This paper presents a review of the basic, model-independent differences between the pre-big bang scenario, arising naturally in a string cosmology context, and the standard inflationary scenario. We use an unconventional approach in which the introduction of technical details is avoided as much as possible, trying to focus the reader’s attention on the main conceptual aspects of both scenarios...

In 1998, astronomers discovered that the expansion of universe is accelerating. Somehow, something must have made gravity repulsive on cosmological scales. This was called dark energy; it described by Einstein’s constant; and amounts to about 70% total mass universe. It has been conjectured constant a form vacuum energy, but its prediction from quantum field theory failed many orders magnitude,...

We give an introduction into quantum cosmology with emphasis on its conceptual parts. After a general motivation we review the formalism of canonical gravity which discussions are usually based. then present minisuperspace Wheeler--DeWitt equation and elaborate problem time, imposition boundary conditions, semiclassical approximation, origin irreversibility, singularity avoidance. Restriction i...