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Although special relativity prevents objects from moving faster than the speed of light, the expansion of the universe, which increases the farther objects are from one another, produces observed superluminal motion. This does not violate the theory because galaxies are moving with the expanding space, rather than through it.
In the 1920s, Hubble showed that the Andromeda “nebula” was another galaxy. He later discovered that galaxies move away from us at speeds proportional to their distance—a breakthrough now known as the Hubble–Lemaître law, foundational to Big Bang cosmology.
One of the most surprising discoveries about our universe over the past century is that it's expanding. Even more surprising is that it’s speeding up, and scientists aren’t entirely sure why. The rate should be roughly dependent on the total energy contained in the universe, but calculations keep coming up short. Explore the so-called “Hubble Tension,” along with potential candidates to solve the riddle.
The amount of normal matter in the universe is most accurately determined through measurements of cosmic microwave background. Quantities of dark matter and dark energy are calculated through the observed bending of light around galaxies and the expansion of the universe, respectively.
In 1931, Georges Lemaître presented his radical idea that the universe came from a “primeval atom,” challenging the long-held belief that the universe had always existed. Einstein initially dismissed Lemaître’s work as having “abominable” physical insight, only to accept it later.
Astronomers discovered in the late 1990s that galaxies move apart faster over time, contradicting expectations that gravity should slow cosmic expansion. This has prompted the revival of the "cosmological constant," a type of repulsive gravity theorized by Albert Einstein.
The Big Bang theory describes the universe beginning 13.7 billion years ago as an ultra-dense point, followed by stages, like the radiation- and matter-dominated eras. Each era was responsible for a unique element, such as the cosmic microwave background and atomic nuclei.
In 1964, Arno Penzias and Robert Wilson accidentally detected the CMB while trying to eliminate persistent static from their radio antenna. The pair initially blamed the signal on pigeon droppings inside their antenna—cleaning it didn’t help, but it did win them a Nobel Prize.
The term “Big Bang” was initially coined by a skeptic of the theory, but the name stuck despite the theory not depicting a "bang." The event took place everywhere in the universe at once, and the theory explains the abundance of hydrogen and helium in the universe.
Multiple independent lines of evidence—galactic redshift, element ratios, cosmic microwave background radiation, and large-scale cosmic structure—all point to a single explosive origin 13.8 billion years ago. Before Edwin Hubble, many astronomers believed the entire universe was contained within our galaxy.
From roughly 60 miles above the Earth's surface to farther than light has traveled during the entire age of the universe, space has captured human imagination for millennia. Explore the final frontier with the best resources curated from across the internet.