The universe is 13.8 billion years old, to within 0.3%

The universe is about 13.8 billion years old, a figure cosmologists get by measuring the cosmic microwave background, the leftover light of the Big Bang, and running its pattern of tiny temperature ripples through a model of how the cosmos has expanded and cooled.

Separate instruments, built years apart, land on nearly the same number; the last two rows are the star that seemed to contradict them:

What was measured	Source	Age
Universe	WMAP (NASA, data analysis completed 2013)	13.77 billion years, to within half a percent
Universe	Planck (ESA, 2013 results)	13.82 billion years
Universe	Atacama Cosmology Telescope (2020)	13.77 billion years, give or take 40 million (about 0.3%)
HD 140283, the “Methuselah star”	estimates dating back to 2000	up to 16 billion years
HD 140283	Hubble Space Telescope distance, 2013	14.5 billion years, plus or minus 800 million

An age is only as good as its cross-checks, and the hardest one is the oldest stars, which obviously cannot be older than the universe that holds them. In the 1990s star ages and expansion ages seemed to clash. As UCLA cosmologist Ned Wright notes, better HIPPARCOS distances in 1997 made the oldest stars younger, WMAP in 2003 made the universe older, and the discrepancy disappeared.

The Methuselah star’s revised age still looks too old at first glance, but the error bars now overlap the universe’s age.

“Maybe the cosmology is wrong, stellar physics is wrong, or the star’s distance is wrong.” — Howard Bond, Pennsylvania State University, 2013

The debate has not entirely gone away. In 2019 a team measuring galaxy motions calculated that the universe is hundreds of millions of years younger than Planck’s figure. The 2020 ACT measurement sided with Planck, but the disagreement over how fast the universe is expanding remains a live question.