‘Dark matter and dark energy are the two most outstanding questions facing physicists today’

Ganesan Srinivasan
Ganesan Srinivasan, 70, Astrophysicist
Photo: Arun Sehrawat

WHEN THE 20th century dawned, the electron had just been discovered. We knew that the electron went around the atom, but no one knew what was inside it. Then Ernest Rutherford discovered that there is a nucleus in the centre, then Niels Bohr came up with his theory of atomic structure, then there was the quantum revolution. We have made tremendous progress in the past 100 years.
Towards the end of the 20th century, cosmology, which had never been considered a science at all, was accepted as one. This is because for most of the century, we had only one fact: that the galaxies are moving away. Nothing was known about where the universe came from. It is really the theorem of Roger Penrose and Stephen Hawking that gave a scientific basis for the thinking that maybe there was a beginning.
This very great progress itself, which made cosmology a science, has led us to the unknown. We ask, what is dark matter, which makes up 85 percent of the universe? We know it is there, because it exerts gravity, but we have not been able to detect it. And we know little about it. The acceleration of the universe tells us that the energy, which is causing this cosmic repulsion, cannot be the energy of matter. For one, there isn’t enough matter: the energy per unit volume is four times as much as what would be generated if all the matter was converted into energy. That leaves us with only vacuum. We used to think about vacuum as only empty space. But now, we find that this empty space has energy and pressure associated with it. But what is this energy? We don’t know. These — dark matter and dark energy — are the two most outstanding questions facing physicists today.
Interestingly, the most outstanding questions for physics at the turn of the 20th century came from the microscopic world of atoms and nuclei, whereas these questions now concern the infinite universe. And what is known about them is that dark matter and energy exist because of what happened when the universe was 10-45 seconds old and 10-33 cm in diameter. By contrast, an atom is an enormous 10-8 cm in size. What is extraordinary, in my mind, is that what is happening to the universe at a scale of billions of light years today, seems to have been decided by the laws of microscopic physics when the universe was 10-45 seconds old. For Rutherford and Bohr and the other great physicists of the early 20th century, this notion would have shocked them. But I suspect they may have enjoyed the beauty of it.
As Told To Ajachi Chakrabarti
Ajachi Chakrabarti is a Correspondent with Tehelka.