How Large is the Universe?

The universe has long captivated humanity with its immense scales of distance and time. But how big is it? Where does it end, and what lies beyond its star fields and streams of galaxies extending as far as telescopes can see?

The Universe is all of space and time and their contents, including that of planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire Universe is still unknown, it is theoretically possible to measure the observable universe.

The size of the Universe is somewhat difficult to define. According to the general theory of relativity, some 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.

Distant regions of space are assumed to exist and to be part of reality as much as we are, even though we can never interact with them. The spatial region that we can affect and be affected by is the observable universe. The observable universe depends on the location of the observer. By traveling, an observer can come into contact with a greater region of spacetime than an observer who remains still.

Nevertheless, even the most rapid traveler will not be able to interact with all of space. Typically, the observable universe is taken to mean the portion of the Universe that is observable from our vantage point in the Milky Way.

How big is the universe?

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 91 billion light-years (28×109 pc).

The distance the light from the edge of the observable universe has travelled is very close to the age of the Universe times the speed of light, 13.8 billion light-years (4.2×109 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 is located roughly 2.5 million light years away.

Because we 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 for the total size of the universe, if finite, reach as high as 10^{10^{10^{122}}} megaparsecs, implied by one resolution of the No-Boundary Proposal.