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Cosmic microwave background (CMB)

The cosmic microwave background (CMB) is a faint glow of radiation that fills the universe, serving as a remnant from the birth of the cosmos approximately 13.8 billion years ago during the Big Bang. This radiation represents a snapshot of the universe when it first became transparent to light, roughly 380,000 years after the Big Bang, allowing for the formation of atoms. While the CMB is not visible to the naked eye, it can be detected with specialized instruments and manifests primarily in the microwave spectrum.

The existence of the CMB was first theorized in the 1940s but was serendipitously discovered in 1964 by radio astronomers Robert Wilson and Arno Penzias, who initially mistook the radiation for equipment noise. This groundbreaking discovery provided significant support for the Big Bang theory and has been pivotal in understanding the early universe's conditions. Over the years, various missions, including NASA's COBE, WMAP, and the European Space Agency's Planck observatory, have studied the CMB in depth, revealing insights about the universe's composition and structure, including the enigmatic dark energy and dark matter that dominate its makeup. The CMB thus stands as a crucial piece of evidence for cosmologists, enhancing our grasp of the universe's origins and development.

Full Article

The cosmic microwave background (CMB) is the radiation signature left over from the birth of the universe about 13.8 billion years ago. Astronomers believe the universe formed in a sudden and violent explosion called the Big Bang. The CMB is a snapshot of the young universe as it existed shortly after the Big Bang. It is invisible to the naked eye but can be observed by human-made instruments as a faint and constant glow of radiation across the sky from all directions. While the existence of the CMB was theorized in the 1940s, it was discovered accidentally in 1964 by radio astronomers in New Jersey.

Background

The prevailing theory about the birth of the universe holds that about 13.8 billion years ago, all matter in existence was packed together into an infinitesimally small point. This point suddenly exploded and expanded in all directions in what astronomers refer to as the Big Bang. The infant universe was a white-hot cloud of matter and energy that reached temperatures of about 10 billion degrees Fahrenheit (5.6 billion degrees Celsius)—so hot that subatomic particles such as electrons, protons, and neutrons could not form atoms. Protons and neutrons eventually began to combine to form atomic nuclei, but electrons roamed freely in the early universe.

Light emitted by the universe was scattered by the free electrons and could not penetrate the fog of subatomic particles. The process is similar to how water droplets in clouds scatter light on Earth. About 380,000 years after the Big Bang, the universe cooled enough that electrons could join with atomic nuclei to form atoms. This allowed the first visible light from the universe to shine.

It was not until about 100 million years after the Big Bang that the first stars and galaxies are believed to have formed. The most distant objects observable by human telescopes would have formed when the universe was about 600 million years old. It took about 9 billion years—about 4.6 billion years ago—for the Sun, Earth, and the rest of the solar system to form.

Overview

In the early twentieth century, American astronomer Edwin Hubble discovered that all the distant galaxies in the universe seemed to be moving away from our galaxy. He also found that the more distant the galaxy, the faster it was receding. Hubble's findings supported an earlier theory from Belgian astronomer and Catholic priest Georges Lemaître. Lemaître theorized that if the universe is expanding, then it must at one point have been confined into a single point, which he called the primordial atom. Lemaître and Hubble's work was refined by other scientists and was eventually referred to as the Big Bang theory. Ironically, an outspoken opponent of the theory gave it that name as an insult.

In the 1940s, American physicists Ralph Alpher, Robert Herman, and George Gamow predicted that the explosion that formed the universe should have left behind some trace of its existence in the form of radiation. Astronomers had never seen any sign of the CMB radiation, and many at the time believed it was too difficult to detect. It was not apparent as visible light, but the light seen by the naked eye falls along a very narrow band of the electromagnetic spectrum. The CMB would more likely appear on another range of the spectrum, such as lower-frequency radio waves, microwaves, or infrared radiation.

In 1964, Robert Wilson and Arno Penzias, two radio astronomers working for Bell Laboratories in Holmdel Township, New Jersey, began picking up a strange buzzing "hiss" seemingly originating from every part of the sky. At first, they thought it might be malfunctioning equipment, signals from nearby New York City, or even pigeons nesting in the antenna. After a year of troubleshooting and retesting the equipment, Wilson and Penzias realized that what they were hearing was the cosmic signature left over by the Big Bang. Their discovery earned them the Nobel Prize in Physics in 1978.

The CMB signifies the moment light from the early universe "switched on" 380,000 years after its creation. Its discovery not only helped support the Big Bang theory but also gave astronomers a key piece of evidence to discover clues about the infant universe. As the universe expanded, it cooled, but in its early days, the CMB was still hotter than the surface of a star. At one hundred-millionth its current size, matter in the universe was about as dense as air on the surface of Earth, and its temperature was about 273 million degrees Fahrenheit (152 million Celsius).

