As sound is made up of wave-like changes in density, it's impossible to hear anything in a true vacuum. Even so, the early Universe wasn't a vacuum, being filled instead with hydrogen and ionised gas. The resulting sound of the Big Bang would be far too low-frequency to hear directly, but using real cosmic data and boosting the frequency, cosmologist Professor John Cramer of the University of Washington has simulated the sound, which can be heard here.
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