Still, after four years, this is quite the moment (although I still can’t help feeling that I really should be preparing for a Ph. D. defence rather than an M. Sc. defence). Despite unappreciative TACs, error-prone service mode observations, and the best efforts of my committee, I’m actually at the point of finishing my degree, which is quite the change from last year, when I was expecting an attempt to kick me out of the program entirely at any moment.

So, the thesis. Available here. The title’s still a bit dull, but it works. And, after this long, it’s nice to see everything together in one place.

So, what’s going on with my life at the moment (other than writing my thesis of course)? Not too much, I’m afraid. I am making progress on my thesis though, so that’s definitely good news. Three of the five chapters either done or mostly one, and the fourth started, and I’m on target to defend at the end of July. And, my committee has been convinced to continue my funding through the end of August (for those who know something about how things have been going, this counts as a major victory).

Even with my thesis work though, I’m still learning new things. For example, a couple of weeks ago I discovered an excellent tip for those of you who cook. When one is trying to cook something (even if it’s just a simple “dump onions, vegetables, and maybe meat into a frying pan, cook it, and serve with rice, noodles, potatoes, whatever”), it is important to put the frying pan on the stove before turning the stove on, and vitally important to put the frying pan on the stove before adding olive oil and starting to cook your onions. Otherwise, the process becomes a bit messy. Or so it seems to me. Theoretically at least. Because, after all, I haven’t actually done anything like that of course.

Moving on now…

There’s an old physics joke where, when someone asks you “What’s new?” you reply “c/λ.” (to ruin it completely for those who haven’t heard it, physicists use the greek letter ν (“nu”) to represent frequency, and c to represent the speed of a wave (and λ (“lambda”) to represent the wavelength of a wave). The wave equation is c = λ ν, so by transformation ν = c/λ, and thus “What’s new?” may be interpreted as “What’s ν?”, and answered appropriately). In my spare time (ok, ok, I don’t actually have spare time, so this is the time during which I can’t write coherently anymore, but my mind is active enough that I need to do something other than reading) I’ve been learning a new programming/scripting language (mostly for MacOS X, but also now usable for Linux), known as “Nu”. Nu is essentially a lisp-based Objective-C interpreter (hence the same — this is really c/λ (if you don’t know the relationship between Lisp and λ, this page will serve as a useful primer)).

So, why do this in the first place? Well, Lisp is an extremely powerful language, in part due to its format (s-expressions make it trivial to treat code as data, which in turn makes it trivial to do some very clever things with your code), but it has a few weaknesses. One of Lisp’s weaknesses is its lack of libraries (while I have learned some Common Lisp (ok, another of Lisp’s weaknesses is that Common Lisp was not really put together in an elegant way, at least as far as I can see), I would use Python instead of Lisp for many purposes just because Python actually has a bunch of libraries that let me not have to worry about writing my own functions to process text, or access a network, or build a decent GUI application, or whatever). MacOS, however, has an incredible number of very useful libraries, mostly written in Objective-C (a combination of useful utility functions and excellent object-oriented GUI-building tools, known as “Cocoa”). Nu, by acting as (sort-of) an Objective-C interpreter, gets most of these libraries essentially “for free”.

Nu also has advantages over many existing Cocoa bridges, including PyObjC and RubyCocoa (etc.) in that it isn’t really a bridge, it’s more a different way of writing Objective-C code. A Nu class is an Objective-C class, and Nu code can be compiled into Objective-C code (or libraries, or frameworks) with almost no overhead.

Objective-C

Now, Objective-C itself has its good points and bad points. It’s very close to C (classic K&R C no less), which may be either good or bad depending on how you feel about C. It takes a lot of its object model (and syntax) from SmallTalk, which again may be either good or bad depending on how you look at it. Objective-C tends to do a lot of things at runtime that C++ (for example) would do at compile time. For example (to quote Andrew M. Duncan’s “Objective-C Pocket Reference”)

Objective-C differs from C++, another object-oriented extension of C, by deferring decisions until runtime that C++ would make at compile time. Objective-C is distinguished by the following key features:

• Dynamic dispatch
• Dynamic typing
• Dynamic loading

The end result of all of this is that, when you send a message to an Objective-C object [myObject doSomething:withParameter], the code that gets called by the message isn’t decided until runtime, and depends on the type of the message receiver (which doesn’t even need to be specified at compile time). For example, if you were writing a paint program, you could say:

id myShape;
myShape = [[shapeSelect alloc] init];
[myShape display];

without having to worry about what type of shape was selected by shapeSelect (presumably a routine that waits until the user selects a shape from the drawing palette and then returns the type of shape that they selected). As long as every shape can receive a display message, you’re fine (id is the Objective-C type of a generic object — an id is always a class of some sort, but may be any class). Objective-C also does dynamic loading, so any libraries that your program uses won’t actually be loaded until you need them (which can make your program more resource efficient).

