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

Pair of superdense neutron stars collide with explosive gravitational waves

The artist’s impression of the explosion and burst of gravitational waves emitted when a pair of superdense neutron stars collide. New observations with radio telescopes show that such events can be used to measure the expansion rate 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 many different ways, but some of their results do not agree with each other. This disagreement, or voltage, 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 of 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 shown how a combination of gravitational waves and radio observations, along with theoretical modeling, can transform mergers of neutron star pairs into a “cosmic ruler” capable of measuring expanding the Universe and solving an outstanding question about its speed.

Astronomers used the NSF’s Very Long Baseline Array (VLBA), the Karl G. Jansky Very Large Array (VLA) and the Robert C. Byrd Green Bank Telescope (GBT) to study the consequences of the collision of two neutron stars. which produced the gravity waves detected in 2017. This event offered a new way to measure the expansion rate of the Universe, known by 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 an essential tool for interpreting observations of objects in other parts of the Universe.

Orientation of the orbital plane

Radio observations of a jet of material ejected in the aftermath of the neutron star meltdown were critical in allowing astronomers to determine the orientation of the stars’ orbital plane prior to their merger, and thus the “brightness” of the emitted gravitational waves. in the direction of the Earth. This can make such events an important new tool for measuring the expansion rate of the Universe. Credit: Sophia Dagnello, NRAO / AUI / NSF

Two main methods to determine the Hubble constant use the characteristics of the cosmic microwave background, the residual radiation from the 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 fusion offers us a new way to measure the Hubble constant and hopefully solve the problem, ”said Kunal Mooley, of the National Radio Astronomy Observatory (NRAO) and Caltech.

The technique is similar to that using supernova explosions. Type Ia supernova explosions are thought to all have an inherent luminosity that can be calculated by how quickly they light up and then vanish. The measurement of brightness seen from Earth therefore indicates the distance to the supernova explosion. Measurement of the Doppler shift of light from the supernova’s host galaxy indicates the speed at which the galaxy is moving away from Earth. Speed ​​divided by distance produces the Hubble constant. To get an accurate figure, many of these measurements must be made at different distances.

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

“This is a completely independent means of measurement that we hope will clarify what the true value of the Hubble constant is,” said Mooley.

However, there is a twist. The intensity of gravitational waves varies with their orientation with respect to the orbital plane of the two neutron stars. Gravitational waves are strongest in the direction perpendicular to the orbital plane and weakest if the orbital plane is edge seen from the Earth.

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

Over a period of months, astronomers used radio telescopes to measure the motion of a jet of superfast material ejected by the explosion. “We used these measurements together with detailed hydrodynamic simulations to determine the orientation angle, thus enabling the use of gravitational waves to determine 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, the scientists said, but the technique can now be applied to future neutron star mergers detected with gravitational waves. .

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

The international science team led by Hotokezaka is reporting its findings in the journal Nature Astronomy.

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

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

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