A topological extension of GR: Black holes induce dark energy

A topological extension of general relativity is presented. The superposition principle of quantum mechanics, as formulated by the Feynman path integral, is taken as a starting point. It is argued that the trajectories that enter this path integral are distinct, despite any quantum uncertainty in geometry, and thus that space-time topology is multiply connected. Specifically, space-time at the Planck scale consists of a lattice of three-tori that facilitates many distinct paths for particles to travel along. To add gravity, mini black holes are attached to this lattice. These mini black holes represent Wheelers quantum foam and result from the fact that GR is not conformally invariant. The stable creation of such mini black holes is found to be caused by the existence of macroscopic (so long-lived) black holes. This connection, by which macroscopic black holes induce mini black holes, is a topological expression of Machs principle. The proposed topological extension of GR can be tested because, if correct, the dark energy density of the universe should be linearly proportional to the total number of macroscopic black holes in the universe at any time. This prediction, although strange, agrees with current astrophysical observations.

Simulations of minor mergers. II. The phase-space structure of thick discs

We analyse the phase-space structure of simulated thick discs that are the result of a significant merger between a disc galaxy and a satellite. Our main goal is to establish what would be the characteristic imprints of a merger origin for the Galactic thick disc. We find that the spatial distribution predicted for thick disc stars is asymmetric, seemingly in agreement with recent observations of the Milky Way thick disc. Near the Sun, the accreted stars are expected to rotate more slowly, to have broad velocity distributions, and to occupy preferentially the wings of the line-of-sight velocity distributions. The majority of the stars in our model thick discs have low eccentricity orbits (in clear reference to the pre-existing heated disc) which gives rise to a characteristic (sinusoidal) pattern for their line of sight velocities as function of galactic longitude. The z-component of the angular momentum of thick disc stars provides a clear discriminant between stars from the pre-existing disc and those from the satellite, particularly at large radii. These results are robust against the particular choices of initial conditions made in our simulations, and thus provide clean tests of the disc heating via a minor merger scenario for the formation of thick discs.

Simulations of minor mergers. I. General properties of thick disks

We present simulations of the formation of thick disks via the accretion of two-component satellites onto a pre-existing thin disk. Our goal is to establish the detailed characteristics of the thick disks obtained in this way, as well as their dependence on the initial orbital and internal properties of the accreted objects. We find that mergers with 10-20% mass of the mass of the host lead to the formation of thick disks whose characteristics are similar, both in morphology as in kinematics, to those observed. Despite the relatively large mass ratios, the host disks are not fully destroyed by the infalling satellites: a remaining kinematically cold and thin component containing ~15-25% of the mass can be identified, which is embedded in a hotter and thicker disk. This may for example, explain the existence of a very old thin disk stars in the Milky Way. The final scale-heights of the disks depend both on the initial inclination and properties of the merger, but the fraction of satellite stellar particles at ~4 scale-heights directly measures the mass ratio between the satellite and host galaxy. Our thick disks typically show boxy isophotes at very low surface brightness levels (>6 magnitudes below their peak value). Kinematically, the velocity ellipsoids of the simulated thick disks are similar to that of the Galactic thick disk at the solar radius. The trend of sigma_Z/sigma_R with radius is found to be a very good discriminant of the initial inclination of the accreted satellite. In the Milky Way, the possible existence of a vertical gradient in the rotational velocity of the thick disk as well as the observed value of sigma_Z/sigma_R at the solar vicinity appear to favour the formation of the thick disk by a merger with either low or intermediate orbital inclination.

Group infall of substructures on to a Milky Way-like dark halo

We report the discovery that substructures/subhaloes of a galaxy-size halo tend to fall in together in groups in cosmological simulations, something that may explain the oddity of the MW satellite distribution. The original clustering at the time of infall is still discernible in the angular momenta of the subhaloes even for events which took place up to eight Gyrs ago, $z sim 1$. This phenomenon appears to be rather common since at least 1/3 of the present-day subhaloes have fallen in groups in our simulations. Hence, this may well explain the Lynden-Bell & Lynden-Bell ghostly streams. We have also found that the probability of building up a flattened distribution similar to the MW satellites is as high as $sim 80%$ if the MW satellites were from only one group and $sim 20%$ when five groups are involved. Therefore, we conclude that the `peculiar distribution of satellites around the MW can be expected with the CDM structure formation theory. This non-random assignment of satellites to subhaloes implies an environmental dependence on whether these low-mass objects are able to form stars, possibly related to the nature of reionization in the early Universe.

Infall of substructures onto a Milky Way-like dark halo

We analyse the dynamical properties of substructures in a high-resolution dark matter simulation of the formation of a Milky Way-like halo in a $Lambda$CDM cosmology. Our goal is to shed light on the dynamical peculiarities of the Milky Way satellites. Our simulations show that about 1/3 of the subhalos have been accreted in groups. We quantify this clustering by measuring the alignment of the angular momentum of subhalos in a group. We find that this signal is visible even for objects accreted up to $z sim 1$, i.e. 8 Gyr ago, and long after the spatial coherence of the groups has been lost due the host tidal field. This group infall may well explain the ghostly streams proposed by Lynden-Bell & Lynden-Bell to orbit the Milky Way. Our analyses also show that if most satellites originate in a few groups, the disk-like distribution of the Milky Way satellites would be almost inevitable. This non-random assignment of satellites to subhalos implies an environmental dependence on whether these low-mass objects are able to form stars, possibly related to the nature of reionization in the early Universe. With this picture, both the ``ghostly streams and the ``disk-like configuration are manifestations of the same phenomenon: the hierarchical growth of structure down to the smallest scales.

Voronoi Tessellations and the Cosmic Web: Spatial Patterns and Clustering across the Universe

The spatial cosmic matter distribution on scales of a few up to more than a hundred Megaparsec displays a salient and pervasive foamlike pattern. Voronoi tessellations are a versatile and flexible mathematical model for such weblike spatial patterns. They would be the natural asymptotic result of an evolution in which low-density expanding void regions dictate the spatial organization of the Megaparsec Universe, while matter assembles in high-density filamentary and wall-like interstices between the voids. We describe the results of ongoing investigations of a variety of aspects of cosmologically relevant spatial distributions and statistics within the framework of Voronoi tessellations. Particularly enticing is the finding of a profound scaling of both clustering strength and clustering extent for the distribution of tessellation nodes, suggestive for the clustering properties of galaxy clusters. Cellular patterns may be the source of an intrinsic ``geometrically biased clustering.