"When it comes to the physical Universe, the notion of "nothing" may truly be possible only in theory, not in practice. As we see the Universe today, it appears full of stuff: matter, radiation, antimatter, neutrinos, and even dark matter and dark energy, despite the fact that we don't truly know the ultimate, fundamental nature of the latter two. Yet even if you took away every single quantum of energy, somehow removing it from the Universe entirely, you wouldn't be left with an empty Universe."
"It's like the Universe itself doesn't understand our idea of "nothing" at all; if we were to remove all the quanta of energy from our Universe, leaving behind only empty space, we would immediately expect that the Universe would be at absolute zero: with no energetic particles anywhere to be found. Yet that's not the case at all. No matter how "empty" we artificially make the expanding Universe, the fact that it's expanding would still spontaneously and unavoidably generate radiation."
The observable Universe contains matter, radiation, antimatter, neutrinos, dark matter, and dark energy, plus stars, gas, dust, galaxies, clusters, quasars, cosmic rays, and background radiation. Expansion of space unavoidably generates radiation even if all quanta of energy are removed, preventing absolute emptiness or absolute zero. Gravitational waves and unknown dark-sector signals would still be present given sufficient detection capability. The expanding spacetime itself leads to spontaneous production of energetic quanta, ensuring that arbitrarily far into the future or before the hot Big Bang the Universe never becomes truly empty.
Read at Big Think
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