Title:	The Simons Observatory: forecasted constraints on primordial gravitational waves with the expanded array of Small Aperture Telescopes
Authors:	Abril-Cabezas, I Adachi, S Ade, P Adler, A E Agrawal, P Aguirre, J Aiola, S Alford, T Rao, M Sathyanarayana + 246 Co-Authors
Keywords:	CMBR experiments cosmological parameters from CMBR gravitational waves CMBR polarization
Issue Date:	16-Apr-2026
Publisher:	Journal of Cosmology and Astroparticle Physics
Citation:	Journal of Cosmology and Astroparticle Physics, 2026, Vol. 2026(4), AR No. 051
Abstract:	We present updated forecasts for the scientific performance of the degree-scale (0.5 deg FWHM at 93 GHz), deep-field survey to be conducted by the Simons Observatory (SO). By 2027, the SO Small Aperture Telescope (SAT) complement will be doubled from three to six telescopes, including a doubling of the detector count in the 93 GHz and 145 GHz channels to 48,160 detectors. Combined with a planned extension of the survey duration to 2035, this expansion will significantly enhance SO's search for a B-mode signal in the polarisation of the cosmic microwave background, a potential signature of gravitational waves produced in the very early Universe. Assuming a 1/f noise model with knee multipole ℓknee = 50 and a moderately complex model for Galactic foregrounds, we forecast a 1σ (or 68% confidence level) constraint on the tensor-to-scalar ratio r of σr = 1.2 × 10-3, assuming no primordial B-modes are present. This forecast assumes that 70% of the B-mode lensing signal can ultimately be removed using high resolution observations from the SO Large Aperture Telescope (LAT) and overlapping large-scale structure surveys. For more optimistic assumptions regarding foregrounds and noise, and assuming the same level of delensing, this forecast constraint improves to σr = 7 × 10-4. These forecasts represent a major improvement in SO's constraining power, being a factor of around 2.5 times better than what could be achieved with the originally planned campaign, which assumed the existing three SATs would conduct a five-year survey.
Description:	Open Access. Also available at arXiv.org (one of the alternative locations)
URI:	http://hdl.handle.net/2289/8716
ISSN:	1475-7516
Alternative Location:	https://doi.org/10.48550/arXiv.2512.15833 http://doi.org/10.1088/1475-7516/2026/04/051
Copyright:	© 2026 The Author(s)
Appears in Collections:	Research Papers (A&A)

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