2,392 research outputs found
Quintessence dynamics with two scalar fields and mixed kinetic terms
The dynamical properties of a model of dark energy in which two scalar fields
are coupled by a non-canonical kinetic term are studied. We show that overall
the addition of the coupling has only minor effects on the dynamics of the
two-field system for both potentials studied, even preserving many of the
features of the assisted quintessence scenario. The coupling of the kinetic
terms enlarges the regions of stability of the critical points. When the
potential is of an additive form, we find the kinetic coupling has an
interesting effect on the dynamics of the fields as they approach the
inflationary attractor, with the result that the combined equation of state of
the scalar fields can approach -1 during the transition from a matter dominated
universe to the recent period of acceleration.Comment: 14 pages, 9 figures. Includes a extra section describing the case of
a field-dependent coupling with two additional figures and minor changes to
match published versio
Using the GCA 4800 DSW Wafer Stepper as a Photorepeater for the Fabrication of Chrome and Aluminum Masks
Utilization of the GCA DSW 4800 as a PHOTOREPEATER enables the production of masks with flexibility, resolution, speed, and precision. A new chuck directly interchangeable with a standard GCA 100 Millimeter wafer chuck was machined for simple conversion from STEPPER to PHOTOREPEATER. Commercial Chrome plates and Quartz plates with thermally evaporated Aluminum films were coated with KTI-820, exposed, and etched. Chrome film resolved to 1 micron and the Etched Aluminum resolved to 3 microns
Construction of asymptotically good low-rate error-correcting codes through pseudo-random graphs
A novel technique, based on the pseudo-random properties of certain graphs known as expanders, is used to obtain novel simple explicit constructions of asymptotically good codes. In one of the constructions, the expanders are used to enhance Justesen codes by replicating, shuffling, and then regrouping the code coordinates. For any fixed (small) rate, and for a sufficiently large alphabet, the codes thus obtained lie above the Zyablov bound. Using these codes as outer codes in a concatenated scheme, a second asymptotic good construction is obtained which applies to small alphabets (say, GF(2)) as well. Although these concatenated codes lie below the Zyablov bound, they are still superior to previously known explicit constructions in the zero-rate neighborhood
New interactions in the dark sector mediated by dark energy
Cosmological observations have revealed the existence of a dark matter sector, which is commonly assumed to be made up of one particle species only. However, this sector might be more complicated than we currently believe: there might be more than one dark matter species (for example, two components of cold dark matter or a mixture of hot and cold dark matter) and there may be new interactions between these particles. In this paper we study the possibility of multiple dark matter species and interactions mediated by a dark energy field. We study both the background and the perturbation evolution in these scenarios. We find that the background evolution of a system of multiple dark matter particles (with constant couplings) mimics a single fluid with a time-varying coupling parameter. However, this is no longer true on the perturbative level. We study the case of attractive and repulsive forces as well as a mixture of cold and hot dark matter particles
Racetrack inflation with matter fields and cosmic strings
We consider the coupling of racetrack inflation to matter fields as realized in the D3/D7 brane system. In particular, we investigate the possibility of cosmic string formation in this system. We find that strings can form before or at the onset of racetrack inflation is possible, but they are then inflated away. Furthermore, string formation at the end of inflation is prevented by the presence of the moduli sector. As a consequence, no strings survive racetrack inflation
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