Primordial Gravitational Wave Background Research Articles

Advances in String Theory in Curved Backgrounds: A Synthesis Report

A synthetic report of the advances in the study of classical and quantum string dynamics in curved backgrounds is provided, namely : the new feature of Multistring solutions; the mass spectrum of Strings in Curved backgrounds; The effect of a Cosmological Constant and of Spacial Curvature on Classical and Quantum Strings; Classical splitting of Fundamental Strings; The General String Evolution in constant Curvature Spacetimes; The Conformal Invariance Effects; Strings on plane fronted and gravitational shock waves, string falling on spacetime singularities and its spectrum. New Developments in String Gravity and String Cosmology are reported: String driven cosmology and its Predictions; The primordial gravitational wave background; Non-singular string cosmologies from Exact Conformal Field Theories; Quantum Field Theory, String Temperature and the String Phase of de Sitter space-time; Hawking Radiation in String Theory and the String Phase of Black Holes; New Dual Relation between Quantum Field Theory regime and String regime and the "QFT/String Tango"; New Coherent String States and Minimal Uncertainty Principle in string theory.

Studying the anisotropy of the gravitational wave stochastic background with LISA

A plethora of gravitational wave stochastic backgrounds populate the sensitivity window of the Laser Interferometer Space Antenna (LISA). We show that LISA can detect the anisotropy of the background corresponding to the multipole moments of order $l=2$ and $4.$ The signal-to-noise ratio generated by galactic white dwarf binary systems could be as high as $\ensuremath{\approx}60$ for 3 yr of integration, and LISA could provide valuable information on the spatial distribution of a variety of galactic sources. We also show that the cross correlation of the data sets from two interferometers could marginally lead to meaningful upper limits on the degree of isotropy of the primordial gravitational wave background.

High energy physics and the very early universe with LISA

Gravitational wave experiments will play a key role in the investigation of the frontiers of cosmology and the structure of fundamental fields at high energies, by detecting, or setting strong upper-limits to, the primordial gravitational wave background produced in the early-Universe. Here we discuss the impact of space-borne laser interferometric detectors operating in the low-frequency window ($\sim 1 \mu$Hz - 1 Hz); the aim of our analysis is to assess the detectability of a primordial background characterized by a fractional energy density $\Omega \sim 10^{-16} - 10^{-15}$, which is consistent with the prediction of "slow-roll" inflationary models.

Entropy of gravitons produced in the early universe

Gravitons produced from quantum vacuum fluctuations during an inflationary stage in the early Universe have zero entropy as far as they reflect the time evolution (squeezing) of a pure state, their large occupation number notwithstanding. A non-zero entropy of the gravitons (classical gravitational waves (GW) after decoherence) can be obtained through coarse graining. The latter has to be physically justified {\it and} should not contradict observational constraints. We propose two ways of coarse graining for which the fixed temporal phase of each Fourier mode of the GW background still remains observable: one based on quantum entanglement, and another one following from the presence of a secondary GW background. The proposals are shown to be mutually consistent. They lead to the result that the entropy of the primordial GW background is significantly smaller than it was thought earlier. The difference can be ascribed to the information about the regular (inflationary) initial state of the Universe which is stored in this background and which reveals itself, in particular, in the appearance of primordial peaks (acoustic peaks in the case of scalar perturbations) in the multipole spectra of the CMB temperature anisotropy and polarization.

Large primordial gravitational wave background in a class of BSI ΛCDM models

We consider the primordial gravitational wave (GW) background in a class of spatially flat inflationary cosmological models with cold dark matter (CDM), a cosmological constant, and a broken scale-invariant (BSI) step-like primordial (initial) spectrum of adiabatic perturbations produced in an exactly solvable inflationary model where the inflaton potential has a rapid change of its first derivative at some point. In contrast to inflationary models with a scale-free initial spectrum, these models may have a GW power spectrum the amplitude (although not the shape) of which is arbitrary for fixed amplitude and shape of the adiabatic perturbation power spectrum. In the presence of a positive cosmological constant, the models investigated here possess the striking property that a significant part of the large-angle cosmic microwave background (CMB) temperature anisotropy observed in the COBE experiment is a result of primordial GW. Confronting the models with existing observational data on CMB angular temperature fluctuations, galaxy clustering and peculiar velocities of galaxies, we find that, for the best parameter values ΩΛ≈0.7 and h≈0.7, the GW contribution to the CMB anisotropy can be as large as that of the scalar fluctuations.

