In recent years, the field of astronomy has faced a significant challenge: Despite our knowledge that the universe is expanding and a general understanding of its rate, there exists a notable discord between the two primary methods used to measure this expansion. Astrophysicists at the Niels Bohr Institute have proposed a novel approach that might offer a solution to this discrepancy.
This quandary has been evident since the pioneering work of Edwin Hubble and other astronomers roughly a century ago. Their observations involved measuring the velocities of various galaxies within our cosmic neighborhood. As a result of the universe’s expansion, galaxies appear to be moving away from each other, causing them to recede.
The rate at which two galaxies move apart is directly proportional to the distance between them, and quantifying this precise rate holds immense importance in contemporary cosmology. This crucial value is known as “the Hubble constant” and plays a central role in a multitude of equations and models describing the universe and its constituents.
Hubble trouble
To fathom the mysteries of the universe, a precise understanding of the Hubble constant is paramount. There exist various methods to determine this constant, each offering independent insights and, fortunately, yielding almost identical results.
However, a caveat lingers in this cosmic quest. The most intuitively graspable method, akin to Edwin Hubble’s century-old approach, involves pinpointing galaxies, gauging their distances, and measuring their velocities. In practice, this involves identifying galaxies hosting supernovae, which serve as cosmic beacons. This method works in tandem with another approach that scrutinizes irregularities in the ancient cosmic background radiation, a light remnant from the dawn of the cosmos shortly after the Big Bang.
Historically, these two methods—the supernova method and the background radiation method—produced slightly discrepant outcomes. Initially, uncertainties clouded these results, but with advancements in measurement techniques, the uncertainties dwindled. We’ve now arrived at a juncture where it’s apparent that both methods cannot be simultaneously correct.
This “Hubble trouble” raises profound questions. Is it due to unforeseen factors systematically skewing one set of results, or does it hint at the emergence of new and uncharted realms of physics yet to be unveiled? This enigma currently stands as one of the most scintillating topics in the realm of astronomy.
Crashing neutron stars may help with the answer
One of the major hurdles in astronomy lies in the precise determination of distances to galaxies. However, a recent study published in Astronomy & Astrophysics, authored by Albert Sneppen, a Ph.D. student specializing in astrophysics at the Cosmic Dawn Center, Niels Bohr Institute in Copenhagen, presents a groundbreaking method to address this ongoing dispute.
Sneppen’s innovative approach revolves around the fascinating phenomenon of ultra-compact neutron stars, remnants of supernovae, orbiting each other and eventually merging, resulting in a spectacular explosion known as a kilonova. He elucidates, “We’ve recently unveiled the remarkable symmetry of this explosion, and it turns out that this symmetry isn’t just aesthetically pleasing but incredibly practical.”
In another study recently published in The Astrophysical Journal, this industrious Ph.D. student demonstrates that despite the intricate nature of kilonovae, they can be characterized by a single temperature. Surprisingly, this symmetry and simplicity empower astronomers to precisely determine the amount of light emitted by kilonovae.
By comparing this luminosity with the light received on Earth, researchers can calculate the distance to the kilonova. This groundbreaking approach provides a fresh and independent means of gauging the distances to galaxies housing kilonovae.
Darach Watson, an associate professor at the Cosmic Dawn Center and a co-author of the study, elaborates: “Supernovae, traditionally employed for galaxy distance measurements, exhibit variability in their light emission. Moreover, their use necessitates calibration through another class of stars known as Cepheids, which introduces additional uncertainties. With kilonovae, we can sidestep these complexities that have long plagued distance measurements.”
Confirms one of the two methods
To showcase the potential of their innovative approach, the astrophysicists put it to the test with a kilonova discovered back in 2017. The outcome yielded a Hubble constant estimate more closely aligned with the background radiation method. However, as to whether the kilonova method can definitively resolve the enigmatic Hubble trouble, the researchers exercise caution in drawing any hasty conclusions.
Albert Sneppen wisely advises, “We have only examined this method with a single case study thus far, and we require a substantial number of additional examples before we can establish a robust outcome. Nevertheless, our method offers the advantage of circumventing certain known sources of uncertainty. It presents a remarkably ‘clean’ system for investigation, demanding no calibration or correction factors.”
Source: Niels Bohr Institute
