Echo of the Universe

Nobel Prizes in Astrophysics & Cosmology - Part 7

(A Twelve Part Series)

John Mather & George Smoot

Cosmology is a science which has only a few observable facts to work with. The discovery of the cosmic microwave background radiation added one — the present radiation temperature of the universe. This, however, was a significant increase in our knowledge since it requires a cosmology with a source for the radiation at an early epoch and is a new probe of that epoch.

-          Robert W Wilson

Temperature map of the CMB measured by the Planck spacecraft

 

The Nobel Prize is equated with the pinnacle of human achievement in both popular perception and professional esteem.  Since it was first awarded in 1901, the annual Nobel Prize for Physics has gone to major contributions in Astrophysics and Cosmology related fields only on eleven occasions. The first six awards (1967, 1974, 1978, 1983, 1993, 2002) were the subjects of earlier articles (see here 1,2,3,4,5,6). The next was in 2006, shared by John C Mather and George F Smoot for their momentous discoveries related to the Cosmic Microwave Background (CMB) Radiation, the afterglow of the Universe.

 

What is CMB

The Cosmic Microwave Background is a faint, nearly uniform glow of microwave radiation that fills the entire universe. It’s the thermal remnant of the hot, dense state of the early universe about 380,000 years after the Big Bang, when electrons and protons combined to form neutral atoms (a moment called recombination). At that time, light could travel freely for the first time — and that light has redshifted over billions of years into microwaves that we detect today.

Discovery

• In 1964, Arno Penzias and Robert Wilson accidentally detected a persistent microwave signal with a radio antenna at Bell Labs (see article on their Nobel Prize winning discovery here).
• They couldn’t explain the noise; it was isotropic (same in all directions) and corresponded to a temperature of about 3 K.
• Simultaneously, theoretical work by Robert Dicke, Jim Peebles, and others predicted such a background as a relic of the Big Bang.
• Penzias and Wilson’s observation provided strong evidence for the Big Bang theory and earned them the Nobel Prize in Physics (1978).

Follow-up Work

Over decades, more precise measurements beginning with COBE (Cosmic Background Explorer) have mapped the CMB in great detail:

COBE (1989–1993)

COBE is the ‘Cosmic Background Explorer’ satellite, a NASA probe launched in 1989 to study the properties and distribution of the CMB. It

• Detected the blackbody spectrum of the CMB with high precision, and
• Found tiny temperature fluctuations (anisotropies).

Graph of CMB spectrum around its peak in the microwave frequency range, a near perfect fit

 

Work of John Mather and George Smoot

They were awarded the 2006 Nobel Prize in Physics “for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation.” Their work was based on results from NASA’s COBE (Cosmic Background Explorer) probe, launched in 1989.

Scientific Background

The Cosmic Microwave Background (CMB) is relic radiation from the epoch of recombination (~380,000 years after the Big Bang) when photons decoupled from matter.

Theoretical prediction (1940s–1960s):

• Universe should contain residual radiation.
• Spectrum should be thermal (‘Planckian’).
• Tiny fluctuations (~10⁻⁵ level) should exist as seeds of structure formation.

Before COBE:

• CMB was detected in 1964.
• Temperature was measured to be ~2.7 K.
• But precision was insufficient to:
• Confirm exact blackbody shape.
• Detect primordial anisotropies.

COBE changed all that.

Blackbody Spectrum with FIRAS, the Far Infrared Absolute Spectrophotometer

An artist’s view of FIRAS

Scientific Question: Is the CMB exactly a blackbody spectrum?

A blackbody spectrum follows Planck’s law:

where h = Planck’s constant, k = Boltzmann constant, ν = frequency and T = temperature.

Any deviation from this would indicate:

• Energy injection after recombination
• Exotic early-universe physics

FIRAS compared:

• Radiation from the sky, and
• Radiation from a precisely controlled onboard blackbody calibrator

It measured the frequency range: 1–95 cm⁻¹ (≈30–3000 GHz)

Result:

• Temperature: T=2.725±0.002 K
• Spectrum matches Planck curve to better than 1 part in 10⁵

This is the most perfect blackbody spectrum ever measured in nature.

