What's the matter with $\Sigma m_{\nu}$?

Understand

Due to non-zero neutrino rest masses we expect the energy density today in non-relativistic matter, $\omega_{\rm m}$, to be greater than the sum of baryon and cold dark matter densities, $\omega_{\rm cb}$.

• We also expect the amplitude of deflections of CMB photons due to gravitational lensing to be suppressed relative to expectations assuming massless neutrinos.
• The combination of CMB and BAO data, however, appear to be defying both of these expectations.
• Here we review how the neutrino rest mass is determined from cosmological observations, and emphasize the complementary roles played by BAO and lensing data in this process.