At about one-hundredth its present size, the CMB had cooled to 32 degrees Fahrenheit (0 degrees Celsius)—the temperature at which water freezes. At the current size of the universe, the temperature of the CMB is about –454.8 degrees Fahrenheit (–270.4 Celsius), slightly above absolute zero, the temperature at which atoms stop moving. The immense distance of the CMB has stretched its light waves so much that they are strongest in the low-frequency microwave part of the electromagnetic spectrum.

In 1989, the National Aeronautics and Space Administration (NASA) launched the Cosmic Background Explorer (COBE), the first orbital mission designed to study the CMB. The spacecraft determined that the temperature of the CMB was not constant but exhibited very small fluctuations. In 2001, NASA launched the Wilkinson Microwave Anisotropy Probe (WMAP) to further study the CMB. The probe found that the temperature variations represented areas of slightly different densities and revealed the first hints of future galaxy and star formation.

In 2009, the European Space Agency sent the Planck space observatory into orbit to study the CMB in the far-infrared, microwave, and radio areas of the electromagnetic spectrum. The more detailed information sent back by Planck, which operated until 2013, helped astronomers estimate the age of the universe at 13.8 billion years. Research has revealed a disagreement between measurements of the universe’s expansion rate based on the CMB and those based on nearby galaxies, a problem known as the “Hubble tension,” which may suggest gaps in the current understanding of cosmology. Planck also helped confirm theories that most of the universe is made of a mysterious force called dark energy. Scientists are unsure what dark energy is, but data from Planck showed that it makes up about 68 percent of the universe. Results from the Dark Energy Spectroscopic Instrument (DESI) in 2025 suggest that dark energy may not be constant over time, based on measurements of millions of galaxies and quasars, indicating that its effects on the expansion of the universe could be changing. Equally mysterious dark matter makes up about 26.8 percent, and regular matter—all the stars, planets, and galaxies—accounts for only 4.9 percent. Observations from the Atacama Cosmology Telescope have produced more detailed maps of the CMB than Planck, including high-precision measurements of polarization that help scientists study the early formation of galaxies and test theories of cosmic inflation. In 2025, scientists using the Cosmology Large Angular Scale Surveyor (CLASS), a ground-based telescope in Chile, detected a polarized microwave signal dating back about 13 billion years to the period known as the “cosmic dawn,” when the first stars formed. This signal provides new evidence of how early starlight interacted with matter and left an imprint on the CMB.


Bibliography

“Atacama Cosmology Telescope Captures Clearest Images Yet of Cosmic Microwave Background.” Sci‑News, 18 Mar. 2025, www.sci.news/astronomy/atacama-cosmology-telescope-cosmic-microwave-background-images-13763.html. Accessed 21 Mar. 2026.

Creighton, Jolene. "The Timeline of the Big Bang and Everything We Know." Futurism, 28 Feb. 2018, futurism.com/cosmology-the-timeline-of-everything-we-know/. Accessed 21 Mar. 2026.

Dreifus, Claudia. "How Two Pigeons Helped Scientists Confirm the Big Bang Theory." Smithsonian Magazine, 19 Feb. 2014, www.smithsonianmag.com/smithsonian-institution/how-scientists-confirmed-big-bang-theory-owe-it-all-to-a-pigeon-trap-180949741/. Accessed 21 Mar 2026.

"The Electromagnetic Spectrum." NASA, Mar. 2013, imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html. Accessed 21 Mar. 2026.

Evans, Rhodri. The Cosmic Microwave Background. Springer, 2015.

Lea, Robert. "Astronomers See the 1st Stars Dispel Darkness 13 Billion Years Ago at 'Cosmic Dawn'." Space.com, 11 June 2025, www.space.com/astronomy/astronomers-see-the-1st-stars-dispel-darkness-13-billion-years-ago-at-cosmic-dawn. Accessed 21 Mar. 2026.

Lea, Robert. "How Cold Is Space? Physics behind the Temperature of the Universe." Space.com, 12 Jan. 2026, www.space.com/how-cold-is-space. Accessed 21 Mar. 2026.

Leitch, Erik M. "What Is the Cosmic Microwave Background Radiation?" Scientific American, 1 Nov. 2004, www.scientificamerican.com/article/what-is-the-cosmic-microw/. Accessed 21 Mar. 2026.

Shu, Frank K. "Cosmic Microwave Background (CMB)." Britannica, 24 Jan. 2026, www.britannica.com/science/cosmic-microwave-background. Accessed 21 Mar. 2026.

Stuart, Colin. "The Dark Energy Survey Weighs in on Cosmic Tensions." Sky & Telescope, 27 Jan. 2026, skyandtelescope.org/astronomy-news/the-dark-energy-survey-weighs-in-on-cosmic-tensions/. Accessed 21 Mar. 2026.

