Deuterium at the dawn of time

The middle-weight cousin of simple hydrogen could be a key to understanding the beginnings of the Universe according to research carried out at MIT’s Haystack Observatory. The team there have made the first radio observations of this isotope of hydrogen.

Many of the chemical elements exist in different isotopic forms – same number of protons in the nucleus and so same atomic number, but more or less neutrons. Hydrogen is no exception. Normal hydrogen atoms have a single proton and no neutron, deuterium has a proton and a neutron and tritium, as its name suggests, has three particles in its nucleus – a proton and two neutrons.

Haystack Observatory (Credit: Alan Rogers, Haystack Observatory)

Alan Rogers and colleagues Kevin Dudevoir, Joe Carter, Brian Fanous and Eric Kratzenberg of Haystack, and Tom Bania of Boston University, worked with the football-field sized radio telescope array to gather data over the course of a year, and finally saw what they were looking for on 30th May.

The key to deuterium’s importance in understanding the Universe lies in its relation to the amount of dark matter present and the way deuterium was created in the Big Bang. The light elements hydrogen and helium along with traces of deuterium, helium 3 and lithium are formed in the Big Bang nucleosynthesis, while all other elements are formed in stars, Rogers told Spotlight. A measurement of the abundance of deuterium determines the baryon density of the Universe.

Overall view of the 25 5x5 crossed dipole stations of the deuterium array (Credit: Alan Rogers, Haystack Observatory)

Overall view of the 25 5×5 crossed dipole stations of the deuterium array (Credit: Alan Rogers, Haystack Observatory)

The researchers knew that accurate observations of this isotope would allow them to set constraints on models of the Big Bang. The same measurements would also allow them to obtain a more accurate estimate of the density of cosmic baryons, which would in turn indicate whether ordinary matter is dark and found in regions such as black holes, gas clouds or brown dwarfs, or is luminous and can be found in stars.

Alan Rogers

Until now the deuterium atom has been extremely difficult to detect with instruments on Earth. Overall emissions from deuterium atoms are weak as the isotope is rare at just one deuterium atom for every 100,000 hydrogen atoms in space. Hydrogen and deuterium also emit at very similar optical wavelengths so that spectroscopic observations are almost impossible. At radio frequencies, however, deuterium is more distinct from hydrogen and so more consistent results are possible.

Tom Bania

The only remaining problem was interference from mobile phones, power lines, pagers, fluorescent lights, televisions and other electrical equipment. To overcome this problem, the researchers used a circle of yagi antennae to locate the sources of spurious signals so that they could be eliminated. On occasion, they asked Haystack’s neighbours to replace a certain brand of answering machine that was producing an interfering radio signal. A stereo system was also modified by the manufacturer to remove another spurious signal.

Once the team had a clearer view of the radio waves reaching us from space, they were able to make their measurements. We measured the abundance of deuterium in the interstellar gas of our Galaxy and obtained a result which implies that only about 4% total mass/energy density is in the form of baryons, Rogers adds. This is consistent with the ‘standard model’ of a spatially flat Universe which requires that the remaining mass/energy is made up of ‘dark matter’ and ‘dark energy’.

Deuterium at the dawn of time

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