Spectrum Of Background Radiation Research Articles

Spectral estimation on a sphere in geophysics and cosmology

We address the problem of estimating the spherical-harmonic power spectrum of a statistically isotropic scalar signal from noise-contaminated data on a region of the unit sphere. Three different methods of spectral estimation are considered: (i) the spherical analogue of the one-dimensional (1-D) periodogram, (ii) the maximum likelihood method, and (iii) a spherical analogue of the 1-D multitaper method. The periodogram exhibits strong spectral leakage, especially for small regions of area $A\ll 4\pi$, and is generally unsuitable for spherical spectral analysis applications, just as it is in 1-D. The maximum likelihood method is particularly useful in the case of nearly-whole-sphere coverage, $A\approx 4\pi$, and has been widely used in cosmology to estimate the spectrum of the cosmic microwave background radiation from spacecraft observations. The spherical multitaper method affords easy control over the fundamental trade-off between spectral resolution and variance, and is easily implemented regardless of the region size, requiring neither non-linear iteration nor large-scale matrix inversion. As a result, the method is ideally suited for most applications in geophysics, geodesy or planetary science, where the objective is to obtain a spatially localized estimate of the spectrum of a signal from noisy data within a pre-selected and typically small region.

The cosmic microwave background radiation in a non-expanding universe

During the nineteenth century, it was common for physicists to believe in the existence of a material vacuum composed of an incompressible fluid that fills the whole universe. This fluid was called the aether. Its original purpose was to provide an elastic tenuous medium for light propagation through space. Although it is well understood today that no such medium is needed for light propagation, the existence of a cosmic aether medium in space is still possible and its physical properties can be understood on models of cosmology that have nothing to do with Big-Bang cosmology. It is possible that electromagnetic radiation emitted by the cosmic aether medium has already been detected. The low-frequency electromagnetic radiation emitted by the aether is called the cosmic microwave background radiation. The present study outlines a model for an aether medium that explains the genesis of the microwave background radiation in a closed static (nonexpanding) universe. It is shown that the spectrum of the microwave background radiation is a perfect blackbody with a temperature Trad=2.77 K in harmony with the perfect cosmological principle. It is further shown that the aether medium is opaque at radio and microwave frequencies. This particular feature of the model does not contradict any observations regarding the existence of distant radio galaxies and quasars.

A Test of the Copernican Principle

The blackbody nature of the cosmic microwave background (CMB) radiation spectrum is used in a modern test of the Copernican principle. The reionized universe serves as a mirror to reflect CMB photons, thereby permitting a view of ourselves and the local gravitational potential. By comparing with measurements of the CMB spectrum, a limit is placed on the possibility that we occupy a privileged location, residing at the center of a large void. The Hubble diagram inferred from lines of sight originating at the center of the void may be misinterpreted to indicate cosmic acceleration. Current limits on spectral distortions are shown to exclude the largest voids which mimic cosmic acceleration. More sensitive measurements of the CMB spectrum could prove the existence of such a void or confirm the validity of the Copernican principle.

The Hard VHE γ-Ray Emission in High-Redshift TeV Blazars: Comptonization of Cosmic Microwave Background Radiation in an Extended Jet?

Observations of very-high-energy (VHE; E > 250 GeV) γ-ray emission from several blazars at z > 0.1 have placed stringent constraints on the elusive spectrum and intensity of the intergalactic infrared background radiation (IIBR). Correcting the observed VHE spectrum for γγ absorption, even by the lowest plausible level of the IIBR, provides evidence for a very hard (photon spectral index Γph < 2) intrinsic source spectrum out to TeV energies. Such a hard VHE γ-ray spectrum poses a serious challenge to the conventional synchrotron self-Compton (SSC) interpretation of the VHE emission of TeV blazars and suggests the emergence of a separate emission component beyond a few hundred GeV. Here we propose that such a very hard, slowly variable VHE emission component in TeV blazars may be produced via Compton upscattering of cosmic microwave background (CMB) photons by shock-accelerated electrons in an extended jet. For the case of 1ES 1101–232, this component could dominate the bolometric luminosity of the extended jet if the magnetic fields are of the order of typical intergalactic magnetic fields (B ∼ 10 μG) and if electrons are still being accelerated out to TeV energies (γ ≳ 4 × 106) on kiloparsec scales along the jet.

