primordial fluctuations
Look at the universe today and it is wonderfully lumpy: stars clump into galaxies, galaxies into clusters, clusters into vast filaments threaded across empty voids. But run the clock back and the universe was almost perfectly smooth, the same density everywhere to about one part in 100,000. The lumps grew from those faint, almost imperceptible bumps. Primordial fluctuations are those original tiny ripples in the density of the early universe — the seeds from which every galaxy, including our own, eventually grew.
These fluctuations are slight overdensities and underdensities — places where the early cosmic soup was a whisker more or less dense than average. We see them imprinted directly on the cosmic microwave background as tiny hot and cold spots, differences of only about 30 millionths of a degree. Their power is gravity: a region just slightly denser than its surroundings pulls in a bit more matter, growing denser still, which pulls in more — a runaway process called gravitational instability. Over hundreds of millions of years these one-part-in-100,000 ripples amplified into the first stars, galaxies, and the great cosmic web. The remarkable thing is their statistical pattern: they are very nearly the same strength on all size scales, a feature called a near-scale-invariant spectrum.
Where did such uniform, scale-spanning ripples come from? The leading answer is inflation, and it is one of inflation's most beautiful claims. During inflation, the unavoidable quantum jitter of the inflaton field — tiny, random fluctuations demanded by quantum uncertainty even in 'empty' space — was stretched along with space itself to astronomical size and frozen in as real variations in density. In this picture the largest structures in the universe are quantum hiccups, enormously magnified. The precise statistics of the fluctuations (their near-scale-invariance and their close-to-random, Gaussian character) match inflation's predictions strikingly well, which is among the strongest evidence we have for the idea.
The mottled hot-and-cold map of the microwave sky is, quite literally, a baby photo of the seeds of everything. A spot that was a hundred-thousandth denser than average 13.8 billion years ago could, given enough time and gravity, become a galaxy cluster of a quadrillion suns. The cosmic web we live in is those whispers of density, grown loud.
The faint hot and cold spots in the CMB are the seeds from which all galaxies grew.
These fluctuations were tiny — about one part in 100,000 — not dramatic clumps. The universe started astonishingly smooth; structure had to be patiently amplified by gravity over hundreds of millions of years.