Archive for March 28, 2017

The Threepenny Opera

Posted in Biographical with tags , on March 28, 2017 by telescoper

Yesterday’s announcement of the launch of a new 12-sided pound coin reminded me of the old three-penny bit, which had the same number of sides but a different composition and overall design.

Threepenny bit

The old coin had been around since the 16th century, but was phased out when the United Kingdom switched to decimal currency back in 1971. Youngsters won’t remember the old currency, but a pound used to be divided into 20 shillings, each of which was 12 `old’ pence. A threepenny bit was therefore worth 1/80 of a pound sterling. Other old coins of note were the tanner (sixpence), the shilling (one bob) and the half-crown (‘two and six’, i.e. two shillings and sixpence). There was also a penny (which was a rather large coin), the halfpenny and even the farthing (half a halfpenny). Pound coins didn’t exist in those days, only pound notes. There were also `ten bob’ notes, corresponding to half a pound, which converted to 50p coins on decimalization.

Tomorrow the Prime Minister of the United Kingdom will trigger Article 50 of the Lisbon Treaty which will begin our country’s regression into the past. I can’t help feeling that it won’t be long until go back to the old money too.

Anyway, I have specific memories of the threepenny bit because once, when I was little, I swallowed one and had to go to hospital. Don’t ask me how or why this happened. I didn’t feel particularly unwell but they did an X-ray and there it was, bold as brass, in all its dodecagonal glory. I don’t remember the eventual emergence of the coin but when we went back to the hospital a few days later it had vanished from the radar. Nature had clearly taken its course.

That little episode wasn’t as funny as my cousin Gary, who once had to go to hospital because he got a marble stuck up his nose. In Newcastle the word for a a marble is a `liggy’, by the way. I was with Gary when this happened (!) and ended up going along to casualty with him. He was in some discomfort as we sat in the waiting room, then a rather burly nurse came in. She looked at him carefully, then raised her right hand and delivered a resounding smack on the back of his head, whereupon the liggy stotted out across the floor. Job done.

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Is there a kinematic backreaction in cosmology?

Posted in The Universe and Stuff with tags , , , , , on March 28, 2017 by telescoper

I just noticed that a paper has appeared on the arXiv with the confident title There is no kinematic backreaction. Normally one can be skeptical about such bold claims, but this one is written by Nick Kaiser and he’s very rarely wrong…

The article has a very clear abstract:

Kaiser

This is an important point of debate, because the inference that the universe is dominated by dark energy (i.e. some component of the cosmic energy density that violates the strong energy condition) relies on the assumption that the distribution of matter is homogeneous and isotropic (i.e. that the Universe obeys the Cosmological Principle). Added to the assumption that the large-scale dynamics of the Universe are described by the general theory of relativity, this means that we evolution of the cosmos is described by the Friedmann equations. It is by comparison with the Friedmann equations that we can infer the existence of dark energy from the apparent change in the cosmic expansion rate over time.

But the Cosmological Principle can only be true in an approximate sense, on very large scales, as the universe does contain galaxies, clusters and superclusters. It has been a topic of some discussion over the past few years as to whether the formation of cosmic structure may influence the expansion rate by requiring extra terms that do not appear in the Friedmann equations.

Nick Kaiser says `no’. It’s a succinct and nicely argued paper but it is entirely Newtonian. It seems to me that if you accept that his argument is correct then the only way you can maintain that backreaction can be significant is by asserting that it is something intrinsically relativistic that is not covered by a Newtonian argument. Since all the relevant velocities are much less than that of light and the metric perturbations generated by density perturbations are small (~10-5) this seems a hard case to argue.

I’d be interested in receiving backreactions to this paper via the comments box below.