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Markov Chain Monte Carlo reconstruction of the large-scale structure with the Lyman alpha forest

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Understanding the space in Space

An EU-funded project introduced a new approach to analysis of the intergalactic structure on both small- and large-scales.

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How the large scale structure (LSS) of the Universe evolved and understanding the nature of the 'space' between galaxies (the intergalactic medium or IGM) are two fundamental issues in cosmology today. When cosmologists look at the distribution of matter at large distances across the observable Universe, they are observing structures in the distant past. This enables them to construct a picture of the Universe at a time when structures were younger and closer to the linear regime in which galaxies were actively forming. Non-linear processes have destroyed information about the origin of our Universe over time, but this information is encoded at very large scales where structures remain close to the original linear regime. To get to this information, the hydrogen absorption spectra of the IGM can be used. This requires an extremely luminous source to radiate the IGM and make it visible to the Earth-bound observer. Such objects are called quasars: believed to be galaxies with an active galactic centre. The ultraviolet radiation emitted by a quasar is scattered by hydrogen in the IGM. This results in a loss of signal and the measurement of multiple quasar absorption spectra allows the LSS to be traced. However, the explicit relationship between the underlying matter field (i.e. the LSS) and the observed quasar spectra is very complex. The 'Markov chain Monte Carlo reconstruction of the large scale structure with the Lyman alpha forest' (MCMCLYMAN) project developed a novel one-dimensional technique to recover the non-linear density field. A strong correlation was found between the spectral flux density and that of matter. This enabled the project to establish a statistical one-to-one relation between the probability densities of the flux and matter. This new approach reduces a number of problems in the analysis of LSS at both the (relatively) small scale and the larger scale. In addition, further novel reconstruction techniques allowed an accurate solution to be achieved despite the incompleteness of data due to the sparse distribution of quasar spectra. Project outcomes have the potential to allow new insights on the evolution of the early Universe.


Space, cosmology, intergalactic medium, Universe, quasar

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