Fabio Silva

Self-accelerating universe in Galileon cosmology

We present a cosmological model with a solution that self-accelerates at late times without signs of ghost instabilities on small scales. The model is a natural extension of the Brans-Dicke (BD) theory including a nonlinear derivative interaction, which appears in a theory with the Galilean shift symmetry. The existence of the self-accelerating universe requires a negative BD parameter but, thanks to the nonlinear term, small fluctuations around the solution are stable on small scales. General relativity is recovered at early times and on small scales by this nonlinear interaction via the Vainshtein mechanism. At late time, gravity is strongly modified and the background cosmology shows a phantomlike behavior and the growth rate of structure formation is enhanced. Thus this model leaves distinct signatures in cosmological observations and it can be distinguished from standard LCDM cosmology.

Nonlinear interactions in a cosmological background in the Dvali-Gabadadze-Porrati braneworld

We study quasistatic perturbations in a cosmological background in the Dvali-Gabadadze-Porrati braneworld model. We identify the Vainshtein radius at which the nonlinear interactions of the brane bending mode become important in a cosmological background. The Vainshtein radius in the early universe is much smaller than the one in the Minkowski background, but in a self-accelerating universe it is the same as the Minkowski background. Our result shows that the perturbative approach is applicable beyond the Vainshtein radius for weak gravity by taking into account the second-order effects of the brane bending mode. The linearized cosmological perturbations are shown to be smoothly matched to the solutions inside the Vainshtein radius. We emphasize the importance of imposing a regularity condition in the bulk by solving the 5D perturbations and we highlight the problem of ad hoc assumptions on the bulk gravity that lead to different conclusions.

Modelling the Spread of Farming in the Bantu-Speaking Regions of Africa: An Archaeology-Based Phylogeography

We use archaeological data and spatial methods to reconstruct the dispersal of farming into areas of sub-Saharan Africa now occupied by Bantu language speakers, and introduce a new large-scale radiocarbon database and a new suite of spatial modelling techniques. We also introduce a method of estimating phylogeographic relationships from archaeologically-modelled dispersal maps, with results produced in a format that enables comparison with linguistic and genetic phylogenies. Several hypotheses are explored. The ‘deep split’ hypothesis suggests that an early-branching eastern Bantu stream spread around the northern boundary of the equatorial rainforest, but recent linguistic and genetic work tends not to support this. An alternative riverine/littoral hypothesis suggests that rivers and coastlines facilitated the migration of the first farmers/horticulturalists, with some extending this to include rivers through the rainforest as conduits to East Africa. More recently, research has shown that a grassland corridor opened through the rainforest at around 3000–2500 BP, and the possible effect of this on migrating populations is also explored. Our results indicate that rivers and coasts were important dispersal corridors, but do not resolve the debate about a ‘Deep Split’. Future work should focus on improving the size, quality and geographical coverage of the archaeological 14C database; on augmenting the information base to establish descent relationships between archaeological sites and regions based on shared material cultural traits; and on refining the associated physical geographical reconstructions of changing land cover.

Between China and South Asia: A Middle Asian corridor of crop dispersal and agricultural innovation in the Bronze Age

The period from the late third millennium BC to the start of the first millennium AD witnesses the first steps towards food globalization in which a significant number of important crops and animals, independently domesticated within China, India, Africa and West Asia, traversed Central Asia greatly increasing Eurasian agricultural diversity. This paper utilizes an archaeobotanical database (AsCAD), to explore evidence for these crop translocations along southern and northern routes of interaction between east and west. To begin, crop translocations from the Near East across India and Central Asia are examined for wheat (Triticum aestivum) and barley (Hordeum vulgare) from the eighth to the second millennia BC when they reach China. The case of pulses and flax (Linum usitatissimum) that only complete this journey in Han times (206 BC–AD 220), often never fully adopted, is also addressed. The discussion then turns to the Chinese millets, Panicum miliaceum and Setaria italica, peaches (Amygdalus persica) and apricots (Armeniaca vulgaris), tracing their movement from the fifth millennium to the second millennium BC when the Panicum miliaceum reaches Europe and Setaria italica Northern India, with peaches and apricots present in Kashmir and Swat. Finally, the translocation of japonica rice from China to India that gave rise to indica rice is considered, possibly dating to the second millennium BC. The routes these crops travelled include those to the north via the Inner Asia Mountain Corridor, across Middle Asia, where there is good evidence for wheat, barley and the Chinese millets. The case for japonica rice, apricots and peaches is less clear, and the northern route is contrasted with that through northeast India, Tibet and west China. Not all these journeys were synchronous, and this paper highlights the selective long-distance transport of crops as an alternative to demic-diffusion of farmers with a defined crop package.

