before that small tangent at the end of the first post, i mentioned that hubble discovered that galaxies were moving away from each other. let me make a small clarification here: it isnt entirely true that all galaxies are moving away from each other. in fact the milky way and the andromeda galaxy, the biggest galaxy in our local cluster of galaxies, are moving closer together and will collide in a couple of billion years. it is actually CLUSTERS of galaxies, which are basically groups of galaxies held together by gravity with vast expanses of empty space between groups, that are moving apart. the reason for this is that at close distances like the distance between two galaxies in a cluster (remember, closeness is relative; we are still talking about MILLIONS of light years here) gravity plays a larger role in the motion of galaxies than the expansion of the universe does. however at a bigger scale, the expansion takes precedence and thus all the galaxy clusters are moving away from each other. thus, trillions of years from now, if we were somewhere in the milky way, the only stars we would observe would be the ones from our local group of galaxies, most of which presumably have collided due to gravity and combined to form one super galaxy. all of the other galaxies would be so far away and moving so fast (due to the universe's acceleration) that light from their stars would never reach us.
theres another clarification i need to make. the fact that a cluster moving away from us does not mean they are moving closer to another cluster beyond the first one. all clusters are moving apart from each other equally. if that doesnt make sense, then picture a balloon without air. imagine you take a marker and randomly make points on the skin of the balloon and then blow air into it. as the balloon expands, no two points get closer. the distance between every two point increases. and that balloon is the perfect analogue for the universe. just as the balloon is a two dimensional surface expanding in three dimensions, our universe is three dimensional space expanding in four dimensions, which is impossible to visualise but it is what it is. to make a further point using the balloon analogy, just as the surface of the balloon does not have a centre or an edge, neither does the universe. imagine a spaceship that can fast enough to traverse large sections of the universe (which, just to remind you, is impossible as nothing can travel faster than light and light itself takes four YEARS to get to our NEAREST star. there are also trillions of stars in just our OBSERVABLE universe), has infinite fuel and is somehow unaffected by gravity. if the ship travels in a perfectly straight line for long enough, it would eventually come back to earth. it would essentially 'circle' (or, since the universe is three dimensional, 'sphere') the universe and come back to its starting point like what would happen if one was able to run in a straight line across the earth. this fact is the basis of a rather interesting theory which ill touch upon later.
but ENOUGH WITH THESE TANGENTS. lets get back to the big bang. so hubble discovered that galaxy CLUSTERS were moving away from each other. if they were moving away from each other then naturally, unless they suddenly started moving randomly (which was unlikely), they must've been together at one point. so from the speeds of the moving galaxies, hubble calculated a point in time where all the matter in the universe was brought to a single point. his actual value was wrong because he wrongly estimated the speeds of the red-shifted stars but he built the foundation for later, more accurate, calculations which set the age of the universe at about 13.5 billion years.
this discovery sparked a lot of curiosity amongst cosmologists about the beginning of the universe. there was a flurry to form a set of theories which reconciled with hubble's observations that were the forerunners of the big bang theory. surprisingly however, a lot of astrophysicists disliked the idea of a 'beginning' to space and time, because of their religious/philosophical undertones, and instead chose to believe in the steady-state/static model (like einstein did before hubble merked him), proving that even scientists can make the error to sticking to their beliefs even when presented with contrary evidence.
the biggest confirmation of the big bang came in the second half of the 20th century when two physicists, arno penzias and robert wilson, began testing out a sensitive microwave detector in bell laboratories. they were concerned that the device was picking up more noise than it ought to. they checked it for malfunctions, cleared out bird droppings inside the device but still the noise persisted. it didnt come in any particular direction. it didnt change from day or night. it remained consistent as the earth revolved around the sun. they thus inferred that the radiation being detected was coming from beyond the solar system, and even beyond the galaxy, as it didnt change even when the motion of the earth pointed in different directions. about the same time that this machine was messing around with the pair, two physicists at princeton unfortunately named bob dicke and jim peebles were interested in microwave radiation because of a theory proposed by a dude named george camow that the early universe must have been very hot and dense, glowing with heat.
before i move on i want to explain another well known and interesting scientific fact that some of you might not know. as mentioned before, light is the fastest thing in the universe and it has a finite speed. because light takes a certain amount of time to reach us, when we see distant objects, we're not actually looking at what they are now. we're looking at what they were when the light first started to travel from them. for example, the sun is eight light minutes away from the earth (light takes eight minutes to travel from the earth to the sun). so if you look at the sun (use sunglasses!) youre not actually seeing the sun as it is now, youre seeing it as it was eight minutes ago. so if the sun suddenly vanished, you would still see the sun in the sky for eight minutes after it disappears. also you'd freeze to death after eight minutes because heat, which is transmitted as electromagnetic radiation like light, also takes eight minutes to reach us. if an alien from a galaxy a billion years away observed our solar system, all he would see is a primitive earth devoid of life. so in a sense telescopes work as time machines. when you observe an object far enough that light from it takes a significant amount of time to reach us, youre observing it in the past.
now dicke and peebles argued that if the universe was indeed glowing at a point in the past as camow predicted, we should be able to observe it as the light from that period should now be reaching us but we cant see the glow because the light would have been so red shifted that it would now appear to us microwave or gamma radiation (which have very long wavelengths). so they proposed to search for this radiation, which penzias and wilson had already unwittingly discovered. when p&w heard of d&p's work, they realised they'd already found the background radiation. penzias and wilson received the nobel prize for it, with dicke and peebles receiving only shit to eat.
the reason the discovery of cosmic microwave background radiation is considered a landmark event in the history of the formulation of the big bang was that the hot big bang model of the universe was the only theory that would explain the background radiation. since then, the big bang became pretty much the only game in town. later detailed observations of the background radiation correlated perfectly with the predictions made by the big bang theory, further setting in stone that it was here to stay and be rebuked for its awkward name by fundamentalists across the globe.