Dark energy, to be precise.
And not even dark energy as such because, let's face it, both my own small skill as an elucidator and the period of time I have available for this endeavor are dramatically inadequate for tackling so deep a mystery. Instead, in the classic talk-about-the-talking postmodernism of blogs, my attention turns to the investigations seeking to understand this phenomena rather than the phenomena itself.
I have, after all, always been much more of an experimentalist than a theoretician.
But to recap, dark energy is a postulated force that would explain some rather odd behavior of the universe. The oddity in question (for there are several oddities about our universe that require postulated things to explain them) is that the universe seems to be expanding at an ever increasing rate. Now that the universe is expanding is not at all odd. We've known about this since Edwin Hubble, a man brilliantly characterized as a "large mass of ego" by Bill Bryson, noticed that all the galaxies in the universe are expanding away from each other. Subsequently, a series of theories beginning with the "big bang" and moving on to modern inflationary cosmology have homed in on the idea of the universe originating at some sort of very small beginning (there are a few variations) and expanding outward from some sort of initial impulse (again, there are a few variations).
This is all fine and good and if you want some ideas about the how/why on that it won't surprise anyone that I now recommend Brian Greene's The Fabric of the Cosmos. But this expansion should be slowing -- as the shared gravitational attraction of all the, well, stuff in the universe gathers together and pulls on itself. And for a few billion years, it appears that it did. But then a few billion years ago, the rate of expansion began accelerating again. There is no good reason for this, not with the rule that we've been playing by.
It is as if, to invoke a classic Feynmanism, we were watching a chess game, thought we'd got the rules and moves pretty much figured out, and then someone castled. Uh-oh, what the hell was that?
Since then, cosmologists, astrophysicists, particle physicists, and plain old ordinary physicists have all gotten in on the bandwagon to try and explain why. The lure of being the first to explain a new (or dramatically revised) physical force is a pretty big one!
Alright, enough of that back-explanation. I said that actually trying to explain dark energy was beyond me. Oh, but it is different from dark matter. I know. They could have come up with some more varied names. Like "The Smuckers Effect" or perhaps "The Universal Choo-Choo." Either one might have been better.
So this dark energy stuff, whatever it is, is suddenly pushing the universe apart faster and faster. Or not so suddenly. Or it remains constant or decreases as a quadratic function while gravity decreases as a cubic function. Sorry. Got distracted again. The point is, we have no idea what this force, this dark energy, is. We can only observe what it does. And that makes it a wonderful place to study and understand the interplay between observation (experiment, if you like) and theory.
Science proceeds, in an idealized and perfect world, as a series of iterative steps. Someone observes a phenomena (say the increased rate of expansion of the universe). That person (and a few others) say "Damn, we didn't expect that!" Everyone then retires to their chalkboards and starts thinking of theories to explain what is causing this phenomena. The theories will span a broad range. Some might involve zero point energy, others extra dimensions, still others giant turtles. As the theorists theorize, the experimenters begin to contemplate the next round of experiment or observation (I think of experiment as an active act -- where we do something, such as at a particle collider -- while observation is a passive act where we take data on what the universe is already up to -- as with a telescope).
Theorists and experiments/observers are different. The former are the ones with the unkempt hair, the latter the ones with the dirty clothes and coffee addictions (particularly in astronomy).
Anyhow, while the theorists are using their imaginations and running the numbers, the experimenters/observers are doing their thing and building the next generation of machines. What proceeds then is something like a lottery. Or perhaps a reality TV show, though I doubt "Survivor: CERN" or "America's Next Top Scientists" or "Theorizing with the Stars" will take off anytime soon.
Any good theory brings a few ingredients to the table. It must offer an explanation for why the phenomenon under consideration occurs. It ideally should offer a mechanism to explain how it occurs. And it should provide some sort of mathematical formula that can fit the observed data to a high degree of accuracy. Lastly, that mathematical rigor should allow for some degree of prediction of as yet unobserved phenomena that can test the accuracy of the theory. This prediction might simply involve taking the measured predictions to a few orders of magnitude more precision. Or it might involve a wholly new physical manifestation. Either way, it provides some way of telling if the theory will have the winning number come lottery time -- the return of experimental results.
We go round and round like this. The results from each round of experiment feed the next round of theory. The predictions of a given round of theory guide the direction in which the experimenters/observers turn their searching. Rarely, however, are things so precisely beautiful as this, like turns in a board game. Usually, after a while, everything gets all out of synch and the experimental and theoretical processes get all overlapped.
But dark energy is new. It was accidental in a wonderful way, and the demands of further experiment have allowed for a long and fruitful phase of theoretical contemplation. And now the experimenalists are about to have their day. And by now I mean in about eight years, because that is how long it takes to get a space mission from budgetary contemplation to launch pad. And then a few more years of taking data.
Science is for the patient, these days.
