Almost Nothing

February 25, 2011

More astronomical hyperbole. I just remembered a story about Glenn White from days of yore at QMW. Glenn, as many of you will know, was and is Mister Molecular Cloud. He was getting fed up with us extragalactic types stealing the limelight. I think we’d just been in the Daily Mirror with the Most Luminous Object in the  Universe, aka IRAS F10214+4724. So when he came back from Hawaii with some pretty random CO data, he put out a press release saying he’d found a cloud with enough Carbon to make all the pencils on the planet. By golly it worked ! Snaffled up.

We are used to thinking of everything in astronomy as bigger and bolder than our humdrum earthly existence. Recently I heard someone talking with passion about the enormous amount of obscuring material towards some object, with an extinction of hundreds of magnitudes… But hang on there. At a normal gas-to-dust ratio, thats a hydrogen column of maybe 10^24 atoms per cm^2. But thats peanuts; the column through the Earth’s atmosphere is much more. If it wasn’t, we’d be able to do X-ray astronomy without those expensive rockets. That obscuring muck is pretty pathetic.

I was expressing these thoughts to Eric Tittley at lunchtime. Indeed, quoth he, there is more stuff in the 10km above our heads than there is all the rest of the way to the edge of the universe. That just made me go quiet for a bit. What a thought.

One of the most striking things about the universe is that it is transparent. And how nice that is; otherwise we couldn’t study those lovely high redshift galaxies. Every X-ray astronomer knows that you have to include the Galactic Column in your spectral fit – but do you add the Intergalactic Column ? Nope.

Ahh, the learned among you cry, this is because the IGM is ionised. However, the WMAP polarisation measurements of the microwave background tell us that the average electron scattering optical depth from here to the re-ionisation epoch is around tau=0.1. So we know how many free electrons, and so how many protons, there are in a typical line of sight from here to redshift ten-ish. If you drill a tube about a thumbnail across from here to eternity, you get about 3 grams of stuff. About as much as a teaspoon of water.

Almost nothing.