Periodic Reporting for period 4 - COSFORM (Cosmological Structure Formation in the Multiverse)
Período documentado: 2020-04-01 hasta 2021-09-30
The COSFORM grant was 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. Possibly 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; or 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.
But how is galaxy formation 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? At a minimum, there is much of interest to be learned regarding the robustness of current theories of galaxy formation by 'stress-testing' them by making predictions for universes very different to our own.
This motivates two distinct strands of research:
• 1: Exploitation of new galaxy redshift surveys, and planning for future surveys.
• 2: Theoretical investigation of galaxy formation in the multiverse.
This programme has been successfully executed. New measurements have been made of redshift-space clustering distortions, using the VIPERS and SDSS-IV surveys, and the results are consistent with standard gravity. Searches have been made for correlations between galaxy properties and their location in the cosmic web, with a null result. Investigation of the future of cosmic star formation have been completed both by analytic means and by numerical simulation. We have measured how the star-formation efficiency depends on the cosmological constant, showing that it would be heavily suppressed if the cosmological constant was more than 10-100 times its observed value. The multiverse approach to explaining why the cosmological constant is so small therefore seems to have succeeded: we are here because formation of stars and hence of observers would have been extremely unlikely if the cosmological constant were to take the much larger typical value predicted by quantum mechanics.
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.
But how is galaxy formation 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? At a minimum, there is much of interest to be learned regarding the robustness of current theories of galaxy formation by 'stress-testing' them by making predictions for universes very different to our own.
This motivates two distinct strands of research:
• 1: Exploitation of new galaxy redshift surveys, and planning for future surveys.
• 2: Theoretical investigation of galaxy formation in the multiverse.
This programme has been successfully executed. New measurements have been made of redshift-space clustering distortions, using the VIPERS and SDSS-IV surveys, and the results are consistent with standard gravity. Searches have been made for correlations between galaxy properties and their location in the cosmic web, with a null result. Investigation of the future of cosmic star formation have been completed both by analytic means and by numerical simulation. We have measured how the star-formation efficiency depends on the cosmological constant, showing that it would be heavily suppressed if the cosmological constant was more than 10-100 times its observed value. The multiverse approach to explaining why the cosmological constant is so small therefore seems to have succeeded: we are here because formation of stars and hence of observers would have been extremely unlikely if the cosmological constant were to take the much larger typical value predicted by quantum mechanics.
The majority of the work 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 optical ground-based data with infrared results from space. 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.
The most important statistical analyses of such data 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 signature using the GAMA, Vipers, and SDSS-IV surveys; all the results match the predictions of standard gravity. A large part of this effort lies in developing infrastructure for generating appropriate mock surveys for the various datasets being analysed by the team.
We have also focused on understanding the relation of galaxies to dark-matter haloes, asking how different astrophysical tracers of clustering (galaxies with different star-formation rates; quasars) populate the haloes, and whether this population is affected by tidal forces that arise in different locations in the cosmic web. Despite past suggestions that such effects are present, we see no sign of an effect, making modelling of galaxy clustering a simpler task.
The other part of the effort concerns modelling galaxy formation. Part of this is 'semianalytic' - estimating physically how gas will behave in the gravitational field of dark matter - but we can also implement the physics of star formation in numerical hydrodynamical simulations. The focus is on the long-term efficiency of star formation, while varying the cosmological parameters. In three papers using the ENZO cosmological hydrodynamical code, we have shown how cosmic star formation would be suppressed in universes with a larger cosmological constant.
The most important statistical analyses of such data 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 signature using the GAMA, Vipers, and SDSS-IV surveys; all the results match the predictions of standard gravity. A large part of this effort lies in developing infrastructure for generating appropriate mock surveys for the various datasets being analysed by the team.
We have also focused on understanding the relation of galaxies to dark-matter haloes, asking how different astrophysical tracers of clustering (galaxies with different star-formation rates; quasars) populate the haloes, and whether this population is affected by tidal forces that arise in different locations in the cosmic web. Despite past suggestions that such effects are present, we see no sign of an effect, making modelling of galaxy clustering a simpler task.
The other part of the effort concerns modelling galaxy formation. Part of this is 'semianalytic' - estimating physically how gas will behave in the gravitational field of dark matter - but we can also implement the physics of star formation in numerical hydrodynamical simulations. The focus is on the long-term efficiency of star formation, while varying the cosmological parameters. In three papers using the ENZO cosmological hydrodynamical code, we have shown how cosmic star formation would be suppressed in universes with a larger cosmological constant.
(1) The DESI project (Dark Energy Spectroscopic Instrument) started survey observations in 2021, building up spectroscopic catalogues that are 10 times larger than those currently available. We will continue to use simulations to develop analysis tools, based on our success with Vipers & SDSS-IV, ready for science analysis of the first DESI data releases.
(2) We have modelled 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). We have also studied whether these different amounts of feedback can alter the redshift-space distortion signature, and it seems there are subtle effects that could be important in future surveys.
(3) The more challenging part of the simulation work was 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. Currently, the semianalytic results predict larger amounts of star formation than the numerical calculations, for cosmological constants 10-100 times the observed value. Both calculations will require further study to establish a consensus.
(2) We have modelled 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). We have also studied whether these different amounts of feedback can alter the redshift-space distortion signature, and it seems there are subtle effects that could be important in future surveys.
(3) The more challenging part of the simulation work was 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. Currently, the semianalytic results predict larger amounts of star formation than the numerical calculations, for cosmological constants 10-100 times the observed value. Both calculations will require further study to establish a consensus.