arXiv subjects

George F. Smoot

Publications and source records attributed to George F. Smoot.

At least 55 records · Page 3Linked to original sources

arXiv2018

Reionization in the dark and the light from Cosmic Microwave Background

We explore the constraints on the history of reionization from Planck 2015 Cosmic Microwave Background (CMB) data and we derive the forecasts for future CMB observations. We consider a class of monotonic histories of reionization as parametrized by two additional extra parameters with respect to the average optical depth used in the instantaneous reionization modeling. We investigate the degeneracies between the history of reionization and selected extensions of the standard cosmological model. In particular, we consider the degeneracies with the total mass of the neutrino sector and we discuss the possible correlation between the dark matter annihilation and the duration of reionization in the CMB. We use an extension to poly-reion model that was proposed in Hazra and Smoot, JCAP 1711, 028 (2017). We compare the constraints from Planck 2015 data with the predicted constraints from possible future CMB mission as LiteBIRD, and we also use the proposed CORE-like specifications as an example of what higher resolution can bring in addition. We find that the degeneracy between the average optical depth and the duration of reionization will be substantially removed by both concepts. Degeneracies between the reionization history and either the total neutrino mass and properties of dark matter annihilation will also be improved by future surveys. We find only marginal improvement in the constraints on reionization history for the higher resolution in the case of long duration of reionization.

astro-ph.CO↗

arXiv2018

Probing features in inflaton potential and reionization history with future CMB space observations

We consider the prospects of probing features in the primordial power spectrum with future Cosmic Microwave Background (CMB) polarization measurements. In the scope of the inflationary scenario, such features in the spectrum can be produced by local non-smooth pieces in an inflaton potential (smooth and quasi-flat in general) which in turn may originate from fast phase transitions during inflation in other quantum fields interacting with the inflaton. They can fit some outliers in the CMB temperature power spectrum which are unaddressed within the standard inflationary ${\mathrmΛ}$CDM model. We consider Wiggly Whipped Inflation (WWI) as a theoretical framework leading to improvements in the fit to the Planck 2015 temperature and polarization data in comparison with the standard inflationary models, although not at a statistically significant level. We show that some type of features in the potential within the WWI models, leading to oscillations in the primordial power spectrum that extend to intermediate and small scales can be constrained with high confidence (at 3$σ$ or higher confidence level) by an instrument as the Cosmic ORigins Explorer (CORE). In order to investigate the possible confusion between inflationary features and footprints from the reionization era, we consider an extended reionization history with monotonic increase of free electrons with decrease in redshift. We discuss the present constraints on this model of extended reionization and future predictions with CORE. We also project, to what extent, this extended reionization can create confusion in identifying inflationary features in the data.

astro-ph.CO↗

arXiv2017

Witnessing the reionization history using Cosmic Microwave Background observation from Planck

We constrain the history of reionization using the data from Planck 2015 Cosmic Microwave Background (CMB) temperature and polarization anisotropy observations. We also use prior constraints on the reionization history at redshifts $\sim7-8$ obtained from Lyman-$α$ emission observations. Using the free electron fractions at different redshifts as free parameters, we construct the complete reionization history using polynomials. Our construction provides an extremely flexible framework to search for the history of reionization as a function of redshifts. We present a conservative and an optimistic constraint on reionization that are categorized by the flexibilities of the models and datasets used to constrain them, and we report that CMB data marginally favors extended reionization histories. In both the cases, we find the mean values of optical depth to be larger ($\approx0.09$ and $0.1$) than what we find in standard steplike reionization histories ($0.079\pm0.017$). At the same time we also find that the maximum free electron fraction allowed by the data for redshifts more than 15 is $\sim0.25$ at 95.4\% confidence limit in the case of optimistic constraint.

