13 mins read 30 Jan 2021

Mapping The Dark Energy Across The Universe

With almost 700 million astronomical objects catalogued, the Dark Energy Survey has now wrapped up, with the hopes that the massive data collection will help shed light on the mysterious nature of dark energy - the elusive, repulsive force that makes up most of the composition of the Universe. 

The barred spiral galaxy NGC 1365 in Fornax, captured by DECam, mounted on the Blanco Telescope at the Cerro Tololo Observatory in Chile. Credit: DES Collaboration.

A new international study, which includes the contributions of several Australian astrophysicists, has catalogued nearly 700 million astronomical items to produce one of the world’s most extensive collections of deep space objects, drastically improving science knowledge about the nature and evolution of our Universe.

The second and latest data release (DR2) from the Dark Energy Survey (DES) has just been made available, with the survey covering an area roughly equivalent to an eighth of the night sky – and stretching back into deep time when the Universe was at a much younger age.

It’s hoped the data produced by DES will start to shine a light on some of the biggest unanswered questions about our Universe’s past and future – such as what is dark energy, how did our Universe begin, and ultimately – what will be its fate?

Scientists from the Australian National University (ANU), Macquarie University, and the University of Queensland worked as part of a truly global collaboration which includes over 400 scientists from 25 institutions across seven countries (USA, Australia, UK, Germany, Switzerland, Brazil, and Spain), for the release of DR2 and preprint paper.  

The Anglo-Australian Telescope at Siding Spring Observatory. Credit: Australian Astronomical Observatory.

The Australian contingent, led by ANU astronomer Dr. Christopher Lidman and Prof. Tamara Davis from the University of Queensland, played a critical role in measuring the exact distances out to many of the objects and to confirm the nature of supernovae – using the 3.9-metre Anglo-Australian Telescope, located at the Siding Spring Observatory in northern, central NSW.

“This is the culmination of years of effort. In addition to mapping hundreds of millions of galaxies, thousands of supernovae (exploding stars) have been discovered," Dr Lidman said. “Hundreds of researchers from many countries have worked together over two decades to achieve this common goal.”

According to Professor Davis, the huge volume of data will allow the research team to measure the history of cosmic expansion and the growth of large-scale structures in the universe, “both of which reflect the nature and amount of dark energy in the universe”.
“I’m excited to use the data to investigate the nature of dark energy, which should reveal what’s behind the acceleration of the expansion of the universe – one of the biggest mysteries in science,” Professor Davis said.

Science Check: What Is Dark Energy?

Simulations of cosmological structure formations. Credit: Springel, V. & Hernquist, L. 2003, MNRAS, 339, 312/MPA.

The Universe we see around us is only a small fraction of what it actually consists off. In fact, we can’t even see most of the ‘stuff’ that exists in the Universe, because our current technology cannot detect it.

Everything that you can see: stars, planets, humans, oceans, chocolate bars, gigantic clouds of hydrogen gas, is known as baryonic matter (sometimes termed “normal matter”) made from protons, neutrons, electrons (atoms). But this accounts for roughly 5% of the mass-energy density of the Universe (that is, the total sum of matter and energy. Neutrinos, and photons (also observable), account for a tiny amount as well).

The rest – the remaining 95% - well, that is the mystery.

It’s thought that approximately, 26% of this is made up of a yet to be detected form of matter that does not interact with electromagnetic radiation but leaves a gravitational influence on structures in the Universe like galaxies. This stuff is known as Dark Matter.

And the last remaining 69% - the majority of the Universe’s mass-energy density – is something even more mysterious and elusive, creatively known as 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/Planck Observatory.

Dark Energy is the yet-to-be-discovered force that is causing the expansion of our Universe to accelerate as time goes on, rather than slowing down. It is thought to exist everywhere in space and not be very dense, and like dark matter, it doesn’t interact with any matter or the fundamental forces (like electromagnetic, strong, and weak nuclear forces), except for gravity. If you wanted to imagine it, it could be thought of as a repulsive force that is spread across the entire Universe (hence why it makes up so much of that matter-energy density).

Funny enough, when Einstein wrote his now-famous Theory of General Relativity around 1915, he proposed a ‘cosmological constant’ that would balance the scales to keep the Universe from collapsing in on itself, due to all the matter that was within it (the thinking here is that matter, which has gravity, would eventually cause the Universe to contract).

To keep the Universe stable and static, he listed the cosmological constant as a term (with the symbol of a capital lambda) – but allegedly later regrated inserting this into his beautiful theorem, having said to call it his “greatest blunder” after evidence started emerging that the Universe was not in fact stable, but rather dynamic (at the time, Edwin Hubble showcased that the Universe was expanding).

