Pre-post comment: This is the first, in what would *hopefully* be a series of posts on standard problems (and proposed/accepted solutions) in Cosmology. It’ll explore what we know about the Universe (a sizeable amount) and what we don’t know about the Universe (a whole awful lot) and essentially drive home the point that we as a species are insignificant in the grand scale of things and therefore, should stop acting like complete imbeciles to Planet Earth and Each Other.
Also, physics, of course.
So let’s begin.
A good storyteller knows the exact point from which to begin a story. Me? I’ll start at the beginning. Or at least as close to the beginning as we can get before the current known laws of physics meet their end in a stunning blaze of incomprehension. A common (and misplaced) criticism of the Big Bang theory is that it seems to imply that the universe came out of nothing. When we talk about the Big Bang theory, however, we are actually talking about the history and evolution of the Universe starting from the time when the laws of physics that we know today became relevant. This is roughly 10-43 seconds after the initial singularity. Therefore, the Big Bang theory says nothing about the creation/birth of the universe. Using it to explain the origin of the universe is like using a fork to eat soup: messy, inefficient and quite frankly, not the reason forks were invented.
Now let’s come back to Earth for a second. In the Dark Ages, it was believed that Earth, and by extension mankind, was the center of the universe. (And although I’d like to believe that we have since come a long way, recent events provide a slew of uncomfortable evidence to the contrary.) This idea was challenged by the Cosmological Principle which says that the universe is homogeneous and isotropic with no particular privileged point anywhere. So when astronomers discovered that far-off galaxies seem to be moving away from us, they were rightfully puzzled. This kind of movement suggested expansion and expansion implied, cursorily at least, a center. However, since Earth is most definitely not the center of the Universe and humankind is definitely one with a penchant for hubris, a second and more scientifically sound explanation was offered. The Cosmological Principle would still be valid in a dynamic universe if the velocities of arbitrary objects are proportional to their position. This would ensure that the universe would continue to remain homogeneous even if it expanded. (Consider stretching a rubber sheet. It expands even though it has no center.)
This proposal was first experimentally tested and verified by Hubble. So we know for a fact that the universe is expanding. Which means that far in the past, the universe was much smaller and denser. The Big Bang Theory details this expansion and cooling down of the universe, along with explaining the formation of various structures (galaxies, stars, nebulae etc) that we see in the universe today.
In the spirit of keeping these posts short, that’s all for today. Up next: looking into the past with the CMB and reminiscing about the good old days before the development of intelligent life.
Until next time.
 At cosmological scales (very, very large scales) a major component of this velocity is because of the expansion of the universe. The ‘motion’ of these objects gives rise to so-called ‘peculiar velocities’ which become more and more insignificant as the distance (or scale) increases.
 Hubble’s results were biased because of certain errors in his assumptions but proved to be a valuable starting point for future experiments. Today, much better experiments have independently verified this relation between distance and velocity.
 The early universe was also very hot. How do we know this? It has to do with a process called Nucleosynthesis which is responsible for the formation of various elements. In the Cold Big Bang Scenario, the formation of Hydrogen is suppressed relative to that of heavier elements. However, the present universe has an abundance of Hydrogen. This anomaly disappears in the Hot Big Bang Scenario.
Post-Post Comment: If you think that this post reeks of thinly veiled aggression/cynicism, it’s because I’m infuriated by the persecution of the Rohingya Muslims in Myanmar and the apathetic response it has garnered from nations worldwide. It saddens me that I live in a world where such massive human rights violations have become commonplace and people everywhere continue to remain aloof and passive in the face of these disgusting atrocities.