astro-ph.CO↗

arXiv2016

21 cm intensity mapping with the Five hundred metre Aperture Spherical Telescope

This paper describes a programme to map large-scale cosmic structures on the largest possible scales by using the Five hundred metre Aperture Spherical Telescope (FAST) to make a 21 cm (red-shifted) intensity map of the sky for the range $0.5 < z < 2.5$. The goal is to map to the angular and spectral resolution of FAST a large swath of the sky by simple drift scans with a transverse set of beams. This approach would be complementary to galaxy surveys and could be completed before the Square Kilometre Array (SKA) could begin a more detailed and precise effort. The science would be to measure the large-scale structure on the size of the baryon acoustic oscillations and larger scale, and the results would be complementary to its contemporary observations and significant. The survey would be uniquely sensitive to the potential very large-scale features from inflation at the Grand Unified Theory (GUT) scale and complementary to observations of the cosmic microwave background.

astro-ph.CO↗

arXiv2016

General Relativity and Cosmology: Unsolved Questions and Future Directions

For the last 100 years, General Relativity (GR) has taken over the gravitational theory mantle held by Newtonian Gravity for the previous 200 years. This article reviews the status of GR in terms of its self-consistency, completeness, and the evidence provided by observations, which have allowed GR to remain the champion of gravitational theories against several other classes of competing theories. We pay particular attention to the role of GR and gravity in cosmology, one of the areas in which one gravity dominates and new phenomena and effects challenge the orthodoxy. We also review other areas where there are likely conflicts pointing to the need to replace or revise GR to represent correctly observations and consistent theoretical framework. Observations have long been key both to the theoretical liveliness and viability of GR. We conclude with a discussion of the likely developments over the next 100 years.

gr-qc↗

arXiv2016

A Brief History of Gravitational Waves

This review describes the discovery of gravitational waves. We recount the journey of predicting and finding those waves, since its beginning in the early twentieth century, their prediction by Einstein in 1916, theoretical and experimental blunders, efforts towards their detection, and finally the subsequent successful discovery.

physics.hist-ph↗

arXiv2016

Primordial features and Planck polarization

With the Planck 2015 Cosmic Microwave Background (CMB) temperature and polarization data, we search for possible features in the primordial power spectrum (PPS). We revisit the Wiggly Whipped Inflation (WWI) framework and demonstrate how generation of some particular primordial features can improve the fit to Planck data. WWI potential allows the scalar field to transit from a steeper potential to a nearly flat potential through a discontinuity either in potential or in its derivatives. WWI offers the inflaton potential parametrizations that generate a wide variety of features in the primordial power spectra incorporating most of the localized and non-local inflationary features that are obtained upon reconstruction from temperature and polarization angular power spectrum. At the same time, in a single framework it allows us to have a background parameter estimation with a nearly free-form primordial spectrum. Using Planck 2015 data, we constrain the primordial features in the context of Wiggly Whipped Inflation and present the features that are supported both by temperature and polarization. WWI model provides more than $13$ improvement in $χ^2$ fit to the data with respect to the best fit power law model considering combined temperature and polarization data from Planck and B-mode polarization data from BICEP and Planck dust map. We use 2-4 extra parameters in the WWI model compared to the featureless strict slow roll inflaton potential. We find that the differences between the temperature and polarization data in constraining background cosmological parameters such as baryon density, cold dark matter density are reduced to a good extent if we use primordial power spectra from WWI. We also discuss the extent of bispectra obtained from the best potentials in arbitrary triangular configurations using the BI-spectra and Non-Gaussianity Operator (BINGO).

astro-ph.CO↗

arXiv2015

Probing Massive Stars Around Gamma-Ray Burst Progenitors

Long Gamma-Ray Bursts (GRBs) are produced by ultra-relativistic jets launched from core collapse of massive stars. Most massive stars form in binaries and/or in star clusters, which means that there may be a significant external photon field (EPF) around the GRB progenitor. We calculate the inverse-Compton scattering of EPF by the hot electrons in the GRB jet. Three possible cases of EPFs are considered: the progenitor is (I) in a massive binary system, (II) surrounded by a Wolf-Rayet-star wind, and (III) in a dense star cluster. Typical luminosities of 10^46 - 10^50 erg/s in the 1 - 100 GeV band are expected, depending on the stellar luminosity, binary separation (I), wind mass loss rate (II), stellar number density (III), etc. We calculate the lightcurve and spectrum in each case, taking fully into account the equal-arrival time surfaces and possible pair-production absorption with the prompt gamma-rays. Observations can put constraints on the existence of such EPFs (and hence on the nature of GRB progenitors) and on the radius where the jet internal dissipation process accelerates electrons.

