Is our universe built up out of warm or cold dark matter? The standard model assumes cold dark matter particles, but astronomer Sylvia Ploeckinger is now testing the possibility of a warm counterpart: sterile neutrino’s. For this project, she received an NWO Physics/F grant, a special grant for women in physics. ‘I want to find out which elementary particle is the strongest dark matter candidate.’
Stars, planets, meteorites, nebulas… all combined they only account for less than 15 percent of all matter in the Universe. The remaining matter is called ‘dark matter’. But what is this mysterious matter made of? ‘The standard model of cosmology usually assumes cold dark matter particles,’ tells Ploeckinger. ‘But since the discovery that neutrinos have a mass, hypothetical particles from the neutrino family have become strong candidates for a theory with warm dark matter particles.’
Hot or cold?
Cold and warm refers to the velocities of the particles in the early universe. Cold particles moved more slowly and could form bound structures called haloes already on a very small scale. Due to their larger velocity, warm particles on the other hand formed bound structures only on a much larger scale.
In both cases, these haloes grow hierarchically by merging into larger and larger structures. These larger structures still contain smaller haloes from the beginning of a universe that have not yet fully merged. Today, the Milky Way Galaxy would be surrounded by millions of haloes with a large range of masses (down to the mass of the Earth) in a cold dark matter universe. In a warm dark matter universe, our galactic neighbourhood would be more quiet: only a few tens of the most massive satellite dark matter haloes would be orbiting the Milky Way.
A cube of universe
Ploeckinger has taken a cube out of our Universe with a side length of 80 million light years. A cube like this contains about 100 galaxies with the mass of the Milky Way. For this cube, she will simulate its evolution, assuming the dark matter is made up of warm dark matter particles. ‘These simulations start when the universe is just a few million years old and follow the formation and evolution of haloes and the galaxies that form from them for 13.8 billion years, until today.’ In an earlier project, Ploeckinger already did the same in a cold dark matter simulation. ‘I am very curious to find out which simulated universe looks most like the Universe we live in today. That will help us to find out which elementary particle is the strongest dark matter candidate. In the best case, we can find clear differences between the simulated cold and warm dark matter universes, that could confirm or rule out one of the two scenarios!’
In any case, Ploeckinger hopes the high resolution simulations will provide new clues about which direction astronomers should look to constrain the nature of the dark matter particle.
A flight through the universe
As a researcher doing basic science, Ploeckinger experiences it’s sometimes difficult to ‘give back to society’. ‘That’s because the data we produce usually doesn’t directly benefits people’s everyday lives.’ She is therefore very pleased the research results will be presented in a variety of easily accessible ways, such as animations and movies, as well as virtual reality flights through the Universe. ‘A special database with galaxies will allow for students to find their favourite galaxy, trace it back in time, and learn how it formed and evolved. I believe this helps to spark interest in particle physics, cosmology and astrophysics, as well as science in general.’
The work of Ploeckinger namely is at the intersection of all these disciplines. ‘Investigating the nature of the dark matter particle affects both the standard model of particle physics and the standard model of cosmology. In parallel to experiments on Earth that are trying to detect these particles, I investigate which signals in outer space could give us more insights into their properties. This way, we complement each other perfectly.’
Coverpicture: A massive dark matter halo in a cold dark matter universe (left) and a warm dark matter universe (right). In the left figure, millions of tiny haloes are visible, but they are missing in the warm dark matter halo in the right figure. Only the most massive few are present in warm dark matter. Source: Lovell et al. (2012).