Models of galaxy formation are built upon several key assumptions:
1. Cosmic Inflation: The universe underwent a rapid expansion phase known as cosmic inflation shortly after the Big Bang, leading to the initial distribution of matter.
2. Dark Matter: Dark matter is a significant component of the universe, and it plays a crucial role in galaxy formation. It provides the gravitational framework for galaxies to form within.
3. Dark Energy: Dark energy is responsible for the accelerated expansion of the universe and affects the large-scale structure of the cosmos.
4. Gravity: Gravity is the force that governs the formation of structures in the universe. It causes matter to clump together, forming galaxies.
Now, let's discuss how a spiral galaxy is thought to form:
Formation of a Spiral Galaxy:
1. Initial Density Fluctuations: In the early universe, tiny density fluctuations existed due to quantum fluctuations during inflation. These fluctuations served as the seeds for structure formation.
2. Dark Matter Halo Formation: Dark matter, which doesn't interact with light or matter except through gravity, began to collapse under its gravitational influence. Dark matter halos formed, and they provided the gravitational scaffolding for galaxies.
3. Baryonic Matter Attraction: Normal (baryonic) matter, such as gas and dust, was drawn into the gravitational potential wells created by the dark matter halos.
4. Angular Momentum Conservation*: As the gas fell into these gravitational wells, it conserved its angular momentum, resulting in the formation of a spinning disk-like structure.
5. Star Formation: Within the disk, gas clouds cooled and condensed, leading to the formation of stars. These stars are distributed in a spiral pattern due to the conservation of angular momentum.
6. Spiral Arm Formation: Density waves, caused by gravitational interactions between stars and gas, give rise to spiral arms. These arms are regions of enhanced star formation and can appear as the distinctive spiral pattern observed in galaxies like the Milky Way.
7. Evolution: Over billions of years, spiral galaxies continue to evolve through processes like mergers, interactions with other galaxies, and ongoing star formation.
It's important to note that the formation of galaxies is a complex process influenced by various factors, and our understanding of it is continually refined as new observations and simulations become available.