[Paper Review] Inflation and Early Dark Energy with a Stage II Hydrogen Intensity Mapping Experiment

Cosmic Visions 21 Cm Collaboration, R. Ansari+30 more|arXiv (Cornell University)|Oct 22, 2018

Cosmology and Gravitation Theories348 references64 citations

TL;DR

The paper outlines a Stage II 21 cm intensity mapping project with 32,000 six-meter dishes to probe expansion history, growth of structure, and inflationary features from z~0.3 to 6, including cross-correlations with optical surveys.

ABSTRACT

This white paper envisions a revolutionary post-DESI, post-LSST dark energy program based on intensity mapping of the redshifted 21cm emission line from neutral hydrogen at radio frequencies. The proposed intensity mapping survey has the unique capability to quadruple the volume of the Universe surveyed by optical programs, provide a percent-level measurement of the expansion history to $z \sim 6$, open a window to explore physics beyond the concordance $Λ$CDM model, and to significantly improve the precision on standard cosmological parameters. In addition, characterization of dark energy and new physics will be powerfully enhanced by cross-correlations with optical surveys and cosmic microwave background measurements. The rich dataset obtained by the proposed intensity mapping instrument will be simultaneously useful in exploring the time-domain physics of fast radio transients and pulsars, potentially in live "multi-messenger" coincidence with other observatories. The core dark energy/inflation science advances enabled by this program are the following: (i) Measure the expansion history of the universe over $z=0.3-6$ with a single instrument, extending the range deep into the pre-acceleration era, providing an unexplored window for new physics; (ii) Measure the growth rate of structure in the universe over the same redshift range; (iii) Observe, or constrain, the presence of inflationary relics in the primordial power spectrum, improving existing constraints by an order of magnitude; (iv) Observe, or constrain, primordial non-Gaussianity with unprecedented precision, improving constraints on several key numbers by an order of magnitude. Detailed mapping of the enormous, and still largely unexplored, volume of cosmic space will thus provide unprecedented information on fundamental questions of the vacuum energy and early-universe physics.

Motivation & Objective

• Measure the expansion history of the universe with percent-level precision up to z~6 to constrain dark energy and pre-acceleration physics.
• Characterize the growth rate of structure from z~0.3 to 6 to test gravity and cosmic acceleration.
• Constrain or detect inflationary relics in the primordial power spectrum and probe primordial non-Gaussianity.
• Leverage cross-correlations with optical surveys and CMB measurements to enhance cosmological constraints.
• Provide a practical, scalable roadmap (Stage I-III) for a post-reionization 21 cm intensity mapping program.

Proposed method

• Fiducial Stage II design: 32,000 six-meter dishes in a hexagonally close-packed array with 50% fill factor and FFT correlation across 200–1100 MHz.
• Three-dimensional 21 cm intensity mapping to map large-scale structure without resolving individual galaxies.
• Forecasts target expansion-history via BAO and spectroscopy of growth-rate through redshift-space distortions and related statistics.
• Exploitation of cross-correlations with optical surveys (DESI, Euclid, LSST) and CMB data to enhance constraints.
• A forward-modeling approach addressing foregrounds, calibration, and systematics to extract robust cosmological information.

Experimental results

Research questions

• RQ1 Can Stage II achieve percent-level measurements of the expansion history out to z~6?
• RQ2 How well can the growth rate be constrained across 0.3<z<6 to test modified gravity?
• RQ3 What constraints can Stage II place on inflationary features and primordial non-Gaussianity, compared to existing limits?
• RQ4 How do cross-correlations with optical and CMB data improve parameter constraints and mitigate systematics?
• RQ5 What is the viable design pathway (Stage I–Stage II–beyond) for a scalable, cost-effective post-reionization 21 cm program?

Key findings

• Stage II aims for percent-level expansion history measurements up to z~6, providing a crucial cross-check with low-redshift probes.
• The growth-rate extraction from 21 cm intensity mapping across 0.3<z<6 offers potential sensitivity to modified gravity scenarios.
• The survey can constrain inflationary relics in the primordial power spectrum and improve primordial non-Gaussianity limits, including equilateral and orthogonal shapes.
• Cross-correlation with optical and CMB surveys significantly enhances cosmological constraints and leverages complementary information.
• The program proposes a three-stage roadmap (Stage I–Stage II–beyond) and emphasizes the synergy with DOE capabilities and large-scale computation.