We test a cosmological model which the only component is a pressureless fluid with a constant bulk viscosity as an explanation for the present accelerated expansion of the universe. We classify all the possible scenarios for the universe predicted by the model according to their past, present and future evolution and we test its viability performing a Bayesian statistical analysis using the SCP ``Union'' data set (307 SNe Ia), imposing the second law of thermodynamics on the dimensionless constant bulk viscous coefficient and comparing the predicted age of the universe by the model with the constraints coming from the oldest globular clusters. The best estimated values found for and the Hubble constant H0 are: = 1.922±0.089 and H0 = 69.62±0.59 (km/s)Mpc−1 with a χ2min = 314 (χ2d.o.f = 1.031). The age of the universe is found to be 14.95±0.42 Gyr. We see that the estimated value of H0 as well as of χ2d.o.f are very similar to those obtained from ΛCDM model using the same SNe Ia data set. The estimated age of the universe is in agreement with the constraints coming from the oldest globular clusters. Moreover, the estimated value of is positive in agreement with the second law of thermodynamics (SLT). On the other hand, we perform different forms of marginalization over the parameter H0 in order to study the sensibility of the results to the way how H0 is marginalized. We found that it is almost negligible the dependence between the best estimated values of the free parameters of this model and the way how H0 is marginalized in the present work. Therefore, this simple model might be a viable candidate to explain the present acceleration in the expansion of the universe.

Can a matter-dominated model with constant bulk viscosity drive the accelerated expansion of the universe?

We explore the viability of a bulk viscous matter-dominated Universe to explain the present accelerated expansion of the Universe. The model is composed by a pressureless fluid with bulk viscosity of the form ? = ?0+?1H where ?0 and ?1 are constants and H is the Hubble parameter. The pressureless fluid characterizes both the baryon and dark matter components. We study the behavior of the Universe according to this model analyzing the scale factor as well as some curvature scalars and the matter density. On the other hand, we compute the best estimated values of ?0 and ?1 using the type Ia Supernovae (SNe Ia) probe. We find that from all the possible scenarios for the Universe, the preferred one by the best estimated values of (?0,?1) is that of an expanding Universe beginning with a Big-Bang, followed by a decelerated expansion at early times, and with a smooth transition in recent times to an accelerated expansion epoch that is going to continue forever. The predicted age of the Universe is a little smaller than the mean value of the observational constraint coming from the oldest globular clusters but it is still inside of the confidence interval of this constraint. A drawback of the model is the violation of the local second law of thermodynamics in redshifts z1. However, when we assume ?1 = 0, the simple model ? = ?0 evaluated at the best estimated value for ?0 satisfies the local second law of thermodynamics, the age of the Universe is in perfect agreement with the constraint of globular clusters, and it also has a Big-Bang, followed by a decelerated expansion with the smooth transition to an accelerated expansion epoch in late times, that is going to continue forever.

Exploring a matter-dominated model with bulk viscosity to drive the accelerated expansion of the Universe

We explore the viability of a bulk viscous matter-dominated Universe to explain the present accelerated expansion of the Universe. The model is composed by a pressureless fluid with bulk viscosity of the form \zeta = \zeta_0 + \zeta_1 * H where \zeta_0 and \zeta_1 are constants and H is the Hubble parameter. The pressureless fluid characterizes both the baryon and dark matter components. We study the behavior of the Universe according to this model analyzing the scale factor as well as some curvature scalars and the matter density. On the other hand, we compute the best estimated values of \zeta_0 and \zeta_1 using the type Ia Supernovae (SNe Ia) probe. We find that from all the possible scenarios for the Universe, the preferred one by the best estimated values of (\zeta_0, \zeta_1) is that of an expanding Universe beginning with a Big- Bang, followed by a decelerated expansion at early times, and with a smooth transition in recent times to an accelerated expansion epoch that is going to continue forever. The predicted age of the Universe is a little smaller than the mean value of the observational constraint coming from the oldest globular clusters but it is still inside of the confidence interval of this constraint. A drawback of the model is the violation of the local second law of thermodynamics in redshifts z >= 1. However, when we assume \zeta_1 = 0, the simple model \zeta = \zeta_0 evaluated at the best estimated value for \zeta_0 satisfies the local second law of thermodynamics, the age of the Universe is in perfect agreement with the constraint of globular clusters, and it also has a Big-Bang, followed by a decelerated expansion with the smooth transition to an accelerated expansion epoch in late times, that is going to continue forever.

We test a cosmological model which the only component is a pressureless fluid with a constant bulk viscosity as an explanation for the present accelerated expansion of the universe. We classify all the possible scenarios for the universe predicted by the model according to their past, present and future evolution and we test its viability performing a Bayesian statistical analysis using the SCP "Union" data set (307 SNe Ia), imposing the second law of thermodynamics on the dimensionless constant bulk viscous coefficient ˜ � and comparing the predicted age of the universe by the model with the constraints coming from the oldest globular clusters. The best estimated values found for ˜

constant bulk viscosity drive the accelerated expansion of the universe

The study of particle creation phenomena at the expense of the gravitational field is of great research interest. It might solve the cosmological puzzle singlehandedly, without the need for either dark energy or modified theory of gravity. In the early universe, it serves the purpose of reheating which gave way to escape from inflationary phase to the hot Big Bang model. In the late universe, it instigates late-time cosmic acceleration, without affecting standard Big Bang Nucleosynthesis (BBN), Cosmic Microwave Background Radiation (CMBR), or Structure Formation. In this chapter, we briefly review the present status of cosmic evolution, develop the thermodynamics for irreversible particle creation phenomena, and study its consequences at the early as well as at the late universe.

M. Heller

Papers

Dust-filled viscous universes.