1,260 research outputs found
Principles, Progress and Problems in Inflationary Cosmology
Inflationary cosmology has become one of the cornerstones of modern
cosmology. Inflation was the first theory within which it was possible to make
predictions about the structure of the Universe on large scales, based on
causal physics. The development of the inflationary Universe scenario has
opened up a new and extremely promising avenue for connecting fundamental
physics with experiment. This article summarizes the principles of inflationary
cosmology, discusses progress in the field, focusing in particular on the
mechanism by which initial quantum vacuum fluctuations develop into the seeds
for the large-scale structure in the Universe, and highlights the important
unsolved problems of the scenario. The case is made that new input from
fundamental physics is needed in order to solve these problems, and that thus
early Universe cosmology can become the testing ground for trans-Planckian
physics.Comment: 8 pages, 8 figures commissioned article for the AAPPS Bulletin,
appeared in 200
Cosmology of the Very Early Universe
In these lectures I focus on early universe models which can explain the
currently observed structure on large scales. I begin with a survey of
inflationary cosmology, the current paradigm for understanding the origin of
the universe as we observe it today. I will discuss some progress and problems
in inflationary cosmology before moving on to a description of two alternative
scenarios - the Matter Bounce and String Gas Cosmology. All early universe
models connect to observations via the evolution of cosmological perturbations
- a topic which will be discussed in detail in these lectures.Comment: 38 pages, 15 figures, to be published in the proceedings of the XIV
Special Course in Astronomy, Observatorio Nacional, Brazi
Trans-Planckian Physics and Inflationary Cosmology
Due to the quasi-exponential red-shifting which occurs during an inflationary
period in the very early Universe, wavelengths which at the present time
correspond to cosmological lengths are in general sub-Planckian during the
early stages of inflation. This talk discusses two approaches to addressing
this issue which both indicate that the standard predictions of inflationary
cosmology - made using classical general relativity and weakly coupled scalar
matter field theory - are not robust against changes in the physics on
trans-Planckian scales. One approach makes use of modified dispersion relations
for a usual free field scalar matter theory, the other uses some properties of
space-time non-commutativity - a feature expected in string theory. Thus, it is
possible that cosmological observations may be used as a window to explore
trans-Planckian physics. We also speculate on a novel way of obtaining
inflation based on modified dispersion relations for ordinary radiation.Comment: 14 pages, 1 figure invited plenary talk at CosPA2002 (National Taiwan
Univ., Taipei, Taiwan), to be publ. in the proceedings. Two references added,
discussion extende
Matter Bounce in Horava-Lifshitz Cosmology
Horava-Lifshitz gravity, a recent proposal for a UV-complete renormalizable
gravity theory, may lead to a bouncing cosmology. In this note we argue that
Horava-Lifshitz cosmology may yield a concrete realization of the matter bounce
scenario, and thus give rise to an alternative to inflation for producing a
scale-invariant spectrum of cosmological perturbations. In this scenario,
quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and
the spectrum is transformed into a scale-invariant one on super-Hubble scales
before the bounce because the long wavelength modes undergo squeezing of their
wave-functions for a longer period of time than shorter wavelength modes. The
scale-invariance of the spectrum of curvature fluctuations is preserved during
and after the bounce. A distinctive prediction of this scenario is the
amplitude and shape of the bispectrum.Comment: 6 pages, 1 figure, a couple of minor wording change
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