Using the cosmic background radiation – the “echo of the Big Bang*” – as a random number generation isn't a new idea, but a couple of scientists have run the slide-rule over measurements of the CMB power spectrum and reckon it offers a random number space big enough to beat any current computer.
Not in terms of protecting messages against any current decryption possibility: the CMB's power spectrum offers a key space “too large for the encryption/decryption capacities of present computer systems”.
A straightforward terrestrial radio telescope, this Arxiv paper states, should be good enough to make “astrophysical entropy sources accessible on comparatively modest budgets”.
The Baylor University (in Waco, Texas) researchers, Jeffrey Lee and Gerald Cleaver, also note that even if Eve (attacker) watched the same bit of sky at the same time as Alice, she wouldn't get the same random number, “due to random variations in photon energy at any sky frequency, spurious signals within the detectors, interference from other sources of stellar radio noise, etc.
“Therefore, the digitised CMB power spectrum obtained by Alice is unique and cannot be acquired through “identical” power spectrum observations of the CMB by Eve”.
Apart from the maths by which Lee and Cleaver demonstrate the CMB power spectrum's randomness, another interesting wrinkle in the paper is that they suggest it could meet the requirements of America's Federal Information Processing Standard 140-2.