An enormous worldwide undertaking has simply delivered outcomes that might redefine the destiny of the universe
The structure of everything
When you consider that Earth is just a planet around a single star in a galaxy with about 100 billion such stars, it's understandable that people consider galaxies to be quite large objects. Even a ship that can travel at the speed of light would do more than 105,000 years to cross the Milky Way. And yet the best guess right now is that there could be two trillion galaxies in the observable universe.
The Dark Energy Survey is about examining how these galaxies are arranged in the universe. Are they randomly distributed? Is the distribution “smooth” and even or “lumpy” and uneven? Are there larger structures of which galaxies are only a part?
In all honesty, we already knew the answers to these questions. Galaxies are not evenly distributed across the universe, but are grouped into clusters, and these clusters are in turn arranged in large "strands" that span millions of light years. Much of the popular science literature on the subject refers to this structure as the "cosmic web," but it is not like every spider web you have ever seen. Instead, imagine a large mass of sponge. Then imagine how all the ragged, uneven holes in that sponge keep getting bigger, until there is all of the sponge and the cups and curves that trace the outlines of those voids. This is our universe on, as Carl Sagan once said, “the greatest imaginable scale”.
But while we've understood this large-format structure for some time, we also know that the measurements we take on these not-so-airy halls pose a problem. Actually two problems.
One of these can be seen when we look at the galaxies themselves. Just as planets orbit a star, stars in most galaxies orbit the center of mass, which is often, or perhaps always, a supermassive black hole. The closest of these, the black hole in the center of the Milky Way, is about 25,800 light years from Earth. Every 240 million years or so, our sun orbits this central mass … and that is the problem. It’s too fast.
If we add up and add up the total mass of all stars, planets, nebulae, black holes, and every other object we can see or discover, the gravity generated is still insufficient to explain how stars move around the earth Galaxy. There is simply a lack of mass. Based on everything we can see, they should turn into space.
This is just one example of why scientists believe that there is something we cannot see; a type of matter that does not reflect light and does not block light, but can alter the path of light by gravity. This stuff that we can't see has been called "dark matter," and math shows that it makes up about 85% of all matter in the universe. The other 15% is common stuff – from black holes to chewing gum.
It is still possible that there is no dark matter. There might be something about the nature of gravity on giant scales that we just don't understand. But Einstein's model of relativistic gravity was so damn good at predicting certain cases, be it planetary orbits or neutron star collisions, that it's hard to imagine how it could be optimized. Lots and lots (and lots of) people have tried it, and lots and lots still are. But there are really good reasons to believe that dark matter, strange as it may seem, is a real thing that makes up most of our universe.
And that's still not what this survey is about. Since the name is not "Dark Matter Survey", it is the "Dark Energy Survey". Dark energy is a name applied to a second discrepancy between what scientists expected when focusing their instruments on the universe.
Since the time of Edwin Hubble (with a lot of work from the "Hey, why didn't he have a telescope named after him" Alexander Friedman, it's all based on the "You should be far more celebrated") Henrietta Swan Leavitt) we understand that the universe is expanding. For decades, astronomers have been trying to figure out how fast this expansion really is.
And they have been working to see how quickly that expansion slows down. Because that would of course be slower. Everything that is important, whether dark or not, has to load the system so that it at least comes to a standstill at some point and … wait a minute? What are you saying, Mr. Telescope? Is the rate of expansion accelerating?
Yes. Any experiment attempted to measure the change in the rate of expansion over time shows that the speed at which all of these galaxies are rushing apart is actually getting faster. Despite an ever better understanding of how the universe was formed and the various phases of expansion that took place in the first milliseconds of reality, measurements continue to insist that things are accelerating out there.
Something is counteracting gravity. This force is actually larger enough than the total force of gravity generated by all matter. This is "dark energy," and there is a lot of it. In fact, if you take all of the dark energy and push it back to the material side of the equation using Einstein's most famous equation, it's roughly 68% of all there is.
So 68% of the universe consists of dark energy and 85% of what is left is dark matter. Everything we have ever touched, seen, or otherwise discovered other than the response of these very large objects to gravity fits into the <5% of things that are not "dark".
And now … back to the show. The Dark Energy Survey is designed to help scientists understand the nature of dark energy (and dark matter, too) by taking precise measurements that indicate the relative position of hundreds of millions of galaxies. over 400 scientists from over 25 institutions worldwide are participating in this survey and are working on converting the images taken in Chile into data that enable the most detailed and accurate measurements of large-scale structures. Would you like to get an impression of how big and international this effort is? Take a look at the credits of just one of thirty new papers.
In his six years of photography DES about a quarter of the southern sky. Ultimately, those images should provide positions of roughly half a billion galaxies (why not half a trillion? Blame it on the limitations of the scope, camera, and the way the damn Milky Way blocks so much of view).
But what the survey already announced is a three-year data collection, much of which required a great deal of sheer manual effort on the part of researchers and a not-so-small army of graduates who kept going through images, finding galaxies, and taking measurements. A wealth of potential information can be extracted from this information. One look at the structure can potentially explain a lot about how dark energy works, the history of the universe, and the ultimate fate of all things (which seemed like a sentence that deserves upper limits).
