In the vast expanse of the cosmos, where black holes reign supreme, neutron stars emerge as the next most enigmatic entities. These remnants of exploded stars, though not as dense as black holes, still pack an extraordinary punch in terms of density. When they collide, they create ripples in the very fabric of space and time, offering a unique window into the universe's expansion. This is where the Hubble constant comes into play, a fundamental yet elusive property that scientists have been striving to measure for decades. The Hubble constant, representing the universe's expansion rate, is crucial for astronomers as it sets the cosmic scale for measurements, enabling them to determine the true distance and size of astrophysical objects. It also provides profound insights into the universe's origins and potential fate. However, the quest to measure the Hubble constant has been fraught with challenges, leading to a growing debate known as the 'Hubble tension'. This tension arises from the discrepancy between measurements from the distant universe and those from the nearby universe, with the former yielding a Hubble constant of 67-68 km/s per megaparsec and the latter providing a higher value of around 72-74 km/s per megaparsec. This discrepancy has become one of the most significant challenges in modern cosmology, prompting scientists to seek new and independent methods for measurement. Gravitational waves, produced by the collision of extremely dense objects like black holes, offer a novel approach to measuring the universe's expansion. The detection of gravitational waves from a neutron star collision in 2017, known as GW170817, marked a significant milestone. By combining the gravitational wave signal with the light produced by the collision, researchers aimed to make a precise measurement of the Hubble constant based on Einstein's theory of gravity. However, the initial measurement fell in between the competing values, leaving everyone frustrated. Over the years, astronomers have worked to improve the precision of the GW170817 measurement, with the best results coming from tracking the aftermath of the collision using a worldwide network of radio telescopes. This revealed the motion and structure of the jet of charged particles produced by the collision, reducing uncertainty but still leaving the measurements consistent with both sides of the Hubble tension. In our new study, we have developed a novel approach to measuring the Hubble constant using the gravitational wave method. By re-examining the gravitational waves caused by the neutron star collision and employing more sophisticated models, improved statistical techniques, and a careful treatment of key sources of uncertainty, we have achieved the most accurate Hubble constant measurement yet from GW170817: 61-70 km/s per megaparsec. Interestingly, our result aligns more closely with measurements from the distant universe than those from the nearby universe, suggesting that the Hubble tension may not stem from a flaw in our understanding of the universe, but rather from subtle calibration issues affecting other nearby universe methods. While our result is still four times less precise than the leading nearby-universe measurements, it provides an important new clue in one of astronomy's biggest problems. The quest to settle the Hubble tension using gravitational waves continues, with the need to detect more neutron star collisions. This endeavor is challenging due to the rarity of such events, but it promises to shed new light on the mysteries of the cosmos.
