# Measurement of the primordial helium abundance from the intergalactic medium

## Abstract

Almost every helium atom in the Universe was created just a few minutes after the Big Bang through a process commonly referred to as Big Bang nucleosynthesis1,2. The amount of helium that was made during Big Bang nucleosynthesis is determined by combining particle physics and cosmology3. The current leading measures of the primordial helium abundance (YP) are based on the relative strengths of H i and He i emission lines emanating from star-forming regions in local metal-poor galaxies4,5,6,7. As the statistical errors on these measurements improve, it is essential to test for systematics by developing independent techniques. Here we report a determination of the primordial helium abundance based on a near-pristine intergalactic gas cloud that is seen in absorption against the light of a background quasar. This gas cloud, observed when the Universe was just one-third of its present age (zabs = 1.724), has a metal content around 100 times less than that of the Sun, and at least 30% less metal content than the most metal-poor H ii region currently known where a determination of the primordial helium abundance is possible. We conclude that the helium abundance of this intergalactic gas cloud is $$Y = 0.250_{ - 0.025}^{ + 0.033}$$, which agrees with the standard model primordial value8,9,10, YP = 0.24672 ± 0.00017. Our determination of the primordial helium abundance is not yet as precise as that derived using metal-poor galaxies, but our method has the potential to offer a competitive test of physics beyond the standard model during Big Bang nucleosynthesis.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. The Keck HIRES data are available as a high-level science product from the W. M. Keck Observatory Data Archive at https://koa.ipac.caltech.edu/cgi-bin/KOA/nph-KOAlogin. The fully reduced HST FOS data are available from https://archive.stsci.edu/prepds/fos_agn/. The reduced HST COS data are available at http://archive.stsci.edu/proposal_search.php?mission=hst&id=13491.

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## Acknowledgements

We thank G. Worseck for discussions about the HST and COS data of this sightline. During this work, R.J.C. was supported by a Royal Society University Research Fellowship. We acknowledge support from STFC (ST/L00075X/1 and ST/P000541/1). This project has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 757535). This work used the DiRAC Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment was funded by BIS National E-Infrastructure capital grant ST/K00042X/1, STFC capital grant ST/H008519/1, STFC DiRAC Operations grant ST/K003267/1 and Durham University. DiRAC is part of the National E-Infrastructure. This research has made use of NASA’s Astrophysics Data System.

## Author information

Both authors participated in the interpretation and commented on the manuscript. R.J.C. led the project and analysis, and was responsible for the text of the paper. M.F. was responsible for the ionization calculations.

Correspondence to Ryan J. Cooke.

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### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Table 1, Supplementary Figures 1–5

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Cooke, R.J., Fumagalli, M. Measurement of the primordial helium abundance from the intergalactic medium. Nat Astron 2, 957–961 (2018). https://doi.org/10.1038/s41550-018-0584-z

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