2,017 research outputs found
Cross-Correlation of the Cosmic Microwave Background with Radio Sources: Constraints on an Accelerating Universe
We present a new limit on the cosmological constant based on the absence of
correlations between the cosmic microwave background (CMB) and the distribution
of distant radio sources. In the cosmological constant-cold dark matter models
currently favored, such correlations should have been produced via the
integrated Sachs-Wolfe effect, assuming that radio sources trace the local
(z=1) matter density. We find no evidence of correlations between the COBE 53Hz
microwave map and the NVSS 1.4 GHz radio survey. The implied 95% CL limit on
the cosmological constant is Lambda < 0.74, in marginal agreement with the
values suggested by recent measurements of the CMB anisotropy and type-IA
supernovae observations, 0.6 < Lambda < 0.7. If the cosmological model does lie
in this range, then the integrated Sachs-Wolfe effect should be detectable with
upcoming CMB maps and radio surveys.Comment: 5 pages; 3 figures; submitted to PR
Cosmic concordance and the fine structure constant
Recent measurements of a peak in the angular power spectrum of the cosmic
microwave background appear to suggest that geometry of the universe is close
to being flat. But if other accepted indicators of cosmological parameters are
also correct then the best fit model is marginally closed, with the peak in the
spectrum at larger scales than in a flat universe. Such observations can be
reconciled with a flat universe if the fine structure constant had a lower
value at earlier times, which would delay the recombination of electrons and
protons and also act to suppress secondary oscillations as observed. We discuss
evidence for a few percent increase in the fine structure constant between the
time of recombination and the present.Comment: 12 Page
A Causal Source which Mimics Inflation
How unique are the inflationary predictions for the cosmic microwave
anisotropy pattern? In this paper, it is asked whether an arbitrary causal
source for perturbations in the standard hot big bang could effectively mimic
the predictions of the simplest inflationary models. A surprisingly simple
example of a `scaling' causal source is found to closely reproduce the
inflationary predictions. This letter extends the work of a previous paper
(ref. 6) to a full computation of the anisotropy pattern, including the Sachs
Wolfe integral. I speculate on the possible physics behind such a source.Comment: 4 pages, RevTex, 3 figure
Photoproduction of Quarkonium in Proton-Proton and Nucleus-Nucleus Collisions
We discuss the photoproduction of and at high energy
, and heavy ion colliders. We predict large rates in
interactions at the Fermilab Tevatron %and in heavy-ion interactions at the
CERN LHC. These reactions can be and in and heavy-ion interactions at the
CERN LHC. The is also produced copiously at RHIC. These reactions can
be used to study the gluon distribution in protons and heavy nuclei. We also
show that the different CP symmetries of the initial states lead to large
differences in the transverse momentum spectra of mesonsComment: 4 pgs. with 3 figure
Signature of Gravity Waves in Polarization of the Microwave Background
Using spin-weighted decomposition of polarization in the Cosmic Microwave
Background (CMB) we show that a particular combination of Stokes and
parameters vanishes for primordial fluctuations generated by scalar modes, but
does not for those generated by primordial gravity waves. Because of this
gravity wave detection is not limited by cosmic variance as in the case of
temperature fluctuations. We present the exact expressions for various
polarization power spectra, which are valid on any scale. Numerical evaluation
in inflation-based models shows that the expected signal is of the order of 0.5
, which could be directly tested in future CMB experiments.Comment: 4 pages, 1 figure, RevTeX, matches the accepted version (to appear in
Phys. Rev. Lett.); code available at
http://arcturus.mit.edu:80/~matiasz/CMBFAST/cmbfast.htm
Computing challenges of the Cosmic Microwave Background
The Cosmic Microwave Background (CMB) encodes information on the origin and evolution of the universe, buried in a fractional anisotropy of one part in 100,000 on angular scales from arcminutes to tens of degrees. We await the coming onslaught of data from experiments measuring the microwave sky from the ground, from balloons and from space. However, we are faced with the harsh reality that current algorithms for extracting cosmological information cannot handle data sets of the size and complexity expected even in the next few years. Here we review the challenges involved in understanding this data: making maps from time-ordered data, removing the foreground contaminants, and finally estimating the power spectrum and cosmological parameters from the CMB map. If handled naively, the global nature of the analysis problem renders these tasks effectively impossible given the volume of the data. We discuss possible techniques for overcoming these issues and outline the many other challenges that wait to be addressed
