Periodic Reporting for period 1 - BEHOMO (Cosmology Beyond Homogeneity and Isotropy)
Reporting period: 2020-09-01 to 2022-02-28
According to the standard model of cosmology – confirmed by the latest results from ESA’s Planck mission – about 5% of the energy content of the universe is made of ordinary baryons, that is, of particles belonging to the standard model of particle physics, recently glorified by the discovery of the Higgs boson. The dark sector accounts for the remaining 95%. More precisely, roughly 25% consists of a yet-undetected matter component, which is thought to be a massive particle of non-baryonic nature that interacts through weak interaction and gravity only. It is dubbed "cold dark matter." Finally, dark energy is responsible for the missing 70% of the energy content. The best candidate for dark energy to date is the so-called "cosmological constant" Λ, which is basically the energy of the vacuum and, in general relativity, is an arbitrary constant of nature. Its fundamental property – gravitational repulsion – causes the acceleration of the expansion of the universe. This is the ΛCDM model.
Although phenomenologically successful, the ΛCDM model relies on the poorly understood dark energy and dark matter for observations to fit the model: one cannot deny that cosmology itself is at the moment built on shaky foundations. This provides the main motivation for testing basic principles in cosmology, such as the assumption of homogeneity and isotropy. These are indeed among the objectives of many major space- and earth-based international collaborations, which will produce detailed maps of a good fraction of the observable universe. Moreover, several anomalous signals in cosmological observables have been emerging, possibly challenging the foundations of the standard model of cosmology. Particularly relevant here are the Hubble crisis, the CMB anomalies and the cosmic dipoles.
It is generally assumed that the universe is statistically homogeneous on very large scales. Consequently, cosmologists often adopt the homogeneous and isotropic FLRW metric to model the spacetime of the universe, on top of which perturbations are added. These perturbations come from inflation and generate, via gravity, the cosmic web of voids, walls, filaments and clusters that we see in the large-scale structure of the universe.
The goal of the BEHOMO project is to study, via the methods of numerical cosmology, the evolution of the large-scale structure on an inhomogeneous background. The basic idea is to add non-standard large-scale perturbations on top of the FLRW metric, and understand how cosmological structures evolve and how the corresponding observables are modified in an inhomogeneous universe. This will allow us to constrain, with present and future data, deviations from large-scale homogeneity and isotropy and place the standard model of cosmology on more solid grounds. Testing basic hypotheses in cosmology is essential if we want to make progress at the fundamental level. Indeed, the ability of discovering the nature of dark matter and dark energy is tightly connected to the scientific paradigm one adopts and it is crucial to test the current FLRW paradigm.
The BEHOMO project produced a suite of simulations for the simplest possible inhomogeneous cosmologies: the spherically symmetric ΛLTB models. We used the Lemaître-Tolman-Bondi (LTB) metric to model a spherical inhomogeneity on top of the standard ΛCDM model. Though still a toy model, on a first approximation, one may regard the inhomogeneity of the ΛLTB model as an archetype for more realistic structures. We obtained a set of high-resolution simulations with varying inhomogeneity size and depth, the two main physical parameters describing such a structure. This is the first set of simulations of the ΛLTB model ever produced. These Newtonian N-body simulations can perfectly reproduce the general relativistic evolution even for deep and large (Hubble-sized) inhomogeneities.
Although phenomenologically successful, the ΛCDM model relies on the poorly understood dark energy and dark matter for observations to fit the model: one cannot deny that cosmology itself is at the moment built on shaky foundations. This provides the main motivation for testing basic principles in cosmology, such as the assumption of homogeneity and isotropy. These are indeed among the objectives of many major space- and earth-based international collaborations, which will produce detailed maps of a good fraction of the observable universe. Moreover, several anomalous signals in cosmological observables have been emerging, possibly challenging the foundations of the standard model of cosmology. Particularly relevant here are the Hubble crisis, the CMB anomalies and the cosmic dipoles.
It is generally assumed that the universe is statistically homogeneous on very large scales. Consequently, cosmologists often adopt the homogeneous and isotropic FLRW metric to model the spacetime of the universe, on top of which perturbations are added. These perturbations come from inflation and generate, via gravity, the cosmic web of voids, walls, filaments and clusters that we see in the large-scale structure of the universe.
