How can the universe be 13.7bln years old if time depends on your frame of reference? Could others “think” it’s older or younger?
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The age of 13.7b years is for a specific reference frame. If you imagine the expanding universe as an expanding grid, it is a reference frame that lies on one of the grid lines. In other words, it moves perfectly with the expansion of the universe, without any other additional velocity. Such a reference frame is called a co-moving frame.
In a reference frame that has an additional velocity, the universe will appear younger. The Earth actually has a small velocity relative to the co-moving frame of around 370 km/s, but the resulting age difference is smaller than the measurement error on the 13.7 b years.
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