September 24, 2011 § Leave a comment
in the late 70s, the start of the universe was described with the hot big bang theory; starting from a singularity and expanding, being filled by baryonic matter as well as non-baryonic matter. this model has successfully explained the existence of the cosmic microwave background and the amount of light being generated by nucleosynthesis. however, it had fundamental problems that it was unable to answer: the horizon problem (the homogeneity and isotropy of the universe in large-scale), the flatness problem (small curvature of the universe), the monopole problem (missing heavy stable magnetic monopoles), and the structure formation problem (the lack of support for the formation of the structures we can see today in the sky). in early 80s, research showed that the universe inflation could explain the above-mentioned problems even though there was still no concrete proof to support it .
among the different inflation theories proposed in the late 70s and early 80s, “old inflation” theory proposed by guth was the most popular, and easiest to understand, and was standardized as a textbook theory of inflation. however, this theory has shortcomings in the sense that it results in an inhomogeneous universe at the end. this led to the invention of the “new inflation” theory and later chaotic inflation scenario . the inflation theory was supported by many practical data. after hubble space telescope was launched in the late 90s, the observation of distant supernovae became possible. it showed that distant supernovae appear to be fainter than expected in a flat matter dominated universe, which means at fixed redshift, they are at large distances than expected in such a context and thus the universe is accelerating . another important proof is based on the observation and studies of the cosmic microwave background, which determines the total energy budget of the universe and proves that there is a shortage of about 70%, which leads to a missing exotic form of energy with negative pressure . dynamics of our universe is defined by einstein’s equations, which shows that the contribution of energy content of universe that is represented by energy momentum tensor is related to the geometry given by the curvature of space-time. there are two ways to expand this equation to take into account the acceleration; either by supplementing an extra energy momentum tensor or by modifying the geometry itself. first method adds an extra momentum tensor, which is sourced by energy with negative pressure. the simplest candidate is the cosmological constant. however, due to its being non-evolving, cosmological constant theory has fine tuning implications. hence, dynamically evolving scalar field models such as phantoms, k-essence, tachyonic scalar fields and quintessence are introduced .
“the quintessence models were proposed to circumvent the two fundamental problems of the cosmological constant: a) a value of ΩΛ~0.7 is about 122 orders of magnitude from the naïve theoretical calculation b) it seems somewhat unnatural that we happen to live in a time when ≈ 2 since this ratio depends on the third power of the redshift. …in “quintessence” models, the “dark energy” density tracks the development of the leading energy term making both comparable” [goobar 53], this modifies the late time evolution of the expansion rate of the universe, and explains the luminosity of supernovae and the angular distance of cosmic microwave background patterns. quintessence theory has its own shortcomings; the main problem lies in the fact that the quintessence field must be weakly coupled to ordinary matter. due to the coupling to ordinary matter, the quintessence potential does not decrease to zero in infinity as theory predicts .
looking at the present status of the studies and research on dark energy, it is fair to say that due to its being around for about 30 years, the dark energy could be a viable answer to the acceleration of the universe. however, its nature is still a mystery, and even though theories such as quintessence and tachyons are very successful in approaching it, it is still one of the biggest mysteries of cosmology.
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