We still don't know how "hot" the hot Big Bang was

"This tremendously successful theory gave us everything from gravitational waves to black holes based on one profound insight: that the fabric of spacetime itself would evolve, curve, and even ripple based on the properties and behavior of the matter and energy within it. When we applied Einstein's equations to the entire Universe as a whole, along with the idea that the Universe was filled nearly uniformly with matter and energy on the largest scales, we wound up with an expanding Universe."

"And yet, if we look at the evidence that persists in the Universe today, left over from those early cosmic stages, we find that the hot Big Bang couldn't have risen all the way up to the hottest possible temperatures: temperatures corresponding to the Planck scale. Those high energies, corresponding to around 10 19 GeV or 10 32 K, would have imprinted signatures in the cosmic microwave background (CMB) that simply aren't present."

"We still don't know how hot the Big Bang was in its hottest, earliest stages, but we do have both upper and lower limits that guide our quest for further knowledge about our cosmic beginnings. The story of the Big Bang began back in the 1920s, when two important parallel developments occurred. On the theoretical side, Alexander Friedmann took Einstein's field equations and considered what would happen if the Universe were uniformly filled with any form of energy at all."

Einstein's General Relativity implies that spacetime evolves, curves, and ripples in response to matter and energy. Applying Einstein's equations to a nearly uniform Universe yields cosmic expansion and extrapolates to a hot, dense early state with all matter and energy concentrated into a tiny volume. Observations of the cosmic microwave background lack signatures expected from Planck-scale temperatures, indicating the Big Bang did not reach those extreme energies. The CMB instead shows evidence consistent with a prior inflationary epoch followed by a comparatively less hot early expansion. Upper and lower observational limits constrain the maximum temperature. Early theoretical work by Friedmann revealed nonstatic cosmological solutions.

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