🔭 astrophysics

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

This white paper advocates for the SIRMOS mission, a UK-led space spectroscopic survey designed to map over 100 million high-redshift galaxies to address fundamental cosmological questions regarding inflation, dark energy, and neutrino masses while leveraging advanced instrumentation to strengthen the national space economy.

Original authors: Florian Beutler, Eva-Maria Mueller, Seshadri Nadathur, Yun Wang, David Alonso, Tessa Baker, Sownak Bose, Rebecca Canning, Shaun Cole, Fergus Cullen, Willem Elbers, Pedro Ferreira, Carlos Frenk, Oscar

Published 2026-01-26

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CC BY 4.0

Original authors: Florian Beutler, Eva-Maria Mueller, Seshadri Nadathur, Yun Wang, David Alonso, Tessa Baker, Sownak Bose, Rebecca Canning, Shaun Cole, Fergus Cullen, Willem Elbers, Pedro Ferreira, Carlos Frenk, Oscar Gonzalez, Or Graur, Boryana Hadzhiyska, Alex Hall, Catherine Heymans, Sergey Koposov, Kazuya Koyama, Ofer Lahav, Baojiu Li, Avery Meiksin, Johannes Noller, John Peacock, Alkistis Pourtsidou, Giorgio Savini, Andy Taylor, Rita Tojeiro, David Wands, Massimo Robberto, Gregory Wirth, Mark Dickinson, Thomas Greene, Jeffrey Kruk, Will Percival, Andreas Faisst, Lynne Hillenbrand, Jeyhan Kartaltepe, Nikhil Padmanabhan, Lado Samushia, Lee Armus, Andrew Benson, Micol Bolzonella, Samuel Brieden, Jarle Brinchmann, Robert Content, Emanuele Daddi, Kyle Finner, Andrew Hearin, Cullan Howlett, Jon Lawrence, Gregory Mosby, Zoran Ninkov, Ken Osato, Casey Papovich, Jack Piotrowski, Lucia Pozzetti, Alvise Raccanelli, Jason Rhodes, Shun Saito, Hee-Jong Seo, Zachary Slepian, Steve Smee

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Universe as a giant, ancient library. For decades, astronomers have been reading the books on the lower shelves (nearby galaxies) and the very first pages of the oldest books (the Cosmic Microwave Background). But there is a massive, dusty middle section of the library that we haven't been able to read properly yet. This is the era between 1 and 4 billion years after the Big Bang.

This white paper proposes a new mission called SIRMOS (Satellite for InfraRed Multi-Object Spectroscopy) to finally clean off the dust and read those missing pages. Here is the simple breakdown of what they want to do and why it matters.

The Big Questions: What are we trying to solve?

The paper says there are three massive mysteries in physics that SIRMOS aims to solve:

The "Inflation" Mystery: Did the Universe start with a single, simple "push" (like a single balloon inflating), or was it a chaotic mix of many forces? SIRMOS wants to find a tiny fingerprint in the galaxy distribution that tells us exactly how the Universe began.
The "Dark Energy" Mystery: We know the Universe is expanding faster and faster, but we don't know why. Is it a constant force, or does it change over time? SIRMOS wants to map the expansion history to see if this "Dark Energy" is behaving strangely in the past.
The "Neutrino" Mystery: Neutrinos are ghost-like particles that pass through us constantly. We know they have mass, but we don't know how much. SIRMOS wants to weigh them by seeing how their gravity (or lack thereof) affects the growth of galaxies.

The Solution: A Cosmic "Super-Scanner"

To solve these mysteries, you can't just look at a few galaxies. You need to look at 100 million of them.

The Problem with Earth: If you try to do this from the ground, Earth's atmosphere acts like a pair of sunglasses with cracks in them. It blocks certain colors of light and adds a lot of "noise" (like static on a radio), making it hard to get a clear picture of the distant, red-shifted light from the early Universe.
The SIRMOS Fix: SIRMOS is a satellite that goes into space. It has a 60-centimeter telescope (about the size of a large trash can lid) and a special "eye" that sees infrared light (heat) from 1.25 to 2.5 microns. Because it's in space, it has a clear, uninterrupted view.

