Relic Radiation Explained: The History of Discovering the Cosmic Microwave Background

The relic radiation of the Universe — known today as the Cosmic Microwave Background (CMB) — is the oldest detectable light in existence, a faint glow of microwaves filling the entire sky and left over from the hot, dense state of the Universe shortly after the Big Bang. It was discovered by accident in the mid-1960s, although its existence had already been predicted theoretically. This article traces what the CMB is, how it was found, how it formed, and why it remains the single most important piece of evidence for the Big Bang theory.

What Is the Relic Radiation of the Universe?

The relic radiation of the Universe is thermal radiation that fills all of space almost perfectly uniformly, observed today in the microwave part of the electromagnetic spectrum at a temperature of about 2.7 degrees Kelvin. It is the cooled remnant of the intense radiation that permeated the early Universe, stretched to centimeter wavelengths by billions of years of cosmic expansion. Because it comes from every direction at the same intensity, it is not associated with any single star or galaxy but with the Universe as a whole.

Definition and Key Properties of Cosmic Microwave Background

The Cosmic Microwave Background is defined as the oldest electromagnetic radiation in the Universe, emitted roughly 380,000 years after the Big Bang and detectable in the microwave domain. Its defining properties make it unmistakable as a cosmological signal rather than local interference:

• Near-perfect uniformity — the intensity is the same to within tiny fractions of a percent across the whole sky.
• Blackbody (Planckian) spectrum — its energy distribution follows an almost ideal thermal curve at about 2.7 K.
• Microwave wavelengths — peaking around the centimeter and millimeter range, which is why it can only be observed in the microwave band.
• Faint anisotropies — minute temperature variations of about one part in 100,000 that seed the large-scale structure we see today.

Each cubic centimeter of real space contains roughly 500 photons of this radiation, an enormous number compared with ordinary matter — if all the matter of the visible galaxies were smeared evenly through space, there would be only about one hydrogen atom per three cubic meters.

Why It Is Called Fossil Radiation and First Light

Relic radiation earns the name "fossil radiation" because, like the fossils of ancient animals and plants preserved to our day, these photons are the surviving remnants of the most ancient epoch of the cosmos. The quanta of centimeter radiation are the oldest of all possible relics, their formation dating back roughly 13–15 billion years. It is also called the "first light" because it represents the moment the Universe first became transparent, releasing photons that had until then been trapped, scattering endlessly off free electrons.

History of the discovery of relic radiation

The history of the discovery of relic radiation began in 1964. Employees of the American laboratory Bell Telephone Laboratories were developing a communication system with the help of an artificial Earth satellite. This system was to work on waves of length 7.5 centimeters.

Penzias and Wilson's Accidental Discovery in 1964

Such short wavelengths for satellite radio communication have some advantages, but until Arno Penzias and Robert Woodrow Wilson, no one had solved this problem. They were pioneers in this field and had to take care that there was no strong interference on the same wavelength, or that the communicators knew about such interference in advance. Their persistent, careful work at Bell Telephone Laboratories ultimately led to one of the most important findings in the history of physical cosmology, for which the pair later received the Nobel Prize in Physics.

Sources of Radio Waves and the Horn Antenna Experiment

At that time it was thought that the source of radio waves coming from space could only be point objects like radio galaxies or stars.

The scientists had at their disposal an exceptionally accurate receiver and a rotating horn antenna — the Holmdel Horn Antenna. With these, scientists could listen to the entire sky in much the same way that a doctor listens to a patient's chest with a stethoscope.

A signal from a natural source

And now, as soon as the antenna was pointed at one of the points of the sky, as the oscilloscope screen danced a curved line. A typical natural source signal. Probably, the experts were surprised by their luck: in the first measured point - the source of radio emission!

But no matter where they pointed their antenna, the effect was the same. The scientists checked the equipment again and again, but it was in good working order. And finally they realized that they had discovered a previously unknown phenomenon of nature: the entire universe was as if filled with radio waves of centimeter length.

If we could see radio waves, the sky would appear to us as glowing from edge to edge.

Penzias and Wilson's discovery was published, and the result was soon connected with predictions made years earlier by George Gamow, Ralph Alpher, and Robert Herman, who had argued that a hot early Universe should leave behind a measurable background of radiation.

