We all belong to the universe. However, we are neither sure of its size nor its definition. We use the terms such as cosmos, world or nature interchangeably to denote universe. But the universe is much more than these words could convey.
The Universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Therefore, in comparison with the planet earth, the universe is very vast. Like our solar system, which is composed of several planets, the Sun and some other bodies; several such formations like our solar system make a galaxy like ‘Milky Way’. It is thus a cluster of billions of stars. And the universe is made of several of such galaxies. This has been the revelation of centuries long research and observation.
Since there is no recorded history of the creation of the universe, there are different models of the universe. One of them is the prevailing scientific model of the universe, known as the ‘Big Bang’, the universe expanded from an extremely hot, dense phase called the Planck epoch, in which all the matter and energy of the observable universe was concentrated. Since the Planck epoch, the universe has been expanding to its present form, possibly with a brief period (less than 10−32 seconds) of cosmic inflation. As space expanded, the universe cooled and matter formed.
Recent observations however, indicate that the universe is believed to be mostly composed of dark energy and dark matter, which is responsible for the process of acceleration of this expansion of the universe. And it is said that most of such matters called ‘dark matter’ in the universe is in a form which cannot be detected by present instruments. These are poorly understood at present. The imprecision of current observations has thus hindered predictions of the ultimate fate of the universe. Therefore, the universe has not been the same at all times in its history; for example, the relative populations of quasars and galaxies have changed and space itself appears to have expanded.
Mostly we agree that there are three elements of the universe such as space-time, matter-energy, and physical law.
According to general relativity, space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the universe is finite or infinite.
Current interpretations of astronomical observations indicate that the age of the universe is ranging from 11 to 20 billion years to 13–15 billion years, and that the diameter of the observable universe is at least 93 billion light years.
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