Andrew Strominger

And so astronomers think that on large scales, even though for thousands of years we hadn't noticed it, beginning with Hubble, we started to notice that space-time was curved.

Space is expanding in time means that space-time is curved.

And the nature of this curvature is affected by the matter in it.

because matter itself causes the curvature of space-time.

But as it expands, the matter gets more and more diluted.

And one might ask, when it's all diluted away, is space-time still curved?

And astronomers believe they've done precise enough measurements to determine this.

And they believe that the answer is yes.

The universe is now expanding.

Eventually, all the matter in it will be expanded away, but it will continue to expand because, well, they would call it the dark energy.

Einstein would call it a cosmological constant.

In any case, in the far future, matter will be expanded away and we'll be left with empty desider space.

Dark energy is synonymous with positive cosmological constant.

And...

we think it's there because astronomers have told us it's there, and they know what they're doing.

It's a really, really hard measurement, but they really know what they're doing, and we have no friggin' idea why it's there.

Another big mystery.

Another reason it's fun to be a physicist.

And if it is there, why should it be so small?

Why should there be so little?