The Simons Observatory: forecasted constraints on primordial gravitational waves with the expanded array of Small Aperture Telescopes

Abril-Cabezas, I., Adachi, S., Ade, P. ORCID: https://orcid.org/0000-0002-5127-0401, Adler, A. E., Agrawal, P., Barry, P., Braithwaite, C., Brien, T. L. R., Calabrese, E. ORCID: https://orcid.org/0000-0003-0837-0068, Desai, S., Doyle, S. ORCID: https://orcid.org/0000-0002-9054-986X, Hargrave, P. ORCID: https://orcid.org/0000-0002-3109-6629, Harrison, I. ORCID: https://orcid.org/0000-0002-4437-0770, Jense, H. T., Lyons, M., Negrello, M. ORCID: https://orcid.org/0000-0002-7925-7663, Papageorgiou, A., Rowe, S., Sudiwala, R. ORCID: https://orcid.org/0000-0003-3240-5304, Tucker, C. ORCID: https://orcid.org/0000-0002-1851-3918 and Vina, M. 2026. The Simons Observatory: forecasted constraints on primordial gravitational waves with the expanded array of Small Aperture Telescopes. Journal of Cosmology and Astroparticle Physics 2026 (04) , 051. 10.1088/1475-7516/2026/04/051

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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.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Physics and Astronomy
Additional Information:	Full list of authors can be seen at https://doi.org/10.1088/1475-7516/2026/04/051 License information from Publisher: LICENSE 1: URL: http://creativecommons.org/licenses/by/4.0/, Type: cc-by
Publisher:	IOP Publishing
Date of First Compliant Deposit:	23 April 2026
Date of Acceptance:	4 February 2026
Last Modified:	05 Aug 2026 04:29
URI:	https://orca.cardiff.ac.uk/id/eprint/186598

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