[Paper Review] The Large-Scale Polarization Explorer (LSPE)

The LSPE collaboration, Simone Aiola+55 more|arXiv (Cornell University)|Aug 1, 2012

Cosmology and Gravitation Theories32 references11 citations

TL;DR

The Large-Scale Polarization Explorer (LSPE) is a balloon-borne mission designed to measure large-scale cosmic microwave background (CMB) polarization, particularly B-mode signals from primordial gravitational waves during inflation. Using dual instruments—coherent HEMT polarimeters at 43/90 GHz and bolometric polarimeters with rotating half-wave plates at 95/145/245 GHz—it aims to constrain the tensor-to-scalar ratio r down to 0.03 at 99.7% confidence while mapping Galactic foreground polarization for magnetic field and dust studies.

ABSTRACT

The LSPE is a balloon-borne mission aimed at measuring the polarization of the Cosmic Microwave Background (CMB) at large angular scales, and in particular to constrain the curl component of CMB polarization (B-modes) produced by tensor perturbations generated during cosmic inflation, in the very early universe. Its primary target is to improve the limit on the ratio of tensor to scalar perturbations amplitudes down to r = 0.03, at 99.7% confidence. A second target is to produce wide maps of foreground polarization generated in our Galaxy by synchrotron emission and interstellar dust emission. These will be important to map Galactic magnetic fields and to study the properties of ionized gas and of diffuse interstellar dust in our Galaxy. The mission is optimized for large angular scales, with coarse angular resolution (around 1.5 degrees FWHM), and wide sky coverage (25% of the sky). The payload will fly in a circumpolar long duration balloon mission during the polar night. Using the Earth as a giant solar shield, the instrument will spin in azimuth, observing a large fraction of the northern sky. The payload will host two instruments. An array of coherent polarimeters using cryogenic HEMT amplifiers will survey the sky at 43 and 90 GHz. An array of bolometric polarimeters, using large throughput multi-mode bolometers and rotating Half Wave Plates (HWP), will survey the same sky region in three bands at 95, 145 and 245 GHz. The wide frequency coverage will allow optimal control of the polarized foregrounds, with comparable angular resolution at all frequencies.

Motivation & Objective

• To measure the large-scale polarization of the cosmic microwave background (CMB) with high sensitivity to detect primordial B-mode signals from cosmic inflation.
• To improve the upper limit on the tensor-to-scalar ratio r to 0.03 at 99.7% confidence, testing predictions of inflationary cosmology.
• To produce wide-area maps of Galactic polarized foregrounds from synchrotron and dust emission to study interstellar magnetic fields and dust properties.
• To achieve optimal control of foreground contamination through wide frequency coverage (43–245 GHz) with comparable angular resolution across bands.
• To conduct a long-duration, circumpolar balloon flight observing the northern sky using Earth as a solar shield, enabling stable, continuous observations.

Proposed method

• Employ a dual-instrument payload: coherent HEMT-based polarimeters operating at 43 and 90 GHz for high sensitivity and low noise.
• Integrate bolometric polarimeters with large-throughput multi-mode bolometers and rotating half-wave plates (HWPs) at 95, 145, and 245 GHz for frequency-matched polarization mapping.
• Use a spinning platform in azimuth to scan a large fraction of the northern sky (25% of the sky) during the polar night, leveraging Earth's shadow to reduce thermal loading.
• Achieve coarse angular resolution (~1.5° FWHM) optimized for large-scale CMB modes, particularly the B-mode signal at multipoles l < 10.
• Apply frequency-multiplexed observations across three bands to disentangle CMB signals from polarized Galactic foregrounds via spectral fitting.
• Utilize cryogenic HEMT amplifiers and bolometers to maintain low system temperature and high dynamic range for precise polarization measurements.

Experimental results

Research questions

• RQ1 Can the LSPE mission detect or place a tighter constraint on the primordial B-mode signal from cosmic inflation, specifically limiting the tensor-to-scalar ratio r to 0.03 at 99.7% confidence?
• RQ2 How accurately can LSPE map the large-scale polarization of Galactic synchrotron and dust emission across multiple frequency bands?
• RQ3 To what extent can multi-frequency observations at 43–245 GHz suppress confusion from polarized foregrounds in CMB B-mode measurements?
• RQ4 What is the performance of a long-duration, circumpolar balloon mission in achieving stable, continuous sky coverage for CMB polarization studies?
• RQ5 How well can the combination of HEMT and bolometric polarimeters with rotating half-wave plates enable high-fidelity, frequency-matched polarization measurements?

Key findings

• LSPE is designed to achieve a sensitivity limit on the tensor-to-scalar ratio r of 0.03 at 99.7% confidence, significantly improving upon previous constraints.
• The mission will map 25% of the sky with coarse angular resolution (~1.5° FWHM), focusing on large-scale CMB polarization modes.
• The dual-instrument approach—HEMT polarimeters at 43/90 GHz and bolometric polarimeters at 95/145/245 GHz—ensures comparable angular resolution and optimal foreground control across all bands.
• The use of rotating half-wave plates in the bolometric array enables modulation of polarization signals, enhancing sensitivity and reducing low-frequency noise.
• The long-duration, circumpolar balloon flight during the polar night enables continuous observation of the northern sky with minimal thermal and pointing disturbances.
• The mission will produce high-fidelity maps of Galactic polarized foregrounds, enabling studies of interstellar magnetic fields and dust grain alignment properties.