The Niels Bohr Institute proposed using kilonovas (explosions resulting from the merger of neutron stars) to overcome the discrepancy in measuring the rate of expansion of the universe. Initial results are promising, but more cases are needed to validate them.
In recent years, astronomy has experienced a bit of a crisis: Even though we know that the universe is expanding, and even though we know roughly how fast it is going, two fundamental ways of measuring that expansion don’t agree. Now astrophysicists from the Niels Bohr Institute are proposing a new method that could help resolve this tension.
The universe is expanding
We have known this since Edwin Hubble and other astronomers, about 100 years ago, measured the velocities of a number of nearby galaxies. The galaxies in the universe are “pushed” apart by this expansion, and therefore they move away from each other.
The greater the distance between two galaxies, the faster they move apart, and the rate of this movement is one of the most fundamental quantities in modern cosmology. The number that describes the expansion is called the Hubble constant, and it appears in various equations and models of the universe and its components.
Galaxies are more or less stationary in space, but space itself is expanding. This causes galaxies to move away from each other at ever-increasing speeds. However, exactly how fast is still a mystery. Credit: ISO/L. Calada. Galaxies are more or less stationary in space, but space itself is expanding. This causes galaxies to move away from each other at ever-increasing speeds. However, exactly how fast is still a mystery. Credit: ISO/L. Calada
Hubble Problem
To understand the universe, we must know the Hubble constant as precisely as possible. There are several ways to measure it; These methods are independent of each other but fortunately give almost the same results.
That means, almost…
The easiest intuitive way to understand it, in principle, is the same method that Edwin Hubble and his colleagues used a century ago: finding the location of a group of galaxies, and measuring their distance and speed. In practice, this is done by looking for galaxies with exploding stars, or so-called Supernova. This method is complemented by another method that analyzes the so-called deviations Cosmic background radiation; An ancient form of light that appeared shortly after the big bang.
The two methods – the supernova method and the background radiation method – always give slightly different results. But any measurement always comes with uncertainty, and a few years ago, those uncertainties were large enough that we could blame them for the gaps.
The left hemisphere shows the expanding supernova remnant discovered by Tycho Brahe in 1572, here viewed with X-rays (Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.). On the right is a map of the cosmic background radiation emanating from half the sky, observed in microwaves. Credit: NASA/WMAP Science Team
However, as measurement techniques have advanced, uncertainty has reduced, and we have now reached the point where we can state with a high degree of confidence that neither is true.
The roots of this “Hubble problem” – whether an unknown effect is systematically biasing one of the results, or whether it points to new, yet to be discovered physics – is currently one of the hottest topics in astronomy.
Persistent Hubble paradox
The expansion of the universe is measured in “speed per distance”, which is more than 20 kilometers per second per million light years. This means that a galaxy 100 million light years away is moving away from us at a speed of 2000 km/s, while another galaxy 200 million light years away is moving away from us at a speed of 4000 km/s.
However, using supernovae to measure galaxy distances and velocities yields 22.7 ± 0.4 km/s, while analyzing the cosmic background radiation yields 20.7 ± 0.2 km/s.
Paying attention to such minor disputes may seem tedious, but they can be very important. For example, the number appears in calculations of the age of the universe, and the two methods yield ages of 12.8 and 13.8 billion years, respectively.
Kilonova: a new approach to measurement
One of the biggest challenges lies in accurately determining the distance to galaxies. But in a new study, Albert Snippen, an astrophysics doctoral student at the Center for Cosmic Dawn at the Niels Bohr Institute in Copenhagen, proposes a new way to measure distance, which could help resolve the ongoing dispute.
“When two very compact neutron stars – which are supernova remnants – orbit each other and eventually merge, they explode in a new explosion; this is called a kilonova,” explained Albert Snepen. “We recently showed how symmetrical these explosions are, and it turns out “This symmetry is not only beautiful, but also very useful.”
inside Third study Just published, this prolific PhD student shows that kilonovae, although complex, can be explained by a single temperature. It turns out that the symmetry and simplicity of the kilonova allows astronomers to deduce exactly how much light it emits.
By comparing this brightness to the amount of light reaching Earth, researchers can calculate how far away a kilonova is. They then obtained a new, independent method for calculating the distance to galaxies containing kilonovae.
Darach Watson is a professor at the Cosmic Dawn Center and one of the authors of the study. “Supernovae, which have been used to measure distances between galaxies, do not always emit the same amount of light,” he explained. “In addition, supernovae require us to first calibrate the distance using another type of star, called a Cepheid star. which in turn must be calibrated as well.” By using kilonovas we can avoid complications that cause uncertainty in measurements.
Preliminary results and future steps
To prove its potential, astrophysicists applied this method to a kilonova discovered in 2017. The result is a Hubble constant that is close to the background radiation method, but whether the kilonova method is able to solve the Hubble problem, researchers do not yet dare to say:
“So far we only have one case study, and we need more examples before we can draw strong conclusions,” warns Albert Sneben. “But our method bypasses at least some known sources of uncertainty, and is a very ‘clean’ system to study. Requires no calibration or correction factors.
Reference: “Measurement of the Hubble constant in kilonovates using the photospheric broadening method” by Albert Snepen, Darach Watson, Dovi Poznanski, Oliver Gast, Andreas Bauzayn, and Radoslaw Wojtak, 2 October 2023, Astronomy and astrophysics.
doi: 10.1051/0004-6361/202346306
2023-10-09 04:57:48
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