"Tests of Big Bang: The CMB." NASA, 20 Feb. 2025, map.gsfc.nasa.gov/universe/bb_tests_cmb.html. Accessed 21 Mar. 2026.

Wall, Mike. "Cosmic Anniversary: 'Big Bang Echo' Discovered 50 Years Ago Today." Space.com, 20 May 2014, www.space.com/25945-cosmic-microwave-background-discovery-50th-anniversary.html. Accessed 21 Mar. 2026.

Full Article

The cosmic microwave background (CMB) is the radiation signature left over from the birth of the universe about 13.8 billion years ago. Astronomers believe the universe formed in a sudden and violent explosion called the Big Bang. The CMB is a snapshot of the young universe as it existed shortly after the Big Bang. It is invisible to the naked eye but can be observed by human-made instruments as a faint and constant glow of radiation across the sky from all directions. While the existence of the CMB was theorized in the 1940s, it was discovered accidentally in 1964 by radio astronomers in New Jersey.

Background

The prevailing theory about the birth of the universe holds that about 13.8 billion years ago, all matter in existence was packed together into an infinitesimally small point. This point suddenly exploded and expanded in all directions in what astronomers refer to as the Big Bang. The infant universe was a white-hot cloud of matter and energy that reached temperatures of about 10 billion degrees Fahrenheit (5.6 billion degrees Celsius)—so hot that subatomic particles such as electrons, protons, and neutrons could not form atoms. Protons and neutrons eventually began to combine to form atomic nuclei, but electrons roamed freely in the early universe.

Light emitted by the universe was scattered by the free electrons and could not penetrate the fog of subatomic particles. The process is similar to how water droplets in clouds scatter light on Earth. About 380,000 years after the Big Bang, the universe cooled enough that electrons could join with atomic nuclei to form atoms. This allowed the first visible light from the universe to shine.

It was not until about 100 million years after the Big Bang that the first stars and galaxies are believed to have formed. The most distant objects observable by human telescopes would have formed when the universe was about 600 million years old. It took about 9 billion years—about 4.6 billion years ago—for the Sun, Earth, and the rest of the solar system to form.

Overview

In the early twentieth century, American astronomer Edwin Hubble discovered that all the distant galaxies in the universe seemed to be moving away from our galaxy. He also found that the more distant the galaxy, the faster it was receding. Hubble's findings supported an earlier theory from Belgian astronomer and Catholic priest Georges Lemaître. Lemaître theorized that if the universe is expanding, then it must at one point have been confined into a single point, which he called the primordial atom. Lemaître and Hubble's work was refined by other scientists and was eventually referred to as the Big Bang theory. Ironically, an outspoken opponent of the theory gave it that name as an insult.

In the 1940s, American physicists Ralph Alpher, Robert Herman, and George Gamow predicted that the explosion that formed the universe should have left behind some trace of its existence in the form of radiation. Astronomers had never seen any sign of the CMB radiation, and many at the time believed it was too difficult to detect. It was not apparent as visible light, but the light seen by the naked eye falls along a very narrow band of the electromagnetic spectrum. The CMB would more likely appear on another range of the spectrum, such as lower-frequency radio waves, microwaves, or infrared radiation.

In 1964, Robert Wilson and Arno Penzias, two radio astronomers working for Bell Laboratories in Holmdel Township, New Jersey, began picking up a strange buzzing "hiss" seemingly originating from every part of the sky. At first, they thought it might be malfunctioning equipment, signals from nearby New York City, or even pigeons nesting in the antenna. After a year of troubleshooting and retesting the equipment, Wilson and Penzias realized that what they were hearing was the cosmic signature left over by the Big Bang. Their discovery earned them the Nobel Prize in Physics in 1978.

The CMB signifies the moment light from the early universe "switched on" 380,000 years after its creation. Its discovery not only helped support the Big Bang theory but also gave astronomers a key piece of evidence to discover clues about the infant universe. As the universe expanded, it cooled, but in its early days, the CMB was still hotter than the surface of a star. At one hundred-millionth its current size, matter in the universe was about as dense as air on the surface of Earth, and its temperature was about 273 million degrees Fahrenheit (152 million Celsius).

At about one-hundredth its present size, the CMB had cooled to 32 degrees Fahrenheit (0 degrees Celsius)—the temperature at which water freezes. At the current size of the universe, the temperature of the CMB is about –454.8 degrees Fahrenheit (–270.4 Celsius), slightly above absolute zero, the temperature at which atoms stop moving. The immense distance of the CMB has stretched its light waves so much that they are strongest in the low-frequency microwave part of the electromagnetic spectrum.

