Estimating Hubble Constant with Gravitational Observations: A Concise Review

Abstract: The Hubble constant is of paramount importance in astrophysics and cosmology. A large number of methods have been developed with different electromagnetic probes to estimate its value. The most recent results show a tension between values obtained from Cosmic Microwave Background observations and supernovae. The simultaneous detection of gravitational waves and electromagnetic radiation from GW170817 provided a direct estimation of the Hubble constant that did not depend on the astronomical distance ladder. This concise review will present the methods to estimate the Hubble constant with the gravitational observations of compact binary mergers, discussing both bright and dark sirens and reporting the state of the art of the results.

Estimating Hubble Constant with Gravitational Observations: A Concise Review

This review by Rosa Poggiani explores the estimation of the Hubble constant ($H_0$) using both electromagnetic and gravitational observations. The Hubble constant, a crucial parameter in cosmology, dictates the rate of expansion of the Universe. Poggiani emphasizes the existing tension between the $H_0$ values derived from the Cosmic Microwave Background (CMB) and those obtained through observations of supernovae. This disparity, known as Hubble tension, poses significant challenges for cosmological models.

Electromagnetic Methods and Observations

Traditionally, the estimation of the Hubble constant relies on electromagnetic observations, notably the CMB and supernova data. The Planck Collaboration's CMB-based value ($$H_0 = 67.4 \pm 0.5 \, \text{km s}^{-1} \text{Mpc}^{-1}$$) contrasts with the value derived by the SH0ES Collaboration ($$H_0 = 73.2 \pm 1.3 \, \text{km s}^{-1} \text{Mpc}^{-1}$$), which utilizes Cepheids and type Ia supernovae. This inconsistency represents a substantial challenge in cosmological measurements, demanding attention from the scientific community.

Gravitational Wave Contributions

Poggiani's review highlights the pioneering use of gravitational wave observations to estimate the Hubble constant. Gravitational waves provide distance measurements independent of the electromagnetic distance ladder, leveraging the concept of standard sirens. The seminal event, GW170817, a binary neutron star merger, provided a gravitational wave-based $H_0$ estimate that aligns well with other methods. The value obtained was $$H_0 = 70^{+12}_{-8} \, \text{km s}^{-1} \text{Mpc}^{-1}$$, indicating broad compatibility with other estimates but failing to resolve the tension.

Bright and Dark Sirens

The paper discusses bright and dark sirens as tools for $H_0$ measurement. Bright sirens, such as GW170817, are events with identifiable electromagnetic counterparts, allowing precise host galaxy determination and redshift measurements. Conversely, dark sirens lack such counterparts but can use galaxy catalogs to estimate host galaxies, albeit with limitations due to catalog completeness and localization accuracy.

Recent implementation of dark sirens and catalogs like GLADE suggests encouraging precision in $H_0$ estimates, remarked as $$H_0 = 68^{+8}_{-6} \, \text{km s}^{-1} \text{Mpc}^{-1}$$ when combined with GW170817 measurements. These results suggest promising potential for gravitational waves to contribute robust, alternative measurements of the Hubble constant.

Implications and Future Directions

The implications of this research are significant. Not only do these gravitational methods provide independent verification of cosmological parameters, but they also pave the way for novel insights into alternative theories of gravity and potentially offer a solution to the Hubble tension. With the increasing frequency of gravitational wave detections, future developments could refine $H_0$ estimates further, enhancing their precision and potentially reconciling discrepancies found in electromagnetic observations.

Given that reliable $H_0$ determinations are vital for understanding the Universe's expansion history, the methodologies discussed in this paper remain central to advancing this field. Continued observation runs, improved detector sensitivity, and expansive galaxy catalogues could collectively yield more precise measurements, augmenting our comprehension of cosmic expansion and its underlying mechanisms.

Rosa Poggiani's review offers a comprehensive examination of the methods for estimating $H_0$ using gravitational wave observations, highlighting the nuances and potential breakthroughs within this innovative domain of astrophysics and cosmology.