No matter what the two did, they couldn't rid the receivers of background radio noise that, puzzlingly, seemed to be coming from all directions at once. Penzias contacted Princeton University physicist Robert Dicke who suggested that the radio noise might be cosmic microwave background radiation (CMB), which is primordial microwave radiation that fills the universe.
And that is the story of the discovery of CMB. Simple and elegant.
For their discovery, Penzias and Wilson received the 1978 Nobel Prize in Physics, and for good reason. Their work ushered us into a new age of cosmology, allowing scientists to study and understand our universe as never before.
Yet, this discovery also led to one of the most surprising findings in recent history: Unique features in the CMB could be the first direct evidence we've ever had of the multiverse — of an infinity of worlds and alien peoples that exist beyond the known universe.
However, to properly understand this extraordinary claim, it's necessary to first take a journey back to the beginning of space and time.
The history of the universe
According to the broadly accepted theory for the origin of our universe, for the first several hundred thousand years after the Big Bang, our universe was filled with a ferociously hot plasma comprised of nuclei, electrons, and photons, which scattered light.
By around 380,000 years of age, the continued expansion of our universe caused it to cool to below 3000 degrees K, which allowed electrons to combine with nuclei to form neutral atoms, and the absorption of free electrons allowed light to illuminate the dark.
Evidence of this, in the form of radiation from the cosmic microwave background (the previously mentioned CMB), is what was detected by Penzias and Wilson, and it helped establish the Big Bang theory of cosmology.
Over the eons, continued expansion cooled our universe to a temperature of just around 2.7 K, but that temperature isn't uniform. Differences in temperature arise from the fact that matter is not uniformly distributed throughout the universe. This is thought to be caused by tiny quantum density fluctuations that occurred right after the Big Bang.
One spot, in particular, seen from the Southern Hemisphere in the constellation Eridanus, is particularly cold, around 0.00015 degrees colder than its surroundings. Dubbed the "Cold Spot", scientists originally thought it was a "supervoid," an area that contains far fewer galaxies than normal.
Then, in 2017, researchers at the UK's Durham University Centre for Extragalactic Astronomy published research they say suggests that the Cold Spot isn't a supervoid after all.
Instead? It may be evidence of alien universes.
Durham Professor Tom Shanks proposed what he described as a "more exotic" explanation for the Cold Spot. In his work, Shanks argued that the Cold Spot was "caused by a collision between our universe and another bubble universe...The Cold Spot might be taken as the first evidence for the multiverse - and billions of other universes may exist like our own."
Previously, physicists including Anthony Aguirre, Matt Johnson, and Matt Kleban had pointed out that a collision between our bubble universe and another bubble in the multiverse would, in fact, produce an imprint on the cosmic background radiation. Additionally, they noted that it would appear as a round spot having either a higher or a lower level of radiation intensity.
Shanks' proposal seems to fit the bill, but could this feature really be evidence of an infinite multitude of universes that exist beyond our own?
The laws of the multiverse
Today, there are three main contenders that explain how the multiverse may function: the Copenhagen Interpretation, the "many worlds" or "branches of the wave function" interpretation, and the "parallel branes" of string theory.
We're going to leave string theory for another day and focus on the other two explanations.
The total of all possible states in which an object can exist is called an object's coherent superposition, and it is made up of what's known as the object's "wave function".
Quantum mechanics necessitate a smooth, fully deterministic wave function — a mathematical expression that conveys information about a particle in the form of numerous possibilities for its location and characteristics. It also requires something that realizes one of those possibilities and eliminates all the others.
Opinions differ about how that happens, but in the most common theory, known as the Copenhagen Interpretation, this occurs through observation of the wave function or by the wave function encountering some part of the "classical" world. This causes the probability, or wave function, to "collapse", and forces the particle into one state.
The Copenhagen Interpretation was worked out in the 1920s by physicists Niels Bohr and Werner Heisenberg, who argued that a particle does not have a material existence until it is subjected to measurement (observation).
The Copenhagen Interpretation was essentially a fudge and, for many, an unsatisfactory one at that.
In 1935, Austrian-Irish physicist Erwin Schrödinger articulated the problem with Copenhagen Interpretation with his famous thought experiment known as Schrödinger's Cat.
In this theoretical experiment, a cat is placed in a sealed box along with a bit of radioactive material and a Geiger counter. If the Geiger counter detects the decay of the radioactive material, it triggers the release of a poison gas which kills the cat.
