Bill Hendry said:
We have set the Earths' Voltage at zero volts but is this 'earth voltage' applicable throughout the Universe. If not, is it relative in the same manner as spacetime and is it calculatable?
For most applications, the choice of which voltage to choose to be zero is a convention rather than a fundamental fact about reality. It's less that the Earth has a voltage of zero, but rather that it has a consistent voltage that we can rely upon. When doing calculations for how most electric systems operate, you can add a constant voltage to every component in the system and its behavior will not change one iota. You could easily say, "The Earth's voltage is 3V," and pretty much nothing would change.
It's differences in voltage that are meaningful (again: most of the time), not the absolute value. If you pick up a 1.5V battery, for instance, that voltage isn't the "voltage value of the battery". Rather, it's the difference in voltage between the + and - terminals. That voltage difference is maintained by the chemical behavior of the materials inside the battery. The absolute voltage of either terminal will be determined by its environment. For example, if you take two 1.5V batteries and place them end-to-end (with + of one battery contacting - of the other), then the voltage differences add, such that the voltage difference from the - of the first battery to the + of the second is now 3V.
This is, in fact, how many 9V batteries are constructed: they're a set of six AAAA batteries with their terminals connected in series.
Caveat:
You may notice that I left open the possibility that some things might depend upon the absolute voltage. It's unfortunately been way too long since my electrodynamics course, but I do recall that the absolute value makes a difference in some contexts. But I also remember very distinctly that this is extremely obscure and not relevant 99.999% of the time.