Results to the middle
Three years at DES have produced a series of astronomical works that are already beginning to affect our understanding of cosmology.
A number of the publications took on the critical but not headline-grabbing task of validating various aspects of the experiment. From lens calibration to identifying distortions that are implicit in the images themselves, this is all critical, if unannounced, work. It is absolutely necessary to give the necessary precision for everything that follows. So … good for you, calibration teams.
A second group of papers consists of "catalogs". Going through a list of over 200 million galaxies will surely bring up many examples, as well as more than a few unexpected results and real weirdos. These papers are indispensable for anyone who wants to find an object in the extensive picture catalog and trace it back to its source.
Several of the publications involve searching for dark matter through gravitational lenses. In the theory of relativity, gravity is less of a force than a distortion of space-time. Heavy objects such as galaxies "dent" space-time and everything, including light, is affected by the dent. The result is that light reaching Earth can be shaped by the gravity of a galaxy or star as if it were falling through a lens. By selecting images that were distorted by gravitational lenses, researchers were able to study not only the position of visible galaxies, but also the position of dark matter by examining the shape of light.
In particular, the survey wanted to take a look at “Weak Lensing”. This is a situation where the distortion created by gravity is not quite as obvious. A weak lens effect does not cause one galaxy to appear as a ring around another or parts of an image to double, but only slightly changes the apparent shape. Understanding the faint lens flare is critical to using the data to measure dark matter, so it is not surprising that there are calibration, catalog, and theory papers all of which focus on this one area.
Some areas of the sky have also been selected for repeated recordings with the most delicate instruments. These sensitive instruments could detect weaker, more distant galaxies. Because they are further away, these galaxies and the shapes they belong to are older than most of the galaxies pictured. This enables a comparison between the structure of the universe over a period of several billion years and shows how the structures that we recognize "evolved". (However, given the distance to these objects, we can't really say that this represents the universe as it is today.)
And of course, some of that sudden plethora of new papers generated by the Dark Energy Survey is focused on – surprise! – dark energy. And that includes a look at one of the most amazing and large-scale terrifying possibilities for the future of the universe.
To be or not to be
We don't know what dark energy is and its behavior is contrary to expectations. A number of theories have been proposed and rejected over the past two decades. However, there are some top contenders. One of these is known as "Phantom Energy" due to an unfortunate coincidence between the first publication of the theory and the publication of a particular Star Wars prequel. It postulates not only an expanding universe, but an ever increasing level of dark energy.
This theory would explain why the observed rate of expansion appears to be increasing, and the proposed solution suggests that it will accelerate to infinity. Worst of all, it suggests ω <-1 (that's not a & # 39; W & # 39; over there, that's the lower case of the Greek letter omega, so give it proper respect for the scary movie score). In this case, this little omega represents the relationship between the pressure of the dark energy and the density of the dark energy. That's … really, the rabbit hole is there you want to climb into.
Without going too deep into the "equation of state", you simply know that ω <-1 is a bad thing. Because if the dark energy confirms the theory of phantom energy and the phantom energy continues to increase, it not only means that galaxies are tearing apart at every rate of acceleration. It means that you will ultimately reach a state where outer space – all of space – expands to the point where galaxies are torn apart. Then solar systems are torn apart. Then stars are torn apart. Then planets. Then molecules. Then atoms, then even individual particles.
That terrifying end-state for the universe is known as the Big Rip, and a string of new measurements from some of the most prestigious surveys of existing astronomical data fell on the blatant side of the line. Others have fallen into the "wheh" level ω > -1 where the universe just … continues to expand until everything gets dark and cold. So … yay?
In each case, the first set of results calculated from DES data gives a ω > -1, so you can drop those pre-rip party plans in about a trillion years. Frustratingly, however, the value is ω > -1 … which is still chaotic. Because if dark energy is actually to play well with the rest of the universe and with the laws of physics as we know them, the real answer should be: ω = -1.
Unfortunately, there is now a whole bunch of measures, some of them ω > -1 and have some of them ω <-1 and all have supposed error areas that do not overlap ω = -1. Which … let's just say it's frustrating. But the results from the three-year data set at DES are very close ω = -1. It seems entirely possible that the difference between almost -1 and -1 could disappear with the whole thing on the table.
But for now the DES vote is “no rip”.
What did we learn
The answer is … I don't know. In the finest dunning process – Krüger Vogue, I understand just enough about most of the published papers to be able to make definitive statements that are almost certainly false. This first set of papers suggests that the universe is not only safe from the great rift, but also strangely smoother than expected. Not smooth – that big holey sponge still exists – but smoother than any previous model predicted. A little less lumpy. It also seems to indicate that the relationship between the placement of dark matter and visible matter is more problematic than in previous results.
What is clear is that the DES is generating a wealth of information that could shed light on the two great mysteries that govern most of the universe. The published papers are only the tip of a 226 million galaxy iceberg, most of which serve to define the wealth of data available.
What has been published so far is likely to knock down many cherished ideas and generate a million more. And there is much more to come.
Just a few of the 400+ scientists involved in the Dark Energy Survey