We said that at this present moment, the universe is expanding. If the universe is gradually expanding as time goes by, the universe should become smaller and smaller if we go back in time. If we go back in time even further, then in the end, it becomes a single point. In fact, this is the beginning of the universe called the "Big Bang." We will talk about this in more depth in the section "The History of the Universe" but the most reliable theory now is that the beginning of the universe was a massive explosion from a single point.

Future Expansion of Universe

In the beginning, the force that inflated and expanded space, was the force of this massive explosion. This was the only explosion with such power; there has been no extra force added towards the expansion of space since the Big Bang. At the same time, space is continuously pulled inward by universal gravitation caused by the mass of matter in the universe. Thus, the universe today is expanding using only the force gained in the first massive explosion, and the force gained is used to work inversely to universal gravitation. In this way,it is possible to imagine that the universe is expanding while gradually decreasing its speed. Such expansion is called decelerating expansion, and cosmologists generally believe the universe today is on the phase of decelerating expansion.

Someday, perhaps the universe will use up all of the force from the Big Bang, and will not be able to oppose universal gravitation. The universe will contract due to its own gravity, and collapse in the end. Such conclusion is possible to happen.

Let's think about this question after we learn a bit more about the universe. If you want to think about the end, you must know the beginning. We will talk about the destiny of universe in future in "The Future Universe," section after we learn about the beginning of the universe in "The History of the Universe" section.

Accelerating Expansion

This is where we encounter some of the most recent findings in cosmology. We said that universe's expansion rate is decreasing in speed, meaning universe is expanding at a decelerating rate. However after analyzing the most recent observation results, it seems that the universe is accelerating. There was a time in the past when the universe was expanding at an accelerating rate. We will explain about this later, but it was during a time of inflation. In this case, the energy emitted from a vacuum was accelerating the expansion. Therefore, in order for the universe to accelerate its expansion, a force to encourage the expansion was necessary. It was gravity that changed the velocity of expansion, but because gravity is a force that tries to pull things together, it works against expansion. Therefore, the force that encourages expansion must be working opposite gravity.

Repulsive force

Gravity is a force when two matters with masses are attracted to each other. There is also a force between two matters that make them repel each other. This is called the repulsive force. If you bring the same two poles of two magnets (south pole and south pole, or north pole and north pole) toward each other, you will feel this repulsive force. The force works on two approaching matters, making them repel. Recently in cosmology it is said that, this repulsive force is affecting every bit of matter and it is thought of as the cause of universe's accelerating expansion. At this point in time, we know very little about this repulsive force. As a simple answer to the question "if universe had changed into accelerating expansion, what is its cause?" it is vaguely thought as the repulsive force, but it has not been proved that the repulsive force is the definite answer.

The concept of repulsive force that works in universe, came from the "cosmological constant" made by Einstein. As the man who created the theory of relativity, Einstein made a great step in the study of cosmology. In the first place, the theory of relativity was not made for cosmology, but as a simple law of physics. When he tried the theory of relativity on the universe, what a surprise! It indicated that the size of the universe was changing (In Einstein's time, the universe was believed to be still and there was no proof of universe expansion). Einstein himself strongly believed that the universe was still, so in order to solve the inconsistency, he remodelled the formula so that the force would not work towards expansion. What was added to the formula at that time was called the "cosmological constant." Einstein's idea was that, the repulsive force of the cosmological constant would keep the universe from crushing due to its own gravity.

The Hidden Repulsive Force

Then why was universe on the verge of decelerating expansion until now? If the repulsive force was working, then universe should keep on accelerating, right? The answer is easy. Until now, the repulsive force was overpowered by the force of gravity and could not be seen. This means that the force of gravity was so strong that the effect of repulsive force did not appear at all. Gravity that works between two matters is stronger if the distance between the two is shorter. The nearer they are, the easier it is to affect each other with their forces.

If universe is expanding, the distance between two matters will be longer and longer. In fact, the galaxies are moving away from each other. Thus, the gravity that works on the whole universe becomes smaller and smaller. Unlike gravity which reduces its power according to its parting distance, repulsive force does not weaken. Repulsive force works not on the matter itself but on the surrounding space. With gravity we say "this much gravity in relation to this much mass," while with repulsive force we say, "this much repulsive force in relation to this much space." Therefore, it does not matter if the distance between masses become longer due to expansion. The amount of repulsive force does not change. Actually, because the universe itself is increasing, the repulsive force must be increasing too. The decreasing gravity and increasing repulsive force are currently thought to be the reasons for the beginning of accelerating expansion.

The Age Problem

In the universe, there is the age problem. As you can guess from its name, it is a problem based on the age of the universe, which is said to be about 15 billion years old. If the age of the universe is 15 billion years, then the galaxies and stars within the universe must be younger. If by any chance, a galaxy or star older than 15 billion years was found, it would mean that the age of universe is incorrect.

A problem actually was found. From the observation of a globular cluster, it was discovered that a fixed star in that cluster had an age of about 15 billion years. Therefore, we must conclude that the age of universe must be older than 15 billion years, In determining the age of the universe, we must take into account whether or not the universe is contracting or expanding. In decelerating expansion, it is expected to be about 15 billion years old but it is difficult to imagine that the universe was born 15 billion years ago and, at the same time, the stars were born. But in accelerating expansion, the age of the universe is a no problem because the age would be over 15 billion years old. Because of these questions, the theory of the accelerating expansion of the universe is strongly supported.

But due to new developments in observational technology, scientists know that the actual age of globular cluster is about 12 billion years. So with the new Hubble constant gained from the values gained through observation, it could be possible to solve the age problem without assuring the accelerating expansion of the universe.

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• 10^-34? - How large was the universe about 10^-34 seconds after the Big Bang? (Change and Delete)
• What will happen in the future? - What are the speculations about the future of the universe? (Change and Delete)
• Cosmological constant - How does the discovery of the cosmological constant relate to missing mass and an open universe? (Change and Delete)
• Is the infinite universe expanding? - How can an infinite universe expand? (Change and Delete)
• Theories of the early universe - The great excitement of modern cosmology comes from the confrontation of theoretical ideas with a rapidly growing body of data. (Change and Delete)
• Extragalactic astronomy - The main emphasis of the extragalactic group is in infrared,thus investigating the history of star formation in the universe. (Change and Delete)
• What came before the Big Bang? - The big bang is just one of many inflating bubbles in a spacetime foam. (Change and Delete)
• Can stars be older than universe? - How can the oldest stars in the universe be older than the universe?Determining the age of the oldest stars requires a knowledge of their luminosity, which depends on their distance. (Change and Delete)
• Age of the universe - There are at least 3 ways that the age of the universe can be estimated. (Change and Delete)
• Hot big bang - About ten billion years ago, the universe began in a gigantic explosion - the Hot Big Bang. (Change and Delete)

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