A gravitational-wave standard siren measurement of the Hubble constant (1710.05835v1)

Abstract: The detection of GW170817 in both gravitational waves and electromagnetic waves heralds the age of gravitational-wave multi-messenger astronomy. On 17 August 2017 the Advanced LIGO and Virgo detectors observed GW170817, a strong signal from the merger of a binary neutron-star system. Less than 2 seconds after the merger, a gamma-ray burst (GRB 170817A) was detected within a region of the sky consistent with the LIGO-Virgo-derived location of the gravitational-wave source. This sky region was subsequently observed by optical astronomy facilities, resulting in the identification of an optical transient signal within $\sim 10$ arcsec of the galaxy NGC 4993. These multi-messenger observations allow us to use GW170817 as a standard siren, the gravitational-wave analog of an astronomical standard candle, to measure the Hubble constant. This quantity, which represents the local expansion rate of the Universe, sets the overall scale of the Universe and is of fundamental importance to cosmology. Our measurement combines the distance to the source inferred purely from the gravitational-wave signal with the recession velocity inferred from measurements of the redshift using electromagnetic data. This approach does not require any form of cosmic "distance ladder;" the gravitational wave analysis can be used to estimate the luminosity distance out to cosmological scales directly, without the use of intermediate astronomical distance measurements. We determine the Hubble constant to be $70.0^{+12.0}_{-8.0} \, \mathrm{km} \, \mathrm{s}^{-1} \, \mathrm{Mpc}^{-1}$ (maximum a posteriori and 68% credible interval). This is consistent with existing measurements, while being completely independent of them. Additional standard-siren measurements from future gravitational-wave sources will provide precision constraints of this important cosmological parameter.

• The paper presents a novel measurement of the Hubble constant using gravitational-wave standard sirens from binary neutron star mergers.
• It details a method that independently derives luminosity distance from GW data and recession velocity from electromagnetic observations.
• Results consistent with previous estimates pave the way for precision cosmology free from traditional distance ladder dependencies.

A Gravitational-Wave Standard Siren Measurement of the Hubble Constant

The paper, entitled "A gravitational-wave standard siren measurement of the Hubble constant," presents an approach to measuring the Hubble constant using gravitational-wave (GW) observations. The research marks a significant advancement in employing gravitational-wave signals, specifically from binary neutron star (BNS) mergers, to calculate cosmological parameters, independently of traditional electromagnetic (EM) distance ladders.

Overview

The detection of GW170817, a gravitational wave from a binary neutron-star merger observed by the Advanced LIGO and Virgo collaborations, serves as the focal point of this research. Because this event was also detected as a gamma-ray burst (GRB 170817A) and observed in the optical spectrum, it offers a unique opportunity for multi-messenger astronomy. The authors leverage this multi-source data by using GW170817 as a "standard siren," a GW analog to the EM standard candles, for computing the Hubble constant. This constant, which indicates the universe's expansion rate, is pivotal for cosmological models.

Methodology

The paper outlines the methodology for deriving the Hubble constant through combined GW and EM observations:

1. Distance Measurement: The distance to the source is inferred solely from the gravitational-wave data. This method makes it possible to estimate the luminosity distance directly, without relying on intermediate distance measurements common in traditional cosmic distance ladders.
2. Recession Velocity: The recession velocity is measured using the redshift derived from EM observations of the host galaxy, NGC 4993. The identification of the host galaxy, aided by the GW and optical counterpart data, allows for a precise estimate of the Hubble flow velocity.
3. Independent Approach: This method does not rely on a cosmic distance ladder or intermediate astronomical distance measurements, making it a direct and independent approach to determining the Hubble constant.

Results and Implications

The paper details the computation of the Hubble constant as $$70.0^{+12.0}_{-8.0} \, \text{km s}^{-1} \text{Mpc}^{-1}$$ based on this method, which is consistent with previous findings but achieved through a completely independent approach. The ability to measure the Hubble constant independently of the EM distance ladder addresses potential sources of systematic error associated with traditional measurements and heralds a new era of precision cosmology based on GW observations.

Future Prospects

The authors propose that as more GW detections occur, particularly standard sirens with EM counterparts, the ensemble of such events will refine the precision of cosmological parameters significantly. This bi-modal capability of gravitational-wave observatories promises to provide tighter constraints on the Hubble constant, resolving existing tensions between measurements using different distance indicators and offering new insights into universal expansion and evolution.

In summary, the research highlights the transformative potential of GW observations in cosmology, presenting a promising framework that could lead to more precise and reliable measurements of fundamental cosmological parameters in the future. The deployment of this methodology in forthcoming observational runs promises to be an exciting development in the field.