astro-ph.HE↗

arXiv2014

Wiggly Whipped Inflation

Motivated by BICEP2 results on the CMB polarization B-mode which imply primordial gravitational waves are produced when the Universe has the expansion rate of about $H \approx 10^{14}$ GeV, and by deviations from a smooth power-law behaviour for multipoles $\ell <50$ in the CMB temperature anisotropy power spectrum found in the WMAP and Planck experiments, we have expanded our class of large field inflationary models that fit both the BICEP2 and Planck CMB observations consistently. These best-fitted large field models are found to have a transition from a faster roll to the slow roll $V(ϕ)=m^2 ϕ^2/2$ inflation at a field value around 14.6~${\rm M_{Pl}}$ and thus a potential energy of $V(ϕ) \sim (10^{16}\,{\rm GeV})^4$. In general this transition with sharp features in the inflaton potential produces not only suppression of scalars relative to tensor modes at small $k$ but also introduces wiggles in the primordial perturbation spectrum. These wiggles are shown to be useful to explain some localized features in the CMB angular power spectrum and can also have other observational consequences. Thus, primordial GW can be used now to make a tomography of inflation determining its fine structure. The resulting Wiggly Whipped Inflation scenario is described in details and the anticipated perturbation power spectra, CMB power spectra, non-Gaussianity and other observational consequences are calculated and compared to existing and forthcoming observations.

astro-ph.CO↗

arXiv2014

Whipped inflation

Motivated by the idea that inflation occurs at the GUT symmetry breaking scale, in this paper we construct a new class of large field inflaton potentials where the inflaton starts with a power law potential; after initial period of relative fast roll that lasts until after a few e-folds inside the horizon, it transits to the attractor of the slow roll part of the potential with a lower power. Due to the initial fast roll stages of inflation, we find a suppression in scalar primordial power at large scales and at the same time the choice of the potential can provide us a tensor primordial spectrum with high amplitude. This suppression in scalar power with a large tensor-to-scalar ratio helps us to reconcile the Planck and BICEP2 data in a single framework. We find that a transition from a cubic to quadratic form of inflaton potential generates an appropriate suppression in power of scalar primordial spectrum that provides significant improvement in fit compared to power law model when compared with Planck and BICEP2 data together. We calculate the extent of non-Gaussianity, specifically, the bispectrum for the best fit potential and show that it is consistent with Planck bispectrum constraints.

astro-ph.CO↗

arXiv2014

A Small, Rapid Optical-IR Response Gamma-Ray Burst Space Observatory (The NGRG)

Here we propose a new gamma-ray burst (GRB) mission, the Next Generation Rapid-Response GRB Observatory (NGRG). As with Swift, GRBs are initially located with a coded-mask X-ray camera. However, the NGRG has two new features: First, a beam-steering system to begin optical observations within ~ 1 s after location; second, a near-IR (NIR) camera viewing the same sky, for sensitivity to extinguished bursts. These features allow measurement of the rise phase of GRB optical-NIR emission. Thus far, the rise time and transition between prompt and afterglow in the optical and NIR are rarely measured. Rapid-response measurements explore many science topics including optical emission mechanisms (synchrotron vs. SSC, photospheric emission) and jet characteristics (reverse vs. forward shock emission, baryon-dominated vs. magnetic dominated). Rapid optical-NIR response can measure dynamic evolution of extinction due to vaporization of dust, and separate star system and galaxy dust extinction. We discuss these measurements, giving reliable detection rate estimates from analysis of Swift data and scaled Swift performance. The NGRG will explore optical/NIR emission measured earlier than ever before, and potentially fainter, more extinguished GRBs than ever before. In the current era, costs are important. Our proposed modest NGRG can still produce new GRB science, while providing rapid GRB alerts for the entire community for post-Swift GRB science. We show that an X-ray instrument barely 1/5 the area of Swift BAT will yield a significant fraction of Swift's detection rate: more than 65 X-ray, and with a 30 cm optical-IR telescope and modern cameras, more than 19 NIR and 14 optical detections each year. In addition, active feedback control of the beam-steering would remove the need for arc sec stabilization of the spacecraft, for a substantial cost saving.

