Optical survey pinpoints dark-matter structure

22 September 2017

During the last two decades the WMAP and Planck satellites have produced detailed maps of the density distribution of the universe when it was only 380,000 years old – the moment electrons and protons recombined into neutral hydrogen, producing today’s cosmic microwave background (CMB). The CMB measurements show that the distribution of both normal and dark matter in the universe is inhomogeneous, which is explained via a combination of inflation, dark matter and dark energy: initial quantum fluctuations in the very early universe expanded and continued to grow as gravity pulled matter together while dark energy worked to force it apart. Data from the CMB have allowed cosmologists to predict a range of cosmological parameters such as the fractions of dark energy, dark matter and normal matter.

Now, using new optical measurements of the current universe from the international Dark Energy Survey (DES), these predictions can be tested independently. DES is an ongoing, five-year survey that aims to map 300 million galaxies and tens of thousands of galaxy clusters using a 570 megapixel camera to capture light from galaxies eight billion light-years away (see figure). The camera, one of the most powerful in existence, was built and tested at Fermilab in the US and is mounted on the 4 m Blanco telescope in Chile.

The DES data sample is set to grow from 26 million to 300 million galaxies

To measure how the clumps seen in the CMB evolved from the early universe into their current state, the DES collaboration first mapped the distribution of galaxies in the universe precisely. The researchers then produced detailed maps of the matter distribution using weak gravitational lensing, which measures small distortions of the optical image due to the mass between an observer and multiple sources. The galaxies observed by DES are elongated by only a few per cent due to lensing and, since galaxies are intrinsically elliptical, it is not possible to measure the lensing from individual galaxy measurements.

The first year of DES data, which includes measurements of 26 million galaxies, has allowed researchers to measure cosmological parameters such as the matter density with a precision comparable to those made using the CMB data. The matter-density parameter, which indicates the total fraction of matter in the universe, measured using optical light is found to be fully compatible with Planck data based on measurements of microwave radiation emitted around 13 billion years ago. Combining the measurements of Planck and DES places further constraints on this crucial parameter, indicating that only about 30% of the universe consists of matter while the rest consists of dark energy. The results are also compatible with other important cosmological parameters such as the fluctuation amplitude, which indicates the amplitude of the initial density fluctuations, and further constrain measurements of the Hubble constant and even the sum of the neutrino masses.

The DES results allow for a fully independent measurement of parameters initially derived using a map of the early universe. With the DES data sample set to grow from 26 million to 300 million galaxies, cosmological parameters will be measured with even higher precision and allow more detailed comparisons with the CMB data.

bright-rec iop pub iop-science physcis connect