The idea of curved space-time is so abstract that it’s hard to describe in a single video. A number of different approaches exist, so it’s worth finding some more. The explanations can seem quite different; none of them (including ours) can be perfect representations of the underlying mathematics. Here’s another one that we like.

Experimental checks of Einstein’s theories were critical to their acceptance. Two early successes were that (1) Einstein’s theory overcame a long-standing difficulty with understanding the orbit of Mercury and (2) his theory predicted precisely how light would be deflected by massive objects, which was later confirmed. More information can be found on this Wikipedia page.

These days, we continue to put Einstein’s ideas to the test. We discussed attempts to find gravitational waves in the animation. But there have been other experiments that have tested exactly how gravity operates. For instance Gravity Probe B successfully verified a subtle consequence of the Earth’s spin on space-time, an effect known as "frame dragging." The next spacecraft to pitch Einstein against the real Universe is known as LISA Pathfinder.

At the end of the video, we introduce black holes – regions of space where gravity doesn’t let anything escape, even light. These can be constructed even in Newton’s theory of gravity; the original idea is attributed to John Michell in 1783, more than a hundred years before Einstein. But when people started studying Einstein’s equations for gravity, a great mathematical interest arose in understanding black holes as regions of very large space-time curvature.

Early pioneers, including Einstein himself, studied black holes as theoretical objects, hardly believing such things could exist in the real universe. These days we have very strong evidence that black holes are actually out there in huge abundance in the real universe. Stars can collapse into black holes at the end of their lives. We also think there are "supermassive" black holes, each with the mass of millions of stars, sitting within many galaxies– including our own. For example, images taken of the stars right at the center of our Milky Way show them orbiting around an invisible, very massive object which is almost certainly a black hole.

So these objects exist, and yet Einstein’s theory does not fully describe them. Therefore, there is no doubt that – despite being spectacularly good at describing and predicting reality – Einstein’s theory is not the final word on space-time or gravity.

To find out more about the accelerated expansion, you could start on the Nobel Prize website.

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