The Low Redshift survey at Calar Alto (LoRCA)
The Baryon Acoustic Oscillation (BAO) feature in the power spectrum of
galaxies provides a standard ruler to measure the accelerated expansion of the
Universe. To extract all available information about dark energy, it is
necessary to measure a standard ruler in the local, z<0.2, universe where dark
energy dominates most the energy density of the Universe. Though the volume
available in the local universe is limited, it is just big enough to measure
accurately the long 100 Mpc/h wave-mode of the BAO. Using cosmological N-body
simulations and approximate methods based on Lagrangian perturbation theory, we
construct a suite of a thousand light-cones to evaluate the precision at which
one can measure the BAO standard ruler in the local universe. We find that
using the most massive galaxies on the full sky (34,000 sq. deg.), i.e. a
K(2MASS)<14 magnitude-limited sample, one can measure the BAO scale up to a
precision of 4\% and 1.2\% using reconstruction). We also find that such a
survey would help to detect the dynamics of dark energy.Therefore, we propose a
3-year long observational project, named the Low Redshift survey at Calar Alto
(LoRCA), to observe spectroscopically about 200,000 galaxies in the northern
sky to contribute to the construction of aforementioned galaxy sample. The
suite of light-cones is made available to the public.Comment: 15 pages. Accepted in MNRAS. Please visit our website:
http://lorca-survey.ft.uam.es
Cosmic Texture from a Broken Global SU(3) Symmetry
We investigate the observable consequences of creating cosmic texture by
breaking a global SU(3) symmetry, rather than the SU(2) case which is generally
studied. To this end, we study the nonlinear sigma model for a totally broken
SU(3) symmetry, and develop a technique for numerically solving the classical
field equations. This technique is applied in a cosmological context: the
energy of the collapsing SU(3) texture field is used as a gravitational source
for the production of perturbations in the primordial fluids of the early
universe. From these calculations, we make predictions about the appearance of
the anisotropies in the cosmic microwave background radiation (CMBR) which
would be present if the large scale structure of the universe was
gravitationally seeded by the collapse of SU(3) textures.Comment: 28 pages, latex, 11 figures, submitted to Phys. Rev.
CMB Anisotropies: Total Angular Momentum Method
A total angular momentum representation simplifies the radiation transport
problem for temperature and polarization anisotropy in the CMB. Scattering
terms couple only the quadrupole moments of the distributions and each moment
corresponds directly to the observable angular pattern on the sky. We develop
and employ these techniques to study the general properties of anisotropy
generation from scalar, vector and tensor perturbations to the metric and the
matter, both in the cosmological fluids and from any seed perturbations (e.g.
defects) that may be present. The simpler, more transparent form and derivation
of the Boltzmann equations brings out the geometric and model-independent
aspects of temperature and polarization anisotropy formation. Large angle
scalar polarization provides a robust means to distinguish between isocurvature
and adiabatic models for structure formation in principle. Vector modes have
the unique property that the CMB polarization is dominated by magnetic type
parity at small angles (a factor of 6 in power compared with 0 for the scalars
and 8/13 for the tensors) and hence potentially distinguishable independent of
the model for the seed. The tensor modes produce a different sign from the
scalars and vectors for the temperature-polarization correlations at large
angles. We explore conditions under which one perturbation type may dominate
over the others including a detailed treatment of the photon-baryon fluid
before recombination.Comment: 32 pg., 10 figs., RevTeX, minor changes reflect published version,
minor typos corrected, also available at http://www.sns.ias.edu/~wh
Challenges for creating magnetic fields by cosmic defects
We analyse the possibility that topological defects can act as a source of
magnetic fields through the Harrison mechanism in the radiation era. We give a
detailed relativistic derivation of the Harrison mechanism at first order in
cosmological perturbations, and show that it is only efficient for temperatures
above T ~ 0.2 keV. Our main result is that the vector metric perturbations
generated by the defects cannot induce vorticity in the matter fluids at linear
order, thereby excluding the production of currents and magnetic fields. We
show that anisotropic stress in the matter fluids is required to source
vorticity and magnetic fields. Our analysis is relevant for any mechanism
whereby vorticity is meant to be transferred purely by gravitational
interactions, and thus would also apply to dark matter or neutrinos.Comment: 9 pages, 1 figure; minor corrections and additions; accepted for
publication in Physical Review
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