131 research outputs found
Spectral index of the Galactic foreground emission in the 50-87 MHz range
Radiometry using individual dipole antennas is a potentially effective way to
study the cosmological epoch referred to as Cosmic Dawn (z~20) through
measurement of sky brightness arising from the 21~cm transition of neutral
hydrogen, provided this can be disentangled from much stronger Galactic and
extragalactic foregrounds. In the process, measured spectra of integrated sky
brightness temperature can be used to quantify properties of the foreground
emission. In this work we analyze data from the Large-aperture Experiment to
Detect the Dark Age (LEDA) in the range 50-87 MHz to constrain the spectral
index of foreground emission in the northern sky. We focus on two
zenith-directed LEDA radiometers and study how estimates of vary with
local sidereal time (LST). We correct for the effect of gain pattern
chromaticity and compare estimated absolute temperatures with simulations.
During times with the best observing conditions, for a "reference" radiometer,
we estimate that varies from -2.55 to -2.58, consistent with previous
measurements of the southern sky and simulated sky models. Using data from the
second, experimental, radiometer, we observe a similar trend vs. LST although
with slightly smaller , in the range. We infer
good instrument stability from consistency in computed spectral indices at a
level of 1-2 for LST=9-12.5h, using data distributed between mid-2018
to mid-2019. Evidence for spectral curvature is weak owing to residual
systematic errors, other than when the Galactic Center is in the sky, at which
time we find evidence for negative curvature, ~-0.4.Comment: 13 pages, 19 figure
The Cosmic Microwave Background in an Inhomogeneous Universe - why void models of dark energy are only weakly constrained by the CMB
The dimming of Type Ia supernovae could be the result of Hubble-scale
inhomogeneity in the matter and spatial curvature, rather than signaling the
presence of a dark energy component. A key challenge for such models is to fit
the detailed spectrum of the cosmic microwave background (CMB). We present a
detailed discussion of the small-scale CMB in an inhomogeneous universe,
focusing on spherically symmetric `void' models. We allow for the dynamical
effects of radiation while analyzing the problem, in contrast to other work
which inadvertently fine tunes its spatial profile. This is a surprisingly
important effect and we reach substantially different conclusions. Models which
are open at CMB distances fit the CMB power spectrum without fine tuning; these
models also fit the supernovae and local Hubble rate data which favours a high
expansion rate. Asymptotically flat models may fit the CMB, but require some
extra assumptions. We argue that a full treatment of the radiation in these
models is necessary if we are to understand the correct constraints from the
CMB, as well as other observations which rely on it, such as spectral
distortions of the black body spectrum, the kinematic Sunyaev-Zeldovich effect
or the Baryon Acoustic Oscillations.Comment: 23 pages with 14 figures. v2 has considerably extended discussion and
analysis, but the basic results are unchanged. v3 is the final versio
CMB Imprints of a Pre-Inflationary Climbing Phase
We discuss the implications for cosmic microwave background (CMB)
observables, of a class of pre-inflationary dynamics suggested by string models
where SUSY is broken due to the presence of D-branes and orientifolds
preserving incompatible portions of it. In these models the would-be inflaton
is forced to emerge from the initial singularity climbing up a mild exponential
potential, until it bounces against a steep exponential potential of "brane
SUSY breaking" scenarios, and as a result the ensuing descent gives rise to an
inflationary epoch that begins when the system is still well off its eventual
attractor. If a pre-inflationary climbing phase of this type had occurred
within 6-7 e-folds of the horizon exit for the largest observable wavelengths,
displacement off the attractor and initial-state effects would conspire to
suppress power in the primordial scalar spectrum, enhancing it in the tensor
spectrum and typically superposing oscillations on both. We investigate these
imprints on CMB observables over a range of parameters, examine their
statistical significance, and provide a semi-analytic rationale for our
results. It is tempting to ascribe at least part of the large-angle anomalies
in the CMB to pre-inflationary dynamics of this type.Comment: 38 pages, LaTeX, 11 eps figures, references added, matches version to
appear in JCA
Decoupling Inflation From the String Scale
When Inflation is embedded in a fundamental theory, such as string theory, it
typically begins when the Universe is already substantially larger than the
fundamental scale [such as the one defined by the string length scale]. This is
