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Gravitational wave scientists propose a new method for improving the Hubble constant – the expansion and age of the universe



Gravitational wave scientists propose a new method for improving the Hubble constant - the expansion and age of the universe

Illustration of an artist of a pair of merging neutron stars. Credit: Carl Knox, OzGrav-Swinburne University

A team of international scientists led by the Galician Institute of High Energy Physics (IGFAE) and the ARC Center of Excellence for Gravitational Wave Detection (OzGrav) has proposed a simple and new method to reduce the accuracy of Hubble̵

7;s continuous measurements to 2%. using an observation of a pair of merging neutron stars.

The universe is constantly expanding. Therefore, distant objects such as galaxies are moving away from us. In fact, the farther they are, the faster they move. Scientists describe this expansion through a number known as the Hubble constant, which tells us how fast objects in the universe move away from us based on their distance from us. By measuring the Hubble constant, we can accurately determine some of the most fundamental properties of the universe, including its age.

For decades, scientists have measured the Hubble constant with increasing accuracy, collecting electromagnetic signals emitted from the universe, but reaching a challenging result: the two best current measurements give conflicting results. Since 2015, scientists have been trying to meet this challenge with the science of gravitational waves, waves in the fabric of space-time that move at the speed of light. Gravitational waves are generated in the most violent cosmic events and provide a new channel for information about the universe. They emit during the collision of two neutron stars – the dense nuclei of collapsed stars – and can help scientists delve into the constant mystery of Hubble.

Unlike black holes, merging neutron stars produce both gravitational and electromagnetic waves, such as X-rays, radio waves, and visible light. While gravitational waves can measure the distance between the fusion of neutron stars and Earth, electromagnetic waves can measure how fast an entire galaxy is moving away from Earth. This creates a new way to measure the Hubble constant. However, even with gravitational waves, it is still difficult to measure the distance to neutron star mergers – this is partly why current measurements of the Hubble constant based on gravitational waves have an uncertainty of ~ 16%, much larger than existing measurements using other traditional techniques.

In a recently published article in Astrophysical Journal Letters, a team of scientists led by the ARC Center for Excellence for Gravitational Wave Detection (OzGrav) and Monash University alumni Prof. Juan Calderon Bustilo (now a junior leader of La Caixa and an associate of Marie Curie at the Galician Institute of Higher Physics Energy at the University of Santiago de Compostela, Spain), proposed a simple and new method to reduce the accuracy of these measurements to 2% using an observation of a pair of merging neutron stars.

According to Prof. Calderon Bustilo, it is difficult to interpret how far these mergers take place, because “at the moment we cannot say whether the binary file is very far and faces the Earth, or is much closer to the Earth in its orbital plane. . “To decide between these two scenarios, the team proposed to study secondary, much weaker components of gravitational wave signals emitted by neutron star fusions, known as higher modes.

“Just as an orchestra plays different instruments, mergers of neutron stars emit gravitational waves through different modes,” explains Prof. Calderon Bustilo. “When the merging neutron stars are facing you, you will hear only the strongest instrument. However, if you are close to the orbital plane of the fusion, you should also hear the secondary ones. This allows us to determine the tilt of the neutron fusion star and better measure the distance. “

However, the method is not entirely new: “We know that this works well in the case of very massive black hole mergers, because our current detectors can record the moment of the merger, when higher modes are most pronounced. But in the case of neutron mergers. stars, the height of the fusion signal is so high that our detectors cannot record it. We can only record earlier orbits, “says Prof. Calderon Bustilo.

Future gravitational wave detectors, such as the proposed Australian NEMO project, will be able to gain access to the actual fusion phase of neutron stars. “When two neutron stars merge, the nuclear physics that control their matter can cause very rich signals that, if detected, can allow us to know exactly where the Earth is in relation to the orbital plane of the fusion,” he said. and OzGrav Principal Investigator Dr. Paul Lasky of Monash University. Dr. Laski is also one of the leaders in the NEMO project. “A detector like NEMO can detect these rich signals,” he added.

In their study, the team performed computer simulations of neutron star fusions that could reveal the effect of stellar nuclear physics on gravitational waves. Studying these simulations, the team found that a detector such as NEMO could measure the Hubble constant with an accuracy of 2%.

Co-author of the study, Professor Tim Dietrich of the University of Potsdam, said: “We have found that the fine details describing the way neutrons behave in a star create fine signatures in gravitational waves that can significantly help determine the rate of expansion of a star. It is fascinating to see how effects on the smallest nuclear scale can infer what happens to the largest possible cosmological. “

Samson Leong, a student at the Chinese University of Hong Kong and co-author of the study, said, “One of the most exciting things about our result is that we’ve made such a big improvement while looking at a rather conservative scenario. While NEMO will indeed do so be sensitive to neutron star fusion, more evolved detectors such as the Einstein Telescope or the Space Explorer will be even more sensitive, allowing us to measure the expansion of the universe with even better accuracy! “

One of the most notable consequences of this study is that it can determine whether the universe is expanding evenly in space, as currently assumed. “Previous methods of achieving this level of accuracy relied on combining many observations, assuming that the Hubble constant was the same in all directions and throughout the history of the universe,” says Calderon Bustilo. “In our case, each individual event would give a very accurate estimate of” Hubble’s own constant, “which allows us to check whether this is actually a constant or whether it varies in space and time.”


Deciphering the life of double neutron stars in radio and gravitational wave astronomy


More information:
Juan Calderon Bustilo et al. Mapping the expansion of the universe: Enabling measurements of percentages of the Hubble constant with a single binary detection of neutron star fusion, Astrophysical Journal Letters (2021). DOI: 10.3847 / 2041-8213 / abf502

At Arxiv: arxiv.org/abs/2006.11525

Provided by the ARC Center of Excellence for gravitational wave detection



Quote: Gravitational Scientists Propose New Method for Improving Hubble’s Constant – The Expansion and Age of the Universe (2021, May 4), obtained on May 4, 2021 from https://phys.org/news/2021-05 -gravitational-wave- scientists-method-improvement-hubble.html

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