[Arnavion]

Well, you can cross the universe in any amount of time. The human lifetime isn't special in that regard. A photon crosses the universe in zero time, and anything with non-zero mass can get arbitrarily close to zero time.

[dredmorbius]

As I've seen this point made in the past, it's that travelling at a constant 1g acceleration the subjective time for an astronaut would be about 45 years in traversing the observable Universe (~14 billion light years).

That is, a comfortable and familiar acceleration gives a human-lifetime-equivalent journey (subjective time).

If you're planning a round-trip, you'll find rather more time has transpired on what used to be Earth.

<https://www.universetoday.com/129086/far-can-travel/>

[saalweachter]

> If you're planning a round-trip, you'll find rather more time has transpired on what used to be Earth.

Or less, depending on the rotation of the universe as a whole.

[ben_w]

Or no return may be possible at any speed/acceleration given the expansion of the universe.

[dredmorbius]

Given that the outbound journey is already technically challenging, that's very nearly quibbling, despite cosmological accuracy.

[heavenlyblue]

That 1g constant acceleration requires practically infinite amounts of energy

[dredmorbius]

True.

However it is also the specific context in which the lifetime-universe-traversal concept emerges.

[denton-scratch]

> A photon crosses the universe in zero time

That is not consistent with my observations.