The age of the universe is the time elapsed since the Big Bang

Big Bang

The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 posited by the most widely accepted scientific model of cosmology

Cosmology

Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...

. The best current estimate of the age of the universe is 13.75 ± 0.13 billion

1000000000 (number)

1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001.In scientific notation, it is written as 109....

 years (433.6 x 10¹⁵ seconds in SI units, or 13.75 Gigayears) within the Lambda-CDM concordance model

Lambda-CDM model

ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...

. It is not known if something existed before the singularity

Gravitational singularity

A gravitational singularity or spacetime singularity is a location where the quantities that are used to measure the gravitational field become infinite in a way that does not depend on the coordinate system...

 that appeared at the moment of the Big Bang, nor if time is linear, since the expansion estimated by Hubble's law

Hubble's law

Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...

 assumed a linear expansion, and later work indicates there may have been variations.