This whole process of theory-experiment (or observation) is crucial to the scientific quest for understanding. It always galls me when people talk about how scientists don't actually know anything -- they just have a bunch of guesses. This points to a fundamental misunderstanding of what a theory is. It isn't a guess. If it was, there might be some credence to the idea of giant turtles playing a role in dark energy. Rather, a theory is an educated attempt to explain a phenomena. It is a look by a very experienced observer at a set of behaviors, an assessment of what those behaviors might mean, and an attempt to predict what they might mean for the future.
We all form theories all the time. When we spot a car swerving erratically while driving at 2am, we say "Woah, look how that dude is driving. I betcha' he's drunk. Look out, he might miss that turn..." We observed phenomena, offered an explanation, and attempted a prediction. The depth of prediction can be tricky. If we only say "This driver will keep swerving around" it may not eliminate other possibilities such as looking for something he dropped, having an epileptic seizure, or making out with the passenger. But it is entirely possible that a drunk will in fact make that next turn too. Or never intend to take it.
The scientific process is nothing different. It is not (and does not pretend to be) a fixed rulebook. It is an evolving set of understanding of the universe. Science is not a set series of answers as it is so often (and so wrongly) presented. Rather it is a process, a pursuit of those answers.
And so when more accurate measurements gave rise to results that disagreed with the predictions inspired by Hubble's results, the result was not joy and frustration, but excitement at the opportunity to solve a new puzzle. An Asimov quote that those who have read email coming from my work address will recognize summarizes this mood better than anything:
The most exciting phrase in science, the one that heralds new discoveries, is not "Eureka!" but "That's funny..."
And now the prospect of dark energy is out there, proposing a grand enough prize and an exciting enough pursuit that seemingly everyone is getting into the game. Established scientists, cranks, those hawking ideas from the fringe, conspiracy theorists, random posters on the Internet: each one has some idea, spun slightly to reflect individual specialities and biases, for what might be at work.
The observational guys have been at it just as enthusiastically, constantly devising new approaches to reflect the latest ideas of the theorists and the latest technological developments in measurement apparatus. Dark energy isn't something we can test in a laboratory with a dark-energy-ometer or create with a steel cased apparatus connected to several thick cables. It acts, by all accounts, over vast distances and only manifests to a measurable degree when other forces (namely gravity) are at their most feeble. And so an earth-bound measurement (even if we knew what to look for) seems doomed to be swamped by noise.
This results in observatories. For various reasons, these would be observatories best sited in space, at the L2 point about a billion miles from earth, where it is dark and cold and not much gets in the way. The idea would be to observe, with great precision, the distances and recessional velocities of several thousand (or million) objects in the middle distance of the universe, the distances over which dark energy starts to manifest -- out to about twenty billion light years (117,580,000,000,000,000,000,000 miles).
A few approaches have shown up. The first involved hunting for something called Type 1A supernovae. These moderately rare explosions function through a well understood mechanism that has the handy feature of producing a reliably predictable brightness. The result is something called a "standard candle" -- an object of known intrinsic brightness which allows the estimation of its distance by comparing that source brightness with the observed brightness. That's good -- and Type 1A's are how this whole dark energy thing got started -- but it turns out it is not good enough.
Clouds of gas and dust can get in the way and it always is possible that we don't understand the Type 1A quite as well as we thought we did. So more recent approaches to understanding dark energy have tried to invoke several different techniques of measurement. Acoustic Baryonic Oscillations (I'm still trying to figure out what those are, but they sound really interesting), weak lensing, and a few others have all surfaced. The result is that any dark energy space mission that actually gets flown will end up as a fantastic multi-disciplinary observatory, quite different from the specialist that was originally envisioned.
The glory of all of these approaches, and of all of the missions that seek to imlement them, is that they will conduct their work through massive "wide and deep" surveys. Taking vast numbers of long exposure images across a large area of the sky, in other words. This is the advantage of a dedicated mission -- Hubble or the James Webb could do the same science, but are general purpose instruments contended over by the entirity of the vast astronomic (and astrophysic) community. But a dedicated mission, running a pre-planned scheme of observation, can produce the staggering amount of data that is necessary for the statistical analysis upon which dark energy studies must be based.
But this vast survey, while intended to specifically test a signle scientific concept, will also have enormous implications for the rest of the community. Currently, we stare through straws, looking across the vast night sky to find things that are interesting. Sometims we do so by chance, but more often we do so by looking at areas that we've already identified as interesting. The terabytes of data coming back from SNAP, DESTINY, ADEPT, JDEM, SPACE, Euclid, or whatever mission or missions end up flying will end up producing an astronomic and astrophysic legacy ready for the picking. A generation or more of astronomers and astrophysicists will mine this legacy to confirm and clarify their theories and hypotheses. And, here and there, they might discover something completely new, something entirely unexpected, something funny, and start the whole glorious process over again.