What was a second in the early universe? I have read some popsci articles and documentaries about the early universe and they often explain how various features of the universe came about and at what time. For example hydrogen atoms came about after hundreds of thousands of years.
Now, the official SI time unit, the second, is based on the caesium atom's properties. How can we talk about time lengths in seconds or years if there are no atoms yet in the universe? (Or going back further where there aren't even electrons.)
Clearly, there must be a way to do it. Then the next question is: why don't we use those field properties or whatever that existed before atoms to define the SI second? If atoms are not needed to get "the beat of time" in the early universe because something else can provide the tick then we could also use those things today instead of relying on the caesium atom for the definition of the second.
I hope it's clear what I mean. Thanks!
 A: How a unit is chosen to be defined depends in large part on how precisely the unit can be reproduced based on that definition.  Two different atomic clocks built using the best currently possible methods will produce almost exactly the same answer for how long a second is, to within about 1 part in $10^{14}$.  The second is defined in terms of a property of cesium 133 because no one has developed an alternative technology for measuring time that's more precisely reproducible than by using a cesium 133 atomic clock.
We can meaningfully talk about time that occurred before any cesium atoms existed, because the kinds of physical phenomena that occurred prior to the existence of cesium atoms still occur, so the speed at which those phenomena take place can be compared to the second as defined using the cesium atom, using experiments that are performed today.
There is a limit to the extent to which that statement is true, because the conditions that existed very near when the universe began are difficult to reproduce.  Increasingly powerful particle accelerators are able to probe conditions that existed increasingly closer to when the universe began.  But we can talk about time even prior to when the conditions we can reproduce existed, by assuming that at least the laws of physics were the same at earlier times as they are now, so that we can successfully predict how things must have behaved back then based on the equations that describe the laws of physics.
There is a limit to the extent to which that statement is true, too.  At an early enough time, certainly no earlier than the end of the Planck epoch, the equations that we use to describe physics are expected to no longer be accurate, because the conditions that existed in the universe at those earlier times are too different from the conditions that are assumed to exist in order for the equations to hold.
