An event in the vast distant space of our universe helped astronomers refine one of the universe’s most important features. So, how the collision of two neutron stars helped astronomers measure the Hubble Constant.
In August 2017, when astronomers around the globe observed two neutron stars collide, it taught us a lot of cool things about the Universe. Now there’s another cool thing. Astronomers used it to refine one of the Universe’s most important features-the Hubble Constant.
The Hubble Constant is the name given to the rate at which the Universe expands, and for some time, discrepancies in this measurement have been giving grief to cosmologists.
How the rate of expansion of universe was measured?
Earlier expansion of the universe was measured through different methods. Some of them are as below:
According to the data from the Planck satellite measuring the cosmic microwave background (the conditions of the early Universe only 380,000 years after the Big Bang), the Hubble Constant should be 67.4 kilometers (41.9 miles) per second per megaparsec. That’s one measuring method.
Another is by studying the Type Ia supernovae nebulae left behind. Way back when Edwin Hubble noted their Doppler shift-that is, the changes in light’s wavelength as the nebula moves away. Recently, this technique returned 72.78 kilometers per second per megaparsec.
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Another, the more latest method utilizes standard candles such as Cepheid variable stars, whose known luminosity enables precise range calculations. And that’s where trouble strikes -because these measurements turn into a faster Hubble Constant. The dilemma is, a measurement based on motions of 70 Cepheid variables lately produced an outcome of 74.03 kilometers per second per megaparsec.
“Either one of them is wrong, or the models of the physics that underpin them are wrong. We’d like to understand what’s really going on in the Universe, so we need an independent check,” said Swinburne University of Technology astrophysicist Adam Deller.
Now the merger of two neutron stars gave us another way to measure the rate of expansion of the universe.
Cue GW170817, the gravitational wave event that allowed astronomers around the world for the first time to observe a neutron star collision in multiple ways. Including gravitational wave astronomy, optical astronomy, and radio astronomy.
“Neutron star mergers are phenomenally energetic processes. Two stars each more huge than our Sun whips hundreds of times a second around each other before combining and generating a huge explosion of material at immense velocity, as well as a burst of gravitational waves,” explained Deller.
“This burst of gravitational waves can be used as a ‘ standard siren’. Based on the gravitational wave signal shape, we can say how ‘bright’ the event should have been in gravitational waves. We can then take how bright the event was actually seen and work out what the distance must have been.”
This can only be performed, if we know the collision’s orientation. We need more information to do that than just the event itself.
That came in the form of the narrow, collimated plasma jet ejected from the collision, which radio telescopes observed over time. And they discovered that it had what is known as superluminal motion. That is when something seems to move faster than light-speed, depending on the angle we observe it.
By comparing tiny changes in the location and shape of this plasma, the team was able to calculate the neutron star orientation-which, in turn, allowed them to calculate their precise distance.
The collision took place 130 million light-years away in a galaxy, and the nice thing here is that we know the speed at which this galaxy moves away from us. So, they were able to derive the Hubble Constant when the team compared the distance of GW170817 to the speed of the galaxy. They finished with a figure of 70.3 kilometers per second per megaparsec.
As you can see, we’re at least in the right ballpark between the Planck measurement and the standard candle measurements-even if the new measurement isn’t sure enough to tell us if Planck, supernovae, or Cepheid variables are more precise.
“We have shown that more merging neutron stars will be able to make that discrimination in the near future,” said Deller.
The study was published in Nature Astronomy.