44 research outputs found
On the detection of Spectral Distortions in the CMB: Recombination to Reionization
The LCDM model of cosmology predicts inevitable, weak distortions
in the spectrum of the Cosmic Microwave Background (CMB) from
that of a blackbody. However, no such deviations have been
measured to date. This thesis focuses on CMB spectral distortions
arising from the cosmological epochs of recombination and cosmic
dawn & reionization. A detection and measurement of these CMB
spectral distortions will enable a better understanding of the
thermal and ionization history of the Universe and help us probe
redshifts that have never been directly observed thus far.
I present a feasibility study for a ground-based detection of
extremely weak, ripple-like additive features in the CMB spectrum
created by photons emitted during cosmological recombination (900
< z < 7000). I identify an octave band in the frequency range
2–6 GHz to be optimal for a detection of this CMB spectral
distortion. This band maximizes signal-to-noise ratio and has
sufficient spectral structure in the signal to aid foreground
separation. I introduce the Maximally Smooth (MS) function, an
algorithm to distinguish smooth foregrounds from the ripple like
signal. Using synthetic spectra, I demonstrate the efficacy of
using MS functions over polynomials to separate foregrounds from
the cosmological recombination signal. Using Bayesian tests I
estimate that using an array of 128 cryogenically cooled, ideal
radio-telescopes, spectral ripples from the recombination epoch
can be detected with 90% confidence in 255 observing days. Thus,
it is in principle possible to detect these cosmological
recombination signals in realistic observing times.
Among others, astronomical foregrounds pose challenges to the
detection of CMB spectral distortions. It is thus necessary to
have a realistic expectation of the Galactic and extragalactic
foreground spectra towards any given direction in the sky. I
present GMOSS: Global Model for the Radio Sky Spectrum, a
physically motivated model of the radio sky over 22 MHz–23 GHz.
GMOSS describes foreground spectra towards all sky directions
over 5 pixels using processes including synchrotron emission
with possible spectral break, emission from composite source
populations, free-free emission and thermal absorption.
Using GMOSS I investigate the spectral complexity expected in
foregrounds and the effect of the same on the detection of the
global redshifted 21-cm signal from cosmic dawn & reionization (6
< z < 150). I find that over large beamwidths foregrounds are
spectrally smooth and describable using MS functions for various
samplings of sky- coverage. However, it is more computationally
challenging to describe foreground spectra towards the galactic
plane, which is best avoided by experiments seeking to detect CMB
spectral distortions from cosmic dawn & reionization
(collectively EoR). Once again, I demonstrate the advantage of
using MS functions over polynomials to separate foregrounds from
the global EoR signal in mock-sky spectra. I find that using MS
functions to separate foregrounds, the global signal from EoR can
be detected using an ideal instrument with 95% confidence in 10
minutes observing time.
I conclude the thesis with a brief discussion on design criteria
for radio-telescopes seeking to detect distortions in the CMB
spectrum arising from the epochs of recombination through
reionization
GMOSS: All-sky model of spectral radio brightness based on physical components and associated radiative processes
We present Global MOdel for the radio Sky Spectrum (GMOSS) -- a novel,
physically motivated model of the low-frequency radio sky from 22 MHz to 23
GHz. GMOSS invokes different physical components and associated radiative
processes to describe the sky spectrum over 3072 pixels of
resolution. The spectra are allowed to be convex, concave or of more complex
form with contributions from synchrotron emission, thermal emission and
free-free absorption included. Physical parameters that describe the model are
optimized to best fit four all-sky maps at 150 MHz, 408 MHz, 1420 MHz and 23
GHz and two maps at 22 MHz and 45 MHz generated using the Global Sky Model of
de Oliveira-Costa et al. (2008). The fractional deviation of model to data has
a median value of and is less than for of the pixels.
