In the beginning, the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move.—Douglas Adams, The Hitchhiker’s Guide to the Galaxy
Humans have many strengths, but chief among them may be our endless curiosity. We are the Universe trying to understand itself. How, and why, did we come to be? Why does the Universe take the shape it does? To understand our place in the cosmos, we look back to see what has gone before. Wind the clock back through the history of the Earth, the Solar System, and even the galaxy, and most experts now agree that space and time originated in a violent and all-encompassing explosion known as the Big Bang.
Big Bang theory, in science, is the idea that all the matter and energy of the Universe was once crushed into a single point, a single infinitesimal granule of everything. The Big Bang itself is the moment when everything began.
(In this article we’ll use the scientific definition of “theory”: an explanation of some aspect of the natural world that can be tested and reproducibly corroborated.)
Credit: NASA
Timeline of the Big Bang
The currently accepted model of the Big Bang begins, chronologically, at t = 0, a threshold defined as the very first instant of meaningful time as we experience it. Before that threshold (specifically, before the end of the first Planck time after the Big Bang began), it’s tough to make any informed conclusions because we have no way of testing anything at all. All conclusions about what may have happened before the Planck threshold, therefore, are philosophical.
By the Numbers
At t = 0, the Big Bang itself begins. Ordinary (three-dimensional) space and time emerge from a primordial state thought to be described by a quantum theory of gravity, also known as a “theory of everything.” Immeasurably hot and dense, that which would become our Universe was crammed into a volume just a tiny fraction the size of a proton.
During the first picosecond of time (10⁻¹² seconds) after the Big Bang, the laws of physics as they currently apply emerged from a state in which they may have looked very different. Atoms literally did not exist yet. First gravity decoupled, then the strong force, then finally the electromagnetic and weak forces. There is no specific measure of the temperature of the Universe as a whole as it began, but at 10⁻³⁶ seconds after the Big Bang, it had cooled to just 10²⁸ Kelvin. Cosmic inflation hadn’t even begun yet; that had to wait until t + 10⁻³³ seconds, lasting only until t + 10⁻³². During cosmic inflation, the Universe expanded by a factor of 10²⁶, whereafter it slowed to a relative crawl. If you think about the Big Bang as an explosion, this tiny interval is when the explosion in volume occurred.
Protons and neutrons began to form after about a microsecond (10⁻⁶ seconds). One second in, the Universe had supercooled to just 10⁹ (one hundred million) kelvin. Once about 10 seconds had passed, electrons emerge, followed by the first photons. But charged particles impede the motion of photons, which meant that light couldn’t really radiate away to escape.
Cosmic Dawn
Neutrinos are a tiny, low-mass particle that interact only weakly with normal matter; huge numbers of neutrinos have passed through your body without ever interacting with it, in the time it took you to read this. But things were very different when the Universe was still packed into its preinflation form. With matter so close together, there was nowhere for neutrinos to get away from it, so they were forced to interact. Within one second after the Big Bang, the Universe had expanded enough that neutrinos could move, so they all but stopped interacting with normal matter. The state of things at that time is physically imprinted onto the distribution of neutrinos we see today on the celestial dome, a nearly undetectable cosmic neutrino background.
Nucleosynthesis began three minutes after the Big Bang. For the next 17 minutes, all was fusion. At 20 minutes after the Big Bang, that fusion ceased, but because it was still so hot that most particles were charged, it was too hot for photons to move much. Subatomic particles of various species were born and annihilated en masse.
If you’d been around to observe, for the first few hundred millennia, you wouldn’t have been able to see anything in any meaningful sense—just a slightly glowing, all-permeating, mostly opaque fog. But within the cooling glow, the first neutral hydrogen atoms were condensing out of the primordial quark-gluon soup.
A hundred thousand years after the Big Bang, helium hydride formed the first molecule. By about t + 370,000 years, neutral hydrogen formation had wound itself down, leaving the Universe transparent for the first time. But not all the matter formed by Big Bang nucleosynthesis condensed in its lowest-energy state. As high-energy matter settled into its lowest-energy state, it released that energy in the form of light. Indeed, there was a titanic burst of light, incomprehensible in its magnitude, enough to fill the entire Universe with a pale peachy reddish-orange. Photons released in that pulse linger today, visual echoes of the Big Bang redshifted down into the microwave band—now known as the cosmic microwave background.
An anisotropic quantity is different depending on the direction in which you measure it.
Credit: ESA/Planck Collaboration
After that massive flash, the light slowly faded into a deep red, and then to the purest black. This period is known as the cosmic Dark Age. Only hundreds of millions of years later did the first new sources of light—stars and galaxies—flare into being.
Shedding Light
One way scientists look into the early Universe is by looking into the deep sky. Satellites such as NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck spacecraft study the cosmic microwave background. Missions like the Gaia Project seek to map the three-dimensional structure of the Milky Way and even the visible Universe. And observatories like the James Webb Space Telescope can resolve tiny, faraway objects, whose light was emitted long ago and has only now reached Earth.
Top left: radial velocity; bottom left: radial velocity and proper motion. Top right: dust, by density; bottom right: chemical composition (redder stars are richer in metal, bluer stars have lower metallicity).
Credit: ESA/GAIA/DIPAC
Looking back into the sky means looking back in time, and the JWST was designed for the purpose. Its actively cooled infrared sensors stay so close to absolute zero that they can detect the faintest shiver of infrared radiation, such as the dim light from truly ancient galaxies. The instrument has detected galaxies whose redshift is so extreme that they are thought to have been formed within 300 million years of the Big Bang.
This unassuming red blob is Maisie’s Galaxy, one of the oldest ever discovered, and named after its discoverer’s daughter (he found it on her birthday).
Credit: NASA/STScI/CEERS
There’s a kind of luminous veil drawn across the history of the Universe at about a second after its birth. Bound as we are by the laws of physics as they apply today, physicists who seek to part the veil can still resort to creating tiny pockets of extremely high-energy conditions, such as those created in particle beams at facilities like Lawrence Livermore and CERN.
TIM is a spy patrolling robot working in the 27-km tunnel of the Large Hadron Collider (LHC). As a mini vehicle, it can transport a set of instruments along tracks suspended from the tunnel’s ceiling. This smart machine is used for real-time monitoring of the LHC tunnel: the tunnel structure, the oxygen percentage, the communication bandwidth and the temperature.
Credit: CERN
Neutrino detectors are another way of turning back the clock. But because neutrinos interact only weakly with matter, it takes an awful lot of matter to catch them in useful numbers. Consequently, neutrino detectors are usually huge. One standout is the University of Wisconsin’s IceCube project, a kilometer-scale array near the Amundsen research base at the South Pole. Another is Japan’s Super-Kamiokande detector, a massive array of flashbulbs buried deep underground in what used to be a salt mine.
Credit: Super-Kamiokande
Upcoming neutrino projects include China’s TRIDENT array, scheduled to debut in the tropical West Pacific in 2030, and IceCube Gen2, which is planned to be 10 times the volume of its predecessor and buried under a mile of Antarctic ice. Meanwhile, looking outward, NASA is in the early stages of work on the planned Nancy Grace Roman infrared space telescope, which the agency intends to launch in 2027.