naturally explained by postulating a pre-inflationary era, during which the
size of the Universe grew from the fundamental scale to the initial
inflationary scale. The problem then arises of maintaining the [presumed]
initial spatial homogeneity throughout this era, so that, when it terminates,
Inflation is able to begin in its potential-dominated state. Linde has proposed
that a spacetime with compact negatively curved spatial sections can achieve
this, by means of chaotic mixing. Such a compactification will however lead to
a Casimir energy, which can lead to effects that defeat the purpose unless the
coupling to gravity is suppressed. We estimate the value of this coupling
required by the proposal, and use it to show that the pre-inflationary
spacetime is stable, despite the violation of the Null Energy Condition
entailed by the Casimir energy.Comment: 24 pages, 5 eps figures, references added, stylistic changes, version
to appear in Classical and Quantum Gravit
Recommended from our members
The Cosmic Dawn and Epoch of Reionisation with SKA
Concerted effort is currently ongoing to open up the Epoch of Reionization (z ∼15-6) for studies with IR and radio telescopes. Whereas IR detections have been made of sources (Lyman-α emitters, quasars and drop-outs) in this redshift regime in relatively small fields of view, no direct detection of neutral hydrogen, via the redshifted 21-cm line, has yet been established. Such a direct detection is expected in the coming years, with ongoing surveys, and could open up the entire universe from z ∼6-200 for astrophysical and cosmological studies, opening not only the Epoch of Reionization, but also its preceding Cosmic Dawn (z ∼30-15) and possibly even the later phases of the Dark Ages (z ∼200-30). All currently ongoing experiments attempt statistical detections of the 21-cm signal during the Epoch of Reionization, with limited signal-to-noise. Direct imaging, except maybe on the largest (degree) scales at lower redshifts, as well as higher redshifts will remain out of reach. The Square Kilometre Array (SKA) will revolutionize the field, allowing direct imaging of neutral hydrogen from scales of arc-minutes to degrees over most of the redshift range z ∼6-28 with SKA1-LOW, and possibly even higher redshifts with the SKA2-LOW. In this SKA will be unique, and in parallel provide enormous potential of synergy with other upcoming facilities (e.g. JWST). In this chapter we summarize the physics of 21-cm emission, the different phases the universe is thought to go through, and the observables that the SKA can probe, referring where needed to detailed chapters in this volume. This is done within the framework of the current SKA1 baseline design and a nominal CD/EoR straw-man survey, consisting of a shallow, medium-deep and deep survey, the latter probing down to ∼1 mK brightness temperature on arc-minute scales at the end of reionization. Possible minor modifications to the design of SKA1 and the upgrade to SKA2 are discussed, in addition to science that could be done already during roll-out when SKA1 still has limited capabilities and/or core collecting area
HERA Phase i Limits on the Cosmic 21 cm Signal: Constraints on Astrophysics and Cosmology during the Epoch of Reionization
Recently, the Hydrogen Epoch of Reionization Array (HERA) has produced the experiment's first upper limits on the power spectrum of 21 cm fluctuations at z ∼ 8 and 10. Here, we use several independent theoretical models to infer constraints on the intergalactic medium (IGM) and galaxies during the epoch of reionization from these limits. We find that the IGM must have been heated above the adiabatic-cooling threshold by z ∼ 8, independent of uncertainties about IGM ionization and the radio background. Combining HERA limits with complementary observations constrains the spin temperature of the z ∼ 8 neutral IGM to 27 K 630 K (2.3 K 640 K) at 68% (95%) confidence. They therefore also place a lower bound on X-ray heating, a previously unconstrained aspects of early galaxies. For example, if the cosmic microwave background dominates the z ∼ 8 radio background, the new HERA limits imply that the first galaxies produced X-rays more efficiently than local ones. The z ∼ 10 limits require even earlier heating if dark-matter interactions cool the hydrogen gas. If an extra radio background is produced by galaxies, we rule out (at 95% confidence) the combination of high radio and low X-ray luminosities of L r,ν /SFR > 4 × 1024 W Hz-1 yr and L X /SFR < 7.6 × 1039 erg s-1 yr. The new HERA upper limits neither support nor disfavor a cosmological interpretation of the recent Experiment to Detect the Global EOR Signature (EDGES) measurement. The framework described here provides a foundation for the interpretation of future HERA results
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