B-mode targets

The CMB-S4 Science Book [1], Simons Observatory report [2], Astro2020 Science White Paper on gravitational waves [3], and PICO report [4] contain an extensive discussion of many ongoing and planned efforts in the search for gravitational waves produced by inflation. Here we give a short executive summary of the results obtained in our papers [5-7], which specify the simplest available inflationary models providing physically motivated targets for these searches. Our conclusions are specific for the 10^-3 ≲ r ≲10^-2 range, where we present the B-mode benchmarks of the U-duality symmetric class of α-attractors, and for r ≲10^-3, where we present B-mode targets, for which the future precision measurements of n_s will be decisive. We show that a combination of the simplest α-attractors and KKLTI models of D-brane inflation covers most of the area favored by Planck 2018.