The Cosmic Microwave Background

The night sky looks dark.

But the universe is filled with ancient radiation.

In every direction, space contains microwave photons left over from the early universe.

This radiation is the cosmic microwave background, or CMB.

It is one of the strongest pieces of evidence for the hot Big Bang model.

It is also one of the richest datasets in all of cosmology.

The CMB is not merely a faint glow.

It is a fossil record of the early universe.

The Universe Was Once Opaque

Today, light can travel enormous distances through intergalactic space.

The early universe was different.

Matter existed as a hot plasma containing:

• free electrons,
• atomic nuclei,
• photons.

Free electrons scatter photons efficiently.

Light could not travel freely over cosmological distances.

The universe behaved more like a glowing opaque fog.

Cooling Through Expansion

As the universe expanded, it cooled.

Eventually the temperature fell enough for electrons to become bound to nuclei, forming neutral atoms.

This process is called recombination, though the name is historical because these electrons and nuclei were not necessarily being reunited after having previously formed stable atoms.

Once free-electron density dropped dramatically, photons could travel much farther without scattering.

The universe became transparent.

Last Scattering

The photons we observe as the CMB last scattered significantly during this transition.

They then traveled through the expanding universe for billions of years.

We therefore speak of a surface of last scattering.

This is not a physical shell centered on Earth.

It is the set of distant regions from which CMB photons reaching us today last interacted strongly with matter.

Every observer has their own corresponding last-scattering surface.

When Did This Happen?

Recombination and photon decoupling occurred roughly 380,000 years after the hot Big Bang began its evolution.

That is extremely early compared with the universe’s present age of about 13.8 billion years.

There were no ordinary stars or galaxies yet.

The CMB therefore gives us a view of the universe before familiar cosmic structure developed.

Why Microwaves?

The radiation was much hotter when it was released.

Cosmic expansion stretched its wavelengths.

As wavelength increased, the radiation cooled.

Today, the CMB has a nearly perfect blackbody spectrum with a temperature of about 2.7 kelvin.

Radiation that once had much shorter wavelengths has been redshifted into the microwave range.

Blackbody Spectrum

The CMB’s spectrum is extraordinarily close to that of an ideal blackbody.

This is crucial evidence for a hot, dense early universe.

A thermalized plasma naturally produces such a spectrum.

The later expansion preserves the blackbody form while lowering the temperature.

The observed spectral precision is one of the great successes of Big Bang cosmology.

Prediction Before Discovery

The existence of relic radiation from a hot early universe was predicted before its accidental observational discovery.

In 1965, Arno Penzias and Robert Wilson detected a persistent microwave noise that seemed to come from every direction.

At the same time, researchers at Princeton were preparing to search for exactly such cosmological radiation.

The two lines of work quickly converged.

The mysterious noise was cosmic.

Why the Discovery Mattered

Before the CMB discovery, competing cosmological models remained more viable.

The steady-state model, for example, proposed an eternally expanding universe with continuous creation of matter maintaining constant average density.

The CMB strongly favored a hot early phase.

It did not answer every cosmological question.

But it transformed the evidential landscape.

The CMB Is Almost Uniform

The temperature of the CMB is nearly the same in every direction.

This tells us that the early universe was remarkably homogeneous.

The uniformity is so strong that it created a puzzle:

how did widely separated regions reach nearly the same temperature if they were not once in causal contact?

This is the horizon problem that inflation helps address.

But Not Perfectly Uniform

The CMB contains tiny temperature fluctuations.

The variations are on the order of tens of microkelvin around the mean temperature.

These tiny differences are cosmologically priceless.

They reveal small variations in early density and gravitational potential.

Those fluctuations later grew into:

• galaxies,
• clusters,
• large-scale structure.

The universe’s enormous present complexity was seeded by extremely small early irregularities.

Anisotropies

These directional variations are called anisotropies.

Their statistical pattern depends on cosmological parameters.

By studying how fluctuation strength varies with angular scale, cosmologists can infer properties such as:

• matter density,
• baryon density,
• spatial curvature,
• expansion history,
• primordial fluctuation spectrum.

The CMB turns the early universe into a precision laboratory.

Acoustic Oscillations

Before recombination, photons and ordinary matter were tightly coupled.

Gravity tried to compress denser regions.

Radiation pressure pushed back.

The result was acoustic oscillation in the primordial plasma.

These oscillations left characteristic peaks in the CMB angular power spectrum.

The pattern is sometimes described as the sound of the early universe frozen into radiation.

The First Acoustic Peak

The location of the first major acoustic peak is especially sensitive to spatial geometry.

Observations show that the observable universe is very close to spatially flat.

Other peaks constrain additional physical quantities.

The shape of the spectrum is not arbitrary.

It encodes the contents and dynamics of the early cosmos.

Polarization

The CMB is also polarized.

Scattering processes in the early universe create characteristic polarization patterns.

These are commonly decomposed into:

• E-modes,
• B-modes.

E-mode polarization has been measured in detail.

B-modes can arise from gravitational lensing and, potentially, from primordial gravitational waves.

Searching for a primordial B-mode signal is one way cosmologists test inflationary physics.

Dipole Anisotropy

The largest CMB anisotropy is a dipole pattern.

One side of the sky appears slightly hotter.

The opposite side slightly cooler.

This is primarily interpreted as arising from our motion relative to the cosmological rest frame defined by the CMB.

The dipole provides a way to measure our local motion with respect to the large-scale universe.

A Preferred Frame?

Does the CMB define an absolute frame of the kind relativity forbids?

No.

Special relativity says there is no preferred inertial frame built into the laws of physics.

A universe filled with matter and radiation can still define a physically useful frame relative to that cosmic material distribution.

The CMB rest frame is cosmologically distinguished by the state of the universe, not by a violation of relativistic symmetry.

Foregrounds

Measuring the CMB is difficult because our detectors also see radiation from:

• the Milky Way,
• dust,
• synchrotron emission,
• radio sources,
• other astrophysical foregrounds.

Cosmologists observe at multiple frequencies and model these contaminants.

The famous CMB maps are therefore products of sophisticated data analysis, not raw photographs.

COBE, WMAP, and Planck

Several satellite missions transformed CMB science.

COBE measured the blackbody spectrum and detected large-scale anisotropies.

WMAP mapped temperature fluctuations with much greater precision.

Planck improved angular resolution and parameter constraints further.

Together, these missions turned cosmology into a precision science.

What the CMB Does Not Show

The CMB is not an image of the Big Bang singularity.

It is not light from time zero.

It does not directly show inflation.

It does not tell us what happened before the hot Big Bang phase.

It reveals the universe after it became transparent.

Earlier physics must be inferred from indirect signatures.

The Oldest Light We Can See

In ordinary electromagnetic astronomy, the CMB forms a natural observational wall.

Earlier than recombination, photons were repeatedly scattered.

We cannot simply build a stronger optical telescope and see through that plasma.

Other messengers may reach us from earlier times, including neutrinos or gravitational waves.

But those observations are much harder.

From Radiation to Matter

The CMB records a universe in which atoms had just become stable enough for light to travel freely.

But those atoms themselves required a long chain of earlier processes.

Quarks had to become confined into protons and neutrons.

Nuclei had to form.

Electrons later had to bind to nuclei.

So the next step is to rewind slightly and follow matter’s construction.

How did the universe go from quarks to atoms?