Spatial periodicity of galaxy number counts, CMB anisotropy, and SNIa Hubble diagram based on the universe accompanied by a non-minimally coupled scalar field

We have succeeded in establishing a cosmological model with a non-minimally coupled scalar field φ that can account not only for the spatial periodicity or the picket-fence structure exhibited by the galaxy N-z relation of the 2dF survey but also for the spatial power spectrum of the cosmic microwave background radiation (CMB) temperature anisotropy observed by the WMAP satellite. The Hubble diagram of our model also compares well with the observation of Type Ia supernovae. The scalar field of our model universe starts from an extremely small value at around the nucleosynthesis epoch, remains in that state for sufficiently long periods, allowing sufficient time for the CMB temperature anisotropy to form, and then starts to grow in magnitude at the redshift z of ∼1, followed by a damping oscillation which is required to reproduce the observed picket-fence structure of the N-z relation. To realize such behavior of the scalar field, we have found it necessary to introduce a new form of potential V(φ)∝ φ 2exp (−q φ 2), with q being a constant. Through this parameter q, we can control the epoch at which the scalar field starts growing.

Nobel Lecture: From the Big Bang to the Nobel Prize and beyond

NASA's Cosmic Background Explorer satellite mission, the COBE, laid the foundations for modern cosmology by measuring the spectrum and anisotropy of the cosmic microwave background radiation and discovering the cosmic infrared background radiation. I describe the history of the COBE project, its scientific context, the people who built it, and the scientific results. The COBE observed the universe on the largest scales possible by mapping the cosmic microwave and infrared background radiation fields and determining their spectra. It produced conclusive evidence that the hot Big Bang theory of the early universe is correct, showed that the early universe was very uniform but not perfectly so, and that the total luminosity of post--Big Bang objects is twice as great as previously believed. The COBE concept was developed by a Mission Definition Study Team appointed by NASA in 1976, based on three competing proposals submitted in 1974. The COBE was built in-house by Goddard Space Flight Center, with a helium cryostat provided by Ball Aerospace, and was launched on a Delta rocket built by McDonnell Douglas. It is in a circular orbit $900\phantom{\rule{0.3em}{0ex}}\mathrm{km}$ above the Earth, in a plane inclined 99\ifmmode^\circ\else\textdegree\fi{} to the equator and roughly perpendicular to the line to the Sun. It carried three instruments, a far infrared absolute spectrophotometer (FIRAS), a differential microwave radiometer with three channels (DMR), and a diffuse infrared background experiment (DIRBE). The helium cryostat cooled the FIRAS and DIRBE for $10\phantom{\rule{0.3em}{0ex}}\text{months}$ until the helium was exhausted, but operations continued for a total of $4\phantom{\rule{0.3em}{0ex}}\text{years}$. Subsequent observations have confirmed the COBE results and led to measurements of the main cosmological parameters with a precision of a few percent.

Constraining interacting dark energy models with flux destabilization

A destabilization in the transfer energy flux from the vacuum to radiation, for two vacuum decay laws relevant to the dark energy problem, is analyzed using the Landau–Lifshitz fluctuation hydrodynamic theory. Assuming thermal (or near thermal) equilibrium between the vacuum and radiation, at the earliest epoch of the Universe expansion, we show that the law due to renormalization-group running of the cosmological constant term, with parameters chosen not to spoil the primordial nucleosynthesis scenario, does soon drive the flux to fluctuate beyond its statistical average value thereby distorting the cosmic background radiation spectrum beyond observational limits. While the law coming from the saturated holographic dark energy does not lead the flux to wildly fluctuate, a more realistic non-saturated form shows again such anomalous behavior.

Relativistic modified Newtonian dynamics from string theory?

We argue that TeVeS-like vector fields appear naturally in certain string theory backgrounds involving D0-branes, as a result of the recoil velocity field, expressing the interaction of neutrino string matter with point-like branes. However, the similarity with TeVeS models is restricted only to the bi-metric properties of space–time, namely the difference of the background metric from the one felt by (some) matter fields interacting, in a topologically non-trivial manner, with the D0-brane defects. In our approach, neutrinos appear as dark matter candidates that could be “captured” by the D0-branes, as a result of stringy properties, and thus couple with the recoil-vector fields. Moreover, we argue in support of a possibly preferential rôle of neutrinos in inducing novel non-perturbative contributions to “vacuum” (dark) energy, in addition to their ordinary dark matter contribution. In fact, the rôle of neutrinos as providing substantial contributions to dark matter and dark energy components of the Universe, suggested by our approach, appears also to be necessitated by the need to reproduce the peaks in the Cosmic Microwave Background radiation spectrum, as claimed recently in the literature. Thus, our framework may be viewed as providing a microscopic explanation of such phenomenological conclusions concerning TeVeS-like, Lorentz-violating models.

