Structure Formation and Inflation
Galaxies and the cosmic web grew by gravity from tiny primordial fluctuations, which inflation, a brief burst of exponential expansion, is thought to have stretched from quantum origins.
Definition
Structure formation is the process by which small primordial density fluctuations grow under gravity into galaxies, clusters, and the cosmic web; inflation is a hypothesized epoch of accelerated expansion in the very early universe that explains its large-scale uniformity and flatness and seeds the fluctuations.
Scope
This area covers the theory of cosmic inflation and the problems it solves, the origin and statistics of the primordial density perturbations it generates, the gravitational growth of those perturbations into galaxies and clusters, and the resulting large-scale structure of the universe as mapped by galaxy surveys.
Sub-topics
Core questions
- What is cosmic inflation and what problems does it solve?
- Where did the initial density fluctuations come from?
- How did gravity turn small fluctuations into galaxies and the cosmic web?
Key concepts
- Inflation
- Primordial power spectrum
- Scale invariance
- Gravitational instability
- Cosmic web
- Horizon and flatness problems
- Quantum fluctuations
Key theories
- Cosmic inflation
- A brief epoch of accelerated expansion in the early universe naturally explains its observed flatness, homogeneity, and lack of relic monopoles, and generates a nearly scale-invariant spectrum of perturbations.
- Gravitational instability
- Regions slightly denser than average attract more matter and grow, so gravity amplifies primordial fluctuations into the galaxies, clusters, and filaments observed today.
Clinical relevance
Structure formation and inflation connect the smooth early universe to the rich cosmos we observe: inflation provides the leading account of the initial conditions and is tested by the cosmic microwave background, while the growth of structure links those initial conditions to galaxy surveys, providing powerful tests of dark matter, dark energy, and the standard model.
History
Peebles and others developed gravitational instability theory in the 1970s; Guth proposed inflation in 1981 to solve the horizon and flatness problems, and Mukhanov, Linde, and others showed it could generate the perturbations later seen in the cosmic microwave background and traced by galaxy surveys.
Debates
- Status of the inflationary paradigm
- Inflation elegantly explains many features of the universe and has passed key tests, but the many possible inflationary models and questions about predictivity and initial conditions keep its theoretical status under active debate.
Key figures
- Alan Guth
- Andrei Linde
- Viatcheslav Mukhanov
- James Peebles
- Yakov Zeldovich
Related topics
Seminal works
- guth1981
- peebles1980
Frequently asked questions
- What problems does inflation solve?
- Inflation explains why the universe looks the same in all directions even between regions that could not have exchanged signals, the horizon problem, why space is so nearly flat, the flatness problem, and the absence of certain relics, while also seeding the fluctuations that grew into structure.
- How did galaxies form from a nearly uniform universe?
- Tiny density variations, around one part in one hundred thousand, were present in the early universe; gravity caused denser regions to grow over billions of years, pulling in matter to form galaxies, clusters, and the filamentary cosmic web.