Cosmic microwave background

The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a normal optical telescope, space between stars and galaxies looks almost dark. But a special radio telescope can find a soft glow from everywhere. This glow is not from stars, galaxies, or any other object. It is strongest in the microwave part of the electromagnetic spectrum.

Temperature map of the cosmic microwave background measured by the Planck spacecraft

The CMB gives us good proof for the Big Bang theory about how the universe started. In the very early universe, it was full of a hot, tight plasma of tiny parts. As the universe grew bigger and cooler, protons and electrons joined to make neutral atoms, mostly hydrogen. This let the universe see clearly for the first time, and light could move freely. This moment is called the recombination epoch.

Even though the CMB looks very even, careful tools have found very small temperature changes. Experiments like COBE, WMAP, and Planck measured these changes. They show a pattern of tiny waves that tell us about the early universe, like its shape and how much dark matter there is. Looking at the CMB helps scientists learn how the universe began and how it has changed.

Features

The cosmic microwave background radiation is a steady glow of energy from all directions in space. It has an even temperature, measured in kelvin, and looks almost the same everywhere. Scientists found very small changes in this temperature, which helps us learn about how the universe began.

The radiation also shows a special kind of light called polarization. This gives us clues about the early universe. Even though these changes are tiny, they are important for understanding how everything started. The cosmic microwave background holds most of the light particles, or photons, in the universe. If the universe had not expanded and cooled, the night sky would be as bright as the Sun.

History

Early speculations

In 1931, Georges Lemaître thought that old parts of the universe might look like radiation. In 1948, Ralph Alpher and Robert Herman guessed that we might find this kind of radiation everywhere in space.

Discovery

In 1964, two scientists named Arno Penzias and Robert Wilson found a strange glow coming from everywhere in the sky. This glow came from long ago, when the universe was very young. They won a prize for finding it.

Cosmic origin

The Holmdel Horn Antenna on which Penzias and Wilson discovered the cosmic microwave background.

At first, people weren't sure where this glow came from. But by the 1970s, scientists showed that the glow matched what we expect from the very early universe.

Progress on theory

In the 1970s, scientists realized that tiny changes in this glow could tell us about what happened right after the universe began.

COBE

In the 1990s, a satellite called COBE measured these tiny changes very carefully. It helped scientists understand how the universe grew and changed.

Precision cosmology

After COBE, many more experiments looked at these changes in even more detail. These helped scientists figure out that the universe is flat, not curved.

Observations after COBE

More experiments followed COBE, measuring these changes on even smaller scales. These helped scientists learn more about the shape and contents of the universe.

Wilkinson Microwave Anisotropy Probe

In 2001, NASA launched a new satellite called WMAP. It made very careful maps of the sky and helped scientists understand the universe even better.

Planck Surveyor

In 2009, a European satellite called Planck made even more detailed maps of this ancient glow. It helped scientists learn that the universe is a little older than they thought.

Theoretical models

The cosmic microwave background radiation shows us how far away objects are and supports the Big Bang. This radiation tells us that the universe started with a Big Bang and has been growing since then.

The Big Bang model says that soon after the universe began, it grew very fast. This growth made most things even, but some tiny differences stayed because of quantum effects. Before stars and planets existed, the universe was smaller, much hotter, and glowed brightly from particles and energy.

As the universe grew, it cooled down. Later, electrons and protons came together to make hydrogen atoms. About 380,000 years after the Big Bang, the universe became clear, and the light from that time has been moving through space since then. This light, now very cool, is what we call the cosmic microwave background. It shows us a picture of the early universe and helps prove the Big Bang theory.

Polarization

The cosmic microwave background has a special kind of glow called polarization. There are two types: E-mode and B-mode. These names come from ideas about electric and magnetic fields.

E-modes

E-modes come from tiny particles moving in the early universe. Scientists first saw these in 2002 with a special telescope.

B-modes

B-modes are weaker than E-modes. They might be created by special waves from right after the Big Bang. They can also come from the bending of stronger E-modes. Finding the original B-modes needs careful study.

Primordial gravitational waves

Artist impression of the gravitational lensing effect of massive cosmic structures

Some ideas about the very early universe predict special waves that would leave a pattern in the B-mode polarization. Finding this pattern would help prove these ideas. Early reports of this pattern were affected by dust in space. Recent studies have looked at this more carefully.

