102 research outputs found
How Many Universes Do There Need To Be?
In the simplest cosmological models consistent with General Relativity, the
total volume of the Universe is either finite or infinite, depending on whether
or not the spatial curvature is positive. Current data suggest that the
curvature is very close to flat, implying that one can place a lower limit on
the total volume. In a Universe of finite age, the "particle horizon" defines
the patch of the Universe which is observable to us. Based on today's best-fit
cosmological parameters it is possible to constrain the number of observable
Universe sized patches, N_U. Specifically, using the new WMAP data, we can say
that there are at least 21 patches out there the same volume as ours, at 95%
confidence. Moreover, even if the precision of our cosmological measurements
continues to increase, density perturbations at the particle horizon size limit
us to never knowing that there are more than about 10^5 patches out there.Comment: 5 pages, 1 figure; received "honourable mention" in 2006 GRF essay
contest; v2: improved analysis with newly available WMAP Monte Carlo Markov
Chain; version published in IJMP
Testing physical models for dipolar asymmetry with CMB polarization
The cosmic microwave background (CMB) temperature anisotropies exhibit a
large-scale dipolar power asymmetry. To determine whether this is due to a
real, physical modulation or is simply a large statistical fluctuation requires
the measurement of new modes. Here we forecast how well CMB polarization data
from \Planck\ and future experiments will be able to confirm or constrain
physical models for modulation. Fitting several such models to the \Planck\
temperature data allows us to provide predictions for polarization asymmetry.
While for some models and parameters \Planck\ polarization will decrease error
bars on the modulation amplitude by only a small percentage, we show,
importantly, that cosmic-variance-limited (and in some cases even \Planck)
polarization data can decrease the errors by considerably better than the
expectation of based on simple -space arguments. We project
that if the primordial fluctuations are truly modulated (with parameters as
indicated by \Planck\ temperature data) then \Planck\ will be able to make a
2 detection of the modulation model with 20--75\% probability,
increasing to 45--99\% when cosmic-variance-limited polarization is considered.
We stress that these results are quite model dependent. Cosmic variance in
temperature is important: combining statistically isotropic polarization with
temperature data will spuriously increase the significance of the temperature
signal with 30\% probability for \Planck.Comment: 18 pages, 11 figures, 2 tables. Version updated to match PRD versio
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