The Hubble rate in averaged cosmology

The calculation of the averaged Hubble expansion rate in an averaged perturbed Friedmann-Lemaitre-Robertson-Walker cosmology leads to small corrections to the background value of the expansion rate, which could be important for measuring the Hubble constant from local observations. It also predicts an intrinsic variance associated with the finite scale of any measurement of H_0, the Hubble rate today. Both the mean Hubble rate and its variance depend on both the definition of the Hubble rate and the spatial surface on which the average is performed. We quantitatively study different definitions of the averaged Hubble rate encountered in the literature by consistently calculating the backreaction effect at second order in perturbation theory, and compare the results. We employ for the first time a recently developed gauge-invariant definition of an averaged scalar. We also discuss the variance of the Hubble rate for the different definitions.Comment: 12 pages, 25 figures, references added, clarity improved, frame switching subtlety fixed, results unchanged, v3 minor typos fixe

Sosiaalisen kompetenssin yhteys häiriökäyttäytymisen kokemiseen kainuulaisilla 5.–6. luokan oppilailla

Tiivistelmä. Tutkielman tarkoituksena oli selvittää 5.–6.-luokkalaisten oppilaiden sosiaalisen kompetenssin yhteyttä häiriökäyttäytymisen kokemiseen luokkatilanteessa. Sosiaalista kompetenssia tutkittiin MASK-monitahoarvioinnin perusteella, jossa tarkastelun kohteena olivat oppilaiden itsearviot sekä opettajien arviot. Häiriökäyttäytymistä tutkittiin tarkkaavaisuushäiriön ydinoireiden; tarkkaamattomuuden, levottomuuden ja impulsiivisuuden pohjalta. Tutkimuksessa tutkittiin myös tyttöjen ja poikien sekä yleisopetuksen ja erityisopetuksen oppilaiden välisiä eroja sosiaalisessa kompetenssissa ja häiriökäyttäytymisen kokemisessa. Lisäksi tutkittiin opettajan keinoja puuttua häiriökäyttäytymiseen. Tutkimuksen otos muodostui seitsemän kainuulaisen alakoulun 5.–6.-luokan oppilaista (N=457), joista tyttöjä oli 222 ja poikia 234. Lisäksi tutkimukseen osallistuivat oppilaiden opettajat. Tutkimusaineisto kerättiin kyselylomakkeella joulukuussa 2011 ja se analysoitiin SPSS for Windows -ohjelman avulla käyttäen analyysimenetelminä frekvenssijakaumia, korrelaatiota, t-testiä, Cohenin d:tä sekä yksisuuntaista varianssianalyysia (ANOVA) ja post hoc -testiä. Kainuulaiset 5.–6.-luokkalaiset oppilaat arvioivat oman sosiaalisen kompetenssinsa yleisesti ottaen heikommaksi kuin heidän opettajansa. Korrelaatio oppilaan itsearvion ja opettajan arvion välillä oli pieni. Tytöt arvioivat sosiaalisen kompetenssinsa paremmaksi kuin pojat ja yleisopetuksen oppilaat arvioivat sosiaalisen kompetenssinsa paremmaksi kuin erityisopetuksen oppilaat. Oppilaat kokivat impulsiivisen käytöksen kaikista häiritsevimpänä ja vähiten oppilaita häiritsi tarkkaamaton käytös. Sosiaaliselta kompetenssiltaan vahvoja oppilaita häiriökäyttäytyminen häiritsi eniten ja he myös puuttuivat siihen herkimmin. Sosiaaliselta kompetenssiltaan heikkoja oppilaita häiriökäyttäytyminen puolestaan häiritsi vähiten ja he myös puuttuivat siihen vähiten. Oppilaat kokivat opettajan käyttävän eniten kehumista saadakseen häiriökäyttäytymisen loppumaan. Sosiaalisen kyvykkyyden kehittäminen on yksi koulujen ja opettajien tehtävistä. Heikon sosiaalisen kompetenssin ja käytöshäiriöiden välillä on todettu olevan yhteys ja se voi pahimmillaan johtaa koulun kesken jättämiseen. Opettajilla tulee olla välineitä tukea ja arvioida oppilaan sosiaalisen kyvykkyyden kehittymistä. Tutkimustulosten pohjalta opettajat saavat hyödyllistä tietoa siitä, miten sosiaaliselta kompetenssiltaan eritasoiset oppilaat kokevat häiriökäyttäytymisen luokassa. Näin opettajat pystyvät herkemmin puuttumaan sellaiseen käytökseen, joka häiritsee oppilaita eniten ja suuntaamaan tukitoimia juuri niille oppilaille, jotka niitä tarvitsevat

Light propagation in statistically homogeneous and isotropic universes with general matter content

We derive the relationship of the redshift and the angular diameter distance to the average expansion rate for universes which are statistically homogeneous and isotropic and where the distribution evolves slowly, but which have otherwise arbitrary geometry and matter content. The relevant average expansion rate is selected by the observable redshift and the assumed symmetry properties of the spacetime. We show why light deflection and shear remain small. We write down the evolution equations for the average expansion rate and discuss the validity of the dust approximation.Comment: 42 pages, no figures. v2: Corrected one detail about the angular diameter distance and two typos. No change in result

Is backreaction really small within concordance cosmology?

