[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
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.
When you take a rubber band and stretch it, how is the new rubber created?
Conservation of energy only applies to a closed system. An expanding universe is not a closed system. Further more the energy ‘created’ in the new space isnt the same thing as we typically think of when talking about energy. In the case of dark energy the ‘new’ energy isn’t doing any work, rather it is a property of empty space itself. The energy density never changes, just the absolute amount.
Think of it like a balloon of air. If I bring a cold balloon into a warm room it expands. There aren’t any more nitrogen atoms in the balloon, but they have more energy. This means that the pressure they exert on the membrane requires the balloon’s fabric to stretch to contain it.
Most of space is empty of nitrogen atoms, so that’s not exactly what’s happening. But space is full of quantum field. The field itself is expanding, moving the atoms slight apart. Some atoms are clumped together by gravity into planets, so the planet isn’t expanding because the gravity keeps pressing the atoms together.
However, when things get far enough apart there isn’t a force to clump them. The empty space between galaxies expands, and the galaxies move (because of their momentum).
There’s no easy way to conceptualize but imagine a balloon being blown up – between any two points there’s more distance than there was before – the distance itself is “created” by the balloon material (the rubber); so just like “empty” space is not empty, think of space being the rubber, or material, of space.
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?
Just wanted to touch on everyone’s else’s answers. They’re all attempting to answer your question of how it happens specifically, when we don’t know that. All we know is that *it happens*, and these are the best descriptors and models for that behavior. We have no idea how or why
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”
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