Nu

Having given you a (very) quick introduction to Objective-C, I’m now going to very briefly describe how Nu works. I’ll be using “nush” (essentially a Nu interpreter which lets you use Nu as a scripting language) as my environment (if it actually matters). So, in nush, I could write a simple program to load a (hard-coded) file, and output the number of characters in each line (all of the classes I’ll be using are Objective-C classes, so you can already begin to see the advantages of Nu’s close relationship with Objective-C):

(set input ((NSString stringWithContentsOfFile:"/Users/shared/sharedfile.txt") componentsSeparatedByString:"\n"))
(set output (NSFileHandle fileHandleWithStandardOutput))
(set i 0)
(while (< i (input count))
    (output writeData:("#{((input i) length)}" dataUsingEncoding:NSUTF8StringEncoding)))
(output closeFile)

Oh, incidentally, the #{} in the middle of the text string is a Nu convention that was borrowed from Ruby. Basically, it takes whatever Nu code lies between the curly braces (which can be anything), processes it, and then substitutes the output into the string. At the moment, the output is the length of the string in characters, but really it’s arbitrary code.

Now, that’s not especially beautiful, and it’s a bit hard to see why I care about using Nu instead of Objective-C for this task (unless I really like parentheses (which I (obviously) do, but that’s not the point)). It becomes more obvious when I decide that, really, the output method is ugly — the NSFileHandle class has only one output method, and it takes an object of type NSData. I can convert an NSString object to an NSData object using the dataUsingEncoding: message, but it’s ugly to have to do that every time. So, what I can do instead is this:

(class NSFileHandle
    (- writeToString:(id)str is
        (self writeData:(str dataUsingEncoding:NSUTF8StringEncoding))))

Now again, that doesn’t seem very special, until you realize something — NSFileHandle class is a base class in the Cocoa framework. It’s available in a library, but it isn’t available in source code. So, in my Nu script, I’m adding a new method to an existing class for which I don’t have the source code. Naturally you can also do this in Objective-C itself too, but with Nu it’s effortless. And, what’s even more interesting, this is an interpreted script. So I’m adding a new function to a library class in an interpreted script. What’s more, because code is data and data is code, my script can actually take input in the form of s-expressions and evaluate them, no matter what code they actually contain.

So, after some struggle, I’m beginning to see that this is the Nu way of doing things. Yes, the Objective-C libraries tend to be fairly low-level, but you can add utility functions to the classes to make them do what you want them to do. So, when writing a Nu script (and, right now, I’m using Nu primarily for writing scripts, generally little data processing scripts that take spectra (encoded as text files with wavelength and flux columns) and do useful things with them, like scaling them, redshifting them, whatever). If you run into something complex then, especially if you might be doing it in multiple places, turn it into a function (or a macro). My rule so far has been to build functions when what I want is to make a class do something it wasn’t designed to do (just add a new method to it), and macros otherwise, but there are likely lots of different ways to go about this.

So that’s what I’m doing right now, and it’s surprisingly fun. And once I’ve finished making my little utility scripts (rather, converting them from python), I’ll be able to use the exact same code to make myself really nice MacOS applications. Not to mention that it keeps me programming, and makes sure I keep my mind flexible in the particular way that coding demands.

Thesis lite

There are still a few things left to do before it’s finished (in particular, I need to figure out what signal-to-noise I’d need to get an equivalent width limit of 0.3 angstroms for each of the Mg II systems, and how long it would take to reach that SNR with various telescopes). Once I have the times together, I need to put them in a table, and put it in the research plan).

Now, as I said, it’s mostly up there for interest, but if any of you feel like commenting or asking questions, that would also be good. The plan is aimed at the level of the non-specialist astronomer, but I’m hoping that it’s not too specialized for the chemist on my committee. In particular, if you see any astronomical jargon that I use without explanation, then I should probably deal with it. And, of course, if my “motivations” section doesn’t motivate, that’s a problem.

It feels really good to be finished this thing finally. Well, mostly finished anyway. And I don’t even have to send it in until the 20th, so I have a bit of time to revise it (if necessary). And, hopefully, I get a good thesis out of it.

The story of this idea starts this morning, at what is known as “coffee time” to the UVic astronomers. Given that only a third of us (if that) drink coffee regularly, and many of us don’t even bring a drink to coffee time, the name is somewhat inaccurate, but names have a tendency to be that way. Coffee time is an opportunity for us to get together at around 11 in the morning, talk about what we’re doing, meet visitors, or just talk about life in general (or, for some of the faculty who were born elsewhere, to rant gently about how, if there are multiple ways to do something, the people in North America always choose one of the stupid ways, and Canada is almost as bad as the US for choosing the stupidest way possible). In any case, this morning there was a discussion involving a faculty member, an NSERC summer undergraduate researcher working for them (who happens to be working out of my office), and (of course) myself. We were talking about the project this student is working on, which involves (through somewhat arcane means, and if anyone would like I can give a fuller description through e-mail) figuring out the masses (well, the maximum possible masses anyway) of a number of galaxies that just happen to be really close to quasars (well, they don’t “just happen” to be close by, and by “close” I mean “on the sky” rather than “in space”, but those are some of the arcane details). Anyway, I asked about absorption lines (since, after all, galaxies close to quasars on the sky may well be what causes DLAs in the first place, and DLAs are my area). Well, the project isn’t about that, and the faculty member involved doesn’t find quasar absorption systems to be the most interesting part of astronomy (I can’t imagine why), so the answer is basically that he doesn’t know. Well, this could be Science.

Moving off on (sort-of) another topic, there are people working out how the equivalent width of Mg II (Magnesium⁺) systems vary with the mass of the associated galaxy. Bouché et al. (2006), Ledoux et al. (2006), and Nestor, Turnshek & Rao (2005) have written about this idea, and how it relates to the mass-metallicity relationship found in galaxies in general. So, if I can get Mg II measurements of these systems, I can likely add something to this discussion (in particular, I can likely add more accurate galaxy masses to test the relationship better). I may be able to do this with SDSS data alone, I may need more spectra of my own, and I might not be able to get the necessary data, but it’s still an interesting idea. Albeit possibly a confusing one as described here.

