Can Supermassive Black Holes Really Exist and What Evidence Supports Them?

[Ultrastar 1]

One topic that interests me is supermassive black holes. Do you think they can exist? Also, have any been reported?

 

[Chronos]

A supermassive black hole happens to reside in the center of our galaxy. Much larger ones have been detected in neighboring galaxies. It is currently believed these monsters reside at the core of nearly every galaxy. Their origins are a mystery. See, for example
http://www.dailygalaxy.com/my_weblog/2009/08/monster-of-the-milky-way-is-a-supermassive-black-hole-at-our-galaxys-core.html

 

[Matterwave]

One speculative origin of SMBHs is that they formed from the merger of many smaller black holes over a long period of time.

 

[Lsos]

Supermassive black holes have been shown to exist at the center of galxies. Whether they're there or not has already been pretty much answered...

 

[Jonathan Scott]

Supermassive objects have been shown to exist in the cores of galaxies. However, as far as I know, there is currently no experimental evidence which allows us to determine for certain whether those objects are actually the weird "black hole" objects, involving event horizons and singularities, which arise from pushing GR theory to its limits.

These objects as currently observed are of course far from black, in that they emit huge amounts of electromagnetic radiation and are some of the most luminous objects in the universe, but black hole theory explains that this emission is coming from accretion disks around the hole rather than the object itself, and the nature of a large part of these emissions is consistent with this hypothesis. However, accretion disks could also be present if the central object was not in fact a black hole.

There is also some evidence suggesting that such objects have strong intrinsic magnetic fields, which is not consistent with the currently predicted properties of a black hole.

 

[Ultrastar 1]

One speculative origin of SMBHs is that they formed from the merger of many smaller black holes over a long period of time.

I tought that was impossible. I thought that if two black holes came within proximity of each other's acression disks, they would be destroyed or just reppled from each other. Does anyone know what would happen if two black holes got too close together? Is black hole merging even possible?

 

[Nabeshin]

Supermassive objects have been shown to exist in the cores of galaxies. However, as far as I know, there is currently no experimental evidence which allows us to determine for certain whether those objects are actually the weird "black hole" objects, involving event horizons and singularities, which arise from pushing GR theory to its limits.

These objects as currently observed are of course far from black, in that they emit huge amounts of electromagnetic radiation and are some of the most luminous objects in the universe, but black hole theory explains that this emission is coming from accretion disks around the hole rather than the object itself, and the nature of a large part of these emissions is consistent with this hypothesis. However, accretion disks could also be present if the central object was not in fact a black hole.

There is also some evidence suggesting that such objects have strong intrinsic magnetic fields, which is not consistent with the currently predicted properties of a black hole.

I was under the impression we were quite certain they were indeed black holes, for no amount of matter could be compressed in such a small volume to form anything but a black hole.

 

[Matterwave]

The periapsis of the stars around the SMBH gives us an upper limit to the radius of the object. I think, but I'm not 100% sure, that this radius is within the Schwarzschild radius.

 

[Jonathan Scott]

I was under the impression we were quite certain they were indeed black holes, for no amount of matter could be compressed in such a small volume to form anything but a black hole.

That only applies if the standard extrapolation of GR to this situation is correct, and so far we have no direct evidence of that. There are plans to try to observe the black hole candidate at the core of the Milky Way over the next few years in sufficient resolution (using VLBI) to see refraction rings as predicted near the event horizon. However, I'm not sure that even that would rule out alternate explanations which allow an extremely dense body without collapse.

In GR, the collapse occurs because of a factor in the metric which becomes negative. If this factor merely tends towards zero, then it would be possible for a supermassive object to be located in a very small amount of space as seen by an external observer without having to be unphysically dense as seen locally. (Even in GR, if you treat the Schwarzschild radius like a "real" physical radius and use it to calculate a "volume", then the density of a supermassive black hole is much lower than that of a stellar-mass one).

 

[Jonathan Scott]

The periapsis of the stars around the SMBH gives us an upper limit to the radius of the object. I think, but I'm not 100% sure, that this radius is within the Schwarzschild radius.

Surely if it were that would mean the star had fallen into the black hole?

 

[Matterwave]

ah, you are correct. I must remember wrong...sorry

 

[russ_watters]

These objects as currently observed are of course far from black, in that they emit huge amounts of electromagnetic radiation and are some of the most luminous objects in the universe, but black hole theory explains that this emission is coming from accretion disks around the hole rather than the object itself, and the nature of a large part of these emissions is consistent with this hypothesis. However, accretion disks could also be present if the central object was not in fact a black hole. [emphasis added]

Is there actually a competing theory that describes the emissions without the central object being a black hole? Or is this just a quibble about the definition of "black hole"? We've had that discussion before...

 

[Jonathan Scott]

Is there actually a competing theory that describes the emissions without the central object being a black hole? Or is this just a quibble about the definition of "black hole"? We've had that discussion before...

The accretion disk mechanism is basically independent of whether the central object is gravitationally collapsed, as it is based on a lower order of accuracy of GR theory. There are of course some differences, in that for example if gravitational collapse did not occur so there is something emitting radiation from a "surface", that could obviously have effects on the accretion disk (for example through radiation pressure) and would be expected to contribute additional components in the spectrum, and it is also possible that the central object could have a very significant magnetic field (whereas a black hole cannot according to standard theory).

If there were a surface, then even if it were rotating near break-up speed, the combined effect of the time dilation at the surface due to gravitational potential and rotational velocity can still be a less than the time dilation due to these effects for the lowest orbits above the surface, where the velocity is higher. This could in some cases mean that the accretion disk region would be at a higher redshift than the surface, so one might see evidence of (surface) emissions at a slightly lower redshift being partially absorbed by material (in the accretion disk) at a higher redshift. I believe that features of this nature have been observed in some quasar spectra, although there are other possible explanations.