A hiss in your radio. Static between stations. That faint noise holds the age of the universe. It is the cosmic microwave background, the afterglow of the Big Bang. It permeates every corner of space. It whispers secrets from 13.8 billion years ago. Let us tune in.
The Discovery
In 1964, Arno Penzias and Robert Wilson worked at Bell Labs in New Jersey. They struggled with mysterious static in their horn antenna. They cleaned pigeon droppings. They adjusted equipment. The noise persisted. Across town, Jim Peebles and his colleagues had predicted a relic radiation from the Big Bang. Penzias and Wilson realised the noise was that relic. They shared the Nobel Prize in 1978.
The cosmic microwave background is remarkably uniform. Its temperature is about 2.725 kelvin, just above absolute zero. Yet tiny fluctuations exist at the level of one part in one hundred thousand. Those fluctuations encode density variations in the early universe, the seeds of galaxies and clusters.
Physics of the Early Universe
Moments after the Big Bang, the universe was a hot plasma of photons, electrons and baryons. Photons scattered off free electrons, creating a dense, opaque fog. About 380,000 years later, the universe cooled to about 3,000 kelvin. Electrons combined with protons to form neutral hydrogen. Photons could then travel freely. The universe became transparent. These photons have stretched with cosmic expansion, shifting from visible wavelengths to microwaves.
Mapping the Fluctuations
COBE, launched in 1989, confirmed the cosmic microwave background’s blackbody spectrum and detected the first fluctuations. NASA’s Wilkinson Microwave Anisotropy Probe, from 2001 to 2010, mapped these fluctuations with greater precision. The European Space Agency’s Planck satellite, operating from 2009 to 2013, delivered the most detailed map yet, measuring fluctuations down to microkelvin scales.
These fluctuations reveal the universe’s composition and evolution. The peaks and troughs in the power spectrum show the relative amounts of ordinary matter, dark matter and dark energy. They pin down the universe’s age with remarkable accuracy. Planck’s data give an age of 13.799 billion years, with only a tiny margin of uncertainty.
Acoustic Oscillations: The Universe’s First Music
Before recombination, photons and baryons moved together. Gravity pulled matter inward. Radiation pressure pushed it outward. This interplay created sound waves, called acoustic oscillations, in the primordial plasma. The fluctuations in the cosmic microwave background capture these waves frozen in time.
The first acoustic peak in the power spectrum corresponds to regions where one full compression occurred. Its position reveals the universe’s curvature. Measurements show that space is flat, to within a fraction of a percent.
Polarisation and Gravitational Waves
Light from the cosmic microwave background is slightly polarised. Thomson scattering off electrons created this polarisation. Two patterns emerge: E-modes and B-modes. E-modes have been mapped extensively. They confirm the standard cosmological model.
B-modes are more elusive. They may arise from gravitational lensing of E-modes by large-scale structures. Or, more intriguingly, from primordial gravitational waves generated by cosmic inflation, the rapid expansion in the universe’s first fractions of a second. Experiments such as BICEP2 and the South Pole Telescope hunt for this faint signal.
In 2014, BICEP2 announced a potential detection of primordial B-modes. Excitement soared. Yet further analysis showed most of the signal came from galactic dust. The quest continues.
Foregrounds and Challenges
Observing the cosmic microwave background from Earth is difficult. The Milky Way emits microwaves from dust and synchrotron radiation. Scientists observe at multiple frequencies to separate these foregrounds from the true cosmic signal. They build telescopes at high, dry sites such as Antarctica and the Atacama Desert to minimise atmospheric noise.
Space missions avoid the atmosphere completely. Planck orbited at Lagrange Point 2, 1.5 million kilometres from Earth. It circled the Sun along with Earth, in a stable, cold environment ideal for precision measurements.
What the CMB Tells Us
The cosmic microwave background reveals the universe’s composition. Ordinary matter makes up only about 5 per cent. Dark matter comprises about 27 per cent. Dark energy dominates at about 68 per cent, driving accelerated expansion. The background also confirms the Big Bang model and cosmic inflation. It rules out many alternative theories.
It sets the stage for structure formation. It shows where matter clusters would grow under gravity. Over billions of years, galaxies, stars and planets formed around those regions of slightly higher density.
Future Prospects
Next-generation experiments aim for even greater sensitivity. Projects such as CMB-S4, a global ground-based array, will map polarisation with unprecedented detail. LiteBIRD, a planned Japanese space mission, will target primordial B-modes. Together, they seek to answer whether inflation occurred, and if so, how it operated.
The Human Connection
Listening to the cosmic microwave background connects us to the universe’s origin. At microwave frequencies near 160 gigahertz, instruments can detect the faint whispers from the dawn of time. These photons were released before any star shone, before any galaxy spun, before any planet formed.
As cosmologist John C. Mather, Nobel laureate and senior project scientist for COBE, said: “We are star stuff. But beyond that, we are listening to the universe’s own first heartbeat.” His words remind us that science can evoke wonder as deeply as myth.
Standing under a dark sky, you might glimpse the Milky Way’s dusty band. You might spot a shooting star. Yet behind that familiar night lies the persistent glow of the cosmic microwave background. It is invisible, silent and eternal. It is the oldest light we can see. It echoes through space. It colours every direction. It is the song of creation itself.
