Age of the universe
In physical cosmology, the age of the universe is the time elapsed since the Big Bang. Astronomers have derived two different measurements of the age of the universe:[1] a measurement based on direct observations of an early state of the universe, which indicate an age of 13.787±0.020 billion years as interpreted with the Lambda-CDM concordance model as of 2021;[2] and a measurement based on the observations of the local, modern universe, which suggest a younger age.[3][4][5] The uncertainty of the first kind of measurement has been narrowed down to 20 million years, based on a number of studies that all show similar figures for the age. These studies include researches of the microwave background radiation by the Planck spacecraft, the Wilkinson Microwave Anisotropy Probe and other space probes. Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang,[6] and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time. The range of the estimate is also within the range of the estimate for the oldest observed star in the universe.
This article is about scientific estimates of the age of the universe. For religious and other non-scientific estimates, see Dating creation.WMAP[edit]
NASA's Wilkinson Microwave Anisotropy Probe (WMAP) project's nine-year data release in 2012 estimated the age of the universe to be (13.772±0.059)×109 years (13.772 billion years, with an uncertainty of plus or minus 59 million years).[6]
This age is based on the assumption that the project's underlying model is correct; other methods of estimating the age of the universe could give different ages. Assuming an extra background of relativistic particles, for example, can enlarge the error bars of the WMAP constraint by one order of magnitude.[27]
This measurement is made by using the location of the first acoustic peak in the microwave background power spectrum to determine the size of the decoupling surface (size of the universe at the time of recombination). The light travel time to this surface (depending on the geometry used) yields a reliable age for the universe. Assuming the validity of the models used to determine this age, the residual accuracy yields a margin of error near one per cent.[14]
In 2015, the Planck Collaboration estimated the age of the universe to be 13.813±0.038 billion years, slightly higher but within the uncertainties of the earlier number derived from the WMAP data.[28]
In the table below, figures are within 68% confidence limits for the base ΛCDM model.
Legend:
In 2018, the Planck Collaboration updated its estimate for the age of the universe to 13.787±0.020 billion years.[2]
Assumption of strong priors[edit]
Calculating the age of the universe is accurate only if the assumptions built into the models being used to estimate it are also accurate. This is referred to as strong priors and essentially involves stripping the potential errors in other parts of the model to render the accuracy of actual observational data directly into the concluded result. This is not a valid procedure in all contexts, as noted in the accompanying caveat: "on the assumption that the project's underlying model is correct". The age given is thus accurate to the specified error, since this represents the error in the instrument used to gather the raw data input into the model.
The age of the universe based on the best fit to Planck 2018 data alone is 13.787±0.020 billion years. This number represents an accurate "direct" measurement of the age of the universe, in contrast to other methods that typically involve Hubble's law and the age of the oldest stars in globular clusters. It is possible to use different methods for determining the same parameter (in this case, the age of the universe) and arrive at different answers with no overlap in the "errors". To best avoid the problem, it is common to show two sets of uncertainties; one related to the actual measurement and the other related to the systematic errors of the model being used.
An important component to the analysis of data used to determine the age of the universe (e.g. from Planck) therefore is to use a Bayesian statistical analysis, which normalizes the results based upon the priors (i.e. the model).[14] This quantifies any uncertainty in the accuracy of a measurement due to a particular model used.[29][30]