B Spotting a Black Hole in the Microwave Background

[tionis]

If we were to take a microwave picture of a region of space said to have a black hole, would we be able to spot the black hole? Would we see a cold spot surrounded by a more hotter background?

 

[Nugatory]

A black hole in otherwise near-empty space so we won't see extraneous (and very obvious) phenomena like accretion disks of incandescent gas?

That ideal black hole is a black hole no matter what wavelength we're looking at. Even if you're considering that black hole will be radiating Hawking radiation, the effective temperature of the event horizon of a stellar-mass black hole is only a tiny fraction of a degree above absolute zero, and far colder than the cosmic background radiation around it.

 

[Vanadium 50]

If the cold spot were big enough to see, you're probably too close.

 

[tionis]

A black hole in otherwise near-empty space so we won't see extraneous (and very obvious) phenomena like accretion disks of incandescent gas?.

Yes, that is the scenario. So, a sat is mapping an empty region of space in microwaves, and it sees no significant temperature fluctuations except where the black hole is. Would the satellite capture a drop in the temperature where the BH is even if there are microwaves in the space between the sat and the BH? Would the picture reveal the shape of the BH?

 

[tionis]

If the cold spot were big enough to see, you're probably too close.

Even for a supermassive black hole that is not accreting anything?

 

[Vanadium 50]

If a SMBH had the angular size of the moon, you'd be about a billion miles away. The gravity would be about 20g's, and in a few hours your day is going to get very bad.

 

[tionis]

If a SMBH had the angular size of the moon, you'd be about a billion miles away. The gravity would be about 20g's, and in a few hours your day is going to get very bad.

Wow! I thought supermassive black holes tidal gravity was less destructive because of the size of the BH.

 

[PeterDonis]

I thought supermassive black holes tidal gravity was less destructive

He's not talking about tidal gravity. He's talking about the proper acceleration you would need to maintain in order to "hover" at that altitude above the supermassive BH. You have to hover because at that distance there are no possible free-fall orbits around the hole.

 

[tionis]

He's not talking about tidal gravity. He's talking about the proper acceleration you would need to maintain in order to "hover" at that altitude above the supermassive BH. You have to hover because at that distance there are no possible free-fall orbits around the hole.

Ah, Ok, I got it now. So, Peter, If the sat were to hover at that distance where the BH would look like the moon like Vanadium said, would it be able to capture a noticeable cool, round spot in the microwave background?

 

[PeterDonis]

If the sat were to hover at that distance where the BH would look like the moon like Vanadium said, would it be able to capture a noticeable cool, round spot in the microwave background?

If it were looking directly towards the hole, yes, I believe so.

 

[tionis]

If it were looking directly towards the hole, yes, I believe so.

It just occurred to me that micowaves near the black hole would shift to the blue end of the spectrum, no? Wouldn't that make the BH hotter than the background?

 

[PeterDonis]

micowaves near the black hole would shift to the blue end of the spectrum, no?

As seen by an observer very far away, they would blueshift as they got close to the hole, but they would redshift again as they came back out. The net effect would be zero.

As seen by an observer close to the hole, such as you proposed (close enough for the hole to have the same angular size as the moon), there would be some net blueshift of the CMB radiation as it came to you around the sides of the hole. But it wouldn't be that much; and there would still be a huge cold spot right in front of you, since no microwaves would be coming from the hole itself.

 

[Khashishi]

Is it possible to see an Einstein ring in the CMB?