There hasn’t been much progress in actually finding something to do for the project, but there has been some progress in goals and methods. In particular, I now have a couple of guiding principles.

Focus on the Science

This, I think, is an important principle. Instead of looking for things I can do with the data I have, I should really be looking for scientific questions that I want to answer. Sure, it’s important that I be able to answer them quickly enough to finish them up during the next couple of years, but it might well be easier to read up on what’s being done, find the holes, and figure out how to use the data I have to fill them, than to look at what I have and then look for how I can use it to answer a question no one has asked/answered before.

That said, it’s not likely to be an easy job, just a possible job. And I’m sure there will be plenty of setbacks along the way, because that’s how it seems to work. I know, in any of my non-science university experience (e.g. choosing topics for a final paper) I’ve always passed through a certain amount of panic at the thought that I wouldn’t be able to find anything to write about. Same thing here I suppose. Of course, there’s also another guiding principle that could help me narrow things down.

Focus on the Story

By this I mean focus on the fact that my thesis, when all is said and done, should be telling a unified story. At the moment, I’ll have a chapter or two on the Diffuse Interstellar Bands (both in “standard” DLAs and in Ca II-selected systems), and then a chapter or two on 21-cm observations, and then a chapter or two on something else. Now, that doesn’t sound very unified. In fact, it isn’t very unified. But, to make up a good thesis, it should be unified. So what’s the connection?

One possibility is the idea of physical conditions in DLA absorbers. After all, when I started the whole DIBs in DLAs work, the thought was that DIBs might serve as a proxy for the conditions in the DLA. In the Milky Way, for example, there’s a nice tight correlation between the strength of the 5780 angstrom DIB and the H I (neutral hydrogen) column density along the same line of sight. If that had held in the DLAs, not only would DIBs have been easy to find, it would also have allowed us to estimate the N(H I) for absorption systems where the Lyman-α transition is in the UV (i.e. systems with a redshift less than 1.8 or so, which are really important systems because the path between z=0 and z=1.8 encompasses 2/3 of the age of the universe). Of course, a UV telescope could also do that, but UV telescopes have to be space telescopes, and all of Hubble’s UV spectrographs are dead (although the next servicing mission will add one and might fix another), and the new James Webb Space Telescope won’t have any UV capability. Of course, this relationship didn’t end up holding up. Nor did a nice simple version of it (say scaled for metallicity), so that’s a dead end in practice (although it can still go in the thesis, because nothing requires your theories to always be right, as long as they’re plausible, and you actually tested them to find out).

Another way in which DIBs might have traced physical conditions in the absorbers is their relative strengths. Of course, to use that we’d need to have detected a lot of DIBs. More than 3 (all in the same DLA) anyway. But, again, it can serve as a connecting thread, because 21-cm absorption also tells us about the physical conditions in the absorber.

What else could I do with physical conditions? Well, there’s always molecular hydrogen, although I’m not sure that’s a sufficiently original idea (or even necessarily possible with the systems I have). There might be the idea of looking for molecules (CO for example). I’m not quite sure, but there are bound to be possibilities.

So, anyone have any other ideas for how I can connect everything together? For each different story I can tell, there’s a different type of science question that I can be asking myself. And having as many questions as possible seems like a good idea.

Elsewhere, work-related

Well, after a few recent events, I’m being pushed into doing something I probably should have done some time ago — actually going through a formal process of testing for Asperger’s. It’s going to be time-consuming, expensive, and probably annoying, but my supervisory committee has strongly recommended it, and knowing exactly what my strengths and weaknesses are there will help them to figure out ways to help me get through the program. In particular, the part about establishing a leading role in my research. There have also been memory issues which I need ways to deal with.

Elsewhere, non-work-related

After all, I have to do something for fun. At the moment, I’m working on a few comments about a recently-released study funded by the US government. A study which hasn’t been much publicized, and which, for some reason, was released without a press release (or any attempt to draw attention to it). A study describing the (lack of) effectiveness of four different abstinence-based sex education programs. I expect my responses will be a bit, well, sarcastic, but I hope people might be interested in reading my commentary (complete with figures and tables taken from the report). Short version, it doesn’t work. For the longer version, you’ll have to read my post (hopefully in the next week or so).

So that’s about it for me, for now. So far at least, I’m managing to post (sort-of) once a week or so, which is good. Let’s see if I can keep it up.

Well, my original intention to spend the week working on my research plan foundered on health. To be more specific, lack of health. My health, in particular.

I’d already planned to lose a day or so from the week getting Morgan to and from her surgery (and associated doing things around the house), but it didn’t help that I also ended up with probably the worst cold I’ve ever had. My symptoms started last Monday, and still haven’t stopped. Thus, instead of planning out research, I was planning out how to deal with the usual (fever, sore throat, coughing, etc. (of which the fever and coughing, especially the coughing, are still going strong)). Oh, and the ear infection. Which is in behind the ear drum, so I’m taking pills rather than ear drops. My ear closed down this past Wednesday, and I’ve already started to forget what it was like to have actual stereo hearing (and actual balance too).

