Browsing by Subject "gravity"

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  • Rasanen, Syksy (2014)
  • Montanari, Francesco; Räsänen, Syksy (2017)
    If the FRW metric is a good approximation on large scales, then the distance and the expansion rate, as well different notions of distance, satisfy certain consistency conditions. We fit the JLA SNIa distance data to determine the expected amplitude of the violation of these conditions if accelerated expansion is due to backreaction. Adding cosmic clock and BAO expansion rate data, we also model-independently determine the current observational limits on such violation. We find that the predicted maximum backreaction amplitude vertical bar k(H)vertical bar less than or similar to 1 (95% C.I.) is of the same order as the current observational constraints vertical bar k(H)vertical bar less than or similar to 1, the precise numbers depending on the adopted fitting method (polynomials or splines) and stellar population evolution model. We also find that constraints on the value of Ho determined from expansion rate data are sensitive to the stellar evolution model. We forecast constraints from projected LSST+Euclid-like SNIa plus Euclid galaxy differential age data. We find improvement by factor of 6 for the backreaction case and 3 for the model-independent case, probing an interesting region of possible signatures.
  • Montanari, Francesco; Räsänen, Syksy (2017)
    We evaluate the effect of structure formation on the average expansion rate with a statistical treatment where density peaks and troughs are modelled as homogeneous ellipsoids. This extends earlier work that used spherical regions. We find that the shear and the presence of filamentary and planar structures have only a small impact on the results. The expansion rate times the age of the universe Ht increases from 2/3 to 0.83 at late times, in order of magnitude agreement with observations, although the change is slower and takes longer than in the real universe. We discuss shortcomings that have to be addressed for this and similar statistical models in the literature to develop into realistic quantitative treatment of backreaction.
  • Räsänen, Syksy; Wahlman, Pyry (2017)
    We compare Higgs inflation in the metric and Palatini formulations of general relativity, with loop corrections treated in a simple approximation. We consider Higgs inflation on the plateau, at a critical point, at a hilltop and in a false vacuum. In the last case there are only minor differences. Otherwise we find that in the Palatini formulation the tensor-to-scalar ratio is consistently suppressed, spanning the range 1 x 10-(13) <r <7 x 10(-5), compared to the metric case result 2 x 10(-5) <r <0.2. Even when the values of n(s) and r overlap, the running and running of the running are different in the two formulations. Therefore, if Higgs is the inflaton, inflationary observables can be used to distinguish between different gravitational degrees of freedom, in this case to determine whether the connection is an independent variable. Non-detection of r in foreseeable future observations would not rule out Higgs inflation, only its metric variant. We conclude that in order to fix the theory of Higgs inflation, not only the particle physics UV completion but also the gravitational degrees of freedom have to be explicated.
  • Enckell, Vera-Maria; Enqvist, Kari; Räsänen, Syksy; Wahlman, Lumi-Pyry (2019)
    We study scalar field inflation in F(R) gravity in the Palatini formulation of general relativity. Unlike in the metric formulation, in the Palatini formulation F(R) gravity does not introduce new degrees of freedom. However, it changes the relations between existing degrees of freedom, including the inflaton and spacetime curvature. Considering the case F(R) = R + alpha R-2, we find that the R-2 term decreases the height of the effective inflaton potential. By adjusting the value of a, this mechanism can be used to suppress the tensor-to-scalar ratio r without limit in any scalar field model of inflation without affecting the spectrum of scalar perturbations.
  • Olsson, Per-Anders; Breili, Kristian; Ophaug, Vegard; Steffen, Holger; Bilker-Koivula, Mirjam; Nielsen, Emil; Oja, Tõnis; Timmen, Ludger (Oxford University Press, 2019)
    Geophysical Journal International
    For the first time, we present a complete, processed compilation of all repeated absolute gravity (AG) observations in the Fennoscandian postglacial land uplift area and assess their ability to accurately describe the secular gravity change, induced by Glacial Isostatic Adjustment (GIA). The dataset spans over more than three decades and consists of 688 separate observations at 59 stations. Ten different organisations have contributed with measurements using 14 different instruments. The work was coordinated by the Nordic Geodetic Commisson (NKG). Representatives from each country collected and processed data from their country, respectively, and all data were then merged to one dataset. Instrumental biases are considered and presented in terms of results from international comparisons of absolute gravimeters. From this dataset, gravity rates of change (g_dot) are estimated for all stations with more than two observations and a timespan larger than two years. The observed rates are compared to predicted rates from a global GIA model as well as the state of the art semi-empirical land uplift model for Fennoscandia, NKG2016LU. Linear relations between observed g_dot and the land uplift, h_dot (NKG2016LU), are estimated from the absolute gravity observations by means of weighted least squares adjustment (WLSA) as well as weighted orthogonal distance regression (WODR). The empirical relations are not significantly different from the modelled, geophysical relation g_dot = 0:03 - 0:163(+-0.016)h_dot. We also present a g_dot -model for the whole Fennoscandian land uplift region. At many stations, the observational estimates of g_dot still suffer from few observations and/or unmodelled environmental effects (e.g. local hydrology). We therefore argue that, at present, the best predictions of GIA-induced gravity rate of change in Fennoscandia are achieved by means of the NKG2016LU land uplift model, together with the geophysical relation between g_dot and h_dot.
  • Jimenez, Jose Beltran; Heisenberg, Lavinia; Koivisto, Tomi (2020)
    The geometrical formulation of gravity is not unique and can be set up in a variety of spacetimes. Even though the gravitational sector enjoys this freedom of different geometrical interpretations, consistent matter couplings have to be assured for a steady foundation of gravity. In generalised geometries, further ambiguities arise in the matter couplings unless the minimal coupling principle (MCP) is adopted that is compatible with the principles of relativity, universality and inertia. In this work, MCP is applied to all standard model gauge fields and matter fields in a completely general (linear) affine geometry. This is also discussed from an effective field theory perspective. It is found that the presence of torsion generically leads to theoretical problems. However, symmetric teleparallelism, wherein the affine geometry is integrable and torsion-free, is consistent with MCP. The generalised Bianchi identity is derived and shown to determine the dynamics of the connection in a unified fashion. Also, the parallel transport with respect to a teleparallel connection is shown to be free of second clock effects.
  • Di Dio, Enea; Durrer, Ruth; Maartens, Roy; Montanari, Francesco; Umeh, Obinna (2019)
    We compute the redshift-dependent angular bispectrum of galaxy number counts at tree-level, including nonlinear clustering bias and estimating numerically for the first time the effect of redshift space distortions (RSD). We show that for narrow redshift bins the amplitude of nonlinear RSD is comparable with the matter density perturbations. While our numerical results only include terms relevant on sub-horizon scales, the formalism can readily be extended to the full tree-level bispectrum. Our approach does not rely on the flat-sky approximation and it can be easily generalized to different sources by including the appropriate bias expansion. We test the accuracy of Limber approximation for different z-bins. We highlight the subtle but relevant differences in the angular bispectrum of galaxy number counts with respect to CMB, due to the different scale dependence of perturbations. Our formalism can also be directly applied to the angular HI intensity mapping bispectrum.