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

We call it dark energy, but it’s really just a name for something that we know absolutely nothing about. It’s just the name we give to whatever is causing the accelerating expansion of the universe. We have no idea what it is or how it works.

Also, it’s important to note that conservation of energy is not applicable here. Conservation of energy relies on the symmetry of your system under time translation (see Noether’s Theorem). In a system that is not time translation invariant, e.g., an expanding universe, energy doesn’t have to be conserved.

Einstein discovered it by accident, and in fact, was initially wrong, because he believed the universe could not be expanding, so he came up with the cosmological constant to achieve a static universe. After Hubble proved the universe was expanding, Einstein came to acknowledge his error and admitted it was the greatest mistake he ever made, however, even in being wrong, he was was right, because it turns out that the cosmological constant almost perfectly describes the expansion of the universe under dark energy.