5,053 research outputs found
Constraining inverse-curvature gravity with supernovae
We show that models of generalized modified gravity, with inverse powers of the curvature, can explain the current accelerated expansion of the Universe without resorting to dark energy and without conflicting with solar system experiments. We have solved the Friedmann equations for the full dynamical range of the evolution of the Universe and performed a detailed analysis of supernovae data in the context of such models that results in an excellent fit. If we further include constraints on the current expansion of the Universe and on its age, we obtain that the matter content of the Universe is 0.07 <=omega(m)<= 0.21 (95% C.L.). Hence the inverse-curvature gravity models considered cannot explain the dynamics of the Universe just with a baryonic matter component
Education choices in Mexico: using a structural model and a randomised experiment to evaluate PROGRESA
In this paper we evaluate the effect of a large welfare program in rural Mexico. For such a purpose we use an evaluation sample that includes a number of villages where the program was not implemented for evaluation purposes. We estimate a structural model of education choices and argue that without such a framework it is impossible to evaluate the effect of the program and, especially, possible changes to its structure. We also argue that the randomized component of the data allows us to identify a more flexible model that is better suited to evaluate the program. We find that the program has a positive effect on the enrollment of children, especially after primary school. We also find that an approximately revenue neutral change in the program that would increase the grant for secondary school children while eliminating for the primary school children would have a substantially larger effect on enrollment of the latter, while having minor effects on the former
On the self-consistent physical parameters of LMC intermediate-age clusters
The LMC clusters are unique templates of simple stellar population (SSP),
being crucial to calibrate models describing the integral light as well as to
test the stellar evolution theory. With this in mind we analyzed HST/WFPC2 (V,
B--V) colour-magnitude diagrams (CMDs) of 15 populous LMC clusters with ages
between ~0.3 Gyr and ~4 Gyr using different stellar evolutionary models
(Padova, PEL or Pisa, BaSTI or Teramo). Following the approach described by
Kerber, Santiago & Brocato (2007), we determined accurate and self-consistent
physical parameters (age, metallicity, distance modulus and reddening) for each
cluster by comparing the observed CMDs with synthetic ones. We found
significant trends in the physical parameters due to the choice of stellar
evolutionary model and treatment of convective core overshooting. In general,
models that incorporate overshooting presented more reliable results than those
that do not. Comparisons with the results found in the literature demonstrated
that our derived metallicities are in good agreement with the ones from the
spectroscopy of red giants. We also confirmed that, independent of the adopted
stellar evolutionary library, the recovered 3D distribution for these clusters
is consistent with a thick disk roughly aligned with the LMC disk as defined by
field stars. Finally, we also provide new estimates of distance modulus to the
LMC center, that are marginally consistent with the canonical value of 18.50.Comment: 6 pages, 4 figures, conference contribution to IAU Symposium 256, van
Loon J.T. & Oliviera J.M., ed
- âŠ