Australian astrophysicist Tamara Davis wants to unlock the secrets of dark energy
The universe has a dark secret and an Australian astrophysicist is at the vanguard of a worldwide effort to try and unravel it.
Professor Tamara Davis of the University of Queensland has just been given five years and a multi-million-dollar Laureate Fellowship from the federal government to explore the dark side of the universe.
To help her she has an international coterie of the some of the smartest minds on planet earth and two phenomenally powered cameras on huge telescopes.
The secret hiding from us in plain sight is called dark energy, a mysterious anti-gravitational force that is pushing the universe to expand at an accelerated rate.
It’s the abyss between the stars — making up 70 per cent of everything. Combined with dark matter, which in contrast pulls things towards it, they make up 95 per cent of everything.
Understanding dark energy may be the key to figuring out the most vexing issue in physics, the problem Albert Einstein spent the last 50 years of his life trying to solve; the fact that two fundamental theories don’t make sense together.
If that makes your head hurt a bit, don’t worry. Professor Davis is good at explaining ridiculously complex concepts to people with non-astrophysicist-sized brains.
The DESI project uses spectral imaging to help map the universe. (Berkeley Lab: Marilyn Chung)
“There’s something causing the expansion to speed up. So something out there is behaving as though it has anti-gravity. It’s causing gravity to push, not pull.
“We give it the name dark energy. We don’t know what that is, but we’re trying to find out.”
Fundamental physics can’t currently explain why the universe’s expansion would be accelerating.
It also can’t explain dark matter (dark matter clumps and pulls things toward it, while dark energy seems smooth and pushes everything away from it).
“We basically rely on two extremely successful theories: The theory of particle physics we have, which is the standard theory of quantum physics, and general relativity, which is our theory of gravity,” Professor Davis says.
“We know that those two don’t mix. They work enormously well in their separate domains but they both can’t be right.
“The only force that’s out there, dangling, without being unified with the others, is gravity. So we want to find a theory to bring it all together.
“Einstein tried and failed. From my perspective, that’s the biggest hole in fundamental physics at the moment.
“We know we’re missing stuff, and who knows what we’ll be able to do once we find it.”
So … why should we care what dark energy is?
Professor Davis and her colleagues are in the process of imaging the sky, and measuring dark energy more precisely, which will hopefully answer a number of questions.
“There’s hints from quantum physics that the gravitational effect of the vacuum of empty space could be causing the acceleration of the expansion of the universe.
“So dark energy may be the first piece of strong observational evidence that we have that might be explained by joining these two theories, which is why it’s so exciting.”
These questions about dark energy loom large:
- Is it the same everywhere in space?
- Was it stronger in the past and is weakening, is it strengthening?
- Is it changing with time?
- Is it smooth throughout space?
- How BIG is it?
- Does it change as a function of scale?
As scientists begin to find out more about dark energy, they can start to eliminate theories as to what it is.
And this is a major element to Professor Davis’s project — coming up with a theory to explain the unexplainable.
Seeing through time with DES and DESI
In order to discover the truth behind dark energy, Professor Davis needs to look back in time — right back to about 10 billion years ago, far closer to the Big Bang.
In August, she was awarded an Australian Research Council Laureate Fellowship, worth $2.9 million over five years to, “measure and explain the dark side of the universe by performing new theoretical analyses of two ground-breaking surveys”.
Those two surveys, the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI), are centred on what are essentially enormously powerful digital cameras mounted on telescopes in Chile and Arizona, backed up by our own Anglo-Australian telescope in Australia.
The projects will chart 300 million galaxies in total, and 30 million in precise detail. It will be the biggest, most precise map ever of the distribution in the universe and they will use it to measure the properties of dark energy very precisely.
The DES is a more than 400-person collaboration spread across five continents.
With a telescope in Chile they are imaging an enormous patch of space — about an eighth of the entire sky — to measure the positions, in two dimensions, of 300 million galaxies.
“If you take pictures of the sky you can see where galaxies are … but you can’t really tell how far they are away,” Dr Davis explains.
