Going Dark

Established in 2012, the Dark Energy Survey has had many successes – from helping identify Planet 9 to LIGO backup – but will it fulfil its ultimate aim of spotting the mysterious dark energy? Professor Ofer Lahav fills us in…     

One of the greatest mysteries in the whole of science is the prospect that 70% of the Universe is made from a mysterious substance known as ‘dark energy’, which causes an acceleration of the cosmic expansion. A further 25% of the Universe is made from invisible ‘cold dark matter’ that can only be detected through its gravitational effects, with the ordinary atomic matter making up the remaining 5%.

The Dark Energy Survey (DES) is an imaging survey of 5000 square degrees of the Southern sky, utilising a 570MP camera on the 4m Blanco telescope in Chile.

The ‘accelerating Universe’ was established with the award of the 2011 Nobel Prize in Physics to three astronomers who, back in 1998-9, led the discovery of the acceleration of the Universe using Supernovae type 1a as ‘standard candles’. The cosmic acceleration was confirmed by other cosmological probes such as the galaxy distribution and the cosmic microwave background mapped by the Planck satellite.  A somewhat intuitive way to think about dark energy is as a ‘repulsive linear force’, opposing the inverse squared gravitational force. However, we still don’t know if the acceleration is due to a cosmological constant (similar to the one proposed by Einstein in 1917 for a different reason), a more complicated time-varying scalar field, or that perhaps Einstein’s theory of General Relativity requires some modifications. Some cosmologists even argue for the radical possibility of a ‘multiverse’, i.e. that our Universe, with its rather strange set of cosmological parameters, is only one of many universes.

[caption id="attachment_53490" align="alignnone" width="450"] The DES started observations in September 2013.[/caption]

The Dark Energy Survey (DES) is an imaging survey of 5000 square degrees of the Southern sky, utilising a 570MP camera on the 4m Blanco telescope in Chile. Photometric redshifts are obtained from the multi-band photometry to produce a three dimensional map of 300 million galaxies. The main goal of DES is to determine the dark energy equation of state w (the ratio of pressure to density) and other key cosmological parameters to high precision. DES will measure w using four complementary techniques in a single survey: counts of galaxy clusters, weak gravitational lensing, galaxy distributions and thousands of type 1a supernovae in a ‘time domain’ survey. DES is an international collaboration, with more than 400 scientists from the US, the UK, Spain, Brazil, Germany, Switzerland and Australia involved.

The survey had its first light in September 2012 and started observations in September 2013. It will run for 525 nights spread over five years. Three out of five observing seasons of DES are now complete, with the cosmology analysis well underway. DES has already ‘seen’ dark matter¹. The camera (DECam) can also capture many other celestial objects. This resulted in expected and unexpected discoveries² including solar system objects, 17 new Milky Way companions, galaxy evolution, galaxy clusters, high-redshift objects and gravitational wave follow ups. We highlight here two contributions of DES to two new research frontiers. It has recently been suggested that there might be a ninth planet in the solar system. One of the six minor planets to predict ‘Planet 9’ was discovered by DES. The DECam camera is well placed to monitor other minor planets that would help in constraining Planet 9, and of course to search for Planet 9 itself.

DES is an international collaboration, with more than 400 scientists from the US, the UK, Spain, Brazil, Germany, Switzerland and Australia involved.

In February 2016, the LIGO collaboration reported the first detection of gravitational waves, resulting from the merger of two black holes. This remarkable measurement confirms Einstein’s prediction 100 years ago. DES provided optical follow up of this event. There were no optical detections, which is not surprising, as in the conventional model a binary black hole merger is not expected to have any optical counterparts, and the DES observations covered only part of the sky where the event was likely to happen. However, DES will be vital for future LIGO follow ups. DES is also providing valuable experience and training of early career scientists for the next generation of large surveys, including the ground based Large Synoptic Survey Telescope (LSST) and Dark Energy Spectroscopic Instrument (DESI) and space missions Euclid and the Wide Field Infrared Survey Telescope (WFIRST).

References:

1 DES Collaboration 2015, arXiv:1507.05552 2 DES Collaboration 2016; arXiv:1601.00329

Author: Professor Ofer Lahav is the Perren Chair of Astronomy at the Department of Physics and Astronomy University College London