That said, I did manage to get a bit of thought in over the past week. Centred mostly around what I have, and what I can do with it. In particular, I have:

• A few bright, well-studied quasars with known low-redshift DLAs that can’t really be studied further without a UV telescope, one of which has diffuse interstellar bands (DIBs) in its spectrum.
• A few relatively bright quasars with Ca II (singly-ionized Calcium (in astronomy, Ca I is neutral, Ca II is the same as Ca⁺, etc.) metal line absorption systems (almost certainly DLAs, but with no UV telescopes we can’t tell for sure), of which (say it with me) one has diffuse interstellar bands in its spectrum (repetitive, no?)
• A bunch of assorted quasars that happen to be radio-loud, but are mostly fairly faint in the optical, which have no detectable (i.e. z>1.8) DLAs in them.
• Several quasars, mostly faint, but all radio-loud, with detectable DLAs. One, so far, has a complete spin temperature (T_s) measurement.

So, really, not much. And not much for follow-up either, since the promising systems (the last two sets) are generally either not interesting for follow-up, or too faint in the optical for high-resolution follow-up. The single bright system with a DLA has lousy RFI at the critical frequency, so we can’t even get a spin temperature measurement. So what to do with them?

Well, the first obvious answer is use all that radio flux. Look for something that you can see in the radio. Which probably means molecules. Which have pretty much never been seen in DLAs before, but I might figure out something (Is it original? Yes — as far as I know, there’s never been a survey. Is it possible? I don’t know — which might be why there’s never been a survey).

Another answer is to look at the quasars themselves. If only I understood more about quasars, and had a clue of what to do with them anyway. That said, it’s a possibility, since I expect I can learn about what isn’t known about quasars (that I might discover) without too much trouble. Fitting it in to the radio, though, could be difficult.

Or, of course, I could do something else entirely. The question (naturally) is what. And I don’t know yet.

So, 1 week down, 9 to go. A few interesting ideas, no real tests of practicality. But then again, it was a sick week (and still is).

Of course, the research plan isn’t the experiment. It’s just something I need if I’m planning to actually get a doctorate and start doing astronomy as a career. Writing about it is the experiment. Specifically, it’s my plan to produce at least ten posts (one per week, minimum) talking about how exactly one goes about producing such a proposal, blind alleys included. It’s my hope that having to write about my progress will keep me working on the thing, and that having a record of what I’ve been doing will help me to avoid becoming discouraged as time goes by with no direct progress towards the final goal. And, of course, I’m experimenting on whether I can actually post once a week or more without missing.

So, on with the experiment.

Incidentally, I’m told that the flight from Seattle to Copenhagen has some nice views. I’m sure it does, but they certainly aren’t obvious when you’re flying overnight. And, when I land, I get to head on to Madrid, where I have to go through customs and successfully recognize my suitcase before I get to actually call the hotel I’m staying at tonight (tomorrow night? Stupid time zones). Oh well, at least I’ve been able to check my e-mail on the way….

First, I’m going to start you with a picture (stolen from Sara Ellison, my advisor) which shows what it is that I’m actually looking at. The basic idea is that, while we can see galaxies very far away (redshifts of 6 or even higher), at any significant distance (even redshift 0.5, for example) we see a very biased sample of galaxies — we can see bright galaxies, but not faint galaxies. The obvious result of this is that we end up with a biased view of what galaxies were like in the early universe, especially since faint galaxies are much more numerous than bright galaxies.

Now, with Damped Lyman-α (DLA) systems (the galaxy in the figure above, named because the Lyman-α line of neutral hydrogen (H I) is strong enough to display damping wings, more on that later), we don’t have that problem. These systems aren’t selected by brightness, by redshift, or by anything other than random chance — whether or not they happen to be on a sightline that intersects with a quasar. As such, we can see all types of galaxies, bright and faint, with this technique.

So, we aim the telescope at a quasar, send the light through a slit, pass it through a prism (actually, generally either a grating or a mirrored grating (called a “grism”), and record a spectrum in which wavelength is our x co-ordinate, and position on the sky (in one dimension, the one you happened to set when deciding how to orient the telescope) is your y co-ordinate. Well, you’re done, right? Sadly, not yet. You first have to reduce the spectrum, and then check to see if there is in fact a DLA there (since nature was not kind enough to provide us with a list).

Now, reducing the data (going from the raw data as the telescope recorded it to the processed data from which you can actually extract something useful) is different at every telescope (and every part of the spectrum), but I’ll walk you through a generic reduction of optical (near UV, visible light, and near IR) spectra (data with wavelength as one axis, as opposed to imaging, which actually takes a picture of the sky). Later on, I’ll go through the reduction procedure for low-frequency radio data. Note that this will be a very brief walkthrough, skip a few steps, and assume that everything works perfectly — things can, in reality, get a lot worse than this.

Now, the CCDs used in telescopes are very good compared to the CCDs in digital cameras. They’re more sensitive to low light, more linear, cooled (to avoid thermal jitter and like problems), etc. But they still have a basic amount of error that you can’t get rid of, including random noise in each pixel. Now, the problem with this is that, when your pixels don’t have very many counts (and, for a good astronomical CCD, the ratio isn’t too far off 1 count = 1 photon), your noise is always in the same direction — up (and easy to mistake for signal), since a pixel with only 1-2 counts in it can’t really read low. The solution to this is to run a current through all of your pixels, to set their zero value (the value they would (in theory) read out even with no light at all) to about 100 counts. That way, reading low won’t really matter. This is called the “bias voltage”, and to figure out what it is you take a bunch of short exposures with the shutter closed, average them together, and subtract them away from each data frame.

