The Dark Energy Survey

The following sections are included:

• Contents
• Foreword
• Preface
• Acknowledgements

A ‘Chapter Minus One’? ‘One before zero’ suggests ‘before we begin’. But it also hints towards the endpoint - the equation of state parameter, w, that the Dark Energy Survey (DES) may confirm as w = −1 (Einstein’s cosmological constant) or as something different. In this chapter, we review the long journey since 2013, from the initial idea for a survey, to obtaining funding, building the instrument, observation with the Blanco Telescope, processing the data and extracting the science. The main conclusion from a multi-probe analysis of just the first year of DES data is that dark energy is consistent with a cosmological constant, contributing at present about 70% of the total mass–energy density of the universe. But at this time (mid-2019) the bulk of the data remains to be analyzed, and possibilities for surprises remain. Other exciting results from early DES data include the discoveries of new Milky Way dwarf satellites, of new objects in the outer solar system, and of an optical flash from the first binary neutron star gravitational wave event.

We describe how the Dark Energy Survey (DES) came to life, from the early days when it started as an idea, through the struggle for funding, the renovation of the Blanco Telescope, the building of the camera, to the ingenious solutions for running the survey and dealing with the unprecedented amount of new data.

The Dark Energy Survey (DES) may not have happened at all if not for a unique combination of factors. One might say that all the stars were aligned. This chapter records the story of these different circumstances and how they brought together a diverse set of people and organizations from around the world to work towards a common goal: to increase our knowledge of the universe and understand the nature of dark energy. The path was not straightforward, but the time was right for DES, and with enough belief and hard work it gradually started to become a reality. In 2008 the US Department of Energy authorized full funding for the Dark Energy Camera Project and the National Science Foundation agreed to support the Data Management System; after that there was no going back.

This chapter focuses on the Dark Energy Camera (DECam) [Flaugher et al. (2015)] and its five giant lenses: the optical corrector. We describe how they work and the story behind their construction, from planning to installation on the Blanco Telescope. DECam is the result of nearly a decade of collective effort by hundreds of people who used their ingenuity to push forward the limits of technology at the time.

Once the Dark Energy Camera (DECam) was built and delivered to Chile, it needed to be installed on the telescope – a major undertaking in itself – take its first images, and undergo a series of commissioning tests to ensure that it would meet the stringent requirements demanded by the Dark Energy Survey (DES) to reach its science goals. These steps required a sustained, coordinated effort by many groups of people from disparate science and engineering traditions. This chapter is written from the perspective of three of those groups: Cerro Tololo Inter-American Observatory (CTIO), Fermilab and the survey commissioning team. No one narrative tells the whole story, and there is an overlap between them, but in combining all three a richer and fuller picture of the process is painted.

For t he Dark Energy Camera (DECam), we defined the commissioning phase as starting at the time of installation of the imager on the telescope, together with all ancillary equipment needed to support its operation. At the formal completion of commissioning, that is, when the instrument and telescope are basically functional, activities move to a ‘science verification’ (SV) phase, where scientists attempt to do a variety of astronomy projects (in this case including a proto-Dark Energy Survey [DES]) designed to test the whole instrument-telescope-facility system, in order to flush out problems and identify inefficiencies. In this chapter, we describe this phase in more detail, including the use of ‘donuts’ to focus DECam.

After the Dark Energy Camera (DECam) was completed and installed on the camera, there was a commissioning period to establish the basic functionality of the instrument. This was followed by a science verification (SV) phase, with the purpose of providing an end-to-end test of all the systems needed to conduct the survey, including the camera, telescope, operations and data-handling systems. The initial allocation for SV was only twenty-three nights, from November 1 to 24, 2012, split in half between Dark Energy Survey (DES) activities and observations selected from a call for proposals from the astronomical community. After the completion of SV, DES would officially begin, and we would be ‘on the clock’, using up our 525-night allocation of Blanco time. It was critical, therefore, that by the end of SV we could be confident we were taking images that met the standards desired for the survey, and that we could run the survey efficiently. Telescope time is a precious resource, and we could not afford to waste it. Continuing improvements to camera and telescope systems were another key component of the SV program. SV would also be our first chance to take data in the patterns of the full survey, allowing us also to test DES Data Management (DESDM) and community pipelines (see Chapter 6). Ideally, these test runs during SV would also jump-start DES science programs. The capabilities of DECam were such that just ten nights of good data could produce a survey competitive with the best previous weak lensing surveys. In this chapter, we describe in more detail what SV in a large astronomy survey involves, the particular challenges faced in DES and how these issues were overcome. We review the state of the instrument and survey readiness at the end of SV and close with a brief summary of the surprising science analyses enabled by the SV data set.

