Hubble Constant

Hubble Constant

News: Recently, some researchers from India and the US have proposed a novel method to determine the Hubble constant and the Rate of Expansion of the Universe.

What is Hubble Constant?
• The Hubble constant is a measure of how fast the universe is expanding. It is named after Edwin Hubble, an American astronomer who discovered in 1929 that distant galaxies are moving away from us at speeds proportional to their distances.
• This means that the farther a galaxy is, the faster it recedes from us. This phenomenon is known as the Hubble law or the Hubble-Lemaître law.

What measurement details are required to calculate value of Hubble constant?
• The distance between the observer and astronomical objects,
• The velocity at which these objects are moving away from the observer as a result of the expansion of the universe.
• So far, scientists have used three methods to get these details
• They compare the observed brightness of a stellar explosion, called a supernova, with its expected brightness to figure how far away it could be. Then they measure how much the wavelength of the light from the star has been stretched by the expansion of the universe – i.e. the redshift – to figure how much it’s moving away.
• They use changes to the Cosmic Microwave Background (CMB) – radiation leftover from the Big Bang event – to estimate the Hubble constant. The CMB is a faint, nearly uniform glow of microwave radiation that fills the observable universe. It is often referred to as the 'afterglow' of the Big Bang.
• They use gravitational waves, ripples in spacetime produced when massive astronomical objects – like neutron stars or black holes – collide with each other. Detectors that observe gravitational waves record the data in the form of curves. Using the shape of these curves, astronomers can calculate the amount of energy that the collision released. Comparing this with the amount of energy the waves had when they reached earth allows researchers to estimate the distance between these objects and earth.

What discrepancy have been observed in measurement?
• Measurements from the first method have reported a Hubble constant about two units higher than the one derived by the second method; the third method hasn’t yet matured enough to provide a precise measurement.
• The discrepancy could be due to a mistake in the methods used – or it could indicate that the Hubble constant is itself evolving with time. This possibility arises because the three methods estimate the Hubble constant today based on information from different stages of the universe.
• The CMB way is based on a much younger universe while the other two are based on an older universe (i.e. closer to the one today).

What is the new approach?
• Researchers proposed analyzing a collection of lensed gravitational waves and their time delays to derive information about the universe's rate of expansion. Gravitational lensing is a phenomenon in which the gravitational field of a massive object, such as a galaxy or a cluster of galaxies, bends and distorts the light from objects located behind it
• This method offers an independent estimation of the Hubble constant and could help determine other cosmological parameters such as matter density. Experts in the field find the study fascinating and see it as a significant cosmological application of gravitational waves.