The next problem you face is that different pixels respond slightly differently to the light that hits them. Some will read a bit higher than others, and you want to be able to deal with that. Some will also respond differently to different wavelengths, and for spectra you want to deal with that too. So you take a few exposures of a bright, uniform source (like the sky at twilight, or the inside of the dome, or special lamps designed for the purpose), and then divide every frame by your “flatfields” (as these frames are called). Of course, this, well, flattens out the field, and gets rid of the per-pixel effects.

You then extract out your spectrum, combining as much light as you can together (while hopefully subtracting off any interference, including emission and absorption from the night sky, which can be really nasty). Now you have a rough idea of which wavelength is there (based on the settings you gave the telescope), but a rough idea isn’t good enough. So you take some more exposures, this time of special lamps that excite specific elements that give off light at known frequencies (like neon lamps, but usually mercury, copper, titanium, argon (etc.) instead of neon). Then you calibrate the wavelength scale on these images, and apply the calibration to your own images. Finally, you combine all the exposures you took together (since you’ve removed the noise on each one individually, this has the net effect of increasing your signal-to-noise), and you’re left with something like this (assuming you’re looking for DLAs):

This is, actually, a picture of the quasar spectrum the does have a DLA in it (right around 4000 Angstroms, can you spot it?). In fact, this is a very special DLA, for reasons that will become clear in a later part. Now, the DLA itself looks very much like this:

In this case, the black histogram is the same spectrum as the figure above, while the dashed line is a fit to a damped profile with H I column density of 2.0×10²⁰ cm^-2. You can see the broad shallow “wings” on either side of the central absorption. This is all based on the so-called “curve of growth” that relates line width and column density. In the low-density or “linear” region, column density and equivalent width are directly related, by

Column Density = 1.13×10²⁰ × EW / (λ² × f)

where EW is the equivalent width, f the oscillator strength (how relatively strong that line is), and λ the wavelength of the line. As lines become stronger (around a column density of 1×10¹⁵ cm^-2, they enter the “saturated” regime, where column density is only related to the logarithm of EW (which is what you measure), and column density also depends on internal velocity (which is impossible to measure), making the actual column density impossible to determine. At a very high column density (around 1×10²⁰ cm^-2), the damping wings start to appear, and the column density is now based on the square root of the EW (easier to find), and velocity disappears again. This generally means that you can measure either very weak or very strong lines, but nothing in the middle.

So now that we’ve found our DLA (and now that you all know what the “damped” part of damped Lyman-α system means), you can do something with it. But, since this has turned out to be another long post, I’ll discuss that in the next part, hopefully in a couple of days.

It’s the third week of the course, and the overall theme is about how to talk with children, and (of course) there’s homework, (mostly) involving actually trying to apply what you’ve been working on. So, at the start of the class, we were asked to share about if (and how) we’d applied the ideas from the previous class. My response?

“Well, I didn’t apply any of these techniques in talking with my child, but I did use them in talking with an employee of the Ministry of Children and Families. That’s the same thing, right?” The instructor, who has some experience with the ministry, agreed (of course).

Ok, that’s it for now. I’m off again to look at the case of the curiously high metals content.

So now, cold dark matter, via gravitational lensing, neutrinos, relativistic free-streaming, and structure formation. Basically, all the neat stuff that turned the universe from a photon-baryon coupled fluid into the mostly-isotropic-but-with-some-structure (galaxies, clusters, and so forth) thing it is today.

So previously I had shown how some very good research had led to the conclusion that there was definitely some dark matter out there. Now, the power spectrum of the cosmic microwave background (basically analyzing the size and distribution of the temperature fluctuations) implies that there is definitely non-baryonic matter out there, but we’ll ignore that for now (it’s a fairly recent result). Instead, we’ll first look at the various types of baryonic particle which have been proposed for dark matter.

First up, we have MACHOs (MAssive Cosmic Halo Objects). These are brown dwarfs (~0.1 M_⊙¹, “stars” too small to initiate fusion), free Jupiters (gas giants which aren’t orbiting around stars), white dwarfs (burned-out stars <1.4M_⊙ which are no longer emitting light since they’ve fused all they can), and black holes (do I really need to explain what these are?). So one theory was that there were a whole bunch of these things floating around the galaxies (especially in the galactic halos), and that they were the dark matter.

Of course, once these ideas were proposed, they had to be tested. So a search had to be mounted for MACHOs, with the usual difficulties inherent in looking for things that are, well, invisible (or pretty much so). Hubble is able to see white dwarfs fairly well, but the others are essentially impossible to see with telescopes. So, enter gravitational lensing.

Gravitational lensing comes as a result of the theory of relativity. If, as Einstein suggested, gravity is a distortion of the geometry of space (rather than a force), it should act on light as well as on matter, because light passes through the space between the source and the observer (Newtonian gravity could have been defined as a force acting on light as well as matter, but then light would have to be given a mass for the purposes of calculating the force, and the equivalence of mass and energy is another of those relativity things you may have heard of). In Astronomy, lensing is divided into “microlensing” and “macrolensing”³, of which microlensing is the important one here (macrolensing also is very useful, but it would add a bit too much to this discussion to mention it here).

