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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Can We Find Out Where The Big Bang Started?

Is there a reason why we can’t extrapolate the expansion of the universe backwards to determine where it all started in the Big Bang? Thanks!
A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Image credit: NASA / WMAP Science Team

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Image credit: NASA / WMAP Science Team

Originally posted at Forbes!

Nope – we can, in fact, trace the universe back to where it all started. Unfortunately, it rapidly gets complicated, because the answer is that it began where you are sitting. And also where I am sitting. And also where everyone else on the planet is sitting. And also at the center of our galaxy, and at the center of every other galaxy.

The idea here is that our current universe is expanding, so the universe must have been smaller in the past. So if you take every point in space that exists now, and trace it backwards, all those points get closer and closer together until they reach a mathematical and physical stopping point – a singularity. A singularity is an infinitesimally small point, which can contain quite a large amount of matter or energy – the centers of black holes are also singularities.

The singularity we reach if we trace back the whole universe must have contained all the energy that now exists in our universe, as either mass or light, or dark matter, or dark energy. But it also contained all the space – so all the points of space that we now see as very widely separated were present within that singularity. So the “where” of the Big Bang is, quite literally, everywhere.

We have quite a lot of evidence pointing us to this idea of a very tiny universe at the very beginning of our universe; one of the more important being the detection of the cosmic microwave background (or CMB for short). This background radiation is called “background” because our universe has a fundamental glow in the microwave that you can’t escape – any other observations you’re making at this wavelength will be in addition to the CMB.

Critically, the CMB is very precisely almost the exact same in every direction that we look, and even though this glow is the oldest light in the universe, and the universe was much, much smaller than it is now, you would still not expect it to be the exact same everywhere — unless the universe had been even smaller previously. The theory of the Big Bang produces this naturally, because in between all space being compressed into the singularity, and the production of the light we see as the CMB, there is predicted to be a period of super fast expansion — inflation. Or, if you’re tracing the universe backwards in time, the universe shrinks dramatically down.

The thing to keep in mind with the Big Bang and the expansion of the universe is that it wasn’t an “explosion” like a detonation here on earth, with a definite center, and the universe spooling outwards into a pre-existing space. The closest you can get while thinking of conventional explosions would be if you managed to really effectively explode a tiny object, and then asked “Where was this piece when the explosion happened?” It was at the center, with all the other scattered pieces. For our universe’s expansion, each of those pieces would have to be markers in space itself. Where did the universe’s big bang happen? It happened where the universe was small, and each fragment of our current universe was there to witness it.

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