Our expertise and observations complemented those from Europe: Gopakumar Achamveedu

Gopakumar Achamveedu (Fifth from right in middle row)

On June 29, an international group of scientists recorded the hum of the gravitational waves of the universe using six of the world’s most sensitive radio telescopes, including India’s Giant Metrewave Radio Telescope (GMRT), located near Narayangaon, Pune.

Gravitational waves were proposed by Albert Einstein in 1916 but were first detected by Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015.

Radio telescopes detected the gravitational hum by studying pulsars, a type of rapidly rotating neutron stars, which form when a massive star runs out of fuel and collapses. A pulsar acts as a cosmic lighthouse, which emits radio beams that hit Earth.

Gravitational waves are thought to change the arrival times of these radio flashes, allowing researchers to document the hum.

The Indian researchers involved in this collaboration were from the National Centre for Radio Astrophysics (Pune), Tata Institute of Fundamental Research TIFR (Mumbai), Indian Institute of Technology (IIT, Roorkee), Indian Institutes of Science Education and Research-Bhopal, IIT-Hyderabad, Institute of Mathematical Sciences (Chennai) and Raman Research Institute (Bengaluru).

Down To Earth caught up with Gopakumar Achamveedu from TIFR, who is one of the researchers involved in this collaboration, to understand the significance of these findings. Edited excerpts:

Rohini Krishnamurthy: Can you describe this gravitational hum?

 

Gopakumar Achamveedu: Imagine a pond and you throw a stone. You see small waves travelling across the pond’s surface and after a while, the surface becomes still again. This explains LIGO’s gravitational waves, first detected in 2015. 

Now imagine you go to this pond, while it is raining. So what happens is that this rain is falling on the surface of the pond. You see several drops falling and disturbing the surface in many places.

These waves on the surface of the pond are essentially mixing with each other and such a pattern is similar to the background that we are talking about. The only difference is that instead of the pond, we are looking at the fabric of the universe made of space and time. Gravitational waves are essentially perturbations, so they vibrate space-time or the fabric of the universe.

In other words, the LIGO events vibrate the local fabric of the universe for a very short duration and they never return. Pulsar Timing Arrays (describes a set of pulsars scattered across the galaxy that could be used as a detector) experience continuous galaxy-wide vibrations due to space-time perturbations coming from all over the universe.

This is the hum of our universe that we detect by using radio telescopes and pulsars. And only extremely large masses can perturb this fabric of our universe to create such gravitational waves.

RK: What is the significance of listening to the gravitational waves? How is this going to help us understand the universe better?

GA: Let me begin by telling about regular telescopes. When we observed the universe only with optical telescopes, we saw a very quiet place. During World War II, people were using radars to look for enemy aircraft with the help of radio antennas.

These radio antennas were then used to view the sky. We then realised that the universe is a very active and violent place. Later, people started probing the heavens using X-ray and gamma-ray detectors, which are part of the electromagnetic spectrum.

We could even see radiation coming from the beginning of the universe. They found noise coming from everywhere and they could not remove it. And that turned out to be the observational evidence for the occurrence of the Big Bang (we were essentially seeing the relic of that Big Bang). It’s called the cosmic microwave background.

The universe is dominated by mass. Galaxy has a mass, and so does the sun. So just like the charged particle — electron or proton — gives you electromagnetic radiation, all accelerated masses will generate gravitational waves.

Black holes emit gravitational waves. And when they are paired up with a companion black hole, we get gravitational waves of specific nature. We are excited about them because these black holes are usually associated with galaxies. We are in a galaxy, and the fate of humans is tied to our galaxy. 

For example, the Andromeda galaxy is rushing towards us as it will collide with the Milky Way. Both galaxies have monster black holes at the centre. When galaxies collide, the black holes are also going to collide. 

Many galaxies in our universe have collided in the past. Their black holes are spinning around each other and they will also merge at some point in time. And they are emitting gravitational waves.

We know that galaxies evolve by merging with each other.  So gravitational waves act as a signature of the evolution of the galaxy.

When they are merging, we can tune into and listen to these background noises to see how many of them were actually colliding with each other.

Our universe is estimated to be 10 billion years old. And there are 100 billion galaxies out there.

We routinely see galaxies colliding with each other using the Hubble space telescope. Pulsar Timing Array observations are telling us that such galaxy collisions lead to collisions of monster black holes that reside at the centre of such galaxies (our Milky Way galaxy hosts a four million solar mass black hole at its centre).

RK: Is it possible to tune into one particular merger? 

GA: There are many candidates, which we suspect are going around each other. We are already suspecting something in our data. The papers actually talk about that.

But we don’t have the confidence to call this a detection. The likelihood of a noise mimicking a gravitational wave is one in 1,000. We can only be very confident if the chance of a fluke is one in a million of not seeing a fluke.

We can improve this by including more data. Different observatories bring in a different set of pulsar data. And that’s what we are going to do now. It could take us a year or two. 

RK: This is an international collaboration. What was India’s contribution?

GA: Michael Kramer, who is the director of the Max Planck Institute in Bonn, reached out to us when we were beginning to routinely observe pulsars using the upgraded Giant Metrewave Radio Telescope (GMRT), located near Narayangaon, Maharashtra. 

The European team lacked the frequency window used by GMRT, which can simultaneously observe a pulsar at two radio frequency windows. This is also not possible with many other telescopes.

We could observe or monitor pulses in 300 megahertz and around 1.2 gigahertz. We call them band 3 and band 5 observations. Pulsars shine very brightly at 300 megahertz and that was very useful for these studies. 

The luminosity of the pulsars is inversely proportional to their observational frequency. So lower the radio frequency, the brighter the pulsar is, loosely speaking.   

Our expertise and observations sort of complemented those from Europe.

Pulsars act as celestial clocks to measure gravitational waves. But radio waves that are used to observe pulsars get dispersed and scattered, which makes it difficult to use pulsars as a very accurate clock. Dispersion in optical frequencies gives rise to a rainbow. The same thing is happening with radio waves.  

So we have to correct them. This is exactly like noise cancellation. 

It is here that our data becomes important. We helped reduce the uncertainties in what we were measuring. The GMRT data was able to make better clocks to measure gravitational waves.  

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