Best answer:
Space expands everywhere (including inside individual photons of light).
Over small distances, the rate of expansion is extremely small.
However, over long distances, it adds up. Over a distance of one million parsec (1 Mpc = 3.26 million light-years), that rate amounts to approximately 70 km/s. Every second, the...
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Best answer: Space expands everywhere (including inside individual photons of light).
Over small distances, the rate of expansion is extremely small.
However, over long distances, it adds up. Over a distance of one million parsec (1 Mpc = 3.26 million light-years), that rate amounts to approximately 70 km/s. Every second, the amount of space between two objects separated by 1 Mpc increases by 70 km.
The rate of change appears to be the same everywhere. 1 Mpc measured from Earth would expand at the same rate as 1 Mpc measured from Jupiter or from another galaxy.
Now, apply the rate of expansion in reverse. A distance of 1 Mpc of space would decrease by 70 km every second. How long does it take to reach zero? Approximately 13.8 billion years.
It does not matter what length you begin with.
Over a distance of 1 light-year, the rate of expansion is only 2.1 cm per second. If you apply the same approach, how long would it take (in reverse) for that length of 1 light-year to reach zero if you decrease it by 2.1 cm every second? The same 13.8 billion years.
The reason that we say the radius of the Observable Universe** is 13.8 billion light-years is connected. If you apply this rate of change (70 km/s for each Mpc), you find that at a distance of 13.8 billion light-years, the rate of expansion is equal to the speed of light... and it is impossible for us to see anything that would be further out (the light from it would never be able to reach us, since the amount of space left to travel would increase faster than the light can move). Thus, calculating the radius of the Observable Universe comes AFTER we calculate the age. We do not need it for the calculation of the age.
Both the age and the radius can be calculated directly from the rate of expansion of space (plus other things used to apply corrections that give us a more precise value -- we know that the rate does change over time).
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**The Observable Universe is the portion we can perceive. The whole universe seems to be bigger than that, and we do not know its actual size.
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