An amateur review of the beginning

Our speculations and imaginations have been truly impressive since the dawn of humanity, even when we just evolved from monkeys, which led us to conquer the most profound questions of the universe. A few questions became able to triumph over them, but we should also acknowledge that a vast cosmic ocean of different questions is always left to be unraveled. - Carl Sagan

Only a few tiny footprints of our universe are given to us, describing whether or not the universe has a beginning. With some existing clues, we tend to increase our ideas, and with those ideas, we come to a conclusion. In our grand quest to understand the significance of the Big Bang theory, we will take a quick tour of the theory!

Einstein's General Theory of Relativity revealed something so fundamental about spacetime that it could expand or contract depending on its geometry. In the beginning, it was believed to have some mathematical absurdities, and Einstein even tried solving them (one of them was later regarded as the biggest mistake of all time!). A Russian physicist, Alexander Friedmann, took this realization of space seriously. And what he came up with was a unique and never-seen-before possibility that our universe might just have started from the big bang. This realization was so revolutionary that physicists could only briefly defend the big bang theory with a mountain of reliable astronomical data and observations. But when Edwin Hubble observed something extraordinary happening in the cosmos and revealed his discovery of the expansion of the universe through the reddening of the spectrum of stars and galaxies out there at a constant rate in every direction, this all concluded something very unfamiliar about spacetime: that it could expand and it was expanding! It was the most absurd declaration of the 20th century, which baffled every astronomer and physicist. It was a never-before-thought-of experience for some of them.

At the time in the 1940s, the concept of the big bang came to light with the interpretations and solutions of the General Theory of Relativity by Alexander Friedmann. The Big Bang Theory's concept matched Hubble's observations. Later on, when some scientists, like Ralph Alfer and George Gamow, took their interest to proceed further with the journey of the Big Bang. They published a paper regarding "Big Bang Nucleosynthesis," popularly known as "Alpher-Bethe-Gamow paper." In this paper, they provided a backbone to this theory by introducing the term "Big Bang nucleosynthesis."

They said that the Big Bang would have created an infinite amount of matter and energy and that as space grew, subatomic particles came together to form hydrogen. When given enough heat, like in the cores of stars, hydrogen can fuse into helium. This discovery explained the abundance of hydrogen and helium, which had been a riddle to contemporary astronomers. Because when they looked around in the cosmos, they used to observe a lot of hydrogen, which makes up three-fourths of the visible cosmos. And again, helium was regarded as the second-most abundant element in the universe. This nucleosynthesis successfully supported the Big Bang Theory.

We've seen two elegant reasons to believe in the Big Bang theory. Now let's move further:

Our scientific society in the 1960s became a hypocritical and two-faced monster because some folks believed in Steady State Theory, and some considered the Big Bang Theory. The famous physicist George Gamow (who was an orthodox supporter of the Big Bang Theory) predicted that "There may be a phase of the universe when the universe was too hot that light emitted would not come out due to ultra-high density, but when the universe would cool, due to expansion, much enough then, it might have released photons that should be called as 'afterglow of Big Bang'. With the calculations, he found that photons released after 380,000 years after the bang was of ultra-high frequency or gamma photons that have traveled across spacetime and reached us; then, due to expansion, their wavelength might have increased, causing them to shift to the microwave spectrum. This was hypothetically named "Cosmic Microwave Background."

Later, in 1964, Arno Penzias and Robert Wilson accidentally detected that. That relic of the Big Bang was something no one could ignore. With this, steady state theory got its final blow, as it was confirmed that the universe was different from earlier times. Despite that, it was hotter, smaller, and more compact.

The far more idealistic reason we believe in the Big Bang could be the absence of multi-wavelength quasars. You would probably ask, "How?"

To be clear, the farther we look in space, the farther we look in time because we detect events with light, which is an absolute measurement of the universe's speed. This is how we correlate space with time. Now, as you asked, why can't quasars exist around ours? Hey, what's a quasar?

Quasar, by its definition, is simply a galaxy with extreme luminosity due to superactive galactic nuclei. So far, we have consistently observed a supermassive black hole, which stabilizes a universe. When a star passes around it, generally due to its immense gravitational pull, it attracts its matter, generally in the form of plasma, eventually devours the whole star. When it starts feeding the case, it becomes violently spectacular, which makes it fatal for all life forms as it releases x-rays and sometimes, gamma rays and super relativistic jets or cosmic rays too. As it feeds on, its mass increases, and eventually, the event horizon grows even larger.

If we look at a quasar, then we can say that in a galaxy where matter is packed in a congested manner and when a supermassive black hole stays at the center, as is evident, then it is probable that it would continuously devour all the matter, releasing an immense amount of high energy radiation. But we don't have any when we still look all around to find any of these active galactic nuclei galaxies. They only exist significantly farther distance, that is, about billions of light years far. That means only billions of years ago. Quasars were persisting in the cosmos. As Big Bang theorists manipulated this statement, they tried to add their answers: "As the universe was too hot, too small, and too compact, galaxies existed too close to each other. There could have been a lot of gas and matter to feed on at these active galactic centers."

Of this cause also, The Big Bang was supported and became reasonable: "Why should one believe in The Big Bang?"

Now, let's look at another reasonable question by which two marvel theories of gravity of its times have got it wrong; that is - Olber's Paradox.

 Why is the night sky so dark? Can't there be endless stars to illuminate them?

