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Size and regions

According to the general theory of relativity, far regions of space may never interact with ours even in the lifetime of the universe due to the finite speed of light and the ongoing expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the universe were to exist forever: space may expand faster than light can traverse it.

The spatial region that can be observed with telescopes is called the observable universe, which depends on the location of the observer. The proper distance —the distance as would be measured at a specific time, including the present—between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs), making the diameter of the observable universe about 93 billion light-years (28 billion parsecs). The distance the light from the edge of the observable universe has traveled is very close to the age of the universe times the speed of light, 13.8 billion light-years (4.2 × 10 pc), but this does not represent the distance at any given time because the edge of the observable universe and the Earth have since moved further apart.

For comparison, the diameter of a typical galaxy is 30,000 light-years (9,198 parsecs), and the typical distance between two neighboring galaxies is 3 million light-years (919.8 kiloparsecs). As an example, the Milky Way is roughly 100,000–180,000 light-years in diameter, and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light-years away.

Because humans cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite. Estimates suggest that the whole universe, if finite, must be more than 250 times larger than a Hubble sphere. Some disputed estimates for the total size of the universe, if finite, reach as high as 10 10 10 122 {\displaystyle 10^{10^{10^{122}}}} megaparsecs, as implied by a suggested resolution of the No-Boundary Proposal.