What is in the universe? We currently think there are between a few hundred billion and a couple of trillion galaxies in the universe. Each of these galaxies has about 100 billion stars, and all those stars have planets, and those planets have moons.
All of that, though - the atoms, molecules, and gas that make it all up - accounts for only 5 per cent of the universe. Another 25 per cent is in something called dark matter, which makes up most of the mass of galaxies. We think it is a weakly interacting massive particle. It does not emit its own light, but we can measure it through its interactions with normal matter via gravity.
The last 70 per cent is dark energy. Discovered by two teams in 1998, one led by Brian Schmidt, it is causing the universe to accelerate in its expansion. What exactly it is a bit of a mystery. It acts like gravity in reverse - instead of pulling things together - it pushes it apart. If gravity is like a space vacuum, dark energy is the leaf blower.
How do we measure what dark energy is and how fast it is causing the universe to grow? We use objects whose brightness we know - standard candles.
Imagine standing at night on a long road with a few light poles. The poles all have the same light bulb, and the poles further away are fainter than the nearby ones. Since light fades with distance, if we know the how bright the bulb is, and how bright it should be, we can measure a distance to the pole.
Type Ia supernova are a unique type of stellar explosion. These stars suck mass from a nearby star, and when they reach a critical mass, they explode. As they explode with about the same mass, they have about the same brightness - our cosmic light bulb.
The Dark Energy Survey, a project involving over 400 scientists, across multiple continents, operated for over five years, repeatedly imaging regions of the southern sky. By repeatedly imaging the sky, you can look for changes like new exploding stars. It found thousands of supernovae to measure the properties of the universe.
When Einstein developed his general theory of relativity, he originally added a term, the cosmological constant, to counteract gravity, else his equation meant the universe expanded.
However, when the expansion of the universe was discovered in the 1920s, he removed, calling it his biggest blunder.
Our past measurements of dark energy appear to show it is constant. As more universe grows, the strength of dark energy is the same - it doesn't weaken like gravity. It acts very similarly to Einstein's cosmological constant, vacuum energy - the energy of empty space.
This is strange as every other force we know of, like gravity and magnetism, the further apart things get, the weaker the force is. This is why the Earth pulls you down despite the sun have a lot more gravity than Earth. The sun is much further away and its pull of gravity much weaker.
However, now with thousands of supernovae and a decade of work we find something close to vacuum energy, but not exactly. It may be that dark energy is more complex and actually changes with time.
A strange, mysterious universe.
- Brad Tucker is an astrophysicist and cosmologist at Mt Stromlo Observatory and the National Centre for the Public Awareness of Science at the ANU.
