暗物质Rap
上周末从学弟那里掏来了一首暗物质Rap，其实我是不大听Rap的，不过这首颇有意思，所以破例。暗物质Rap是俄亥俄州立大学天文学教授David Weinberg所作，内容涉及暗物质的发现史及理论研究情况，形式近于讲座，不过内容也颇为专业，听之前最好能先了解点背景知识。
歌词：
My name is Fritz Zwicky,
I can be kind of prickly,
This song had better start
by giving me priority.
Whatever anybody says,
I said in 1933.
Observe the Coma cluster,
the redshifts of the galaxies
imply some big velocities.
They’re moving so fast,
there must be missing mass!
Dark matter.
Dark matter: Do we need it? What is it? Where is it? How much?
Do we need it? Do we need it? Do we need it? Do we need it?
For nearly forty years,
the dark matter problems sits.
Nobody gets worried ’cause, “It’s only crazy Fritz.”
The next step’s not ’til the early 1970s,
Ostriker and Peebles,
dynamics of the galaxies,
cold disk instabilities.
They say: “If the mass, were sitting in the stars,
all those pretty spirals, ought to be bars!
Self-gravitating disks? Uh-uh, oh no.
What those spirals need is a massive halo.
And hey, look over here, check out these observations,
Vera Rubin’s optical curves of rotation.
They can provide our needed confirmation:
those curves aren’t falling, they’re FLAT!
Dark matter’s where it’s AT!
Dark matter: Do we need it? What is it? Where is it? How much?
What is it? What is it? What is it? What is it?
And so the call goes out for the dark matter candidates:
black holes, snowballs, gas clouds, low mass stars, or planets.
But we quickly hit a snag because galaxy formation
requires too much structure in the background radiation
if there’s only baryons and adiabatic fluctuations.
The Russians have an answer: “We can solve the impasse.
Lyubimov has shown that the neutrino has mass.”
Zel’dovich cries, “Pancakes! The dark matter’s HOT.”
Carlos Frenk, Simon White, Marc Davis say, “NOT!
Quasars are old, and the pancakes must be young.
Forming from the top down it can’t be done.”
So neutrinos hit the skids, and the picture’s looking black.
But California laid-back, Blumenthal & Primack
say, “Don’t have a heart attack.
There’s lots of other particles.
Just read the physics articles.
Take this pretty theory that’s called supersymmetry.
What better for dark matter than the L-S-P?
The mass comes in at a ~keV,
and that’s not hot, that’s warm.”
Jim Peebles says, “Warm? Don’t be half-hearted.
Let’s continue the trend that we have started.
I’ll stake out a position that’s bold:
dark matter’s not hot, not warm, but COLD.”
Well cold dark matter causes overnight sensations:
hand-waving calculations,
computer simulations,
detailed computations of the background fluctuations.
Results are good, and the prospects look bright.
Here’s a theory that works! Well, maybe not quite.
Dark matter: Do we need it? What is it? Where is it? How much?
Where is it? How much? Where is it? How much?
We have another puzzle that goes back to Robert Dicke.
Finding a solution has proven kind of tricky.
The CMB’s so smooth, it’s as if there’d been a compact
between parts of the universe that aren’t in causal contact.
Alan Guth says, “Inflation,
will be our salvation,
give smoothness of the universe a causal explanation,
and even make the galaxies from quantum fluctuations!
There is one prediction, from which it’s hard to run.
If inflation is correct, then Omega should be one.”
Observers say, “Stop, no, sorry, won’t do.
Look at these clusters, Omega’s point 2.”
The theorists respond, “We have an explanation.
The secret lies in biased galaxy formation.
We’re not short of critical mass density.
Just some regions are missing luminosity.”
Observers roll their eyes, and they start to get annoyed,
But the theorists reply, “There’s dark matter in the voids.”
Dark matter: Do we need it? What is it? Where is it? How much?
Do we need it? Do we need it? Do we need it? Do we need it?
Along comes Moti Milgrom,
who’s here to tell us all:
“This dark matter claptrap
has got you on the wrong track.
You’re all too mired in conventionality,
wedded to your standard theory of gravity,
seduced by the elegance of General Relativity.
Just change your force law, that’s the key.
Give me one free parameter, and I’ll explain it all.”
“Not so,” claim Lake, and Spergel, et al.,
“On dwarf galaxies, your theory does fall.”
The argument degenerates; it’s soon a barroom brawl.
Dark matter: Do we need it? What is it? Where is it? How much?
What is it? What is it? What is it? What is it?
New observations hit the theory like an ice cold shower.
They show that cold dark matter has too little large scale power.
Says Peebles: “Cold dark matter? My feeblest innovation.
An overly aesthetic, theoretical aberration.
Our theories must have firmer empirical foundation.
Shed all this extra baggage, including the carry-ons.
Use particles we know, i.e., the baryons.
Others aren’t convinced, and a few propose a mixture
of matter hot and cold, perhaps with strings or texture.
And nowadays some physicists are beginning to wonder
if it’s time to ressurrect Einstein’s “greatest blunder.”
Why seek exotic particles instead of just assume
that the dark matter’s all around us – it’s what we call the vacuum?
Who’s right? It’s hard to know, ’til observation or experiment
gives overwhelming evidence that relieves our predicament.
The search is getting popular as many realize
that the detector of dark matter may well win the Nobel Prize.
So now you’ve heard my lecture, and it’s time to end the session
with the standard closing line: Thank you, any questions?
末了发泄几句，本想这周把《E. E. 巴纳德的银河》发上来的，谁料上周末莫名其妙地把第12期Sky & Telescope连同夹在其中的译稿丢掉了，一并还丢了本推导量子力学用的演算本，书包也差点没了（我把包捡回来的时候发现其他东西包括现金和计算器一件都没少，怪哉！），着实让我哭笑不得。丢东西的那天晚上迷迷糊糊地看电动力学，听觊觎已久的帝国反击战的碟子，郁闷中居然一个字也没看进去，一首曲子的调子也没听出来。不过庆幸的是，笔记和课本没丢，否则就更惨了……
