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Big Bang Conundrum

By Stephen Luntz

An unexpected consistency in the concentration of deuterium atoms in the distant universe might be a curious coincidence, or it could rewrite our understanding of the Big Bang.

Deuterium atoms, which combine a neutron with the proton and electron of an ordinary hydrogen atom, were formed during the Big Bang. To the best of our knowledge no natural processes since then have created any more, while on the other hand deuterium is sometimes destroyed within stars.

Cosmologists are interested in finding out how much deuterium was initially created in the Big Bang. In the youngest galaxies we can observe it at about 30 parts per million hydrogen atoms.

Besides existing in atomic form, deuterium can bond with ordinary hydrogen atoms to form hydrogen–deuterium (HD) molecules. The concentration of these HD molecules in distant galaxies has shown an unexpected and unexplained consistency. The researchers who discovered this think it’s merely a coincidence borne from a small sample size, but it’s just possible they have stumbled on a pattern of great significance.

Isaac Asimov once said: “The most exciting thing to hear in science is not ‘Eureka!’ but ‘That’s odd’”. It’s certainly what Dr Michael Murphy of Swinburne University’s Centre for Astrophysics and Supercomputing and PhD student Adrian Malec thought when they compared the concentration of HD molecules with molecular hydrogen (H2) in four distant parts of the universe.

In all four cases the HD shows up as roughly 30 parts per million hydrogen molecules. This is strange because the four patches of the universe appear very different in other ways. Murphy and his colleagues had not expected the HD:H2 ratio to be consistent between them.

Even more oddly, the concentration matches the proportion of atomic deuterium to atomic hydrogen in the early universe. Given the number of processes that can form and destroy HD, it was not expected that the two ratios would be similar.

“It’s probably just a coincidence,” Murphy says. “However, if we get more readings that all show the same thing we’ll have to look for an explanation.”

At the moment Murphy admits that he and his colleagues wouldn’t know where to start trying to explain it. A consistent correspondence of HD and atomic deuterium just doesn’t make sense.

The Search for Deuterium
Finding the deuterium concentrations in the early universe would test our models of what went on in the first moments of the universe’s existence. Lithium was also created in the Big Bang, and the so-called “lithium anomaly” relates to the fact that our measurements show much less lithium in the early universe than the models predict. It’s not clear whether the models or measurements are wrong, but discrepancies such as this certainly make cosmologists keen for other methods of testing our theories.

However, measuring deuterium is not easy. Deuterium’s spectrum is very similar to that of hydrogen, so when seeking deuterium at great distances it is easy for any observations to be drowned out by the far more common hydrogen.

The spectrum of HD shows more difference from H2, so it can be easier to observe. However, Murphy says that “even finding clear molecular hydrogen signals from the early universe can be hard. You have to have a quasar shining through an absorption cloud bright enough to produce clear spectra.” As a result, the measurements of HD concentrations are scarce. Up to this point, only four such measurements have been taken from the early universe, two of them by Murphy, Malec and colleagues.

That’s Odd
Some researchers have discussed using HD abundance as a proxy for the abundance of deuterium, but Murphy believes that this is unwise. "You would expect the abundance of HD to vary dramatically from place to place in the universe,” Murphy says. HD can be formed and destroyed in multiple ways, such as through collisions with ultraviolet photons. Within our own galaxy there are large variations in the abundance of HD relative to H2, and these variations appear to relate to many more factors than the abundance of the atomic deuterium in the same areas.

The Swinburne researchers were all set to point this out when they checked their own data for HD concentrations and found that in the two galaxies they had measured the HD:H2 ratio was pretty much the same as the ratio of D:H. This seemed an odd coincidence, but Murphy says: “We then looked at the only other two existing measurements of HD in distant galaxies and found almost exactly the same thing.”

All this might not be surprising if most of the deuterium in these galaxies was bound up in HD molecules, or even if the proportion of deuterium in such molecules was consistent, but it’s not. In some of the galaxies 30–50% of the deuterium present was in HD, whereas in others it was less than 1%. “This is a key indicator that the physical conditions in these galaxies vary,” Murphy says, which would have been expected to produce variations in the HD:H2 ratio.

The next logical step is to go looking for more galaxies with a visible HD signal. These galaxies could then be used to produce further measurements of the abundance of HD. Murphy says the Swinburne team is currently doing just that. While such clear signals will not be easy to find, Murphy says: “We’ll be able to find more examples, but not ten times more”.

Murphy remains confident these data will produce large variations in HD concentrations, revealing the initial consistency is an accident. However, if further galaxies show the same pattern he admits to being stumped. He says: “We don’t have any theories on why the consistency would occur” if it’s not a coincidence.

Without such explanations it is hard to know just how important the confirmation of such a pattern would be. However, it seems likely that an explanation would require a significant reworking of our ideas about either the formation of the early universe or the astronomical processes that produce and destroy HD. Either could have profound effects on cosmology in general.

About Deuterium

Deuterium is an isotope of Hydrogen, and is technically written 2H since it has both a proton and a neutron. However, its importance is such that it is often given the chemical symbol D as if it was an element in its own right. Most isotopes are relatively similar in mass (e.g. the difference between Uranium-235 and Uranium-238 is less than 2%). However, deuterium is almost twice as heavy as neutron-less ordinary hydrogen (also known as protium), making the differences in its behaviour more stark than heavier elements.

Like protium, deuterium can exist as an independent atom, in molecular bonds with another hydrogen atom, or bonded to other atoms (e.g. in water).

Heavy water comprises an oxygen atom with either one protium atom and one deuterium atom or two deuterium atoms. Deuterium’s ratio to hydrogen in the Earth’s oceans is 156 parts per million.

Deuterium is not radioactive, but very high quantities of heavy water are toxic to eukaryote life, including animals.

Heavy water is a moderator used in some nuclear reactors, and was considered essential for nuclear energy and nuclear weapons research in Germany during the Second World War. Although it can be produced from any seawater, the only facility to do so at the time was in Norway. This may have contributed to the Nazi decision to invade Norway, and certainly led to a major Allied operation to bomb the facility and blow up a ferry transporting heavy water to Germany.