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6 mins read 29 Jun 2020

Australian Eyes on the Dark Cosmos

New results from six-years of research and operations from the Australian Dark Energy Survey have been published - providing a data set that will help scientists measure the expansion history of the Universe.

Credit: Hubble.org

Since the late 1990s, astronomers have realised that the universe is dominated by dark energy, a strange substance that pervades the universe and is accelerating its expansion. The nature of dark energy remains mysterious, with physicists scrabbling for an answer in theories of fundamental physics. 

Astronomers are still scanning the heavens to explore the nature of dark energy, looking for evidence whether it has remained constant over cosmic history, or if it has changed and evolved. One of the largest programs is the Dark Energy Survey (DES), which has been mapping the skies with a superb camera on the 4-m Blanco Telescope in Chile. 

With the superb images from DES, astronomers are searching for supernovae, exploding stars that can be used to chart the expansion of the universe. Combined with other cosmological measurements, including the counts of galaxies and the distortion of the distant universe through gravitational lensing, astronomers are beginning to uncover the true nature of dark energy.

Science Check: What is Dark Energy?

Energy density composition of the Universe, highlighting the dominant roles of Dark Energy and Dark Matter relative to ordinary matter made from atoms. Result pre and post Planck Telescope observations. Credit: NASA

The familiar objects that we know and see around us - that is the galaxies, stars, gas, planets, moons, humans, oceans, cookies, and more - account for only a small fraction of all the things that make up the Universe. In fact, this value is just under 5% of what is known as the energy density of the Universe. 

The remaining 95% is split into two components - both of which are yet to be directly observed. The first is something called Dark Matter - an invisible form of matter that exerts only a gravitational influence on the regions around it but does not interact with any electromagnetism. As such, we can’t see it - there’s no emission, no reflection, no absorption - a truly invisible source of mass that holds entire galaxies together and is responsible for their origin and formation. 

The second, and even more elusive component - Dark Energy. This force seems to not only be causing the Universe to expand into the forever but also accelerate. Like dark matter, no one yet understands how this can be, only that it is. 

The evidence for dark energy is outlined by a number of different observed factors. This includes observations that the Universe is particularly flat (when reviewing the Cosmic Background Radiation) and not curved, meaning that something must be adding to the total energy density when accounting for ordinary and dark matter to reach critical density. In addition to this, the study of supernovae explosions across the Universe has indicated that the Universe has expanded more (or accelerated) in the latter half of its existence.  

Data in the light

A deeper analysis of the spectrum of light (spectroscopy) from objects in space can reveal lots of information about the source. Credit: Lund Observatory.

The power of DES is its impressive field of view, covering an area about 14 times the size of the full Moon in a single shot, allowing it to monitor millions of galaxies for the burst of light from supernovae. But images are limited, only providing astronomers with limited information. What astronomers need is spectra, dispersing light to allow astronomers to accurately measure speeds and chemistry of cosmological sources. 

Since the days of Newton, light has been known to be made up of a variety of colourful bands - known as the spectrum. These bands are related to the wavelength of electromagnetic energy and only make up a small portion of the entire electromagnetic spectrum.

The electromagnetic spectrum and Earth’s ‘atmospheric windows’. Not all of the EM spectrum comes through to the surface. Visible light only occupies a small portion of the greater spectrum. Credit: Uni. of Delaware.

By analysing the spectrum of light, astronomers are able to derive an enormous amount of information about the light source - such as the density, mass, and luminosity of objects, what elements are present where the light is produced, what elements exist between the observer and the source, if the source is moving towards or away from the observer and at what velocity, if magnetic fields are present, the temperature of objects, if an object is part of a binary system, the chemical composition of atmospheres, and so much more.

The study of this spectrum, known as spectroscopy, has revolutionised human understanding of the Universe and allowed astronomers to conduct detailed remote sensing analysis on nearly every object in the cosmos (black holes remain, black - though their influence on their surroundings can be studied). 

Putting the Oz in DES

Here, DES’s immense field acts against it, as obtaining spectra of many sources over a large area is more challenging than simply imaging. Here, an aging telescope in outback New South Wales comes to the rescue. When it was opened in 1974, the 3.9-metre Anglo-Australian Telescope (AAT) was amongst the largest in the world, but today, in the era of 10-metre facilities, it is a minnow. 

The AAT’s secret is its technology, especially the development of the 2-degree-Field (2dF) which is a robot positioner that can place fibres over a field equivalent to DES, obtaining spectra for up to 400 objects simultaneously. A new collaboration between DES and Australian astronomer, OzDES, was born.

Plot showing the relative allocation for the full six years of OzDES. The percentages represent the fraction of fibre hours. Credit: Lidman et al. 2020.

OzDES has been running since 2012, obtaining spectra in select DES fields, measuring the cosmological speeds, their redshift, of almost 30,000 distant sources, including the galaxies hosting supernovae, as well as gravitational lenses and galaxy clusters to provide new clues to the nature of dark energy. Excitingly, OzDES has also been measuring the spectra of quasars, hyperactive galaxies powered by the accretion of matter onto supermassive back holes. Through a technique known as “reverberation mapping”, OzDES is charting how bursts of energy from the central regions of quasars propagate through surrounding gas, providing a measurement of the mass of the black hole.   

OzDES has been an intense effort for Australian astronomers, planning and taking the observations, as well as working with data to extract the physical properties of sources. Results are now flowing, but the observations represent a goldmine of potential science, and OzDES is releasing their data to the community, available to anyone to download and work with. With this, it continues astronomy’s long history of sharing data to maximize the impact of their observations, with the goal of ultimately revealing the secrets of the universe.

Video credit: AAO/Dr. Andy Green