Colin Hill, Adrien La Posta and Even McDonough tell us about the state of play with early dark energy (EDE). They cover both observation, primarily the CMB; and fundamental physics models that might generate EDE phenomenology.

Colin gives an intro to EDE, the Hubble tension, how the CMB is sensitive to EDE, and EDE models, and then shows how the observational evidence for/against EDE from the CMB is… curious.

The Atacama Cosmology Telescope (ACT) + large scales of Planck/WMAP appear to favour, or even detect(!?), EDE at more than 3 sigma, more or less “solving” the Hubble tension (if you ignore large scale structure, which you shouldn’t!). However, the full Planck data sees no evidence at all, even excluding Hubble tension solving values of EDE at about 3 sigma. ACT+Planck also excludes EDE, because small scale Planck is just so constraining that any ACT preference is washed out; there’s just so much more statistical information in Planck small scales!

Adrien then talks about how one can eliminate at least one possible CMB systematic via using the ratio of power spectra. When doing this there is still a Hubble tension, and the results are completely consistent with conventional CMB analyses (albeit with larger error bars because some info is thrown out when taking this ratio). He then covers the same analysis as Colin’s ACT analysis, but this time with the South Pole Telescope (SPT). SPT also allows large EDE amplitudes, but isn’t constraining enough to either favour or rule them out.

The small additional constraining power of SPT does mean though that ACT+SPT+WMAP seems to almost conclusively detect EDE (or some equivalent phenomena). It makes me think that *if* Planck didn’t exist, we’d be almost convinced this is *the* answer to where the Hubble tension is coming from. Unfortunately, Planck does exist and doesn’t see any evidence for this at all – suggesting that this is instead either some systematic in ACT, or a statistical fluke.

So, all bets are off, but this is something to really keep an eye on. ACT will have much more data to say much more about this “within a year” (everything is blind at the moment though so nobody knows what the data will point at in the end).

Finally, Evan talks about theory space and how one might tackle this mystery of EDE and Hubble tensions and *crucially* large scale structure. He talks about a “swampland” motivated principle that might allow the mass of dark matter to be influenced by the early dark energy, meaning that the “S8 tension” would not be made worse by early dark energy, but possibly even made better. Coupling EDE to dark matter might also help explain why any EDE transition happens to happen close to when the CMB is formed, maybe… time will tell.

Adrien: https://inspirehep.net/authors/1889152
Adrien’s paper: https://arxiv.org/abs/2112.10754

Colin: http://user.astro.columbia.edu/~jch/
Colin’s paper: https://arxiv.org/abs/2109.04451

Evan: https://www.evanmcdonoughphysics.com/
Evan’s paper: https://arxiv.org/abs/2112.09128

Colin’s earlier video on EDE and large scale structure: https://www.youtube.com/watch?v=5JRHFGuPAV8&t=0s