Resolving Cosmological Inconsistencies: Neutron Star Collisions as a New Tool for Measuring Universal Expansion

Visual representation of the collision between two neutron stars, termed as a “kilonova” event. Credit: Elizabeth Wheatley (STScI)

Astrophysicists suggest that the observation of colliding neutron stars, or kilonovae, could be an innovative technique to reconcile the persisting discrepancies in calculations of the Universe’s rate of expansion, often referred to as the “Hubble Tension.”

The astrophysical community has been facing what some term as a “Cosmological Crisis” in recent times. While it is widely accepted that the Universe is expanding, there have been irregularities in quantifying the pace of this expansion, also known as the Hubble Constant. This inconsistency stems from the Cosmic Distance Ladder, a framework in which various methods are used for measuring distances across large scales, including parallax measurements, nearby variable stars, and supernovae, often referred to as “standard candles.”

Additionally, astronomers employ redshift analyses of the Cosmic Microwave Background (CMB), the residual radiation from the Big Bang, to establish cosmological distances. The variance between these approaches has led to the issue commonly described as the “Hubble Tension,” a problem that astronomers are keen to resolve.

A Groundbreaking Method for Calculating the Hubble Constant

A recent academic paper, spearheaded by an international collective of astrophysicists from the Niels Bohr Institute, posits a groundbreaking technique to measure the Universe’s expansion rate. They contend that the observation of kilonovae can alleviate the Hubble Tension and yield consistent determinations of the Hubble Constant.

This research was a collaborative effort that included scholars from the Cosmic Dawn Center (DAWN), the Niels Bohr Institute at the University of Copenhagen, Tel Aviv University, the Cahill Center for Astrophysics at the California Institute of Technology, the GSI Helmholtz Centre for Heavy Ion Research, the Astrophysical Big Bang Laboratory, Helmholtz Research Academy Hessen for FAIR, and the DARK research group at the Niels Bohr Institute. The findings were published in the journal Astronomy & Astrophysics.

Although galaxies appear stationary, the fabric of space itself is expanding, causing galaxies to drift apart at an accelerating pace. The exact rate of this phenomenon remains somewhat enigmatic. Credit: ESO/L. Calçada.

Historical Perspective on Universal Expansion

The notion that the Universe is expanding is well over a century old, credited to Edwin Hubble’s observations and redshift analyses of galaxies. His work confirmed aspects of Einstein’s Theory of General Relativity, specifically that the Universe is either expanding or contracting. By measuring the speed at which galaxies are receding from the Milky Way, scientists have endeavored to calculate the Hubble Constant.

Modern measurements estimate this rate to be slightly over 20 km/s per million light-years. Yet, there is a noticeable discrepancy between calculations based on supernovae and those based on the CMB—22.7 ± 0.4 km/s versus 20.7 ± 0.2 km/s, respectively. This seemingly minor difference leads to significantly varying estimates of the Universe’s age—12.8 versus 13.8 billion years.

Innovative Solutions and Recent Studies

While early 20th-century measurements were understandably fraught with uncertainties, advances in technology have refined these calculations considerably. Consequently, it has reached a point where astronomers assert that both values cannot be accurate. This has led to speculation on whether systematic biases or unique early-Universe physics, such as early Dark Energy, could be factors.

In their academic paper, the researchers propose an innovative approach for distance measurements that could mitigate the existing disagreements. Albert Sneppen, a Ph.D. student in astrophysics at the Cosmic Dawn Center at the Niels Bohr Institute, elaborated on this:

“When two ultra-compact neutron stars, remnants of supernovae, orbit one another and finally coalesce, they result in a new type of explosion known as a kilonova. Recent findings indicate that this event is remarkably symmetric. This symmetry is not just aesthetically pleasing but has practical implications.”

Prior research has uncovered that kilonovae produce perfectly spherical explosions, contradicting earlier assumptions and offering a new avenue for measuring the Universe’s age. Another study demonstrated that, despite their complexity, kilonovae can be characterized by a single temperature.

Practical Implications of Kilonova Observations

This unique characteristic of kilonovae, along with their observed symmetry, has allowed researchers to predict the amount of light emitted during such an event accurately. By comparing this with the light that reaches Earth, they can calculate the distance to the kilonova, providing a new and independent way to gauge distances to galaxies containing these phenomena. As Darach Watson, an associate professor at the Cosmic Dawn Center and a co-contributor to the study, stated:

“Unlike supernovae, which have been traditionally used for such measurements and whose emitted light can vary, kilonovae do not require prior calibration of distance using other types of stars like Cepheids. This avoids additional uncertainties in the measurements.”

To validate the effectiveness of this new approach, the researchers applied it to a kilonova observed in 2017. The calculated Hubble Constant aligned more closely with the value obtained using the CMB method. However, whether this technique can definitively resolve the Hubble Tension is yet to be ascertained. “We only have this one instance to study thus far and will require a larger sample size to confirm the efficacy of our method,” noted Sneppen. “However, this system eliminates some known sources of error and does not require any calibration or correction factors.”

For additional details on this research, refer to the article titled ‘Neutron Star Collisions Illuminate the Expansion of the Universe.’

Adapted from an article originally published on Universe Today.

Frequently Asked Questions (FAQs) about Neutron Star Collisions and Cosmological Expansion

More about Neutron Star Collisions and Cosmological Expansion

• Hubble Constant Explained
• Cosmic Distance Ladder
• Neutron Stars and Kilonovae
• Cosmic Microwave Background
• Niels Bohr Institute Research
• Einstein’s Theory of General Relativity
• The Expansion of the Universe: A Historical Perspective
• Understanding Redshift
• Early Dark Energy Theories
• Supernovae as Standard Candles