Eli5: How did we map the timeline of the big bang?


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

Mathematic extrapolation and educated guesswork. We essentially ran a simulation of the universe and ran that simulation backwards and found that it all seems to come from one extremely small, dense point that exploded.


The reason it’s hard to give a fulfilling answer is because the current picture is based on decades of work and numerous sources of evidence. The basic idea of the Big Bang comes from observations that distant stars and galaxies are all moving away from us and each other. This led people to investigate, based on a mixture of theory and experiment, what a universe filled with extremely hot, dense matter would be like. The most important prediction was that there would be an intense burst of radiation at the point when the universe had expanded and therefore cooled enough that atoms could form. This was detected exactly as predicted, in the form of the cosmic microwave background, which has since been studied in great detail and essentially gives a snapshot of the universe at that point. Over time, more details have been added to the theory based on that and other sources of evidence, such as particle physics experiments and observations of the relative abundance of various isotopes and particles. I don’t know about the Higgs field in particular as I know very little about particle physics.

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.