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Importance of gravitational force in the universe
Earth is a planet and it has mass. That’s why it also has gravity. Sun, earth, and moon have their gravity. There is also a mutual force of attraction in which the sun is attracting the earth towards its center and the Earth is also attracting the moon towards its center as well as the moon is also attracting the Earth towards its center.nIn this way, there is a mutual force of attraction among all the celestial bodies in the space. This force of attraction is called gravitational force. Due to this gravitational force, the earth rotates on its axis as well as revolves in its orbit. The moon also revolves around the Earth. Similarly, every celestial body in the solar system revolves around the sun. Gravitational force is responsible for maintaining the alignment of the planets in their orbits and making them revolve around the sun.
Study of the Universe
Have you ever thought about how the universe originated? What is the shape of the universe?
With similar questions, scientists have carried out various research about it at different times. Universe science is the branch of science that deals with the study of various facts of the universe including the history and future of the universe. One of its branches, Astrophysics, studies the origin, structure, and future of the universe.
Big-bang Theory
Our solar system lies at a distance of about 30,000 light years from the center of a spiral galaxy named Milky Way Galaxy having a diameter of about 100,000 light years. It is estimated mathematically that there are about 1.5 billion stars in this galaxy alone. Many theories about the origin of the universe have been proposed, the Big Bang theory is considered the most reliable theory. This theory is believed to have been born from the observation that all the galaxies along with the Milky Way are moving far away from each other with tremendous speed. According to this theory, the universe is believed to have originated from the big explosion of a single atom.
The four fundamental forces existing in the universe gravitational force, electromagnetic force, strong nuclear force, and weak nuclear force are considered to have unified as a single force before the explosion. Therefore, in the first stage of existence, the universe is considered to be in a very compressed state. At that time, the universe is believed to be in a very energetic state in the size of a small single atom. According to the big bang theory, due to excessive force and pressure, a huge explosion of the atom took place and the universe originated. All celestial bodies in the universe are believed to have originated from the explosion.
Just as the particles scatter and move away from each other in the explosion of an explosive, all the celestial bodies are moving far away from each other after the huge explosion of that dense atom. So, the size of the universe is also increasing day by day. Its expansion is slow but continuous. But, the speed of these bodies is decreasing due to the force of gravity.
Hubble’s Study
It has been discovered that the galaxies in space are moving away from each other. In 1929 BC, the American astronomer Edwin Hubble tried to calculate the velocities of various galaxies using the 100-inch Hooker telescope on Mount Wilson. During his research, he discovered that every galaxy is moving away from other galaxies. His estimation showed that the farther the celestial bodies are from the earth the smaller they appear, and also have a greater velocity.
He then plotted the velocities of the constellations against their distances on a graph and explained the relationship between the velocities between the constellations and their distances. This relation is represented by the equation v=Hd, and is known as Hubble’s law. Here, v represents the velocity with which galaxies are moving away from each other, d represents the distance between them and H is the Hubble’s constant. The value of this constant is 73 km/s/Mpc (Kilometer per second per Megaparsec). This means, if any two galaxies are at a distance of one mega parsec, they are separating away from each other with a velocity of 73 km/s.
If the distance between any galaxies is expressed in parsec and then multiplied by Hubble’s constant, the velocity with which these galaxies are moving away from each other is obtained. Based on such calculation, we came to know the fact that the galaxies are moving very far away very quickly. It can also be known from Hubble’s equation that the farther the galaxies are, the faster they are separating.
From this calculation, just as every dot moves away from each other as the size of the air-filled balloon increases, the size of the area occupied by the entire universe is increasing. It is clear that the constellations in the universe are moving away from each other. Therefore, it can be assumed that they were close to each other in the past, which in turn means they were all at the same point a long time ago. Thus, Hubble’s study helps to confirm the Big Bang theory.
Future of the Universe
As discussed above, all the galaxies in the universe are moving away from each other at a certain velocity and the size of the universe is increasing, but due to the gravitational pull of the masses in the universe, the velocity of all the galaxies is decreasing. As the distance between the bodies increases, the effect of gravity also decreases. Now the question remains whether the velocity becomes zero before the effect of gravity, the effect of gravity becomes zero before the velocity becomes zero, or both of them become zero at the same time. This determines the future of the universe.
According to Newton’s universal law of gravitation, every mass exerts a gravitational force on every other mass and they pull each other toward themselves. The magnitude of that force is directly proportional to the product of their masses and inversely proportional to the square of distance between their centers. Hence, it is obvious that the net gravitational force on any celestial body depends on the total mass in the universe and the volume of the universe. Since the ratio of mass to volume is called density, the net magnitude of the gravitational force depends on the average density of the universe. The average density of the universe required to stop the expanding universe, which could be after billion of years, is called the critical density.
Let Us Know
Open Universe
If the celestial bodies retain some velocity when the gravity becomes zero, they will then continue to move away from each other forever and the size of the universe continue to increase. This situation is considered an open universe. If the average density of the universe is less than the critical density, gravity cannot hold the expanding process and the universe which is continuously expanding now will continue to expand in the future too.
Flat Universe
If the gravity and the velocity of the celestial bodies become zero at the same time, the celestial bodies remain in the same position after that and the universe remains stable. Such a state is called a flat universe. If the average density of the universe is equal to the critical density, then the expansion rate of the continuously expanding universe gradually decreases and finally stops. Till then, there will be an infinite distance between each celestial body because of this they will not be affected by mutual gravitational force. Such universe is called a flat universe.
Closed Universe
If the gravitational effect remains non-zero when the speed of the celestial bodies becomes zero, the celestial bodies will start to come closer and the universe will come to a single point. The phenomenon in which the vast universe filled with unlimited mass and energy will at one point shrink and collapse is called the Big Crunch. If this happens, the universe created by the explosion of a single point will end up being a point and explode again to form a new universe like the present one. This means, just like the life cycle of a star, the age of the universe is fixed and it also has a life cycle. For the life cycle of the universe to continue, the Big Bang and the Big Crunch must keep appearing in turn. Such a universe is called a closed universe.
The above facts can be presented in graphs. Open, flat, and closed universes are named according to the nature of the graphs. The main factor that determines the future of the universe is gravity. The magnitude of gravity depends on the mass and size, i.e. density and the universe. The density of the universe is estimated based on the masses seen in space.
