca Smethurst aka Dr. Becky is an astrophysicist specializing in
research on Supermassive Black Holes aka SMBHs, arbitrarily defined as
having at least 10^5 solar masses.

The following is a 16-minute Youtube video that describes an
interesting conundrum about SMBHs:

<https://www.youtube.com/watch?v=6tXIoViA_0g>

Short version: There is good evidence that almost all galaxies have
SMBHs near their centers, and their masses are strongly correlated to
the size of their associated galaxies, which suggests they co-evolved.
There is also good evidence that about 10% of galaxies have merged
with at least one other of about the same size in their lifetime, as Andromeda and Milky Way are expected to do in the next few billion
years. This suggests these merged galaxies have either:

1. multiple SMBHs, or
2. the SMBHs merged into one.

The conundrum is the math implies the SMBH's can't get closer to each
other than about a parsec within the age of the universe. In the
process of orbiting ever closer to each other, the SMBHs should kick
out from the merged galaxies objects that would otherwise reduce their orbital velocity and allow them to continue to move closer to each
other. Without these objects, the SMBHs can't slow down and so can't
get closer. OTOH merging black holes are the only known way to create
SMBHs.

That video says there's a regime where neither dynamical friction with
gas and stars, nor gravitational radiation, is effective at reducing the distance between two SMHBs on a timescale of billions of years.

I thought of a third mechanism. If you have three SMBHs, dynamical
friction between the black holes will kick out the smallest, while
bringing the other two closer together. Dynamical friction with gas and
stars then brings the third SMBH back to the vicinity of the other two.
Rinse and repeat. But this is obviously a process which requires a long timescale, and perhaps it also can't work fast enough for SMBH mergers
to have occurred at the current age of the universe.

I have previously posted links which discuss Pulsar Timing Arrays,
which can detect gravitational waves created by merging SMBHs. The
Laser Interferometer Space Antenna aka LISA is also designed to
directly detect the extremely long wavelength gravitational waves of
merging SMBHs. However, this is an extremely complex and expensive
system, and is scheduled to be launched in 2037. Considering how many
times JWST was delayed, I would be surprised if LISA was launched
before 2050.

--
alias Ernest Major

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On Thu, 17 Aug 2023 14:17:02 +0100, Ernest Major
<{$to$}@meden.demon.co.uk> wrote:

On 17/08/2023 12:17, jillery wrote:

Rebecca Smethurst aka Dr. Becky is an astrophysicist specializing in
research on Supermassive Black Holes aka SMBHs, arbitrarily defined as
having at least 10^5 solar masses.

The following is a 16-minute Youtube video that describes an
interesting conundrum about SMBHs:

<https://www.youtube.com/watch?v=6tXIoViA_0g>

Short version: There is good evidence that almost all galaxies have
SMBHs near their centers, and their masses are strongly correlated to
the size of their associated galaxies, which suggests they co-evolved.
There is also good evidence that about 10% of galaxies have merged
with at least one other of about the same size in their lifetime, as
Andromeda and Milky Way are expected to do in the next few billion
years. This suggests these merged galaxies have either:

1. multiple SMBHs, or
2. the SMBHs merged into one.

The conundrum is the math implies the SMBH's can't get closer to each
other than about a parsec within the age of the universe. In the
process of orbiting ever closer to each other, the SMBHs should kick
out from the merged galaxies objects that would otherwise reduce their
orbital velocity and allow them to continue to move closer to each
other. Without these objects, the SMBHs can't slow down and so can't
get closer. OTOH merging black holes are the only known way to create
SMBHs.

That video says there's a regime where neither dynamical friction with
gas and stars, nor gravitational radiation, is effective at reducing the >distance between two SMHBs on a timescale of billions of years.

I thought of a third mechanism. If you have three SMBHs, dynamical
friction between the black holes will kick out the smallest, while
bringing the other two closer together. Dynamical friction with gas and >stars then brings the third SMBH back to the vicinity of the other two. >Rinse and repeat. But this is obviously a process which requires a long >timescale, and perhaps it also can't work fast enough for SMBH mergers
to have occurred at the current age of the universe.

I have previously posted links which discuss Pulsar Timing Arrays,
which can detect gravitational waves created by merging SMBHs. The
Laser Interferometer Space Antenna aka LISA is also designed to
directly detect the extremely long wavelength gravitational waves of
merging SMBHs. However, this is an extremely complex and expensive
system, and is scheduled to be launched in 2037. Considering how many
times JWST was delayed, I would be surprised if LISA was launched
before 2050.

Good on you to think of that third mechanism independently. However,
the cited video does mention it:
****************************************
@8:32
Now there have been a few studies over the years that have claimed
just that that our maths isn't quite right and the two supermassive
black holes can actually lose enough energy if for example you have
another galaxy merger which brings in a third supermassive black hole
which can interact with the other two and then get slingshotted out
or perhaps that the supermassive black holes will have an interaction
with a gas disc much closer in or by just recycling the stars back in
close towards the two supermassive black holes on more realistic
orbits but there's still a lot of uncertainty there and there's no
General agreement in the scientific literature yet ****************************************

--
To know less than we don't know is the nature of most knowledge

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