Cosmic ray origin of the diffuse gamma-ray background radiation

As a test of our unified theory of cosmic rays and high energy astrophysical phenomena, we have calculated the diffuse gamma ray background radiation produced by inverse Compton scattering of cosmic-microwave-background and starlight photons by Galactic and extragalactic cosmic ray electrons. The main sources of such electrons are the highly relativistic jets from supernova explosions and active galactic nuclei. We show that our calculations well reproduce the spectrum and intensity of the diffuse gamma-ray background radiation measured by EGRET in directions away from the Galactic disk and centre.

Gauss-Bonnet quintessence: Background evolution, large scale structure, and cosmological constraints

We investigate a string-inspired dark energy scenario featuring a scalar field with a coupling to the Gauss-Bonnet invariant. We discuss extensively the cosmological and astrophysical implications of the coupled scalar field. Such coupling can trigger the onset of late dark energy domination after a scaling matter era. The universe may then cross the phantom divide and perhaps also exit from the acceleration. The evolution of fluctuations in the scalar field and their impact on the clustering of matter are studied in detail and model independently. The small-scale limit is derived for the perturbations and their stability is addressed. The general equations for scalar perturbations are also presented and solved numerically, confirming that the Gauss-Bonnet coupling can be compatible with the observed spectrum of cosmic microwave background radiation as well as the matter power spectrum inferred from large-scale surveys. Data from the solar system, supernovae Ia, cosmic microwave background radiation, large-scale structure, and big bang nucleosynthesis are used to constrain the parameters of the model. The geometric constraints from background expansion favor exponential potentials with a shallow slope, which is in tension with the nucleosynthesis bound on early quintessence. Also, high values for the present matter density are required. Including the baryon oscillation scale, one could rule out the model at about 99% confidence level. A discussion of how to overcome such possible problems in more elaborate models is included, together with considerations of the validity of these constraints in the present context. Interestingly, one also finds that a good Newtonian limit may require fixing the coupling.

Cosmological dark energy: prospects for a dynamical theory

We present an approach to the problem of vacuum energy in cosmology, based on dynamical screening of Λ on the horizon scale. We review first the physical basis of vacuum energy as a phenomenon connected with macroscopic boundary conditions, and the origin of the idea of its screening by particle creation and vacuum polarization effects. We next discuss the relevance of the quantum trace anomaly to this issue. The trace anomaly implies additional terms in the low-energy effective theory of gravity, which amounts to a non-trivial modification of the classical Einstein theory, fully consistent with the Equivalence Principle. We show that the new dynamical degrees of freedom the anomaly contains provide a natural mechanism for relaxing Λ to zero on cosmological scales. We consider possible signatures of the restoration of conformal invariance predicted by the fluctuations of these new scalar degrees of freedom on the spectrum and statistics of the cosmic microwave background radiation (CMB), in light of the latest bounds from WMAP. Finally we assess the prospects for a new cosmological model in which the dark energy adjusts itself dynamically to the cosmological horizon boundary, and therefore remains naturally of order H2 at all times without fine tuning.

Discharge on boiling in a channel: Effect of channel geometry on the performance characteristics of determining metals in a liquid flow by atomic emission spectrometry

Discharge on boiling in a channel was studied as a new atomization and excitation source for spectrochemical analysis in a flow of electrolyte solutions. The discharge arises between the liquid walls of a vapor lock formed in the channel of a dielectric membrane because of the rapid Joule heating of the liquid in the channel. The effect of channel geometry on the reproducibility of the integrated light intensity was studied. The background radiation spectrum was measured over the range 220–900 nm, and the possibility of determining alkali and alkaline earth metals in a flow was studied. The parameters of linear calibration equations and the detection limits for these metals are given.

Planck scale inflationary spectra from quantum gravity

We derive the semiclassical evolution of massless minimally coupled scalar matter in the de Sitter space-time from the Born-Oppenheimer reduction of the Wheeler-DeWitt equation. We show that the dynamics of trans-Planckian modes can be cast in the form of an effective modified dispersion relation and that high energy corrections in the power spectrum of the cosmic microwave background radiation produced during inflation remain very small if the initial state is the Bunch-Davies vacuum.