Cosmological Significance

This ruled out many alternative cosmological models (e.g., steady-state variants).
It confirmed that the early universe was in thermal equilibrium.

The energy density of CMB:

ρ_γ​=aT⁴

where a is the radiation constant.

The FIRAS study provided direct evidence of hot Big Bang.

Detection of Anisotropies through DMR (Differential Microwave Radiometers)

Gorge Smoot led this experiment.

Scientific Question

Are there primordial fluctuations in the CMB?

Theoretical prediction: ΔT​∼10⁻⁵ and density fluctuations of the same order.

These tiny fluctuations would later grow via gravitational instability into galaxies and clusters.

Observational Results (1992)

COBE detected ΔT≈30μK

After subtracting dipole anisotropy due to Earth’s motion ~3.3 mK and foreground emission, they did indeed reveal the primordial anisotropies just as expected.

John C Mather (1946 - ) – A Biographical Sketch

Early Life and Education

John Cromwell Mather was born on 7 August 1946, in Roanoke, Virginia, USA. Raised in a family that valued education and intellectual curiosity, Mather developed an early fascination with science. He earned his B S in physics from Swarthmore College (1968) and went on to complete his Ph D in physics at the University of California, Berkeley (1974).

His doctoral work focused on precision measurements in experimental cosmology—skills that would later prove decisive in one of the most important discoveries in modern astrophysics.

Mather’s name is permanently linked with the study of the cosmic microwave background (CMB) — the faint afterglow of the Big Bang.

In the 1970s and 1980s, he became the Project Scientist for NASA’s COBE (Cosmic Background Explorer) mission. Launched in 1989, COBE carried three key instruments:

• FIRAS (Far Infrared Absolute Spectrophotometer) – led by Mather
• DMR (Differential Microwave Radiometers) – associated with George Smoot
• DIRBE (Diffuse Infrared Background Experiment)

Mather’s instrument, FIRAS, made an extraordinarily precise measurement of the CMB spectrum and showed that it is an almost perfect blackbody radiation curve at a temperature of about 2.725 K.

This result was monumental because:

• It confirmed a central prediction of the Big Bang theory.
• It ruled out alternative steady-state cosmological models.
• It established that the early universe had once been in a hot, dense, thermal equilibrium state.

For this achievement, Mather shared the 2006 Nobel Prize in Physics with George F Smoot.

The Nobel citation specifically recognized the discovery of the blackbody form and anisotropy of the cosmic microwave background radiation.

Mather’s contribution specifically concerned the blackbody spectrum confirmation, often described as one of the most precise measurements ever made in physics.

Leadership in Space Astronomy

After COBE, Mather continued to play a central role in space-based astronomy. He became Senior Project Scientist for NASA’s flagship infrared observatory, later to become the James Webb Space Telescope (JWST). Mather was one of the earliest scientific advocates for this and helped guide the mission for decades, through technical and political challenges. The telescope’s extraordinary success stands as testimony to his enduring legacy.

Mather is known for:

• Exceptional experimental precision
• Strong collaborative leadership
• Long-term commitment to major scientific missions
• Ability to bridge theoretical cosmology and instrumentation

His work transformed cosmology from a speculative discipline into a precision science.

Honors and Recognition

Beyond the Nobel Prize, Mather has received:

• The NASA Distinguished Service Medal
• The Shaw Prize in Astronomy
• Membership in the U.S. National Academy of Sciences

He remains a leading voice in cosmology and space science.

Legacy

Mather’s confirmation of the CMB’s perfect blackbody spectrum is often compared to a “Rosetta Stone” for cosmology. It provided the thermodynamic fingerprint of the Big Bang and cemented modern cosmology on firm experimental foundations.

George F Smoot (1945 – 2025) – A Biographical Sketch

Early Life and Education

George Fitzgerald Smoot III was born on 20 February 1945, in Yukon, Florida, USA. He studied mathematics and physics at the Massachusetts Institute of Technology (MIT), earning his undergraduate degree in 1966. He then completed his Ph D at MIT in 1970, working on experimental particle physics.

