Amount of black hole entropy inside the Universe?

[bbbl67]

Now, it's been said that the majority of the entropy in the universe resides within the cumulative entropy of black holes inside the universe. How do they know that?
Now, I'm not so interested in how they determine the black hole's entropy, I know there's a relatively simple formula for that. What I'm curious to know is how do they know what the absolute entropy of the rest of the universe is to compare it to the black holes within it?

 

[Vanadium 50]

Now, it's been said

By whom? Where?

 

[jim mcnamara]

In order to answer we need references please.

 

[Dale]

Please PM me when you have the reference you wish to discuss and I will reopen the thread for you. In the meantime, I have closed it to reduce the number of “we need references” replies
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The OP has a reference and the thread is reopened

 

[bbbl67]

Here's the reference:
Entropy in the Universe | Azimuth
https://johncarlosbaez.wordpress.com/2020/01/25/entropy-in-the-universe/

The entropy in all the photons in the Universe is even more! The Universe is full of radiation left over from the Big Bang. The photons in the observable Universe left over from the Big Bang have a total entropy of about 10⁹⁰ bits. It’s called the ‘cosmic microwave background radiation’.

Black holes have immensely more entropy than anything listed so far. Egan and Lineweaver estimate the entropy of stellar-mass black holes in the observable Universe at 10⁹⁸ bits. This is connected to why black holes are so stable: the Second Law says entropy likes to increase.

 

[PeterDonis]

how do they know what the absolute entropy of the rest of the universe is

Because we know what "the rest of the universe" is made of: ordinary matter and photons. And we know how to calculate the entropy of those things.