Indian group proposes radical new way to settle universe expansion dispute

Source: The post is based on the article “Indian group proposes radical new way to settle universe expansion dispute”  published in The Hindu on 17th July 2023

What is the News?

Researchers from the International Centre for Theoretical Sciences (ICTS), Bengaluru, the Inter-University Centre for Astronomy and Astrophysics (IUCAA),Pune, and University of California, Santa Barbara (UCSB), have proposed a new way to determine the Hubble constant.

What is Hubble Constant?

The Hubble constant is a unit that describes how fast the universe is expanding at different distances from a particular point in space. 

It is one of the keystones in our understanding of the universe’s evolution.

It gets its name from Edwin Hubble who was first to calculate the constant from his measurements of stars in 1929.

How is Hubble Constant measured?

Two details are required to calculate the value of the Hubble constant 1) distance between the observer and astronomical objects and 2) 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:

– First method: 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.

– Second method: They use changes to the cosmic microwave background (CMB) – radiation leftover from the Big Bang event – to estimate the Hubble constant.

– Third method: They use gravitational waves, ripples in spacetime produced when massive  astronomical objects – like neutron stars or black holes – collide with each other. 

Drawbacks with these methods: 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.

What is the new method proposed by scientists to understand universe expansion rate?

​​The new method proposed by the researchers uses gravitational lensing. 

Gravitational lensing is when massive objects in space can warp spacetime. This bends all kinds of waves that travel near the objects, distorting them. The phenomenon is typically used to describe when light waves are bent.

As we have seen before, sometimes, extreme lensing of some objects can make them visually appear as multiple objects in the sky. Just like that, lensing can produce multiple copies of the same gravitational signal that can reach Earth at different times.

The researchers believe that the delays between the signals could be used to calculate the universe’s expansion rate.

– Note: Scientists are yet to find lensed gravitational waves but there’s good reason to believe that they will in the next two decades. This is because the upcoming generation of gravitational wave detectors are expected to be able to sense about a million gravitational waves a year.