How small was the Universe when the hot Big Bang began?Ethan Siegel
24 December 2021
Big Think...No matter how tempting it may be to think that the Universe arose from a singular point of infinite temperature and density, and that all of space and time emerged from that starting point, we cannot responsibly make that extrapolation and still be consistent with the observations that we’ve made. We can only run the clock back a certain, finite amount until the story changes, with today’s observable Universe — and all the matter and energy within it — allowed to be no smaller than the wingspan of a typical human teenager. Any smaller than that, and we’d see fluctuations in the Big Bang’s leftover glow that simply aren’t there.
Before the hot Big Bang, our Universe was dominated by energy inherent to space, or to the field that drives cosmic inflation, and we have no idea how long inflation lasted for or what set up and caused it, if anything. By its very nature, inflation wipes our Universe clean of any information that came before it, imprinting only the signals from inflation’s final fractions-of-a-second onto our observable Universe today. To some, that’s a bug, demanding an explanation all its own. But to others, this is a feature that highlights the fundamental limits of not only what’s known, but what’s knowable. Listening to the Universe, and what it tells us about itself, is in many ways the most humbling experience of all.
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