And not only they, but also scientists of many other countries began to search for sources of mysterious radio waves, captured by all adapted for this purpose antennas and receivers, wherever they were and at whatever point in the sky would be aimed, and the intensity of radio emission at a wave of 7.5 centimeters at any point was absolutely the same, it was as if smeared across the sky evenly.

How Soviet Scientists Predicted Relic Radiation

Soviet scientists predicted relic radiation before its discovery through detailed theoretical work on the radiation budget of the cosmos. Working at the Institute of Applied Mathematics of the USSR Academy of Sciences, researchers in the circle of Ya. B. Zel'dovich — including the calculations associated with A. G. Doroshkevich and I. D. Novikov — showed that a uniform microwave glow was an expected consequence of a hot early Universe.

Calculations by Doroshkevich and Novikov

Soviet scientists A. G. Doroshkevich and I. D. Novikov, who predicted relic radiation before its discovery, made the most complicated calculations. They took into account all the available sources of radiation in our Universe, took into account and how the radiation of certain objects has changed over time. And it turned out that in the area of centimeter waves all these radiations are minimal and, therefore, for the detected glow of the sky is not responsible. Their analysis even pointed to the horn antenna at Bell Telephone Laboratories as an instrument capable of detecting such a background — a remarkably specific prediction.

Density of the Photon Background vs. Matter in the Universe

Further calculations showed that the density of smeared radiation is very high. Here is a comparison of the photon "kisel" (so called by scientists for this mysterious radiation) with the mass of all matter in the Universe. If all the matter of all visible galaxies is evenly "smeared" over the entire space of the Universe, there will be only one hydrogen atom per three cubic meters of space (for simplicity, we will consider all the matter of stars as hydrogen).

And at the same time, each cubic centimeter of real space contains about 500 photons of radiation. Not much, even if we compare not the number of units of matter and radiation, but directly their masses. Where did such intense radiation come from? The answer lies in the hot, dense origin of the cosmos itself.

Origin of Relic Radiation and the Big Bang

Relic radiation originated in the searing heat of the early Universe and was released when expansion finally cooled matter enough for it to become transparent. The Big Bang theory — the Hot Big Bang Model — explains both the existence and the temperature of the CMB as direct consequences of an expanding, cooling cosmos.

Friedman's Equations and the Expanding Universe

In his time, the Soviet scientist A. A. Friedman, solving the famous Einstein equations, discovered that our universe is in constant expansion. His solutions required setting initial conditions, and Friedman proceeded from the assumption that the Universe is homogeneous and isotropic — that matter within it is distributed uniformly.

Hubble's Discovery of Galaxy Expansion

Soon confirmation of Friedman's result was found. The American E. Hubble discovered the phenomenon of galaxy expansion. Extrapolating this phenomenon into the past, we can calculate the moment when all the matter of the Universe was in a very small volume and its density was incomparably greater than now.

Cooling and Expansion of the Universe Over Time

In the course of the expansion of the Universe there is also a lengthening of the wavelength of each quantum proportional to the expansion of the Universe; thus the quantum as if "cools down" — in fact, the smaller the wavelength of the quantum, the "hotter" it is. Today's centimeter radiation has a brightness temperature of about 3 degrees Kelvin on the absolute scale.

And ten billion years ago, when the Universe was incomparably smaller and the density of its matter very large, these quanta had a temperature of about 10 billion degrees. Since then our Universe has been "covered" with quanta of continuously cooling radiation. That is why the "smeared" centimeter radio emission received the name of relic radiation.

Relics, as it is known, are the remains of the most ancient animals and plants, preserved to our days. Quanta of centimeter radiation are certainly the oldest of all possible relics. In fact, their formation dates back to an epoch about 15 billion years from us.

The Surface of Last Scattering

The surface of last scattering is the boundary in space and time from which the CMB photons we observe today were finally set free. So, a second had passed since the zero moment. The matter of our Universe had a temperature of 10 billion degrees and consisted of a kind of "porridge" of relic quanta, electrons, positrons, neutrinos and antineutrinos. The density of the "porridge" was huge — more than a ton per each cubic centimeter. In such "crowded" conditions, collisions transformed protons into neutrons and vice versa.

