Explanation
The Lambda-CDM concordance model describes the evolution of the universe from a very uniform, hot, dense primordial state to its present state over a span of about 13.75 billion years of cosmological time. This model is well understood theoretically and strongly supported by recent high-precision astronomical observations such as WMAP. In contrast, theories of the origin of the primordial state remain very speculative. If one extrapolates the Lambda-CDM model backward from the earliest well-understood state, it quickly (within a small fraction of a second) reaches a singularity called the "Big Bang singularity." This singularity is not understood as having a physical significance in the usual sense, but it is convenient to quote times measured "since the Big Bang" even though they do not correspond to a physically measurable time. For example, "10−6 seconds after the Big Bang" is a well-defined era in the universe's evolution. If one referred to the same era as "13.75 billion years minus 10−6 seconds ago," the precision of the meaning would be lost because the minuscule latter time interval is swamped by uncertainty in the former.
Though the universe might in theory have a longer history, the International Astronomical Union presently use "age of the universe" to mean the duration of the Lambda-CDM expansion, or equivalently the elapsed time since the Big Bang in the current observable universe.
Read more about this topic: Age Of The Universe
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