One of the most interesting questions considered by astrophysicists deals with the start of our universe. Indeed, there is a great deal of speculation on the subject, with different theories about how the universe began, and what may have existed before the universe came into being.
Science has advanced to the point where we can infer something about the entire universe. This has been a great challenge considering how unimaginably vast the universe is. The countless stars you see in the darkest sky constitute merely 3000 neighbors out of about 300,000,000,000 stars in our galaxy, and as many as 100,000,000,000 galaxies exist in the universe. Humans have always wondered: Has the universe always existed like we see it now, or did it somehow start all of a sudden? In the beginning of this past century, we found out in amazement that the entire universe is expanding. This led physicists to deduce that the universe started out in the finite past with a minuscule size. Realizing that the universe had a beginning, and awed by its vastness and its creations, people have asked: How did the universe begin? After all, we are here to be amazed by it because the universe eventually created lives like us. Now, after decades of observing and thinking, we have come to answer confidently the question of the origin of our universe... with what is known as the "big bang".
What is the Big Bang?According to the big bang theory, the universe began by expanding from an infinitesimal volume with extremely high density and temperature. The universe was initially significantly smaller than even a pore on your skin. With the big bang, the fabric of space itself began expanding like the surface of an inflating balloon – matter simply rode along the stretching space like dust on the balloon's surface. The big bang is not like an explosion of matter in otherwise empty space; rather, space itself began with the big bang and carried matter with it as it expanded. Physicists think that even time began with the big bang. Today, just about every scientist believes in the big bang model. The evidence is overwhelming enough that in 1951, the Catholic Church officially pronounced the big bang model to be in accordance with the Bible.
Until the early 1900s, most people had assumed that the universe was fixed in size. New possibilities opened up in 1915, when Einstein formulated his famous general relativity theory that describes the nature of space, time, and gravity. This theory allows for expansion or contraction of the fabric of space. In 1917, astronomer Willem de Sitter applied this theory to the entire universe and boldly went on to show that the universe could be expanding. Aleksandr Friedmann, a mathematician, reached the same conclusion in a more general way in 1922, as did Georges Lemaître, a cosmologist and a Jesuit, in 1927. This step was revolutionary since the accepted view at the time was that the universe was static in size. Tracing back this expanding universe, Lemaître imagined all matter initially contained in a tiny universe and then exploding. These thoughts introduced amazing new possibilities for the universe, but were independent of observation at that time.
Why Do We Think the Big Bang Happened?Three main observational results over the past century led astronomers to become certain that the universe began with the big bang. First, they found out that the universe is expanding—meaning that the separations between galaxies are becoming larger and larger. This led them to deduce that everything used to be extremely close together before some kind of explosion. Second, the big bang perfectly explains the abundance of helium and other nuclei like deuterium (an isotope of hydrogen) in the universe. A hot, dense, and expanding environment at the beginning could produce these nuclei in the abundance we observe today. Third, astronomers could actually observe the cosmic background radiation—the afterglow of the explosion—from every direction in the universe. This last evidence so conclusively confirmed the theory of the universe's beginning that Stephen Hawking said, "It is the discovery of the century, if not of all time."
Expansion of Universe -
In 1929, Edwin Hubble discovered that farther galaxies are going away from us at higher speeds, proportional to their distance. In other words, the spectra of more distant galaxies had higher redshifts. From distant galaxies, light takes millions or even billions of years to reach us. This means we are seeing an image from millions or billions of years ago. In redshift, the spectrum is shifted from shorter wavelength to longer wavelength as the light travels from the galaxy to us. This increase in wavelength is due to expansion of the very fabric of space itself over the years that the light was traveling. If the wavelength had doubled, space must have expanded by a factor of two. Thus, Hubble's discovery was that this expansion factor was roughly proportional to the distance light traveled, or equivalently, to how far back in time you looked. This means that the universe was smaller and smaller earlier and earlier. The universe has been expanding.
Conclusion -The 20th century saw a giant leap in how humans perceive the cosmos. No longer did people assume that the universe was static in size. By looking at how distant galaxies recede from us, we learned instead that the universe is expanding in volume. Tracing the expanding universe backward in time, we imagined a dense, hot beginning of our universe in a finite past. In the middle of the century, we found out that the nuclear reactions in this hot early universe accurately account for the previously mysterious abundance of helium and deuterium. Moreover, we detected a faint afterglow of the big bang that occurred billions of years ago. That the universe began with a big bang is essentially conclusive and may stand as the most profound discovery humans have ever made.
The big bang, however, is merely a global description of the origin of the universe. Today, particle physicists have consistent theories about the history of the universe down to only a trillionth of a second after its birth or even earlier. They can test their theories experimentally with particle accelerators that can simulate events involving enormous energies similar to the condition at the beginning. To learn more about how exactly the universe began, physicists must develop a theory that works at even earlier times after the big bang. Such theory must combine both the general relativity (because of the extreme gravitational field at the beginning) and quantum mechanics (because of the extreme compactness of the universe at the beginning). The goal of physics today is to develop this quantum theory of gravity so that we may one day understand what exactly happened around the moment of the big bang to get the universe started.