A 2 per cent Hubble constant measurement from standard sirens within 5 years (1712.06531v2)

Abstract: Gravitational wave coalescence events provide an entirely new way to determine the Hubble constant, with the absolute distance calibration provided by the theory of general relativity. This standard siren method was utilized to measure the Hubble constant using LIGO-Virgo's detection of the binary neutron-star merger GW170817, as well as optical identifications of the host galaxy, NGC 4993. The novel and independent measurement is of particular interest given the existing tension between the value of the Hubble constant determined using Type Ia supernovae via the local distance ladder ($73.24 \pm 1.74$) and that from Cosmic Microwave Background observations ($66.93 \pm 0.62$) by $\sim 3$ sigma. Local distance ladder observations may achieve a precision of $1\%$ within 5 years, but at present there are no indications that further observations will substantially reduce the existing discrepancies. In addition to clarifying the discrepancy between existing low and high-redshift measurements, a precision measurement of the Hubble constant is of crucial value in elucidating the nature of the dark energy. Here we show that LIGO and Virgo can be expected to constrain the Hubble constant to a precision of $\sim2\%$ within 5 years and $\sim1\%$ within a decade.

• The paper demonstrates that gravitational-wave standard sirens can achieve a 2% precision measurement of the Hubble constant within five years.
• It employs simulations of 30,000 binary neutron-star and 60,000 binary black hole mergers to realistically account for observational uncertainties.
• The study highlights gravitational-wave astronomy’s potential to resolve the tension between local and CMB-derived Hubble constant estimates.

Precision Measurement of the Hubble Constant Using Gravitational-Wave Standard Sirens

The paper by Hsin-Yu Chen, Maya Fishbach, and Daniel E. Holz discusses a method to refine the measurement of the Hubble constant ($H_0$) utilizing gravitational-wave standard sirens. This approach is rooted in using gravitational-wave events such as binary neutron-star (BNS) and binary black hole (BBH) mergers as standard candles, providing a direct measurement of luminosity distances through general relativity. The methodology is significant due to the increasing tension between differing $H_0$ values obtained from electromagnetic observations of Type Ia supernovae and the Cosmic Microwave Background (CMB).

Numerical Results and Analysis

The authors predict that the LIGO and Virgo gravitational-wave observatories could achieve a 2% precision in measuring $H_0$ within five years, using gravitational-wave standard sirens. They suggest this level of precision could arbitrate current discrepancies between local and CMB-derived $H_0$ estimates. Their simulation, assuming a network including KAGRA and LIGO-India, forecasts a precision of 1% should be achievable within a decade. Such accuracy is instrumental for understanding cosmological parameters and the nature of dark energy.

The core approach relies on a dataset featuring 30,000 BNS mergers and 60,000 BBH mergers, integrating realistic observational uncertainties and selection effects. In scenarios where an electromagnetic counterpart of a BNS event allows for distinct host galaxy identification, they note the fractional $H_0$ uncertainty decreases proportionally to $15\%/\sqrt{N}$, where $N$ is the number of detections. The incorporation of additional detectors, such as KAGRA and LIGO-India, could moderately enhance the signal resolution, refining distance measures through better source inclination data.

Theoretical Implications

The promise of gravitational-wave observations is broadening the toolkit available to cosmologists. Unlike traditional ladders, which rely on properties of light-emitting astronomical objects with potential systematic errors, gravitational-wave standard sirens introduce a novel, independent approach directly tied to the physics of general relativity. This can provide more stringent constraints on $H_0$ without intermediate steps such as cepheid variable calibration in the traditional cosmic distance ladder.

Future Directions and Speculation

This work points towards exciting developments for gravitational-wave astronomy. With adequate numbers of BNS events, the precision measurement of $H_0$ using standard sirens may become as reliable as current electromagnetic methods. However, the detection of BNS mergers and their electromagnetic counterparts remains a bottleneck, influenced by galaxy distribution and the increasing sensitivity and duty cycle of observatories, which will dictate how rapidly the requisite number of events is compiled.

Beyond astronomy, achieving this level of precision—including resolving inherent biases from peculiar velocities or galaxy catalog completeness—will also necessitate advancements in data analysis techniques and observing capabilities. Future work may include addressing systematic detection biases more rigorously and exploring the integration of gravitational-wave and electromagnetic constraints to further refine the cosmological model.

In summary, this paper provides a detailed roadmap whereby gravitational-wave standard sirens can contribute to one of the most debated topics in cosmology: the precise value of $H_0$. The prospect of arbitrating existing tensions with an independent, purely relativistic measure heralds a significant advancement in the field, with implications spanning observational techniques, cosmological modeling, and understanding of the fundamental composition and evolution of the universe.