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Welcome to our Physics lesson on Overview, this is the first lesson of our suite of physics lessons covering the topic of Formation of Galaxies and Solar System. Actual Problems, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
The picture we have provided so far regarding the first 300 000 years of Universe's existence illustrates a huge amount of gas containing hydrogen and helium which are uniformly distributed in the space around the position in which the Big Bang occurred. At the beginning of this stage this mixture of gas was at temperature T = 3000 K and it was in continuous expansion.
Now, after billions of years, the picture of the Universe is quite different. New structures such as galaxies that contain billions of stars, planets, natural satellites, etc., are present in the space, which on the other hand has expanded a lot since the end of primordial stage of Universe. Matter is no longer uniformly distributed; there are neutron stars in which the density reaches the values of atomic nuclei, but there are also large spaces where there is no matter. This empty space is much larger than the dimensions of galaxies. Galaxies have the tendency to gather in groups and super-groups, where each of these structures increases the distance from the others, making the structure of the Universe highly hierarchic. This results in a continuous expansion of the Universe.
The question that naturally arises at this point is: "How is it possible that from a homogenous Universe (as it was initially), it transformed to a highly partialized Universe as it is now? Maybe the atomic plasma was not as homogeneous as initially thought?"
The presence of structures with varying densities such as galaxies etc., excludes a Universe perfectly homogenous at the initial stage of its existence. The homogeneity must have been either deformed or only partial. These small deformations have acted as "seeds" for later structures we now know. In other words, regions where the density of gas was slightly higher than in the surroundings have acted as centres of new celestial structures such as galaxies etc. This is because gravitational force (that depends on the mass of objects) in these regions, was greater than in the surroundings due to higher density (i.e. higher mass concentration). As a result, matter from the surrounding gathered around these regions, forming the galaxies we now know. All actual models that try to explain the formation of galaxies are based on this assumption.
You have reached the end of Physics lesson 22.12.1 Overview. There are 7 lessons in this physics tutorial covering Formation of Galaxies and Solar System. Actual Problems, you can access all the lessons from this tutorial below.
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