Almost Nothing

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.

10 Responses to Almost Nothing

  1. Tigran Khanzadyan says:

    Just be careful in mentioning that to the public next time:) They might somewhat object to provide funding for the observations and study of “Almost nothing” …

  2. Newspapers love superlatives. And the great thing about space stories is that there will always be something else hotter / brighter / closer / more distant / older / bigger etc coming along later.

  3. It reminds me of Geoff Macdonald’s ethyl alcohol detection in an HII region that added up to 300,000 pints of beer each for everyone on Earth, every day for the next billion years.

  4. Mrs Trellis says:

    Dear Syd,

    I saw a very clear explanation of the emptiness of space from an expert on cosmology and quantum physics on Youtube recently. I’m sure you’ll find it very informative.

    Yours sincerely,

    Mrs Trellis
    Yours sincerely

  5. Dr Charlatan Werner says:

    Thank you for the wonderful publicity Mrs Trellis. You have the gravitas that some of your colleagues unfortunately lack

  6. […] Crowther mentioned my recent blog post about how the Universe is almost empty. He said he likes to set his classes ballpark estimate […]

  7. Bill Keel says:

    This put me in mind of a line I’d been meaning to look up (every time I refer to Osterbrock’s books):
    “To devote the greater part of one’s adult life to the lonely recording of
    the terrible emptiness between the stars is more than can be asked of someone normal.” (Isaac Asimov, The Currents of Space)

  8. […] is another one of those occasional posts with mazin facks bout stronomy. (See also Almost Nothing and A Dim […]

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