So, gravity bends light. Now, imagine a heavy (but not too heavy) object passing between us an a distant light source (a star, a galaxy, whatever). Since the light source emits light in all directions, for every light ray that goes directly towards us, there are several that are aimed slightly away from us, and so we just barely don’t see them. Now, when the object moves directly between us and the source, it bends some of those rays a little bit towards us. Sure, it blocks out the rays that we were seeing before, but it bends even more rays from “just missing” to “just hitting”, so we see the light source get brighter for a moment (with a slight dip to “fainter” (not as faint as at the start, but a bit fainter than the maximum brightness) right when the object crosses the line of sight. On the off chance that the explanation was a bit confusing, it’s figure time. The first figure is taken from the University of Oregon, the second from UBC.

So, as you can see, even if a MACHO is “invisible” (not emitting light), it can still be detected if it passes in front of something. And, if there are enough of them in the galactic halo to act as dark matter, then if we monitor a bunch of stars in a nearby galaxy (let’s say one of the Magellanic clouds), we should see a few lensing events. And we do, but not nearly enough. So, while there are definitely large massive objects in the galactic halo, there aren’t enough of them to be dark matter.

The other category of dark matter is so-called WIMPs (Weakly Interacting Massive Particles). Now, there’s one possible WIMP that’s actually known to exist now, and that’s the neutrino. Neutrinos are (almost) massless particles which were first invented to deal with problems that were found in nuclear reactions, and have since been detected. In particular, let’s look at a couple of decay reactions and show where the neutrino belongs:

Each of these reactions has a slight problem. The first problem is that, well, mass-energy isn’t conserved. When you add up the total mass-energy of the products, it’s lower than the mass-energy of the reactants. The second problem is that particle number isn’t conserved. The electron (e⁻) has a baryon number of 1, while the positron (e⁺) has a baryon number of −1 (it’s antimatter, which is defined as having a negative baryon number). So, in the first case you’re creating a particle from nowhere, and in the second destroying a particle. Not a good thing to do. As a result, the neutrino was created to “balance out” the equation (and then, eventually, detected). Now, things look like:

Incidentally, the overlined text indicates that the particle in question is antimatter, here an electron anti-neutrino (now, neutrinos and anti-neutrinos are effectively the same, but the important thing is making the particle number come out right). Lots of nuclear reactions (including the fusion reactions that happen inside stars) emit neutrinos, so there are a lot of them around, and they don’t interact with much. They can be detected, however, and scientists were actually quite quick to detect solar neutrinos &emdash; there just weren’t enough of them. While many theories were created to explain this, the answer turned out to be that neutrinos change type. In theory there are three types &emdash; electron (ν_e), muon (ν_μ) and tau (ν_τ). And the early experiments could only detect electron neutrinos (which should have been the ones the Sun was producing). Instead, though, some were changing type on the way, and thus weren’t being detected. What this type-changing (“oscillating”) means is that neutrinos have mass.⁴ Unfortunately, they don’t have enough mass to be dark matter. The electron neutrino has a mass <2.2 eV, the muon neutrino <170 keV, and the tau neutrino <28 MeV.⁵ Considering that an electron has a mass of 0.51 MeV, and the proton 938 MeV, you’d need a lot of neutrinos to be six times as heavy as the rest of matter combined. Not to mention, of course, that neutrinos would be hot dark matter (and next I’ll tell you what the difference is).

Whether dark matter is “cold” or “hot” depends on the mass of whatever particle(s) make up dark matter. Heavy particles are “cold”, while light particles are “hot” (except axions, which I believe are both light and cold, because they work very oddly). What the distinction means is that, essentially, dark matter particles start out moving at relativistic speeds (near the speed of light), and eventually slow down, and heavier particles slow down sooner. The reason that this is important is that dark matter is most of the matter in the universe. Now, relativistic free-streaming dark matter tends to “smooth out” normal matter, getting rid of structure (like, say, galaxies or clusters) on a size scale that gets bigger for lighter particles (that stay relativistic longer). Cold dark matter is thus the currently accepted default, because hot dark matter wouldn’t have allowed galaxies to form at all.

Of course, cold dark matter has its problems. Specifically, the models predict that there should be dozens of dwarf companion galaxies around every regular galaxy (like, say, the Milky Way), and instead there are two. Three if you count the one currently being digested. Well, a few more than that, really, but not as many as there should be. This has led some physicists to speculate on the possibility of “warm” dark matter, which is hot enough to have smoothed out most of the companion galaxies, but cold enough to have left everything else in place. Of course, balancing things just right is difficult to do, and simulating warm dark matter is much more difficult than simulating the cold stuff. And, since no one knows what dark matter is yet, there’s no real way to resolve the debate either.

This has gotten to be a really long article, so I think I’m going to close things here, at least for now. Hope it was entertaining, and at least somewhat comprehensible. Next time, I might even talk about what it is that I, personally, do in Astronomy.

Many years ago (circa 1934), a smart but somewhat eccentric astronomer (strange how often those go together) named Zwicky pointed out that some of the galaxy clusters we see have too much kinetic energy. Specifically, when you estimate their mass (based on the amount of light present), it’s too small to keep them from flying apart. Of course, this wasn’t too much of a problem, since what it mostly meant was that we weren’t very good at measuring the mass of galaxy clusters based on their light content. But that wasn’t all.

