Observational and Experimental status
The observation of the Cosmic Microwave Background started in 1964 with its discovery by Penzias and Wilson, two radio astronomers who first detected an isotropic, homogeneous excess radiation over the observable sky. Since then, a series of space observatories (COBE, WMAP and Planck), balloons and ground-based projects scrutinized this radiation and characterized in details the physics of the primordial plasma.
Beyond the measurement of total intensity fluctuations of the radiation, the community has been looking into the anisotropies in polarization for ~ 15-20 years, as it opens a complementary window onto the physics of the early universe. Physicists are particularly interested in an excess of power on the largest scales of the so-called B-modes polarization patterns. As mentioned in the “early universe” section, observing this signal would constitute the detection of primordial gravitational waves, possibly generated by the cosmic inflation.
So far, the latest observations only led to direct measurements of the so-called lensing B-modes, induced by the presence of large scale structures between us and the last scattering surface. Here is a compilation of the most recent measurements of the B-modes power spectrum:
(credits: Yuji Chinone, POLARBEAR collaboration)
The primordial B-modes, not detected yet, have an amplitude parametrized by the so-called tensor-to-scalar ratio, r, which is directly related to the energy involved in the inflation. Latest observations from the BICEP/Keck team set an upper bound on this parameter: r < 0.07 with 95% confidence level.
The instrumental and observational community is now in the process of developing very complex instruments in order to reach the required sensitivity to reach the primordial B-modes, along with the other science goals carried by CMB photons.
Among this global instrumental efforts, here is a list of observational projects that we are involved in:
- POLARBEAR/Simons Array (2012-2021): http://bolo.berkeley.edu/polarbear/
- Simons Observatory (2021-2026): https://simonsobservatory.org/
- CMB-S4 (~2025-): https://cmb-s4.org/
- LiteBIRD (2027-2030): http://litebird.jp/eng/