4,089 research outputs found
The cosmic microwave background: observing directly the early universe
The Cosmic Microwave Background (CMB) is a relict of the early universe. Its
perfect 2.725K blackbody spectrum demonstrates that the universe underwent a
hot, ionized early phase; its anisotropy (about 80 \mu K rms) provides strong
evidence for the presence of photon-matter oscillations in the primeval plasma,
shaping the initial phase of the formation of structures; its polarization
state (about 3 \mu K rms), and in particular its rotational component (less
than 0.1 \mu K rms) might allow to study the inflation process in the very
early universe, and the physics of extremely high energies, impossible to reach
with accelerators. The CMB is observed by means of microwave and mm-wave
telescopes, and its measurements drove the development of ultra-sensitive
bolometric detectors, sophisticated modulators, and advanced cryogenic and
space technologies. Here we focus on the new frontiers of CMB research: the
precision measurements of its linear polarization state, at large and
intermediate angular scales, and the measurement of the inverse-Compton effect
of CMB photons crossing clusters of Galaxies. In this framework, we will
describe the formidable experimental challenges faced by ground-based,
near-space and space experiments, using large arrays of detectors. We will show
that sensitivity and mapping speed improvement obtained with these arrays must
be accompanied by a corresponding reduction of systematic effects (especially
for CMB polarimeters), and by improved knowledge of foreground emission, to
fully exploit the huge scientific potential of these missions.Comment: In press. Plenary talk. Copyright 2012 Society of Photo-Optical
Instrumentation Engineers. One print or electronic copy may be made for
personal use only. Systematic reproduction and distribution, duplication of
any material in this paper for a fee or for commercial purposes, or
modification of the content of the paper are prohibite
Common-mode rejection in Martin-Puplett spectrometers for astronomical observations at mm-wavelengths
The Martin-Puplett interferometer (MPI) is a differential Fourier transform
spectrometer (FTS), measuring the difference between spectral brightness at two
input ports. This unique feature makes the MPI an optimal zero instrument, able
to detect small brightness gradients embeddend in a large common background. In
this paper we investigate experimentally the common-mode rejection achievable
in the MPI at mm wavelengths, and discuss the use of the instrument to measure
the spectrum of cosmic microwave background (CMB) anisotropy
A multi-frequency study of the SZE in giant radio galaxies
Radio-galaxy (RG) lobes contain relativistic electrons embedded in a tangled
magnetic field that produce, in addition to low-frequency synchrotron radio
emission, inverse-Compton scattering (ICS) of the cosmic microwave background
(CMB) photons. This produces a relativistic, non-thermal Sunyaev-Zel'dovich
effect (SZE). We study the spectral and spatial properties of the non-thermal
SZE in a sample of radio galaxies and make predictions for their detectability
in both the negative and the positive part of the SZE, with space experiments
like Planck, OLIMPO, and Herschel-SPIRE. These cover a wide range of
frequencies, from radio to sub-mm. We model the SZE in a general formalism that
is equivalent to the relativistic covariant one and describe the electron
population contained in the lobes of the radio galaxies with parameters derived
from their radio observations, namely, flux, spectral index, and spatial
extension. We further constrain the electron spectrum and the magnetic field of
the RG lobes using X-ray, gamma-ray, and microwave archival observations. We
determine the main spectral features of the SZE in RG lobes, namely, the
minimum, the crossover, and the maximum of the SZE. We show that these typical
spectral features fall in the frequency ranges probed by the available space
experiments. We provide the most reliable predictions for the amplitude and
spectral shape of the SZE in a sample of selected RGs with extended lobes. In
three of these objects, we also derive an estimate of the magnetic field in the
lobe at the muG level by combining radio (synchrotron) observations and X-ray
(ICS) observations. These data, together with the WMAP upper limits, set
constraints on the minimum momentum of the electrons residing in the RG lobes
and allow realistic predictions for the visibility of their SZE to be derived
with Planck, OLIMPO, and Herschel-SPIRE. [abridged]Comment: 26 pages, 21 figures; Astronomy and Astrophysics, in pres
Multiparameter ergodic Cesà ro-α averages
Let (X,F,Îœ) be a Ï-finite measure space. Associated with k Lamperti operators on Lp(Îœ), T1,âŠ,Tk, nË=(n1,âŠ,nk)âNk and αË=(α1,âŠ,αk) with 0<αjâ€1, we define the ergodic CesĂ ro-Î±Ë averages
RnË,αËf=1âkj=1Aαjnjâik=0nkâŻâi1=0n1âj=1kAαjâ1njâijTikkâŻTi11f.
