by Prasad Ganti

The Nobel prize in Physics for 2019 was announced for two separate advances. One was for Cosmology which moulded our view of the Universe over the last five decades. The second advance was the discovery of planets outside of our Solar system. Both of these advances have expanded our ideas of the Universe much beyond our Solar system, far out into the distant galaxies and far back into the time of the birth of the Universe itself.

Expansion of the Universe was confirmed by Edwin Hubble closer to the mid twentieth century. That is each galaxy is moving away from every other galaxy. Further away the galaxy is, faster it is moving away. Playing this story in reverse, leads to a point in time when the Universe was born. It is called the moment of the Big Bang. It sprouted forth from a tiny point with incredible amount of energy about 13.8 billion years ago. And suddenly expanded in a fraction of a second (called inflation) to a big space where the temperatures cooled down considerably enabling the creation of radiation (like light) and atoms, leading to formation of Hydrogen and Helium. Stars formed in due course and eventually galaxies.

Such a framework of the Cosmos was being constructed by several luminaries. One of them was James Peebles of Princeton, New Jersey, who was awarded this year’s Nobel prize. Peebles realized that the temperature of this background radiation, called CMB (Cosmic Microwave Background), could provide clues about the Big Bang and confirm its happening. This background radiation is omnipresent and is seemingly uniform in all directions as viewed from the Earth. The hunt for such a radiation was on. In stepped Bob Wilson and Arnos Penzias, two radio engineers from AT&T Bell Labs in New Jersey. They accidentally stumbled upon the background radiation while testing microwave antennas. They found the “noise” whichever direction the antenna was tilted. It could not go away with any amount of tweaking of the antenna. The noise manifests itself in form of random dots seen on a TV channel which is not receiving any signal. Penzias and Wilson were looking for an explanation and reached out to Princeton University where Peebels and team were very happy to connect and exchange notes. Each side got what they were looking for. The Big Bang theory was put on a firm foundation.

Another aspect of the background radiation is that Peebles thought that some amount of tiny non-uniformity must have been present to enable the formation of stars and galaxies. It is not the same in all the directions all the time. Tiny microscopic fluctuations must be present. Such measurements needed very high level of precision. Confirmation came from the COBE satellite (COsmic Background Explorer leading to Nobel prizes for John Mather and George Smoot in 2006), later from the satellites WMAP (Wilkinson Microwave Anisotropy Probe) and Planck.

Peebles also thought about the energy of empty space, which eventually came to be know as dark energy. Dark energy remained just a theory for fourteen years, until the universe’s accelerating expansion was discovered in 1998 (leading to a Nobel Prize in Physics 2011 to Saul Perlmutter, Brian Schmidt and Adam Riess).

Second part of the Nobel prize was awarded to Michel Mayor and Didier Queloz who found the first planet outside of our Solar system. Stars in our Milky Way galaxy are relatively easier to find as they emit sharp points of light. Other galaxies are seen as blurs of light as they contain a collection of stars. But planets orbiting other stars are very difficult to find. They do not emit any light, instead just reflect the light of its star. This reflected light is nearly impossible to detect.

The planet pulls on the star as much as the star pulls the planet towards itself. The tug of the planet on the star is small. As a result of this tug, the light coming from the star is doppler shifted, that it is blue shifted when the star moves towards us and red shifted when the star is moving away from us. The doppler shift tells us how long the planet takes to go around the star and how heavy the planet is. To detect this light, a very sensitive spectrograph is required. Also, as the planet moves in front of the star, the amount of light reaching us reduces. Known as transit photometry, it provides the size of the exoplanet. The size and mass of the planet leads to knowing the density and in determining its structure, if it is a rocky place or full of gas.

Mayor and Queloz found the planet labelled as 51 Pegasis b, which moves rapidly around its star 51 Pegasis, which is fifty light years from the Earth. It takes four days to complete its orbit. Only eight million kilometers (comparatively Earth is 150 million kilometers from the Sun) from it, the star heats the planet to more than a thousand degrees centigrade. The planet is a gaseous ball, similar to our Jupiter. To date, close to four thousand planets have been found, but Mayor and Queloz remain the harbingers of this quest.

A lot has been learnt about our Universe and the distant stars and planets in the last few decades. And very appropriately, recognition came in form of a Nobel prize, the highest honor awarded by mankind.