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A holistic approach to large-scale structure cosmology

Periodic Reporting for period 2 - BAHAMAS (A holistic approach to large-scale structure cosmology)

Reporting period: 2019-12-01 to 2021-05-31

The standard model of cosmology, which is the framework upon which our understanding of the evolution of the Universe is based, is remarkably successful at explaining a wide range of observations. However, it is now being subjected to much more stringent tests than ever before, and recent large-scale structure (LSS) measurements appear to be in tension with its predictions. Is this tension signalling that new physics is required? For example, time-varying dark energy, or perhaps a modified theory of gravity? A contribution from massive neutrinos? Before coming to such bold conclusions we must be certain that all of the important systematic errors in the LSS tests have been accounted for.

Presently, the largest source of systematic uncertainty is from the modelling of complicated astrophysical phenomena associated with galaxy formation. In particular, energetic feedback processes associated with star formation and black hole growth can heat and expel gas from collapsed structures and modify the large-scale distribution of matter. Furthermore, the LSS field is presently separated into many sub-fields (each using different models, that usually neglect feedback), preventing a coherent analysis.

Cosmological hydrodynamical simulations (are the only method which) can follow all the relevant matter components and self-consistently capture the effects of feedback. With the “BAHAMAS” programme we are leading the development of large-scale simulations with physically-motivated prescriptions for feedback that are unrivalled in their ability to reproduce the observed properties of massive systems. Our team is exploiting these developments to produce a large suite of simulations designed specifically for LSS cosmology applications with the effects of feedback realistically accounted for and which will allow us to unite the different LSS tests. The primary objective is to perform the first self-consistent comparisons with the full range of LSS cosmology tests, and critically assess the evidence for physics beyond the standard model.
The first half of the BAHAMAS programme has been focused on the development of state-of-the-art cosmological hydrodynamical simulations, primarily in the context of the standard model of cosmology. The main midterm objectives are to develop a suite of such simulations to characterise the impact of processes associated with galaxy and black hole formation on various large-scale structure observables (such as galaxy clustering, cosmic shear, galaxy cluster counts, etc.) and to assess whether claimed tensions between constraints on certain cosmological parameters from large-scale structure observations and those of the cosmic microwave background are significant (i.e. cannot be explained by said galaxy formation processes). As of the end of 2020, our group has published 44 articles in top international, peer-reviewed journals that deliver on the midterm goals and nicely set the stage for the second half of the BAHAMAS programme.

A brief summary of the major objectives completed thus far is as follows. We have explored in great detail the impact of processes associated with galaxy formation on a wide variety of large-scale structure observables, concluding that its effects are significant and cannot be ignored in cosmological analyses. Furthermore, we have carefully characterised the potential degeneracies between cosmological parameters and those associated with the modelling of galaxy formation physics (particularly feedback processes). We have demonstrated that what is required to accurately constrain cosmological parameters is a systematic and simultaneous (with the cosmological variations) exploration of the “feedback landscape”.
Current large-scale structure tests of cosmology rely almost exclusively on so-called collisionless (or “gravity only”) simulations that ignore the important role that baryons and processes associated with galaxy formation play. However, ours and other groups have demonstrated that these effects are likely to be significant and cannot be ignored if one wishes to rigorously test models for the evolution of the Universe.

Part of the problem of including their effects is that we do not yet have a complete theory of galaxy formation and there are significant uncertainties in the modelling of feedback processes in particular. The BAHAMAS programme has taken a novel and significant step forward, where instead of using a single model for galaxy formation, we explore the full “feedback landscape”, by varying the parameters associated with the feedback modelling over a wide range of theoretically-plausible values. Through comparisons with observations, we select from this parameter space the range of feedback models that are consistent with the real Universe. This range of feedback models will be incorporated within our large cosmological grid, meaning that one can, for the first time, simultaneously constrain models of galaxy formation and cosmology and rigorously characterise the uncertainties in the parameters and their correlations.

By end of the project we expect to have a number of emulators which will predict various large-scale structure observables for any plausible evolution scenario for the Universe and which rigorously accounts for the uncertainties inherent in models of galaxy formation.
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