Late-time cosmology with 21cm intensity mapping experiments

Abstract: We present a framework for forecasting cosmological constraints from future neutral hydrogen intensity mapping experiments at low to intermediate redshifts. In the process, we establish a simple way of comparing such surveys with optical galaxy redshift surveys. We explore a wide range of experimental configurations and assess how well a number of cosmological observables (the expansion rate, growth rate, and angular diameter distance) and parameters (the densities of dark energy and dark matter, spatial curvature, the dark energy equation of state, etc.) will be measured by an extensive roster of upcoming experiments. A number of potential contaminants and systematic effects are also studied in detail. The overall picture is encouraging -- if autocorrelation calibration can be controlled to a sufficient level, Phase I of the SKA should be able to constrain the dark energy equation of state about as well as a DETF Stage IV galaxy redshift survey like Euclid, in roughly the same timeframe.

• The paper introduces a Fisher matrix forecast to assess the precision of 21cm intensity mapping in measuring key cosmological parameters.
• It demonstrates that experiments like SKA Phase I can achieve constraints rivaling DETF Stage IV optical galaxy surveys.
• The study emphasizes the crucial role of foreground subtraction and HI density uncertainty in optimizing survey design and dark energy constraints.

Analysis of Late-time Cosmology with 21cm Intensity Mapping Experiments

The paper "Late-time cosmology with 21cm intensity mapping experiments" lays out a framework for forecasting cosmological constraints using future neutral hydrogen (HI) intensity mapping experiments, particularly at low to intermediate redshifts. This framework allows for a detailed comparison between such surveys and classical optical galaxy redshift surveys, facilitating an understanding of the capabilities and limitations of upcoming 21cm intensity mapping initiatives.

Framework and Methodology

The authors develop a forecasting formalism based on the Fisher matrix technique, which is a standard approach in cosmology for estimating the sensitivity of an experiment to various parameters. They use this technique to model the expected cosmological signal and assess its contamination by systematic effects and noise. The signal in question is the HI 21cm emission line, which maps out the large-scale structure of the Universe by tracing underlying dark matter distributions from the cosmic dawn to the present.

Experimental Configurations

The study evaluates various experimental setups, from smaller, specialized HI initiatives to large-scale projects like the Square Kilometer Array (SKA). The SKA, in particular, is touted as a game-changer, with its Phase I expected to achieve constraints comparable to those forecasted for state-of-the-art optical galaxy surveys, including the ability to tightly constrain the dark energy equation of state.

Results and Comparisons

Significantly, if autocorrelation calibration can be controlled effectively, SKA Phase I could rival the constraints from a DETF Stage IV galaxy redshift survey such as Euclid. The study also emphasizes the competitive edge of intensity mapping experiments at wider redshift ranges, which is crucial for unraveling the geometry and expansion history of the Universe. This capability opens up prospects for probing late-time acceleration and dark energy dynamics with an unprecedented level of precision.

Key Findings

• Cosmological Parameters: The study demonstrates the potential of 21cm intensity mapping to measure cosmological parameters including the expansion rate, growth rate, and angular diameter distance with high precision.
• HI Bias and Density: A notable challenge highlighted is the uncertain evolution of the HI density fraction ($\Omega_{\mathrm{HI}}$) over redshift, which significantly impacts the signal-to-noise ratio and, consequently, the cosmological constraints achievable.
• Foreground Subtraction: The paper intensively discusses the burden posed by foreground emissions, especially given that they vastly overwhelm the 21cm signal. Efficient foreground subtraction will be critical to success, demanding sophisticated algorithms and robust calibration strategies.
• Survey Design: The authors present a detailed discussion on survey design, emphasizing the balance between dish size, frequency range, survey area, and duration to optimize the figure of merit for dark energy constraints.

Implications and Future Developments

The research underscores that late-time cosmology via 21cm intensity mapping holds significant promise, particularly as a complementary probe alongside optical surveys and CMB data. The unique ability of these experiments to probe the Universe over vast scales and redshifts positions them as a powerful tool for exploring dark energy and testing models of gravity. Looking to the future, overcoming technical challenges such as autocorrelation calibration and foreground contamination will be decisive in maximizing the potential of 21cm surveys.

In conclusion, the paper provides a comprehensive framework for understanding and optimizing the role of 21cm intensity mapping in modern cosmology. With ambitious projects like the SKA on the horizon, the methodologies and insights presented in this work will be instrumental in guiding the development of next-generation cosmological surveys.