Gravitational lensing

In 2013, scientists found the second type of B-modes using telescopes at the South Pole and in space. In 2014, another team measured this B-mode polarization and showed it came from the universe itself, not just from dust, with high confidence.

Multipole analysis

The cosmic microwave background (CMB) shows very small temperature differences across the sky. Scientists study these differences by breaking them into parts called multipoles. This helps us learn about the early universe.

Example Multipole Power Spectrum. WMAP Data are represented as points, curves correspond to the best-fit LCDM model

The simplest part, called the monopole (ℓ = 0), shows the average temperature of the CMB. This temperature is about 2.7255 Kelvin. The next part, the dipole (ℓ = 1), shows how the Earth moves through space. It is not related to the early universe.

Higher multipoles (ℓ ≥ 2) show tiny temperature changes that began in the very early universe. These changes happened when the universe was hot and dense. They give us clues about how the universe began.

Some patterns in the CMB are still being studied, like unusual alignments. Scientists are working to understand if these patterns are real or just chances in the data.

Future evolution

If the universe keeps getting bigger and does not end in a big collapse, the cosmic microwave background will keep changing. It will move to longer wavelengths until we can no longer see it. After that, the light we see today will be replaced by light from stars. Much later, other processes might create new kinds of background radiation.

Timeline of prediction, discovery and interpretation

Thermal (non-microwave background) temperature predictions

In 1896, Charles Édouard Guillaume thought the heat from stars was between 5–6 K.
In 1926, Sir Arthur Eddington estimated the energy from starlight in the galaxy to be about 3 K.
In the 1930s, cosmologist Erich Regener calculated that energy from space had a temperature of 2.8 K.
In 1931, the word microwave was first used in writing.
In 1934, Richard Tolman showed that energy in space stays the same even as the universe grows.
In 1946, Robert Dicke predicted we might find energy from the universe at about 2.7 K.
In 1955, Émile Le Roux reported finding energy in space at about 3 K.
In 1957, Tigran Shmaonov reported finding energy in space at about 4 K.
In 1964, A. G. Doroshkevich and Igor Dmitrievich Novikov suggested looking for this energy with telescopes.
In 1964–65, Arno Penzias and Robert Woodrow Wilson measured the temperature to be about 3 K. Robert Dicke, James Peebles, P. G. Roll, and D. T. Wilkinson said this was evidence of the Big Bang.
In 1966, Rainer K. Sachs and Arthur M. Wolfe predicted how the energy would look based on gravity.
In 1968, Martin Rees and Dennis Sciama also predicted how the energy would look.
In 1969, R. A. Sunyaev and Yakov Zel'dovich studied how very hot material changes the energy.
In 1983, researchers first saw this change in energy from groups of galaxies.
In 1983, the RELIKT-1 Soviet experiment to study the energy was launched.
In 1990, the Cosmic Background Explorer satellite measured the energy very precisely.
In January 1992, scientists reported finding small differences in the energy.
In 1992, scientists using COBE data reported finding small differences in the energy.
In 1995, the Cosmic Anisotropy Telescope made detailed observations of the energy.
In 1999, experiments measured sound waves in the energy.
In 2002, scientists found a special kind of energy pattern called polarization.
In 2003, the Wilkinson Microwave Anisotropy Probe created a detailed map of the whole sky.
In 2006, two scientists from COBE won a Nobel Prize for their work.
In 2010, the first map of the whole sky from the Planck telescope was released.
In 2013, an improved map from the Planck telescope was released.
In 2014, scientists thought they found evidence of the universe’s fastest growth, but later found it was caused by dust.
In 2018, the final data from the Planck telescope was released.

In popular culture

The cosmic microwave background has appeared in several stories and shows. In the TV series Stargate Universe, an ancient spaceship named Destiny studied patterns in the cosmic microwave background. In the novel Wheelers by Ian Stewart & Jack Cohen, the cosmic microwave background was shown as hidden messages. In the book The Three-Body Problem by Liu Cixin, an alien probe interfered with instruments that monitor the cosmic microwave background. The Swiss 20 francs bill from 2017 lists several space objects, including the cosmic microwave background. In the 2021 Marvel series WandaVision, a strange television broadcast was found within the cosmic microwave background.