Planck 2018 Results: The Universe Is 13.8 Billion Years Old

The Planck 2018 results are the final measurements of the universe’s basic numbers from ESA’s Planck satellite. Released in July 2018 and published in 2020, they put the age of the universe at 13.787 ± 0.020 billion years and the Hubble constant at 67.4 ± 0.5 km/s per megaparsec, from the cosmic microwave background.

The question before the paper

The cosmic microwave background is light released about 400,000 years after the Big Bang. Its tiny temperature ripples, first seen by COBE in 1992 and mapped by WMAP in 2003, encode the universe’s geometry, contents and expansion. Planck was built to read that record as precisely as physics allows. Its final question: what are the parameters of the standard model of cosmology, and does that model still fit?

What the paper found

• Age of the universe: 13.787 ± 0.020 billion years.
• Hubble constant: H₀ = 67.4 ± 0.5 km/s per megaparsec, the expansion rate today predicted from the early universe.
• Matter: a matter density parameter Ω_m = 0.315 ± 0.007, with dark matter density Ω_ch² = 0.120 ± 0.001 and ordinary (baryon) density Ω_bh² = 0.0224 ± 0.0001.
• Primordial ripples: spectral index n_s = 0.965 ± 0.004, slightly below the exactly scale-invariant value of 1, as cosmic inflation models generally predict.
• Neutrinos: the sum of the three neutrino masses is below 0.12 eV (95% confidence).

The simple Lambda-CDM model, a flat universe of cold dark matter, ordinary matter and a cosmological constant, fits the data.

How they did it

Planck mapped the whole microwave sky in temperature and polarization. The collaboration combined three kinds of information: the temperature ripples, the polarization pattern, and gravitational lensing, the subtle distortion of the background light by all the matter it passed on its way to us. The statistical pattern of ripples at different angular sizes, with its series of peaks from sound waves in the young universe, was fitted with cosmological models to extract the parameters and their uncertainties.

Why it mattered

Planck’s numbers are the benchmark “early universe” values in cosmology. When someone says the universe is 13.8 billion years old, this is the measurement behind the number. The values also serve as the baseline that every new survey compares against, which is how tensions are found.

What we know now

The biggest one is the Hubble tension. Measuring the expansion directly, with Cepheid stars and supernovae, the SH0ES team found 73.04 ± 1.04 km/s per megaparsec, a 5-sigma disagreement with Planck’s 67.4. Either a measurement has an unrecognized error or Lambda-CDM is missing something; see our Hubble tension explainer.

A second test comes from galaxy maps. The DESI first-year results and DESI’s 2025 second release, from the Dark Energy Spectroscopic Instrument, combined their data with Planck’s and found a 3.1-sigma preference for dark energy that weakens over time, short of a discovery. The age of the universe remains secure; see how old the universe is.

Who wrote it

The Planck Collaboration is a large international team built around ESA’s Planck mission. Papers in the series list the collaboration first, followed by individual members, beginning with N. Aghanim and Y. Akrami. This paper, number VI in the 2018 series, is the one that reports the cosmological parameters.

Read the original

Planck Collaboration, Aghanim, N., Akrami, Y., et al. (2020). “Planck 2018 results. VI. Cosmological parameters.” Astronomy & Astrophysics. doi:10.1051/0004-6361/201833910. First posted in July 2018, published in September 2020. Free on arXiv: 1807.06209. Semantic Scholar lists about 5,400 citations (October 2026).

Questions people ask

How old is the universe according to Planck?

Planck’s final 2018 analysis gives 13.787 ± 0.020 billion years, usually rounded to 13.8 billion. The age is inferred from the cosmic microwave background within the standard Lambda-CDM model of cosmology, so it depends on that model being right.

What Hubble constant did Planck measure?

Planck’s 2018 value is 67.4 ± 0.5 km/s per megaparsec. Strictly, it is a prediction of today’s expansion rate from the early universe, assuming Lambda-CDM. Direct local measurements give about 73, a 5-sigma gap called the Hubble tension.

Why is the universe 13.8 billion years old and not 13.7?

Both numbers come from cosmic microwave background missions. WMAP’s first-year results in 2003 gave 13.7 ± 0.2 billion years. Planck’s more precise 2018 analysis gives 13.787 ± 0.020 billion years, which rounds to 13.8. The difference reflects better data, not a contradiction.