WMAP First-Year Results (2003): The Universe's Age, Shape and Contents

The WMAP first-year results, published by David Spergel and the WMAP team in 2003, measured the age, shape and contents of the universe to a few percent. Using NASA’s WMAP all-sky maps of the cosmic microwave background, they found a flat universe 13.7 billion years old, dominated by dark energy and dark matter.

The question before the paper

By 2002 cosmology had a rough consensus. COBE had found ripples in the cosmic microwave background, and supernova surveys had shown in 1998 that the expansion is accelerating. But the key numbers, the expansion rate, the amount of ordinary and dark matter, the geometry of space, were still known only roughly. Could one simple model fit everything, with every parameter pinned down?

What the paper found

• A simple model fits. A flat universe dominated by a cosmological constant (Lambda), with cold dark matter (CDM) and seeded by nearly scale-invariant, Gaussian fluctuations, matches the data. This is the model now called standard Lambda-CDM.
• Age: 13.7 ± 0.2 billion years.
• Expansion rate: h = 0.71 (+0.04/-0.03), meaning a Hubble constant of about 71 km/s per megaparsec, combining WMAP with other CMB and galaxy data.
• Contents: baryon density Ω_bh² = 0.0224 ± 0.0009 and matter density Ω_mh² = 0.135 (+0.008/-0.009).
• Shape and dark energy: total density Ω_tot = 1.02 ± 0.02, a flat universe, and a dark energy equation of state w < -0.78 at 95% confidence.

How they did it

The Wilkinson Microwave Anisotropy Probe mapped the temperature of the microwave sky over the whole sky for a full year. The team measured how strong the temperature ripples are on each angular size, the “power spectrum”, and fitted it with models of the early universe. The peaks in that spectrum record sound waves in the young universe, and their positions and heights depend sensitively on geometry and composition.

Why it mattered

WMAP turned cosmology into a precision science. Its parameter set became the reference model that later surveys test and try to break, and the paper is one of the most cited in astrophysics, with about 9,400 citations in Semantic Scholar as of October 2026. Its agreement with galaxy clustering, including the 2005 detection of baryon acoustic oscillations, made Lambda-CDM the standard framework.

What we know now

ESA’s Planck satellite refined every number. The Planck 2018 results give an age of 13.787 ± 0.020 billion years and a Hubble constant of 67.4 ± 0.5 km/s per megaparsec. That lower expansion rate now conflicts with direct measurements such as SH0ES, the Hubble tension. Galaxy surveys such as DESI now test the WMAP and Planck model from a completely different direction. Read how the age is measured in our age of the universe guide.

Who wrote it

David Spergel is the first author of this parameter paper, written with L. Verde, H. V. Peiris and the WMAP science team. The satellite itself is named after David Wilkinson, one of the Princeton physicists who first interpreted the cosmic microwave background in 1965.

Read the original

Spergel, D. N., Verde, L., Peiris, H. V., et al. (2003). “First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters.” The Astrophysical Journal Supplement Series. doi:10.1086/377226. Free on arXiv: astro-ph/0302209.