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