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Welcome to our Physics lesson on Dark Matter, this is the seventh 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.
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. Gravitational force that is responsible for the creation of galaxies does not justify the actual equilibrium of these systems. It is actually much smaller than the force necessary to make galaxies rotate around their centre.
For this reason, 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.
There are several hypothetic versions that try to explain the identity of this dark matter. It probably consists partially of small white dwarfs that are not observable or other extinct celestial bodies. Many not observable of the primordial Universe may partially be part of this dark matter as well. As an example in this regard we can mention neutrinos and antineutrinos detached from matter in the first second of the universe existence. Their number is very large, so they may produce a considerable gravitational effect. The same role is probably played by other primordial particles of the first instants of the Universe.
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. In this case, everything discussed so far will occur in the reverse way; the Universe will experience a colossal contraction until it reaches very small dimensions similar to those prior to Big Bang. This process is known as the Big Crunch.
On the other hand, if dark matter is less than believed, the Universe will continue expanding at infinity.
What is the minimum mass of universe if it is closed? Express the answer in solar masses. The actual mass of observable universe is Mu = 1.5 × 1053 kg and the mass of Sun as about Ms = 2 × 1030 kg.
The condition for a closed universe is that it must be slightly more massive than it actually is However, we take the actual mass of Universe as a lower limit for this condition. Hence, we have
You have reached the end of Physics lesson 22.12.7 Dark Matter. 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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