Without the work of a Ukrainian physicist named George Gamow, astronomers might never have been able to prove definitively that the universe originated in an unimaginably enormous explosion called the Big Bang 13.4 billion years ago.
Belgian cosmologist Georges Lemaitre first proposed the Big Bang hypothesis in 1929. He suggested that the universe originated in a highly dense “primeval atom.” That “atom,” which is now called a singularity, began to expand rapidly and has been doing so ever since.
Lemaitre’s proposition made sense. During the 1920s, America’s Edwin Hubble determined the relative distances to galaxies other than our Milky Way. He also determined that, as a rule, the galaxies were receding from each other at a rapid rate.
The most distant galaxies seemed to be moving away from us at nearly the speed of light. Given the time it took the light from those galaxies to get to Earth, the universe must be vast.
Run the universal clock backward, and you get to an initial condition where everything was crammed into an extraordinarily dense and hot singularity.
Some astronomers were skeptical. The Big Bang theory had to compete with the Steady State model (SSM).
The Big Bang theory implies that the universe is getting less dense over time as the universe expands. The SSM hypothesizes that the universe does not change in its density. Stars and galaxies have always existed and will continue to exist infinitely in the past and future.
To maintain the universe’s current density as it expands, it must be creating hydrogen atoms at the slow but steady rate of one atom of hydrogen in six cubic kilometers of space per year or one atom per cubic meter every 100 billion years.
To answer the skeptics, Big Bang proponents had to prove that the universe changed over time and that the Big Bang model predicts those changes.
Enter George Gamow. He was born in 1904 in Odesa, Ukraine. He studied physics at the University of Leningrad.
In 1933, he attended a conference in Brussels and used the occasion to defect from an increasingly repressive Soviet Union. From there, he migrated to George Washington University in the United States.
In 1948, Gamow realized that the early universe must have been extremely hot soon after the initial explosion. Such high temperatures must have left a distinct signature in the sky.
Gamow wanted to know just how hot, so he set his graduate assistants Ralph Alpher and Robert Herman to do the math.
Astronomers express temperatures in the Kelvin scale, i.e., degrees above absolute zero. Gamow’s team determined that a critical moment occurred about 380,000 years after the initial explosion.
Before 380,000 years, the universe could not radiate light. It was self-contained and opaque.
Around that time, the universe became transparent to light. It erupted into view as what astronomers rather dramatically refer to as “fireball radiation.”
The universe’s temperature had cooled to about 3,000 degrees Kelvin, just cool enough for the whizzing mass of atomic nuclei that formed moments after the Big Bang to combine with stray electrons to form hydrogen and helium.
Alpher and Herman calculated that after 13 billion years, that temperature had decreased to just three degrees Kelvin. That cooled “fireball radiation” was still bouncing around, and it should be coming at us from all directions.
It should behave like a theoretical object called a black body, which absorbs all the energy that hits it and thus reflects none of that energy. As energy is absorbed, the object’s temperature increases.
Another characteristic of a black body is that it should have reached thermal equilibrium. No energy should pass from one part of it to another. The temperature throughout should remain the same.
Astronomers measure temperature by looking at the radiation’s spectrum. As temperature increases, specific colors of the spectrum dominate.
Heat an iron bar, and it first turns red-hot in visible light, about 1,000 degrees F. Heat it to yellow, and it glows at 2,000 degrees.
At every temperature, a different wavelength of color predominates. By identifying that wavelength, astronomers can determine the temperature of an astronomical object with accuracy.
In an object that is nearly a black body, the spectrum spikes at a particular wavelength. That spike yields a precise indication of its temperature.
Recall that Gamow’s team predicted that the radiation should now have a temperature of just three degrees above absolute zero. The best way to measure its spectrum is in the low-frequency energies called microwaves and radio waves.
After 13 billion years, the proposed radiation would be extremely faint. Gamow believed that radio telescopes could not differentiate its quiet hiss from the energy coming from stars and galaxies, the sky’s more energetic sources. In a universe full of screaming stars, astronomers would be looking for a whisper.
Other astronomers thought otherwise, and the search was on. Notable among them were Robert Dick and James Peebles of Princeton University.
They enlisted the aid of David Wilkinson and Peter Roll to build the radio telescope.
The key was to find the proper wavelengths. Earth and sky are filled with radiation across the spectrum.
They had to look in the relatively dark areas in the spectrum where astronomical objects don’t radiate much. One such was the three-centimeter band in the microwave portion of the spectrum.
By the spring of 1964, Wilkinson and Roll had begun to look.
Meanwhile, astronomers Robert Wilson and Arno Penzias were looking for a halo of cold gas around the Milky Way galaxy.
They were using the Holmdel Horn, built by Bell Laboratories for use in satellite communication, to look for the heat signature of the gas in the seven-centimeter microwave range.
However, a low-level hiss was spoiling their measurements. They tried everything to find the cause of the hiss. They pointed away from New York City, a significant source of electromagnetic interference. They dusted everything. They checked and rechecked their circuits.
They removed nesting pigeons from the horn and scraped away a considerable accumulation of what they called “white dielectric material” (“pigeon poop” to you and me).
No matter what they did, the hiss never varied.
In desperation, they called James Peebles, one of the radio astronomers at Princeton. Peebles knew precisely what they were detecting — the “radiation fireball,” or cosmic microwave background (CMB), as astronomers now call it.
Peebles put down the phone and said to his team, “Well, boys, we’ve been scooped.”
The fundamental assumption of the Steady State model — that the universe doesn’t change much over time — had been disproven. The Big Bang theory was now the accepted model.
It’s not that the steady-staters didn’t try. They suggested that the CMB resulted from scattered starlight from distant galaxies.
To prove that the CMB was indeed the remnant of that critical moment in the evolution of the Big Bang, astronomers had to show that the CMB fulfilled Alpher and Herman’s second criterion.
The CMB must behave similarly to a black body. Of course, there are no actual black bodies in nature, but the CMB is about as close as nature gets.
Like a black body, the CMB must have achieved thermal equilibrium. No heat must be flowing from one part to another.
During the 1970s, physicist Hans Gush launched sounding rockets into the upper atmosphere. He was attempting to get measurements of the CMB without the disturbing effects of Earth’s blanket of air.
His results indicated that the CMB was at thermal equilibrium. However, the rockets’ hot exhausts often interfered with the results.
In 1989, NASA launched the Cosmic Background Explorer (COBE). It determined that the CMB behaved like a black body at thermal equilibrium. COBE also precisely measured the CMB’s temperature at nearly 2.7 degrees Kelvin.
It also found faint differences in the CMB’s density from one part to another. At 380,000 years after the Big Bang, the universe was already lumpy. Subsequent spacecraft have mapped that unevenness of density in great detail.
The unevenness of the early universe slowly evolved to our current lumpiness as matter collapsed into galaxies separated by vast gulfs of space.
The universe was once very hot, but now it is far cooler. It once consisted of disconnected atomic nuclei and stray electrons but currently consists of stars, planets, and even the miracle of life.
And the Big Bang remains the fundamental model to explain how all that happened.