Dongwon Han, Niall MacCrann, Mathew Madhavacheril, Frank Qu and Blake Sherwin tell us about the Atacama Cosmology Telescope’s latest measurement of CMB weak lensing (“Advanced ACT” to be precise).

It is quite the measurement. Despite ACT being on the ground, the measurement now rivals the best Planck lensing measurement. This means the additional complications of the atmosphere have been overcome, and the better technology of telescope possible from not having to put something in space are starting to win.

The results are fascinating in the era of S₈ tensions. They match both Planck CMB and Planck lensing (and ACT primary CMB). The CMB lensing measurements are sensitive to larger scales and higher redshifts than other large scale structure probes (because the CMB is further away, so is lensed by stuff further away). This means that if the S₈ tension is new physics their result seems to be suggesting it must happen at late times and/or on small scales. Perhaps this is consistent with what other probes are suggesting too?

In this video we hear about the hard work done to eliminate systematic effects and make sure noise isn’t being mistaken for signal, and then we hear about the cosmology results and their implications for the future.

Paper 1: https://arxiv.org/abs/2304.05203
Paper 2: https://arxiv.org/abs/2304.05202
Paper 3: https://arxiv.org/abs/2304.05196

Dongwon: https://dwhan89.github.io/
Niall: https://www.maths.cam.ac.uk/person/nm746
Mathew: https://live-sas-physics.pantheon.sas.upenn.edu/people/mathew-madhavacheril
Frank: https://www.linkedin.com/in/frankqu7/
Blake: http://www.damtp.cam.ac.uk/person/bds30

[00:00] Intro bits
[02:50] Why is this important now?
[14:32] ACT and its maps
[18:35] ACT lensing maps
[27:56] The power spectrum from the map (and challenges!)
[40:54] Cosmology Results! (and implications)
[1:01:39] What comes next (and discussion)