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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)https://www.theguardian.com/science/2020/jan/25/has-physicists-gravity-theory-solved-impossible-dark-energy-riddle …
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)pic.twitter.com/7Y38aPPQOa
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)pic.twitter.com/Y5DyRN2qMg
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)pic.twitter.com/cgEyMtE73l
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)pic.twitter.com/bN8NKU4jAp
I’m actually rather proud of this mini explanation of gauge gravity
If graviton isn't massless then how does gravity travel at the speed of light?
it wouldn't in this case
when you see a fresh berger thread and wait for them good posts to appearpic.twitter.com/WziRBYfUHo
Please can you interpret for lay person, thanks. 
We have extremely good reasons to think that gravitons should be massless. But if they are NOT masseless, then this fact alone might explain the observations that lead us to introduce dark energy. So, no need for dark energy!
(Tons of caveats apply to everything I just wrote 
)
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