The age of the universe is estimated based on various scientific observations and models, including the measurement of the cosmic microwave background radiation, the study of the large-scale structure of the universe, and the observations of distant objects. While our ability to observe the universe is limited to the distance light has traveled since the Big Bang (known as the observable universe), scientists use this information to make inferences about the age of the entire universe.
The most widely accepted current estimate for the age of the universe is approximately 13.8 billion years. This value is derived from multiple lines of evidence, including the observations of the cosmic microwave background radiation, which provides a snapshot of the universe when it was only about 380,000 years old. By studying the properties of this radiation, scientists can extrapolate back in time and estimate the age of the universe.
It is important to note that our understanding of the universe's age is based on the best scientific knowledge and observations available to us. While our observations are limited to the observable universe, the models and theories that underpin our understanding have been extensively tested and refined over time. However, scientific knowledge is subject to revision as new data and observations become available, and our understanding of the age of the universe may be refined in the future as our techniques and technology improve.