Re: Oh my god, this is physics

In message <X9SaH$FRK$kDFwnx@xxxxxxxxxxxxxxxxxxxx>, Oz
<Oz@xxxxxxxxxxxxxxxxxxxx> writes
>Charles Francis <charles@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> writes
>>I mean real physics with incredibly messy calculations that I don't know
>>how to do, using (whispers) numbers and formulae that you probably have
>>to remember. Arrgh.
>
><RANT>
>
>The 'remembering' goes in circles. One minute you have to remember every
>formula and next minute you get a booklet with everything in.

I mean remembering formulae from one chapter to the next. Liddle doesn't
rigorously refer back to every formula by number.
>
>IMHO its absolute stupidity for students to be expected to remember a
>bookload of formulae when nobody with any sense would not check first
>unless they regularly worked in that sector of physics. I wouldn't trust
>an engineer (or mathematician) who didn't check his formulae so why
>should undergraduates be any different?
>
>I imagine charles already has isbn 0-521-57507-9, and all undergraduates
>should be given a copy to use in all examinations.

The Cambridge Handbook of Physics Formulas. Of course I recognise it
immediately from the ISBN. (Not - I'm a bloody matmo remember, if I
wanted a formula in an exam I worked it out in my head from first
principles, which was quicker than looking it up. Of course that doesn't
work with physics formulae)

> Those with excellent
>memories will obtain a speed advantage if they do not need to look
>things up. Those who get a sudden mental block can look it up, as they
>would in real life.
>
><End RANT>
>
>phew!
>
>>>>The H-He-Li (IIRC) is highly critical of the exact conditions and rate
>>>>of expansion.
>>
>>I've been screwing up again. As you rightly remark the rate of expansion
>>is a vital ingredient in the calculation. Liddle is an undergrad text,
>>and only does a simplified version of the calculation of He-4 abundance.
>>H gets 22%, compared to the real figure of 23-24%. As I have half the
>>rate of expansion the calculation has to be redone and comes out at 15%,
>>which I suppose means 16-17% really.
>
>Ooops, buggered!
>
>That can't be so precise or people wouldn't have been arguing about the
>rate of expansion for the last 50 years. I can remember when 8->16 GY
>was the accepted range ferchristsake.

I've gone back over it, and uncovered some of the fiddle factors. See
below.
>
>Indeed. Well, by next year sometime I should be relatively unemployed

I thought you already were. Next year you will have found more ways to
use your spare time so that you have even less of it.

>>>>Its the *only* way Li can be made in astrophysical
>>>>situations. ISTR the result is NOT well modelled by current (when I read
>>>>about it)
>>
>>Where did you read about it, btw?
>
>Probably SciAm, it was quite a big deal 15-odd years ago then vanished
>into the astronomical black hole of "doesn't quite fit and we don't
>quite know why but maybe someone will explain it later".

Liddle has it that this was all sorted out in the late 1980's. Actually
its a pretty impressive result. Not quite as impressive as he makes it
appear, because of the way he bases it on fiddle factors and kludges,
but damn impressive nonetheless.

First impressive result, which I had heard of; you can only get the
number densities of light elements if there are three neutrinos. I
expect you knew that.

There are in fact five light elements whose mass densities are to be
compared

H He-4 D He-3 Li

At first sight this means 4 degrees of freedom, so getting them all
right has to be rather more than coincidence. But, the measurement of
He-3 abundance is only an upper bound, so that is only 3.5 degrees of
freedom. And now the bit where the fudge factor comes in. Liddle puts
the age of the universe in by hand. This determines, in all big bang
models, the rate of expansion in the early universe, which in turn
determines the He-4 abundance. Really this bit of the calculation is
backwards. If we knew the density of matter and the cosmological
parameters we could work out the age of the universe from the current
expansion rate, but actually these are the things we are trying to
determine.

So, what we are really doing is determining the rate of expansion in the
early universe from the He-4 abundance. This is actually pretty solid,
as all big bang models do obey the same expansion rate at that time. The
result has to be broadly consistent with the current age of the
universe, 14bill years, although he plugs in 12 bill and waves his hands
as to the explanation (you have to use more accurate formulae, taking
into account cosmological constant etc). It is also broadly consistent
with my age of universe, about 15bill years (depending on Hubble)
assuming zero cosmological constant (which is not essential, but I
prefer it).

Anyway, net result we are actually down to only 2.5 degrees of freedom,
for He-3, Li and D. There is only one input parameter, density, so
getting them right is pretty damn good. D abundance falls of rapidly
with density and is much more accurate measure than Li which is near a
minimum.

>Try asking on sci.astro (or spr), for a reference.

Not a lot of response to my last question. No one even told me to read a
book.

There is a review paper, which I am about to download, but haven't yet
read. It just might be easier than Dodelson.

astro-ph/9706069 :
Title: Big-bang Nucleosynthesis Enters the Precision Era
Authors: David N. Schramm, Michael S. Turner (Chicago/Fermilab)
Journal-ref: Rev.Mod.Phys. 70 (1998) 303-318


--
Best regards

Charles Francis



.

Relevant Pages

  • Re: "Tired" light
    ... the accelerated expansion is a mere hypothesis. ... I pointed out that I neglect the acceleration in my argument ... >> twin paradox, in an expanding universe, Terence is not at rest ... As I said, he works in particle physics, not cosmology. ...
    (sci.astro)
  • Re: If everything started 14bn years ago how come we can still see
    ... to cause them (and hence the expansion of the universe) to slow down. ... Mainstream physics, has given the name "dark energy" to this ... theory of 3-space is applied to Hubble expansion dynamics, ...
    (sci.physics)
  • Re: If everything started 14bn years ago how come we can still see
    ... to cause them (and hence the expansion of the universe) to slow down. ... Mainstream physics, has given the name "dark energy" to this ...
    (sci.physics)
  • Re: How Can Gods Existence Be Proven?
    ... mathematics gets intractable and exact calculation isn't possible, ... principle--is that nobody knows what the universe would be like if the ... laws of physics were different or even what different laws of physics ...
    (alt.usage.english)