The structure of everything
Considering that the Earth is just one planet, around a single star, in a galaxy that contains about 100 billion such stars, it’s understandable that people ten to think of galaxies as rather large objects. Even a ship capable of traveling at the speed of light would require more than 105,000 years to cross the Milky Way. And yet, the best current guess is that there may be two trillion galaxies in the observable universe,
The Dark Energy Survey is about looking at how those galaxies are arrayed across the universe. Are they distributed randomly? Is the distribution “smooth” and regular, or is it “clumpy” and uneven? Are there grander structures of which galaxies are only a part?
Truthfully, we already knew the answers to these questions. Galaxies are not equally spread across the universe, but are grouped into clusters, and those clusters are in turn arrayed in great “strands” that sweep through millions of light years. In much of the popular science literature around the topic, this structure is referred to as a “cosmic web,” but it’s not like any spider web you’ve ever seen. Imagine instead a great mass of sponge. Then imagine all the irregular, unevenly spaced holes in that sponge getting larger and larger, until all that’s left of the sponge and shells and curves that trace the outlines of those voids. That’s our universe on, as Carl Sagan once said, “the grandest scale imaginable.”
But while we’ve understood this large scale structure for some time, we also understand that the measurements we make of these not-so-airy halls has a problem. Actually, two problems.
One of these can be seen when we look at galaxies themselves. Just as planets orbit around a star, in most galaxies stars orbit around the center of mass—which is often, or maybe even always, a supermassive black hole. The nearest of these, the black hole at the center of the Milky Way, is about 25,800 light years from Earth. About every 240 million years, our Sun makes an orbit around that central mass … and that’s the problem. It’s too fast.
If we took all the mass of all the stars, planets, nebulae, black holes, and every other object we can see or detect, and add it all together, the gravity it would generate is still not enough to explain how stars move around the galaxy. There simply isn’t enough mass. Based on everything we can see, they should go spinning off into the void.
This is just one example of why scientists believe there is something we can’t see; some kind of matter which doesn’t reflect light and doesn’t block light, but which can alter the path of light through the force of gravity. This stuff we can’t see is what was tagged “dark matter,” and the math indicates that it’s about 85% of all the matter in the universe. The ordinary stuff—from black holes to bubblegum—makes up the other 15%.
It’s still possible that dark matter does not exist. There could be something about the nature of gravity on vast scales that we simply don’t understand. But Einstein’s model of relativistic gravity has been so damn good at predicting specific cases, whether that’s planetary orbits or neutron star collisions, that it’s hard to think of how it could be tweaked. Lots and lots (and lots) of people have made a go at this, and lots and lots still are. But there are really good reasons to believe that dark matter, strange as it seems, is a real thing which makes up the majority of our universe.
And … that’s still not what this survey is mostly about. Because the name isn’t the “Dark Matter Survey” it’s the “Dark Energy Survey.” Dark energy is a name applied to a second discrepancy between what scientists expected to find when they focused their instruments on the universe.
Since the time of Edwin Hubble (with a lot of work by the “hey, why doesn’t he have a telescope named after him” Alexander Friedman, all of which was based on the “she should be more far more celebrated” Henrietta Swan Leavitt) we’ve understood that the universe is expanding. For decades, astronomers have been looking to find out just how fast that expansion really is.
And they’ve been working to see how quickly that expansion is slowing down. Because of course it would be slowing down. All that matter, dark and otherwise, has to be putting a drag on the system so that eventually there will at least be stasis and … wait a sec? What’s that you say Mr. Telescope? The rate of expansion is speeding up?
Yes. Every experiment that’s attempted to measure change in the rate of expansion over time shows that the rate if which all those galaxies are rushing apart is actually getting faster. Despite an ever improving sense of how the universe was formed, and the various expansion phases that happened in the opening milliseconds of reality, measurements continue to insist that things are speeding up out there.
Something is providing a counter-force to gravity. That force is actually enough greater than all the gravity generated by all the matter. That’s “dark energy,” and there is a lot of it. In fact, if you were to take all the dark energy and squeeze it back to the matter side of the equation using Einstein’s most famous equation, it’s roughly 68% of everything there is.
So 68% percent of the universe is dark energy, and 85% of what’s left is dark matter. Everything we’ve ever touched, seen, or detected in any way other than by looking at how these very large objects are reacting to gravity, fits in the < 5% of stuff that isn’t “dark.”
And now … back to the show. The Dark Energy Survey is designed to help scientists learn about the nature of dark energy (and also dark matter) by obtaining precise measurements giving the relative locations of hundreds of millions of galaxies. Over 400 scientists from over 25 institutions around the world are engaged in this survey, working to turn the images taken in Chile into data that can provide the most detailed and precise measure of large-scale structures. Want to get a glimpse of just how large and international this effort is? Take a peek at the credits on just one of out thirty new papers.
In it’s six years of photo snapping DES about a quarter of the southern sky. Ultimately, those images should provide positions of around half a billion galaxies (Why not half a trillion? Blame the limits of the scope, the camera, and the way that darn Milky Way blocks so much of the view).
But what the survey has already announced is three-years worth of data cranking, much of that involving a lot of sheer manual effort on the part of researchers and a not-so-small army of grad students who went over images again and again to locate galaxies and make measurements. There is a wealth of potential information to be gleaned from this information. Looking at the structure can potentially explain much about how dark energy works, about the history of the universe, and about the Ultimate Fate of All Things (which seemed like a phrase that deserves caps).
