A Cosmic Murder Mystery
Image courtesy of the Space Telescope Science Institute.
“…would you believe it? … I stepped right into the middle of a baffling murder mystery and they put me to work…”
–Nick Charles (William Powell), The Thin Man
They say there are a million ways to die in the big city. Sometimes a body goes out quietly. Sometimes, it goes out with a bang.
Either way, it’s our jurisdiction, and we’re on the case.
I can tell you about one we just had. Actually, it’s more like a serial. About once a day, Earth-bound telescopes catch a glimpse of bright flashes coming from distant regions of the universe. Sometimes, if they got enough in them, we can even see the flash with our bare eyes. The Hubble Space Telescope caught one of these things going off (top image).
This bright flash – one last cry before a star shoves off this coil – tells us how it lived.
In this artist’s rendering, jets of high-energy radiation shoot out from a Gamma-ray burst, signaling the death of a massive star. (Image courtesy of NASA/Swift/Mary Pat Hrybyk-Keith and John Jones, via Wikipedia)
Before they die, stars teeter on a delicate balance between gravity and nuclear fusion. Gravity pulls the matter of the star inward – close enough for the atoms to fuse together. The fusion produces the light we see, and it pushes outward on the matter of the star.
But this detente can only last so long. Once a star runs out of nuclear “fuel,” gravity takes over, and fusion slows down. Then, in a few short cosmic moments – as long as you can hold your breath – the dying star gives up more energy than our sun will produce in all its 10 billion years of burning. One more blast, a final scream to let us know it once lived.
These momentary flares are Gamma-Ray Bursts (GRBs), the death throes of some massive stars. At the top of this case file, the series of images shows the afterglow of the GRB (lower middle) next to its host galaxy (center).
Why do these things depart so violently?
Well, it ain’t the colonel with the candlestick.
The biggest clue is the blast. Since we see GRBs from all the way across the universe, the energy released by a GRB must be enormous, more powerful than a supernova. We deduce that much of the star’s mass and energy are converted to light and other particles, like neutrinos, during its final moments. A sketch artist’s rendering shows these jets of energy shooting out from the center of the burst. The only way this could happen is if a whole star’s worth of matter gets pulled close to a black hole in just a few seconds.
Now we know the perp (a black hole), and that its weapon is gravity.
We’ve seen these types before. Now, we’re ready, we’re watching for it.
Our detectives developed the DESAlert system as a way to put out an APB to fellow astronomers. When a GRB is detected – for example, by the Swift satellite – DESAlert uses data automatically generated by Swift to find the GRB’s location on the sky. DESAlert then looks within Dark Energy Survey data for observations in the same region of the sky.
Massive stars that end with a burst rarely die alone: usually, they’re near or inside a galaxy. Our fellow detectives use information about its location to look for a GRB’s galactic accomplice amongst the line-up of nearby galaxies.
Adriano Poci (left) and Raymond Lam (right), students at Macquarie University in Sydney, Australia, were among the first Dark Energy Detectives to work on the case of the dying stars. They are two of the creators of the computer program that controls the DESAlert “galactic APB” system.
For the suspect galaxies in DES data, we have entire astrophysical profiles – shape, size, brightness, distance from Earth – ready for comparison: we share this data with other GRB detectives, who continue the search, trying to catch the bursters.
Dark Energy Detectives and their astronomer colleagues can learn even more about how stars form, how they gather together into galaxies, and how they change during their lifetime leading up to their spectacular fiery death.
They say there are a million ways to die in the big city. How many could there be in the dark reaches of the cosmos?
–Det. Kyler Kuehn @kwkuehn [Australian Astronomical Observatory]
Our dark, tangled web: Clues of dark energy
Lurking beneath a sea of light, an intricate pattern rustles and changes ever so slowly. It is built from dark, and nearly invisible, cosmic forces. Amidst the clumps and knots of galaxies lay empty, usually fallow spaces. While each galaxy, with its billions of stars, has a unique story of birth and evolution, we don’t miss the forest for the trees. Taken as a whole, the pattern of clusters and voids in our galaxy maps can tell us about the dark forces that shape our universe.
Mapping of galaxies by the Sloan Digital Sky Survey out to 2 billion light-years away. Red and green points indicate positions of galaxies, with red points having a larger density of galaxies. The fully black areas on the sides are parts of the sky inaccessible to the survey. (See also the SDSS fly-through.)
Looking at the image from the Dark Energy Camera (above), we can see a plethora of celestial objects, including many blue, red and yellow smudges, many of which are distant galaxies. It may appear that these galaxies are randomly strewn about the cosmos. However, astronomers charting the locations of these galaxies across large distances have found that galaxies are organized into structures, into cosmic patterns that can span swaths of space and time much larger than what is seen in this image. The figure on the right, from the Sloan Digital Sky Survey, shows a map of millions of galaxies. These galaxies appear to cluster into knots and filaments (areas with many galaxies), and leave behind voids (areas with few or no galaxies). Some filamentary structures stretch across a billion light-years – 60 trillion times the distance from the Earth to the Sun!
Like any good detective, we cannot ignore a pattern. How do galaxies, separated by up to billions of light-years, eventually coalesce into the great cosmic structures we see today? It turns out the ‘mastermind’ of this cosmic operation is a familiar friend (and foe) to us on Earth: the force of gravity.
