What the model actually says
The Big Bang model says that the universe was once extremely hot and dense, and has been expanding and cooling for roughly 13.8 billion years. It is a description of how the universe evolved from an early hot state, not a claim about an explosion at one point in space. Space itself is expanding everywhere, and there is no center from which everything flew outward.
The model also does not claim to explain the very first instant or why there was a universe at all. At the earliest times, densities and temperatures go beyond what current physics can describe reliably. The evidence below concerns the period from a tiny fraction of a second after the start (more cautiously, from about the first few minutes) to the present.
Evidence 1: distant galaxies are moving away
In the 1920s, astronomers found that light from distant galaxies is shifted toward the red end of the spectrum, and that the shift is larger for more distant galaxies. This redshift is interpreted as the stretching of light as space expands. Run the expansion backward, and everything must have been closer together in the past.
This is the oldest of the main pillars, and it is still supported by many measurements. Later observations that the expansion is speeding up refined the story without overturning the basic picture of a universe that was once much denser.
Evidence 2: the cosmic microwave background
In the early universe, matter was so hot that atoms could not hold together, and light was constantly scattered by free electrons, making the universe opaque like a fog. As expansion cooled things, around 380,000 years after the beginning, electrons and nuclei combined into neutral atoms and light could travel freely for the first time. That light has been traveling ever since.
Because the universe has expanded enormously since then, the light has stretched into microwaves. In 1965, Arno Penzias and Robert Wilson detected a faint, uniform microwave glow coming from every direction while working with a radio antenna, and after eliminating other explanations they realized it matched earlier predictions. It is called the cosmic microwave background, or CMB. Its temperature is about 2.7 degrees above absolute zero.
Later space missions measured the spectrum of this glow with great precision and found that it matches that of an almost perfect thermal source. They also mapped tiny variations in temperature, around one part in a hundred thousand. These variations are the seeds of later structure and carry information about the contents and geometry of the universe. Alternatives to the Big Bang have a hard time explaining such a precise thermal spectrum.
Evidence 3: the abundance of light elements
When the universe was a few minutes old, it was hot enough for nuclear fusion to occur throughout space, as in the core of a star, but only for a short time before cooling shut it off. Calculations based on well-tested nuclear physics predict what should have come out: mostly hydrogen, about a quarter helium by mass, and tiny traces of deuterium and lithium.
Observations of the oldest gas clouds, which have not been much altered by stars, show a helium fraction near that prediction. The agreement is good for helium and deuterium. There is a known discrepancy for one lithium isotope, which researchers have not fully resolved. A model that matches several numbers independently is considered to be strongly supported, and the early universe left the raw material that stars later processed.
Evidence 4: the structure of the universe
Galaxies are not scattered randomly. They form clusters, filaments and large voids, often described as a cosmic web. Computer simulations that begin with the small CMB-era variations, and then let gravity act over billions of years (with dark matter included), produce web-like patterns that resemble what telescopes see in galaxy surveys.
Telescopes also look back in time, because light takes time to arrive. Observations of very distant galaxies show younger, smaller and more chaotic systems than nearby ones, consistent with galaxies growing over time. No galaxies are found with stars older than the age given by the model.
What about other explanations?
Over the decades, rival ideas, such as steady-state models in which the universe does not change over time, have been proposed. They struggled to account for the CMB, the changing galaxy populations and the light element abundances together, and the Big Bang model became the mainstream view.
Open questions
The model has gaps, and scientists say so openly.
- What set the initial conditions? A proposed rapid expansion in the first instant, called inflation, would explain why the universe looks so uniform and flat, but it is supported by indirect hints rather than being settled.
- What are dark matter and dark energy, which together make up most of the universe’s contents in the standard version of the model?
- Why is there more matter than antimatter?
These questions refine the Big Bang model rather than reject it. They are central topics in cosmology, and they relate to what we know about the fundamental forces and the contents of space.