We are in the age of big data, with massive amounts of unstructured data being continuously generated through science alone. Astronomy is not yet dealing with the same level of information processing as particle physics experiments, but the volume and complexity of the data set is ever-increasing. Besides which, the particular nature of data produced by an astronomy survey calls for unique and creative solutions. This chapter and the following describe some of the ways in which the Dark Energy Survey (DES) is evolving to understand and negotiate these challenges.

This chapter introduces the Dark Energy Survey (DES) Science Portal, an innovative collection of software systems developed by the DES-Brazil Consortium. The Science Portal provides scientists with straightforward, efficient methods to evaluate, explore and analyze the vast amounts of data collected by Dark Energy Camera (DECam). In Section 7.2, Emmanuel Bertin describes his Source Extractor (SExtractor) package, which is used to build a catalogue of objects from an astronomical image. For obvious reasons, this is of fundamental importance to astronomy.

In this chapter, we discuss the nuts and bolts of the survey: the plan we follow to carry out the Dark Energy Survey (DES) objectives as efficiently and cost-effectively as possible while still collecting data of the highest quality attainable by the Dark Energy Camera (DECam). Integral to this overall strategy is photometric calibration, which in essence is the process of converting DES data to a standard (externally calibrated) system. Without accurate, unbiased calibration, our science analysis is of no value, so we are continuously refining and improving our plan.

In this section, we discuss the core dark energy science in more detail, with chapters on Type Ia supernovae, large-scale structure, weak gravitational lensing, galaxy clusters, photometric redshifts, spectroscopy, simulations and theory and combined probes.

The observation of Type Ia supernovae is the most mature technique for investigating dark energy and is one of the four main cosmological probes in the Dark Energy Survey (DES). Unlike the studies of large-scale structure, galaxy clustering and gravitational lensing, the study of supernovae involves measuring the expansion rate of the universe, rather than the formation of structure. In this chapter, we describe what Type Ia supernovae are, why they are useful for cosmology and what new discoveries we are making in DES. In the final section, we reveal the preliminary cosmological parameters we derived from our Year 3 season supernova analysis.

The Dark Energy Survey (DES) is one of the most important of the current large galaxy surveys and it is investigating the nature of dark energy using four independent and complementary methods. Among them is the precise study of the spatial distribution of galaxies, or large-scale structure (LSS) of the universe. This chapter describes how LSS is sensitive to dark energy and other cosmological parameters and how DES is improving our current understanding of LSS, with the goal of solving the problem of the nature of dark energy.

Weak gravitational lensing is a unique probe of the dark side of the universe. Measurements of the weak lensing-induced distortions of the shapes of distant galaxies provide a direct way to map the distribution of dark matter and to probe dark energy. We discuss how these challenging measurements are made and interpreted. In particular, we describe the work done on weak lensing in the Dark Energy Survey (DES) and the results from the first season of data.

In the previous chapter, we explored the importance to the Dark Energy Survey (DES) of weak gravitational lensing. In this chapter, we see how combining this knowledge with measurements from galaxy clusters allows us to learn about the formation of structure and the amount of matter in the universe, which helps us to measure the effects of dark energy on the expansion history of the universe.

Multi-probe analysis is the most promising way to constrain cosmology. In Chapter 11, we described how light from a distant source is ‘deflected’ by the ‘lens’ of intervening mass on its journey towards us. As a result of this, a distant ‘source’ appears distorted by the mass of the foreground underlying dark matter structures – a process known as ‘weak lensing’. In Chapter 10 we described how the clustering pattern of galaxies can constrain cosmological parameters. Here we use the weak lensing and galaxy position catalogues, which were produced from the first full year of the Dark Energy Survey (DES) observations, to describe and illustrate the DES ‘combined-probe’ analysis theory and method. We will describe the three two-point (‘3 × 2pt’) functions, the importance of understanding the correlation between their parameters and how we fit cosmological models to the data – as well as why we do this blind. In so doing, this chapter charts the journey from ‘data’ to ‘knowledge’ and understanding the dark side of the universe.

