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

A pair of superdense neutron stars collide with an explosion Gravitational waves

The artist’s impression of the explosion and explosion of gravitational waves emitted by a collision of a pair of superdense neutron stars. 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 don’t know for sure.

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

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

Astronomers using the National Science Foundation’s (NSF) radio telescopes have demonstrated how a combination of gravitational waves and radio observations, along with theoretical modeling, can turn fusion of pairs of neutron stars into a “space ruler” capable of measuring the expansion of the universe and solving unresolved question about its speed.

Astronomers used the very long NSF (VLBA) source array, the Karl G. Jansky (VLA) very large array, and the Robert C. Byrd Green Bank (GBT) telescope to study the effects of a collision of two neutron stars that produce gravity. waves discovered in 2017. This event offered a new way to measure the rate of expansion of the universe, known to scientists as the Hubble constant. The rate of expansion of the universe can be used to determine its size and age, as well as serve as a basic tool for interpreting observations of objects elsewhere in the universe.

Orbital plane orientation

Radio observations of a jet of material ejected after the fusion of neutron stars were key to allowing astronomers to determine the orientation of the orbital plane of stars before they merged and thus the “brightness” of gravitational waves emitted in the direction of the Earth. This could make such events an important new tool for measuring the rate of expansion of the universe. Credit: Sofia Danelo, NRAO / AUI / NSF

Two leading methods for determining the Hubble constant use the characteristics of the cosmic microwave background, the residual 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.

“The neutron star the merger gives us a new way to measure the Hubble constant and hopefully solve the problem, ”said Kunal Muli of the National Radio Astronomical Observatory (NRAO) and Caltech.

The technique is similar to that of supernova explosions. All type Ia supernova explosions are thought to have an inherent brightness that can be calculated based on the rate at which they illuminate and then disappear. The measurement of the brightness seen from Earth then shows the distance to the supernova explosion. The measurement of the Doppler displacement of light from the receiving supernova galaxy shows the rate at which the galaxy is moving away from Earth. The speed divided by the distance gives the Hubble constant. To obtain an accurate figure, many such measurements must be made at different distances.

When two massive neutron stars collide, they create an explosion and an explosion of gravitational waves. The shape of the gravitational wave signal tells scientists how “bright” this explosion of gravitational waves was. Measuring the “brightness” or intensity of gravitational waves received on Earth can give the distance.

“It’s a completely independent measurement tool that we hope can clarify the true value of the Hubble constant,” Mulley said.

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

“To use gravitational waves to measure distance, we had to know this orientation,” said Adam Deler of Swinburne Technical University in Australia.

For months, astronomers have used radio telescopes to measure the motion of a superfast jet of material ejected by an explosion. “We used these measurements along with detailed hydrodynamic simulations to determine the angle of orientation, thus allowing the use of gravitational 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 still not enough to resolve the uncertainty, scientists said, but the technique can now be applied to future mergers of neutron stars detected by gravitational waves.

“We believe that 15 more such events, which can be observed with both gravitational waves and many details 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 results to the journal Natural astronomy.

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

The National Radio Astronomical Observatory is a facility of the National Science Foundation operating under a collaborative agreement with Associated Universities, Inc.

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