How Did the Universe Get Started?

The timeline of the universe
This represents a timeline of the universe from the Big Bang through the present. At the left is the "birth event" of the cosmos, known as the "Big Bang". NASA / WMAP Science Team

How did the universe begin? That's a question scientists and philosophers have pondered throughout history as they looked at the starry sky above. It's the job of astronomy and astrophysics to provide an answer. However, it's not an easy one to tackle.

Big Bang, conceptual image
An artist's concept of what the Big Bang might have looked like, if anybody had been around to see it. HENNING DALHOFF / Getty Images

The first major glimmerings of an answer came from the sky in 1964. That's when astronomers Arno Penzias and Robert Wilson discovered a microwave signal buried in data they were taking to look for signals being bounced from Echo balloon satellites. They assumed at the time that it was simply unwanted noise and attempted to filter out the signal.

Holmdel Horn
The antenna that Penzias and Wilson were using when they stumbled across the signals from the cosmic background radiation heralding the birth of the universe. Fabioj, CC BY-SA 3.0

However, it turns out that what they detected was coming from a time shortly after the beginning of the universe. Although they didn't know it at the time, they had discovered the Cosmic Microwave Background (CMB). The CMB had been predicted by a theory called the Big Bang, which suggested that the universe began as a densely hot point in space and suddenly expanded outward. The two men's discovery was the first evidence of that primordial event.

The Big Bang

What started the birth of the universe? According to physics, the universe sprang into existence from a singularity — a term physicists use to describe regions of space that defy the laws of physics. They know very little about singularities, but it's known that such regions exist in the cores of black holes. It's a region where all the mass gobbled up by a black hole gets squeezed into a tiny point, infinitely massive, but also very, very small. Imagine cramming Earth into something the size of a pinpoint. A singularity would be smaller.

That's not to say the universe began as a black hole, however. Such an assumption would raise the question of something existing before the Big Bang, which is pretty speculative. By definition, nothing existed prior to the beginning, but that fact creates more questions than answers. For instance, if nothing existed prior to the Big Bang, what caused the singularity to be created in the first place? It's a "gotcha" question astrophysicists are still trying to understand. 

However, once the singularity was created (however it happened), physicists have a good idea of what occurred next. The universe was in a hot, dense state and began to expand through a process called inflation. It went from very small and very dense, to a very hot state. Then, it cooled as it expanded. This process is now referred to as the Big Bang, a term first coined by Sir Fred Hoyle during a British Broadcasting Corporation (BBC) radio broadcast in 1950.

Although the term implies some kind of explosion, there really wasn’t an outburst or a bang. It was really the rapid expansion of space and time. Think of it like blowing up a balloon: as someone blows air in, the exterior of the balloon expands outward.

The Moments after the Big Bang

The very early universe (at a time a few fractions of a second after the Big Bang began) was not bound by the laws of physics as we know them today. So, no one can predict with great accuracy what the universe looked like at that time. Yet, scientists have been able to construct an approximate representation of how the universe evolved.

First, the infant universe was initially so hot and dense that even elementary particles such as protons and neutrons could not exist. Instead, different types of matter (called matter and anti-matter) collided together, creating pure energy. As the universe began to cool during the first few minutes, protons and neutrons began to form. Slowly, protons, neutrons, and electrons came together to form hydrogen and small amounts of helium. During the billions of years that followed, stars, planets, and galaxies formed to create the current universe.

Evidence for the Big Bang

So, back to Penzias and Wilson and the CMB. What they found (and for which they won a Nobel Prize), is often described as the “echo” of the Big Bang. It left behind a signature of itself, just like an echo heard in a canyon represents a “signature” of the original sound. The difference is that instead of an audible echo, the Big Bang's clue is a heat signature throughout all of space. That signature has been specifically studied by the Cosmic Background Explorer (COBE) spacecraft and the Wilkinson Microwave Anisotropy Probe (WMAP). Their data provide the clearest evidence for the cosmic birth event. 

The detailed, all-sky picture of the infant universe created from seven years of WMAP data. The image reveals 13.7 billion year old temperature fluctuations (shown as color differences) that correspond to the seeds that grew to become the galaxies. NASA / WMAP Science Team

Alternatives to the Big Bang Theory

While the Big Bang theory is the most widely accepted model that explains the origins of the universe and is supported by all the observational evidence, there are other models that use the same evidence to tell a slightly different story.

Some theorists argue that the Big Bang theory is based on a false premise — that the universe is built on an ever-expanding space-time. They suggest a static universe, which is what was originally predicted by Einstein’s theory of general relativity. Einstein’s theory was only later modified to accommodate the way the universe appears to be expanding. And, expansion is a big part of the story, particularly as it involves the existence of dark energy. Finally, a recalculation of the mass of the universe seems to support the Big Bang theory of events. 

While our understanding of the actual events is still incomplete, CMB data are helping shape the theories that explain the birth of the cosmos. Without the Big Bang, no stars, galaxies, planets, or life could exist. 

Fast Facts

  • The Big Bang is the name given to the birth event of the universe.
  • The Big Bang is thought to have occurred when something kicked off the expansion of a tiny singularity, some 13.8 billion years ago.
  • Light from shortly after the Big Bang is detectable as the cosmic microwave radiation (CMB). It represents light from a time when the newborn universe was lighting up some 380,000 years after the Big Bang occurred.


  • “The Big Bang.” NASA, NASA,
  • “The Origins of the Universe.” National Geographic, National Geographic, 24 Apr. 2017,

Updated and edited by Carolyn Collins Petersen.

mla apa chicago
Your Citation
Millis, John P., Ph.D. "How Did the Universe Get Started?" ThoughtCo, Apr. 5, 2023, Millis, John P., Ph.D. (2023, April 5). How Did the Universe Get Started? Retrieved from Millis, John P., Ph.D. "How Did the Universe Get Started?" ThoughtCo. (accessed June 1, 2023).