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Department of Physics

The Cavendish Laboratory
96 million light years slices

In their latest research on dark matter scientists have collected vital clues about the ‘coldness’ of the mysterious component making up 80% of all matter, revealing a deep connection between dark matter and the formation of galaxies in the early Universe.

With the help of powerful telescopes, the researchers based at the University of Cambridge have studied deeper into the Universe, when it was 1 billion years old, to analyse the spectra of distant quasars. The new findings, published in Physical Review D suggest that the abundance of small galaxies in the cosmos aligns with the presence of ‘cold’ dark matter, while scenarios involving ‘warm’ dark matter lead to a smoother cosmic web, devoid of smaller galactic structures.

Through meticulous numerical simulations, researchers explored various scenarios, from the dense, slow-moving particles of ‘cold’ dark matter to the lighter, faster particles of ‘warm’ dark matter.

“Arriving at a crucial conclusion in this study we have challenged the notion that dark matter is composed of exceptionally light particles,” said Vid Irsic, first author of the paper and Senior Kavli Fellow at Cavendish Laboratory, University of Cambridge.

 “By scrutinising quasar spectra from the Universe's infancy, we have excluded certain particle masses (thousands of times the mass of an electron or lower), narrowing down the possibilities for dark matter's composition.”

What sets this research apart is its comprehensive approach. Previous studies utilised intergalactic gas to probe dark matter, but this analysis employs advanced spectrograph instruments to unprecedented extents. This required meticulous modelling of instrumental effects and observing conditions. The approach has yielded robust measurements, extending to the very limits of the instruments which opens future investigations.

The scientists have also improved on their previous studies by implementing more realistic production of ionising photons in numerical simulations, and their link to the heating and ionisation they cause in the Universe. In particular, researchers have for the first time investigated the impact of variations in the levels of ionising photons coming from different galaxies. The effect this has on the ability to infer the ‘coldness’ of dark matter has been one of the key open questions of previous such studies.

“Through the combination of new data and better simulations, we were able to increase our confidence in ruling out light and fast dark matter species as the primary component of the elusive dark sector (dark matter and dark energy) of the Universe,” said second author of the paper Matteo Viel from SISSA, Italy.

Our results effectively rule out the existence of light dark matter particles as the primary explanation for various effects observed in the local universe, such as missing satellite problem, and problem with the formation of density cores in galaxies. Additionally, we were able to conclude that interpretation of the excess of X-ray photons from Perseus cluster in a narrow emission line (at 3.5 keV) is unlikely to result from conversion of light and fast dark matter into photons. Such models are excluded at 2-3sigma in our analysis.

The findings not only deepen our understanding of dark matter but also provide further clues for solving longstanding cosmic mysteries. By ruling out certain dark matter scenarios, scientists can now focus their efforts on more promising avenues.

 “One of the biggest next steps will focus on obtaining more high-quality spectra of distant quasars, with high resolution instruments such as VLT/UVES and Keck/HIRES. This will also be complemented with data from large spectroscopic surveys such as DESI and WEAVE in which we are actively involved. At the same time, we will be looking to improve on our modelling techniques, in particular related to the hydrodynamic response of small filamentary structures in our simulations,” said Irsic.


 Iršič V. et al. 'Unveiling dark matter free streaming at the smallest scales with the high redshift Lyman-alpha forest', Physical Review D, Feb. 2024; DOI: 10.1103/PhysRevD.109.043511

Image:  The 96 million light years slices through our computer simulations of the Universe showing the gas density distribution for different dark matter models including a “cold” dark matter model (CDM; top left), and several “warm” dark matter (WDM) models varying the particle mass from 4 to 2 keV. The lower the WDM particle mass the smoother the gas density in the simulations. The results are shown at a time when the Universe had an age of 1.4 billion years, roughly 10 % its current age. | Credit: Vid Irsic, Senior Kavli Fellow, Cavendish Laboratory