Smoothing over structures in general relativity leads to a renormalisation of the background, and potentially many other effects which are poorly understood. Observables such as the distance-redshift relation when averaged on the sky do not necessarily yield the same smooth model which arises when performing spatial averages. These issues are thought to be of technical interest only in the standard model of cosmology, giving only tiny corrections. However, when we try to calculate observable quantities such as the all-sky average of the distance-redshift relation, we find that perturbation theory delivers divergent answers in the UV and corrections to the background of order unity. There are further problems. Second-order perturbations are the same size as first-order, and fourth-order at least the same as second, and possibly much larger, owing to the divergences. Much hinges on a coincidental balance of 2 numbers: the primordial power, and the ratio between the comoving Hubble scales at matter-radiation equality and today. Consequently, it is far from obvious that backreaction is irrelevant even in the concordance model, however natural it intuitively seems.Comment: 28 pages. Invited contribution to Classical and Quantum Gravity special issue "Inhomogeneous Cosmological Models and Averaging in Cosmology

EARLY DEVELOPMENT OF SYNCHRONY IN CORTICAL ACTIVATIONS IN THE HUMAN

Peer reviewe

Cosmological Backreaction from Perturbations

We reformulate the averaged Einstein equations in a form suitable for use with Newtonian gauge linear perturbation theory and track the size of the modifications to standard Robertson-Walker evolution on the largest scales as a function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In both cases the effective energy density arising from linear perturbations is of the order of 10^-5 the matter density, as would be expected, with an effective equation of state w ~ -1/19. Employing a modified Halofit code to extend our results to quasilinear scales, we find that, while larger, the deviations from Robertson-Walker behaviour remain of the order of 10^-5.Comment: 15 pages, 8 figures; replaced by version accepted by JCA

Photon Physics in Heavy Ion Collisions at the LHC

Various pion and photon production mechanisms in high-energy nuclear collisions at RHIC and LHC are discussed. Comparison with RHIC data is done whenever possible. The prospect of using electromagnetic probes to characterize quark-gluon plasma formation is assessed.Comment: Writeup of the working group "Photon Physics" for the CERN Yellow Report on "Hard Probes in Heavy Ion Collisions at the LHC", 134 pages. One figure added in chapter 5 (comparison with PHENIX data). Some figures and correponding text corrected in chapter 6 (off-chemical equilibrium thermal photon rates). Some figures modified in chapter 7 (off-chemical equilibrium photon rates) and comparison with PHENIX data adde

Cosmic Acceleration Driven by Mirage Inhomogeneities

A cosmological model based on an inhomogeneous D3-brane moving in an AdS_5 X S_5 bulk is introduced. Although there is no special points in the bulk, the brane Universe has a center and is isotropic around it. The model has an accelerating expansion and its effective cosmological constant is inversely proportional to the distance from the center, giving a possible geometrical origin for the smallness of a present-day cosmological constant. Besides, if our model is considered as an alternative of early time acceleration, it is shown that the early stage accelerating phase ends in a dust dominated FRW homogeneous Universe. Mirage-driven acceleration thus provides a dark matter component for the brane Universe final state. We finally show that the model fulfills the current constraints on inhomogeneities.Comment: 14 pages, 1 figure, IOP style. v2, changed style, minor corrections, references added, version accepted in Class. Quant. Gra

Gradient expansion(s) and dark energy

Motivated by recent claims stating that the acceleration of the present Universe is due to fluctuations with wavelength larger than the Hubble radius, we present a general analysis of various perturbative solutions of fully inhomogeneous Einstein equations supplemented by a perfect fluid. The equivalence of formally different gradient expansions is demonstrated. If the barotropic index vanishes, the deceleration parameter is always positive semi-definite.Comment: 17 pages, no figure

Non-perturbative results for the luminosity and area distances

The notion of luminosity distance is most often defined in purely FLRW (Friedmann-Lemaitre-Robertson-Walker) cosmological spacetimes, or small perturbations thereof. However, the abstract notion of luminosity distance is actually much more robust than this, and can be defined non-perturbatively in almost arbitrary spacetimes. Some quite general results are already known, in terms of dAobserver/d\u3a9source, the cross-sectional area per unit solid angle of a null geodesic spray emitted from some source and subsequently detected by some observer. We shall reformulate these results in terms of a suitably normalized null geodesic affine parameter and the van Vleck determinant, \u394vV. The contribution due to the null geodesic affine parameter is effectively the inverse square law for luminosity, and the van Vleck determinant can be viewed as providing a measure of deviations from the inverse square law. This formulation is closely related to the so-called Jacobi determinant, but the van Vleck determinant has somewhat nicer analytic properties and wider and deeper theoretical base in the general relativity, quantum physics, and quantum field theory communities. In the current article we shall concentrate on non-perturbative results, leaving near-FLRW perturbative investigation for future work