Is the transition redshift a new cosmological number?

Observations from Supernovae Type Ia (SNe Ia) provided strong evidence for an expanding accelerating Universe at intermediate redshifts. This means that the Universe underwent a transition from deceleration to acceleration phases at a transition redshift $z_t$ of the order unity whose value in principle depends on the cosmology as well as on the assumed gravitational theory. Since cosmological accelerating models endowed with a transition redshift are extremely degenerated, in principle, it is interesting to know whether the value of $z_t$ itself can be observationally used as a new cosmic discriminator. After a brief discussion of the potential dynamic role played by the transition redshift, it is argued that future observations combining SNe Ia, the line-of-sight (or "radial") baryon acoustic oscillations, the differential age of galaxies, as well as the redshift drift of the spectral lines may tightly constrain $z_t$, thereby helping to narrow the parameter space for the most realistic models describing the accelerating Universe.Comment: 12 pages, 5 figures. Some discussions about how to estimate the transition redshift have been added. New data by Planck and H(z) data have been mentioned. New references have been adde

New coupled quintessence cosmology

A component of dark energy has been recently proposed to explain the current acceleration of the Universe. Unless some unknown symmetry in Nature prevents or suppresses it, such a field may interact with the pressureless component of dark matter, giving rise to the so-called models of coupled quintessence. In this paper we propose a new cosmological scenario where radiation and baryons are conserved, while the dark energy component is decaying into cold dark matter (CDM). The dilution of CDM particles, attenuated with respect to the usual $a^{-3}$ scaling due to the interacting process, is characterized by a positive parameter $\epsilon$, whereas the dark energy satisfies the equation of state $p_x=\omega \rho_x$ ($\omega < 0$). We carry out a joint statistical analysis involving recent observations from type Ia supernovae, baryon acoustic oscillation peak, and Cosmic Microwave Background shift parameter to check the observational viability of the coupled quintessence scenario here proposed.Comment: 7 pages, 7 figures. Minor corrections to match published versio

Tomografia passiva costiera (TOMPACO). Inversion results with passive data - phase 3

Rep 02/03 - SiPLAB 28/March/2003This report shows the acoustic inversion results obtained on the INTIFANTE’00 data set, Events II, IV, V and VI

Real-time environmental inversion using a network of light receiving systems

This paper reports preliminary environmental inversion results of acoustic data collected simultaneously at two receiving systems during the RADAR’07 sea trial. These receiving systems have communication capabilities that allow for transfering acoustic and telemetric data to a base station with processing capabilities in order to produce environmental estimates during the acoustic experiment. During a large part of the experiment estimates on the temperature field appear to agree with concurrent ground truth data

Matched-field tomography using an acoustic oceanographic buoy

The Acoustic Oceanographic Buoy (AOB) is a light acoustic receiving device that is being developed in the framework of a joint research project and tested during the Maritime Rapid Environmental Assessment (MREA) sea trials. One of the AOB’s application is in Matched-Field Tomography (MFT) when a reduced number of receivers is available in opposition to traditional systems used in tomography. One problem of chief importance in MFT is the degree of uniqueness of the problem’s solution which is highly dependent on the number of receivers and on the number of free parameters. This paper studies the possibility of using matched-field processors with reduced ambiguity levels in comparison to conventional processors with application to acoustic data collected during the MREA sea trials. Two aspects are investigated: (a) the choice of an explicit broadband data model, where the exploitation of the spectral coherence of the acoustic field is seen as a mean to reduce the ambiguity level of the cost function used in the optimization; (b) conventional and high-resolution methods based on the proposed broadband model are implemented and compared.FC

Broadband MFP: coherent vs. incoherent

Matched-Field Processing (MFP) is now a mature technique for source localization and tracking. There are at least two aspects that emerge, by their relevance, to the success of MFP: one is the ability of a given MFP processor to accurately pinpoint the source location while rejecting sidelobes, and the other is the impact of erroneous or missing environmental information (known as model mismatch) in the final source location estimate. This study addresses the first aspect regarding sidelobe rejection while considering that the processor is working on a mismatch free situation. One well known procedure to reduce sidelobes is to use a broadband MFP processor (whenever a band of frequencies is available). There are a number of different ways to combine MFP information across frequency that ran be classified in two broad groups: the conventional incoherent methods, that are based on the direct averaging of the auto-frequency inner products and the, say, less conventional methods, that perform a weighted average of the cross-frequency inner products where the weights are the frequency compensation phase-shifts. The later are generally termed as coherent broadband methods since they combine complex inner products. The coherent broadband methods proposed in the literature are either suboptimal or very computationally Intensive, even for a small number of frequencies. An alternative method is presented that combines cross-frequency information with the same localization performance than the standard coherent methods and a computation load similar to that of the incoherent processor. The performance of the various broadband processors is compared in simulated data

Range-dependent acoustic tomography: modeling an upwelling filament using an asymmetric Gaussian function

SiPLAB Report 05/03, University of Algarve,2003.Acoustic tomography in range-dependent waveguides using source-array pair represents an inverse problem with many potential solutions. The present problem is to model an upwelling filament which is a localized up rise of cold water and introducing a high degree pf range dependence. In this study a parameterization scheme with a reduced number of parameters is proposed in order to represent the spatial evolution of the filament using an asymmetric Gaussian function parameterized by two variances, an amplitude coefficient and a mean value. Using a real data example of the filament of the California current system, this modelling scheme is tested on semi-synthetic data. The results indicate that such an approach can be considered for an efficient modelling of a complex oceanographic feature.FC

Physical limitations of travel time based shallow water tomography

Travel-time-based tomography is a classical method for inverting sound-speed perturbations in an arbitrary environment. A linearization procedure enables relating travel-time perturbations to sound-speed perturbations through a kernel matrix. Thus travel-time-based tomography essentially relies on the inversion of the kernel matrix and is commonly called ‘linear inversion. In practice, its spatial resolution is limited by the number of resolved and independent arrivals, which is a basic linear algebra requirement for linear inversion performance. Physically, arrival independency is much more difficult to determine since it is closely related to the sound propagating channel characteristics. This paper presents a brief review of linear inversion and shows that, in deep water, the number of resolved arrivals is equal to the number of independent arrivals, while in shallow water the number of independent arrivals can be much smaller than the number of resolved arrivals. This implies that in shallow water there are physical limitations to the number of independent travel times. Furthermore, those limitations are explained through the analysis of an equivalent environment with a constant sound speed. The results of this paper are of central importance for the understanding of travel-time-based shallow water tomography