A New Way to Probe the Early Universe

Astronomers have long sought to probe deep into our universe’s early history. What was the nature of matter back then? How did small galactic seeds grow into the gas-siphoning monsters we see today, and what was the nature of the mysterious substance that weighs down their halos yet eludes our earthly detectors? A team of astronomers may have uncovered a new tool that will allow us to probe this mysterious matter on smaller scales than ever before. 

Image showing galaxies of all shapes and sizes scattered on the sky

The Hubble eXtreme Deep Field, which shows galaxies from when the universe was just 500 million years old. [NASA, ESA, G. Illingworth, D. Magee, and P. Oesch, R. Bouwens, and the HUDF09 Team]

Peering Back into the Universe’s History 

One of the key tasks of modern astronomy has been to understand the early universe and how it evolved to get to the state it’s in today. The Hubble Space Telescope took us back to when the universe was just 500 million years old, and the Planck mission allowed us to peer back at the universe when it was just 380,000 years old, using the cosmic microwave background radiation (CMB) (light from the very early universe that’s been stretched to the microwave regime as the universe has expanded). One of the keys to understanding the early universe is understanding how both ordinary matter and dark matter behaved at this time.  

A clue to how dark matter acts on small scales might be found in the dark matter halos surrounding galaxies in the early universe. These dark matter halos were much less massive than those that surround galaxies today, so probing these halos in the early universe would provide us with a new window to look at dark matter on smaller scales and could help us understand the nature of this mysterious substance that pervades our cosmos. 

1+z (redshift) vs. k (wavenumber) showing the range that CMB lensing, CMB, LRG, Cosmic shear, MW Sat. Lyman-alpha, this work, and 21 cm probe (also showing the corresponding halo masses at redshift z = 0 on the y-axis)

Different areas probed by different experiments, showing redshift against wavenumber, which characterizes the spatial scale explored with the measurements. [Sabti et al. 2022]

Probing Dark Matter on Small Scales 

A group of scientists led by Nashwan Sabti from King’s College London has used a decade of observations from the Hubble Space Telescope to study dark matter at very small scales, looking at distant galaxies and their halos using a method complementary to the range of local probes and the CMB. The team first determined the ultraviolet galaxy luminosity function (UV LF), which captures the abundance of galaxies as a function of their UV luminosity. Because the UV LF is dependent on the mass distribution of dark matter halos, this technique allowed the authors to indirectly probe how dark matter is distributed on different scales during this early period in the universe’s history, revealing clues as to how the early structure of our universe formed and evolved. 

Power spectrum plotted as a function of wavenumber, showing a curve that starts ~10^3 Mpc^3 on the y-axis (10^-4 wavenumber), peaks around a power spectrum of 10^5 and wavenumber 10^-2, and then falls off

Power spectrum as a function of wavenumber k for seven different works. Wavenumber is a measure of the spatial scale, and the matter power spectrum indicates what the matter density perturbations look like on any given scale. The results of this study (black crosses) are plotted along with previous measurements, showing that this work probes smaller scales (larger k) than any other experiment has before. [Sabti et al. 2022]

Using the Power of a Wide Range of Measurements 

The authors’ UV LF measurements cover a wide range, from when the universe was 48 million years old all the way up to 156 million years old, and probe scales beyond what the CMB allows us to explore. The authors model the resulting matter power spectrum — a measure of how matter clusters on different spatial scales — with different parameters to test a range of theoretical models describing dark matter. The team found that their modeled power spectra were consistent with the theoretical predictions of the lambda cold dark matter model of cosmology (the standard model of the universe) up to a certain point. The power spectra disfavor other models, such as the warm dark matter model, which doesn’t predict structure consistent with what the team found on small scales.  

These new results show that measuring the UV LF is a unique, powerful technique for probing the nature of dark matter. The newly launched JWST and the Nancy Grace Roman Space Telescope, which is set to launch in mid-2027, will observe galaxies farther back in the universe’s history and probe dark matter halos on smaller scales, making this an exciting time for dark matter astronomers! 


“New Roads to the Small-scale Universe: Measurements of the Clustering of Matter with the High-redshift UV Galaxy Luminosity Function,” Nashwan Sabti et al 2022 ApJL 928 L20. doi:10.3847/2041-8213/ac5e9c