Sean M. Carroll

SUPERNOVA LIMITS ON THE COSMIC EQUATION OF STATE

We use Type Ia supernovae studied by the High-z Supernova Search Team to constrain the properties of an energy component that may have contributed to accelerating the cosmic expansion. We find that for a flat geometry the equation-of-state parameter for the unknown component, αx = Px/ρx, must be less than -0.55 (95% confidence) for any value of Ωm, and it is further limited to αx < -0.60 (95% confidence) if Ωm is assumed to be greater than 0.1. These values are inconsistent with the unknown component being topological defects such as domain walls, strings, or textures. The supernova (SN) data are consistent with a cosmological constant (αx = -1) or a scalar field that has had, on average, an equation-of-state parameter similar to the cosmological constant value of -1 over the redshift range of z ≈ 1 to the present. SN and cosmic microwave background observations give complementary constraints on the densities of matter and the unknown component. If only matter and vacuum energy are considered, then the current combined data sets provide direct evidence for a spatially flat universe with Ωtot = Ωm + ΩΛ = 0.94 ± 0.26 (1 σ).

QUINTESSENCE AND THE REST OF THE WORLD : SUPPRESSING LONG-RANGE INTERACTIONS

A nearly-massless, slowly-rolling scalar field

Quintessence and the rest of the world

phi

Is there evidence for cosmic anisotropy in the polarization of distant radio sources

may provide most of the energy density of the current universe. One potential difficulty with this idea is that couplings to ordinary matter, even if suppressed by the Planck scale, should lead to observable long-range forces and time dependence of the constants of nature. I explore the possibility that an approximate global symmetry serves to suppress such couplings even further. Such a symmetry would allow a coupling of

Domain wall junctions are 1/4 BPS states

phi

Dark matter with time dependent mass

to the pseudoscalar

Interpreting Epsilon Aurigae

F_{munu}widetilde F^{munu}

Collapse of exotic textures

of electromagnetism, which would rotate the polarization state of radiation from distant sources. This effect is fairly well constrained, but it is conceivable that future improvements could lead to a detection of a cosmological scalar field.

Primordial Magnetic Fields that Last

A nearly-massless, slowly-rolling scalar field φ may provide most of the energy density of the current universe. One potential difficulty with this idea is that couplings to ordinary matter, even if suppressed by the Planck scale, should lead to observable long-range forces and time dependence of the constants of nature. I explore the possibility that an approximate global symmetry serves to suppress such couplings even further. Such a symmetry would allow a coupling of φ to the pseudoscalar FμνFμν of electromagnetism, which would rotate the polarization state of radiation from distant sources. This effect is fairly well constrained, but it is conceivable that future improvements could lead to a detection of a cosmological scalar field. (Based closely on astro-ph/9806099 [1].)

Domain Wall Junctions and 1/4-Supersymmetric States

Measurements of the polarization angle and orientation of cosmological radio sources may be used to search for unusual effects in the propagation of light through the Universe. Recently, Nodland and Ralston [Phys.Rev.Lett.{bold 78}, 3043 (1997)] have claimed to find evidence for a redshift- and direction-dependent rotation effect in existing data. We reexamine these data and argue that there is no statistically significant signal present. We are able to place stringent limits on hypothetical chiral interactions of photons propagating through spacetime. {copyright} {ital 1997} {ital The American Physical Society}