A few decades later (we’re moving into the 1960s, 1970s, and even the early 1980s here), astronomers were using spectrographs on the new 4-m class telescopes to look at galaxies. Specifically, they were measuring rotation curves. This involves looking at the stars a certain distance out from the galactic centre, and measuring how fast they were rotating around the centre. They expected to see a nice falling curve as distance increased, similar to the curve you get when you look at the solar system. The reason you get this nice falling curve is that you can treat a solar system as having an ideal point mass in the centre, and a bunch of other point masses orbiting it which interact only with the central mass, and not with each other (to a first approximation, anyway). When you do that, you discover that the orbital speed depends on the distance away from the centre, and is lower the farther away you go (farther away = less gravitational force = move more slowly). Galaxies also have most of their light concentrated at the centre, so the astronomers were expecting to see the same thing. But they didn’t.

Instead, the orbital velocity levelled off, and became effectively constant. No big deal at first, since you can get that type of velocity curve just by having an even distribution of mass all the way out. Sure, the stars looked denser in the centre, and there looked like there was more gas there, but the arms of a spiral galaxy are still pretty bright, and there must be gas in the arms (because that’s where most of the star formation happens), so the new results were possible. But then people started looking out beyond the disk, where there were only a few stars, no star formation, and effectively no gas. And the rotation curve was still constant.

The only way you can get that, with gravity as we know it, is if there were mass, and quite a lot of it, out even in the galactic halo. But there obviously wasn’t any light there, and light traces mass quite well (at least in the solar system). Still, there seemed no choice but to conclude that there was “dark matter” in the galaxies that were making the rotation curves look odd. But not everyone agreed, of course.

A few astronomers decided that the real solution was to change the way gravity worked. This is called MOND (MOdified Newtonian Dynamics), and it introduces a correcting term that only operates at very large distances, and very low mass concentrations (otherwise it would have shown up in the solar system, and it hadn’t). The first MOND theories were, well, Newtonian (that is they didn’t include relativity), which was an obvious problem, but eventually some relativistic MOND theories were created. And there really wasn’t any way to choose between them, since the dark matter was invisible, and it followed the light distribution closely enough that you couldn’t really distinguish. Sure, dark matter was hierarchical in nature (pretty much none at the solar system scale, a bit at the galaxy scale, a lot at the cluster scale, and quite a lot at higher scales), but MOND also had more effect for larger objects (like clusters), so there still wasn’t any good way to tell.

Then came WMAP (Wilkinson Microwave Anisotropy Probe). Now, COBE (Cosmic Background Explorer) found the cosmic background, but it couldn’t really measure any irregularities in the background. For reference, the biggest source of fluctuations is microwave emission from the Milky Way (so you have to filter that out). Then comes other microwave hotspots (from neutron stars, active galaxies, etc.) (so you have to filter those out). Then comes the cosmic dipole (because the Milky Way is moving relative to the cosmic background, so part of it is red-shifted and part is blue-shifted). Then you have the actual fluctuations, which produce a nice map like this. For reference, the overall temperature of the background is 2.73 K. And the difference in temperature between the hottest (red) and the coolest (blue) points on that map is about 0.00001 K. So finding them wasn’t easy.

Now, the fluctuations look random, but they aren’t. There’s a pattern in the spacing, and it tells us a lot about the early universe. In the early universe, you see, photons, protons, and electrons were in equilibrium. Oh yes, and there was dark matter (in the standard model), which only interacts due to gravity. And the dark matter has a clumpy distribution. So the result is that places with dark matter clumps attract in protons and electrons. But these are constantly absorbing and emitting photons, so the clump also has many more photons that the surrounding universe, and that means light pressure, which pushes away the protons and electrons (but not the dark matter, because it doesn’t interact with light). So these temporary matter clumps are constantly forming and breaking apart as the universe expands and cools.

Then, suddenly, the universe becomes cool enough (and big enough) that the photons that are emitted don’t get absorbed right away. So they escape, and keep on going. The coupling is broken, and nothing’s forcing the protons and electrons to break apart again when they clump together. So these little clumps start interacting with one another, and getting bigger and bigger, and eventually structure is formed (stars, galaxies, clusters, etc.) with larger structures taking more time to form.

And those photons that escaped? They’re the cosmic background. And those fluctuations exist because, when matter and radiation decoupled, things were clumpy. And, by looking at the fluctuations, and how intense they are, and how far apart they are, you can tell a lot about the early universe.

One of the things you can tell is how much matter there was. Another is how much baryonic (non-dark) matter there was, because baryonic matter was interacting with the photons, while dark matter wasn’t. And, when you do the calculations, you find there was about six times as much dark matter as baryonic matter. This is, of course, a problem for MOND people, which they dealt with mostly by ignoring it (really — MOND papers seldom reference the WMAP results except to say vague things about them being unreliable, and there are quite a few MOND papers published in the past year that imply (or even state outright) that the only evidence for dark matter is galactic rotation curves). So dark matter now has a fairly substantial majority of scientists convinced, while MOND seems unable to explain the WMAP data, but people are still pushing it.

