For example how did we know or come to assume that the Higgs field formed a trillionth of a second after it happened?
It all seems so specific and I think most topics in physics give unfulfilling answers and we’re just supposed to go with it. Thank you and sorry if I sound like an ignorant fool
In: 1
We measure the temperature of the universe today (see COBE) to be 2.73 degrees above absolute zero. If you compress a gas, it gets hotter – for light, if I shrink a box by a factor of 2, the temperature of the light will go up by a factor of 2. It’s also the case that as I move forward in time, the universe is *unstable*, in the sense that if I change the density of the universe now by a little bit, I change the density of the future universe by a lot. If I look backwards in time, though, that means that large changes in density now correspond to very tiny changes in density in the past.
These two facts combined mean that we can say very accurately what the temperature of the universe was as you go backwards in time. We also have a lot of checks on this – not only the cosmic microwave background (when the universe was about 1/1000th its current size, about 300,000 years after the big bang), but also the first few minutes of the big bang. If you take the temperature and density we see today and predict what would happen, it turns out in the first few minutes of the universe you create most of the helium, deuterium, and helium-3 in the universe. The amount of each you create depends sensitively on the temperature and the density of the universe. What we actually see is that the amounts helium/deuterium etc. agree almost perfectly with what we would have predicted based on the universe we see today. So, we have direct evidence that extrapolating back to just a few minutes after the big bang works just fine.
Based on that, if you believe the laws of physics we have today, then you can continue back in time, because the universe looks just like stuff we have seen in particle accelerators. There actually almost certainly is a time where this all breaks down, but that’s because the laws of physics have to change at very high temperature/density. In particular, we don’t have a quantum mechanical theory of gravity, so when we get to the regime where quantum gravity has to become important, we know our models will break down. They probably break down somewhat later (we don’t understand why there’s more matter than antimatter, for instance), but as long as we use the laws of physics as we currently understand them in the regime where they’re still valid, we can extrapolate backwards quite a long ways in time. I don’t think anyone is nervous about saying what happened a trillionth of a second after the big bang, but I wouldn’t place any bets on what was going on a trillionth of a trillionth of a trillionth of a second.
Latest Answers