When we talk about the Universe's first second, what do we really mean?

In writings about the Big Bang, there are discussions of what happened in the first picosecond, billionth of a picosecond, etc., etc. My question is: what is the measure of time used by the writer? Our time as we experience here on Earth? The instantaneous time passage there, which would be influenced by the infinite concentration of mass and energy (a singularity?)? What is the time scale?
This illustration summarises the almost 14-billion-year long history of our Universe. Credit:  ESA – C. Carreau  

This illustration summarises the almost 14-billion-year long history of our Universe. Credit: ESA – C. Carreau 

This is a really fun question, because the answer is that these time points you’re seeing are for time as we experience it here on Earth, where we’re trying to use an objective ruler of time to describe how rapidly things were changing during those early moments of our Universe. All measurements of time are based on what we use here on Earth, where we humans first developed our timekeeping methods. The second is now a unit of measure used for all sorts of things, though pretty rarely in extragalactic astronomy (with a few exciting exceptions like events that trigger gravitational waves) because the distances involved often mean things happen on billion year timescales. 

But when we’re talking about the very beginning times our Universe went through, a lot of things did happen in the first second - the Universe underwent a lot of dramatic changes in that first second. It went from a soup of energy to filled with protons and neutrons in that time - a dramatic change! And when we say this, we really do mean the second that you could watch tick past on a watch. This second comes from taking the speed of our Earth’s rotation, and dividing it into twenty four hours, dividing each hour into sixty minutes, and each minute into sixty seconds. It’s that second, 1/86,400th of an Earth-spin, that we use to describe the initial changes of our Universe.

It’s fun to think that a fluke of angular momentum that gave us (approximately) a 24 hour day also gave us a useful metric for describing the early state of the Universe in precisely the units that we do. 

As time has wound on, we humans have sought to make our units of measure ever more precise. To do this, we often wind up redefining our units in terms of something more fundamental than where we had begun. The meter was redefined to be the distance that light travels in 1/299,792,458th of a second instead of “one ten-millionth of the distance from the equator to the North Pole”, and the kilogram was recently redefined to be a function of Planck’s constant, instead of a very specific, carefully guarded, lump of metal in a vault in Paris. The second has also undergone this transformation. 

Bell jar display of prototype kilogram replica,  public domain via National Institute of Standards and Technology  

Bell jar display of prototype kilogram replica, public domain via National Institute of Standards and Technology 

As we measured the Earth’s rotation to higher and higher precision, we encountered the need for leap seconds to account for the fact that our Earth’s rotation is intrinsically slowing by a tiny, but measurable amount.  Instead of using the Earth’s rotation speed, then, a more fundamental, reliably measurable feature of our Universe was adopted as the official definition of a second - the length of time it takes a cesium atom to vibrate between two hyperfine states 9,192,631,770 times. While this may seem like a much more complex unit of time, it’s actually a better definition in that anyone, anywhere in the universe, should be able to measure this unit of time consistently. 

During this redefinition of the second, the length of a second wasn’t changed, but now we have a more persistent method of measuring it. So that first nanosecond (10^-9) of the Universe is the same length of time it takes a cesium atom – in a vacuum, at absolute zero – to vibrate 9 times.


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