How is new space created as the universe expands? Einstein discovered that empty space is not nothing, so what is responsible for new space appearing into existence, when energy and matter cannot?

Question

[Nasa.gov](https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy) explains dark energy like a property of space, and go on to say:

> Albert Einstein was the first person to realize that empty space is not nothing. (…) The first property that Einstein discovered is that it is possible for more space to come into existence.

But they never elaborate on how Einstein knows this. I know this is a field without concrete answers, but I’m curious about Einstein’s explanation.

In: Physics

Answer 1

Observing an expanding universe (even if we can’t yet quite nail down Hubble’s constant), we must conclude that it was once (much?) smaller. We simultaneously observe a “horizon”, apparently resulting from not being able to see beyond the energy that is radiating from the origin (and having to wait for it to be reflected to the observer. What’s beyond that horizon? Are we in a black hole?