In the Face Book group:
When was the beginning of time?
In our 4-dimensional universe, XYZ relate to the physical location in 3-D space. Time, the 4th dimension, is not a physical dimension. It is just a 4th coordinate, allowing someone to define the time when something is at a physical location. Mankind has agreed upon a standard measurement for time based on an atomic clock.
This time coordinate needs a zero reference, to define the start of its count.
Similarly the XYZ coordinates need a zero reference to define the start of motion along that axis. (since there is no defined X0,Y0,Z0 in the universe, in cosmology these XYZ positions are in the observer's frame of reference).
From an online reference:
We can model this [big bang] process remarkably closely (at least until the very early nanoseconds or less), and physicists have been able to piece together the major events in the evolution of universe, beginning with the tiniest fractions of a second after the Big Bang:
Planck Epoch (the first 5.39 x 10-44 seconds after the Big Bang) – events (if any) occurring within this time must necessarily remain pure speculation.
Time did not begin with the Big Bang. That is just the event used by cosmologists to set the zero reference for the cosmological time coordinate.
When cosmologists are referring to x nanoseconds after the big bang they must be assuming this time coordinate is increasing at the rate assumed by that atomic clock standard.
For cosmologists clearly 'time' did not start with the big bang; it is just when the time coordinate started counting.
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