Scientists develop a new method to calculate the Hubble constant

A precise measurement of the value of Hubble's constant has been a challenge to scientists for decades. Its value describes how fast the universe is expanding. Hence, a precise value of this constant would bring us one step closer towards understanding the origin, evolution and the fate of the universe. Now the researchers have found a new way to calculate this value using the gravitational waves.

So far, two approaches have been made to determine the value of Hubble's constant. The first method involves the observation of the redshifts in the distant light sources like stars, galaxies, etc. When any such object moves away due to expansion of universe, its light shift towards the red wavelength in the visible spectrum. By calculating the amount of redshifts, scientists can estimate the amount of acceleration with which that light source is moving away, and in turn the amount of expansion is calculated.

The second method involves the study of CMB (cosmic microwave background) - the blueprint of the radiations just after the Big Bang. Precise measurements in the temperature variations in the CMB using the Planck space telescope, allows the scientists to calculate the Hubble's constant.
However, the problem with these techniques is that the estimated values don't match with each other in the two methods. The value from redshift method comes out to be 73 km per sec per megaparsec, while that calculated through CMB is 68. This made scientists to believe that the difference is due to error in measurements, but so far, they have been unable to find and explain any such source of error. One more explanation may be that redshift calculates the constant for recent universe, while the CMB predicts it for the early universe - the same reason behind the accelerated expansion caused by an unknown force what we call the dark energy.
Now, the researchers have developed a new technique to calculate this value. In this method, they have combined the data from two different experiments. In the first method they have used the gravitational wave signals from the merger of two distant black holes. Astronomers have named them the "standard sirens". In this method, they calculate the intrinsic amplitude of these gravitational waves and then compare it to their apparent amplitude, which gives the measurement of the distance of these binary systems of merging black holes. This value came out to be 1.8 billion ly. The LIGO was first to detect a gravitational wave back in September 2015, which were predicted by Einstein in his general theory of relativity, a century ago in 1915. A few more than a dozen such observations have been made till now.
In the second method, they calculated the redshifts in the same galaxy that contains these merging binary system of black holes. The survey conducting this redshift observation was known as the "dark energy survey".
Combining the two data from these two different observations, provided an estimated range for the value of Hubble's constant. This value comes out to be 75.2 with a large amount of error though (+39.5 and -32.4, which means it could be as large as 114.7 and as small as 42.8). This large margin of error is due to the fact that it is calculated from one single measurement. Hence, researchers will continue to take more such measurements in future from multiple events, and then combine them to draw any conclusions.
This new method is submitted to "the astrophysical journal letters".