Searching for a baby photo of the universe

17 December 2021

A red-hot soup of minuscule particles continually colliding with each other. According to the prevailing theories, that is what our universe looked like immediately after the Big Bang. The NWA project ‘One second after the Big Bang’ is searching for new witnesses from the very beginning of the universe: exotic ghost particles called neutrinos.

‘The Big Bang is the most important event in world history. And yet we only have a few pieces of experimental evidence that can tell us exactly how it happened’, states project leader Auke Pieter Colijn from the University of Amsterdam and Nikhef in explaining the importance of this research. Based on the cosmic background radiation (light that was emitted by the primaeval soup once this had already cooled down considerably), we can take a photo of the universe when it was 400,000 years old. ‘Let’s say when it was a toddler. Now we want to try and make the first-ever baby photo – an image of what the universe looked like one second after its birth’, explains Colijn.

Capturing elusive particles

In order to make this photo, the scientists need to develop methods with which they can detect particles called neutrinos. In the exotic world of particle physics, neutrinos are among the most elusive of particles. They are invisible, have no electrical charge, weigh almost nothing and fly through anything and anybody largely unhindered. For example, every person on earth is bombarded by billions of neutrinos each second. Fortunately, you hardly notice them because they interact with almost nothing. Scientists are nevertheless determined to catch them. As neutrinos fly unhindered in a straight line from the moment of their creation, they contain a lot of information about the environment and conditions in which they were formed.

‘In our experiment, we will search for the most elusive of these particles’, says Colijn. These are the so-called relic neutrinos: neutrinos that were born during the Big Bang and since then have been finding their way through the universe for more than 13 billion years. Meanwhile, they have cooled down to such an extent that they have very little remaining energy. This makes them far more difficult to detect than their brothers, which originate from the sun, for example, and for which enormous underground and undersea detectors have already been built, such as Super-Kamiokande and KM3NeT.


Hubble Ultra Deep Field infrared image of galaxies billions of light years away from Earth. Photo: NASA

High-risk, high reward

Colijn’s NWA project therefore bears a high risk, he explains straightforwardly. ‘The chance that we can actually make something with which we can observe these neutrinos is not very large. However, if we succeed … then the future will certainly hold a Nobel Prize.’ The idea for the experiment the researchers want to build is not new. ‘The groundwork for our detection method was already done back in 1962 by Nobel Prize Winner Steven Weinberg. We will make use of an existing reaction in a special form of hydrogen called tritium. That is a non-stable substance that exhibits spontaneous radioactive decay as a result of which, amongst other things, an electron is created. If that tritium has by chance absorbed a relic neutrino before it decays, then the electron’s energy is very slightly bigger than normal. And that is the miniscule deviation we will try to measure.’

Although, in theory, it should be possible to make this process work, many things are still very uncertain. ‘For example, is tritium really the most obvious material to conduct this experiment with? We have chosen this material because such a decay reaction occurs a million billion times per second per gram of tritium. And even then, we only expect to be able to detect five neutrinos per year because the vast majority of those particles will quite simply not react with anything. But can we keep that material sufficiently stable to make it last for several years? And then I have not even touched upon the practical issues that we will be confronted with – tritium is also a raw material for nuclear weapons and very heavily regulated.’

Sensitive detector

There is also a major technical challenge involved in the design of a detector that can measure the energy of the sought electrons as accurately as possible. Because, ultimately, the scientists hope five times per year to be able to observe an electron that has very slightly more energy than its ten billion billion brothers and sisters that were also emitted that same year by the spontaneously decaying tritium. ‘Therefore, the first question is: can we actually construct an electron detector that can detect a few needles in such an immense haystack? If we can answer that question with a “yes” at the end of this project, then I will be jumping for joy.’

Although the NWA project offers great opportunities, this challenge is far too big for this project alone. ‘We are therefore part of a much larger international collaboration called PTOLEMY’, says Colijn. ‘In this consortium, which now consists of almost 60 scientists from 23 institutes in 7 countries, there are also colleagues from the American Princeton University, the Italian Laboratorio Nazionale di Gran Sasso and the German Karlsruhe Institute of Technology (KIT) who are all partners in our NWA project as well. Since this is such high-risk research, it is difficult to find funding for it. Therefore, each PTOLEMY consortium partner is trying to find funding for smaller subprojects so that we can realise this idea together.’ In the NWA project, Colijn is collaborating with his Princeton colleagues. ‘Here in the Netherlands, we are building a piece of the electron detector together with TNO and, in Princeton, they are building another part. Meanwhile, a joint study of Radboud University and the KIT serves to discover whether tritium is indeed the right material for capturing neutrinos.’

Disseminating knowledge

Besides fellow scientists, the NWA consortium also encompasses parties who will work on disseminating the knowledge from this project, emphasises Colijn. ‘For example, we have people on board from The Hague University of Applied Sciences who will develop practicals around this project. And we also collaborate with the Netherlands’ Physical Society. Their chair and science journalist Diederik Jekel leads a subproject for outreach since questions concerning the origins of the universe are of interest to a large target group.’

Although this research is a sidetrack for Colijn – his other research projects are aimed at understanding the equally mysterious dark matter – he is very excited about this opportunity. ‘For me, physics and astronomy have always mainly been voyages of discovery into the unknown. With this project, we are trying to go 400,000 years further back in time than anyone has ever managed in the past. Then, at last, we will be able to detect the very first moments of our universe. And just imagine, we might succeed! What could be more fantastic?’


Source: NWO / Tekst: Sonja Knols-Jacobs