What Is the Cosmic Microwave Background? The Oldest Light, Explained

Point a sensitive enough microwave receiver at any patch of sky, day or night, and it picks up a faint, almost perfectly even glow. That glow is the cosmic microwave background (CMB), and it is the oldest light anyone can observe. NASA calls it “the oldest light we can observe in the universe” (NASA Science^[1]). Our sticker Relicta is that relic glow, personified.

What is the cosmic microwave background?

It is radiation left over from the hot early universe, now stretched into microwaves and cooled to just above absolute zero. Its spectrum is that of a near-perfect blackbody at a temperature of 2.72548 ± 0.00057 K (Fixsen 2009^[2]). According to ESA, roughly 400 of these photons fill every cubic centimetre of space (ESA^[3]).

Where did the CMB come from?

A universe that was opaque

For roughly its first 380,000 years, the universe was a hot plasma of protons, electrons and other particles mixed with light. Photons kept scattering off free electrons, so light could not travel far; the universe was opaque, “like being in fog” (ESA^[3]).

The moment it cleared

As the universe expanded it cooled. ESA describes how, after about 380,000 years, it reached around 3000 K, cool enough for electrons to combine with protons into neutral hydrogen atoms (ESA^[3]). With the free electrons gone, light could suddenly travel freely, and the photons from that moment have been streaming through space ever since. Cosmologists call this epoch recombination; the Planck satellite’s analysis puts it at a redshift of z* ≈ 1090 (Planck Collaboration 2020^[4]).

Why it is microwaves now

Since then, the expansion of space has stretched those photons’ wavelengths by a factor of about 1100, to around a millimetre, so their temperature has dropped from about 3000 K to 2.7 K (ESA^[3]). Temperature and redshift track each other: T today ≈ T_then / (1 + z).

Who discovered the cosmic microwave background?

1964: the noise that wouldn’t go away

Arno Penzias and Robert Wilson, radio astronomers at Bell Laboratories in Holmdel, New Jersey, were working with a large horn antenna. In 1964 they found that microwaves at a wavelength of about 7 centimetres were stronger than expected, and the excess would not go away when they looked for faults in their equipment (NobelPrize.org^[5]). Their 1965 paper, modestly titled “A Measurement of Excess Antenna Temperature at 4080 Mc/s,” reported an unexplained extra few kelvin coming from every direction (Penzias & Wilson 1965^[6]).

The team that was looking for it

Down the road at Princeton, Robert Dicke, Jim Peebles, Peter Roll and David Wilkinson had been preparing to search for exactly such radiation. Their companion paper in the same volume of the journal, “Cosmic Black-Body Radiation,” interpreted the signal as relic radiation from a hot early universe (Dicke et al. 1965^[7]). Penzias and Wilson received the 1978 Nobel Prize in Physics “for their discovery of cosmic microwave background radiation” (NobelPrize.org^[8]). The idea of a hot, dense early universe was older still; Alpher, Bethe and Gamow’s 1948 paper on the origin of the elements is one landmark (Alpher, Bethe & Gamow 1948^[9]). For more on how credit was shared (and not) in stories like this, see Who Really Discovered It?

What did COBE, WMAP and Planck find?

A perfect blackbody

NASA’s COBE satellite measured the CMB spectrum with its FIRAS instrument and found it matched a blackbody curve with extraordinary precision; the first results were published in 1990 (Mather et al. 1990^[10]). That spectrum is strong evidence that the early universe really was hot and in thermal equilibrium.

Tiny ripples

The CMB is not perfectly even. It carries temperature differences of only tens of millionths of a kelvin from place to place. COBE first detected these anisotropies, and John Mather and George Smoot shared the 2006 Nobel Prize “for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation” (NobelPrize.org^[11]). WMAP mapped them in detail; NASA describes the WMAP image as showing 13.8-billion-year-old temperature fluctuations, “seeds that grew into the galaxies we see today” (NASA Science^[1]). The Planck satellite measured them more precisely still, and its analysis pins down the universe’s age, composition and expansion rate (Planck Collaboration 2020^[4]).

What does the CMB tell us?

• The Big Bang was hot. A blackbody glow filling all space is what a hot, dense, expanding early universe predicts.
• Where galaxies came from. The ripples are the starting pattern of the cosmic web.
• What the universe is made of. The pattern of ripples is sensitive to the amounts of ordinary matter, dark matter and dark energy (Planck^[4]). Some tensions with other measurements, such as the expansion rate, are still debated.

The CMB also cross-checks the story of the first elements: the ordinary-matter density it implies can be compared with what Big Bang nucleosynthesis needs. Our pillar post How Did the First Elements Form? explains that link.

Wear the oldest light

Relicta also appears with Primordix on the On Cosmic Time sticker sheet. Browse more cosmos in Relativity & Cosmos.

FAQ

How old is the cosmic microwave background?

The light was released about 380,000 years after the Big Bang, roughly 13.8 billion years ago.

What temperature is the CMB?

2.72548 ± 0.00057 K, just under 3 degrees above absolute zero.

Who discovered the CMB?

Arno Penzias and Robert Wilson, in 1964, at Bell Labs; a Princeton group led by Robert Dicke interpreted it.

Why is it microwaves and not visible light?

The expansion of space stretched its wavelength by about 1100 times since it was released.

References

1. NASA Science, “Universe overview”. https://science.nasa.gov/universe/overview/
2. Fixsen (2009) The Temperature of the Cosmic Microwave Background, ApJ 707, 916. https://doi.org/10.1088/0004-637X/707/2/916
3. ESA, “Planck and the cosmic microwave background”. https://www.esa.int/Science_Exploration/Space_Science/Planck/Planck_and_the_cosmic_microwave_background
4. Planck Collaboration (2020) Planck 2018 results. VI. Cosmological parameters, A&A 641, A6 (Table 2: z* = 1089.92 ± 0.25). https://doi.org/10.1051/0004-6361/201833910
5. NobelPrize.org, Arno Penzias, Physics 1978, facts. https://www.nobelprize.org/prizes/physics/1978/penzias/facts/
6. Penzias & Wilson (1965) A Measurement of Excess Antenna Temperature at 4080 Mc/s, ApJ 142, 419. https://doi.org/10.1086/148307
7. R. H. Dicke, P. J. E. Peebles, P. G. Roll & D. T. Wilkinson (1965), “Cosmic Black-Body Radiation,” ApJ 142, 414. https://doi.org/10.1086/148306
8. NobelPrize.org — The Nobel Prize in Physics 1978 (Penzias and Wilson, cosmic microwave background radiation). https://www.nobelprize.org/prizes/physics/1978/summary/
9. Alpher, Bethe & Gamow (1948), “The Origin of Chemical Elements,” Phys. Rev. 73, 803. https://doi.org/10.1103/PhysRev.73.803
10. Mather et al. (1990) A preliminary measurement of the cosmic microwave background spectrum by the Cosmic Background Explorer (COBE) satellite, ApJ 354, L37. https://doi.org/10.1086/185717
11. NobelPrize.org — The Nobel Prize in Physics 2006 (Mather and Smoot, blackbody form and anisotropy of the CMB). https://www.nobelprize.org/prizes/physics/2006/summary/

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.