Weighing the Universe with Deuterium

In this project, we aim to measure the number of atoms in the universe, or the mass of normal matter (baryons), which is a fundamental quantity in cosmology.

Background 

Counting the number of atoms directly is obviously impossible, but a highly precise alternative is to compare the amount of hydrogen and its main isotope, deuterium, in distant, almost pristine clouds of gas.

The deuterium nuclei were created in a short epoch just three minutes after the birth of the universe called Big Bang Nucleosynthesis. Very importantly, that deuterium is all that was ever created.

Much later in the universe's life, stars destroyed some, but they didn't create more. So, the deuterium abundance in pristine gas clouds probes the physics of the three-minute-old universe, including how much normal matter was contained in the rapidly expanding plasma at that time– its total baryonic mass. 

Testing for new physics beyond the Standard Model 

There are other ways to weigh the universe. One is to use the cosmic microwave background radiation, which was emitted when the universe had cooled enough for electrons to combine with protons, forming the first hydrogen atoms.

The large-scale acoustics of this gas imprinted tell-tale patterns into the radiation, revealing many cosmological parameters, including the total baryonic mass.

But, crucially, the physics of that process is entirely different to Big Bang nucleosynthesis, where deuterium nuclei were created: if we get different answers from these two very different epochs in the universe, then it may be evidence of new, unknown physics - beyond our Standard Model of Particle Physics – just after the Big Bang.

Finding evidence for such differences would transform our understanding of physics, and potentially point the way to a unified Theory of Everything. 

Quasar light-houses 

The pristine gas clouds are dark. They lie in the remote outskirts of early galaxies and have few, if any, deuterium-destroying stars themselves.

To probe these dark, starless clouds, we use background quasars as light-houses: the central, supermassive black holes of quasars are fed by discs of gas and dust, heated to such enormous temperatures by friction that they outshine all the stars in their host galaxies by hundreds of times.

As the bright quasar light passes through the clouds, its spectrum is imprinted with the absorption signatures of the hydrogen and deuterium in the gas. 

Project goal and collaboration 

In this project, we aim to make 10 new deuterium abundance measurements by careful, high-resolution spectroscopy of quasars with the biggest optical telescopes in the world– the Keck 10-metre telescopes in Hawaii and eight-metre Very Large Telescopes in Chile – to significantly improve our tests of the Standard Model.

With collaborators in the US, UK and Italy, Professor Michael Murphy began this project in 2019 to study newly-discovered deuterium signatures in existing quasar spectra. The most promising cases will be proposed for new, higher-quality observations on Keck and/or Very Large Telescope to obtained some of the best measurements of the universe's baryonic mass to date.