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How fast is the universe expanding? Measurement of cosmic expansion with radio astronomy and gravitational waves

Pair of Superdense neutron stars collide explosion Gravitational waves

The artist’s impression of the explosion and the eruption of gravitational waves emitted when a pair of super-dense neutron stars collide. New observations with radio telescopes show that such events can be used to measure the rate of expansion of the universe. Credit: NRAO / AUI / NSF

How fast is the universe expanding? We do not know for sure.

Astronomers study cosmic expansion by measuring the Hubble constant. They have measured this constantly in several different ways, but some of their results do not agree with each other. This disagreement, or excitement, in the Hubble constant there is a growing controversy in astronomy. But new observations of colliding neutron stars could provide a solution.

Join our host Melissa Hoffman from the National Radio Astronomy Observatory as she explains how astronomers use radio astronomy and gravitational waves to answer this cosmic mystery.

Astronomers using National Science Foundation (NSF) radio telescopes have demonstrated how a combination of gravitational waves and radio observations combined with theoretical modeling can turn the fusion of pairs of neutron stars into a “cosmic ruler” capable of measuring expansion. of the Universe and solve an outstanding question about its speed.

Astronomers used NSF’s Very Long Baseline Array (VLBA), Karl G. Jansky’s Very Large Array (VLA) and Robert C. Byrd’s Green Bank Telescope (GBT) to study the wake of the collision between two neutron stars that produced the gravitational waves detected in 2017. This event offered a new way to measure the rate of expansion of the universe, known by scientists as Hubble Constant. The rate of expansion of the universe can be used to determine its size and age as well as serve as an important tool for interpreting observations of objects elsewhere in the universe.

Orbital plane orientation

Radio observations of a beam of material ejected in the wake of the neutron star association were the key to allowing astronomers to determine the orientation of the stars’ orbital plane before their fusion, and thus the “brightness” of the gravitational waves emitted in the direction of the earth. This can make such events an important new tool for measuring the rate of expansion of the universe. Credit: Sophia Dagnello, NRAO / AUI / NSF

Two leading methods for determining Hubble Constant use the properties of the cosmic microwave background, the remaining radiation from Big bang, or a specific type of supernova explosion, called Type Ia, in the distant universe. However, these two methods give different results.

“That neutron star fusion gives us a new way to measure Hubble Constant and hopefully solve the problem, ”said Kunal Mooley of the National Radio Astronomy Observatory (NRAO) and Caltech.

The technique is similar to the one that uses the supernova explosions. Type Ia supernova explosions are believed to have an inherent brightness that can be calculated from the rate at which they light up and then fade away. Measuring the brightness seen from the ground then tells the distance to the supernova explosion. Measurement of Doppler displacement of light from the supernova’s host galaxy indicates the rate at which the galaxy returns from Earth. The speed divided by the distance gives the Hubble Constant. To get an accurate figure, many such measurements must be made at different distances.

When two massive neutron stars collide, they produce an explosion and a burst of gravitational waves. The shape of the gravitational wave signal tells researchers how “light” the burst of the gravitational wave was. Measuring the “brightness” or intensity of the gravitational waves as they are received on Earth can provide the distance.

“This is a completely independent measuring instrument that we hope can clarify what the true value of the Hubble Constant is,” Mooley said.

However, there is a twist. The intensity of the gravitational waves varies with their orientation relative to the orbital plane of the two neutron stars. The gravitational waves are stronger in the direction perpendicular to the orbital plane and weaker if the orbital plane is edge-on seen from the ground.

“In order to use the gravitational waves to measure the distance, we had to know that orientation,” said Adam Deller of Swinburne University of Technology in Australia.

Over a period of months, astronomers used the radio telescopes to measure the motion of a super-fast beam of material ejected from the explosion. “We used these measurements together with detailed hydrodynamic simulations to determine the orientation angle, allowing the use of gravity waves to determine the distance,” said Ehud Nakar of Tel Aviv University.

This single measurement of an event about 130 million light-years from Earth is not yet sufficient to solve the uncertainty, the researchers said, but the technique can now be used for future neutron star fusions detected by gravitational waves.

“We believe that another 15 such events, which can be observed both with gravitational waves and in detail with radio telescopes, may be able to solve the problem,” said Kenta Hotokezaka, of Princeton University. “This would be an important step forward in our understanding of one of the most important aspects of the universe,” he added.

The international scientific team led by Hotokezaka reports its findings in the journal Natural astronomy.

Reference: “A Hubble Constant Measurement from the Superluminal Motion of the Ray in GW170817” by K. Hotokezaka, E. Nakar, O. Gottlieb, S. Nissanke, K. Masuda, G. Hallinan, KP Mooley, and AT Deller, July 8, 2019 , Natural astronomy.
DOI: 10.1038 / s41550-019-0820-1

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under a partnership agreement by the Associated Universities, Inc.

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