Piercing the mystery of the cosmic origins of gold
Gold pre-exists the formation of Earth: this is what differentiates it from, for example, diamond. However valuable it may be, this precious stone is born out of mere coal, whose atomic structure is modified by enormous pressure from the earth's crust. Gold is totally different – the strongest forces in the earth's mantle are unable to change the composition of its atomic nucleus. Too bad for the alchemists who dreamed of transforming lead into gold.
Yet there is gold on Earth, both in its deep core, where it has migrated together with heavy elements such as lead or silver, and in the planet's crust, which is where we extract this precious metal. While the gold in the core was already there at the formation of our planet, that in the crust is mostly extraterrestrial and arrived after the formation of Earth. It was brought by a gigantic meteor shower that bombarded the Earth (and the Moon) about 3.8 billion years ago.
How gold is produced in the universe? The elements heavier than iron, including gold, are partially produced by the s process during the ultimate evolution phases of the stars. It is a slow process (s stands for slow) that operates in the core of what are referred to as AGB stars – those of low and intermediate mass (less than 10 solar masses) that can produce chemical elements up to polonium. The other half of the heavy elements is produced by the r process (r stands for rapid). But the site where this nucleo-synthesis process takes place has long remained a mystery.
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The first detection of gravitational waves from the cataclysmic merger of two neutron stars, and the observation of visible light in the aftermath of that merger, finally answer a long-standing question in astrophysics: Where do the heaviest elements, ranging from silver and other precious metals to uranium, come from?
Based on the brightness and color of the light emitted following the merger, which closely match theoretical predictions by University of California, Berkeley and Lawrence Berkeley National Laboratory physicists, astronomers can now say that the gold or platinum in your wedding ring was in all likelihood forged during the brief but violent merger of two orbiting neutron stars somewhere in the universe.
This is the first detection of a neutron star merger by the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the United States, whose leaders were awarded the Nobel Prize in Physics two weeks ago, and the Virgo detector in Italy. LIGO had previously detected gravitational waves from four black hole mergers, and Virgo one, but such events should be completely dark. This is the first time that light associated with a source of gravitational waves has been detected.