Using computer simulations, astronomers have investigated how gravity acts among so many galaxies over such very large distances. The Millennium Simulation, and others like it, show that a mostly random distribution of matter will naturally cluster into filaments and voids through the force of gravity. When we statistically compare the simulation results to our data (observations of many galaxies), the patterns are the same: gravity’s influence throughout the visible universe has fostered this grand filamentary structure, which has been dubbed, “The Cosmic Web.”
The Millennium Simulation: brighter areas are where more matter and galaxies have concentrated. (See more of this simulation in this fly-through video).
What does this mean for the detectives working on the Dark Energy Survey? It turns out that gravity has a nemesis in its goal for creating web-like order across the universe: dark energy, the invisible force causing the accelerated expansion of space throughout the universe. The faster space grows and accelerates, the greater the distances galaxies must travel to form filaments and clusters. If there is more dark energy, gravity needs more time to pull galaxies together, and web-like structure develops slowly. If there is no dark energy, the web gets built quickly. By studying how quickly or slowly the cosmic web was built across time, we learn how strong dark energy has been and if it is growing stronger or weaker.
The battle between gravity and dark energy, manifested in the evolving structure of the cosmic web, is a key way to study dark energy. In fact, the cosmic web is particularly important for answering one specific question: is there even dark energy at all?!
Most astronomers agree that there is overwhelming evidence for the accelerated expansion of the universe. For many reasons, the most plausible source of this acceleration is some new force or otherwise unseen, “dark” energy. The leading alternative theory though is a change in the laws of gravity (specifically, in Einstein’s laws of general relativity). Since physicists and astronomers have tested Einstein’s laws numerous times on Earth, the Solar System, and within galaxies, the change would only manifest itself at much larger distance scales. It could be causing the appearance of cosmic acceleration, such that there might be no dark energy.
This second hypothesis would re-write our case file on the cosmic web. Perhaps instead of fighting against dark energy, gravity is just not carrying quite the influence across billions of light years that we’ve predicted. Measurements of the cosmic web, in conjunction with other measures of cosmic acceleration, will be key in telling us whether our universe is a battleground for dark energy and gravity, or if gravity is just different than previously thought. Either conclusion (or perhaps an even stranger one!) would signify a fundamental revision in how we think about the workings of our universe.
As the Dark Energy Survey collects more beautiful images of hundreds of millions of galaxies over a five-year span, our detectives will be carefully logging their positions, charting out the cosmic web, hoping to identify what forces are at work in the dark.
Detective Ross Cawthon (University of Chicago)
Image: Det.’s Marty Murphy and Reidar Hahn
Distant Wanderer
After a great journey, a long-hidden member of our solar system has returned. Not since the 9th century, when Charlemagne ruled as Emperor of the Holy Roman Empire and Chinese culture flourished under the Tang Dynasty, has this small icy world re-entered the realm of the outer planets.
This distant wanderer is among first of its kind discovered with data from the Dark Energy Survey (DES). Now officially known as 2013 TV158, it first came into view on October 14, 2013, and has been observed several dozen more times over the following 10 months as it slowly traces the cosmic path laid out for it by Newton’s law of gravitation. We see this small object move in the animation to the left, comprised of a pair of images taken two hours apart in August, 2014.
It takes almost 1200 years for 2013 TV158 to orbit the sun, and it is probably a few hundred kilometers across – about the length of the Grand Canyon.
In eight more years, it will make its closest approach to the sun – still a billion kilometers beyond Neptune. At this distance, the sun would shine with less than a tenth of a percent of its brightness here on earth, and would appear no larger than a dime seen from a hundred feet away.
That’s what high noon looks like on 2013 TV158.
Then it will begin its six-century outbound journey, slowly fading from the view of even the most powerful telescopes, eventually reaching a distance of nearly 30 billion kilometers before pirouetting toward home again sometime in the 27th century.
This object is just one of countless tiny worlds that inhabit the frozen outer region of the solar system called the Kuiper Belt, an expanse 20 times as wide and many times more massive than the asteroid belt between Mars and Jupiter. The dwarf planet Pluto also calls the Kuiper Belt its home. The orbits of Jupiter, Pluto and 2013 TV158 around the sun can be seen in the image to the lower right.
Scientists believe that these Kuiper Belt Objects, or KBOs, are relics from the formation of the solar system, cosmic leftovers that never merged into one of the larger planets. By studying them, we can gain a better understanding of the processes that gave birth to the solar system 4.5 billion years ago.
Because they are so distant and faint, KBOs are extremely difficult to detect. The first KBO, Pluto, was discovered in 1930. Sixty-two years would pass before astronomers found the next one. Astronomers have identified well over half a million objects in the main asteroid belt between Mars and Jupiter. To date, we know of only about 1500 KBOs.
DES is designed to peer far beyond our galaxy, to find millions of galaxies and thousands of supernovae, but it can also do much more. DES records images of ten specific patches of the sky each week between August and February. These images are a perfect hunting ground for KBOs, which move slowly enough that they can stay in the same field of view for weeks or even months. This allows us to look for objects that appear in different places on different nights, and eventually track the orbit over many nights of observations.
So far we’ve searched less than one percent of the DES survey area for new KBOs. Who knows what other distant new worlds will wander into view?
Det. D. Gerdes
Dark Energy Detectives 