Thread by @InertialObservr

Prof. Claudia de Rahm's theory of 'Massive Gravity' to explain Dark Energy is getting a lot of press as of late.

So let's dig into it. (thread: 1/n)

theguardian.com/science/2020/j…

Before getting to gravity, recall that forces are transmitted by a 'mediator' which are always bosons

EM: the photon, strong force: gluon, weak force: W± & Z.

When the symmetry corresponding to the force carrier is unbroken the force carriers are massless (m=0).

(2/n)

What about Gravity?

The symmetry corresponding to gravity is the most respected symmetry in all physics & goes by many names

It's the one underlying Einstein's General Theory of Relativity:

General Covariance/Gauge-Lorentz Symmetry/ Diffeomorphism Invariance

(3/n)

Because this symmetry (General Covariance) has never been observed to be violated, physicists were convinced that the gauge field corresponding to gravity corresponded to an exact, unbroken symmetry

I would then immediately follow, then, that the graviton is massless

But wait!, says de Rahm.. Perhaps Lorentz symmetry could just be an *approximate* symmetry of nature

This would happen if the graviton (hμν) has a mass--mass terms for gauge fields always break gauge invariance

But!.. look at the first tweet..

(5/n)

Boom! We see that the graviton (metric; hμν) contains the exponential potential we expected!

TL;DR: If the graviton has a mass~hubble rate (H), then large scale gravity can act repulsive and reproduce the Dark Energy observations, with no Dark Energy at all!

(n/n)

I’m actually rather proud of this mini explanation of gauge gravity