Big Bang
Origin of Observable Universe
Introduction
Big Bang is not a point in space. It is a moment in time. There is no specific location in universe where it happened; it happened everywhere at once. About 13.8 billion years ago, all of space was in an extremely hot, dense state. Whether universe was already infinite at that moment or something unimaginably small, we do not know. What we do know is that space itself began expanding stretching and cooling and it has not stopped since. Three independent lines of evidence confirm this: observed expansion of universe, cosmic microwave background radiation filling all of space, and measured abundances of light elements matching predictions with stunning precision.
Big Bang model does not describe moment zero. It does not tell us what caused initial state or what existed before it, if "before" even means anything when time itself may have begun with expansion. But physicists have ideas. One leading hypothesis, eternal inflation, suggests our Big Bang may be one bubble in an endlessly expanding ocean of space, where new bubble universes constantly form, each with its own beginning. Another proposal is that Big Bangs are recurring - universe expands, slows, contracts back into a hot dense state, and expands again in an endless cycle. Quantum cosmology suggests universe may have appeared spontaneously from a quantum fluctuation something from nothing. None of these have been confirmed by experiment. What Big Bang model does describe, with extraordinary accuracy, is evolution of universe from a fraction of a second onward. That is what we know. And it is remarkable.
First Microsecond
In first trillionth of a trillionth of a trillionth of a second (10-43 seconds), all four fundamental forces may have been one. A single unified interaction governing everything. Our physics cannot describe this moment; it demands a theory of quantum gravity that does not yet exist. By 10-43 seconds, gravity had separated from other three. By 10-36 seconds, strong nuclear force broke away from electroweak force, a moment that may have triggered inflation. By 10-12 seconds, electroweak force split into electromagnetism and weak force as Higgs field settled into its nonzero value.
During first microsecond, universe was a quark-gluon plasma, the hottest, densest state of matter we know of. A screaming-hot soup of free quarks, gluons, and other fundamental particles at temperatures beyond two trillion degrees. As universe cooled below this threshold, quarks bound together into protons and neutrons through strong force. This transition locked quarks permanently inside composite particles. They have never been free since except briefly, in particle colliders where scientists recreate these extreme conditions for a fleeting instant.
First Elements
Imagine a window that opens for exactly ten minutes and then slams shut forever. Between roughly 10 seconds and 20 minutes after expansion began, universe was in a sweet spot, cool enough for protons and neutrons to stick together, but still hot enough to power nuclear fusion. In that brief window, simplest atomic nuclei formed. Result was roughly 75% hydrogen and 25% helium by mass, with tiny traces of deuterium and lithium. Nothing heavier. Every carbon atom in your body, every oxygen atom you breathe, every iron atom in your blood, all of that came later, forged inside stars billions of years afterward.
Here is what makes this remarkable. Scientists calculated exactly how much hydrogen and helium should have formed in that ten-minute window, based purely on physics of early universe. Then they measured actual ratios in the most ancient, distant gas clouds places untouched by stars. Numbers match. This agreement is one of the strongest pieces of evidence that Big Bang model is correct. Universe kept a receipt from its first twenty minutes, and we found it.
The detail is worth pausing on, because it is the kind of prediction that cannot be fudged. Early universe physics predicts helium-4 should make up about 24 to 25 percent of ordinary matter by mass, and it does. Predicts deuterium (a heavy form of hydrogen) should exist at about 25 parts per million relative to regular hydrogen, and it does. Predicts helium-3 at about 10 parts per million, lithium-7 at a few parts per ten billion. Every number is fixed by a single free parameter, how much ordinary matter exists per photon in universe, and four independent abundance measurements all pin down the same value for that parameter. Four separate tests, four agreements. This is why Big Bang nucleosynthesis is often called one of the three pillars of modern cosmology, alongside expansion and cosmic microwave background. No other theory of the early universe comes close to reproducing these numbers.
First Light
For 380,000 years after Big Bang, universe was opaque. Photons could not travel freely; they were constantly scattered by free electrons in hot plasma, like headlights in thick fog. You could not see through it. Then temperature dropped to about 3,000 Kelvin. Electrons combined with nuclei to form neutral atoms for first time. Fog lifted. Photons streamed freely through now-transparent space. This was first light ever released, a glow that filled entire universe at once.
