Gold has been one of the most sought after metal in the history of humankind. In the past, alchemists tried very hard to transform other metals to gold. Even Isaac Newton was fascinated enough to devote a considerable part of his time to those efforts. However, it was only possible when physicist Ernest Rutherford and his associates began doing artificial transmutations in the 20th century.
It was only later that the Big Bang theory and nuclear fusion theory showed how certain elements were made. The former proposed that the universe was created at some instant and that certain elements could only have been created moments after the Big Bang. Hydrogen and some amount of Helium were made about 3,80,000 years after the Big Bang. The latter showed that stars shine because of the nuclear fusion reaction and also make new elements. Elements up to Carbon in the periodic table make up lighter stars, while elements up to Iron in the periodic table make up massive stars.
The end points of heavier stars are interesting since they are accompanied by an outpouring of enormous amount of light which shows up as a supernova. They also leave behind compact and dense objects like neutron stars or black holes. However, the problem was how to make elements like gold with 79 protons and 118 neutrons from iron, with only 26 protons and 30 neutrons. It was obvious that a neutron-rich environment is necessary for the process to happen.
This was proposed in 1957 by the astrophysicists Geoffrey Burbidge and Margaret Burbidge, Fred Hoyle and William Fowler. They stated that heavier elements like gold and uranium could be made at the time of death of heavier stars, that is, when protons and electrons join together in the collapsing core to make a copious number of neutrons. With time, there were doubts cast about this as scientists argued that so many neutrons cannot be present to make heavier elements. So, physicists had to look for alternative processes.
Spotting light and gold
In the 1970s, Russell Hulse and Joseph Taylor had observed a Binary Pulsar, which had two neutron stars circling each other with decreasing separation. It was hypothesised that they would collide after millions of years. These findings provided, for the first time, an indirect evidence for gravitational waves.
There were also theoretical proposals in the 1990s that such neutron star mergers could make heavy elements like gold. These collisions were also given the name Kilonova, which is considered to be more powerful than a supernova.
Gravitational waves had been predicted as early as 1915 by Einstein. Very sensitive instruments - called the Laser Interferometer Gravitational-Wave Observatory (LIGO) - were set up at two locations in USA to detect these waves in the mid-1990s and upgraded in 2014. And, in early 2016 the first observations of gravitational waves were made. Since then, LIGO confirmed three more events. These were said to have occurred as a result of a merger of two massive black holes.
The original LIGO instruments, along with a third one in Europe, again detected gravitational waves on August 17, 2017. But this signal was unique as it was much stronger. It was surmised that the waves occurred due to a collision of neutron stars. The first one to detect the accompanying electromagnetic waves was Fermi, a NASA satellite, which detected a gamma-ray burst. Scientists have long suspected that neutron star mergers could create gamma-ray bursts. There was also a rush among the scientists to work quickly to observe visible and infrared light from the collision's aftermath.
The detailed optical and infrared spectrum of the kilonova were looked forward to since it could contain the fingerprints of the heaviest elements in the universe. The observers were able to spot the signature glow of platinum, gold and other 'r-process' elements for the first time. A Nature paper stated that the early stage of the observed outflow was dominated by lighter elements while later, there was an emergence of a heavy-element composition.
The event, which is called GW170817, has provided alternative theories of gravity, a clear origin for a cosmic explosion and a strong evidence for the formation path for some of the heaviest elements in the universe. Also, for the first time, a gravitational wave detection has been linked to the rest of astronomy! This enormous effort involved more than 4,000 scientists, and papers were published in leading scientific journals.