World news – Whispers from the dark side: what can gravitational waves say about dark matter?


The NANOGrav collaboration recently picked up the first signs of very low frequency gravitational waves. Prof. Pedro Schwaller and Wolfram Ratzinger analyzed the data and particularly considered the possibility of whether this indicates a new physics beyond the Standard Model. In an article published in the journal SciPost Physics, they report that the signal coincides with both a phase transition in the early universe and the presence of a field of extremely light axion-like particles (ALPs). The latter are considered promising candidates for dark matter.

Gravitational waves open a window into the early universe. While the ubiquitous cosmic microwave background provides no clues as to the first 300,000 years of our universe, it does provide some glimpse into what happened during the Big Bang. « It is precisely this very early universe that is so exciting for particle physicists, » explains Pedro Schwaller, Professor of Theoretical Physics at the PRISMA Cluster of Excellence at Johannes Gutenberg University Mainz (JGU).

« This is the time when elementary particles such as quarks and gluons exist and then combine to form the building blocks of atomic nuclei. »

The special thing about the gravitational waves, which the NANOGrav Collaboration discovered for the first time, is that they have a very low frequency of 10-8 Hertz, which corresponds to about one oscillation per year. Due to their correspondingly large wavelength, each detector would also have to be of the same size in order to detect them. Since such a detector is not possible here on Earth, the astronomers at NANOGrav use distant pulsars and their light signals as huge detectors.

Wolfram Ratzinger outlines the motivation behind her work: « Although the data so far only give us an initial indication of the existence of low-frequency gravitational waves, it is still very exciting for us to work with them. This is because such waves could be generated by various processes that took place in the early universe. We can now use the data that we already have to decide which of them to consider and which of which don’t match the data at all

As a result, the Mainz scientists decided to investigate two scenarios that could have caused the observed gravitational waves in particular: phase transitions in the early universe and a dark matter field made up of extremely light axion-like particles (ALPs). Phase transitions like this occur due to the falling temperature in the primordial soup after the Big Bang and lead to massive turbulence – like dark matter, however, they are not covered by the standard model.

Based on the available data, Pedro Schwaller and Wolfram Ratzinger interpret the results of their analysis with relative caution: « Perhaps the scenario of the early phase transition is somewhat more likely. » On the other hand, the two physicists believe that the fact that they can only work out certain possibilities on the basis of limited data proves the potential of their approach. « Our work is a first but important development – it gives us a lot of confidence that with more accurate data we can draw reliable conclusions about the message that gravitational waves send us from the early universe. »

« In addition, » concludes Pedro Schwaller, « we can already begin to define certain characteristics of the scenarios and to impose restrictions on them, in our case the strength of the phase transition and the mass of the axions. »


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