In Hubble's measurement, the jet appeared to be moving seven times the speed of light. Later, radio observations indicated that the jet had decelerated to apparent speeds four times faster than light. This, however is just a "cosmic illusion" since nothing can travel faster than light.
A collision between two neutron stars propelled a jet through space at speed greater than 99.97% the speed of light. This is according to NASA’s Hubble Space Telescope measurements. The explosive event was observed in August 2017 and was designated GW170817. It was estimated that the blast had the same energy as a supernova explosion. As a result of the binary neutron star merger, the first gravitational wave and gamma-ray signals were detected at the same time. These extraordinary collisions marked a significant turning point in the investigation. Seventy observatories around the globe and in space observed this merger’s aftermath. They detected gravitational waves and a broad range of the electromagnetic spectrum. This event took place in 2017.
A historic observation
This marked an extremely significant achievement in time domain and multi-messenger astrophysics. The study of the universe as it changes over time using multiple “messengers” such as light and gravitational waves. Within two days of the explosion, scientists focused Hubble on the explosion site. Upon collapsing, neutron stars created a black hole whose gravity pulled matter toward it. From those poles, jets emerged from a rapidly-spinning disk. During the explosion, the roaring jet smashed into the expanding debris cloud and swept it up. Finally, an emergent jet appeared through a blob of material.
As it turns out, scientists have been analyzing Hubble data and data from other telescopes for years to come up with this complete image. Hubble observations were combined with observations taken by multiple radio telescopes of the National Science Foundation for very long baseline interferometry (VLBI). Data were collected 75 days and 230 days after the explosion. “I’m amazed that Hubble could give us such a precise measurement, which rivals the precision achieved by powerful radio VLBI telescopes spread across the globe,” said Kunal P. Mooley of Caltech in Pasadena, California. He is the lead author of a paper being published in the October 13 journal of Nature magazine.
Extreme precision
Furthermore, the authors combined Hubble data with data from the Gaia satellite and VLBI to achieve extreme precision. This measurement was made after months of careful data analysis, said Jay Anderson of the Space Telescope Science Institute in Baltimore, Maryland. Their combined observations helped them locate the explosion site. In Hubble’s measurement, the jet appeared to be moving seven times the speed of light. Later, radio observations indicated that the jet had decelerated to apparent speeds four times faster than light.
Since nothing can move faster than light, this “superluminal” movement is an illusion. Since the jet is approaching Earth at nearly the speed of light, its light will travel a shorter distance when it emits at a later time. Essentially, the jet chases its own light. As a result, light from the jet has been emitted after a much longer period than what the observer expects. As a result, the object’s speed is overestimated, exceeding light speed. The jet was moving at an average speed of 99.97% of the speed of light at the time it was launched, said Wenbin Lu of the University of California, Berkeley. A combination of Hubble and VLBI measurements announced in 2018 further supports the theory that neutron star mergers are related to short-duration gamma-ray bursts. An emerging fast-moving jet is required for this connection, measured in GW170817.
The Hubble constant
In addition, this work allows for more detailed studies of neutron star mergers detected by the gravitational wave observatories LIGO, Virgo, and KAGRA. A large enough sample of relativistic jets over the next few years may provide another approach to measuring the Hubble constant. This constant is an estimate of the universe’s expansion rate. A discrepancy exists between Hubble constant values for the early universe and the nearby universe. This is one of the greatest mysteries in astrophysics. Differences in values are based on Hubble and other observatories’ extremely precise measurements of Type Ia supernovae, as well as the Planck satellite’s measurements of the Cosmic Microwave Background. Astronomers trying to solve the puzzle may benefit from a deeper understanding of relativistic jets.
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