astro-ph.IM↗

arXiv2014

Ruling out the power-law form of the scalar primordial spectrum

Combining Planck CMB temperature [1] and BICEP2 B-mode polarization data [2,3] we show qualitatively that, assuming inflationary consistency relation, the power-law form of the scalar primordial spectrum is ruled out at more than $3σ$ CL. This is an important finding, since the power-law form of the scalar primordial spectrum is one of the main assumptions of concordance model of cosmology and also a direct prediction of many inflationary scenarios. We show that a break or step in the form of the primordial scalar perturbation spectrum, similar to what we studied recently analyzing Planck data [4] can address both Planck and BICEP2 results simultaneously. Our findings also indicate that the data may require more flexibilities than what running of scalar spectral index can provide. Finally we show that an inflaton potential, originally appeared in [5] can generate both the step and the break model of scalar primordial spectrum in two different limits. The discussed potential is found to be favored by Planck data but marginally disfavored by BICEP2 results as it produces slightly lower amplitude of tensor primordial spectrum. Hence, if the tensor-to-scalar ratio ($r$) quoted by BICEP2 persists, it is of importance that we generate inflationary models with large $r$ and at the same time provide suppression in scalar primordial spectrum at large scales.

astro-ph.CO↗

arXiv2014

See Saw Inflation / Dark Matter / Dark Energy / Baryogenesis

Motivated by BICEP2 results that imply gravitational waves are produced when the universe has an expansion energy of about $Mc^2 \approx 10^{14}$ GeV and that a natural extension to the Standard Model of Particle physics is a right-handed neutrino that would or could be at $m_{ν_R}c^2 \approx 10^{14}$ GeV, I propose here a See-Saw Inflation model which fits into the general class of models we have dubbed "Wiggly Whipped Inflation"\cite{Hazra:2014WWI}. The residual heavy right-handed neutrinos work out to be Dark Matter in reasonable physical scenarios. The same scalar boson that stabilises the heavy right-handed neutrino mass then becomes the Inflaton whose potential is set by self-coupling and the heavy right-handed neutrino mass coupling. Following this See-Saw Inflation one also finds that the seesaw mechanism provides for an after electroweak symmetry breaking offset in the Inflaton potential at the near GUT scale vev by an amount set by the lightest neutrino and thus as a consequence produces a "Dark Energy" at a scale set by the lightest left-handed neutrino mass $m_{ν_L} \sim 10^{-3}$ eV. Since these neutrinos are Majorana particles and violate lepton number conservation, they are the natural mediators of baryogenesis at the electroweak scale. The residual heavy right-handed neutrinos work out to be Dark Matter with some fine tuning. So an unaesthetic fine-tuned model can address the four remaining fundamental issues of cosmology by linking them to the neutrino sector. The resurrected SO(10) GUT model provides a framework for the development of this scenario and allows a specific prediction for the Inflaton potential and a framework for fitting to neutrino as well as cosmological observables.

astro-ph.CO↗

arXiv2014

Some Implications of inverse-Compton Scattering of Hot Cocoon Radiation by relativistic jets in Gamma-Ray Bursts

Long Gamma-Ray Bursts (GRB) relativistic jets are surrounded by hot cocoons which confine jets during their punch out from the progenitor star. These cocoons are copious sources of X-ray photons that can be and are inverse-Compton (IC) scattered to MeV--GeV energies by electrons in the relativistic jet. We provide detailed estimates for IC flux resulting from various interactions between X-ray photons and the relativistic jet, and describe what we can learn about GRB jets and progenitor stars from the detection (or an upper limit) of these IC scattered photons.

