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The Scientific Framework
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The observations in intensity and polarization of the Cosmic Microwave Background Radiation (CMB-R) fluctuations have been playing and will continue to play a crucial role in probing the early stages of the Universe and constrain the cosmological parameters to the sub-percent precision1. In the context of the Big-Bang cosmology, the CMB-R was formed ~380.000 years after the Big Bang, when the temperature of the Universe was low enough to permit the decoupling between the photons from the primordial plasma. The expansion of the Universe has stretched its wavelengths, and today we can observe this radiation like a blackbody emission at 2.7255 K.

Thomson scattering of temperature anisotropies on the last scattering surface generates a linear polarization pattern on the sky. This pattern can be decomposed into even parity component called E-modes and odd parity component called B-modes2. The density perturbations of the primordial plasma result in only E-modes polarization that had been discovered by DASI in the 1992. Nowadays, the CMB community has moved the scientific interest to the detection of the B-modes signal, which originates by primordial inflationary gravitational waves3 - PGW, and by the weak lensing of the foregrounds large scale structures.

The cosmological target is focused on the B-modes correlation patterns at large angular scales where the lensing contributes minimum, and the PGW signal could be maximum. The PGW B-modes have never been observed yet, but the recent measurements have put an upper limit constrain and forecast their signal \(\Delta T / T < 10^{-6} \). The detection of this very faint signal represents a smoking gun for the inflationary theory and a formidable challenge due to the difficulty of keeping the instrumental and environmental systematics under control during the observations.

In this context, a synergistic approach between different experiment philosophies is needed to reach the B-modes detection achievement. Space satellite-like WMAP4, PLANCK5, and the future LiteBIRD6 are essential to cover all the \(4\pi\) sky in order to detect the low-\(l\) PGW B-modes bump on the power spectrum. Similarly, the ground-based telescopes like BICEP2/Keck7, POLARBEAR8, CLASS9, GroundBIRD[^10], QUIJOTE10\ and more recently, the South Pole Telescope (SPT)11 play a crucial role for characterizing the lensing contribute and the polarized foreground like the galactic synchrotron and thermal dust.

Differently from the space satellites, a ground-based telescope presents some positive points: it can be improved in time (with a different focal plane), and the mechanical issues can be repaired in loco. The possibility of a continuous update and the accessibility at the telescope site ensure at the instrument a relatively long life (compared to a space satellite). However, a ground-based telescope has to deal with the spurious atmospheric signal that is particularly strong at the microwave frequencies.

Bibliography

Footnotes

  1. N.Ahganim, et al., Planck 2018 results: VI cosmological parameters, arxiv ID: 1807.06209 ↩

  2. Wayne Hu and Martin White, A CMB polarization premier, New Astronomy 1997 ↩

  3. Alan H. Guth, Inflationary Universe: A possible solution to the horizon and flatness problems, Physical Review D. 1981 ↩

  4. GSFC NASA ↩

  5. Space Science Planck ↩

  6. LiteBIRD Satellite: JAXA's new strategic L-class mission for all-sky survays of Cosmic Microwave Background ↩

  7. P.A.R. Ade et al., Measurement of gravitational lensing from large-scale B-mode polarization, The ApJ 2016 ↩

  8. The POLARBEAR Collaboration, A measurement of the cosmic microwave background B-mode polarization power spectrum at sub-degree scales with POLARBEAR, The ApJ 2014 ↩

  9. ?? ↩

  10. J. A. Rubino-Martin, The QUIJOTE CMB experiment, Highlights of Spanish Astrophysics, 2010 ↩

  11. W. L. K Wu et al, A measurement of the Cosmic Microwave Background Lensing Potential and power spectrum from 500deg2 of SPTpol Temperature and Polarization Data., The ApJ, 2019 ↩