Then comes the result I started with. These people looked at two merging galaxy clusters, because they realized something very important: most of the (non-dark matter) mass in the cluster is in the form of very hot plasma. And, while stars (and galaxies, and dark matter) act like collisionless particles when clusters merge, gas acts like a fluid, so the gas from the two clusters would end up colliding and slowing down (which you can see in their figure). So they could see where the gas was, and it wasn’t where the galaxies were. Next, they measured where the mass was through gravitational lensing (gravity bends light, so when you have mass between you and a source of light the source looks slightly brighter because the light has been focussed (so long as the mass is compact enough not to just block the light of course)). And the mass measurements show that the mass is where the galaxies are. Except that it’s already known that the galaxies are much less massive than the plasma, so there must be something else where the galaxies are. Something more massive than the plasma. Something that acts as collisionless particles. Something that doesn’t emit light. Something like dark matter. And this is a vitally important result, because it’s the first time that someone has shown, conclusively, that the mass distribution and the visible mass distribution are not the same. That’s something that doesn’t work at all in any MOND theory (unless you modify MOND to include dark matter, but really what’s the point).

So, after all this, that’s why this result is so significant. It’ll solve a “controversy” that’s been going on almost as long as “what are the diffuse interstellar bands” (which, I point out modestly, I’m currently working on), and much more relevant to cosmology as a whole.

And what about the “cold” in the title? That’s a distinction for another day.

So, universe design.

I recently had a couple of ideas on how things might work in a particular SF universe that I’m designing (rough outline available in the fudge factor article I wrote about it). Now, some details have changed (mostly I got rid of gravity manipulation, probably, because I don’t think I need it, although it might re-appear, and I’ve firmed up the way the jump drive works). In any case, I had some ideas about the way orbital space would be divided up between various planetary nations, and the way freight and passengers would get from the surface to the starships. And I posted these ideas to sfconsim-l (which is actually on yahoogroups now), and the discussion quickly evolved in many unexpected directions. During the discussion, I quickly realized how important a group like that is for anyone trying to create any type of hard-SF setting, mostly because there are a lot of scarily smart people there, and they’re willing to put your math, physics, biology, sociology, etc. through the wringer, and question your unstated assumptions, and help you tighten things up to no end (even if only by letting you know, unambiguously, where you need to break the laws of physics to make things more fun).

I should mention, also, for anyone who’s interested, that the other absolutely-not-to-be-missed resource for anyone who’s doing anything that’s even remotely hard SF is Nyrath’s incredibly detailed and useful atomic rockets web page. This page recently highlighted one of the strange ironies of my own setting — because of the way I handle FTL travel, the biggest, most powerful engines (~35 gigawatts of thrust power, which is nothing compared to most SF rockets) belong to the orbital tugs, and most actual starships have only chemical maneuvering thrusters, or very low-acceleration, low-power, low-Δ-v engines. And my spaceships resemble trains far more than they do anything else — everything is modular. Not to mention that the weapons carried by warships are essentially useless near planets.

The real advantage of Atomic Rockets is that it explains what you’re doing step-by-step, and it puts the equations together in ways that are much easier for me to look up than my textbooks (which were designed to teach physics rather than science-fiction rocketry), and it also makes it easy to fiddle around with my parameters until I have something that looks right (even if it does mean travelling at several light-years per day between stars, then taking another two days or so to go from (roughly) the distance of the Moon back to the Earth).

And, well, sfconsim-l will tell you when you’re breaking the laws of physics, and will mention some of the consequences you might not have noticed about breaking certain laws (so as to avoid the Star Trek problem of having to explain each time why the transporter (or the replicators, or that gadget they put together in ten minutes last episode) can’t solve the problem). And finally, if you run your assumptions past them, and tell them the results you want to get, they’ll usually tell you where you have to break the rules, and what the good places would be to do that with a minimum impact on the way your universe works.

So, now that I’ve rambled on about ideas (a little bit) and good places to hang out if you’re into science fiction (and note that none of it was work-related, except to the extent that I might be thinking about things while I should be working), I’ll sign off again for a little while.

Actually, mostly it’s because that’s when I’m pretty much guaranteed to have some time with nothing to do except try to stay awake (accompanied by internet access, of course, since astronomers just love running cables up mountains). Not that I’m on a mountain this time….

Actually, right now, I’m in Socorro, New Mexico, home of the VLA (Very Large Array). I just saw it for the first time today (it’s about 100 km outside of Socorro). I’m not actually here to observe on the VLA, of course, much though I might like to.

So, how is radio different from optical? Well, you don’t care about the weather (it’s been cloudy, raining, whatever (even thunderstorms apparently) at Greenbank, but we haven’t even noticed (of course, at higher frequencies you do notice, but we’re not using them). You can observe during the daylight. RFI is nasty (not so much at high frequencies but, again, that’s not what we’re using). The data reduction package (“AIPS++”, now called “CASA”, and we’re using a single-dish package called “DISH”) is just as quirky as IRAF, but involves a whole new learning curve. Of course, you still use FITS files. All hail the mighty FITS file format (seriously, if there’s one development that has made astronomy easier for everyone, the FITS file format is probably that development). Oh, and the telescopes are a lot bigger. Even the VLA’s (individual) antennas are maybe 15 metres across, (and there are 27 of them, spread over about 40 km, which makes for a very impressive sight), and I can’t even imagine Greenbank — a fully-steerable 100 by 110 metre telescope. Araceibo is 300 metres across, but you can’t move it.

Overall, it’s been a productive (if busy) trip, and New Mexico, aside from the heat and dry conditions, is a really nice area (this part of it, anyway). And now I can add another part of astronomy to my list.