The universe is not smooth. Stars are clumped into galaxies. Galaxies are bound in clusters. And the clusters follow a vast universe-wide web of dark matter filaments, with huge voids between them.
Seeing how this web has changed over galactic history is one of the holy grails of astronomy.
A computer model of the filamentary structure of the universe at the age of about 2 billion years. Credit: ESO (CC-BY)
Astronomers now know that the universe is not only expanding, but is doing so at an ever increasing rate. The driver for this expansion, dubbed ‘dark energy’, however, is still a mystery. Even its most basic properties, such as how it has affected the structure of the universe over time, are the subject of continued scientific debate.
If we could turn back the clock, and see snapshots of the how the universe looked at stages throughout its history, we would take a huge step closer to understanding dark energy.
Fortunately, two major projects currently under way are doing this: the Dark Energy Survey (DES) and the Kilo Degree Survey (KiDS). UCL scientists are closely involved with both, and the KiDS project has this week released its first data.
Over the next few years, KiDS will use the ESO VLT Survey Telescope in Chile to produce a detailed colour image of 1500 square degrees of sky (equivalent to a square 80 times the height and width of the full Moon). A parallel project will map the same area in five wavelengths of infrared light.
The ESO VLT Survey Telescope at Paranal Observatory in Chile. Credit: ESO (CC-BY)
As light from distant galaxies and quasars passes through the cosmos, its path is slightly bent by the gravity of objects in the foreground, an effect known as gravitational lensing. Scientists can use these subtle distortions to map where the mass is located in an image – revealing the location not only of the mass of the visible galaxies, but of the dark matter. Dark matter, as its name suggests, neither emits nor reflects light, so its presence can only be inferred from this type of painstaking detective work.
This week’s first data release from KiDS only covers about a tenth of the total area of sky that will be studied in the project, but it has already produced its first useful maps of the location of dark matter.
The first dark matter map from the KiDS survey, showing the inferred location of the dark matter in pink. Credit: Kilo-Degree Survey Collaboration/A. Tudorica & C. Heymans/ESO (CC-BY)
The next step is to move back through time. This is difficult but not impossible. As the universe expands, it stretches the waves of light that pass through it. The further away you look, and the further back in time you see, the redder this makes the objects appear.
KiDS, along with its infrared counterpart, will record the colour of each object through nine different coloured filters – giving enough information on the colour profile to estimate the distance of each galaxy. Through this, it will be possible to dial back time, observing the distribution of mass at various points going back through time, charting how the size and structure of the dark matter filaments has changed throughout cosmic history.
Current theories about dark energy suggest that we should see structures rapidly growing in the early universe, with this gradually slowing down over time – and KiDS will help test whether this is indeed the case.
UCL is involved in KiDS through Benjamin Joachimi, Edo van Uitert (both UCL Physics & Astronomy) and Tom Kitching (UCL Mullard Space Science Laboratory). They work mainly on analysing the gravitational lensing effects detected in the survey.
These lensing effects can be quite dramatic – high resolution images of large galaxy clusters taken with the Hubble Space Telescope show dramatic distortions in the shapes of background galaxies.
Gravitational lensing can dramatically distort the shapes of background galaxies, as can be seen in this Hubble image which shows galaxies distended into arcs around the cluster’s centre of gravity. Credit: NASA/ESA (CC-BY)
But in most cases the effect is actually very subtle – a tiny modification of the shapes of thousands of galaxies, which appear as barely more than dots in the background.
For each one of these dots, it’s impossible to say whether the shape it appears to us is down to it genuinely being slightly flattened – or whether this is a result of the light from the galaxy being distorted by gravitational lensing.
But if you look at thousands of galaxies, you can tease out the statistical likelihood – for instance, if thousands of galaxies all appear flattened in the same direction, it’s likely to be because an unseen mass of dark matter is distorting them all in the same way.
These measurements rely on extremely accurate modelling of how the telescopes work and of precisely how lensing effects occur – to the extent of even producing dummy data to test their assumptions and calibrate the observations.
A simulated image of lensed galaxies, developed by scientists to calibrate their analysis of real telescope data. Credit: R. Herbonnet/E. van Uitert
The primary goal of the KiDS project is to find out more about the evolution of the cosmos and to test the laws of gravity and general relativity.
KiDS is in friendly competition with the Dark Energy Survey to do this. To an outsider this might look like a waste of resources – but this is the cutting edge of cosmology and in truth nobody really knows what we will find. If both KiDS and DES come up with the same result, despite their different telescopes, detectors and methods, then we can have some confidence that the conclusions are accurate.
Of course if they don’t, then we’re back to square one. But at least we’ll know that we don’t know.