The goal of the BEHOMO project is to study, via the methods of numerical cosmology, the evolution of the large-scale structure on an inhomogeneous background. The basic idea is to add non-standard large-scale perturbations on top of the FLRW metric, and understand how cosmological structures evolve and how the corresponding observables are modified in an inhomogeneous universe. This will allow us to constrain, with present and future data, deviations from large-scale homogeneity and isotropy and place the standard model of cosmology on more solid grounds. Testing basic hypotheses in cosmology is essential if we want to make progress at the fundamental level. Indeed, the ability of discovering the nature of dark matter and dark energy is tightly connected to the scientific paradigm one adopts and it is crucial to test the current FLRW paradigm.
The BEHOMO project produced a suite of simulations for the simplest possible inhomogeneous cosmologies: the spherically symmetric ΛLTB models. We used the Lemaître-Tolman-Bondi (LTB) metric to model a spherical inhomogeneity on top of the standard ΛCDM model. Though still a toy model, on a first approximation, one may regard the inhomogeneity of the ΛLTB model as an archetype for more realistic structures. We obtained a set of high-resolution simulations with varying inhomogeneity size and depth, the two main physical parameters describing such a structure. This is the first set of simulations of the ΛLTB model ever produced. These Newtonian N-body simulations can perfectly reproduce the general relativistic evolution even for deep and large (Hubble-sized) inhomogeneities.
The main result of the BEHOMO project is the BEHOMO suite of LLTB N-body simulations described before. We validated the background evolution given by the Newtonian N-body simulations against the general relativistic solution. The large-scale structure of the corresponding ΛCDM simulation has also been validated.
The data products consist of 11 snapshots between redshift 0 and 3.7 for each of the 68 simulations that have been performed, together with halo catalogs and lens planes relative to 21 snapshots, between redshift 0 and 4.2 for a total of approximately 180 TB of data. Data is stored at the data center of the Astronomical Observatory of Trieste and at the Italian Astronomical Archive. It can be obtained upon request. For more informations, see Marra etal arXiv:2203.04009.
Intermediate and final results have been presented at institute seminars and journal clubs, in the pre-print Marra etal arXiv:2203.04009 and in the project website https://valerio-marra.github.io/BEHOMO-project/(opens in new window)
The data products consist of 11 snapshots between redshift 0 and 3.7 for each of the 68 simulations that have been performed, together with halo catalogs and lens planes relative to 21 snapshots, between redshift 0 and 4.2 for a total of approximately 180 TB of data. Data is stored at the data center of the Astronomical Observatory of Trieste and at the Italian Astronomical Archive. It can be obtained upon request. For more informations, see Marra etal arXiv:2203.04009.
Intermediate and final results have been presented at institute seminars and journal clubs, in the pre-print Marra etal arXiv:2203.04009 and in the project website https://valerio-marra.github.io/BEHOMO-project/(opens in new window)
The BEHOMO suite of LLTB N-body simulations is the first set of simulations of the ΛLTB model ever produced, considerably improving the state of the art in the field of inhomogeneous cosmology. As mentioned before, there is increasing interest in testing basic assumptions in cosmology, such as large-scale homogeneity and isotropy, and this project has opened the era of precision cosmology for the field of inhomogeneous cosmologies, on par with the standard model or other alternative models such as modified gravity or coupled dark energy. The results of the BEHOMO project are expected to trigger new research. Besides the projects already in development, we expect to start fruitful collaborations in order to exploit the richness of the BEHOMO suite of simulations and produce tools to accurately model the growth of perturbations in inhomogeneous models.
It is not easy to define a metric to assess the societal implications of a project on fundamental research like BEHOMO. Nevertheless, there is no doubt that testing the basic assumptions on which our understanding of the Universe is based has far-reaching cultural implications that go beyond research in cosmology.
It is not easy to define a metric to assess the societal implications of a project on fundamental research like BEHOMO. Nevertheless, there is no doubt that testing the basic assumptions on which our understanding of the Universe is based has far-reaching cultural implications that go beyond research in cosmology.