We now know the big bang theory is (probably) not how the universe began – “The Big Bang still happened a very long time ago, but it wasn’t the beginning we once supposed it to be.”
Extrapolating back to as far as your evidence can take you is a tremendous success for science. The physics that took place during the earliest stages of the hot Big Bang imprinted itself onto the universe, enabling us to test our models, theories, and understanding of the universe from that time. The earliest observable imprint, in fact, is the cosmic neutrino background, whose effects show up in both the cosmic microwave background (the Big Bang’s leftover radiation) and the universe’s large-scale structure. This neutrino background comes to us, remarkably, from just ~1 second into the hot Big Bang.
But extrapolating beyond the limits of your measurable evidence is a dangerous, albeit tempting, game to play. After all, if we can trace the hot Big Bang back some 13.8 billion years, all the way to when the universe was less than 1 second old, what’s the harm in going all the way back just one additional second: to the singularity predicted to exist when the universe was 0 seconds old?
The answer, surprisingly, is that there’s a tremendous amount of harm. The reason this is problematic is because beginning at a singularity — at arbitrarily high temperatures, arbitrarily high densities, and arbitrarily small volumes — will have consequences for our universe that aren’t necessarily supported by observations.
also btw…
- Why is there a ‘crisis’ in cosmology? – “Early universe versus late universe, global versus local, large scale versus small scale — no matter how you slice it, these two perspectives on the universe should agree, but they don’t. We should have a common, agreed-upon value for the Hubble constant, but we don’t.”
- How old is the universe? Our answer keeps getting better. “After thousands of years, we’re getting more consistent age estimates.”
- Can just one equation describe the entire history of the universe? – “As the first Friedmann equation celebrates its 99th anniversary, it remains the one equation to describe our entire universe.”[*]