How Neutron Star Collisions Could Solve the „Cosmological Crisis”

Artist’s impression of two neutron stars colliding in what is known as a „kilonova” event. Credit: Elizabeth Wheatley (STScI)

Astrophysicists are proposing observations of colliding neutron stars (kilonovae) as a new method to address discrepancies in measuring the expansion rate of the universe, with a view to resolving the current „Hubble tension”.

According to some in the astrophysical community, there has been a „crisis in cosmology” in recent years. Although astronomers all know that the universe is expanding, there are some discrepancies when it comes to measuring its rate (aka. the Hubble constant). This issue arises Cosmic distance ladder, astronomers use different methods to measure relative distances on long scales. This includes making local distance estimates using parallax measurements, nearby variable stars, and supernovae („constant candles”).

They also conduct redshift measurements Cosmic microwave background (CMB), the remnant relic radiation big bang, to determine cosmological distances. The contrast between these two methods is „Hubble tension,” and astronomers are eager to solve it.

A new approach to measuring the Hubble constant

In a recent study, an international team of astrophysicists from the Niels Bohr Institute proposed a new method for measuring cosmic expansion. By observing colliding neutron stars (kilonova), they argue, astronomers can relax the tension and obtain stable measurements of the Hubble constant.

The research was led by astrophysicists Center of Cosmic Dawn (DAWN) and the Niels Bohr Company at the University of Copenhagen. They were joined by researchers from Tel Aviv University Cahill Center for Astrophysics (California Institute of Technology), The GSI Helmholtz Center for Heavy Ion ResearchThe Astrophysical Big Bang Laboratory, Helmholtz Research Academy Hessen for FAIRAnd this Darkness Research Group at Niels Bohr Institute. An article detailing their research recently appeared in the journal Astronomy & Astrophysics.

Galaxies are very still in space, but space itself is expanding. This causes the galaxies to move away from each other at an ever-increasing rate. However, how fast is a mystery. Credit: ESO/L. Pants. Galaxies are very still in space, but space itself is expanding. This causes the galaxies to move away from each other at an ever-increasing rate. However, how fast is a mystery. Credit: ESO/L. Pants

The historical context of the expansion of the universe

The expansion of the universe is something astronomers have known about for over a century, thanks to Edwin Hubble. By observing galaxies and measuring their light curves for redshift, he demonstrated that the farther away a galaxy is, the faster it is receding. milky way. This confirmed the suspicions of many Einstein’s theory of general relativity, which predicts that the universe is either expanding or receding. By measuring the speed at which other galaxies are moving away from us, scientists have tried to measure the Hubble constant.

This rate of expansion is measured in „speed per distance”, and modern estimates are over 20 km/s () per million light-years. This means that a galaxy 100 million light-years away is receding from us at 2,000 km/s (1,242 mps), while another galaxy 200 million light-years away is receding at 4,000 km/s (2,485 mps). However, using the supernova to measure the distances and velocities of the galaxies yields 22.7 ± 0.4 km/s, while the CMP analysis gives 20.7 ± 0.2 km/s. This may not seem like much, but the difference also produces significantly different estimates for the age of the universe -12.8 and 13.8 billion years.

Emerging solutions and new research

Although uncertainties were to be expected in the early 20th century, improvements in measurement techniques have come a long way, and the discrepancy between measurements has decreased. As a result, astronomers and cosmologists are now at a point where they can say with confidence that the two values ​​cannot be right. This has led many scientists to wonder whether systematic biases might affect one of the results, or whether special physics (a la early dark energy) was involved in the early universe.

In their paper, the team proposed a new method for measuring distance, thereby helping to resolve the ongoing controversy. The research was led by Albert Snappen, a Ph.D. student in astrophysics at the Cosmic Dawn Center at the Niels Bohr Institute. As he recently explained:

„When two ultra-compact neutron stars—the remnants of supernovae—come around each other and eventually merge, they go off in a new explosion; called a kilonova. We recently demonstrated how this explosion is symmetric, and this symmetry is not only beautiful, but incredibly useful.

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In a previous study (“Spherical symmetry in Kilonova AT2017gfo/GW170817„), Snappen and several of his colleagues reported the discovery of a „perfect explosion in space” in this recent study. This contradicts previous assumptions about kilonovae, indicating that the collision produced a complete spherical explosion. As they reported at the time, the discovery provided insight into the fundamental physics and the universe. could provide a new way to measure age. Another study published in September („On the blackbody spectrum of kilonovae„), despite their complexity, Snappen demonstrated that kilonovae can be described by a single temperature.

The left hemisphere shows the expanding remnant of the supernova discovered by Tycho Brahe in 1572, observed here in X-rays. Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al. On the right is a map of the cosmic background radiation from one half of the sky seen in microwaves. Credit: NASA/WMAP Science Team

This simple feature of kilonovae, combined with their apparent symmetry, allowed Snappen to determine precisely how much light was emitted by an event. By comparing this luminosity to how much light reaches Earth, astronomers can measure the distance to the kilonova, leading to a new and independent method of calculating distances to galaxies that host kilonovae. As Darach Watson, associate professor at the Cosmic Dawn Center and co-author of the study, explained:

“Supernovae, which have so far been used to measure the distances of galaxies, do not always emit the same amount of light. Also, they first had to calibrate the distance using another class of stars, called Cepheids, which also had to be calibrated. With KiloNova we can avoid these problems that introduce uncertainty in the measurements.”

To demonstrate the potential of the new method, the team applied it to a kilonova observed by astronomers in 2017. The resulting Hubble constant calculation is closer to the value obtained by the CMB method, but whether this method can resolve the Hubble tension remains to be seen. i saw „We only have this one case study so far, and many more examples are needed before we can establish a strong conclusion,” Snappen said. „But our method bypasses at least some known sources of uncertainty, and that’s very important Clean Organization for study. It does not require any calibration or correction factor.

For more on this research, see Neutron star collisions illuminate the expansion of the universe.

Adapted from an originally published article Universe Today.