Age of the Universe
The age of the universe is not measured; it is modeled.
In physical cosmology, the age of the universe is the time elapsed since the Big Bang. The current measurement of the age of the universe is 13.799±0.021 billion years within the Lambda-CDM concordance model. The uncertainty has been narrowed down to 21 million years, based on a number of projects that all give extremely close figures for the age. These include studies of the microwave background radiation, and measurements by the Planck spacecraft, the Wilkinson Microwave Anisotropy Probe and other probes. Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang, and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time.
All the CMB measurements are in doubt because the instruments and analyses ignored water whose molecular bonds have the same signature as the proposed CMB. Because of this mistake the CMB results are not repeatable; the earth's surface is substantially water. Despite this observation the CMB results remain critical in this analysis for the model.
The age of the universe is set by the model. The age results from a calculation with adjustable parameters. The only actual measurement in this process is the suspect CMB and It has parameters dependent on the CMB which is a theoretical result from an event billions of years ago. That entire span of time, except for the Earth's geological history, is just a guess.
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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 13.8 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. This is known as the "initial singularity" or 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.
the time of this initial event is not understood but it still defines a start for the theoretical events to follow, with no measurements over that entire duration.
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The age of the universe can be determined by measuring the Hubble constant today and extrapolating back in time with the observed value of density parameters. Before the discovery of dark energy, it was believed that the universe was matter-dominated (Einstein - de Sitter universe, green curve). Note that the de Sitter universe has infinite age, while the closed universe has the least age.
the calculation of the age of the universe changed with 'the discovery of dark energy'
This invisible theoretical phenomenon has never been discovered, let alone defined to where it could be observed. It remains 'dark.' If the model relies on something invisible then it has no foundation.
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The problem of determining the age of the universe is closely tied to the problem of determining the values of the cosmological parameters. Today this is largely carried out in the context of the LCDM model, where the universe is assumed to contain normal (baryonic) matter, cold dark matter, radiation (including both photons and neutrinos), and a cosmological constant. The fractional contribution of each to the current energy density of the universe is given by the density parameters. The full LCDM model is described by a number of other parameters, but for the purpose of computing its age these three, along with the Hubble parameter, are the most important.
Apart from the Planck satellite, the WMAP Probe was instrumental in establishing an accurate age of the universe, though other measurements must be folded in to gain an accurate number. CMB measurements are very good at constraining the matter content and the curvature parameters, it is not as sensitive to [parameter omega lambda] directly, partly because the cosmological constant becomes important only at low redshift. The most accurate determinations of the Hubble parameter H0 come from Type Ia supernovae. Combining these measurements leads to the generally accepted value for the age of the universe quoted above.
The cosmological constant makes the universe "older" for fixed values of the other parameters. This is significant, since before the cosmological constant became generally accepted, the Big Bang model had difficulty explaining why globular clusters in the Milky Way appeared to be far older than the age of the universe as calculated from the Hubble parameter and a matter-only universe. Introducing the cosmological constant allows the universe to be older than these clusters, as well as explaining other features that the matter-only cosmological model could not.
Comment: the matter-only model failed so this model includes theoretical invisible stuff having no way to observe or measure it.
This is not a better understanding; one problem was replaced by a genie with a magic lamp.
Cosmologists claim with near certainty the age of the universe is known within just a few million years. However the start of this age remains unknown.
As the number is from a mathematical model with few actual measurements (only supernovae), this calculated number has absolutely no credibility.
Cosmology is defined as 'the scientific study of the origin, evolution, and eventual fate of the universe.'
Science is defined as the intellectual and practical activity encompassing the systematic study of the structure and behaviour of the physical and natural world through observation and experiment.
Cosmology does absolutely nothing by experiment; it relies solely on modeling, wholly subject to the whims of the programmers.
I made the big mistake of higher expectations for cosmology when knowing so much new data had been gathered since we began using space probes.
Cosmology is an undisciplined science.
The age of the universe is whatever we want it to be, so I expect soon we will know the age with a claimed uncertainty of less than a year. The value and its range are unfounded with no real data.