319 research outputs found
Inflation after Planck and BICEP2
We discuss the inflationary paradigm, how it can be tested, and how various
models of inflation fare in the light of data from Planck and BICEP2. We
introduce inflation and reheating, and discuss temperature and polarisation
anisotropies in the cosmic microwave background radiation due to quantum
fluctuations during inflation. Fitting observations of the anisotropies with
theoretical realisations obtained by varying various parameters of the
curvature power spectrum and cosmological parameters enables one to obtain the
allowed ranges of these parameters. We discuss how to relate these parameters
to inflation models which allows one to rule in or out specific models of
inflation.Comment: Slightly longer version of a plenary review talk at the XXI DAE-BRNS
High Energy Physics Symposium at IIT Guwahati, Dec.8-12, 2014. 14 pages, 7
fig
A Size of ~10 Mpc for the Ionized Bubbles at the End of Cosmic Reionization
The first galaxies to appear in the universe at redshifts z>20 created
ionized bubbles in the intergalactic medium of neutral hydrogen left over from
the Big-Bang. It is thought that the ionized bubbles grew with time, surrounded
clusters of dwarf galaxies and eventually overlapped quickly throughout the
universe over a narrow redshift interval near z~6. This event signaled the end
of the reionization epoch when the universe was a billion years old. Measuring
the hitherto unknown size distribution of the bubbles at their final overlap
phase is a focus of forthcoming observational programs aimed at highly
redshifted 21cm emission from atomic hydrogen. Here we show that the combined
constraints of cosmic variance and causality imply an observed bubble size at
the end of the overlap epoch of ~10 physical Mpc, and a scatter in the observed
redshift of overlap along different lines-of-sight of ~0.15. This scatter is
consistent with observational constraints from recent spectroscopic data on the
farthest known quasars. Our novel result implies that future radio experiments
should be tuned to a characteristic angular scale of ~0.5 degrees and have a
minimum frequency band-width of ~8 MHz for an optimal detection of 21cm flux
fluctuations near the end of reionization.Comment: Accepted for publication in Nature. Press embargo until publishe
Measurement of Permanent Electric Dipole Moments of Charged Hadrons in Storage Rings
Permanent Electric Dipole Moments (EDMs) of elementary particles violate two
fundamental symmetries: time reversal invariance (T) and parity (P). Assuming
the CPT theorem this implies CP-violation. The CP-violation of the Standard
Model is orders of magnitude too small to be observed experimentally in EDMs in
the foreseeable future. It is also way too small to explain the asymmetry in
abundance of matter and anti-matter in our universe. Hence, other mechanisms of
CP violation outside the realm of the Standard Model are searched for and could
result in measurable EDMs.
Up to now most of the EDM measurements were done with neutral particles. With
new techniques it is now possible to perform dedicated EDM experiments with
charged hadrons at storage rings where polarized particles are exposed to an
electric field. If an EDM exists the spin vector will experience a torque
resulting in change of the original spin direction which can be determined with
the help of a polarimeter. Although the principle of the measurement is simple,
the smallness of the expected effect makes this a challenging experiment
requiring new developments in various experimental areas.
Complementary efforts to measure EDMs of proton, deuteron and light nuclei
are pursued at Brookhaven National Laboratory and at Forschungszentrum Juelich
with an ultimate goal to reach a sensitivity of 10^{-29} e cm.Comment: 8 pages, 2 figure
Broad Brush Cosmos
An innovative approach to map the large-scale structure in the Universe
sidesteps the conventional need to observe millions of galaxies individually,
and holds promise for both astrophysical and cosmological studies.Comment: Invited Nature 'News and Views' Commentary on Chang et al. 2010,
Nature, 466, 463; 6pages, 1 figur
Thermal production of axino Dark Matter
We reconsider thermal production of axinos in the early universe, adding: a)
missed terms in the axino interaction; b) production via gluon decays
kinematically allowed by thermal masses; c) a precise modeling of reheating. We
find an axino abunance a few times larger than previous computations.Comment: 6 pages, 2 figures. Final version, to appear on JHE
Cosmic Hydrogen Was Significantly Neutral a Billion Years After the Big Bang
The ionization fraction of cosmic hydrogen, left over from the big bang,
provides crucial fossil evidence for when the first stars and quasar black
holes formed in the infant universe. Spectra of the two most distant quasars
known show nearly complete absorption of photons with wavelengths shorter than
the Ly-alpha transition of neutral hydrogen, indicating that hydrogen in the
intergalactic medium (IGM) had not been completely ionized at a redshift z~6.3,
about a billion years after the big bang. Here we show that the radii of
influence of ionizing radiation from these quasars imply that the surrounding
IGM had a neutral hydrogen fraction of tens of percent prior to the quasar
activity, much higher than previous lower limits of ~0.1%. When combined with
the recent inference of a large cumulative optical depth to electron scattering
after cosmological recombination from the WMAP data, our result suggests the
existence of a second peak in the mean ionization history, potentially due to
an early formation episode of the first stars.Comment: 14 Pages, 2 Figures. Accepted for publication in Nature. Press
embargo until publishe
The Complete Star Formation History of the Universe
The determination of the star-formation history of the Universe is a key goal
of modern cosmology, as it is crucial to our understanding of how structure in
the Universe forms and evolves. A picture has built up over recent years,
piece-by-piece, by observing young stars in distant galaxies at different times
in the past.
