On February 11, 2016, the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo observatory announced the detection of a gravitational wave for the first time, a phenomenon that was predicted in 1916 by Albert Einstein in his theory of general relativity. Gravitational waves, according to Einstein, are ripples in the fabric of the universe, caused by various cosmic events such as when two black holes merge or during a supernova explosion of stars. Once generated, these waves can travel unhindered throughout the universe, expanding and contracting matter, space and time itself, along its path.
The discovery of gravitational waves has generated huge excitement among the astronomers community and opened up a treasure trove of opportunities for scientists studying the cosmos. “We can see that Einstein’s theory of gravity, yet again, has predicted something exactly correctly, in advance,” exclaims Professor Brian Schmidt, vice-chancellor and president at the Australian National University. Brian is also the winner of the 2011 Nobel Prize in Physics along with Saul Perlmutter and Adam Riess, for showing that the expansion of the universe was accelerating
Rate of expansion
In 1927, Georges Lemaitre, using general relativity, had shown that the universe is expanding. This was later confirmed by Edwin Hubble, who showed how faraway galaxies were receding from the Milky Way. Since then, we’ve known that we live in a universe that was born in the Big Bang and is expanding, but scientists had expected the expansion to be slowing down. Einstein’s theory however showed something else. “If you use general relativity and look at what happens when space is filled with energy, that’s energy which is uniform everywhere, then it will lead to what is called negative pressure. Now we all know what positive pressure is. Take, for example, a bicycle tire. If I fill it up, and try and push on it, it will push back. This is because of positive pressure. Negative pressure means, if I push, the tire will want to shrink, and if I pull it apart, it will want to spring apart. So it amplifies motion instead of dampening it,” explains Brian.
If such energy pervades throughout the universe, the resulting negative pressure could then lead to acceleration in the expansion.
Brian, along with Adam Riess and his team, were studying type 1A supernovae, a type of supernova that occurs in a binary system of stars, when one of the stars in the binary is a white dwarf. Since all of type 1A supernovae have the same brightness, they can be used as standard candles to study various aspects of the universe like the distance from stars or the expansion of the universe. “Supernovae are ending states of some stars and there are different ways a star can end. We know that type 1A supernovae are always burn like a 1043 watt light bulb, so they are bright, and we can use them to measure distances very accurately, with about a six percent error margin,” remarks Brian.
While trying to estimate the rate of expansion of the universe by measuring the distances of type 1A supernovae, they found that instead of slowing down, the expansion of the universe was speeding up. The space between our galaxy and other galaxies was increasing faster than in the past. “The only sensible explanation for the observations is that there is some form of energy that pervades all of space, evenly and we call that dark energy,” says Brian.
According to our current understanding, the universe is made of 70% dark energy, 25% dark matter, and matter that makes up all of life, the oceans, the earth, stars and galaxies and everything we can see constitute the remaining five percent. “We’re in a peculiar place in cosmology. We know a little about the properties of dark matter and dark energy but the fact that we don’t know what they are, although they make up most of the universe, means there is so much to explore,” opines Brian.
Digital map of the sky
Most of our knowledge about the universe comes by studying light at different wavelengths. But light interacts with matter, which means the presence of obstructions like gas, debris, or stars can affect our visibility. Objects like black holes can never be observed by detecting light alone, since black holes don’t emit light. Gravitational waves, on the other hand, are ripples in space-time that can pass through matter completely unaffected. This makes them an ideal tool to explore the mysteries of the universe that we have been blind to due to the limitations of light or electromagnetic radiation.
“Study of the gravitational radiation is something that we’ve been waiting a long time to do. It means we can detect a black hole. I want to study what is creating the black hole, how many of them are out there in the universe, so that we can try and understand the physics of the black hole and the physics of the end of stellar life,” remarks Brian. Apart from black holes, gravitational waves could also help us learn more about neutron stars and properties of matter at high densities, like in a neutron star. Moreover, it could also allow us a glimpse of the earliest time in our universe, a fraction of a second after its birth.
Apart from gravitational waves, a host of new telescopes packed with updated technology will be operational in the coming years. Brian leads the Sky Mapper Southern Sky Survey that aims to map the southern skies. “Sky Mapper is about making a digital map of the southern sky, since the northern sky has already been well mapped. We built the new telescope in 2013 after the old one burned down in a bush fire.We’ve equipped it to look at the oldest stars in the Universe ,” says Brian. Brian has also been working with Raman Research Institute, Bengaluru to build a new generation radio telescope in Western Australia. The telescope will be equipped to see hydrogen before it forms any stars to understand star formation in the early universe.
“Over the next 10 years, we’ll have the James Webb telescope and other radio telescopes that will allow us to see the gas before they form stars. We’ll see the first stars, see the first galaxies and we’ll be able to see the life history of the universe. That’s a really exciting part of cosmology that’s going to emerge in the next 10 years,” signs off Brian.