For decades, the idea of the cosmological constant was retired, or at least given a value equal to zero, thus becoming negligible. Then, in the late 1990s, by studying the light of supernovae – an extraordinary discovery was made.

Not only was the Universe found to be expanding as per Hubble’s discovery, but at some point in its history the speed changed – the expansion started to accelerate. What could have caused this gear shift on the scale of the entire Universe? This is one of the questions many cosmologists and astrophysicists are keen to learn more about.

Graphical representation of the expansion of the Universe over time from the Big Bang through to the current era. Credit: NASA/WMAP Science Team.

Aside from the supernovae used to make this discovery, other forms of evidence have recently started to emerge to support theories of the existence of dark energy. One such example is the data produced by space-based telescope surveys between 2000 – 2010 by the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck Spacecraft 2013 survey.

Both orbiting observatories indicated that the total amount of matter, as measured against the cosmic microwave radiation spectrum, accounted for roughly 30% of the total mass-energy density of the Universe (and in particular, how this value contribute to the overall flat shape of the Universe that is observed). To ensure that the Universe remained in line with observations, the remaining roughly 70% would need to be this invisible, unknown, repulsive force that is pushing everything further away from each other. This is dark energy.

There has also been a number of alternate theories that have been proposed that remove the need for dark energy to account for observations – after all, maybe the 70% that can’t be found is actually not there.

But these theories rely heavily on a changing Einstein’s general relativity, which has for over 100 years stood rigorous tests and assessments, and still continues to deliver a range of diverse experimental and observational results in fierce agreement.

To change general relativity to remove dark energy would mean all that other evidence accumulated over the last 100 years would also need to be tossed out, and as such, most astrophysicists do not agree with this approach. That’s to say, most astrophysicists agree that dark energy exists.

The Dark Energy Survey

Spiral galaxy NGC 1398, captured by DECam, mounted on the Blanco Telescope at the Cerro Tololo Observatory in Chile. Credit: DES Collaboration.

Here’s where the Dark Energy Survey steps in. The project's website describes the study as an international collaboration designed to map millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that reveal the underlying nature of dark energy, including an attempt to explain the accelerated expansion.

The survey ran over a period of six years (from 2013 – 2019) and mapped an enormous swath of the sky, surrounding the southern galactic pole (located in the constellation of Fornax) in both visible and infrared light. The total sky footprint of the survey covers an area roughly about 5,000 square degrees. For comparison, the southern constellation of Crux (also known as the Southern Cross) occupies an area of 68 square degrees.

The DES footprint in equatorial coordinates, covering 5,000 square degrees surrounding the south galactic pole (marked by '+' symbol). The Milky Way plane is shown as a solid line with the galactic centre marked by the 'x' symbol. Blue and red circles represent deep supernovae fields. Credit: DES Collaboration (2021).

To achieve the ability to survey a large portion of the sky, the project team built and utilised a 570-Megapixel digital camera called DECam (dark energy camera), which was mounted on the Blanco 4-metre telescope at the Cerro Tololo Inter-American Observatory in the dry Chilean Andes. Each image captures a field of view as big as 2.2 square degrees, which is over 4 times the size of the full Moon as observed from Earth.

The DECam recorded all images using five filters stretching from visible light (green and red) through to infrared light (400 nm through to approximately 1080 nm in wavelength), with each filter ranging across its wavelength bandwidth. The imager itself is made up of 74 charged-couple devices arranged in a hexagonal patter on the focal plane of the DECam.

The Dark Energy Camera (DECam) mounted onto the Blanco Telescope. Credit: Reidar Hahn/Fermilab).

The four categories that the DES sets out to improve scientific knowledge and understanding about are:

  • Weak gravitational lensing – reviewing how distant light sources are stretched and magnified due to the presence of intermediate dark matter or galaxy clusters, causing their light to bend into our view and reveal information about these objects, like its redshift
  • Galaxy cluster counts – testing theory against reality in the number of galaxy clusters that can be accounted for, relative to what cosmological models predict
  • Baryonic acoustic oscillations – building a stronger understanding of the conditions during a time when the Universe was young, which allowed the large scale structures we see today (like the filaments and walls of galaxies) to be seeded and imprinted
  • Distances to type Ia supernovae – using the bright, consistent light curves from exploding white dwarf stars across the universe to measure how far these events are, and how long it has taken to for the light to arrive at Earth

Composite image of three areas that the DES researched: Type Ia supernovae (SN2011fe) captured in the Pinwheel Galaxy (credit: B.J. Futon, Las Cumbres Observartory); Galaxy cluster RXC J0232.2-4420 (credit: ESA/Hubble/NASA/RELICS); Gravitational lensing arcs surrounding a galaxy cluster (credit: ESA/Hubble/NASA/Rivera-Thorsen et al.)

