Astronomy 101: Cosmic microwave background

The cosmic microwave background is one of the major pieces of evidence that our universe began in what’s called the Big Bang.
By | Published: February 4, 2013 | Last updated on February 20, 2025

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Key Takeaways:

  • The cosmic microwave background (CMB) was discovered serendipitously in the mid-1960s by Arno Penzias and Robert Wilson, providing crucial observational evidence for the Big Bang theory through the detection of a uniform microwave signal.
  • Subsequent space-based telescopes, including the COsmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP), were deployed to precisely measure the CMB's temperature at 2.73 Kelvin and map its subtle temperature variations.
  • These minute temperature fluctuations within the CMB, which depict the universe merely 375,000 years after the Big Bang, reveal primordial density differences that seeded the formation of galaxies and large-scale cosmic structures.
  • WMAP's comprehensive nine-year data release in 2012 refined key cosmological parameters, determining the universe's age at 13.77 billion years, confirming its geometric flatness, and quantifying its composition as 4.6% normal matter, 24% dark matter, and 71.4% dark energy.
Key takeaways sponsored by The Space Store
CMB_1965
The microwave sky as it looked with 1965 technology // NASA/WMAP Science Team
Two scientists, Arno Penzias and Robert Wilson, discovered the cosmic microwave background in the mid-1960s while  testing a large radio antenna for Bell Laboratories. They detected a constant signal no matter what direction they studied. Astrophysicists who had been researching the Big Bang theory of the universe, and possible evidence for it, heard of the engineers’ observations and knew immediately what it was. If the universe had begun in a hot and tiny state, then it would have cooled as it expanded. Eventually, the cosmic temperature would reach just about 3 kelvins above absolute zero, or –270.15° Celsius. This temperature coincides with a microwave radiation glow no matter what direction scientists observed. The discovery of the cosmic microwave background, or CMB, earned Penzias and Wilson the 1978 Nobel Prize in physics.

Since the first observations of the CMB, astronomers have been creating space-based telescopes to study the radiation and see how it varies. The COsmic Background Explorer was the first spacecraft to study the CMB in detail. Then in 2001, the Wilkinson Microwave Anisotropy Probe (or, WMAP) launched to record the microwave sky and create the best map we have of the radiation background. This telescope measured the CMB as 2.73 kelvins (or –270.40° C), and the color variations in the image show temperature differences of just a few hundred-thousandths of a degree. These tiny temperature deviations also provide evidence that the universe is expanding and must have begun in a much smaller state, because those regions of similar temperature would have had to be in contact.

WMAP_map
WMAP’s nine-year data release // NASA/WMAP Science Team
As scientists achieved higher-resolution images, they could learn more about the universe’s properties. This is because the CMB shows the cosmos as it was just 375,000 years after the Big Bang. Slight differences in matter and radiation distributions at that time emerge in the CMB map. Hotter and less-dense regions appear as redder splotches on the image, whereas cooler and denser areas are bluer. These denser regions will evolve into galaxies and clusters, while less-dense areas will eventually become the voids in cosmic structure.

The sizes of the spots can tell astronomers about the universe’s age, shape, and composition: the amount of normal matter (which makes up the stars and galaxies), dark matter (the majority of the universe’s mass), and dark energy (the mysterious “something” that’s accelerating expansion) in the cosmos.

The WMAP team released its final data set in 2012, incorporating nine years of observations. According to those numbers, the universe is 13.77 billion years old and geometrically flat, which means that if you follow parallel lines forever across the universe, they will never meet or diverge. Analysis also shows that 4.6 percent of the universe is normal matter, 24 percent is the mysterious dark matter, and 71.4 percent is dark energy. Astronomers have a lot to learn about our universe, as just 4.6 percent is well understood.


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