The Scholars' Avenue

Douglas Adams, in his famous speech titled “Is there an Artificial God?” at Digital Biota in September 1998, said, “The fact that we live at the bottom of a deep gravity well, on the surface of a gas covered planet going around a nuclear fireball 90 million miles away and think this to be normal is obviously some indication of how skewed our perspective tends to be “. Fortunately or unfortunately, there have been people who did not think this to be normal. Some of the really curious ones among the lot realized that a lot could be learnt about the deep gravity well, the nuclear fireball and the things beyond merely by looking up in the sky, provided, one had eyes that were powerful enough. Modern cosmology is basically a continuation of the work started by that curious lot and the techniques that are being used now are nothing but technologically advanced versions of looking up in the sky.

However, the fact that the thing that makes us capable of “looking” up travels at a finite speed of 3,00,000 km/s has certain advantages and certain disadvantages. For example, there is no way we can figure out what exactly is happening at the sun at this exact moment. On the other hand, we can figure out what exactly happened at the sun approximately eight minutes ago. Similarly, there are electromagnetic radiations that emanated not just a long time ago, but also in a galaxy far far away, that are still hanging around in our atmosphere. These radiations tell us a lot about what the universe looked like when it was younger. Unfortunately, there was a period in the life of the universe that did not release enough radiation to make us capable of observing it. For obvious reasons, it is called The Dark Ages. However, a small trick (21 cm HI radiation) that was recently thought of by a few astrophysicists and now is actively being researched at the Centre for Theoretical Studies (CTS), IIT Kharagpur, by professors and students from the Department of Physics & Meteorology and CTS, has made it possible to probe even into that era.

A timeline of the universe and what’s known till now

Physicists believe that the universe was created roughly 10-20 billion years ago in the grandmother-of-all-explosions dubbed the Big Bang. Till about 3,00,000 years after the Big Bang, all of the protons and electrons in the universe existed as free ions moving around in plasma. Every time a proton grabbed an electron it would be zapped by a photon with high enough energy to rip them apart again. The expansion of the universe reached a stage where the temperature became low enough for neutral atoms and molecules to form. This phase is called the “epoch of recombination”, and the light that emanated at this stage is called CMBR or Cosmic Microwave Background Radiation. This particular kind of radiation permeates the entire universe and was first discovered in the form of a very annoying noise. People had tried even to ascribe it to a “white dielectric substance” (pigeon droppings) in the horn of the antenna, for lack of an explanation.

But CMBR provides information only about a particular stage in the history of the universe. The intervening time between this stage and the formation of stars and galaxies is called the Dark Ages, as there was no method to probe into this period. However, the recent discovery of HI radiations has been very promising with regards to bridging this gap.

21 cm HI radiation
Subatomic particles such as the proton and the electron have an intrinsic property called spin which can either be ‘up’ or ‘down’. In a Hydrogen atom, the proton and electron can either have the same spin or different spins, giving rise to two different configurations. Although the hydrogen atom is neutral, the difference in the energy level of these two configurations results in radiations of 21 cm wavelength. These are called the HI radiations. So, in effect, although the universe was almost invisible during the Dark Ages, it did emit HI radiations and hence telescopes that work with radiations at these frequencies can be used to probe into the era. Not just this, the fact that the universe had different rates of expansion at different points in time makes it possible to observe the HI radiation and predict the time they come from.

Where does IIT come into the picture?
A group of researchers at the CTS, led by Dr. Somnath Bharadwaj has been working on the use of HI 21 cm radiation in cosmology. The group has been working with the data from the GMRT or the Giant Metre Radiowave Telescope (the largest array of radio telescopes in the world), which is located in Pune and is an instrument used by astronomers from all over the world.
Among other things, the group has shown that it is possible to obtain, with the help of this radiation, parameters that characterize the cosmological models that have been proposed to explain the structure of the universe. With the help of theoretical models, it has even been able to predict the kinds of signals the GMRT, should, in principle, be able to detect.
However, HI radiation based research has a practical limitation. The HI radiation that reaches the telescope comes from sources other than the ones intended. Signals from our galaxy and the neighbouring ones are about 1000 times stronger. Detection and removal of these extraneous signals from the HI background is another aspect of the work done by the group at CTS. Their strategy uses the fact that the noise from the foreground sources remains constant in different ranges of wavelengths, whereas the primordial signal fluctuates with wavelength.

Though we are far from detecting the HI radiation with accuracy high enough to be used to corroborate the theory, there is hope that this will be possible soon. When this goal is achieved and a 21 cm map of the universe is obtained, the implications will be enormous. Unlike the Cosmic Microwave Background Radiation, the HI maps will not only reveal the process of formation of galaxies but also how they evolved and interacted with each other to form the present universe. Several projects have been proposed to build powerful telescopes that can achieve this. Even as the theory is being fine tuned, there are plans to send missions to the far side of the moon that can set up a base for measuring HI radiation. It is hoped that future investigations will be able to realise the ultimate cosmological dream of being able to establish the story of the birth and growth of the universe to the stage that we today see it in.

Acknowledgements
We thank all the students and professors at CTS and the Department of Physics and Meteorology for their contribution to the article. We particularly thank Dr. Sayan Kar for his guidance.