But it was the quanta that were most abundant here — 100 million times more than neutrons and protons. Of course, at this density and temperature, no complex nuclei of matter could exist. A hundred seconds passed. The Universe continued to expand, its density continuously decreasing, its temperature falling.

Positrons almost disappeared, neutrons turned into protons. The atomic nuclei of hydrogen and helium began to form. Calculations made by scientists show that about 30 percent of neutrons united to form helium nuclei, while 70 percent of them remained lonely and became hydrogen nuclei. In the course of these reactions there were new quanta, but their quantity was no longer in any comparison with the original, so it can be considered that it did not change at all.

The expansion of the Universe continued. The density of the "porridge," so steeply brewed by Nature at the beginning, decreased proportionally to the cube of linear distance. Years, centuries, millennia passed. Around 380,000 years after the Big Bang — the recombination epoch — the temperature fell to a few thousand degrees, electrons combined with nuclei into neutral atoms, and the Universe became transparent. The photons released at that moment travel freely to us as the relic radiation, marking the surface of last scattering and the limit beyond which we cannot observe with light.

Relic Radiation as Evidence for the Big Bang Theory

Relic radiation is the strongest observational evidence for the Big Bang theory because its existence, its near-perfect blackbody spectrum, and its temperature were all predicted by the hot early-Universe model before being confirmed. No competing explanation accounts so naturally for a uniform thermal glow filling the entire sky at exactly the temperature expansion predicts.

Why CMB Is the Oldest Detectable Radiation

The CMB is the oldest detectable radiation because no light can reach us from before the surface of last scattering. Before recombination, the Universe was opaque — photons could not travel any distance without scattering off free electrons. Only after matter became neutral did the cosmos turn transparent, so the CMB represents the earliest moment the Universe could emit light that survives to today. This is the fundamental limitation on observing the cosmos with electromagnetic radiation; to probe earlier epochs, physicists must turn to neutrinos or, in theory, gravitational waves.

Cosmological Redshift and Photon Travel

Cosmological redshift explains why radiation that began at billions of degrees now appears as cold microwaves. As space itself expands, the wavelength of every photon traveling through it is stretched, lowering its frequency and energy. Photons released when the Universe was about 3,000 degrees have had their wavelengths stretched by a factor of roughly 1,100, cooling the radiation to its present 2.7 K. This wavelength stretching is precisely why the relic radiation is detectable only in the microwave domain rather than as visible or infrared light.

Properties and Structure of the CMB

Although the CMB is remarkably smooth, its tiny departures from perfect uniformity carry detailed information about the composition, geometry, and history of the Universe. These structural features — anisotropies and polarization — are what transform the background from a curiosity into a precision cosmological tool.

Anisotropy and Temperature Variations

Anisotropy in the CMB refers to faint temperature variations of about one part in 100,000 across the sky, the imprint of primordial density fluctuations in the early Universe. These slightly denser and rarer regions were the seeds from which galaxies and large-scale structure eventually grew. Analyzing the angular sizes of these fluctuations produces the CMB power spectrum, whose characteristic peaks reveal the density of ordinary matter, dark matter, and the overall geometry of space.

CMB Polarization Modes

CMB polarization arises because the photons of the last scattering surface were scattered by electrons in a way that imprinted a slight directional preference on the radiation. Scientists classify this polarization into two patterns: E-modes, produced by density variations, and B-modes, a subtler signal that certain inflationary models predict should be generated by primordial gravitational waves. Detecting primordial B-modes would be powerful support for the theory of cosmic inflation in the first instant after the Big Bang.

How Relic Radiation Is Detected and Measured

Relic radiation is detected with extremely sensitive radio receivers and microwave detectors, increasingly placed in space to escape the absorbing and emitting effects of Earth's atmosphere. Space-based experiments have produced progressively sharper maps of the background, each refining the measured temperature, spectrum, and anisotropies. Foreground interference — emission from our own galaxy, dust, and free-free emission from ionized gas — is the central challenge, and much of the analysis involves carefully separating these contaminants from the true cosmological signal.

COBE Satellite Observations and Measurements

The Cosmic Background Explorer (COBE), launched by NASA in 1989 and operated from NASA Goddard Space Flight Center, was the first satellite to measure the CMB spectrum with high precision. COBE confirmed that the background follows an almost perfect blackbody Planckian spectrum and, crucially, made the first detection of the tiny anisotropies, demonstrating that the early Universe contained the density variations needed to form galaxies. These results earned a Nobel Prize and cemented the Hot Big Bang Model.

