Project sheds light on the mysteries of dark energy
It has now been over 20 years since the discovery that the expansion rate of the universe is increasing, which led to the idea that a form of energy called dark energy is pushing the entire universe apart.
We know very little about this energy, but a new survey that will hopefully help us to understand more about this phenomenon has now begun.
One could say that the work in the 1990s resulted in an accidental discovery, so firstly I’ll explain what that work was all about.
The story began a century ago, when astronomer Vesto Slipher in the USA analysed the light from a number of galaxies and found that in general they were moving away from us. This was even before these objects were actually recognised in the 1920s as being star systems external to our galaxy.
The famous astronomers Edwin Hubble (after whom the Hubble Space Telescope is named) and Georges Lemaître noticed that there was a clear connection between the speed of recession of the galaxies and their apparent distance from us, giving astronomers a way of estimating the distance to a galaxy: the faster galaxies were farther away.
This was originally called simply the Hubble Law, because Lemaître’s contribution was not properly recognised until as recently as 2018. It is now called the Hubble-Lemaître law.
Astronomers came to realise that this situation pointed to the fact that the entire universe is expanding. In general, on a large scale, galaxies are moving apart from each other.
However, what about the past and future rate of expansion? It seemed that the expansion should, at least, be slowing down, with the gravity between the galaxies trying to hold the universe together.
But in the 1990s, when two separate teams of astronomers set out to measure this rate of slowing of the expansion, they found the opposite: it was actually speeding up.
Brian Schmidt — current Vice-Chancellor of the Australian National University — and Adam Reiss led a team called the High-Z Supernova Team, while another team (running the Supernova Cosmology Project) was led by Saul Perlmutter.
The three were jointly awarded the 2011 Nobel Prize in Physics for their discovery.
The discovery led to the idea that something that we now call dark energy was causing an acceleration in the expansion rate.
However, even though dark energy is thought to represent about 70 per cent of the energy in the universe, we understand very little about it.
Now, a new device called the Dark Energy Spectroscopic Instrument (DESI) has begun a study that will hopefully tell us more about this mysterious energy.
The instrument will record the spectrum of the light of 35 million galaxies and 2.4 million quasars. This will allow astronomers to find the distance to each one, making a three-dimensional map of a significant portion of the universe.
This will give us a better understanding of what the dark energy is doing.
Amazingly, DESI will be able to measure 5000 galaxies in each single 20-minute observation period.
This would not be possible by observing each galaxy one at a time. Instead, DESI is based on fibre optics — each of which is placed in the correct position in the telescope to see an individual galaxy. The 5000 fibre optics will then be quickly reassigned to different positions to measure the next group of galaxies. It has been estimated that the set-up will be able to record up to 100,000 galaxies in a single night!
DESI’s observations will start in earnest in 2020. The massive instrument has been mounted on one of the world’s great telescopes: the fourmetre-diameter Mayall telescope at Kitt Peak in Arizona in the United States. Testing all seems to be going well.
This 3D map will not be the first of its kind, but it will be the most detailed. It’s very exciting, and it may even answer questions about the universe that have not yet been asked.
As with so many scientific observations, we can never be quite sure what we shall find — and that’s a point to which I shall return in an article in the near future.