While the box is sealed, the cat is in a superposition of being both alive and dead at the same time. It is only when the box is opened that the cat is forced into one state or another. Schrödinger pointed out that this was ridiculous, and that quantum superposition could not work with large objects such as cats, because it is impossible for an organism to be simultaneously alive and dead. Thus, he reasoned that the Copenhagen Interpretation must be inherently flawed.
A number of alternatives to the Copenhagen Interpretation were proposed. For example, the ‘hidden variables’ approach championed by Albert Einstein and David Bohm, among others, suggests that the wave function be treated as a temporary fix until physicists eventually find something better. Late in his life, Heisenberg proposed that the problem is with our notion of reality itself. He suggested that the wave function represents an “intermediate” level of reality.
The most straightforward approach may be that of the "many worlds" interpretation" (MWI) which was first posited in 1957 by a graduate student at Princeton University named Hugh Everett. Everett was studying physics under John Archibald Wheeler, who had envisioned the fabric of the universe as being a churning, sub-atomic realm of quantum fluctuations, which he called "quantum foam".
In his dissertation, entitled The Theory of the Universal Wave Function, Everett contended that the universal wave function is real and doesn't collapse, as in the Copenhagen Interpretation. In that case, then every possible outcome of a quantum measurement is realized in some "world" or universe, and by that logic, there must be a very large, or infinite, number of universes.
Everett's many worlds interpretation of quantum physics received little support from the wider physics community, and Everett spent his entire working life outside of academia. So strongly did Everett believe in his theory that he ate whatever he wanted, smoked three packs of cigarettes a day, drank to excess, and refused to exercise. In July 1982, Hugh Everett died suddenly of a heart attack aged 51.
Per his instructions, Everett was cremated and his ashes thrown into the garbage. In 1996, Everett's daughter, Elizabeth, killed herself, and her suicide note stated that she too wanted her ashes to be thrown into the garbage so that she might "end up in the correct parallel universe to meet up w[ith] Daddy."
Everett's son, Mark Oliver Everett went on to form the rock group "The Eels" whose music is often filled with themes of family, death, and lost love.
Stephen Hawking and the multiverse
Famed British physicist Stephen Hawking died on March 14, 2018, after spending decades confined to a wheelchair and dependent upon a speech synthesizer due to suffering from amyotrophic lateral sclerosis. Hawking's final research paper, published just 10 days before he died, was written along with Thomas Hertog, a professor of theoretical physics at KU Leuven University in Belgium, and it concerned the multiverse.
In the paper entitled, "A Smooth Exit from Eternal Inflation?" Hawking and Hertog proposed that the rapid expansion of space-time after the Big Bang may have happened repeatedly, creating a multitude of universes.
This was an expansion of Inflation Theory, the currently-held theory that the Big Bang was not really the beginning. Inflation Theory suggests that, before the Big Bang, the Universe was filled with energy that was part of space itself, and that energy caused space to expand at an exponential rate. It is that energy that gave rise to Big Bang.
However, because inflation, like everything else, is quantum in nature, it must have ended at different times in different locations, while the space between the locations continued to inflate. This, in turn, means that there would be regions of space where inflation ends and a Big Bang begins, but these regions can never encounter one another, as they're separated by regions of inflating space.
In an interview, Hawking explained his concerns with Inflation Theory, saying, 'The usual theory of eternal inflation predicts that globally our universe is like an infinite fractal, with a mosaic of different pocket universes, separated by an inflating ocean. The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse. But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can’t be tested."
Instead, the pair predict that the universe, at least on the largest scales, is actually smooth and finite. Their theory uses the concept of holography, which describes how physical reality in certain 3D spaces can be mathematically reduced to 2D projections on a surface. By using this concept, they were able to reduce eternal inflation to a timeless state, defined on a spatial surface at the beginning of time itself.
Hertog and Hawking then used their new theory to predict that the universe which emerges from eternal inflation is actually finite and much simpler than the infinite fractal structure predicted by the existing theory of eternal inflation.
Hawking explained that, “We are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes."
This makes the theory not only more predictive but also testable.
And, if Hawking and Hertog, Everett and Laura Mersini-Houghton, Tegmark and Greene, and a multitude of other physicists are right, then somewhere, in another universe at the exact moment that you're reading this article, Hawking is walking and talking animatedly about physics. Let's hope.
Disclaimer: This article has been updated. A previous version incorrectly said "Walter" instead of Werner Heisenberg.