That light has been traveling ever since. Over 13.8 billion years, expansion of space stretched it from a visible glow into faint microwave radiation. But it is still here, arriving from every direction, at every moment. We call it cosmic microwave background.
What followed was a long silence. From roughly 380,000 years to about 200 million years after Big Bang, universe entered what cosmologists call the Dark Ages. No stars existed yet. The cosmic microwave background had redshifted out of visible range. Universe was filled with neutral hydrogen and helium, slowly cooling, utterly dark. Gravity was quietly at work, pulling matter into denser clumps along the scaffolding of dark matter filaments. Eventually, the densest pockets collapsed far enough for hydrogen to ignite. The first stars lit up, ending the Dark Ages and beginning the epoch of reionization, when ultraviolet radiation from those first stars stripped electrons off surrounding hydrogen, making universe transparent again at shorter wavelengths.
Cosmic Microwave Background
This ancient glow was discovered by accident. In 1965, two radio astronomers kept hearing a persistent hiss they could not eliminate from their antenna. They pointed it at every part of sky, at every time of day and night. Hiss never went away. They had stumbled upon afterglow of creation. Today we measure it as a nearly perfect glow at 2.725 Kelvin, filling every direction of sky with extraordinary uniformity.
You might wonder: if we see this glow coming from every direction equally, does that mean we are at center of universe? No. It means entire universe was glowing at that moment. Light has been traveling toward us from every direction for 13.8 billion years. An observer on other side of universe would see exactly same thing, ancient light arriving from all sides, surrounding them in exactly same way. There is no center and no edge. Every point in space sees itself as middle, because light from that hot early era reaches every point from all directions equally.
Satellite missions mapped this glow in extraordinary detail and found tiny temperature variations about one part in 100,000. Slightly hotter spots where matter was a fraction denser, slightly cooler spots where it was thinner. Those faint ripples are seeds of everything. Over billions of years, gravity slowly pulled denser regions together into clumps, filaments, and sheets. Galaxy clusters, superclusters, and vast cosmic web we observe today all grew from those whisper-thin variations.
Is Universe Infinite?
Observable universe has a definite size. Light has been traveling for 13.8 billion years since Big Bang, and because space has been expanding entire time, farthest objects we can detect are now about 46.5 billion light-years away. That gives us an observable sphere roughly 93 billion light-years across. But that is only what we can see. Actual universe almost certainly extends far beyond this boundary and it may go on forever.
How would we even begin to measure that? Geometry. If space is curved, like surface of a sphere, then parallel lines would eventually meet, triangles would add up to more than 180 degrees, and space would wrap around on itself: finite but with no edge. If space is perfectly flat, it could extend infinitely in every direction. Scientists tested this using cosmic microwave background. Physical size of those ancient temperature spots is known from theory they act as a ruler painted on sky. If space were curved, those spots would appear distorted: stretched or squashed compared to what flat geometry predicts. They are not. To highest precision we can measure, space is flat. This means universe is either truly infinite, or so staggeringly large that any curvature is beyond our ability to detect. We may never know which, because information from beyond observable horizon can never reach us.
Edge of Our Understanding
Despite everything above, enormous questions remain. What triggered initial expansion? Inflation theory proposes a burst of exponential stretching in first 10-36 seconds that would explain why universe looks so uniform in every direction, explain why space is flat to within 0.4%, and dilute exotic relics like magnetic monopoles to undetectable density – three independent puzzles solved by the same mechanism. But mechanism driving inflation is still unknown. What is dark energy, mysterious force accelerating expansion today? What is dark matter, invisible substance whose gravity shaped galaxies into existence? Why is there matter at all, when equal amounts of matter and antimatter should have annihilated each other completely in first moments? How did billion-solar-mass black holes already exist when universe was less than a billion years old, far earlier than ordinary growth channels can account for?
These are active frontiers of physics. Some may be answered in our lifetime. Others may require entirely new frameworks we have not yet imagined. Honest answer to many of the deepest questions about our universe is simply: we do not know yet. And that is what makes cosmology one of the most exciting fields in all of science.