“Because you can’t tell whether it’s something really bright far away, or something faint, close to you. You just don’t have that depth perception. You can only get approximate distances.”
And that’s where DESI comes in. DESI is a spectroscopic instrument, which uses 5,000 moveable optical fibres to view the spectrum of light from red to blue, the same way we see rainbows or light refracted through a prism.
It will take spectra of about 30 million galaxies — measuring in detail their brightness across all the colours of the rainbow.
All the different elements of the periodic table leave different colour fingerprints, and scientists can tell by looking at the bright lines in the spectrum what elements are there (copper, for example, glows green when it burns).
“So you can tell what the galaxy’s made of, but more importantly for us, you can also measure how fast they’re moving away,” says Professor Davis.
“Because if something’s moving away from you, it’s spectrum gets shifted, the wavelengths get stretched longer. It’s the Doppler shift, the same thing you hear when you hear a siren or a racing car going past, high-pitched to low-pitched.”
Combining the images seen by different telescopes helps create 3D maps of space. Each colour represents a different map — the longer strands are looking further back in time.
This is how the enormous map of the sky goes from 2D to 3D. The galaxies furthest away are logically moving the fastest, as they have travelled the most distance since the Big Bang.
But that’s not all. Her team is also measuring thousands of exploding stars in those distant galaxies. This gives a completely new way to measure distances because the brightness of their explosions is consistent — so seeing how bright they appear tells us how far they are away.
“We can look at how fast the universe was expanding in the past and compare it to now.
“Because if you look in the deep, dark, distant past, the light from the most distant supernovae we’ve been looking at has been travelling to us for about 10 billion years.
“So you’re actually directly observing the universe as it was billions of years ago, and comparing that to how fast it’s moving now.
“That tells you how that expansion has changed over that time. You look far away, nearby and everything in between, and that gives you a sort of curve of how the expansion is changing.”
Why spend do much time, money and brainpower trying to understand mysterious forces such as dark energy? Couldn’t we just, you know, accept that the universe has its mysteries and get on with paying our taxes and watching The Bachelorette?
According to Professor Davis there are immediately tangible benefits to such research, such as the 10 years of development that went into creating what is essentially an advanced digital camera, DES, which will likely end up making our phone cameras better.
Then you have the offshoots, like WiFi — Australian astronomers looking for black holes developed the algorithm that made WiFi possible.
“Every time we understand more about quantum physics, we can make more energy efficient things, we can make new types of devices and have capabilities that we just never had before,” she says.
“We don’t know what we’ll be able to do once we find out how these theories work together, but it could be spectacular.
“It might be — dark energy appears to be accelerating the entire universe, can we harness that as a form of power generation? Can we harness it as a form of propulsion because it seems to have an anti-gravity effect — harness it to launch space ships?
“We don’t know yet, and that’s the exciting bit about exploring.”
Educating against ignorance and the beauty of science
Attempting to wrap your mind around the secret truth of the universe — devoting your life to science — must take a high level of commitment.
So what motivates Dr Davis? What drives her to keep at it?
“Getting the public to understand science is incredibly important. Possibly more important than all the scientific discoveries that I could make” she says.
“Because, if as a community we can understand and trust the outcomes of science, then we can make clever, science-based decisions on issues such as climate change and vaccinations, medicine, anything that fear-mongering might cause people to misunderstand.
“In terms of the science itself, it’s the fascination of it.
“It is beautiful and the equations are so elegant. I find maths really elegant and nice.
And when you realise that mathematics actually describes the universe that we’re in, and you can actually try and answer questions like ‘how did universe begin?’, ‘does the universe actually have an edge?’, ‘is there life on other planets?’.
“These kind of questions you can actually go an scientifically attack. And when you realise that maths is able to answers questions like this and ‘what happens around a Black Hole’ and all those kind of things, that’s just really exciting. And that’s why I do it.”
Star trails take shape around Kitt Peak National Observatory, which houses the DESI telescope. (NOAO/AURA/NSF: P. Marenfeld)