A new wave of researchers is setting out to help shed light on the unseen forces shaping the universe, and keep Europe at the forefront of experimental cosmology.
Earlier this year, Dr Alexey Boyarsky at Leiden Observatory in the Netherlands discovered what could be indirect evidence of dark matter, while researchers at the European Space Agency (ESA) are now developing a probe that will try to measure dark energy, the unseen force believed to be driving the accelerated expansion of the universe.
They are using Europe’s Planck satellite to map cosmic microwave background radiation. ‘The Planck satellite has been the best tool on offer for these broad scales,’ said physicist René Laureijs of the ESA, who believes Europe has become the global home of experimental cosmology.
To cement Europe’s position, the EU-funded Invisibles project is training over 200 scientists to hunt neutrinos, dark matter and dark energy, which constitute the mysterious ‘dark sector’ of the universe and are essential to its cosmological evolution.
‘Our aim is to lift the curtain on this dark sector, while providing training for the next generation of scientists who will lead this quest in the future,’ said project coordinator Professor Belén Gavela.
The idea of dark matter came about some 80 years ago when researchers realised that galaxies were spinning so fast that centrifugal force should have torn them apart based on the amount of gravitating matter visible inside them.
‘The scientific questions we’re trying to answer go to the core of that human quest to find out what nature’s made of.’
Professor Belén Gavela, Invisibles coordinator, Autonomous University of Madrid, Spain
Swiss astronomer Fritz Zwicky postulated in 1933 that the galaxies must contain some sort of additional ‘dunkle Materie’, or dark matter, to bind them together. Researchers now believe there is more than five times as much dark matter in the universe as normal matter, yet they still do not know what it is.
Using XMM-Newton, an orbiting observatory run by the ESA, Dr Boyarsky and his team recorded a signal of X-ray photons that have about the right energy – 3.5 kiloelectron-volts – to have been produced when ‘sterile neutrinos’ decay somewhere in the distant cosmos.
This is significant because some researchers believe that so-called sterile neutrinos could be components of dark matter.
However, dark matter is not the biggest mystery facing cosmology. Dark energy makes up two-thirds of all mass–energy, researchers believe, and by using the Planck satellite to study the microwave background, cosmologists can test their leading theory of the universe’s evolution, known as the lambda-CDM (cold dark matter) model, where the ‘lambda constant’ refers to dark energy.
The EUCLID Satellite ©ESA - C. CarreauBut ESA physicist Laureijs wants to go one step further. He is the project scientist for the future ESA satellite EUCLID, which will more accurately measure the acceleration of the universe to see whether lambda is indeed a constant – as Albert Einstein allowed for in his famous equations – or whether it varies over time and space.
‘If it’s not a constant, it would be a breakthrough in physics because it will go against what Einstein predicted,’ Laureijs said.
EUCLID is due to launch in 2020, and will set the next benchmark in dark energy observation. Data from the Planck and EUCLID satellites will be used and analysed by researchers of the Invisibles project
The Invisibles project aims at understanding the mysteries of the dark sector of the universe: what are the properties of neutrinos and dark matter? How heavy are they? How do they interact? What is dark energy made of? It will give researchers access to data from experiments such as XENON – an experiment buried in a mountain in Italy to look for hypothetical type of particle that could make up dark matter called weakly interacting massive particles.
‘The scientific questions we’re trying to answer go to the core of that human quest to find out what nature’s made of, what the constituents of the universe are, and how it evolved into what we see in the sky today,’ said Prof. Gavela.
Sterile neutrinos are a possible explanation for dark matter: they could bind galaxies together with gravity, but they would avoid any of the other forces that might have enabled us to glimpse them already.
We already know that ordinary neutrinos exist – they are created in nuclear reactions as well as in nuclear decay. In fact, the sun emits so many neutrinos that about 100 billion of them pass through just the tip of your finger in any one second.
Neutrinos are so elusive because they interact with only two of nature’s forces – the so-called weak force, which is one of the four fundamental forces of nature, and gravity. Sterile neutrinos would be extra neutrinos that, if they do exist, would be even more slippery. These hypothetical particles would feel the weak force extremely weakly, in fact only by mixing with ordinary neutrinos. In a sense, then, sterile neutrinos are some of the ‘darkest’ matter imaginable.
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