His early research included work on antimatter and cosmic rays, but he soon shifted toward cosmology — particularly the experimental study of the early universe.

Pioneer of Cosmic Structure Measurements

Smoot is best known for his leadership in detecting tiny temperature fluctuations (anisotropies) in the cosmic microwave background (CMB), the relic radiation from the Big Bang.

He played a central role in NASA’s COBE (Cosmic Background Explorer) satellite mission, launched in 1989. Smoot led the team responsible for the DMR (Differential Microwave Radiometers) instrument

While John Mather’s FIRAS instrument confirmed the CMB’s perfect blackbody spectrum, Smoot’s DMR instrument made a complementary and equally historic discovery. In 1992, his team announced the first detection of minute temperature variations in the CMB — differences of only about one part in 100,000.

These anisotropies were crucial because:

• They represented the “seeds” of galaxies and large-scale structure.
• They provided direct evidence that small density fluctuations existed in the early universe.
• They strongly supported inflationary cosmology models.

The discovery was famously described as: “Looking at the face of God.”

Nobel Prize

For these discoveries, Smoot shared the 2006 Nobel Prize in Physics with John C Mather.

The Nobel citation recognized:

• The discovery of the blackbody form (Mather)
• The discovery of anisotropy in the cosmic microwave background radiation (Smoot)

Together, these findings transformed cosmology into a precision, data-driven science.

Later Career and Contributions

After COBE, Smoot continued his research at the University of California, Berkeley, contributing to subsequent cosmology missions and large-scale structure studies. He was involved in:

• Balloon-borne and satellite experiments refining CMB measurements
• Cosmological data interpretation from missions like WMAP and Planck
• Theoretical and observational work related to cosmic inflation

He also became known for public engagement in science and international collaborations, including academic positions in Europe and Asia.

Scientific Impact

Smoot’s measurement of CMB anisotropies:

• Confirmed predictions that tiny primordial fluctuations would grow under gravity into galaxies and clusters.
• Provided empirical support for inflationary cosmology.
• Laid groundwork for later high-precision missions such as WMAP and Planck.

Today, the CMB anisotropy map is considered one of the most important images in modern science.

Personality and Public Role

Smoot is known for:

• Strong experimental leadership
• Ambitious, large-scale scientific projects
• Effective communication of cosmology to the public

He has written popular science books explaining the origin and evolution of the universe, helping bring cosmology to a wider audience.

Legacy

If Mather gave cosmology its thermal proof of the Big Bang, Smoot revealed its structure — the tiny ripples that grew into galaxies, stars, and eventually observers like us.

 

Epilogue: Follow-up work on CMB

WMAP (2001–2010)

WMAP is the ‘Wilkinson Microwave Anisotropy Probe’, a NASA mission that

• Produced detailed, full-sky maps of CMB temperature variations.
• Allowed precise determination of cosmological parameters (age of universe, composition, geometry).

WMAP Spacecraft

Planck (2009–2013)

Planck was a European Space Agency (ESA) space observatory that

• Provided the most detailed CMB survey to date.
• Measured the temperature and polarization anisotropies with unprecedented precision.

These anisotropies are the seeds of large-scale structure — the galaxies and clusters we see today.

Planck Spacecraft

Current Status

The CMB is now one of the most well-measured observables in cosmology.
It shows that the universe is:

• ~13.8 billion years old
• Flat (in overall geometry)
• Composed of ~5% ordinary matter, ~27% dark matter, ~68% dark energy

Current work focuses on:

• CMB polarization, especially B-modes, which could reveal primordial gravitational waves from inflation.
• Ground-based telescopes (e.g., the South Pole Telescope, Simons Observatory) and future missions aiming for even finer measurements.

Why it Matters

    The CMB is a snapshot of the early universe — a cornerstone of modern cosmology that underpins our understanding of the origin, composition, and evolution of the cosmos.