Analytic approach to the CMB polarization generated by relic gravitational waves

By Polnarev's method we analytically calculate the polarization spectra of the cosmic microwave background radiation (CMB) generated by cosmic relic gravitational waves (RGW). In this analytic approach the physics involved in this generating process is more transparent. Consequently, the effects due to various elements of physics can be isolated easily. In solving the equation for evolution of RGW in the expanding universe, both the sudden transition and the WKB approximation for the scale factor during the radiation-matter transition have been taken. To describe more precisely the decoupling process, we have introduced an analytic expression for the visibility function, consisting of two pieces of half-Gaussian curves. We also include the damping on polarizations due to the photon diffusion up to the second order of the tight coupling. Analytic polarization spectra ${C}_{l}^{XX}$ have been obtained with the following several improvements over the previous results. 1. The approximate analytic result is quite close to the numerical one evaluated from the cmbfast code, especially, for the first three peaks of the spectrum that are observable. By using the analytic exact solution of RGW in the sudden transition approximation, we have demonstrated the dependence of ${C}_{l}^{XX}$ on the dark energy and the baryons. 2. Our analytic half-Gaussian approximation of the visibility function fits better than the usual Gaussian model, and its time integration yields an analytic damping factor, which is parameter-dependent. This improves the spectra by $\ensuremath{\sim}30%$ around the second and third peaks. 3. The second order of tight coupling reduces the overall amplitude of ${C}_{l}^{XX}$ by 58%, comparing with the tight-coupling limit. 4. The influences of inflation on RGW and on CMB polarization are explicitly demonstrated.

Structure formation and CMBR anisotropy spectrum in the inflessence model

The inflessence model has recently been proposed in an attempt to explain both early inflation and present day accelerated expansion within a single mechanism. The model has been successfully tested against the Hubble diagram of Type Ιa Supernovae, the shift parameter, and the acoustic peak parameter. As a further mandatory test, we investigate here structure formation in the inflessence model determining the evolution of matter density contrast 6 = δρ M /ρ M in the linear regime. We compare the growth factor D(a) ≡ 6/a and the growth index f(z) ≡ dlnδ/dlnα to these same quantities for the successful concordance ACDM model with a particular emphasis on the role of the inflessence parameters (γ,z Q ). We also evaluate the anisotropy spectrum of the cosmic microwave background radiation (CMBR) to check whether the inflessence model may be in agreement with the observations. We find that, for large values of (γ, z Q ). structure formation proceeds in a similar way to that in the ACDM scenario, and it is also possible to nicely fit the CMBR spectrum.

A Blind Estimation of the Angular Power Spectrum of CMB Anisotropy from WMAP

Accurate measurements of the angular power spectrum of cosmic microwave background (CMB) radiation have lead to a marked improvement in the estimates of different cosmological parameters. This has required removal of foreground contamination as well as detector noise bias with reliability and precision. We present the estimation of the CMB angular power spectrum from the multifrequency observations of the Wilkinson Microwave Anisotropy Probe (WMAP) using a novel model-independent method. The primary results of WMAP are the observations of the CMB in 10 independent difference assemblies (DAs) that have uncorrelated noise. Our method utilizes the maximum information available within the WMAP data by linearly combining all the DA maps in order to remove foreground contamination and by estimating the power spectrum from cross-power spectra of clean maps with independent noise. We compute 24 cross-power spectra that are the basis of the final power spectrum. The binned average power matches the WMAP team's published power spectrum closely. A small systematic difference at large multipoles is accounted for by the correction for the expected residual power from unresolved point sources. The correction is small and significantly tempered. Previous estimates have depended on foreground templates built using extraneous observational input. This is the first demonstration that the CMB angular spectrum can be reliably estimated with precision from a self-contained analysis of the WMAP data.

