Title: Niels Bohr Institute Proposes Explosions from Neutron Star Mergers to Solve Cosmic Expansion Rate Debate
Date: October 3, 2023
In a bid to address discrepancies in measuring the rate of expansion of the universe, astrophysicists from the prestigious Niels Bohr Institute have suggested a groundbreaking solution involving kilonovae, which are explosions resulting from merging neutron stars. Although initial results are promising, further study and additional cases are deemed necessary for validation.
The universe’s expansion has been a subject of scientific investigation since Edwin Hubble and his fellow astronomers measured the velocities of surrounding galaxies around a century ago. According to the theory, galaxies are “carried” away from each other due to this expansion, causing them to recede further apart.
The rate of this receding movement, known as the Hubble constant, plays a crucial role in modern cosmology. However, the two primary methods of measuring this expansion, namely the supernova method and the background radiation method, have yielded slightly different results. Over the years, as measurement techniques have improved and uncertainties diminished, it has become clear that the two methods cannot both be correct.
This discrepancy, termed “Hubble trouble,” has ignited intense debates within the field of astronomy. The root cause of this disagreement, whether it indicates unknown biases in one of the methods or potential new physics waiting to be discovered, remains a hot topic of research.
The disparity in measuring the expansion rate translates to small differences in speed per distance units. Using supernovae to measure distances and velocities of galaxies yields approximately 22.7 ± 0.4 km/s, while analyzing the background radiation of the Universe provides a value of roughly 20.7 ± 0.2 km/s. Although seemingly minor, these variations have significant implications, such as differing estimates of the age of the Universe.
To resolve this ongoing debate, Albert Sneppen, a PhD student in astrophysics at the Niels Bohr Institute, proposes a novel approach. Sneppen suggests using kilonovae, which occur when ultra-compact neutron stars resulting from supernovae merge, as a means to accurately measure galaxy distances.
In a recently published study, Sneppen reveals that kilonovae explosions exhibit remarkable symmetry and simplicity, enabling astronomers to precisely determine the amount of light they emit. By comparing this measured luminosity with observed light on Earth, researchers can calculate the distance to galaxies hosting kilonovae. This provides an independent method free from the uncertainties associated with calibrating other types of stars.
Darach Watson, an associate professor at the Cosmic Dawn Center and co-author of the study, highlights the advantages of kilonovae. Watson states that unlike supernovae, which emit varying amounts of light and require calibration with other stars like Cepheids, kilonovae offer a more straightforward and reliable system for distance measurement.
To demonstrate the potential of their method, the astrophysicists applied it to a kilonova event discovered in 2017. The preliminary findings yielded a Hubble constant value closer to that obtained from the background radiation method. However, the researchers remain cautious and emphasize the need for more examples and cases before drawing robust conclusions.
The study, titled “Measuring the Hubble Constant with Kilonovae Using the Expanding Photosphere Method,” was authored by Albert Sneppen, Darach Watson, Dovi Poznanski, Oliver Just, Andreas Bauswein, and Radosław Wojtak. Published in Astronomy & Astrophysics, the research presents a potential breakthrough in resolving the cosmic expansion rate debate but acknowledges the need for further investigation.
As scientists continue to delve into the mysteries of the universe, the Niels Bohr Institute’s proposal offers a new avenue of exploration that could potentially bring astronomers closer to accurately measuring the expansion rate and gaining deeper insights into the nature of our cosmos.