For these averages we prove the almost everywhere convergence on X and the convergence in the Lp(Îœ) norm, when n1,âŠ,nkââ independently, for all fâLp(dÎœ) with p>1/αâ where αâ=min1â€jâ€kαj. In the limit case p=1/αâ, we prove that the averages RnË,αËf converge almost everywhere on X for all f in the OrliczâLorentz space Î(1/αâ,Ïmâ1) with Ïm(t)=t(1+log+t)m. To obtain the result in the limit case we need to study inequalities for the composition of operators Ti that are of restricted weak type (pi,pi). As another application of these inequalities we also study the strong CesĂ ro-Î±Ë continuity of functions.Fil: Bernardis, Ana Lucia. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Santa Fe. Instituto de MatemĂĄtica Aplicada del Litoral. Universidad Nacional del Litoral. Instituto de MatemĂĄtica Aplicada del Litoral; ArgentinaFil: Crescimbeni, Raquel Liliana. Universidad Nacional del Comahue; ArgentinaFil: Ferrari Freire, Cecilia. Universidad Nacional del Comahue; Argentin
Large Radio Telescopes for Anomalous Microwave Emission Observations
We discuss in this paper the problem of the Anomalous Microwave Emission
(AME) in the light of ongoing or future observations to be performed with the
largest fully steerable radio telescope in the world. High angular resolution
observations of the AME will enable astronomers to drastically improve the
knowledge of the AME mechanisms as well as the interplay between the different
constituents of the interstellar medium in our galaxy. Extragalactic
observations of the AME have started as well, and high resolution is even more
important in this kind of observations. When cross-correlating with IR-dust
emission, high angular resolution is also of fundamental importance in order to
obtain unbiased results. The choice of the observational frequency is also of
key importance in continuum observation. We calculate a merit function that
accounts for the signal-to-noise ratio (SNR) in AME observation given the
current state-of-the-art knowledge and technology. We also include in our merit
functions the frequency dependence in the case of multifrequency observations.
We briefly mention and compare the performance of four of the largest
radiotelescopes in the world and hope the observational programs in each of
them will be as intense as possible.Comment: Review accepted for publication in Advances in Astronom
A cryogenic waveplate rotator for polarimetry at mm and sub-mm wavelengths
Mm and sub-mm waves polarimetry is the new frontier of research in Cosmic
Microwave Background and Interstellar Dust studies. Polarimeters working in the
IR to MM range need to be operated at cryogenic temperatures, to limit the
systematic effects related to the emission of the polarization analyzer. In
this paper we study the effect of the temperature of the different components
of a waveplate polarimeter, and describe a system able to rotate, in a
completely automated way, a birefringent crystal at 4K. We simulate the main
systematic effects related to the temperature and non-ideality of the optical
components in a Stokes polarimeter. To limit these effects, a cryogenic
implementation of the polarimeter is mandatory. In our system, the rotation
produced by a step motor, running at room temperature, is transmitted down to
cryogenic temperatures by means of a long shaft and gears running on custom
cryogenic bearings. Our system is able to rotate, in a completely automated
way, a birefringent crystal at 4K, dissipating only a few mW in the cold
environment. A readout system based on optical fibers allows to control the
rotation of the crystal to better than 0.1{\deg}. This device fulfills the
stringent requirements for operation in cryogenic space experiments, like the
forthcoming PILOT, BOOMERanG and LSPE.Comment: Submitted to Astronomy and Astrophysics. v1: 10 pages, 8 figures. v2:
corrected labels for the bibliographic references (no changes in the
bibliography). v3: revised version. 9 pages, 7 figures. Added a new figure.
Updated with a more realistic simulation for the interstellar dust and with
the latest cryogenic test
Can the quintessence be a complex scalar field?
In light of the recent observations of type Ia supernovae suggesting an
accelerating expansion of the Universe, we wish in this paper to point out the
possibility of using a complex scalar field as the quintessence to account for
the acceleration. In particular, we extend the idea of Huterer and Turner in
deriving the reconstruction equations for the complex quintessence, showing the
feasibility of making use of a complex scalar field (instead of a real scalar
field) while maintaining the uniqueness feature of the reconstruction for two
possible situations, respectively. We discuss very briefly how future
observations may help to distinguish the different quintessence scenarios,
including the scenario with a positive cosmological constant.Comment: 9 pages, LaTeX, final versio
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