(This is something I wrote a while back for my University)
Take a look at the night-sky and what do you see? Stars, constellations, a moon, the occasional planet, maybe a shooting star, but everything else is dark. Theory and several observations suggest that the Milky Way, the galaxy in which our humble solar system lies, has millions and millions of stars. Not just that, we have observed several other galaxies with millions of stars. Then, one can ask, why do we not see all, and just a handful? If the universe was static, and infinite, as many believed it to be, then the night sky should be bright and full of wondrous stars, rather than dark abyss that we see. This contradiction between theory and observation is called the Olbers’ Paradox. This Paradox has many solutions, as is standard for paradoxes. Many of them are absurd, as is expected, but I shall focus on exactly one – The Inflationary Model.
Hundred years ago, Albert Einstein was formulating his General Theory of Relativity and the equation he formulated described a universe that is constantly expanding. At that time it was assumed that the universe was static, and not expanding, so Einstein introduced the Cosmological Constant, denoted by Λ (Greek: Lambda) to counteract this expansion. Around the same time Edwin Hubble discovered through observations of galaxies that the universe was expanding, as described by Einstein’s original equation sans the constant. According to George Gamow, Einstein called his failure to recognize the accuracy of his equations the “biggest blunder” of his life. Many assumed the cosmological constant to have a zero value, and this led to a conclusion that the expansion of our universe was decelerating. But observations of galaxies showed them receding away from us at an accelerated rate. This led to the cosmological constant to be brought back, and this constant was said to have a positive value to account for the accelerated expansion.
So, why is the universe expanding at an accelerated rate, and what is resisting and counteracting the attractive nature of gravity all around us? The answer is that something is pushing it and that something is Dark Energy. Dark Energy is this hypothetical “force” that exists in the form of negative pressure causing this accelerated expansion and works against gravity. This constitutes the so-called ΛCDM – Model (Lambda- CDM; and CDM is an abbreviation of Cold Dark Matter), which takes into account Dark Energy as well as Dark Matter.
Dark Matter is that hypothetical substance that occupies a large amount of space in our universe, and it is used to explain the gravitational effects (like gravitational lensing) of very large-scale structures, which cannot be explained by ordinary matter. Cold Dark Matter is a form of Dark Matter, which travels at speeds much smaller than the speed of light (hence the name cold). Dark Matter is described as “non-baryonic” that is to say that it is made up of elementary particles that are not protons or neutrons; dissipationless – that is it cannot cool itself by radiating photons; and collisonless – that is it interacts with each other, and ordinary matter through gravitational forces or the weak force, but not directly.
It is estimated that the total energy density in our universe has the following distribution: Dark Energy – 70%, Dark Matter – 25%, and Ordinary Matter (stuff we are made of) a mere – 5%.
The ΛCDM – model includes a single originating event – the Big Bang or a Singularity where there was no bang but a sudden and unexpected appearance of an expanding space time with a temperature of around 1027 K. The very next instant, about 10-29 seconds after it came into existence, it started expanding at an exponential rate and this is what is known as Cosmic Inflation. For the first several hundred thousand years it was very hot (around 10,000K) and this is detectable through the Cosmic Microwave Background (CMB) Radiation. Cosmic Background Radiation is what is observed in the microwave spectrum in the dark between galaxies where there are no stars.
This Inflationary Model tries to provide a solution to The Horizon Problem. Imagine standing somewhere in space. To your left, at about 10 billion light years away (1 light year is the distance traveled by light in a year) is a galaxy. To your right, again at 10 billion light years is another galaxy. Armed with the knowledge that the universe is 13.8 billion years old, one would assume that, since nothing can travel faster than the speed of light, the two galaxies would not have had any opportunity to communicate with each other as light could not have traveled sufficiently far to reach the other galaxy to transfer information. Here “information” refers to some form of physical interaction.
Now, let’s take something basic like heat transfer. The Zeroth Law of Thermodynamics states that heat from a hot body, keeps flowing into a cold body until they reach thermal equilibrium, and only if they are in thermal contact. That is heat flows from a hot body, to a cold body until they are at the same temperature and the bodies would have to be in some form of physical contact with each other, without which no heat transfer can happen.
The two galaxies that were mentioned before have never been in physical contact and light wouldn’t have travelled fast enough to transfer any information. So, one would expect these galaxies, and the whole universe to have different properties. But this is contrary to the observations made.
Our Universe is highly isotropic, meaning it has roughly the same properties throughout; and it is homogenous, which means that matter is spread quite evenly throughout. The CMB radiation that fills the universe is roughly the same temperature: 2.728 K. The difference in temperature is extremely minute and only recently has human kind developed the technology to detect these differences.Inflation helps resolve this problem.
The universe, at the very beginning was very small, very dense and was causally connected. It is at this stage that all the properties evened out and then there was a very brief period of exponential expansion, which led to an increase in the size of the universe by a massive factor. This didn’t eliminate any irregularities, but greatly reduced them.
This theory of Inflation (originally proposed by Alan Guth in 1980), though it manages to solve several problems that have plagued the field of cosmology, was not welcomed with open arms by everyone. Roger Penrose, a world renowned physicist, is one of the most vocal critics of this theory. He says that for this theory to be a valid explanation, the originating events must have had highly specific initial conditions, and this is otherwise known as the Fine-Tuning Problem.
Andrei Linde of Stanford, another major contributor to the Inflationary Model, proposed something called Chaotic Inflation, a more general theory of inflation (also called Eternal Inflation). He is also responsible for proposing the theory on how matter was created (in a process called reheating that took place right after the inflationary stage). Linde made a prediction that the inflationary model of the universe would inevitably lead to the creation of a multiverse. He suggested that the inflation will go on, in certain parts of the universe, endlessly and this will lead to creation of pocket universes that will be independent of ours. So, our universe, instead of being like a balloon, will be like a huge fractal.
The ΛCDM – Model and the Inflationary Model try to provide an explanation as to why the night sky is dark, instead of bright. The acceleration of the universe is causing something known as redshift. To put it simply, redshift is what causes the emitted light to increase in wavelength, hence pushing it beyond the visible spectrum and into the microwave range, which our eyes cannot register. This redshift causes the energy to reduce by a factor of 1100 and so the light fades into the Cosmic Background Radiation. This causes the night sky to look dark, and not bright, as theory would suggest. Hence providing one of the most beautiful, and plausible explanations to the Olbers’ Paradox.