Planck Mission Technology and Detection Methods

The Planck mission, operated by ESA via the Planck spacecraft, mapped the CMB with the highest resolution and sensitivity yet achieved, building on the earlier NASA/WMAP Science Team results from the WMAP satellite. Planck's detectors measured temperature fluctuations across the whole sky in fine detail, tightening the constraints on the age, expansion rate, and matter content of the Universe. Its measurements placed strict limits on any deviations from a pure Planckian spectrum, constraining exotic energy releases in the early Universe and refining the standard cosmological model.

Alternative Theories of Relic Radiation Origin

Alternative theories of the relic radiation's origin have been proposed, but none match the Big Bang model's success in explaining its blackbody spectrum. Over the decades, physicists explored whether the background could instead come from the integrated light of very early objects — for example, energy released by hypothetical Population III supergiant stars — or whether large-scale structure might arise from topological defects such as cosmic strings rather than inflationary fluctuations. Researchers including R. A. Sunyaev studied how processes like the Compton interaction and free-free emission could distort the spectrum, and work published in journals such as Nuclear Physics B examined defect-based scenarios. Inflationary models ultimately fit the observed CMB power spectrum far better than topological-defect models, while still leaving open deeper questions about quantum gravity, the holographic principle, and frameworks like M-theory that attempt to describe the very origin of the Universe.

Here is one of the hypotheses of the formation of the Universe: most of the matter of our Universe is not in the composition of planets, stars and galaxies, but forms intergalactic gas — 70 percent hydrogen and 30 percent helium, about one hydrogen atom per cubic meter of space.

Then the development of the Universe passed the stage of protostars and entered the stage of matter ordinary to us — ordinary unfolding spiral Galaxies, ordinary stars, the most familiar of which is our Sun.

Around some of these stars formed systems of planets, at least on one of these planets arose life, in the course of evolution gave rise to intelligence. How often there are in the vastness of space stars surrounded by a round dance of planets, scientists do not yet know. Nor can they say anything about how often life arises on the planets.

And the question of how often the plant of life blossoms with the lush flower of reason remains open. The hypotheses known to us today, treating all these questions, are more like ill-founded guesses. But today science develops avalanche-like.

More recently, scientists had no idea how our universe began. Discovered about 70 years ago, relic radiation made it possible to paint that picture. Penetration into outer space, visits to the Moon and other planets, bring new facts — and modern instruments such as the James Webb Telescope now probe the early galaxies that formed in the wake of the events the CMB records.

The Role of CMB in Understanding the Universe's Evolution

The CMB plays a central role in understanding the Universe's evolution because it provides a direct snapshot of cosmic conditions 380,000 years after the Big Bang, anchoring the entire chronology of the Universe. From the homogeneity of the background and the structure of its fluctuations, cosmologists reconstruct the thermal history of the cosmos, test theoretical predictions, and measure the ingredients that govern how galaxies and clusters — from individual spirals to vast assemblies like the Coma Cluster — came to be.

The isotropy of the Universe is evidenced by the amazing uniformity of relic radio emission. A second fact testifies to the same — the distribution of the substance of the Universe between Galaxies and intergalactic gas.

Indeed, the intergalactic gas, which constitutes the bulk of the matter of the Universe, is distributed over it as uniformly as the relic quanta. The discovery of relic radiation provides an opportunity to look not only into the ultra-distant past — beyond the limits of time, when there was neither our Earth, nor our Sun, nor our Galaxy, nor even the Universe itself.

Dark Matter and the Density of the Universe

Dark matter is revealed by the CMB through the way its anisotropies depend on the total amount of gravitating matter, much of which emits no light. The fate of the Universe depends first of all on how much matter it contains. If its total gravitation is enough to overcome the inertia of expansion, the expansion will inevitably be replaced by contraction, in which the galaxies gradually converge. And if the gravitational forces are not enough to brake the inertia of expansion, the Universe will dissipate into space. The precise CMB measurements of cosmic density — including dark matter — are exactly what allow science to weigh these possibilities, a problem the study of relic radiation first made it possible to pose and that further research may yet solve.