1,849 research outputs found

### How to make maps from CMB data without losing information

The next generation of CMB experiments can measure cosmological parameters
with unprecedented accuracy - in principle. To achieve this in practice when
faced with such gigantic data sets, elaborate data analysis methods are needed
to make it computationally feasible. An important step in the data pipeline is
to make a map, which typically reduces the size of the data set my orders of
magnitude. We compare ten map-making methods, and find that for the Gaussian
case, both the method used by the COBE DMR team and various variants of Wiener
filtering are optimal in the sense that the map retains all cosmological
information that was present in the time-ordered data (TOD). Specifically, one
obtains just as small error bars on cosmological parameters when estimating
them from the map as one could have obtained by estimating them directly from
the TOD. The method of simply averaging the observations of each pixel (for
total-power detectors), on the contrary, is found to generally destroy
information, as does the maximum entropy method and most other non-linear
map-making techniques.
Since it is also numerically feasible, the COBE method is the natural choice
for large data sets. Other lossless (e.g. Wiener-filtered) maps can then be
computed directly from the COBE method map.Comment: Minor revisions to match published version. 12 pages, with 1 figure
included. Color figure and links at
http://www.sns.ias.edu/~max/mapmaking.html (faster from the US), from
http://www.mpa-garching.mpg.de/~max/mapmaking.html (faster from Europe) or
from [email protected]

### Removing point sources from CMB maps

For high-precision cosmic microwave background (CMB) experiments,
contamination from extragalactic point sources is a major concern. It is
therefore useful to be able to detect and discard point source contaminated
pixels using the map itself. We show that the sensitivity with which this can
be done can often be greatly improved (by factors between 2.5 and 18 for the
upcoming Planck mission) by a customized hi-pass filtering that suppresses
fluctuations due to CMB and diffuse galactic foregrounds. This means that point
source contamination will not severely degrade the cleanest Planck channels
unless current source count estimates are off by more than an order of
magnitude. A catalog of around 40,000 far infra-red sources at 857 GHz may be a
useful by-product of Planck.Comment: 4 pages, with 2 figures included. Minor revisions to match accepted
version. Color figure and links at http://www.sns.ias.edu/~max/cleaning.html
(faster from the US), from http://www.mpa-garching.mpg.de/~max/cleaning.html
(faster from Europe) or from [email protected], and Angelica's foreground links at
http://www.sns.ias.edu/~angelica/foreground.htm

### Cosmic Microwave Background Anisotropy Observing Strategy Assessment

I develop a method for assessing the ability of an instrument, coupled with
an observing strategy, to measure the angular power spectrum of the cosmic
microwave background (CMB). It allows for efficient calculation of expected
parameter uncertainties. Related to this method is a means of graphically
presenting, via the ``eigenmode window function'', the sensitivity of an
observation to different regions of the spectrum, which is a generalization of
the traditional practice of presenting the trace of the window function. I
apply these techniques to a balloon-borne bolometric instrument to be flown
this summer (MSAM2). I find that a smoothly scanning secondary is better than a
chopping one and that, in this case, a very simple analytic formula provides a
good (40\% or better) approximation to expected power spectrum uncertainties.Comment: Substantial revisions, LaTeX 15 pages including 3 figure

### Non-Gaussianity in Two-Field Inflation

We derive semi-analytic formulae for the local bispectrum and trispectrum in
general two-field inflation and provide a simple geometric recipe for building
observationally allowed models with observable non-Gaussianity. We use the
\delta N formalism and the transfer function formalism to express the
bispectrum almost entirely in terms of model-independent physical quantities.
Similarly, we calculate the trispectrum and show that the trispectrum parameter
\tau NL can be expressed entirely in terms of spectral observables, which
provides a new consistency relation unique to two-field inflation. We show that
in order to generate observably large non-Gaussianity during inflation, the
sourcing of curvature modes by isocurvature modes must be extremely sensitive
to the initial conditions, and that the amount of sourcing must be moderate in
order to avoid excessive fine-tuning. Under some minimal assumptions, we argue
that the first condition is satisfied only when neighboring trajectories
through the two-dimensional field space diverge during inflation.
Geometrically, this means that the inflaton must roll along a ridge in the
potential V for some time during inflation and that its trajectory must turn
slightly (but not too sharply) in field space. Therefore, it follows that
two-field scenarios with attractor solutions necessarily produce small
non-Gaussianity. This explains why it has been so difficult to achieve large
non-Gaussianity in two-field inflation, and why it has only been achieved in a
narrow class of models like hybrid inflation and certain product potentials
where the potential and/or the initial conditions are fine-tuned. Some of our
conclusions generalize qualitatively to general multi-field inflation.Comment: Discussion improved, gNL formula and extra figure included, typos
corrected, references added. 18 pages, 2 figure

### The angular power spectrum of the 4 year COBE data

The angular power spectrum C_l is extracted from the 4 year COBE DMR data
with a 20 degree galactic cut, using the narrowest window functions possible.
The average power in eight multipole bands is also computed, and plotted
together with a compilation of power spectrum measurements from other
experiments. The COBE results are found to be consistent with an n=1 power
spectrum with the normalization Qrmsps=18 microkelvin reported by the COBE DMR
team. Certain non-standard cosmologies, such as "small universe" models with
nontrivial spatial topology, predict power spectra which are not smooth
functions. Rather, they contain bumps and wiggles that may not have been
detected by other data analysis techniques such as the Hauser-Peebles method
(Wright etal 1996), the band-power method (Hinshaw etal 1996) and
orthogonalized spherical harmonics (Gorski etal 1996), since these methods all
give broader window functions that can smear such features out beyond
recognition. On the large angular scales probed by COBE, the Universe thus
appears to be kind to us, presenting a power spectrum that is a simple smooth
function.Comment: Final accepted version, incl. plots with new Saskatoon data. 14
pages, with 4 figures included. Postscript. Latest version at
http://www.sns.ias.edu/~max/cobepow.html (faster from the US), from
http://www.mpa-garching.mpg.de/~max/cobepow.html (faster from Europe) or from
[email protected]

### Experimental neutrino physics

The current experimental status of neutrino physics is reviewed. It contains
the evidences for a non-vanishing neutrino rest mass from neutrino oscillation
searches. In addition an outlook is given on determining the various mixing
matrix elements and mass differences more precisely with new experiments. Of
special interest is the value of the mixing angle \theta_{13} determining the
possibility of detecting leptonic CP violation in the future. The prospect for
absolute mass measurements using beta and double beta decay as well as
cosmological observations is presented.Comment: 11 pages, 11 figures, Inv. talk presented at the DPF 2004 meeting of
the APS, Riverside, Aug. 200

### Learning from observations of the microwave background at small angular scales

In this paper, we focus our attention on the following question: How well can
we recover the power spectrum of the cosmic microwave background from the maps
of a given experiment?. Each experiment is described by a a pixelization scale,
a beam size, a noise level and a sky coverage. We use accurate numerical
simulations of the microwave sky and a cold dark matter model for structure
formation in the universe. Angular scales smaller than those of previous
simulations are included. The spectrum obtained from the simulated maps is
appropriately compared with the theoretical one. Relative deviations between
these spectra are estimated. Various contributions to these deviations are
analyzed. The method used for spectra comparisons is discussed.Comment: 15 pages (LATEX), 2 postcript figures, accepted in Ap

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