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

The question is better answered by a description of how it actually works. A more technical 5 min read at the top, simpler analogy at the bottom.

The context that originally suggested dark energy is essentially the discovery that the expansion of the universe was accelerating (which is a claim that’s recently being somewhat contested). At the time, when Einstein formulated general relativity, he made further discoveries and hypotheses based on the model that he came up with, many of whichj were verified. In the mathematics, for it to be fully correct there was a number that shows up. To account for it, many people reasoned intuitively that the number should be 0. Later on in the 90s, when it was discovered that some data showed the expansion of the universe was accelerating, the theorists realized that this cannot be true if that number were really 0. This number could be a lot less simple than just a number, and in the equations it represented what people called “dark energy”. As for your question, it’s not really about “creating new space” or abstract notions, it’s simply that there was a variable that we thought was very simple (and equal to 0 so no contribution) but actually fairly complex and today we can’t really account for.

The slightly more metaphorical explanation is basically, say you threw a ball into the air, eventually, it falls back down. After the big bang, stars, like the ball and the earth, flew into opposite directions. We later discovered that not only did the stars never fall back down, it kept going faster and faster than it had before. If this happened in your room, you might expect something was wrong. The ball falls *away* from the ground, like a sort of anti-gravity. Einstein’s equations allows for some *thing* that counteracts gravity, ie the expansion of space. This expansion, however, requires energy, since lifting two massive objects away from the other requires energy, so we call this “dark energy”