THE EXPANDING UNIVERSE AND THE BIG BANG

Firoz Ahmed

B.Sc 5^th Sem(Major)

This is a question that has haunt everyone of us atlaest once in our lifetime: Does the sky that we see, has any limit? Is the universe finite, does it have any limiting boundary? Or it has no limit. Has it always existed, or did it come to being sometime in the past. The scientific answers to these questions were given by the American astronomer Edwin Powell Hubble. In 1929, Hubble discovered that the universe is expanding.

Now, what really is the expansion of universe? Well, it is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion in which the scale of space itself changes. The expansion of the universe is different from the expansions and explosions seen inside the universe. It is a property of the universe as a whole rather than a phenomenon that applies to just one part of the universe and, unlike other expansions and explosions, cannot be observed from outside of it.

The ancient Greeks recognized that it was difficult to visualize how an infinite universe might look like. At the same time, they also wondered if the universe were finite, what its size would be? The Greeks’ problems with the universe represented a paradox: The universe had to be either finite or infinite.

In the early 1800s, German astronomer Heinrich Olbers argued that the universe must be finite. According to Olbers, if the universe were infinite and contained stars throughout, if we looked at the sky in any direction, or line-of-sight would eventually fall on the surface of a star, and therefore, the whole surface of the night sky would be as bright as a star. But in reality, we see dark areas in the sky,so the universe must be finite.

But , after the discovery of the laws of Gravity by Sir Isaac Newton, it was realized that every object in the universe attracts every other object. Now, the question arised, if the universe truly were finite, the attractive forces of all the objects in the universe should have caused the entire universe to collapse on itself. This clearly had not happened. So, astronomers came across a paradox.

The Discovery of the Expanding Universe:

In 1929, Edwin Hubble, working at the Carnegie Observatories in Pasadena, California, measured the redshifts of a number of different galaxies. He also measured their relative distances by measuring the apparent brightness of a class of variable stars called Capheids in each galaxies. When he plotted redshift against relative distance, he found that the redshift of distant galaxies increased as a linear function of their distance. This experimentally proved that the universe is ever-expanding.

Most recently, by comparing the apparent brightness of distant standard candles to the red shifts of their host galaxies, the expansion rate of the universe has been measured to be H_o= 73.24 ± 1.74 (km/s)/Mpc. This means that for every million parsecs of distance from the observer, the light received from that distance is cosmologically redshifted by about 73 kilometers per second.

Once scientists understood that the universe was expanding, they immediately realized that it would have been smaller in the past. At some point in the past, the entire universe would have been a single point, later called the big bang, the beginning of the universe as we understand it today.

The universe was born with the Big Bang as an unimaginably hot, dense point. When the universe was just 10^-34 of a second old —it experienced an incredible burst of expansion known as inflation, in which space itself expanded faster than the speed of light. During this period, the universe doubled in size at least 90 times, going from subatomic-sized to golf-ball-sized almost instantaneously.

According to NASA, after inflation, the growth of the universe continued, but at a slower rate. As space expanded, the universe cooled and matter formed. One second after the Big Bang, the universe was filled with neutrons, protons, electrons, anti-electrons, photons and neutrinos. Roughly 380,000 years after the Big Bang, matter cooled enough for atoms to form during the era of recombination, resulting in a transparent, electrically neutral gas, according to NASA. This set loose the initial flash of light created during the Big Bang, which is detectable today as cosmic microwave background radiation. However, after this point, the universe was plunged into darkness, since no stars or any other bright objects had formed yet.

Although the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity, about 5 or 6 billion years after the Big Bang, according to NASA, a mysterious force now called dark energy began speeding up the expansion of the universe again, a phenomenon that continues today.

The Big Bang did not occur as an explosion in the usual way one think about such things.. The universe did not expand into space, as space did not exist before the universe, according to NASA Instead, it is better to think of the Big Bang as the simultaneous appearance of space everywhere in the universe. The universe has not expanded from any one spot since the Big Bang — rather, space itself has been stretching, and carrying matter with it.

The universe is in a state of constant change and it is ever expanding. The new idea of the expanding universe based on modern physics, laid rest to the paradoxes that troubled astronomers from ancient times until the early 20^th Century