Age of the universe

In physical cosmology, the age of the universe is the time elapsed since the Big Bang: 13.8 billion years.

One is based on a particle physics model of the early universe called Lambda-CDM, matched to measurements of the distant, and thus old features, like the cosmic microwave background.

The other is based on the distance and relative velocity of a series or "ladder" of different kinds of stars, making it depend on local measurements late in the history of the universe.

[2] These two methods give slightly different values for the Hubble constant, which is then used in a formula to calculate the age.

[3] In the 18th century, the concept that the age of Earth was millions, if not billions, of years began to appear.

[4] The first scientific theories indicating that the age of the universe might be finite were the studies of thermodynamics, formalized in the mid-19th century.

The concept of entropy dictates that if the universe (or any other closed system) were infinitely old, then everything inside would be at the same temperature, and thus there would be no stars and no life.

In order to remain consistent with a steady-state universe, Einstein added what was later called a cosmological constant to his equations.

In addition, the farther away these galaxies seemed to be (the dimmer they appeared) the greater was their redshift, and thus the faster they seemed to be moving away.

Hubble's initial value for the universe's age was very low, as the galaxies were assumed to be much closer than later observations found them to be.

The first reasonably accurate measurement of the rate of expansion of the universe, a numerical value now known as the Hubble constant, was made in 1958 by astronomer Allan Sandage.

As of 2024, using the latest models for stellar evolution, the estimated age of the oldest known star is 13.8±4 billion years.

[9] The discovery of cosmic microwave background radiation announced in 1965[10] finally brought an effective end to the remaining scientific uncertainty over the expanding universe.

In 1964, Arno Penzias and Robert Woodrow Wilson were trying to detect radio wave echoes with a supersensitive antenna.

The antenna persistently detected a low, steady, mysterious noise in the microwave region that was evenly spread over the sky, and was present day and night.

The two teams realized that the detected noise was in fact radiation left over from the Big Bang, and that this was strong evidence that the theory was correct.

Since then, a great deal of other evidence has strengthened and confirmed this conclusion, and refined the estimated age of the universe to its current figure.

This model is well understood theoretically and strongly supported by recent high-precision astronomical observations such as WMAP.

Since the universe must be at least as old as the oldest things in it, there are a number of observations that put a lower limit on the age of the universe;[15][16] these include Before the incorporation of dark energy in the model of cosmic expansion, the age was awkwardly less than the oldest observed astronomical objects.

This connection can be used in reverse: the oldest objects found constrain the values of the density parameter for dark energy.

The first observation that one can make from this formula is that it is the Hubble parameter that controls that age of the universe, with a correction arising from the matter and energy content.

In general this must be done numerically, and the results for a range of cosmological parameter values are shown in the figure.

are currently believed to come from measured brightnesses and redshifts of distant Type Ia supernovae.

Combining these measurements leads to the generally accepted value for the age of the universe quoted above.

The lookback time depends on the objects redshift and, like the age of the universe, the cosmological parameters selected.

Assuming an extra background of relativistic particles, for example, can enlarge the error bars of the WMAP constraint by one order of magnitude.

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.

[11] 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.

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.