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Cosmological Structure Formation in the Multiverse

Periodic Reporting for period 3 - COSFORM (Cosmological Structure Formation in the Multiverse)

Reporting period: 2018-10-01 to 2020-03-31

The programme of research in the COSFORM grant is directed at the outstanding puzzle of modern cosmology: the strangely small non-zero value of the vacuum density, which is at least 60 powers of 10 smaller than the expected zero-point contribution of quantum fields. This puzzle can be approached in three ways: (1) Evolution; (2) Revision of gravity; (3) Observer selection in the multiverse. In the first case, we allow for the possibility that the density of the vacuum is not just Einstein's 'cosmological constant', but is something that can change with time (termed 'dark energy'). But perhaps dark energy is an illusion: we infer its existence because the universe expands at a rate that accelerates, but it could be that Einstein's relativistic theory of gravity needs to be replaced by something else.

The first two of these targets can be addressed by ongoing and future large galaxy surveys. Part of the research programme is directed at new ways of assuring robust measurements from these surveys of the evolution of the dark energy density and the growth rate of density fluctuations. But so far such tests show no deviation from standard gravity and a cosmological constant, Lambda.

This fact drives interest in a multiverse solution, in which different causally disconnected domains may be able to possess different effective cosmological constants. Such a multiverse arises from the bubbles predicted in some 'inflationary' cosmological models in which the early universe has its expansion driven by a 'scalar field' - a cousin of the Higgs field detected at CERN in 2014.

The astrophysically interesting aspect of this approach is to ask how galaxy formation would be affected by different levels of vacuum energy. Maybe large vacuum densities are natural, but cannot be observed as they would not permit the creation of observers? Such a question has previously been addressed only by oversimplified analytic arguments, and there are many reasons for attempting a more realistic treatment, not least because it is important to see if the predicted exponential sensitivity of galaxy formation efficiency to Lambda holds up. In any case, there is much of interest to be learned regarding the robustness of current theories of galaxy formation by 'stress-testing' them outside the rather restricted parameter regimes normally considered.

The state of affairs summarized above motivates two distinct strands of future research:

• 1: Exploitation of new galaxy redshift surveys, and planning for future surveys. The need here is to develop new methods for testing robustness of measurements of density fluctuations. The intention is to investigate these fundamental-cosmology signatures using the galaxy population split according to its geometrical environment within the cosmic web.

• 2: Theoretical investigation of galaxy formation in the multiverse. This will involve semianalytic modelling of the long-term history of star formation, as well as more detailed hydrodynamical simulations, to act as a cross-check on the semianalytic results and to understand in more detail how the formation of cosmic structure is expected to proceed in non-standard cosmologies.
The majority of the work performed so far has concentrated on mapping and measuring the 'cosmic web' - the network of filaments within which galaxies form. In local regions of the universe, it is possible to survey galaxies over the whole sky. This was done by combining the optical SuperCOSMOS data with infrared results from the WISE satellite. The optical-to-IR data give a rough measure of galaxy redshift (proportional to distance, according to Hubble's law), so the distribution can be cut into 'tomographic' slices, revealing the cosmic web at different distances. But for best accuracy, spectroscopic redshifts are required, and this is the basis for the second picture. This is from the VImos Public Extragalactic Redshift Survey (VIPERS), which was a major project of the European Southern Observatory's 8m telescopes in Chile.

With such data, many statistical analyses are possible. The most important concern the impact of 'peculiar velocities' - motion cause by the growth of the cosmic web. This causes anisotropic clustering that can be used to measure the amplitude of the velocities and hence the strength of gravity. We have measured this anisotropic signature of cosmic voids in redshift space. Other analyses of redshift-space distortions are under way using existing data (the GAMA survey, in particular), and focusing on the cross-correlation between galaxies and groups. A large part of this effort lies in developing infrastructure for generating appropriate mock surveys for the various datasets being analysed by the team.

The other part of the effort has concentrated on modelling galaxy formation. Part of this is 'semianalytic' - applying simple physical arguments to estimate how gas will behave in the gravitational field of dark matter - but other parts of the work are fully numerical: implementing the physics of star formation while solving cosmological hydrodynamics. The focus is on the long-term efficiency of star formation, while varying the cosmological parameters. The interest will be to see in what circumstances global star formation terminates (as happens in the real universe before it becomes Lambda dominated). In particular, we have used the ENZO cosmological hydrodynamical code to estimate the future of galaxies like the Milky Way (which host most of the stars in the real universe).
Many exciting developments can be foreseen in the second half of the ERC project:

(1) Major new datasets will become available. The DESI project (Dark Energy Spectroscopic Instrument) will start taking data in 2019, building up spectroscopic catalogues that are 10 times larger than those currently available. We will continue to use simulations to develop analysis tools, ready for science as soon as observations begin. In the meantime, the final eBOSS data from the SDSS-IV survey will become available.

(2) We will expand our programme of modelling redshift-space distortions in higher precision, in order to be sure that statistical measurements are correct and unbiased. An interesting question here is to what extent the results are limited by the astrophysics of galaxy formation. As galaxies form, energy is returned to the surrounding gas via supernovae and black holes (this process is termed 'feedback'). We know already that such effects can separate gas from dark matter on small scales, leading the overall degree of inhomogeneity to change on scales of typical intergalaxy separations (1 Mpc). What has not yet been studied is whether different amounts of feedback also alter the redshift-space distortion signature.

(3) The more challenging part of the simulation work will be to extend the Milky Way study to galaxies of all masses, and with different cosmological parameters. Limited computing time means that one can only consider a limited set of models, and for a limited period into the future. The key will be to use the semianalytic methods to see how well they can reproduce the direct calculations, where these exist, and then use semianalytic arguments to address the parameter regimes that cannot be accessed directly. Will the results favour a strong role for observer selection in determining the cosmology that we experience? Everything is open at present.
The positions in space of galaxies identified by the VIPERS survey.
The all-sky galaxy distribution in different radial slices of the WIxSC survey.