Hybrid estimation of cosmic microwave background polarization power spectra

This paper generalizes the hybrid power spectrum estimation method developed in a previous paper to the estimation of polarization power spectra of the cosmic microwave background radiation. The hybrid power spectrum estimator is unbiased and we show that it is close to optimal at all multipoles, provided the pixel noise satisfies certain constraints. Furthermore, the hybrid estimator is computationally fast and can easily be incorporated in a Monte Carlo chain for Planck-sized data sets, which may be essential to account for realistic noise covariance and systematic errors. Simple formulae are given for the covariance matrices, including uncorrelated inhomogeneous instrumental noise, and these are tested extensively against numerical simulations. We compare the behaviour of simple pseudo-Cℓ (PCL) estimates with maximum-likelihood estimates at low multipoles. For realistic sky cuts, maximum-likelihood estimates reduce very significantly the mixing of E and B modes. To achieve limits on the scalar–tensor ratio of r≪ 0.1 from sky maps with realistic sky cuts, maximum-likelihood methods, or PCL estimators based on unambiguous E and B modes, will be essential.

Dark energy anisotropic stress and large scale structure formation

We investigate the consequences of an imperfect dark energy fluid on the large scale structure. A phenomenological three parameter fluid description is used to study the effect of dark energy on the cosmic microwave background radiation (CMBR) and matter power spectrum. In addition to the equation of state and the sound speed, we allow a nonzero viscosity parameter for the fluid. Then anisotropic stress perturbations are generated in dark energy. In general, we find that this possibility is not excluded by the present day cosmological observations. In the simplest case when all of the three parameters are constant, we find that the observable effects of the anisotropic stress can be closely mimicked by varying the sound speed of perfect dark energy. However, now also negative values for the sound speed, as expected for adiabatic fluid model, are tolerable and in fact could explain the observed low quadrupole in the CMBR spectrum. We investigate also structure formation of imperfect fluid dark energy characterized by an evolving equation of state. In particular, we study models unifying dark energy with dark matter, such as the Chaplygin gas or the Cardassian expansion, with a shear perturbation included. This can stabilize the growth of inhomogeneities in these models, thus somewhat improving their compatibility with large scale structure observations.

Temperature of the Inflaton and Duration of Inflation from Wilkinson Microwave Anisotropy Probe Data

If the initial state of the inflaton field is taken to have a thermal distribution instead of the conventional zero particle vacuum state then the curvature power spectrum gets modified by a temperature dependent factor such that the fluctuation spectrum of the microwave background radiation is enhanced at larger angles. We compare this modified cosmic microwave background spectrum with Wilkinson microwave anisotropy probe data to obtain an upper bound on the temperature of the inflaton at the time our current horizon crossed the horizon during inflation. We further conclude that there must be additional -foldings of inflation beyond what is needed to solve the horizon problem.

Galaxy merging and number vs. apparent magnitude relation for the universe with a time-decaying cosmological term

Aims. An attempt is made to constrain the values of the cosmological parameters together with the galaxy merging factor η on the basis of a comparison between the observed galaxy number counts vs. their apparent magnitudes relation (N-m relation) with those theoretically constructed for the universe with a time-decaying cosmological term A. Methods. We assume that the galaxy number density evolution can be represented sufficiently well by a function of the redshift z of the form Φ*(z) ∝ (1 + z) η . Three variations of the cosmological term with time T are considered, (1) A oc τ -1 , (2) A ∝ a -m with a being the scale factor, and (3) A oc with H the Hubble parameter. The optimum ranges for the decaying parameters (l, m, and n), the density parameters Ω Λ,0 and Ω m,0 , as well as T mg (the timescale for the merger of a pair of galaxies) and the redshift z mg for the first onset of galaxy merger are sought based on statistical analysis using likelihood functions given by Χ 2 evaluations. Results. In the case of the type I models, for instance, we find that l = 0.75 +0.55 -0.75 , T mg = 0.3 Gyr, z mg = 3, Ω Λ,0 = 0.71 +0.17 -0.09 and Ω m,0 = 0.29 +0.09 -0.17 , from which a cosmic age of 17.2 +1.3 -1.2 Gyr results. These model parameters lead to η = 1.73 +0.14 -0.07 . As a consistency check, we have also carried out computations of the cosmic microwave background radiation (CMBR) spectrum, and have made comparisons with WMAP measurements. We found that it is necessary to somewhat modify the parameter values obtained above on account of the high sensitivity of η to the value of T mg . The final model that was found to account for both the observed N-m relation and the WMAP measurements of the CMBR spectrum is as follows: z mg = 3.0, T mg = 0.2 Gyr, l = 0.04, Ω Λ,0 = 0.77, η = 2.2269. The age of this model universe is 14.6 Gyr, which is still sufficiently high to cope with the new cosmic age problem.