The Niels Bohr Institute proposes using kilonovae, explosions caused by merging neutron stars, to resolve discrepancies in measurements of the expansion rate of the universe. Preliminary results are promising, but more cases are needed to confirm.
Astronomy has been in crisis in recent years: Although we know the universe is expanding, and although we know the approximate rate of expansion, the two main methods of measuring this expansion are at odds. Now, astrophysicists at the Niels-Bohr Institute have proposed a new method that may help resolve this contradiction.
expansion of the universe
We've known this since Edwin Hubble and other astronomers measured the velocities of some surrounding galaxies about 100 years ago. Galaxies in the universe are "carried" away by this expansion, causing them to recede from each other.
The farther apart two galaxies are from each other, the faster they are moving between them, and the precise speed of this movement is one of the most fundamental quantities in modern cosmology. The number describing the expansion of the universe is called the "Hubble constant," and it appears in many different equations and models of the universe and its components.
Galaxies lie quietly in space, but space itself is expanding. This causes galaxies to move away from each other at ever-increasing speeds. But exactly how fast is a mystery. Image credit: ESO/L. The Kalkata galaxy is stationary in space, but space itself is constantly expanding. This causes galaxies to move away from each other at an ever-increasing rate. But exactly how fast is a mystery. Source: ESO/L. Calcada
Hubble conundrum
Therefore, to understand the universe, we must know the Hubble constant as precisely as possible. There are several ways to measure the Hubble constant; these methods are independent of each other, but fortunately they give nearly identical results.
In principle, the easiest intuitive method to understand is the one Edwin Hubble and his colleagues used a century ago: find a bunch of galaxies and measure their distances and velocities. In practice, this is accomplished by looking for galaxies in which stars explode, known as supernovae. This approach is complemented by another that analyzes irregularities in the so-called cosmic background radiation; an ancient form of light that dates back to shortly after the Big Bang.
The two methods - the supernova method and the background radiation method - always give slightly different results. But there are uncertainties in any measurement, and the uncertainties a few years ago were large enough that we can attribute the difference to uncertainty.
The left hemisphere shows the expanding remnants of the supernova discovered by Tycho Brahe in 1572, here viewed with X-rays (Image credit: NASA/CXC/Rutgers/J. Warren & J. Hughes et al.). The picture on the right is a map of the cosmic background radiation of half the sky observed using microwaves. Source: NASA/WMAP Science Team
Nonetheless, as measurement technology continues to improve, uncertainties continue to decrease, and we can now say with a high degree of confidence that both results cannot be correct.
The source of this "Hubble trouble" -- whether an unknown influence systematically deviates from one of the results or hints at new physics that has yet to be discovered -- is one of the hottest topics in astronomy right now.
Hubble constant difference
The expansion of the universe is measured as "velocity per unit distance", which is slightly more than 20 kilometers/second per million light-years. This means that a galaxy 100 million light-years away is receding from us at a speed of 2000 kilometers per second, while another galaxy 200 million light-years away is receding at a speed of 4000 kilometers per second.
However, using supernovae to measure the distance and velocity of galaxies gives a result of 22.7±0.4 kilometers/second, while analyzing the background radiation of the universe gives a result of 20.7±0.2 kilometers/second.
It may sound trivial to care about such a small disagreement, but it can be significant. For example, this number appears in calculations of the age of the universe, with two methods yielding ages of 12.8 billion and 13.8 billion years, respectively.
Accurately determining the distances of galaxies is one of the biggest challenges. But in a new study, Albert Snepen, 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 that could help resolve the current controversy.
"When two ultracompact neutron stars - themselves the remnants of supernovae - orbit each other and eventually merge, they undergo new explosions, so-called kilonovae," explains Albert-Snaben. "We recently demonstrated how such explosions can have remarkable symmetry, which turns out to be not only beautiful, but also very useful."
In a third study just published, the prolific PhD student shows that kilonovae, despite their complexity, can be described by a single temperature. It turns out that the symmetry and simplicity of kilonovae allow astronomers to deduce exactly how much light they emit.
By comparing this luminosity to the luminosity that reaches Earth, researchers can calculate how far away the kilonova is from Earth. This gave them a novel, independent way to calculate the distances of galaxies containing kilonovae.
Darach Watson is an associate professor at the Cosmic Dawn Center and a co-author of the study. He explained: "So far, supernovae have been used to measure the distances of galaxies, but the amount of light emitted by supernovae is not always the same. Furthermore, they first require us to calibrate the distance using another type of star, the so-called Cepheus stars, which also have to be calibrated. With kilonovae, we can get around these complications that bring uncertainty to the measurements."
Preliminary findings and future steps
To demonstrate its potential, astrophysicists applied the method to a kilonova discovered in 2017. The result is that the Hubble constant is closer to the background radiation method, but whether the kilonova method can solve Hubble's troubles, researchers dare not say yet:
"We only have this one case study so far, and more examples are needed to establish a reliable result," cautioned Albert Snepen. But our method bypasses at least some known sources of uncertainty and is a very "clean" system for research. It requires no calibration and no correction factors. "