The Magic Trick: The Digital Mirror

The most clever part of SIRMOS is how it looks at so many galaxies at once.

Imagine you are at a concert and you want to record a song from 4,000 different people in the crowd simultaneously. You can't just hold up one microphone.
SIRMOS uses a Digital Micromirror Device (DMD). Think of this as a tiny, high-speed mirror wall with millions of tiny mirrors.
The telescope takes a picture of the sky. The computer picks 4,000 specific galaxies it wants to study. The DMD flips 4,000 tiny mirrors to catch the light from only those galaxies and bounce it into a spectrometer (a machine that breaks light into a rainbow).
This allows SIRMOS to take the "fingerprint" (spectrum) of 4,000 galaxies at the exact same time. It does this over and over again to build a 3D map of 100 million galaxies.

The Plan: A 3-Year Road Trip

The Route: The satellite will scan a huge patch of the sky (14,000 square degrees, which is about one-third of the entire sky).
The Schedule: It will visit the same spots three times over three years. Each visit takes about 17 minutes.
The Goal: By the end of the mission, it will have measured the distance and speed of 100 million galaxies, creating the most detailed 3D map of the Universe's "teenage years" ever made.

Why the UK is Involved

This is a partnership between NASA and the UK.

The Role: The UK scientists and engineers are leading the charge to build the telescope and the optical systems (the mirrors and lenses).
The Benefit: It uses technology the UK has already helped develop for other missions (like the Euclid satellite). It keeps high-tech jobs in the UK and ensures British scientists are at the front of the line when the big discoveries are made.

In Summary

SIRMOS is like a high-speed, space-based barcode scanner for the Universe. Instead of scanning groceries, it scans 100 million galaxies to measure the speed of the Universe's expansion, the nature of its beginning, and the weight of invisible particles. By doing this from space with a special digital mirror, it hopes to answer questions that have puzzled physicists for decades.

Technical Summary: SIRMOS – Satellite for InfraRed Multi-Object Spectroscopy

Problem Statement
Current cosmological understanding faces three fundamental gaps regarding the Universe's origin and evolution: the specific physics of cosmic inflation, the nature of dark energy (DE) driving current accelerated expansion, and the absolute mass scale of neutrinos. While the Cosmic Microwave Background (CMB) and low-redshift galaxy surveys ( $z \lesssim 2$ ) have advanced the field, they lack the sensitivity to distinguish between competing models in critical regimes.

Inflation: Current measurements of primordial non-Gaussianity ( $f_{NL}$ ) are insufficient to rule out multi-field inflation models against single-field scenarios.
Dark Energy: Recent hints of time-evolving DE equation of state ( $w(z)$ ) and "phantom" behavior ( $w < -1$ ) at high redshifts ( $1.5 \lesssim z \lesssim 4$ ) remain unconfirmed due to a lack of spectroscopic data in this "redshift gap." Existing surveys also rely on phenomenological parametrizations (CPL) that may bias model selection.
Neutrino Mass: Current cosmological constraints on the sum of neutrino masses ( $\sum m_\nu$ ) are not precise enough to detect the minimal mass scenario ( $\sim 0.06$ eV) with high significance, leaving the mass hierarchy undetermined.

Ground-based observations are further limited by atmospheric absorption, variable transmission, and OH sky emission lines, which fragment the near-infrared (NIR) spectral window and introduce systematic uncertainties difficult to model for ultra-large-scale measurements.

Methodology and Mission Concept
The paper proposes SIRMOS (Satellite for InfraRed Multi-Object Spectroscopy), a NASA MIDEX-class mission designed to conduct a wide-area, high-throughput spectroscopic survey.

