Universe, everyone familiar with this term defines it in a unique way. There is no observable boundaries to its extent, there is still a plethora of expeditions and discoveries to be done in it. Some people regard Universe as an empty space where all the cosmological objects float, but for a curious mind the defination may be different and perhaps more ellaborate. Such varying explanations on the true nature of Universe about its most fundamental property, its expansion has been startling the scientific community from a very long. Let's us dive deeper to explore the attempts made to clarify our understanding of Universe.

Before Relativity:
Newton and other scientist of his time, believed that the static universe would soon begin to contract under the influence of gravity. But instead of being static, what if the Universe would have been expanding? There are 2 possibilities in this case: If it was expanding at a very slow rate, gravity would eventually stop it and will begin contracting. Another approach says that if the universe is expanding at more than a certain critical rate, gravity would not be able to stop it and it will continue to expand forever.


Theory of Relativity and Expanding Universe
The behaviour of the Universe could have been predicted from Newtonian laws of physics, which directed towards the static nature of Universe. Even when in 1915, Albert Einstein developed the general theory of relativity he was of the opinion that the universe must be static and subsequently modified his theory to make this idea possible. He did so by introducing a Cosmological Constant into his equations. This was new antigravity force which didn’t came from any source but was built into the very fabric of space time. This constant gave space and time dimension an in-built tendency to expand, which can balance the attraction of all the matter in universe. Thus, leading to a Static Universe.


Image by Guillermo Ferla On Unsplash


FRIEDMANN’s Approach:

He gave two simple assumptions about the universe:
   1. The Universe looks identical from which ever direction we look
   2. This is true also if we were observing the universe from somewhere else.
On the basis on general relativity and above two assumptions, he showed that universe is not Static.





Incident which proved the accuracy of Friedmann’s assumptions

While working with sensitive microwave detector for communication with orbiting satellites, Arno Penzias and Robert Wilson, in 1965 detected some extra noise. This noise didn’t come from some particular direction, but was constant when tested in every direction. It was also not affected by day and night cycle and weather cycle, so was concluded to be coming from a farther (extremely farther, by calculations) distance somewhere in the observable universe. Since they were same in all direction, then universe must also be same in every direction. We should note that all the other galaxies are continuously moving away from us, it is quite reasonable to assume that we are at the centre of the universe. This universe might also look same when observed from any other galaxy. This proved Friedmann’s second assumption.


Inflating Balloon model of Expansion of Universe
Assume a balloon painted with numerous many colourful spots on it. As when a balloon is filled with air, the distance between two spots increases but none of the spot can be regarded as the centre of expansion. Moreover, the farther the balloons are, the faster they will move apart. Similarly, according to Friedmann’s model, galaxies are moving apart from each other. Also, the speed with which two galaxies are moving apart is directly proportional to the distance between them. It predicted that the red shift of the galaxy should be directly proportional to its distance from us.



Models obeying Friedmann’s fundamental assumptions
1.The Universe is expanding so sufficiently slow that the gravitational attraction between the different galaxies causes the expansion to slow down and eventually to stop. The galaxies then start to move towards each other and contracts. The distance between 2 neighbouring galaxies begins from zero, goes to maximum and return back to zero again. 2.The Universe in expanding so rapidly that the gravitational attraction can never stop it, though can surely slow it down a bit. The separation between two neighbouring galaxies starts from zero and eventually moves apart at a steady speed. 3.The Universe in expanding only just fast enough to avoid re-collapse. In this case also, the separation starts from zero and increases forever. Although the speed at which the galaxies are moving apart gets smaller and smaller, although it never reaches zero.
The universe is not infinite in space but neither has any boundary. Gravity is so strong that the space is bent round onto itself, making it quite similar to Earth’s shape. If you move on the surface of Earth, you come exactly where you started without any barrier or wedge. But when we talk about space, we have a finite limitation known as Time (the 4th Dimension). So, if one start anywhere in space, it is impossible to return to starting point before the collapse of Universe.

Which Friedmann’s Model describes our Universe?
To answer this question, we must first find out the present rate of expansion and present average density. To determine the current rate of expansion we can use Doppler’s Effect. By experimentation, we know that universe in expanding by between 5% to 10% every thousand million years. If we add the masses of all the starts of observable universe, the resultant mass will be one-hundredth (1/100) of the mass required to halt the expansion of the universe. But we also know that there exists a large amount of dark matter in our galaxy and between super clusters, detected by its gravitational influence in other orbiting stars. If we add the mass of dark matter as well, we still get only one-tenth (1/10) of the mass required to halt the expansion of Universe. Although there might be some form of matter existing in the universe which is till not detected and can add up in the average density. All these evidences give us the idea that our Universe will be expanding forever.

Further Aspects of Friedmann’s Model
From the solutions given by Friedmann, there must be some point in the history of universe when the distance between neighbouring galaxies was equal to zero. At that time, known as the Big Bang, when the density of the universe and the space-time was infinite. This suggests that there was some singular point in the Universe – also known as the Big Bang Singularity.