[Paper Review] Report of the Topical Group on Cosmic Frontier 5 Dark Energy and Cosmic Acceleration: Cosmic Dawn and Before for Snowmass 2021

C. L. Chang, Laura Newburgh+10 more|arXiv (Cornell University)|Sep 17, 2022

Cosmology and Gravitation Theories7 citations

TL;DR

This Snowmass 2021 report outlines a transformative research program (2025–35) to probe inflation and new physics via precision measurements of early-universe relics, primarily through CMB-S4, upgraded LIGO, and a Stage-V spectroscopic survey. It aims to detect primordial gravitational waves (r < 0.001), constrain non-Gaussianity, and search for the Cosmic Neutrino Background, with key advances in theory, data analysis, and next-gen technology development.

ABSTRACT

This report summarizes the envisioned research activities as gathered from the Snowmass 2021 CF5 working group concerning Dark Energy and Cosmic Acceleration: Cosmic Dawn and Before. The scientific goals are to study inflation and to search for new physics through precision measurements of relic radiation from the early universe. The envisioned research activities for this decade (2025-35) are constructing and operating major facilities and developing critical enabling capabilities. The major facilities for this decade are the CMB-S4 project, a new Stage-V spectroscopic survey facility, and existing gravitational wave observatories. Enabling capabilities include aligning and investing in theory, computation and model building, and investing in new technologies needed for early universe studies in the following decade (2035+).

Motivation & Objective

• To probe the energy scale and dynamics of inflation through precision measurements of primordial gravitational waves and non-Gaussianity.
• To search for new physics beyond the Standard Model via detection of relic radiation, such as the Cosmic Neutrino Background or axions.
• To enable transformative discoveries in early-universe cosmology by constructing and operating next-generation facilities and advancing theoretical and technological foundations.
• To overcome foreground contamination and astrophysical confusion in CMB and gravitational wave data through improved modeling and analysis techniques.
• To prepare for the 2035+ era by developing critical technologies for future CMB, gravitational wave, and intensity mapping surveys.

Proposed method

• Construct and operate the CMB-S4 experiment to achieve sub-microkelvin sensitivity and map the CMB with unprecedented angular resolution and sky coverage.
• Upgrade and operate existing gravitational wave observatories (LIGO, Virgo, KAGRA) and prepare for third-generation detectors (Einstein Telescope, Cosmic Explorer) to detect the stochastic gravitational wave background.
• Develop and operate a Stage-V spectroscopic survey facility to enable 3D large-scale structure measurements and probe primordial non-Gaussianity beyond 2D CMB limits.
• Implement advanced data analysis pipelines to model and remove astrophysical foregrounds, including overlapping binary merger signals in gravitational wave data.
• Field smaller-scale instruments to mature technologies for future CMB detectors, gravitational wave observatories (e.g. CBE), and 21-cm/mm-wave intensity mapping surveys.
• Integrate theoretical modeling, simulation of cosmological signals, and prediction of observable phenomena into a coordinated program aligned with major facilities.

Experimental results

Research questions

• RQ1 What is the amplitude of primordial gravitational waves (r), and can it be measured or constrained to r < 0.001 with CMB-S4?
• RQ2 What is the shape of the primordial power spectrum, and can deviations from scale invariance or non-Gaussian features reveal the dynamics of inflation?
• RQ3 Can the Cosmic Neutrino Background be detected through its imprint on CMB anisotropies or other relic radiation probes?
• RQ4 How can the stochastic gravitational wave background from inflation be disentangled from astrophysical foregrounds in third-generation gravitational wave detectors?
• RQ5 What new physics beyond the Standard Model is revealed by discrepancies in relic radiation energy densities or spectral features?

Key findings

• CMB-S4 is projected to constrain the tensor-to-scalar ratio to r < 0.001, placing inflation near the grand unification scale if detected.
• The combination of CMB and gravitational wave observatories can test or rule out alternatives to inflation by comparing predicted spectra of primordial gravitational waves.
• Future 3D large-scale structure surveys (e.g., Stage-V spectroscopy, 21 cm arrays) will extend constraints on primordial non-Gaussianity beyond the statistical limits of 2D CMB measurements.
• The stochastic gravitational wave background from inflation may be detectable via spectral distortions or deviations from scale invariance in third-generation detectors.
• Foreground removal in CMB and gravitational wave data remains a major challenge, particularly due to systematics and overlapping binary signals, requiring new algorithms and modeling.
• Technology development for next-generation instruments—including advanced CMB detectors, CBE-style gravitational wave observatories, and mm-wave intensity mapping—must begin now to enable the 2035+ science program.