Have you ever wondered, even in your childhood, why the night sky is so dark? Only miserable stars with twinkling lights, a silent moon, and a few planets. Why not our night sky even look brighter due to stars twinkling like they are on each and every section of the sky? Why not our night sky receive infinite radiation due to the infinite amount of stars? Many uprising scientists like Johannes Kepler of those times said that our universe is uniform and infinite.

Allow me to explain.

If we start gazing at any point in the night sky, then, as the universe is infinite, our line of sight must have the possibility that it must require that a countless number of stars fall in that line of sight only. This simple analogy unravels and raises many questions about the nature of the cosmos. To answer this, a straightforward answer might be that dense clouds and nebulae might have blocked the light from other stars. But think to yourself: if that cloud were bombarded with radiation from infinite stars, which we have assumed to be invisible, then it would also glow with a glare like the surface of a star.

Another reasonable statement is that as the universe is so vast and as distance increases, light intensity decreases too. Well, it's true, but it's not the current reason we were asked for. With this answer, we tackle only those who don't think about something too deeply. Because if the universe were stated as uniform and infinite, then distances would have been canceled out due to the infinite number of stars jiggling, flickering, and twinkling all over the night sky.

So, what was the correct answer to this riddle?

The solution to this riddle was discovered in 1901 by Scottish physicist Lord Kelvin. He reasoned that when we look at the night sky, we look at it in the past, not as it is now, because the speed of light is not instantaneous. He calculated that the universe would have to extend hundreds of trillions of light-years for the night sky to be completely white with twinkling stars, as imagined earlier. But because the universe is not trillions of years old, that's why the night sky is black.

Or

The second contributing reason to this paradox was the finite lifespan of stars, which is generally measured in billions of years, not trillions.

"The universe is not only queerer than we suppose; it is queerer than we can suppose."

Big Bang Theory relies on the scientific method of speculating something and then predicting something. When it matches our observations, we proceed with the theory even further. This is how the Big Bang hypothesis emerged in the 1960s, with the prediction of the cosmic microwave background, which is the entire backbone and evidence of the Big Bang Theory. At the time, physicists worldwide debated the universe's fate based on data derived from Alexander Friedmann. There was a vigorous debate in astronomy about whether our universe would die in the "Big Crunch" or the "Big Freeze." At the same time, MIT graduate Alan Guth came out with a new theory named "Inflationary Theory."

The concept of hyperinflation was revolutionary and hotly debated, and it supports the big bang theory. But just as a lame person can help another lame one, this inflation theory was proposed as a hypothesis, but it reveals and solves some key concepts that were trickier to understand. One of the major problems it has tackled is the Flatness problem. The standard picture of the big bang could not explain why the universe is so flat as it seems.

Astronomers (at the time before the proposal of inflationary theory by Alan Guth) observed that the universe's curvature was remarkably close to zero, and they were even surprised by that. Guth explained that the universe underwent vigorous inflation of space and time with speeds much more than the speed of light. He also reasoned that Big Bang was nothing like supernovae or hypernovas explosions. Still, the inflation of space and time gave birth to the universe as we perceive it. He speculated the universe as a balloon that underwent hyperinflation at the instant when time, as we know it, started with a bang. He speculated that we, along with our long-range telescopes, were like the microbes on them. To us, the universe seems flat (unlike 2D paper; it's 4D spacetime that appears to be flat), and the big picture is different. Inflation has stretched spacetime so much that it now seems flat. The main reason for referring to inflation as the "Big Bang Theory" is that it involves the physics of microscopic objects, i.e., Quantum Theory and the Standard Model. It even tries to approach the quantum uncertainty principle, which states that any fluctuation in spacetime could be random, which might have led to hyperinflation.

Not only did the inflation explain the data supporting the flatness of the universe, but it also solved the Horizon problem.

Again, the question arises from the simple realization that the night sky seems relatively uniform, no matter how you look. You take the minor patch of the night sky and observe the amount of matter present, then compare it with another patch. You would be surprised that it is the same density all over there. But how is that even possible if nothing can speed up faster than light and how information travels so accurately all over the night sky? It is a tricky thing that needs consideration.

Just look at this picture. You see our baby universe when it became transparent, or in simpler words, when photons could travel to us through microwave radiation. It's a map created using data from the Planck telescope, which detected microwave radiation to the point where we can see the density of matter in the universe. Hey, did you observe something? Something extraordinary? Yeah, it is uniform in density, even when you look at it. You are seeing matter which is thousands of degrees Celsius. Why is matter (generally in the form of plasma at that time) so even distributed to each region of spacetime? The universe shouldn't have gotten a break, mixing their temperatures uniformly like coffee cup cream.

Let's have a look at it!

So, let's get to the point: when you look at the two opposite ends of this cosmic microwave background picture, you will see they are uniform. However, according to big bang theory calculations, these opposing ends could have been separated by 90 million light years due to the expected expansion of spacetime theorized by Georges Lemaitre. But there is no way light or information could have traveled by 90 million light years in just 380,000 years! This was even trickier than the Flatness problem. Alan Guth's Inflationary theory came into this situation. It explained that after just the big bang, hyperinflation had started extending the universe's size at an unbelievable rate, much faster than light, as spacetime is not like any particle with a rest mass. This inflationary theory is terrific!

But one question Alan Guth has also been trickled with:

How to stop inflation, and what created such hyper-expansion of the universe?

There we leave you left amazed for next time! 

 By Adityadhar Dwivedi (MS22265)