When we examine an image of the night sky on our computers, it is easy to measure the positions of objects that we see on the screen. However, without making further assumptions, we cannot tell if one object is further away than another. Measuring distances in astronomy is hard. Even the concept of distance is somewhat ‘flexible’ when the universe is constantly changing size. Redshifts are far easier to measure and they are closely related to distances in a way that is dependent on the cosmology. In this chapter, we give a qualitative explanation of redshift and how we measure it, and discuss spectroscopic redshifts in the Dark Energy Survey (DES) – in particular the work of OzDES at the Anglo-Australian Telescope.

The Dark Energy Survey (DES) is a photometric survey, which means that incoming light from stars and galaxies passes through one of a set of five filters (g, r, I, z, and Y), each sensitive to a different wavelength of light, before an image is recorded on the charge-coupled device (CCD) array. The aim is for every part of the 5000-square degree survey area to be imaged several times through every filter (see Chapter 8 for more details). As this chapter explains, these filters allow us to estimate the redshifts of distant objects, upon which all of our science analysis relies. Accurate and efficient measurement and calibration of photometric redshifts is, therefore, at the heart of DES. Many people in different science working groups in the collaboration are working to understand as well as possible how the photometric redshifts differ from full spectroscopic redshifts, because this knowledge is central to getting the best results possible from the science.

In this chapter, we look briefly at the role that cosmological simulations play in the study of the universe and how the Dark Energy Survey (DES) uses them to create synthetic galaxy catalogues that the science working groups use to test methodology and analysis.

This section focuses attention on the wide range of research areas that the Dark Energy Survey scientists are working on, aside from dark energy, using the new information flooding in about the universe. This research includes Milky Way science, galaxy evolution, strong gravitational lensing, quasars, transients and moving objects and optical follow-ups to gravitational wave events.

How do galaxies form? Why do they come in different shapes, colours and luminosity? And what can these features tell us about their population, evolution and environment? What do they have to do with the mysterious dark matter? In this chapter, we look at how the extensive catalogue of galaxies assembled by the Dark Energy Survey (DES) can be used to study galaxy evolution.

In this chapter, we discuss work being done in the Dark Energy Survey (DES) to identify and study quasars, which are among the oldest, most powerful and distant objects in the universe. Quasars were first detected in the 1940s and named quasi-stellar radio sources due to their star-like appearance and strong radio emission. They are one of a class of objects called ‘active galactic nuclei’ or AGN. The quasar group in DES work with OzDES (based at the Anglo-Australian Telescope) to estimate the masses of the black holes at the centre of galaxies, using a technique called reverberation mapping.

Strong gravitational lensing is an effect that gives rise to extraordinary images. It is also a powerful tool with which to study objects in the universe and measure cosmological parameters. In this chapter, we describe how strong lensing works and what we can learn from it. We discuss how we find strong gravitational lenses in the Dark Energy Survey (DES), the lensing systems we have already discovered and the advances we are making in strong lensing science.

The Dark Energy Survey (DES) was established to study the properties of hundreds of millions of distant galaxies, in order to learn more about why the expansion of the universe is speeding up. However, it quickly became obvious that the survey could also be used to study objects in and around our home Galaxy, the Milky Way. The Dark Energy Camera (DECam) was the first super-sensitive, wide-field digital camera to operate in the southern hemisphere and this represented an amazing opportunity. Within the first season of observations, scientists using DES data discovered eight new satellite dwarf galaxies of the Milky Way, and by the end of the second year, we had found a total of seventeen dwarf galaxies. In this chapter, we explain how new data is enabling us to characterize the structure, mass assembly and star formation history of our Milky Way Galaxy, and to improve the census of low-mass and low-luminosity stars and sub-stellar objects. DES Milky Way working group attempts to address fundamental questions in Galactic archaeology and near-field cosmology such as what can we learn about the nature of dark matter from its distribution in and around the Milky Way? How was the Milky Way formed? and How do environmental effects influence the formation and evolution of galaxies?