Modelling the Geographical Origin of Rice Cultivation in Asia Using the Rice Archaeological Database

We have compiled an extensive database of archaeological evidence for rice across Asia, including 400 sites from mainland East Asia, Southeast Asia and South Asia. This dataset is used to compare several models for the geographical origins of rice cultivation and infer the most likely region(s) for its origins and subsequent outward diffusion. The approach is based on regression modelling wherein goodness of fit is obtained from power law quantile regressions of the archaeologically inferred age versus a least-cost distance from the putative origin(s). The Fast Marching method is used to estimate the least-cost distances based on simple geographical features. The origin region that best fits the archaeobotanical data is also compared to other hypothetical geographical origins derived from the literature, including from genetics, archaeology and historical linguistics. The model that best fits all available archaeological evidence is a dual origin model with two centres for the cultivation and dispersal of rice focused on the Middle Yangtze and the Lower Yangtze valleys.

Cosmological perturbations in the DGP braneworld: Numeric solution

We solve for the behavior of cosmological perturbations in the Dvali-Gabadadze-Porrati (DGP) braneworld model using a new numerical method. Unlike some other approaches in the literature, our method uses no approximations other than linear theory and is valid on large scales. We examine the behavior of late-universe density perturbations for both the self-accelerating and normal branches of DGP cosmology. Our numerical results can form the basis of a detailed comparison between the DGP model and cosmological observations.

Modelling the diffusion of pottery technologies across Afro-Eurasia: Emerging insights and future research

Abstract Where did pottery first appear in the Old World? Statistical modelling of radiocarbon dates suggests that ceramic vessel technology had independent origins in two different hunter-gatherer societies. Regression models were used to estimate average rates of spread and geographic dispersal of the new technology. The models confirm independent origins in East Asia (c. 16000 cal BP) and North Africa (c. 12000 cal BP). The North African tradition may have later influenced the emergence of Near Eastern pottery, which then flowed west into Mediterranean Europe as part of a Western Neolithic, closely associated with the uptake of farming.

Ghosts in asymmetric brane gravity and the decoupled stealth limit

We study the spectrum of gravitational perturbations around a vacuum de Sitter brane in a 5D asymmetric braneworld model, with induced curvature on the brane. This generalises the stealth acceleration model proposed by Charmousis, Gregory and Padilla (CGP) which realises the Cardassian cosmology in which power law cosmic acceleration can be driven by ordinary matter. Whenever the bulk has infinite volume we find that there is always a perturbative ghost propagating on the de Sitter brane, in contrast to the Minkowski brane case analysed by CGP. We discuss the implication of this ghost for the stealth acceleration model, and identify a limiting case where the ghost decouples as the de Sitter curvature vanishes.

A Tomb with a View: New Methods for Bridging the Gap between Land and Sky in Megalithic Archaeology

Abstract The orientations of European prehistoric structures have been studied independently by landscape archaeologists and archaeoastronomers. Despite their similar interests, the two fields have failed to converge primarily because of their differing epistemologies. This paper argues that archaeology has much to gain by integrating the two fields to provide a fuller and more balanced exploration and understanding of the location and orientation of the European megaliths. It is suggested that prehistoric archaeoastronomy needs to become more grounded on the archaeological record and context of the prehistoric structures it studies. If it is to generate knowledge of value to archaeology it needs to become a “skyscape archaeology.” This paper looks at current archaeoastronomical approaches through the lens of archaeological practice. It identifies some limitations and discusses how landscape archaeology can inform archaeoastronomy on overcoming them. A methodology that attempts this necessary cross-fertilization, by shedding unfounded assumptions and developing a more phenomenological approach to pattern-recognition, is proposed. This methodology is applied to a case study in central Portugal. The emergent narrative, linking a cluster of dolmens to a local mountain range and the star Aldebaran, not only fits the archaeological record, but is mirrored by local folklore, lending further support to the validity of this methodology.

MODELING BOUNDARIES BETWEEN CONVERGING FRONTS IN PREHISTORY

We introduce a modeling framework that can be applied to cases of multiple converging fronts during episodes of population expansion and innovation diffusion, referring to two prehistoric case studies known archaeologically (the spread of pottery-making in Europe, and the spread of farming in southern Africa). We model front propagation using Fast Marching methods, drawing on the analogy with crystallization processes to build compoundly-weighted Voronoi diagrams of a spatially partitioned surface in which the zones of influence of each competing spreading process are determined by their respective front initiation times and propagation rates. We analyze the phase space for the general two-source case, and illustrate the potential of this approach by modeling the evolving interface for the archaeological case studies.