Why is the universe ripping itself apart? A new study of exploding stars shows dark energy may be more complicated than we thought

• Discovered in 1998, this is an unknown form of energy believed to be making the universe expand at an ever-increasing rate.
• In a new study soon to be published in the Astronomical Journal, we have measured the properties of dark energy in more detail than ever before.
• Our results show it may be a hypothetical vacuum energy first proposed by Einstein – or it may be something stranger and more complicated that changes over time.

What is dark energy?

• Read more:
  More than 70% of the Universe is made of 'dark energy', the mysterious stuff even stranger than dark matter

  However, in 1998, two teams of researchers found the expansion of the universe was actually accelerating.

• This implies that something quite similar to Einstein’s cosmological constant may exist after all – something we now call dark energy.
• Until now, these results have shown the density of dark energy in the universe appears to be constant.

Exploding stars as cosmic measuring sticks

How do we measure what is in the universe and how fast it is growing? We don’t have enormous tape measures or giant scales, so instead we use “standard candles”: objects in space whose brightness we know. Imagine it is night and you are standing on a long road with a few light poles. These poles all have the same light bulb, but the poles further away are fainter than the nearby ones.

• For astronomers, a common cosmic light bulb is a kind of exploding star called a Type Ia supernova.
• By measuring how quickly the explosion fades, we can determine how bright it was and hence how far away from us.

The Dark Energy Survey

The Dark Energy Survey is the largest effort yet to measure dark energy. More than 400 scientists across multiple continents work together for nearly a decade to repeatedly observe parts of the southern sky. Repeated observations let us look for changes, like new exploding stars. The more often you observe, the better you can measure these changes, and the larger the area you search, the more supernovae you can find.

• The first results indicating the existence of dark energy used only a couple of dozen supernovae.
• The latest results from the Dark Energy Survey use around 1,500 exploding stars, giving much greater precision.
• Using a specially built camera installed on the 4-metre Blanco Telescope at the Cerro-Tololo Inter-American Observatory in Chile, the survey found thousands of supernovae of different types.

More complicated than the cosmological constant

• To be the cosmological constant, or the energy of empty space, it would need to be exactly –1.
• With the idea that a more complex model of dark energy may be needed, perhaps one in which this mysterious energy has changed over the life of the universe.
• Read more:
  From dark gravity to phantom energy: what's driving the expansion of the universe?

Brad E Tucker receives funding from the Australian Research Council and ACT Government.