The broadly accepted theory for the origin and evolution of our universe is the Big Bang model, which states that the universe began as an incredibly hot, dense point roughly 13.7 billion years ago. So, how did the universe go from being fractions of an inch (a few millimeters) across to what it is today?
Here is a breakdown of the Big Bang to now in 10 easy-to-understand steps.
Step 1: How It All Started
The Big Bang was not an explosion in space, as the theory’s name might suggest. Instead, it was the appearance of space everywhere in the universe, researchers have said. According to the Big Bang theory, the universe was born as a very hot, very dense, single point in space.
Cosmologists are unsure what happened before this moment, but with sophisticated space missions, ground-based telescopes and complicated calculations, scientists have been working to paint a clearer picture of the early universe and its formation.
A key part of this comes from observations of the cosmic microwave background, which contains the afterglow of light and radiation left over from the Big Bang. This relic of the Big Bang pervades the universe and is visible to microwave detectors, which allows scientists to piece together clues of the early universe.
In 2001, NASA launched the Wilkinson Microwave Anisotropy Probe (WMAP) mission to study the conditions as they existed in the early universe by measuring radiation from the cosmic microwave background. Among other discoveries, WMAP was able to determine the age of the universe — about 13.7 billion years old.
Step 2: The Universe’s First Growth Spurt
NASA, ESA, and S. Beckwith (STScI) and the HUDF Team
When the universe was very young — something like a hundredth of a billionth of a trillionth of a trillionth of a second (whew!) — it underwent an incredible growth spurt. During this burst of expansion, which is known as inflation, the universe grew exponentially and doubled in size at least 90 times.
“The universe was expanding, and as it expanded, it got cooler and less dense,” David Spergel, a theoretical astrophysicist at Princeton University in Princeton, N.J., told SPACE.com.
After inflation, the universe continued to grow, but at a slower rate. As space expanded, the universe cooled and matter formed.
Step 3: Too Hot to Shine
Light chemical elements were created within the first three minutes of the universe’s formation. As the universe expanded, temperatures cooled and protons and neutrons collided to make deuterium, which is an isotope of hydrogen. Much of this deuterium combined to make helium.
For the first 380,000 years after the Big Bang, however, the intense heat from the universe’s creation made it essentially too hot for light to shine. Atoms crashed together with enough force to break up into a dense, opaque plasma of protons, neutrons and electrons that scattered light like fog.
Step 4: Let There Be Light
About 380,000 years after the Big Bang, matter cooled enough for electrons to combine with nuclei to form neutral atoms. This phase is known as “recombination,” and the absorption of free electrons caused the universe to become transparent. The light that was unleashed at this time is detectable today in the form of radiation from the cosmic microwave background.
Yet, the era of recombination was followed by a period of darkness before stars and other bright objects were formed.
Step 5: Emerging from the Cosmic Dark Ages
ESA XMM-Newton/EPIC, LBT/LBC, AIP
Roughly 400 million years after the Big Bang, the universe began to come out of its dark ages. This period in the universe’s evolution is called the age of re-ionization.
This dynamic phase was thought to have lasted more than a half-billion years, but based on new observations, scientists think re-ionization may have occurred more rapidly than previously thought.
During this time, clumps of gas collapsed enough to form the very first stars and galaxies. The emitted ultraviolet light from these energetic events cleared out and destroyed most of the surrounding neutral hydrogen gas. The process of re-ionization, plus the clearing of foggy hydrogen gas, caused the universe to become transparent to ultraviolet light for the first time.
Step 6: More Stars and More Galaxies