Formation of Galaxies and Solar System. Actual Problems Revision Notes

In these revision notes for Formation of Galaxies and Solar System. Actual Problems, we cover the following key points:

• How were the galaxies formed? When did this process begin?
• How did the density fluctuations affect the formation of galaxies?
• How cosmic radiation helps us study the Universe?
• How did the Solar System emerge? How much did this process last?
• What is the Observable Universe? How do we know that there are other things beyond it?
• What is the inflation phase? When did it occur?
• What is the photon-matter particles ratio in the universe? What does this mean?
• Why the universe contains more matter than antimatter?
• What is dark matter? Why it is called so?
• What is the dark matter - visible matter ratio in the universe?

Formation of Galaxies and Solar System. Actual Problems Revision Notes

There are many models that try to explain the process of galaxies formation, but all of them have one thing in common: they rely on the process of increase in matter fluctuation.

Since gravitational force in some denser regions universe is greater, they start collecting matter from the surrounding space. As a result, they become even denser with time at the expense of the surroundings, which on the other hand becomes less and less dense, losing matter continuously until only vacuum is left.

One of the most reliable fluctuation-based models of galaxies formation is that of gradual growth of fluctuations. According to this model, the expansion of Universe favours certain fluctuations while eliminating the others.

There are two variables to be considered when dealing with growth of fluctuation: one is the Universe expansion that brings a decrease in matter density and the other is the gravitational attraction that increases the density. After some time, these opposing effects balance each other. As a result, the new structures created (the gas cloud and its components) detach from the rest of cosmic gas. After this critical moment, the galactic cloud collapses as gravitational force prevails over the expansive effect because the expansion process has already stopped. In this way, the cloud takes an elliptical shape while rotating around itself, which eventually takes a disc shape except the central region. Moreover, a number of smaller fragmentations containing dense matter occur inside this structure that eventually form the stars.

Careful observations of cosmic radiation made in 1990s have confirmed that it is not completely isotropic, as it was previously believed. Its density changes very slightly when measured in various directions. This difference in density is not more than 1/10000.

Initially, Solar System has been just a cosmic cloud (or dust) originating from Milky Way galaxy. After attracting matter from surroundings, it began to collapse about 4.6 billion year ago. However, this dust did not contain only hydrogen and helium; it also contained heavier elements ejected from other stars of the galaxy, which now are found in various celestial bodies of the solar system.

The Big Bang theory is insufficient at a certain extent when trying to explain the perfect order in the Universe. This is because we can only see or detect EM waves coming from sources that are closer or equal to 13.7 billion light years away from us. This part of Universe is called the Observable Universe.

Scientists introduced the concept of inflation phase to explain discrepancies in the Big Bang theory. Inflation phase is believed to have occurred between 10^-38 s and 10^-30 s after the Big Bang when the Universe experienced a symmetry break, in which the electroweak and electro-strong interactions separated from each other. This is similar to the phase change from steam to water. At this instant, cosmic bubbles of a new phase began to appear in the cosmos. They enlarged very quickly including a space where much larger regions than those we can observe today were included.

At the first instants of its existence, the Universe contained same number of positive and negative charges. However, when the process of the number of particles freeze took place at the first second after the Big Bang, a number of extra matter particles survived over antimatter, creating a misbalance between them. The most plausible explanation for this issue is the lack of symmetry in weak interaction.

During the annihilation processes of matter and antimatter, a very large number of photons were produced. The ratio of photons to matter particles in the Universe is related to the original particles-antiparticles difference at the first second of the Universe. This ratio is about 10⁹:1 (one billion to one). This means that today there are about one billion times more photons than particles of matter in the Universe.

Studies carried out in the recent years have found that the Universe contains more matter than we can observe in the form of visible light emitted from bright stars. This is because the actual gravitational force produced by the visible universe is not sufficient to make the galaxies rotate around themselves. To explain this phenomenon, scientists have introduced the idea of the existence of an invisible matter, otherwise known as "dark matter", not detectable by our actual devices. It provides the extra gravitational force required for this process. Calculations have found that the amount of this invisible matter is believed to be about 100 times greater than the visible (or detectable) one.

The amount of dark matter determines whether Universe is open or close. If the amount of dark matter is too large the Universe is closed while if the amount of matter is less than believed, the universe is open. If the universe is closed, it will stop expanding after some time and then, it will start contracting. On the other hand, if dark matter is less than believed, the Universe will continue expanding at infinity.