Illuminating the Physics of Cosmic Origin and Evolution: A UK Space Frontiers 2035 White Paper

TL;DR

Illuminating the physics of cosmic origin and evolution identifies inflation, dark energy, and neutrino masses as key problems and proposes a space-based, high-density spectroscopic survey to resolve them. It introduces SIRMOS, a 60 cm space telescope with a DMD-based spectrograph and continuous $1.1$–$2.5~\mu$m coverage to measure $>10^8$ galaxy redshifts in $1<z<4$, achieving $\sigma_z/(1+z)\approx 10^{-4}$. Forecasts show sensitivity to $|f_{\mathrm{NL}}|=1$ at $5\sigma$, strong expansion-history constraints in the high-redshift gap, and $>10\sigma$ detection of the minimal neutrino-mass sum when combined with Planck and multi-tracer analyses. The proposal highlights UK leadership and industrial opportunities via NASA/UK collaboration, leveraging Euclid-descended capabilities to advance fundamental cosmology while strengthening the domestic space economy.

Abstract

Understanding the Universe's origins and evolution remains one of the most fundamental challenges in modern cosmology. This white paper explores three key science priorities in this field: unravelling the physics of cosmic inflation, investigating the accelerating expansion of the Universe, and precisely measuring the sum of the neutrino masses. Achieving these goals requires a dedicated survey to map the large-scale structure at high redshift in unprecedented detail. We describe how this can be achieved through a mission concept called SIRMOS, providing a high-throughput, highly multiplexed spectroscopic capability to obtain accurate redshifts for over 100 million galaxies over a wide sky area. Such a survey would leverage the deepest existing wide-area photometric catalogues for targeting, with spectra offering continuous 1.25-2.5~$μ$m wavelength coverage at moderate resolution, allowing precise redshift measurements in the $1<z<4$ range with minimal bias. We outline the scientific opportunities this presents. Recent years have seen significant advances in instrumentation, including digital micromirror devices, complex telescope mirrors, large detector arrays, and data processing pipelines. While these technologies have been demonstrated in terrestrial applications, such a survey is a unique opportunity to apply these proven capabilities in space to address fundamental questions in cosmology. Participation in such a mission will simultaneously deliver a compelling science case, help align UK Space Agency and STFC strategies, demonstrate the UK's growing capability in end-to-end space missions, and strengthen the national space economy through high-value industrial participation.

Figures (6)

• Figure 1: Forecast constraints on non-Gaussianity parameter $f_\mathrm{NL}$ from various surveys (left), and the achievable measurement significance (right), assuming a target $f_\mathrm{NL}=1$. SIRMOS is designed to be able to detect $|f_{\mathrm{NL}}|=1$ at 5$\sigma$, thus distinguishing between single-field and multi-field inflation.
• Figure 2: Left: Number density of galaxies detectable by different surveys. Centre: Forecast constraints on DE parameters in the CPL parametrisation from SIRMOS together with Planck CMB in two scenarios motivated by the results in Abdul_Karim_2025, compared to equivalent DESI constraints. Right: Time derivative of cosmic scale factor vs. redshift (normalized to 1 at $z=0$), with predicted measurement uncertainties for various surveys. SIRMOS is able to distinguish DE models with more complex high-$z$ behaviour.
• Figure 3: SIRMOS payload Functional Block Diagram.
• Figure 4: Left: SIRMOS telescope elements deliver wide-field, diffraction-limited images to the DMD. Right: DMD directs light from 4000 selected targets to the spectral channel for highly-mutiplexed operation.
• Figure 5: The SIRMOS payload is well-sized to fit within the SPHEREx photon shields.