So after six years of its imaging campaign across that massive region of the sky, the DES project has now gathered data across the visible and infrared range, down to a depth of approximately magnitude 24 with a strong signal-to-noise ratio (S/N) of 10. This in turn has increased the number of catalogued objects from the roughly 400 million captured in the first data release, to about 700 million in this second release.

Australia’s role, and in particular usage of the AAT – which was used in collaboration with the Chilean telescope, was used to measure the exact distances to the many of the objects and to confirm the nature of the supernovae.

The Australian program was known as OzDES and incorporated following up on DES transient events like supernovae to obtain their redshifts, which told astronomers about the distance to the host galaxy it occurred in.

OzDES was also involved in measuring the masses of central supermassive black holes that reside in Active Galactic Nuclei, reviewed the spectroscopic data associated with giant galaxy clusters, measured the redshift of gravitational lensing transient events, and worked with the Australian Telescope Large Area Survey (ATLAS) projects to support the analysis of redshifts from a sample of radio sources.

The Universe is Accelerating!

Red and blue giant stars in the Universe. Credit: ESA/Hubble/NASA.

By studying the correlation of the recessional velocity of galaxies, along with their redshifts, it became apparent to scientists like Lemaîre and Hubble in the late 1920s that the Universe was indeed dynamic (and no longer static). This radical new concept brought its own implication – if it is expanding now (and has been since time began), then at some point in history, everything must have been remarkably close together.

These ideas were the early stages of what would develop into the Big Bang Theory – a cosmological notion that models the observable Universe from its earliest stages and through its evolution until today.

Since then, a reasonable amount of evidence (on top of the Hubble- Lemaîre expansion) has been reported – such as the detail within the cosmic microwave background radiation, the morphology and distribution of galaxies, the detection of primordial gas clouds, and the abundance of primordial elements. By taking all these considerations into account, scientists have dated the Big Bang to a point in history approximately 13.8 billion years ago.

But even the Hubble- Lemaîre expansion, thought to have been linear since the 1930s would be turned on its head in the late 1990s when the discovery that this expansion hit a speed hump and started accelerating somewhere along the way of the history of the Universe.

For their study, type Ia supernovae redshifts were observed to determine a positive cosmological constant value by the science teams, which indicated that the expansion was accelerating. That something, some dark energy, was causing it to get faster.

Three scientists, including the current Vice-Chancellor of the Australian National University – Prof. Brian Schmidt ended up earning the 2011 Nobel Prize for this extraordinary discovery.

So, what does this mean for the ultimate fate of our Universe? There’ve been many cosmologists who have written books about the different scenarios of how it will all come to an end, but application of the observations of dark energy seem to indicate that everything we see around us (i.e. our galaxy and local group), will eventually be pushed well out beyond our visible event horizon, and we’ll be left all alone here in the dark here in the Universe.

A rather depressing, lonely fate – but thankfully, one that will not arrive until much, much, further into the distant future so we don’t need to worry too much about it now (and that’s taking an overly ambitious presumptive position that humans will survive until then).

Additionally, between 2006 and 2011, another Australian project known as the WiggleZ Dark Energy Survey (which also used the AAT), created a detailed analysis of the baryon acoustic oscillations in the distribution of 240,000 southern galaxies by measuring their redshift using the AAOmega spectrograph.

Their results, combined in conjunction with the data captured by the cosmic microwave background observatories in orbit, helped add additional support to the accurate estimations of the matter-density composition estimates of the Universe.

Further Discoveries by the Dark Energy Survey

Artist illustration of trans-neptunium objects located in the far reaches of our Solar system. Credit: DES website.

On top of helping establish the foundational science for dark energy through observations of supernovae, weak gravitational lensing events and more, the DES has also contributed to the population of small, dwarf galaxies surrounding the Milky Way.

Several potential candidate satellite dwarf galaxies have been detected in DES data, with spectroscopic analysis underway to confirm they are indeed independent galaxies or rule this out with alternate confirmation that they are newly discovered star clusters (which already belong to our galaxy).

Additionally, the sensitivity of DEcam has proven successful in even finding Solar system bodies orbiting our Sun, albeit at great distances. Nine Trans-Neptunian Objects have been detected as of October 2019, all with orbits beyond Neptune (approximately 30 astronomical units and greater). The ability to observe this massive portion of the sky over such long periods will likely mean that further minor planets in the far reaches of our own system, might still be detected as yet.


Video Credit: ANU TV YouTube (2012)

The DES Collaboration has now released their final paper (currently on arXiv) and has made the data available for usage by astronomers, scientists as well as the general public.

The data is now available on the DES website.