# Neutron star collisions shed light on expansion of the universe

The Niels Bohr Institute proposes using kilonovas (explosions resulting from the merger of neutron stars) to address discrepancies in measuring the expansion rate of the universe. Initial results are promising, but more cases are needed to validate them.

In recent years, astronomy has seen itself in a bit of a crisis: Although we know that the universe is expanding, and although we know roughly how fast it is going, the two fundamental ways of measuring that expansion don’t agree. Now astrophysicists from the Niels Bohr Institute propose a new method that may help resolve this tension.

#### The universe is expanding

We’ve known this since Edwin Hubble and other astronomers, about 100 years ago, measured the velocities of a number of surrounding galaxies. The galaxies in the universe are being “pushed” apart by this expansion, and are therefore moving away from each other.

The greater the distance between two galaxies, the faster they are moving apart, and the precise rate of this motion 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 many different 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 an ever-increasing rate. However, exactly how fast is a bit of 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 an ever-increasing rate. However, exactly how fast is a bit of 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; The methods are mutually independent but fortunately give almost the same result.

That is, it is almost…

The easiest intuitive way to understand, in principle, is the same method Edwin Hubble and his colleagues used a century ago: locating a group of galaxies, and measuring their distances and velocities. In practice, this is done by searching for galaxies with exploding stars, or so-called Supernovae. This method is supplemented by another method that analyzes irregularities in the so-called Cosmic background radiation; An ancient form of light dating back to shortly after the great explosion.

The two methods – the supernova method and the background radiation method – always gave slightly different results. But any measurement comes with uncertainties, and a few years ago the uncertainties were large enough that we could blame those disparities.

The left hemisphere shows the expanding remnant of the supernova discovered by Tycho Brahe in 1572, here viewed in X-ray (source: 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, which is observed in microwaves. Credit: NASA/WMAP Science Team

However, as measurement techniques have improved, the uncertainties have diminished, and we have now reached a point where we can state with a high degree of confidence that neither can be true.

The root of this “Hubble problem” – whether unknown effects are systematically biasing one of the results, or whether it points to new physics that has not yet been discovered – is currently one of the hottest topics in astronomy.

The expansion of the universe is measured in “velocity per distance,” which is just over 20 kilometers per second per million light-years. This means that a galaxy located 100 million light-years away is moving away from us at a speed of 2000 km/s, while another galaxy located 200 million light-years away is moving away from us at a speed of 4000 km/s.

However, using supernovae to measure distances and velocities of galaxies results in 22.7 ± 0.4 km/s, while analyzing the cosmic background radiation results in 20.7 ± 0.2 km/s.

Paying attention to such a minor disagreement may seem boring, but it can be very important. For example, the number appears in calculating the age of the universe, and the two methods produce an age 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 distances to galaxies. But in a new study, Albert Snippen, a doctoral student in astrophysics at the Center for Cosmic Dawn at the Niels Bohr Institute in Copenhagen, proposes a new way to measure distances, which could help settle the ongoing dispute.

“When two extremely compact neutron stars – themselves supernova remnants – orbit each other and eventually merge, they explode in a new explosion; this is called a kilonova,” explains Albert Snepen. “We recently showed how remarkably symmetric this explosion is, and it turns out that “This symmetry is not only beautiful, it is also incredibly useful.”

in Third study Just published, the prolific PhD student demonstrates that kilonovas, although complex, can be described with a single temperature. It turns out that the kilonova’s symmetry and simplicity enable 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 the kilonova is. They thus obtained a new, independent method for calculating the distance to galaxies containing kilonovas.

Darach Watson is an associate professor at the Cosmic Dawn Center and a co-author of the study. “Supernovae, which have been used so far to measure distances between galaxies, do not always emit the same amount of light,” he explains. “Furthermore, they first require us to calibrate the distance using another type of star, called Cepheid stars, which in turn must be calibrated as well.” Using kilonovas we can circumvent these 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 closer to the background radiation method, but whether the kilonova method is able to solve the Hubble problem, researchers do not yet dare to say:

“We only have one case study so far, and we need more examples before we can come to a strong conclusion,” warns Albert Sneben. “But our method bypasses at least some known sources of uncertainty, and is a very ‘clean’ system to study. It requires no calibration or correction factor.

Reference: “Measurement of the Hubble constant in kilonovates using the expanding photosphere method” by Albert Snepen, Darach Watson, Dovi Poznanski, Oliver Gast, Andreas Bauszyn, and Radoslaw Wojtak, 2 October 2023, Astronomy and astrophysics.
doi: 10.1051/0004-6361/202346306

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