Amplification of gravitational waves in radiation-dominated universes: Relic gravitons in models with matter creation

The formation of a background of primordial gravitational waves is investigated in Friedmann-Robertson-Walker (FRW) universes endowed with adiabatic matter creation. The condition for conformal invariance of the gravitational wave equation on this scenario is established. The relation between conformal non-invariance and the process of superadiabatic amplification of waves is clarified. It is shown that, in such models, the amplification process may occur even during a radiation-dominated phase. Generalizing previous work, the spectrum of the relic gravitons associated with transitions between arbitrary cosmic eras is obtained. The analysis is not restricted to the low-frequency band. It is shown that, for a certain range of the parameters involved, the sudden transition approximation between eras can still give accurate results at frequencies higher than it is usually assumed. A simple expression for the spectrum in this range is obtained. Constraints on the matter creation rate are derived from the possible influence of the relic gravitons on the nucleosynthesis of ${}^{4}\mathrm{He}.$

Mixed Cold-Hot Dark Matter Model with Falling and Quasi-flat Initial Perturbation Spectra

view Abstract Citations (54) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Mixed Cold-Hot Dark Matter Model with Falling and Quasi-flat Initial Perturbation Spectra Pogosyan, Dmitri Yu. ; Starobinsky, Alexei A. Abstract The mixed cold-hot dark matter cosmological model (CHDM) withΩ tot = 1 and a falling power-law initial spectrum of Gaussian adiabatic perturbations (n > 1) is tested using recent observational data. It is shown that its fit to the data becomes worse with the growth of n-1 and may be considered as unreasonable for n> 1.1 for all possible values of the Hubble constant. Thus, the CHDM model with a falling initial spectrum is worse than the same model with the approximately flat (|n-1| <0.1) spectrum. On the other hand, the CHDM model provides a rather good fit to the data if n lies in the range (0.9-1.0), the Hubble constant H0 < 60 km s-1 Mpc-1 (H0 < 55 for n = 1) and the neutrino energy density Ω < 0.25. So, the CHDM model offers the best possibility for the realization of the simplest variants of the inflationary scenario having the effective slope n ≍ (0.95-0.97) between galaxy and horizon scales, including a modest contribution of primordial gravitational wave background to large-angle ΔT/T fluctuations of the cosmic microwave background (resulting in the increase of their total rms amplitude by 5%-10% expected in some variants). A classification of cosmological models according to the number of fundamental parameters used to fit observational data is presented, too. Publication: The Astrophysical Journal Pub Date: July 1995 DOI: 10.1086/175890 arXiv: arXiv:astro-ph/9409074 Bibcode: 1995ApJ...447..465P Keywords: COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; COSMOLOGY: DARK MATTER; COSMOLOGY: OBSERVATIONS; COSMOLOGY: THEORY; Astrophysics E-Print: 18 pages, LaTeX file, YITP/U-94-26, postscript figures on request full text sources arXiv | ADS |

On the agreement between COBE anisotropy results and specific predictions from clusters of galaxies

The recent results from the first year of data from the COBE DMR experiment (Smoot et al. 1992) detect a large angle anisotropy in the Cosmic Microwave Background [CMB]. This first set of data seems to indicate that the scale-invariant (n = 1) spec­ trum is preferred, with an amplitude at the horizon crossing of 'H = (5.4 ± 1.6) .10- 6 (Wright et al. 1992). This level is in remarkable agreement with the value of 'H ~ (1.5/b) (6 ± 2.5) . 10- 6 , which was predicted to be the relevant one from a study of the distribution of Abell clusters which sampled large scales which correspond to angles up to twice the COBE FWHM (Scaramella 1992), where the value b - 1.5 was suggested by agreement with the observed large scale peculiar velocities. This observational agreement indicates that on the very large scales the peculiar gravitational potential has kept nearly constant from the epoch of last scattering to the present. This fact, together with the presence of large scale flows, suggests that we live in a spatially flat Universe of critical density, where optical galaxies are mildly biased with respect to the matter distribution, and with a power spectrum which, depending on the spectral index on small scales, has a turnover to n =1 in the 100 - 200 h-1 Mpc scale range. We also considered a tilted (n < 1) CDM model, and find that this model would need a small spectral index (n - 0.3) to match the cluster normalization. However, such an index would yield a too large CMB anisotropy, even without the the possible additional anisotropy due teJ the presence of a significant background of primordial gravitational waves.

The gravitational wave contribution to cosmic microwave background anisotropies and the amplitude of mass fluctuations from COBE results

A stochastic background of primordial gravitational waves may substantially contribute, via the Sachs--Wolfe effect, to the large--scale Cosmic Microwave Background (CMB) anisotropies recently detected by COBE. This implies a {\it bias} in any resulting determination of the primordial amplitude of density fluctuations. We consider the constraints imposed on $n<1$ (``tilted") power--law fluctuation spectra, taking into account the contribution from both scalar and tensor waves, as predicted by power--law inflation. The gravitational--wave contribution to CMB anisotropies generally reduces the required {\it rms} level of mass fluctuation, thereby increasing the linear {\it bias parameter}, even in models where the spectral index is close to the Harrison--Zel'dovich value $n=1$. This ``gravitational--wave bias" helps to reconcile the predictions of CDM models with observations on pairwise galaxy velocity dispersion on small scales.