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LSPE : the Large-Scale Polarization Explorer

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STRIP Collaboration Papers Photos


CMB polarization at large angular scales

  • The detection of B-modes polarization of the CMB would represent the final confirmation that our universe developed from quantum fluctuations, boosted by an inflation process very early in the history of the universe.

  • The signal from B-modes is extremely small, and is mainly at large angular scales (see figure).

  • LSPE targets are the recombination and reionization bumps in the angular power spectrum of B-modes. A detection of both peaks will be a very convincing argument for the detection of truely primordial B-modes.

  • For this reason, for LSPE, coarse angular resolution (around 1o FWHM) is sufficient, while large sky coverage is mandatory.
large scales
Red line : contribution from each multipole to the total mean square fluctuation of the tensor component of CMB polarization (B-modes, r = 1). Thin blue line : the cumulative of the B-modes, i.e. the variance measured by an experiment sensitive from multipole 2 to a given multipole l. The top blue thick line : the beam function B2 for an experiment with a 1.5o FWHM Gaussian beam. Despite of the coarse angular resolution such an experiment collects most of the polarization signal from B-modes
power
              spectra B & dust
coverage
 
  • Measuring CMB polarization with high precision at large angular scales is very difficult, due to large signals from the interstellar medium (thermal emission from interstellar dust grains and synchrotron emission from electrons in the Galactic magnetic field).
        
  • The only way to separate the cosmological signal from the local one is by exploiting the difference in their spectra, which must be sampled over a wide frequency range.
     
  • Large sky coverage and wide frequency coverage call for a space mission. On a shorter time-scale, experimentation is required to qualify specific instrumentation (optical systems, polarization modulators, detectors …) and methods (sky scan, mapping procedures, polarized foregrounds separation …), and possibly to get detections !
     
  • A balloon-borne instrument can avoid atmospheric noise and loading,
    exploit a wide frequency coverage, access a large fraction of the sky during night-time, offer a stable environment during night-time, reject ground spillover using very large ground-shields
     
  • For this reason LSPE surveys the sky at 5 frequencies (44, 90, 140, 220, 240 GHz) simultaneously. In this way we can separate the cosmological signal from the contaminating galactic signal with high accuracy (see top figure). About 20% of the sky, with low polarized foreground, will be surveyed for each flight. The expected performance, taking into account the components separation process, is detailed below : r=0.03 can be detected at 95% CL.

performance-1
performance-2