astro-ph.HE↗

arXiv2014

Reconstruction of broad features in the primordial spectrum and inflaton potential from Planck

With the recently published Cosmic Microwave Background data from Planck we address the optimized binning of the primordial power spectrum. As an important modification to the usual binning of the primordial spectrum, along with the spectral amplitude of the bins, we allow the position of the bins also to vary. This technique enables us to address the location of the possible broad physical features in the primordial spectrum with relatively smaller number of bins compared to the analysis performed earlier. This approach is in fact a reconstruction method looking for broad features in the primordial spectrum and avoiding fitting noise in the data. Performing Markov Chain Monte Carlo analysis we present samples of the allowed primordial spectra with broad features consistent with Planck data. To test how realistic it is to have step-like features in primordial spectrum we revisit an inflationary model, proposed by A. A. Starobinsky which can address the similar features obtained from the binning of the spectrum. Using the publicly available code BINGO, we numerically calculate the local $f_{\rm NL}$ for this model in equilateral and arbitrary triangular configurations of wavevectors and show that the obtained non-Gaussianity for this model is consistent with Planck results. In this paper we have also considered different spectral tilts at different bins to identify the cosmological scale that the spectral index needs to have a red tilt and it is interesting to report that spectral index cannot be well constrained up to $k \sim 0.01~ {\rm Mpc}^{-1}$.

astro-ph.CO↗

arXiv2012

Entropic Inflation

One of the major pillars of modern cosmology theory is a period of accelerating expansion in the early universe. This accelerating expansion, or inflation, must be sustained for at least 30 e--foldings. One mechanism used to drive the acceleration is the addition of a new energy field, called the Inflaton; often this is a scalar field. We propose an alternative mechanism which, like our approach to explain the late-time accelerating universe, uses the entropy and temperature intrinsic to information holographically stored on a surface enclosing the observed space. The acceleration is due in both cases to an emergent entropic force, naturally arising from the information storage on the horizon.

hep-th↗

arXiv2011

Testing cosmological models using relative mass-redshift abundance of SZ clusters

Recent detection of high-redshift, massive clusters through Sunyaev-Zel'dovich observations has opened up a new way to test cosmological models. It is known that detection of a single supermassive cluster at a very high redshift can rule out many cosmological models all together. However, since dealing with different observational biases makes it difficult to test the likeliness of the data assuming a cosmological model, most of the cluster data (except those with high mass-redshift) stays untouched in confronting cosmological models with cluster observations. We propose here that one can use the relative abundance of the clusters with different masses at different redshifts to test the likeliness of the data in the context of cosmological models. For this purpose we propose a simple parametric form for the efficiency of observing clusters at different mass-redshift and we test if the standard LCDM model can explain the observed abundance of the clusters using this efficiency parameterization. We argue that one cannot expect an unusual and highly parametric form of the efficiency function to fit the observed data assuming a theoretical model. Using many realizations of Monte Carlo simulations we show that the standard spatially flat LCDM model is barely consistent with the SPT cluster data using a simple and plausible two-dimensional efficiency function for detection of the clusters. More cluster data are needed to make any strong conclusion.

astro-ph.CO↗

arXiv2010

Entropic Accelerating Universe

To accommodate the observed accelerated expansion of the universe, one popular idea is to invoke a driving term in the Friedmann-Lemaitre equation of dark energy which must then comprise 70% of the present cosmological energy density. We propose an alternative interpretation which takes into account the entropy and temperature intrinsic to the horizon of the universe due to the information holographically stored there. Dark energy is thereby obviated and the acceleration is due to an entropic force naturally arising from the information storage on the horizon surface screen. We consider an additional quantitative approach inspired by surface terms in general relativity and show that this leads to the entropic accelerating universe.

hep-th↗