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The entropy of the observable universe is mainly in supermassive black holes.

Next comes star-sized black holes.

Then comes the cosmic background radiation: photons, neutrinos and gravitons.

Everything else is a tiny correction - probably even dark matter, if it exists.

(1/n)

These estimates are rough! Egan and Lineweaver's estimate is 100 times that of this earlier paper, because they assume more supermassive black holes: black holes at least 10 million times heavier than the Sun, living at the centers of galaxies.

(3/n)

For more fun facts about entropy and information, check out this page of mine:

https://math.ucr.edu/home/baez/information.html

For example: a 1-gram black hole would have entropy equal to 4.78 gigabytes!

(4/n, n = 4)

as a comparison:

a mini-SD (memory) card is about 2 g. How many GB of memory in your mini-SD card in your phone?

@johncarlosbaez C'mon, what's a 1023 error between friends?

It's not too late to consider an accounting career!

@johncarlosbaez

what's an order of magnitude, or 23, among friends?

John Carlos Baez@johncarlosbaez@mathstodon.xyzPhotons and neutrinos beat gravitons, because gravitons decoupled from other matter earlier and have had more time to cool down!

If you want to see how these estimates are done, see Chas Egan and Charles Lineweaver's paper:

(2/n)

https://arxiv.org/abs/0909.3983