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How Fast Is the Universe Expanding Exactly?

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Exactly how fast is the universe expanding?

Scientists are still not completely sure, but a Princeton-led team of astrophysicists has used the neutron star merger detected in 2017 to come up with a more precise value for this figure, known as the Hubble constant. Their work appears in the current issue of the journal Nature Astronomy

.The collision of two neutron stars (GW170817) flung out an extraordinary fireball of material and energy that is allowing a Princeton-led team of astrophysicists to calculate the Hubble constant, the speed of the universe’s expansion. They used a super-high-resolution radio ‘movie’ (left) that they compared to a computer model (right). To generate their ‘movie,’ the science team combined data from enough radio telescopes spread over a large enough region to generate an image with such high resolution that if it were an optical camera, it could see individual hairs on someone’s head 6 miles away. The movie emphasizes observations taken 75 days and 230 days after the merger. The middle panel shows the radio afterglow light curve.

Video by Ore Gottlieb and Ehud Nakar, Tel Aviv University

“The Hubble 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, the Lyman Spitzer, Jr. Postdoctoral Fellow in Princeton’s Department of Astrophysical Sciences. “So we’d like to know what its value is.”

Currently, the two most successful techniques for estimating the Hubble constant are based on observations of either the cosmic microwave background or stars blowing themselves to pieces in the distant universe.

But those figures disagree: Measurements of exploding stars — Type Ia supernovae — suggest that the universe is expanding faster than is predicted by Planck observations of the cosmic microwave background.

“So either one of them is incorrect, or the models of the physics which underpin them are wrong,” said Hotokezaka. “We’d like to know what is really happening in the universe, so we need a third, independent check.”

He and his colleagues — Princeton’s NASA Sagan Postdoctoral Fellow 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 from Swinburne University of Technology in Australia — found that independent check by using the merger of two neutron stars.

Neutron star mergers are phenomenally energetic events in which two massive stars whip around each other hundreds of times per second before merging in an extraordinary collision that flings out a burst of gravitational waves and an enormous blast of material. In the case of the neutron star merger that was detected on Aug. 17, 2017, the two stars — each the size of Manhattan and with almost twice the mass of the sun — were moving at a significant fraction of the speed of light before they collided.

The gravitational wave burst from a neutron star merger makes a distinctive pattern known as a “standard siren.” Based on the shape of the gravitational wave signal, astrophysicists can calculate how strong the gravitational waves should have been. They can then compare that to the measured strength of the signal to work out how far away the merger occurred.

Radio wave observations and model of fireball from neutron star collision
Credit: Ore Gottlieb and Ehud Nakar, Tel Aviv University
But there’s a catch — this only works if they know how the merging stars were oriented with respect to Earth’s telescopes. The gravitational wave data can’t distinguish between mergers that were nearby and edge-on, distant and face-on, or something in between.

To separate those possibilities, the researchers used a super-high-resolution radio “movie” of the fireball of material that was left behind after the neutron stars merged. To make their movie, they combined data from radio telescopes spread across the world.

“The resolution of the radio images we made was so high, if it was an optical camera, it could see individual hairs on someone’s head 3 miles away,” said Deller.

“By comparing the miniscule changes in the location and shape of this distant bullet of radio-emitting gas against several models including one developed on supercomputers, we were able to determine the orientation of the merging neutron stars,” said Nakar.

Using this, they calculated how far away the merging neutron stars were — and then, by comparing that with how fast their host galaxy is rushing away from ours, they could measure the Hubble constant.

After the 2017 neutron star merger (GW170817) was registered by nearly every astronomical instrument on the planet, astrophysicists calculated that the Hubble constant value was between 66 and 90 kilometers per second per megaparsec. By using tight constraints on the orientation of the collision, published last year by Mooley and several of the same co-authors, including Hotokezaka, the current group of collaborators were able to pin that estimate down further, to between 65.3 and 75.6 km/s/Mpc.

While that precision is “quite good,” said Hotokezaka, it’s still not good enough to distinguish between the Planck and Type Ia models. He and his colleagues estimate that to get that level of precision, they would need data from 15 more collisions like GW170817 — with its helpful abundance of data up and down the entire electromagnetic spectrum — or 50 to 100 collisions that are detected only with gravitational waves.

“This is the first time that astronomers have been able to measure the Hubble constant by using a joint analysis of a gravitational-wave signals and radio images,” said Hotokezaka. “It is remarkable that only a single merger event allows us to measure the Hubble constant with a high precision — and this approach relies neither on the cosmological model (Planck) nor the cosmic-distance ladder (Type Ia).”

 The research was supported by Princeton University, the Israel Science Foundation, the Netherlands Organization for Scientific Research, the National Aeronautics and Space Administration, the National Science Foundation (AST-1654815), and the Australian Research Council (FT150100415).

Contacts and sources:
Liz Fuller-Wright
Princeton University

Citation: “A Hubble constant measurement from superluminal motion of the jet in GW170817″ by K. Hotokezaka, E. Nakar, O. Gottlieb, S. Nissanke, K. Masuda, G. Hallinan, K. P. Mooley and A. T. Deller appears in the current issue of the journal Nature Astronomy (DOI: 10.1038/s41550-019-0820-1.)

 



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  • Counter Analysis

    Some physicists believe the speed of light is slowing. If so, it may be a function of the expanding universe. Light speed is instant travel. It appears to be finite because light is stuck in time and follows its present moment through space. Space is laid out like a time differential matrix. The further the distance, the greater the time difference. A light year’s distance has time that is simultaneous with one year in the future or past, and the present moment, depending on whether your point of reference is origin or destination. If you are the light speed traveler, you arrive anywhere of any distance with no time having passed.

    Assuming the, “Big Bang,” when the universe was a singularity, there was no time differential across the universe, and light if it existed, would be measured to have infinite speed by even an outside observer as it would take no time to go from one end of the universe to the other end (since the universe was a singularity). As the universe stretched and expanded, different points of space in the universe accelerated creating the time differential, though all the differing times remain simultaneous to each other as they all came from the same singularity. The more space-time is stretched out, the slower the speed of light appears, as instant speed is diluted by an increasing time differential.

    In the early universe, its expansion relative to its size was enormous resulting in a very noticeable slowing of light speed. Much later, the aging universe became so huge that the expansion relative to the universe’s size became very much less, resulting in a much slower and harder to detect deceleration of light speed. Or at the very least from our limited perspective, the expansion between two points that we can measure became very much less than the expansion we would have measured in the past.

    If we can measure the deceleration of light speed, we may be able to extrapolate the age and rate of expansion of the universe. It would be very interesting to link this deceleration with the red shift of distant objects. The speed of light and it’s constant are key to a lot of physics including matter. Perhaps when the speed of light slowed to a certain point in the early universe, it allowed matter to form like a precipitate, and the other forces to separate.

    Maybe we can send space craft far enough away from us and each other to measure light from a distant source and analyze the changing red shift and any light speed discrepancy.

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