#cosmic time

Relativity took away the master clock.

Before Einstein, it was natural to imagine time as an invisible current flowing uniformly through the universe. Clocks might run badly, people might lose track of the hour, and trains might fail to respect the timetable, but beneath these local disappointments there was supposedly one true time. If two events happened simultaneously, they happened simultaneously for everyone.

Relativity tells us otherwise. Observers moving relative to one another can disagree about which distant events happened at the same time. Clocks following different paths through spacetime can accumulate different durations. Gravity changes the rate at which clocks tick. There is no clock mounted somewhere above the universe, safely outside the proceedings, displaying the correct cosmic hour.

And then cosmologists say that the universe is about 13.8 billion years old.

This sounds suspiciously like the master clock has been smuggled back into the building.

Whose 13.8 billion years are these? A clock on Earth? A clock drifting between galaxies? A clock moving at nearly the speed of light? There is no inertial reference frame for the entire expanding universe, so where does this age have any truth?

The short answer is that the age of the universe is not the reading on an arbitrary observer's clock. It is an amount of cosmic time: the proper time measured by an ideal family of observers who move with the average expansion of the universe.