“The light signal is a ‘baby photo’ of the universe”
Prof Dr Kai Schmitz's field of work lies at the interface of particle physics and cosmology; his favourite subject is gravitational waves from the early universe. In an interview with Christina Hoppenbrock, the research group leader at the Institute of Theoretical Physics provides insights into the evolution of the universe, methods of research and still unresolved questions about cosmology.
I’d like to talk to you about evolution, but not about evolution in the Darwinian sense ...
The term evolution is also very common in cosmology. It describes the development of the universe from a very early, original state to the present day – or in other words: the sequences of physical processes on cosmological scales of size, length and time.
‘Evolution of the universe’ - that sounds like a huge field of research.
Cosmology as a scientific discipline has grown considerably over the past years and decades. Large-scale research projects bring together hundreds, even thousands of scientists. Modern cosmology attempts to reconstruct evolution from the earliest possible point in time. We are investigating the physical processes that took place in the hot primordial soup 13.8 billion years ago. In doing so, we have a strong overlap with particle and nuclear physics. We then follow the development of the universe over the first fractions of a second, the first seconds, minutes and years. 380,000 years after the Big Bang, the so-called cosmic background radiation was emitted, which is the afterglow of the Big Bang. Important findings from atomic physics play a role in understanding these processes.
And what happened next?
With our physical theories we can describe how structures formed out of the hot primordial soup. Clumps formed, from which the first stars, the first galaxies and the first galaxy clusters emerged. Today, we see a network of galaxies and galaxy clusters in the universe that are not randomly distributed. In this ‘cosmic web’ there are cross-connections, and in between are large cavities where fewer galaxies exist.
How can you investigate something that happened 13.8 billion years ago?
Our idea of the evolution of the universe is based on three pillars. The first are observations by Edwin Hubble, who realised at the beginning of the 20th century that galaxies outside our Milky Way were moving away from us. He correctly interpreted this as an expansion of the universe that began with the Big Bang. Looking back in time, this means the universe must have been denser and hotter in the past.
The second pillar is the cosmic background radiation which I’ve already mentioned. 380,000 years after the Big Bang, the primordial soup had cooled down to such an extent that light particles – photons – were able to move freely for the first time. Some of them were emitted in our direction. We receive this light signal today, in other words, a ‘baby photo’ of the universe. It contains a great deal of information. For example, we can see that the primordial soup was not uniformly hot – the precursor of today’s cosmic web. We can describe such observations statistically and compare them with our theoretical models.
And thirdly?
The Big Bang Theory also makes concrete predictions about the ratio in which the light chemical elements, e.g. hydrogen and helium, were formed in the first few minutes of the Big Bang. We can verify this through astrophysical observations and find very close correlations down to the last detail.
You research focuses on the evolution of the early universe. What especially interests you about it?
Gravitational waves from the Big Bang. There are many physical theories that predict the generation of gravitational waves in the Big Bang. If this were true, there would be a gravitational wave background in the universe in addition to the electromagnetic waves, i.e. the light signals which resulted from the Big Bang. The underlying theories are based on physics beyond the standard model of particle physics – there are many open questions we’d like to find answers to. My group is involved in the ‘Nanograv Pulsar Timing Array Collaboration’. We observe pulsars in the Milky Way to detect gravitational wave noise. ...
Pulsars are the remnants of the cores of massive stars.
... Last year, we published findings showing that such signals do indeed exist. But where do they come from? A popular explanation is that the gravitational waves originate in black holes in the centres of galaxies. We are interested in the less likely but more exciting alternative: the detected gravitational waves could be an ‘echo’ of the Big Bang.
So there are still many unanswered questions about the evolution of the universe ...
There are observations that are virtually incontrovertible. For example, we can say with certainty that the universe was in a hot, dense state 13.8 billion years ago. But in terms of its structural formation, there are phenomena that we don't understand exactly, we call them dark energy and dark matter. We know many of their properties and can use them in computer simulations of the evolution of the universe – that all fits so far. Nevertheless, dark energy and dark matter are more of a placeholder for physical phenomena whose mode of action and nature we do not yet know exactly. Dark energy has properties that contribute to the universe expanding faster and faster. Dark matter interacts with its surroundings through the force of gravity and thus contributes to the formation of structures. Galaxies, for example, are typically located at the centre of large clusters of dark matter. But we do not know what lies behind this. Are they, for example, previously unknown elementary particles? There are many candidates, but we are still in the dark.
In biology, the evolutionary process can be simulated in laboratory experiments. Does this also work in physics?
We are trying to recreate some of the early stages of the universe in the laboratory, for example in accelerator experiments in which particle collisions take place. Otherwise, we have various possibilities in physics and cosmology to reconstruct evolution. We use satellites and telescopes to do this. The cosmic background radiation, for example, is precisely measured using satellites. Satellites and telescopes are the ‘eyes’ of cosmology, so to speak. And more recently, there are also the ‘ears’, by which we mean the experiments to detect gravitational waves.
In current astrophysics and cosmology, attempts are being made to reconstruct certain phenomena, processes and events not only with the help of a single messenger, but through various sources, so-called multi-messenger observations. In addition to the signals mentioned, neutrinos are also important: elementary particles with a very low mass that originate from the universe. We therefore have various messengers from space: light signals, gravitational wave signals and neutrinos.
Can we make predictions about the evolution of the universe?
We can predict with some certainty what to expect in the next few million years, perhaps even a few billion years. But what the final state will look like in hundreds of billions of years or beyond is mere speculation. If we look into the future with our current model, at some point there will be a great emptiness in the universe. Everything will move away from each other, no more new stars will form. In the end, there will only be black holes, which will then slowly disintegrate over unimaginably long periods of time.
In the current standard model of cosmology, we describe dark energy using Einstein’s cosmological constant. But there have been indications in recent months that dark energy could possibly be a time-dependent quantity that becomes weaker over time. Whether this is really the case will only become clear in the future when other observations come to the same conclusion. However, if the current indications prove to be true, we will have to make significant corrections to our predictions about the evolution of the universe.