Minkowski Vacuum Stress Tensor Fluctuations

We study the fluctuations of the stress tensor for a massless scalar field in two and four-dimensional Minkowski spacetime in the vacuum state. Covariant expressions for the stress tensor correlation function are obtained as sums of derivatives of a scalar function. These expressions allow one to express spacetime averages of the correlation function as finite integrals. We also study the correlation between measurements of the energy density along a worldline. We find that these measurements may be either positively correlated or anticorrelated. The anticorrelated measurements can be interpreted as telling us that if one measurement yields one sign for the averaged energy density, a successive measurement with a suitable time delay is likely to yield a result with the opposite sign.Comment: 24 pages, 5 figures; Some additional comments added in Sect. IIB and a more compact argument given in App.

Structurally constrained protein evolution: results from a lattice simulation

We simulate the evolution of a protein-like sequence subject to point mutations, imposing conservation of the ground state, thermodynamic stability and fast folding. Our model is aimed at describing neutral evolution of natural proteins. We use a cubic lattice model of the protein structure and test the neutrality conditions by extensive Monte Carlo simulations. We observe that sequence space is traversed by neutral networks, i.e. sets of sequences with the same fold connected by point mutations. Typical pairs of sequences on a neutral network are nearly as different as randomly chosen sequences. The fraction of neutral neighbors has strong sequence to sequence variations, which influence the rate of neutral evolution. In this paper we study the thermodynamic stability of different protein sequences. We relate the high variability of the fraction of neutral mutations to the complex energy landscape within a neutral network, arguing that valleys in this landscape are associated to high values of the neutral mutation rate. We find that when a point mutation produces a sequence with a new ground state, this is likely to have a low stability. Thus we tentatively conjecture that neutral networks of different structures are typically well separated in sequence space. This results indicates that changing significantly a protein structure through a biologically acceptable chain of point mutations is a rare, although possible, event.Comment: added reference, to appear on European Physical Journal

Estimating Poverty for Indigenous Groups in Chile by Matching Census and Survey Data

It is widely held that indigenous Chileans experience greater rates of poverty and indigence than non-indigenous Chileans, yet the evidence to date has been based on surveys that are not representative by ethnicity. In this paper, we use poverty mapping methodologies that are typically applied to geography to develop statistically precise estimates of poverty, indigence, poverty gaps, and indigence gaps for each of the eight indigenous groups recognized by Chilean law. We find that indigenous people experience higher rates of poverty and indigence and greater depth of poverty and indigence than non-indigenous people. These results hold within individual regions, suggesting that the differential access to economic opportunities in different parts of the country cannot fully explain the results. We also find that the burden of poverty is not shared equally across indigenous groups. Instead, the Mapuche and Aymar· experience disproportionately high poverty rates. We argue that including ethnicity in criteria for identifying poor households may help policy-makers to improve antipoverty targeting.http://deepblue.lib.umich.edu/bitstream/2027.42/64360/1/wp932.pd