The Solar System wasn’t anyone’s focus during the construction of the Dark Energy Survey (DES). Hardly surprising. After all, DES is at its core a cosmological survey, with the intended purpose of observing hundreds of millions of distant galaxies and helping cosmologists seek answers to the greatest mysteries of the Universe. However, while it wasn’t built to be used by mere planetary scientists, its design did also make it an incredibly helpful tool for observing (and discovering) objects far closer to home. From near-Earth asteroids on potentially fatal orbits, to distant dwarf planets with bizarre orbital parameters, DES has been used to probe all reaches of the Solar System, with some of its most exciting discoveries to be found not in the furthest reaches of the Universe but inside our own cosmic backyard.

The joint detection of electromagnetic (EM) and gravitational wave (GW) emission from astrophysical sources is one of the holy grails of present day astronomy. These multi-messenger observations allow for novel measurements, including standard siren measurements of the Hubble constant. In this chapter, we describe how the Dark Energy Survey (DES) GW working group was set up and how the first optical confirmation of a GW event was made.

This final part of the book broadens the focus to consider the Dark Energy Survey from the perspective of a philosopher, an anthropologist and artists. In doing so, it raises more questions than it answers. The section concludes with a chapter speculating on the future of dark energy research.

The Dark Energy Survey (DES) is a scientific investigation that aims to reduce our uncertainty about the universe. It is also a collaboration between hundreds of scientists from all over the world. This creates a remarkable opportunity to investigate how the idea of uncertainty in scientific knowledge relates to the uncertainty in the lives of scientists carrying out the research. The material in this chapter is based on seven months of non-intensive fieldwork carried out by astrophysicist and anthropologist Lucy Calder from January to July 2014, predominantly among the group of DES scientists working in the Cosmology Group at University College London (UCL). Her supervisor in the Department of Physics and Astronomy was Professor Ofer Lahav and her training in anthropology was guided by Professor Martin Holbraad.

In this chapter, philosopher of science Michela Massimi reflects on the use of the ‘Bayesian’ method to select models and estimate parameters in cosmology. She highlights the advantages over the ‘frequentist’ approach, but cautions us to tread carefully if relying on Bayes for scientific ‘evidence’.

As artists-in-residence in the Physics and Astronomy Department at University College London (UCL), Judy Goldhill and Jane Grisewood continue their passion for the interaction between art and science. They are compelled by questions surrounding who we are and why we are here, and how these concerns are expressed through art practice. They are captivated by dark energy, blackness and darkness, quasars and black holes, the visible and the invisible.

Amy Catanzano is Associate Professor of English and poet-in-residence at Wake Forest University in North Carolina. Her work as a writer explores the boundaries between literature, science and art and focuses on quantum physics and cosmology. From December 8 to 14, 2018, with a W.C. Archie Endowed Fund for Faculty Excellence grant from Wake Forest, she conducted research on the Dark Energy Survey at the Cerro Tololo Inter-American Observatory in Chile. As a result of these creative and scholarly investigations, Amy wrote a book-length poem entitled, At the Edge of the Abyss: A Poem for the Dark Energy Survey (2019). We include the opening lines of the poem here, which reference the phrase, ‘mountain at the edge of the abyss’, one English translation of ‘Cerro Tololo’.

With its multi-probe approach, the Dark Energy Survey (DES) has done much to shape our conception of dark energy as an experimental field. Over the next two to three years, DES and other large surveys will, for the first time, sharpen measurements of the amplitude of dark matter clustering to the ∼ 1% level already achieved for measurements of the expansion history, allowing stringent new tests of modified gravity alternatives to dark energy models. The results of DES and its ‘Stage III’ brethren will define the agenda for the still more ambitious ‘Stage IV’ experiments of the 2020s. With rapid growth of observational and modelling capabilities, dramatic changes in our understanding of cosmic acceleration could come quickly, but we are dependent on the kindness of nature as well as our own ingenuity.

The following sections are included:

• Appendix A: The US Department of Energy Approval Process
• Appendix B: The Dark Energy Survey Collaboration Meeting Group Photos
• Index