Cosmologia de Universos Emergentes de Mundos-Brana.

Consideramos diferentes problemas ligados a modelos cosmológicos do tipo mundo- brana com uma (quinta) dimensão, não-compacta, a mais. Investigamos as opções para a introdução de matéria de origem geométrica no nosso universo, realizado como uma 3-brana imersa no espaço-tempo cinco-dimensional criado por um Buraco-Negro em AdS5. Começamos com a ação de Einstein com constante cosmológica negativa e com a 3-brana, posicionada perto da borda do espaço, cuja geometria induzida é a de uma cosmologia de FRW. Depois adicionamos termos descrevendo gases de 1- e 2-branas imersas dentro da 3-brana original. Mostramos que a densidade do fluido efetivo da matéria neste modelo tem os elementos do modelo cosmológico padrão, mas inclui dois novos termos: um gás de cordas cósmicas e um gás de branas. O problema principal estudado nessa dissertação é a descrição detalhada das possíveis evoluções dos universos, dependendo crucialmente das condições iniciais que são determinadas pelas relações entre os parâmetros cosmológicos. Mesmo que a natureza da densidade do fluido efetivo permaneça intacta, nós estabelecemos, nos casos mais simples (na ausência de poeira), que existem onze diferentes tipos de evolução do universo. Os métodos usados para a descrição da evolução dos universos e a descrição das mudanças (transições de fase) entre eles, são baseados no estudo de sistemas dinâmicos equivalentes às equações de Friedmann ou, no caso particular de radiação e constante cosmológica, na forma explícita das soluções exatas. Em todos os casos, a origem das diferenças entre as classes de soluções que encontramos (e da transição de fase entre elas) consiste na existência de pontos e linhas fixas no diagrama planar do sistema dinâmico, devido às diferenças entre as condições iniciais e à forma do potencial escalar na descrição equivalente do fluido efetivo

Small Angle Neutron Scattering from Nanodroplet Aerosols

We report the first measurements of small angle neutron scattering from an aerosol. The aerosol was produced by expanding a D2O-N2 vapor mixture in a supersonic Laval nozzle. The neutron wavelength (0.5 nm) is less than the typical particle size, and we can therefore derive the average particle size (5-8 nm), number density (~1012 cm-3), and polydispersity of the size distribution directly from the experimental data rather than by inferring them from complex models of particle formation and growth. We also predict and observe a Doppler shift-induced anisotropy in the scattering pattern due to the directed motion of the aerosol in the nozzle. Further applications of this new technique are discussed

Doppler Shift Anisotropy in Small Angle Neutron Scattering

The two-dimensional patterns in our small angle neutron scattering (SANS) experiments from rapidly moving aerosols are anisotropic. To test the kinematic theory of two-body scattering that describes the anisotropy, we conducted SANS experiments using a constant source of D2O aerosol with droplets moving at ~440 m/s, and varied the neutron velocity from 267 to 800 m/s. The theoretically predicted anisotropy of the laboratory scattering intensities agrees well with the experimental results. Based on an analysis of the scattering intensity in the Guinier region, we also determined the particle velocity. The results are in very good agreement with independent velocity estimates based on supersonic flow measurements

Boundary Effects in Chiral Polymer Hexatics

Boundary effects in liquid-crystalline phases can be large due to long-ranged orientational correlations. We show that the chiral hexatic phase can be locked into an apparent three-dimensional N+6 phase via such effects. Simple numerical estimates suggest that the recently discovered "polymer hexatic" may actually be this locked phase.Comment: 4 pages, RevTex, 3 included eps figure

Interfaces of Modulated Phases

Numerically minimizing a continuous free-energy functional which yields several modulated phases, we obtain the order-parameter profiles and interfacial free energies of symmetric and non-symmetric tilt boundaries within the lamellar phase, and of interfaces between coexisting lamellar, hexagonal, and disordered phases. Our findings agree well with chevron, omega, and T-junction tilt-boundary morphologies observed in diblock copolymers and magnetic garnet films.Comment: 4 page

A twist in chiral interaction between biological helices

Using an exact solution for the pair interaction potential, we show that long, rigid, chiral molecules with helical surface charge patterns have a preferential interaxial angle ~((RH)^1/2)/L, where L is the length of the molecules, R is the closest distance between their axes, and H is the helical pitch. Estimates based on this formula suggest a solution for the puzzle of small interaxial angles in a-helix bundles and in cholesteric phases of DNA.Comment: 7 pages, 2 figures, PDF file onl

Fluctuation spectrum of fluid membranes coupled to an elastic meshwork: jump of the effective surface tension at the mesh size

We identify a class of composite membranes: fluid bilayers coupled to an elastic meshwork, that are such that the meshwork's energy is a function $F_\mathrm{el}[A_\xi]$ \textit{not} of the real microscopic membrane area $A$, but of a \textit{smoothed} membrane's area $A_\xi$, which corresponds to the area of the membrane coarse-grained at the mesh size $\xi$. We show that the meshwork modifies the membrane tension $\sigma$ both below and above the scale $\xi$, inducing a tension-jump $\Delta\sigma=dF_\mathrm{el}/dA_\xi$. The predictions of our model account for the fluctuation spectrum of red blood cells membranes coupled to their cytoskeleton. Our results indicate that the cytoskeleton might be under extensional stress, which would provide a means to regulate available membrane area. We also predict an observable tension jump for membranes decorated with polymer "brushes"