Though aimed at modeling of foregrounds for the global signal arising from the
redshifted 21-cm line of Hydrogen during Cosmic Dawn and Epoch of Reionization
(EoR) - over redshifts , GMOSS is well suited for any
application that requires simulating spectra of the low-frequency radio sky as
would be observed by the beam of any instrument. The complexity in spectral
structure that naturally arises from the underlying physics of the model
provides a useful expectation for departures from smoothness in EoR foreground
spectra and hence may guide the development of algorithms for EoR signal
detection. This aspect is further explored in a subsequent paper.Comment: 19 pages, 7 figure
On the detection of spectral ripples from the Recombination Epoch
Photons emitted during the epochs of Hydrogen () and Helium recombination ( for HeII
HeI, for HeIII
HeII) are predicted to appear as broad, weak spectral distortions of the Cosmic
Microwave Background. We present a feasibility study for a ground-based
experimental detection of these recombination lines, which would provide an
observational constraint on the thermal ionization history of the Universe,
uniquely probing astrophysical cosmology beyond the last scattering surface. We
find that an octave band in the 2--6 GHz window is optimal for such an
experiment, both maximizing signal-to-noise ratio and including sufficient line
spectral structure. At these frequencies the predicted signal appears as an
additive quasi-sinusoidal component with amplitude about nK that is
embedded in a sky spectrum some nine orders of magnitude brighter. We discuss
an algorithm to detect these tiny spectral fluctuations in the sky spectrum by
foreground modeling. We introduce a \textit{Maximally Smooth} function capable
of describing the foreground spectrum and distinguishing the signal of
interest. With Bayesian statistical tests and mock data we estimate that a
detection of the predicted distortions is possible with 90\% confidence by
observing for 255 days with an array of 128 radiometers using cryogenically
cooled state-of-the-art receivers. We conclude that detection is in principle
feasible in realistic observing times; we propose APSERa---Array of Precision
Spectrometers for the Epoch of Recombination---a dedicated radio telescope to
detect these recombination lines.Comment: 33 pages, 16 figures, submitted to ApJ, comments welcom
SARAS 2: A Spectral Radiometer for probing Cosmic Dawn and the Epoch of Reionization through detection of the global 21 cm signal
The global 21 cm signal from Cosmic Dawn (CD) and the Epoch of Reionization
(EoR), at redshifts , probes the nature of first sources of
radiation as well as physics of the Inter-Galactic Medium (IGM). Given that the
signal is predicted to be extremely weak, of wide fractional bandwidth, and
lies in a frequency range that is dominated by Galactic and Extragalactic
foregrounds as well as Radio Frequency Interference, detection of the signal is
a daunting task. Critical to the experiment is the manner in which the sky
signal is represented through the instrument. It is of utmost importance to
design a system whose spectral bandpass and additive spurious can be well
calibrated and any calibration residual does not mimic the signal. SARAS is an
ongoing experiment that aims to detect the global 21 cm signal. Here we present
the design philosophy of the SARAS 2 system and discuss its performance and
limitations based on laboratory and field measurements. Laboratory tests with
the antenna replaced with a variety of terminations, including a network model
for the antenna impedance, show that the gain calibration and modeling of
internal additives leave no residuals with Fourier amplitudes exceeding 2~mK,
or residual Gaussians of 25 MHz width with amplitudes exceeding 2~mK. Thus,
even accounting for reflection and radiation efficiency losses in the antenna,
the SARAS~2 system is capable of detection of complex 21-cm profiles at the
level predicted by currently favoured models for thermal baryon evolution.Comment: 44 pages, 17 figures; comments and suggestions are welcom
Synthetic Observations with the Square Kilometre Array (SKA) -- development towards an end-to-end pipeline
Detection of the redshifted 21-cm signal of neutral hydrogen from the Cosmic
Dawn and the Epoch of Reionization is one of the final frontiers of modern
observational cosmology. The inherently faint signal makes it susceptible to
contamination by several sources like astrophysical foregrounds and
instrumental systematics. Nevertheless, developments achieved in the recent
times will combine to make signal detection possible with the upcoming Square
Kilometer Array (SKA), both statistically and via tomography. This review
describes an indigenously developed end-to-end pipeline that simulates
sensitive interferometric observations. It mainly focuses on the requirements
for \hi detection in interferometers. In its present form, it can mimic the
effects of realistic point source foregrounds and systematics- calibration
error and position error on 21-cm observations. The performance of the pipeline
is demonstrated for test cases with 0.01\% calibration error and position
error. Its performance is consistent across telescope, foreground, and signal
models. The focus of the simulation pipeline during the initial stages was for
EoR science. But since this is a general interferometric simulation pipeline,
it will be helpful to the entire SKA user community, irrespective of the
science goals.Comment: 24 Pages, 7 Figures, Review Article to appear in Special Issue of
Journal of Astrophysics and Astronomy on "Indian Participation in the SKA'',
comments are welcom
The Simons Observatory: Galactic Science Goals and Forecasts
Observing in six frequency bands from 27 to 280 GHz over a large sky area,
the Simons Observatory (SO) is poised to address many questions in Galactic
astrophysics in addition to its principal cosmological goals. In this work, we
provide quantitative forecasts on astrophysical parameters of interest for a
range of Galactic science cases. We find that SO can: constrain the frequency
spectrum of polarized dust emission at a level of
and thus test models of dust composition that predict that in
polarization differs from that measured in total intensity; measure the
correlation coefficient between polarized dust and synchrotron emission with a
factor of two greater precision than current constraints; exclude the
non-existence of exo-Oort clouds at roughly 2.9 if the true fraction is
similar to the detection rate of giant planets; map more than 850 molecular
clouds with at least 50 independent polarization measurements at 1 pc
resolution; detect or place upper limits on the polarization fractions of
CO(2-1) emission and anomalous microwave emission at the 0.1% level in select
regions; and measure the correlation coefficient between optical starlight
polarization and microwave polarized dust emission in patches for all
lines of sight with cm. The goals and
forecasts outlined here provide a roadmap for other microwave polarization
experiments to expand their scientific scope via Milky Way astrophysics.Comment: Submitted to AAS journals. 33 pages, 10 figure