arXiv subjects

Domenico Sapone

Publications and source records attributed to Domenico Sapone.

41 records · Page 3Linked to original sources

arXiv2007

Constraining the growth factor with baryon oscillations

The growth factor of linear fluctuations is probably one of the least known quantity in observational cosmology. Here we discuss the constraints that baryon oscillations in galaxy power spectra from future surveys can put on a conveniently parametrized growth factor. We find that spectroscopic surveys of $5000 deg^2$ extending to $z \approx 3$ could estimate the growth index $γ$ within 0.06; a similar photometric survey would give $Δγ\approx 0.15$. This test provides an important consistency check for standard cosmological model and could constrain modified gravity models. We discuss the errors and the figure of merit for various combinations of redshift errors and survey size.

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arXiv2007

Measuring the dark side (with weak lensing)

We introduce a convenient parametrization of dark energy models that is general enough to include several modified gravity models and generalized forms of dark energy. In particular we take into account the linear perturbation growth factor, the anisotropic stress and the modified Poisson equation. We discuss the sensitivity of large scale weak lensing surveys like the proposed DUNE satellite to these parameters. We find that a large-scale weak-lensing tomographic survey is able to easily distinguish the Dvali-Gabadadze-Porrati model from LCDM and to determine the perturbation growth index to an absolute error of 0.02-0.03.

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arXiv2007

Crossing the Phantom Divide

We consider fluid perturbations close to the "phantom divide" characterised by p = -rho and discuss the conditions under which divergencies in the perturbations can be avoided. We find that the behaviour of the perturbations depends crucially on the prescription for the pressure perturbation delta-p. The pressure perturbation is usually defined using the dark energy rest-frame, but we show that this frame becomes unphysical at the divide. If the pressure perturbation is kept finite in any other frame, then the phantom divide can be crossed. Our findings are important for generalised fluid dark energy used in data analysis (since current cosmological data sets indicate that the dark energy is characterised by p ~ -rho so that p < -rho cannot be excluded) as well as for any models crossing the phantom divide, like some modified gravity, coupled dark energy and braneworld models. We also illustrate the results by an explicit calculation for the "Quintom" case with two scalar fields.

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arXiv2006

Dark Energy versus Modified Gravity

There is now strong observational evidence that the expansion of the universe is accelerating. The standard explanation invokes an unknown "dark energy" component. But such scenarios are faced with serious theoretical problems, which has led to increased interest in models where instead General Relativity is modified in a way that leads to the observed accelerated expansion. The question then arises whether the two scenarios can be distinguished. Here we show that this may not be so easy, demonstrating explicitely that a generalised dark energy model can match the growth rate of the DGP model and reproduce the 3+1 dimensional metric perturbations. Cosmological observations are then unable to distinguish the two cases.

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arXiv2005

Supernovae type Ia data favour coupled phantom energy

We estimate the constraints that the recent high-redshift sample of supernovae type Ia put on a phenomenological interaction between dark energy and dark matter. The interaction can be interpreted as arising from the time variation of the mass of dark matter particles. We find that the coupling correlates with the equation of state: roughly speaking, a negative coupling (in our sign convention) implies phantom energy ($w_ϕ<-1$) while a positive coupling implies ``ordinary'' dark energy. The constraints from the current supernovae Ia Hubble diagram favour a negative coupling and an equation of state $w_ϕ<-1$. A zero or positive coupling is in fact unlikely at 99% c.l. (assuming constant equation of state); at the same time non-phantom values ($w_ϕ>-1$) are unlikely at 95%. We show also that the usual bounds on the energy density weaken considerably when the coupling is introduced: values as large as $Ω_{m0}=0.7$ become acceptable for as concerns SNIa. We find that the rate of change of the mass $\dot{m}/m$ of the dark matter particles is constrained to be $δ_{0}$ in a Hubble time, with $-10<δ_{0}<-1$ to 95% c.l.. We show that a large positive coupling might in principle avoid the future singularity known as ``big rip'' (occurring for $w_ϕ<-1$) but the parameter region for this to occur is almost excluded by the data. We also forecast the constraints that can be obtained from future experiments, focusing on supernovae and baryon oscillations in the power spectra of deep redshift surveys. We show that the method of baryon oscillations holds the best potential to contrain the coupling.

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