These studies indicated that the stellar birthrate peaked some 8 billion
years ago, and then declined by a factor of around ten to its present value.
Here we report on a new study which obtains the complete star formation history
by analysing the fossil record of the stellar populations of 96545 nearby
galaxies. Broadly, our results support those derived from high-redshift
galaxies elsewhere in the Universe. We find, however, that the peak of star
formation was more recent - around 5 billion years ago. Our study also shows
that the bigger the stellar mass of the galaxy, the earlier the stars were
formed. This striking result indicates a very different formation history for
high- and low-mass formation.Comment: Accepted by Nature. Press embargo until publishe
Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background
Cosmology's standard model posits an infinite flat universe forever expanding
under the pressure of dark energy. First-year data from the Wilkinson Microwave
Anisotropy Probe (WMAP) confirm this model to spectacular precision on all but
the largest scales (Bennett {\it et al.}, 2003 ; Spergel {\it et al.}, 2003).
Temperature correlations across the microwave sky match expectations on scales
narrower than , yet vanish on scales wider than .
Researchers are now seeking an explanation of the missing wide-angle
correlations (Contaldi {\it et al.}, 2003 ; Cline {\it et al.}, 2003). One
natural approach questions the underlying geometry of space, namely its
curvature (Efstathiou, 2003) and its topology (Tegmark {\it et al.}, 2003). In
an infinite flat space, waves from the big bang would fill the universe on all
length scales. The observed lack of temperature correlations on scales beyond
means the broadest waves are missing, perhaps because space itself
is not big enough to support them.
Here we present a simple geometrical model of a finite, positively curved
space -- the Poincar\'e dodecahedral space -- which accounts for WMAP's
observations with no fine-tuning required. Circle searching (Cornish, Spergel
and Starkman, 1998) may confirm the model's topological predictions, while
upcoming Planck Surveyor data may confirm its predicted density of . If confirmed, the model will answer the ancient question of
whether space is finite or infinite, while retaining the standard
Friedmann-Lema\^\i{}tre foundation for local physics.Comment: 10 pages, 4 figures. This is a slightly longer version of the paper
published in Nature 425, p. 593, 200
A correlation of the cosmic microwave sky with large scale structure
We cross correlate the large-scale cosmic microwave background (CMB) sky
measured by WMAP with two probes of large-scale structure at z ~ 1. The hard
X-ray background, measured by the HEAO-1 satellite, is positively correlated
with the WMAP data at the 2.5-3.0 sigma level. The number counts of radio
galaxies in the NVSS survey are also correlated at a slightly weaker level
(2.-2.5 sigma). These correlations appear to arise from both hemispheres on the
sky and are resilient to changes in the levels of masking of the Galaxy and
point sources, suggesting that foregrounds are not responsible for the signal.
The implication is that some of the observed CMB fluctuations arise at low
redshifts. The level of the correlations is consistent with that expected for
the cosmological constant (Omega_Lambda = 0.72) concordance model resulting
from the integrated Sachs-Wolfe effect. Thus, we may be observing dark energy's
effect on the growth of structure.Comment: 8 pages, 3 postscript figure
Vacuum Stability, Perturbativity, and Scalar Singlet Dark Matter
We analyze the one-loop vacuum stability and perturbativity bounds on a
singlet extension of the Standard Model (SM) scalar sector containing a scalar
dark matter candidate. We show that the presence of the singlet-doublet quartic
interaction relaxes the vacuum stability lower bound on the SM Higgs mass as a
function of the cutoff and lowers the corresponding upper bound based on
perturbativity considerations. We also find that vacuum stability requirements
may place a lower bound on the singlet dark matter mass for given singlet
quartic self coupling, leading to restrictions on the parameter space
consistent with the observed relic density. We argue that discovery of a light
singlet scalar dark matter particle could provide indirect information on the
singlet quartic self-coupling.Comment: 25 pages, 10 figures; v2 - fixed minor typos; v3 - added to text
discussions of other references, changed coloring of figures for easier black
and white viewin
- …