Survey Scope: The mission targets a sky area of 14,000 deg², covering the redshift range $1 < z < 4$ . The goal is to obtain spectroscopic redshifts for over $10^8$ (100 million) galaxies, creating a dense, homogeneous 3D map of cosmic structure with a volume of $\sim 500$ Gpc³.
Instrumentation:
- Telescope: A 60 cm diameter, all-reflective, off-axis telescope cooled to 140 K to minimize thermal background. It features a $\sim 1$ deg² field of view (FoV).
- Spectroscopy: Utilizing a Digital Micro-mirror Device (DMD) to simultaneously select and direct light from $\sim 4,000$ targets to a spectrometer. This enables high-multiplexing slit spectroscopy.
- Detectors: The spectrometer uses a Teledyne HAWAII-4RG (H4RG) 4K $\times$ 4K HgCdTe focal plane array, offering continuous wavelength coverage from 1.25 to 2.5 $\mu$ m at moderate resolution.
- Imager: An off-axis imager monitors alignment stars to ensure the DMD slit pattern aligns with sky positions to within 0.1 arcseconds.
Operational Strategy: The mission will operate in a sun-synchronous orbit. To reach the target of $10^8$ spectra, each field will be observed for a total of 51 minutes, split into three 17-minute visits over a 3-year prime mission. Galaxies achieving a signal-to-noise ratio (S/N) $\ge 5$ will be replaced with new targets after each visit.
Redshift Precision: The slit spectroscopy approach aims for a redshift accuracy of $\sigma_z/(1+z) \simeq 10^{-4}$ , representing an order of magnitude improvement over slitless grism missions (like Euclid and Roman) and significantly better than filter-based missions (like SPHEREx).

Key Contributions and Technical Feasibility
The paper outlines a mission architecture that leverages proven technologies to address the limitations of current and planned facilities:

Technology Readiness: The payload relies heavily on the heritage of the SPHEREx mission for the spacecraft bus and thermal design (passive V-groove radiators). The DMD, currently at TRL 5, is scheduled for a 2026 rocket demonstration to reach TRL 6. The remaining payload components are at TRL $\ge 6$ .
Engineering Solutions: The team proposes replacing the DMD's hermetically sealed window with one transparent to NIR radiation to enable operation beyond 1.0 $\mu$ m, identified as a low-risk engineering study.
Data Handling: The mission generates a manageable daily data volume (7.4 GB), well within the capabilities of a standard Ka-band antenna or even a 2,000 kbps S-band receiver for weekly downlinks.
UK Strategic Role: The paper positions SIRMOS as a vehicle for UK leadership, proposing that UK industry and academia lead the construction of the telescope and optical systems, building on expertise gained from the Euclid mission and ground-based DESI.

Projected Results and Scientific Capabilities
Forecasts indicate that SIRMOS will achieve the following scientific milestones:

Inflation Physics: The survey is projected to detect $|f_{NL}| = 1$ at a $5\sigma$ significance level. This precision is sufficient to decisively distinguish between single-field slow-roll inflation ( $f_{NL} \approx 0$ ) and multi-field inflation models ( $|f_{NL}| \gtrsim 1$ ).
Dark Energy Evolution: By measuring the expansion history and structure growth via Baryon Acoustic Oscillations (BAO) and Redshift-Space Distortions (RSD) in the $1 < z < 4$ range, SIRMOS can distinguish between a cosmological constant and evolving DE models (including CPL and phantom scenarios) without relying on Type Ia supernovae data. It aims to resolve the "phantom" behavior hints seen in lower-redshift surveys.
Neutrino Mass: Combining power spectrum, bispectrum, and multi-tracer analysis with Planck CMB priors, the mission forecasts a detection of the minimal neutrino mass scenario ( $\sum m_\nu = 0.057$ eV) at $> 10\sigma$ significance, thereby establishing the absolute neutrino mass scale.

Significance and Claims
The paper asserts that SIRMOS represents a unique opportunity to apply terrestrial-proven technologies (DMDs, large detector arrays) in space to solve fundamental cosmological questions that ground-based surveys cannot address due to atmospheric limitations.

Scientific Impact: It claims to be the first survey capable of mapping the high-redshift Universe ( $1 < z < 4$ ) with the necessary density and spectroscopic precision to reconstruct cosmic evolution during the transition from matter domination to dark energy domination.
Strategic Alignment: The mission is presented as a strategic alignment of UK Space Agency and STFC goals, aiming to demonstrate UK capability in end-to-end space missions and strengthen the national space economy through high-value industrial participation.
Synergy: Beyond the primary cosmological goals, the paper notes the mission's potential to contribute to understanding galaxy evolution in high-redshift environments and probing the Milky Way's dust-enshrouded regions, offering synergies with UK-led ground-based NIR capabilities like MOONS.