Through observations made using advanced astronomical tools, it has been widely accepted that the universe is expanding. There are two primary ways used to measure the speed of this expansion, but they seem to contradict each other. To respond to this challenge, astrophysicists from the Niels Bohr Institute suggested a novel method to resolve this tension.
Understanding the Hubble Constant
About a century ago, Edwin Hubble and other astronomers measured the speed of various galaxies. They found that the galaxies in the universe are carried away by the cosmological expansion and, therefore, recede from each other.
The greater the distance between two galaxies, the faster they move away from each other. The precise speed of this movement is one of the most fundamental quantities in modern cosmology. The number that describes this expansion is called the "Hubble constant," which appears in various equations and universe models.
Understanding the universe means knowing the Hubble constant as precisely as possible. Several methods have been designed to measure it, giving almost the same result even if they are mutually independent.
The easiest method to understand is the same as by Edwin Hubble and his colleagues. This involves locating a group of galaxies and measuring their distances and speeds. In practice, this is carried out by searching for galaxies with exploding stars called supernovae. This approach is complemented by another strategy, which analyzes irregularities in cosmic background radiation.
The supernova method and the background radiation method always provide slightly different results. Any measurement involves some uncertainties; in the past, these uncertainties were substantial enough that scientists could blame those for the disparity. The cause of this "Hubble trouble" remains one of the hottest topics in astronomy. One of the challenges in resolving this dilemma lies in determining the distances to galaxies.
Finding Clues in Crashing Neutron Stars
When two ultra-compact neutron stars orbit each other and merge, they go off in a new explosion called kilonova. In a recent study, astrophysicists demonstrated how this explosion is symmetric, which is beautiful and incredibly useful.
Despite their complexity, researcher Albert Sneppen showed that a single temperature can describe kilonovae. It turns out that its symmetry and complexity allow astronomers to deduce the amount of light that they emit.
By comparing this luminosity with the amount of light that reaches the Earth, scientists can calculate the distance of a kilonova. They have obtained a novel, independent method for calculating the distance to galaxies with kilonovae.
Supernovae do not always emit the same amount of light. They first require experts to calibrate the distance using another type of star called Cepheids, which must also be calibrated. Astronomers can find a way around these complications with kilonovae, introducing measurement uncertainties.
To demonstrate the potential of this approach, astrophysicists applied the method to a kilonova discovered in 2017. This resulted in a Hubble constant closer to the background radiation method, although experts have not shared whether the Kilonova method can resolve the trouble.
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