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Register a new userCosmic Parallax as a probe of late time anisotropic expansion
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Added by
Claudia Quercellini
Publication date
2009
fields
Physics
and research's language is
English
Authors
Claudia Quercellini
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Cosmic parallax is the change of angular separation between pair of sources at cosmological distances induced by an anisotropic expansion. An accurate astrometric experiment like Gaia could observe or put constraints on cosmic parallax. Examples of anisotropic cosmological models are Lemaitre-Tolman-Bondi void models for off-center observers (introduced to explain the observed acceleration without the need for dark energy) and Bianchi metrics. If dark energy has an anisotropic equation of state, as suggested recently, then a substantial anisotropy could arise at $z lesssim 1$ and escape the stringent constraints from the cosmic microwave background. In this paper we show that such models could be constrained by the Gaia satellite or by an upgraded future mission.
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In the current work, we have implemented an extension of the standard Gaussian Process formalism, namely the Multi-Task Gaussian Process with the ability to perform a joint learning of several cosmological data simultaneously. We have utilised the low-redshift expansion rate data from Supernovae Type-Ia (SN), Baryon Acoustic Oscillations (BAO) and Cosmic Chronometers (CC) data in a joint analysis. We have tested several possible models of covariance functions and find very consistent estimates for cosmologically relevant parameters. In the current formalism, we also find provisions for heuristic arguments which allow us to select the best-suited kernel for the reconstruction of expansion rate data. We also utilised our method to account for systematics in CC data and find an estimate of $H_0 = 68.52^{+0.94 + 2.51 (sys)}_{-0.94} $ $textrm{km/s Mpc}^{-1}$ and a corresponding $r_d = 145.61^{+2.82}_{ - 2.82 - 4.3 (sys)} $ Mpc as our primary result. Subsequently, we find constraints on the present deceleration parameter $q_0 = -0.52 pm 0.06$ and the transition redshift $z_T = 0.64^{+0.12}_{-0.09}$. All the estimated cosmological parameters are found to be in good agreement with the standard $Lambda$CDM scenario. Including the local model-independent $H_0$ estimate to the analysis we find $H_0 = 71.40^{ + 0.30 + 1.65 (sys)}_{- 0.30 } $ $textrm{km/s Mpc}^{-1}$ and the corresponding $r_d = 141.29^{ + 1.31 }_{-1.31-2.63 (sys)}$ Mpc. Also, the constraints on $r_d H_0$ remain consistent throughout our analysis and also with the model-dependent CMB estimate. Using the $mathcal{O}m(z)$ diagnostic, we find that the concordance model is very consistent within the redshift range $z lesssim 2$ and mildly discrepant for $z gtrsim 2$.
Current data indicate that the reionization of the Universe was complete by redshift z~6-7, and while the sources responsible for this process have yet to be identified, star-forming galaxies are often considered the most likely candidates. However, the contribution from z>6 galaxies to cosmic reionization critically depends on the fraction of ionizing (Lyman continuum, LyC) photons escaping from these objects and into the intergalactic medium. At z<4, the escaping LyC flux can be measured directly, but the opacity of the neutral intergalactic medium precludes such measurements at higher redshifts. In a recent paper, we argue that since the LyC escape fraction regulates the contribution of nebular emission to the rest-frame optical/UV spectra of galaxies, the James Webb Space Telescope should be able to indirectly assess the LyC escape fraction for galaxies at z~6-9. JWST can, on the other hand, not constrain the fraction of LyC photons directly absorbed by dust, and this is where SPICA comes in. The dust continuum emission from gravitationally lensed LyC-leakers at z=6 may in principle be detectable with SPICA, thereby constraining the level of LyC extinction in these objects.
Refined astrometry measurements allow us to detect large-scale deviations from isotropy through real-time observations of changes in the angular separation between sources at cosmic distances. This cosmic parallax effect is a powerful consistency test of FRW metric and may set independent constraints on cosmic anisotropy. We apply this novel general test to LTB cosmologies with off-center observers and show that future satellite missions such as Gaia might achieve accuracies that would put limits on the off-center distance which are competitive with CMB dipole constraints.
The leading candidate for the very early universe is described by a period of rapid expansion known as inflation. While the standard paradigm invokes a single slow-rolling field, many different models may be constructed which fit the current observational evidence. In this work we outline theoretical and observational studies of non-Gaussian fluctuations produced by models of inflation and by cosmic strings - topological defects that may be generated in the very early universe during a phase transition. In particular, we consider the imprint of cosmic strings on the cosmic microwave background (CMB) and describe a formalism for the measurement of general four-point correlation functions, or trispectra, using the CMB. In addition we describe the application of our methodology to non-Gaussian signals imprinted in the large scale structure of the universe. Such deviations from Gaussianity are generally expressed in terms of the so-called bispectrum and trispectrum.
The detection of a time variation of the angle between two distant sources would reveal an anisotropic expansion of the Universe. We study this effect of cosmic parallax within the ellipsoidal universe model, namely a particular homogeneous anisotropic cosmological model of Bianchi type I, whose attractive feature is the potentiality to account for the observed lack of power of the large-scale cosmic microwave background anisotropy. The preferred direction in the sky, singled out by the axis of symmetry inherent to planar symmetry of ellipsoidal universe, could in principle be constrained by future cosmic parallax data. However, that will be a real possibility if and when the experimental accuracy will be enhanced at least by two orders of magnitude.
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