Dark energy is influencing how galaxies are moving, how fast they are moving, and as a result, how they are distributed across the universe. By looking at where galaxies are in the universe, we should be able to glean new information concerning the nature of dark energy.
Professor Alfonso Aragón-Salamanca
September 2012, the Dark Energy Camera (DECam) captured its first ever images. The 570-megapixel device is the product of eight years of hard work by those involved in the Dark Energy Survey (DES) collaboration, and will be used by more than 120 scientists to conduct the largest galaxy survey ever undertaken. In the hunt for dark energy, the international team will use the data collected by DECam to study galaxy clusters, supernovae, the large-scale clumping of galaxies and weak gravitational lensing.
Following an initial phase of testing, the DES collaboration will begin in December 2012. During the next five years, it will create detailed colour images of one-eigth of the night sky. The Universities of Nottingham, Portsmouth, Cambridge, Edinburgh and Sussex, and University College London, comprise DES:UK – the British contingent of DES. Researchers at these institutions will have access to data from DECam as it becomes available.
I spoke to Alfonso Aragón-Salamanca, Professor of Astronomy at the University of Nottingham, to find out how he and his colleagues hope to exploit the images captured by DECam. I began by asking whether he was excited by the fact that the world’s most powerful camera is now operational.
"Certainly," he replied. "We can now begin to collect the data that will allow us to do the science. After years of planning and fundraising, it is finally happening. The machine has been switched on and it is going to be collecting data for the next five years or so."
I asked Professor Aragón-Salamanca whether he had been impressed by the quality of DECam’s first images.
"Yes," he said. "The most important thing is that they are to spec. They are exactly as we hoped they would be. As you can see from the images that have been released, all of DECam’s detectors are working as they should be."
The concept of a DECam might seem odd to some. As its name suggests, dark energy is not visible. How then can a camera, however sophisticated, be useful in the hunt for this elusive prize?
"Dark energy is similar to dark matter in the sense that it is a component of the universe that cannot be observed directly," explained Professor Aragón-Salamanca. "Dark energy affects the ways in which galaxies move. It makes them fly away from one another at a rate faster than one might expect. The expansion of the universe is accelerating. This is something that nobody
expected to find 15 years ago. We knew that the universe was expanding but – because of gravity – we assumed that the rate at which it was doing so would be slowing. It came as a big surprise when we discovered that the universe was expanding at an accelerated rate
. The Nobel Prize in Physics 2011 shows you how big a discovery this was.
Zoomed-in image from the Dark Energy Camera of the Fornax cluster of galaxies, which lies about 60 million light years from Earth.
"Essentially, dark energy is the name that we have given whatever is causing this acceleration to happen. Dark energy is influencing how galaxies are moving, how fast they are moving, and as a result, how they are distributed across the universe. By looking at where galaxies are in the universe, we should be able to glean new information concerning the nature of dark energy. We are exploring something about which we know precious little. Whilst we can see very clearly what dark energy does
, we have no physical understanding of what dark energy is
. The images captured by DECam will enable us to narrow down the possibilities."
Each image taken by DECam captures an area of the sky 20 times the size of the moon as seen from Earth. The instrument, which is around the size of a phone booth, has 62 charged-coupled devices (CCDs) and is minutely sensitive to red light. Professor Aragón-Salamanca will take a leading role in the planning and implementation of spectroscopic follow-up surveys which will exploit this sensitivity. Essentially, he and his colleagues will use redshift to create a three-dimensional map of the southern sky.
"The spectroscopic surveys are still at the planning stage," he explained. "We will have many, many photographs, and these photographs will tell us whereabouts galaxies are in the sky. However, they will not tell us precisely how far away from us they are. DECam is building a 2D picture of the sky. Observing the colours of galaxies, however, will enable us to add a third dimension to our knowledge. By studying the redshift of galaxies, we can calculate how far away from Earth they are. The colour of a galaxy changes very slightly depending on the speed at which it is moving. Hubble’s law tells us that the farther away a galaxy is, the faster it is moving. The colour of a galaxy, therefore, allows us to accurately calculate these distances. The dimension of distance will allow us to create the map needed to explore the nature of dark energy.
Zoomed-in image from the Dark Energy Camera of the barred spiral galaxy NGC 1365, in the Fornax cluster of galaxies.
"Some three-dimensional galaxy maps already exist. We currently have accurate maps for approximately 1,000,000 galaxies. What we are planning on constructing, however, is a 3D map containing 300,000,000 galaxies. It’s a big step, but it is necessary. The effects of dark energy are very subtle so you need big numbers in order to make the statistical information useful."
I concluded our interview by asking Professor Aragón-Salamanca whether or not he was confident that DECam would help to more precisely define the nature of dark energy.
"Certainly, and I hope that our discoveries surprise us," he answered. "It is our explicit intention to measure the specific properties of dark energy. Technically speaking, I am referring to dark energy’s equation state, which is strong information concerning what it could
be. I am confident that we will obtain this information. Obviously, I don’t know what answers we will find. There would be no need to conduct this experiment if I did. In addition, DECam will give us 300,000,000 galaxies to study in detail. This represents an unprecedented opportunity to look at how galaxies form and evolve. It might not sound as sexy as dark energy, but I find this very exciting as well."