Sydney. What is the universe made of? This question has been troubling astronomers for hundreds of years. For the past quarter century, scientists have believed that the ‘normal’ things like atoms and molecules that make up you, me, the Earth, and almost everything we can see make up only 5 percent of the universe.
The other 25 percent is ‘dark matter’, an unknown substance that we cannot see but that we can detect by how it affects normal matter through gravity. The remaining 70 percent of the universe is made up of ‘dark energy’. Discovered in 1998, it is an unknown form of energy that is believed to be causing the universe to expand at an ever-increasing rate.
In a new study to be published soon in the Astronomical Journal, we have measured the properties of dark energy in more detail than ever before. Our results show that it could be a hypothetical vacuum energy first proposed by Einstein – or it could be something strange and more complex that varies over time.
What is dark energy? : When Einstein developed his general theory of relativity a century ago, he realized that his equations showed that the universe must either be expanding or contracting. This seemed wrong to them, so they added a “cosmological constant” – a type of energy contained in empty space – to balance the force of gravity and keep the universe stable.
Later, when the work of Henrietta Swan Leavitt and Edwin Hubble revealed that the universe was in fact expanding, Einstein discarded the cosmological constant, calling it his “biggest mistake.” However, in 1998, two teams of researchers discovered that the expansion of the universe was actually accelerating. This means that something similar to Einstein’s cosmological constant may exist – what we now call dark energy.
Since those initial measurements, we have been using supernovae and other probes to measure the nature of dark energy. So far, these results have shown that the density of dark energy in the universe appears to be constant. This means that the strength of dark energy remains the same as the universe grows – it does not appear to become more finely dispersed as the universe gets bigger. We measure this with a number called W. Einstein’s cosmological constant is in effect set to -1, and earlier observations showed that this was approximately correct.
Exploding stars as cosmic measuring sticks: How do we measure what’s in the universe and how fast it’s expanding? We don’t have huge tape measures or huge scales, so instead we use “standard candles.” These are those objects present in space, whose brightness we know. Imagine that it is night and you are standing on a long road with some light poles. All these pillars have the same light bulb, but the farthest pillars are brighter than the nearby ones.
This is because light decreases in proportion to distance. If we know the power of the bulb, and can measure how bright the bulb appears, we can calculate the distance to the light pole. For astronomers, a typical cosmic light bulb is a type of exploding star called a Type Ia supernova. These are white dwarf stars that often suck matter from a neighboring star and grow until they reach 1.44 times the mass of our Sun, at which point they explode. By measuring how quickly the explosion fades, we can determine how bright it was and therefore how far away it is from us.
Dark Energy Survey: The Dark Energy Survey is the largest effort to date to measure dark energy. More than 400 scientists from multiple continents worked together for nearly a decade to repeatedly observe parts of the southern sky. By making repeated observations we look for changes such as new exploded 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 few dozen supernovae. The latest results from the Dark Energy Survey used about 1,500 exploded stars, providing very high precision. Using a specially built camera installed on the 4-meter Blanco telescope at the Cerro-Tololo Inter-American Observatory in Chile, the survey found thousands of supernovae of different types.
To find out which type IA is (the kind we need to measure distances), we used the 4-meter Anglo Australian Telescope at Siding Spring Observatory in New South Wales. The Anglo Australian Telescope took measurements that broke down the colors of light from the supernova. This allows us to see the “fingerprint” of different elements in the explosion.
Type LA supernovae have some unique characteristics, such as having no hydrogen and no silicon. And with enough supernovae, machine learning allowed us to efficiently classify thousands of supernovae. More complex than the cosmological constant Finally, after more than a decade of work and studying nearly 1,500 Type LA supernovae, the Dark Energy Survey has produced a new best-ever measurement of W. We found that W = -0.80 ± 0.18, so it is somewhere between -0.62 and -0.98.
This is a very interesting result. It’s close to -1, but not quite there. To be the cosmological constant, or the energy of empty space, it would need to be exactly -1. Where does this leave us? With the idea that a more complex model of dark energy may be needed, perhaps one in which this mysterious energy is transformed into the life of the universe.