On Fri, Mar 29, 2019 at 01:23:37AM -0400, Richard Aiken wrote:
> IIRC, it's premise is that there was a certain mathematical constant that
> Einstein initially felt was needed to make his math balance out, then later
> decided he didn't actually need. But now certain modern physicists are
> beginning to re-think the need for it.

The cosmological constant?  Yes, it's basically the simplest model of
"dark energy", with the same effect as an everywhere constant density
(positive or negative) added to the ordinary matter and energy.

Einstein originally added it to account for a static universe, which
was widely assumed at the time and his model did allow for such a
term.  Thus he missed out on predicting the expansion of the universe,
probably the biggest ever discovery in cosmology.  The concept was
resuscitated later due to more detailed observations that it explains
very well (with a different value for the constant).

> If they're right - at least as I remember the video - then the speed
> of light is not actually constant.

The speed of light is still constant in such models.  There are other
cosmological models in which the speed of light varies, but evidence
isn't good for them and they're on much shakier epistemic grounds.

- Tim