Home SCIENCE Ask Ethan: Are singularities physically real? | by Ethan Siegel | Starts With A Bang! | Oct, 2023

Ask Ethan: Are singularities physically real? | by Ethan Siegel | Starts With A Bang! | Oct, 2023


During cosmological inflation, the space contained in the inflationary region grows exponentially, doubling in all three dimensions with each tiny fraction-of-a-second that passes. Where inflation ends, a hot Big Bang ensues. But due to quantum effects, each region where a Big Bang occurs will be surrounded by more inflating, exponentially expanding space, ensuring that no two regions where hot Big Bang occur ever collide, intersect, or overlap. (Credit: Kavli IMPU)

From the Big Bang to black holes, singularities are hard to avoid. The math definitely predicts them, but are they truly, physically real?

One of the most important advances in all of physics was the development of Einstein’s General Relativity: our greatest and most predictively powerful theory of gravity. Replacing the idea of a “gravitational force” that acts on objects that never physically touch one another with the notion that all objects exist within the fabric of spacetime, and that the curvature of spacetime determines how those objects will move, is a concept that many — even professionals — still struggle to wrap their heads around. However, it comes along with consequences: certain configurations of matter-and-energy within spacetime inevitably lead to a condition that marks an effective “end” or “beginning” to spacetime itself, more commonly known as a singularity.

But are these singularities necessarily physically real, representing something profound that’s occurring within the Universe? Or might there be some way to avoid them, perhaps signaling a very different scenario than space and time themselves ceasing to exist? (At least, as we understand them.) That’s what Patreon supporter Cameron Sowards wants to know, as he writes in to ask:

“Why do we believe that the pre big bang state was not a singularity when it is a much higher concentration of energy than a black hole could possibly have… since the pre big bang universe was not a singularity, could the same mechanisms that prevented it from being a singularity apply to the interior of black holes?”

There’s a tremendous amount to unpack here, so let’s try and do this question justice!

Once you cross the threshold to form a black hole, everything inside the event horizon crunches down to a singularity that is, at most, one-dimensional. No 3D structures can survive intact. However, one interesting coordinate transformation shows that every point in the interior of this black hole maps 1-to-1 with a point on the outside, raising the mathematically interesting possibility that the interior of each black hole gives rise to a baby universe inside of it, and the possibility that our Universe itself may have arisen from a black hole in a pre-existing universe prior to our own. (Credit: vchalup / Adobe Stock)

The Big Bang and the question of a “first” singularity

If you start with just two basic observations — that the Universe is full of matter and energy, and also, is expanding today — you might think there’s no way out of an initial singularity. Indeed, this was first put together nearly a hundred years ago, all the…

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