Most, if not all astronomers will happily tell you that we can only see 4% of the Universe. 22% is believed to be the elusive dark matter, with the remaining 74% being assigned to the troublesome concept of dark energy. So no, we really don’t know much at all about the Universe. But that blissful 4% of baryonic “light” matter – we know what that is, right? I mean surely we must understand at least 4% of the Universe, don’t we? Well, erm… no.
The Diffuse Interstellar Bands
Image courtesy of The McCall Group
To be perfectly honest, the problem of the Diffuse Interstellar Bands (or DIBs for short) is a bit embarrassing. The whole riddle began to unfold back in 1922 with Mary Lea Heger. She was looking at the spectra of some stars, and she noticed some bizarre absorption lines. Much wider and more “diffuse” than the atomic lines that normally show up in stars, these lines seemed to be interstellar in origin.
Most people tend to believe that interstellar space is a vacuous expanse devoid of pretty much anything, but in reality there’s a lot drifting about out there. A lot we don’t understand. We know that there’s a lot of dust out there, blocking out starlight (the effect’s called ‘extinction’), but the DIBs don’t look like dust. Dust doesn’t cause absorption lines. So the other option is that they’re molecules. But the absorption bands are much too broad to be caused by any molecules we’ve seen anywhere else; some are up to ten times as wide as regular molecular peaks. And there are a lot of them. Over 400, in fact.
Astronomers, chemists, spectroscopists and physicists have grappled with this conundrum for nearly 90 years now but to no avail. The only thing a lot of scientists agree on is that whatever these molecules are, they’re likely to contain carbon and hydrogen. The main reason for this belief though, is simply the fact that carbon and hydrogen are two of the most common elements in the Universe. If carbon and hydrogen really are the culprits, then around 20% of the carbon in the galaxy might exist inbetween the stars.
Theories on what might cause the DIBs have ranged from fullerenes, to aromatic hydrocarbon clusters to small lumps of amorphous carbon. None have answered the question well enough to satisfy everyone. Probably the best candidates so far are large polycyclic aromatic hydrocarbon (PAH) molecules (which I’m sure I’ve ranted about more than enough in the past). Some researchers have found PAHs whose spectra are tantalisingly close to certain DIBs. Close, but still no cigar.
The Diffuse Interstellar Band puzzle seems to have persisted for so long that most people have apparently forgotten all about it. There’s still a lot of work to be done in solving this riddle, but given the seemingly endless possibilities for what might exist in the frozen expanses of space, we could be waiting a long time for an answer!
In the meantime, we’re just going to have to admit it — we might think we know what that 4% of the “visible” Universe is made of, but… In reality, we could know next to nothing at all.