In 1989, the National Aeronautics and Space Administration (NASA) launched the Cosmic Background Explorer (COBE), the first orbital mission designed to study the CMB. The spacecraft determined that the temperature of the CMB was not constant but exhibited very small fluctuations. In 2001, NASA launched the Wilkinson Microwave Anisotropy Probe (WMAP) to further study the CMB. The probe found that the temperature variations represented areas of slightly different densities and revealed the first hints of future galaxy and star formation.

In 2009, the European Space Agency sent the Planck space observatory into orbit to study the CMB in the far-infrared, microwave, and radio areas of the electromagnetic spectrum. The more detailed information sent back by Planck, which operated until 2013, helped astronomers estimate the age of the universe at 13.8 billion years. Research has revealed a disagreement between measurements of the universe’s expansion rate based on the CMB and those based on nearby galaxies, a problem known as the “Hubble tension,” which may suggest gaps in the current understanding of cosmology. Planck also helped confirm theories that most of the universe is made of a mysterious force called dark energy. Scientists are unsure what dark energy is, but data from Planck showed that it makes up about 68 percent of the universe. Results from the Dark Energy Spectroscopic Instrument (DESI) in 2025 suggest that dark energy may not be constant over time, based on measurements of millions of galaxies and quasars, indicating that its effects on the expansion of the universe could be changing. Equally mysterious dark matter makes up about 26.8 percent, and regular matter—all the stars, planets, and galaxies—accounts for only 4.9 percent. Observations from the Atacama Cosmology Telescope have produced more detailed maps of the CMB than Planck, including high-precision measurements of polarization that help scientists study the early formation of galaxies and test theories of cosmic inflation. In 2025, scientists using the Cosmology Large Angular Scale Surveyor (CLASS), a ground-based telescope in Chile, detected a polarized microwave signal dating back about 13 billion years to the period known as the “cosmic dawn,” when the first stars formed. This signal provides new evidence of how early starlight interacted with matter and left an imprint on the CMB.


Bibliography

“Atacama Cosmology Telescope Captures Clearest Images Yet of Cosmic Microwave Background.” Sci‑News, 18 Mar. 2025, www.sci.news/astronomy/atacama-cosmology-telescope-cosmic-microwave-background-images-13763.html. Accessed 21 Mar. 2026.

Creighton, Jolene. "The Timeline of the Big Bang and Everything We Know." Futurism, 28 Feb. 2018, futurism.com/cosmology-the-timeline-of-everything-we-know/. Accessed 21 Mar. 2026.

Dreifus, Claudia. "How Two Pigeons Helped Scientists Confirm the Big Bang Theory." Smithsonian Magazine, 19 Feb. 2014, www.smithsonianmag.com/smithsonian-institution/how-scientists-confirmed-big-bang-theory-owe-it-all-to-a-pigeon-trap-180949741/. Accessed 21 Mar 2026.

"The Electromagnetic Spectrum." NASA, Mar. 2013, imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html. Accessed 21 Mar. 2026.

Evans, Rhodri. The Cosmic Microwave Background. Springer, 2015.

Lea, Robert. "Astronomers See the 1st Stars Dispel Darkness 13 Billion Years Ago at 'Cosmic Dawn'." Space.com, 11 June 2025, www.space.com/astronomy/astronomers-see-the-1st-stars-dispel-darkness-13-billion-years-ago-at-cosmic-dawn. Accessed 21 Mar. 2026.

Lea, Robert. "How Cold Is Space? Physics behind the Temperature of the Universe." Space.com, 12 Jan. 2026, www.space.com/how-cold-is-space. Accessed 21 Mar. 2026.

Leitch, Erik M. "What Is the Cosmic Microwave Background Radiation?" Scientific American, 1 Nov. 2004, www.scientificamerican.com/article/what-is-the-cosmic-microw/. Accessed 21 Mar. 2026.

Shu, Frank K. "Cosmic Microwave Background (CMB)." Britannica, 24 Jan. 2026, www.britannica.com/science/cosmic-microwave-background. Accessed 21 Mar. 2026.

Stuart, Colin. "The Dark Energy Survey Weighs in on Cosmic Tensions." Sky & Telescope, 27 Jan. 2026, skyandtelescope.org/astronomy-news/the-dark-energy-survey-weighs-in-on-cosmic-tensions/. Accessed 21 Mar. 2026.

"Tests of Big Bang: The CMB." NASA, 20 Feb. 2025, map.gsfc.nasa.gov/universe/bb_tests_cmb.html. Accessed 21 Mar. 2026.

Wall, Mike. "Cosmic Anniversary: 'Big Bang Echo' Discovered 50 Years Ago Today." Space.com, 20 May 2014, www.space.com/25945-cosmic-microwave-background-discovery-50th-anniversary.html. Accessed 21 Mar. 2026.

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