Results at the mid-point
Three years in the DES has generated a raft of astronomical papers that are already starting to have an impact on our understanding of cosmology.
A number of the papers took on the critical, but non-headline generating task of validating various aspects of the experiment. From lens calibration to working out biases implicit in the images themselves, this is all critical, if unheralded, work. It’s absolutely necessary to give the precision needed for everything that follows. So … good on you, calibration teams.
A second group of papers consist of “catalogs.” Running through a list of 200+ million galaxies is bound to generate a lot of examples, as well as more than a few unexpected results and genuine oddballs. These papers are essential for anyone who wants to find an object and trace it back to its source in the vast catalog of images.
Several of the papers involve looking for dark matter through gravitational lensing. In relativity, gravity isn’t so much a force as a distortion in space time. Heavy objects, like galaxies, “dent” space time, and everything, including light, is affected by that dent. The result is that light reaching Earth can be shaped by the gravity of a galaxy or star as if it were passing through a lens. By picking out images distorted by by gravitational lensing, researchers were able to not just study the location of the visible galaxies, but also the location of dark matter by looking at how it shaped the light.
In particular, the survey wanted to take a look at “weak lensing.” This is a situation where the distortion generated by gravity isn’t quite so obvious. Weak lensing doesn’t cause a galaxy to appear as a ring around another, or causing parts of an image to double up, it just slightly alters the apparent shape. Understanding weak lensing is critical to using the data for dark matter measurements, so it’s not surprising that there are calibration, and catalog, and theory papers all focused on this one area.
Some areas of the sky were also selected for repeated imaging with the most sensitive instruments, Those sensitive instruments could pick up fainter, more distant galaxies. Being more distant, those galaxies and the shapes of which they’re a part are older than most of the galaxies imaged. That allows a comparison between the structure of the universe over a period of several billion years, showing how the structures we recognize “evolved.” (Though, given the distance to these objects, we can’t really say any of this represents the universe as it is today.)
And of course, some of this sudden wealth of new papers generated by the Dark Energy Survey, focus on — surprise! — dark energy. And that includes a look at one of the most amazing, and large-scale horrific, possibilities for the future of the universe.
To be or Not to be
We don’t know what dark energy is, and it’s behavior runs maddingly counter to expectations. Over the last two decades, a number of theories have been proposed and discarded. However, there are a few leading contenders. One of these, due an unfortunate coincidence of timing between when the theory was first published and the release of a certain Star Wars prequel, is known as “phantom energy.” It postulates not just an expanding universe, but an ever increasing level of dark energy.
That theory would explain why the observed rate of expansion seems to be picking up, and the suggested solution indicates that it will keep accelerating, ad infinitum. Worst of all, it indicates that ω < -1 (that’s not a ‘W’ over there, that’s the lowercase form of the Greek letter omega, so give it the proper scary movie music level of respect). In this case, that little omega represents the ration between dark energy pressure, and dark energy density. Which is … really, the rabbit hole is there is you want to climb in.
Without getting too deep into the “equation of state,” just know that ω < -1 is a bad thing. Because if dark energy confirms to the theory of phantom energy, and phantom energy keeps on increasing, it doesn’t just mean galaxies whipping apart at an every accelerating rate. It means that ultimately you reach a state where space—all space—is expanding to where galaxies are ripped apart. Then solar systems are ripped apart. Then stars are ripped apart. Then planets. Then molecules. Then atoms, Then even individual particles.
This terrifying end state for the universe is known as the Big Rip, and a number of recent measurements from some of the most well-respected surveys of existing astronomical data fell on the rippy side of the line. Others have fallen in the “wheh” level of ω > -1 where the universe just … keeps expanding until everything goes dark and cold. So … yay?
In any case, the first batch of results calculated from DES data give a ω > -1, so you can drop those plans for a pre rip party in a trillion years or so. However, frustratingly, the value is ω > -1 … which is still messy. Because if dark energy is actually going to play nice with the rest of the universe, and with physical laws as we know them, the real answer should be ω = -1.
Unfortunately, there are now a whole cluster of measures some of which have ω > -1 and some of which have ω < -1 and all of which have supposed ranges of error that don’t overlap ω = -1. Which is … let’s just say, frustrating. But the results from the three year dataset at DES are very close to ω = -1. It seems entirely possible that when the whole thing is on the table, that difference between almost -1 and -1 could disappear.
But for now, the DES vote goes for “no rip.”
What have we learned
The answer is … I don’t know. In finest Dunning–Kruger fashion, I understand just enough about most of the published papers to made definitive statements that are almost certainly wrong. This first set of papers suggests not just the universe safe from the big rip, it’s also strangely smoother than expected. Not smooth — that big holey sponge still exists — but smoother than any previous model predicted. Slightly less clumpy. It also seems to suggest that the relationship between the placement of dark matter and visible matter is more problematic than in previous results.
What’s clear is that the DES is generating a wealth of information that could shed light on the two great mysteries that define most of the universe. The papers that have been released are just the tip of a 226 million galaxy iceberg, most of which serve to define the wealth of data available.
What’s been released so far is likely to overturn a lot of cherished ideas, and generate a million more. And there’s a lot more to come.