Updated: Jul 19, 2021
If you accept that the universe is infinite in size, then it could not possibly have started as a singularity. A finite size can never grow into an infinite one. No matter how much you add to an finite number, you just get another finite number. But a smaller infinity can expand into a larger version version of itself. If the universe is infinite in size, then it must have always been infinite.
I think that the idea of a singularity was popular, because it made it easier to think of something coming from nothing. But what must be explained is not just the creation of a finite something, but rather an infinite everything, including infinite spacetime.
If spacetime is expanding, scientists figured, then they could calculate the rate of expansion, and work backwards, to find out how everything began. They would rewind the movie of our universe’s history, and discover the very moment when all that space and matter was together, in a very small compact ball, which we call the singularity. That moment, T=0, when all space and matter coincided with each other, that was the Big Bang.
Unless it wasn’t. If the universe at T=0 was infinite in space and matter, maybe we don’t need to rewind the movie quite so far. We don’t need to shrink spacetime down to zero, and in fact, we can’t.
Just as we can’t logically start from a finite space, and grow it into an infinity, so too we cannot logically start from zero space, a singularity, because it would have to pass through finite space on its way to infinite space. And that’s impossible. Logically, the universe had to begin, at T=0, as infinite in size. If, as we believe, the universe is infinite now, there’s no other possibility.
And why would a singularity expand at all? The theoretical singularity would have represented the maximal moment of mutual gravitational attraction, because all the mass of the universe was concentrated in a tiny spot. Black holes, our current day notion of a singularity, are infamous for not letting anything out. Why would the biggest, most dense singularity ever spread out into infinity?
The current kluge to cover this problem is just to explain it away. The Big Bang was a special case singularity; quickly expanding singularities are inherently different then black hole singularities, etc. But of course, we don’t have any direct evidence for this, and we have zero other examples to draw upon. It's just a kluge, to make the model work.
So, a more elegant model might have no singularity at all, and would start with the universe at T=0 being formed as spatially infinite - just a much smaller infinity then we have now. This would allow for expansion, without needing brand new physics and forces that could expand a super-dense singularity, the densest singularity ever.
Veritasium episode, arguing against a singularity.