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How fast is the universe expanding?
Scientists are still not quite sure, but a team of astrophysicists led by Princeton used the neutron star fusion detected in 2017 to get a more accurate value for this figure, called Hubble's constant . Their work appears in the latest issue of the journal Nature Astronomy.
"Hubble's constant is one of the most fundamental pieces of information that describes the state of the universe in the past, present, and future," said Kenta Hotokezaka, Lyman Spitzer Postdoctoral Fellow, Jr. Department of Astrophysical Sciences, Princeton. "So we would like to know what is its value."
Currently, the two most effective techniques for estimating the Hubble constant are based on observations of the cosmic microwave background or stars splintering in the far universe.
But these figures do not agree: the exploded star measurements – supernova type Ia – suggest that the universe is developing faster than predicted by Planck's observations on the microwave cosmic background.
"So, one or the other of them is incorrect, or the models of the physics that underlie them are wrong," Hotokezaka said. "We would like to know what is really happening in the universe, so we need a third independent check."
The collision of two neutron stars projected an extraordinary ball of fire of matter and energy allowing a team of astrophysicists led by Princeton to calculate the Hubble constant, the speed of the extension of the universe. They used a very high resolution "movie" radio (left) that they compared to a computer model (right). To generate their film, the science team combined the data of enough radio telescopes spread over a region large enough to generate an image with a resolution so high that, if it was an optical camera, it could see hair on the head of a person to ten kilometers. The film focuses on observations taken 75 days and 230 days after the merger. The middle panel shows the curve of the reverb light of the radio.
He and his colleagues – NASA's Sagan Postdoctoral Fellow at Princeton Kento Masuda, Ore Gottlieb and Ehud Nakar from Tel Aviv University in Israel, Samaya Nissanke from the University of Amsterdam, Gregg Hallinan and Kunal Mooley from the California Institute of Technology and Adam Deller of the Swinburne University of Technology in Australia – found this independent check using the fusion of two neutron stars.
Neutron star mergers are phenomenally energetic events in which two massive stars whip themselves hundreds of times per second before turning into an extraordinary collision that projects a burst of gravitational waves and a tremendous breath of material. In the case of the neutron star melting detected on August 17, 2017, the two stars, each of the size of Manhattan and weighing nearly twice the mass of the sun, were moving at a significant fraction of the speed of light is collided.
The gravitational wave resulting from a fusion of neutron stars creates a distinctive pattern called "standard siren". On the basis of the shape of the gravitational wave signal, astrophysicists can calculate the strength of gravitational waves. They can then compare this to the measured signal strength to determine the distance of the fusion.
But there is a problem: it only works if they know how the merging stars have been oriented relative to Earth's telescopes. Gravitational wave data do not distinguish between near-edge fusions, edges, face-to-face, or anything in between.
To separate these possibilities, the researchers used a very high-resolution radio "film" on the fireball of material that had been left after the fusion of the neutron stars. To make their film, they combined radio telescope data spread all over the world.
"The resolution of the radio images we had was so high that if it was an optical camera, it could see hair standing on someone's head three kilometers away," Deller said.
"By comparing tiny changes in the location and shape of this far-flung gas-emitting gas balloon compared to several models, including one developed on supercomputers, we were able to determine the orientation of neutron stars." merger, "said Nakar.
Using this, they calculated the distance separating the merging neutron stars – and then, comparing that to the speed at which their host galaxy was separating from ours, they could measure the Hubble constant.
After the merger of the 2017 neutron stars (GW170817) was recorded by almost every astronomical instrument on the planet, astrophysicists calculated that Hubble's constant value was between 66 and 90 km per second per megaparsec. Using strict constraints on the direction of the collision, published last year by Mooley and several of the same coauthors, including Hotokezaka, the current group of collaborators was able to refine this estimate between 65.3 and 75 , 6 km / s. / Mpc.
Hotokezaka believes this accuracy is "good enough", but is not yet sufficient to distinguish Planck models from Type Ia models. He and his colleagues believe that to obtain this level of precision, they would need data from 15 additional collisions such as GW170817 – with its useful abundance of data from the entire electromagnetic spectrum – or from 50 to 100 collisions detected only by gravitational waves. . .
"This is the first time that astronomers have been able to measure the Hubble constant using a joint analysis of gravitational wave signals and radio images," said Hotokezaka. "It is remarkable that a single fusion event allows us to measure the Hubble constant with great precision – and this approach is neither based on the cosmological model (Planck) nor on the cosmic distance scale ( Type Ia). "
"A constant measure of Hubble 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 appears in the issue current journal of Nature Astronomy (DOI: 10.1038 / s41550-019-0820-1.) The research was funded by the University of Princeton, the Israel Science Foundation, the Netherlands Organization for the Scientific research, the National Aeronautics and Space Administration, the National Science Foundation (AST-1654815) and the Australian Research Council (FT150100415).
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