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What was the informational content of the universe at the Planck time?

Physics Asked on June 29, 2021

If the energy of $1 , mathrm{bit}$ of information is $k_{small{text{B}}}T ln{2}$, then the energy of that bit increases with the temperature of the system. When I try to calculate how much energy 1 bit of information would have had around the Planck time, when the temperature of the universe was in the ballpark of ${10}^{32},mathrm{K}$ and the diameter was around ${10}^{-33},mathrm{cm}$, then the energy of that $1,mathrm{bit}$ of information is suspiciously close to the Bekenstein bound. In fact, the result I got was $1.3719436998375747 ,$ which I think means that the energy of $1,mathrm{bit}$ of information at planck time was just a bit less than would have been required to collapse the universe into a black hole before it had even begun (perhaps even before inflation could save it from said fate).

Is it just coincidence that the number I happen to get is so close to the Bekenstein bound? Was the Information (or entropy or negentropy or whatever you want to call it) contained in the big bang really THAT low? I mean, I knew it had to be low, but 1 bit seems to be cutting things a bit fine, doesn’t it?

One Answer

Further to Padmanabhan (page 75) I postulate that the information content of the Universe after inflation (Hot Big Bang) was $4π$ (nats not bits).

Realising that the effective dimension of quantum-corrected space-time becomes $D=2$ at close to Planck scales, we can immediately consider the Universe at this time as an event horizon, modelled as a 2-D black (to us, white) hole with $r=2L_p$ i.e. a de Sitter space-time.

So then the beginning, like the end, is a de Sitter space.

If you don't like this, then don't worry too much about what the universe 'really' was at the beginning. Postulate $4π$ was the initial information content, and discover that you can model the whole history of the Universe remarkably well, as Padmanabhan shows (in huge detail).

Answered by Mr Anderson on June 29, 2021

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