| Lambda-cdm Model |
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ΛCDM or '''Lambda-CDM''' is an abbreviation for '''Lambda-Cold Dark Matter'''. It represents the current concordance model of Big Bang Cosmology that explains Cosmic Microwave Background observations, as well as Large Scale Structure observations and Supernova e observations of the accelerating Expansion Of The Universe . It is the simplest model that is in agreement with all the observations.
These are the simplest assumptions for a consistent, physical model of cosmology. However, ΛCDM is a ''model'' . Cosmologists anticipate that all of these assumptions will not be borne out exactly, after more is learned about the applicable fundamental physics. In particular, cosmic inflation predicts spatial curvature at the level of 10−4 to 10−5. It would also be surprising if the temperature of dark matter were absolute zero. Moreover, ΛCDM says nothing about the fundamental physical origin of dark matter, dark energy and the nearly scale-invariant spectrum of primordial curvature perturbations: in that sense, it is merely a useful parameterization of ignorance. PARAMETERS The model has six parameters. The to reionization determines the Red Shift of Reionization . Information about the density fluctuations is determined by the amplitude of the primordial fluctuations (from cosmic inflation) and the spectral index, which measures how the fluctuations change with scale ( corresponds to a scale-invariant spectrum). The errors quoted are 1σ: that is, there is statistically a 68% likelihood that the true value falls within the upper and lower error bounds. The errors are not Gaussian , and they have been derived using a Markov Chain Monte Carlo analysis by the Wilkinson Microwave Anisotropy Probe collaboration ( Spergel ''et al.'' 2006) which also uses Sloan Digital Sky Survey and Type Ia Supernova data. EXTENDED MODELS Possible extensions of the simplest "vanilla" ΛCDM model are to allow Quintessence rather than a Cosmological Constant . In this case, the Equation Of State of dark energy is different from −1. Cosmic inflation predicts tensor fluctuations ( Gravitational Wave s). Their amplitude is parameterized by the tensor-to-scalar ratio, which is determined by the energy scale of inflation. Other modifications allow for spatial curvature or a running spectral index, which are generally viewed as inconsistent with cosmic inflation. Allowing these parameters will generally ''increase'' the errors in the vanilla parameters quoted above, and may also shift the observed values somewhat. These are consistent with a cosmological constant, , and no spatial curvature . There is some evidence for a running spectral index, but it is not statistically significant. Theoretical expectations suggest that the tensor-to-scalar ratio r should be between 0 and 0.3, and so should be tested in the near future. REFERENCES
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