We have pretty good evidence of The Big Bang (which is that at one point of time our universe was confined in a much smaller volume than today and has been expanding ever since). As elaborated in [[Demystifying the first three minutes of the universe]], we have multiple independent lines of evidence for it: - Light from all galaxies is redshifted, which suggests everything is moving away from us - Observations from the Cosmic Microwave Background (CMB) today match with what we should expect if the Big Bang happened - Measured abundances of helium and hydrogen today matches with what we should expect if the Big Bang happened Here's a rough timeline of what happened during the Big Bang: ![[timeline.jpg]] (via [here](https://slideplayer.com/slide/8565087/)) **How we can we time these events so precisely?** Essentially, we time these by the relation that the smaller the universe, the hotter it will be (since the radiation we see in CMB is much cooler given that the universe is much bigger now). So, effectively we have a relation between the size of the universe, the temperature (or, equivalently, energy) and how long ago from today in time are we talking about. Plus, we have a relationship between time in the past and size of the universe from the Hubble Constant (which tells us how far away everything in our universe is moving from one another). The universe was hot enough until 380k years for electrons to remain unbound from nucleus, so the universe was in a state of plasma which is opaque. Only when the universe cooled did the electrons started binding with nucleus to form the first atoms. This is when the universe started becoming transparent and radiation started streaming freely. This radiation is precisely what we observe in CMB today (although it's much less energetic because space has been expanding ever since). We can go even further back. It is hypothesized that all the four forces were unified and, as temperature started becoming cooler, forces started getting separated one by one. First, gravity split off, then the strong nuclear force and then finally electromagnetic force and the weak nuclear force become separate. We have pretty good evidence (see [this](https://www.physicsforums.com/threads/experimental-confirmation-of-electroweak.522817/) and [this](http://www-pnp.physics.ox.ac.uk/~barr/mphys/notes4_2014.pdf)) from various particle accelerators for the unification of electromagnetic force and the weak force (called the [electroweak force](https://en.wikipedia.org/wiki/Electroweak_interaction)). This unification is an integral part of the standard model. (See [[Demystifying gauge theories and standard model]]). Based on success of the electroweak unification, it's expected that other forces will also unify but the energies needed to test the grant unification is beyond what's accessible from current (or even near term future) particle accelerators. ### Three unexplained mysteries of the Big Bang The Big Bang is pretty well-validated from multiple lines of evidence. But there are three mysteries that it is unable to explain well: #### 1. Distant parts of the universe have a remarkably similar temperature (energy density) We can observe objects that are much further away than the age of the universe. That is, we can measure objects farther than 13.8 billion light years away. Which means one of two possibilities: a) the universe has expanded at a steady rate but at the Big Bang, universe was much larger than a point size; b) the universe has expanded at different rates, and because the universe expanded at a much faster rate initially, the universe could have been much smaller at the Big Bang. The trouble with explanation “a” is that from the CMB, we measure a very uniform average temperature of 2.75 Kelvin even for places that would have been very distant from each other at the Big Bang. So, how would two such distant places end up having similar temperatures? It's very unlikely by itself unless the two places were near each other, which would favor the explanation “b”. #### 2. The measured curvature of the universe is very close to zero The [curvature of our universe](https://en.wikipedia.org/wiki/Shape_of_the_universe) essentially determines whether two parallel lines meet, diverge or converge. ![[curvature.png]] (via [here](https://www.astronomicalreturns.com/2019/06/the-earth-isnt-flat-but-universe-is.html)) It's hard to measure it precisely but our best measurements put the value very near to zero. For a given parameter of the universe, any value above or below zero is possible but having a precise value of zero is highly unlikely. When we roll back time towards the Big Bang, we find that today's curvature value that is close to zero has to be _even smaller and closer to zero_ way back in the past. How did our universe end up having zero curvature? #### 3. The universe is not completely uniform - we have stars and galaxies By default, we should expect a uniform density of matter and energy everywhere in our current universe and hence it should be a uniform cloud of gas. But we find _lumpiness_ of matter in terms of galaxies and stars. What caused this initial differential in matter / energy so that higher density regions could ultimately form structures such as galaxies? ### Cosmic Inflation Cosmic Inflation is a theory that tries to answer these three mysteries of the Big Bang in one swoop. It suggests that very early on, between $10^{-36}$ to $10^{-32}$ seconds after the Big Bang, the universe went through an exponential expansion in which nanometer distances in space got scaled to lightyear distances. Such rapid initial expansion would explain the three mysteries above: 1. Exponential expansion early on would mean we could today see objects much farther away than the age of the universe (which is what we see). 2. Any curvature of the universe when exponentially blown up would have a curvature close to zero today (just like a circle segment when sufficiently zoomed in looks like a straight line). 3. The initial differential in matter/energy was due to quantum mechanical fluctuations which ended up distributing differential density at different places in the universe, which is what led to formation of stars and galaxies. #### Cause of inflation and how it ended Inflation of the universe is hypothesizes to have been caused by vacuum energy which was high when the universe was much smaller. This vacuum energy slowly rolled to one of the more stable values (of zero or close to it) that we see today, hence effectively ending inflation. ![[ljdEY.jpg]] (via [here](https://astronomy.stackexchange.com/questions/648/inflation-cosmology-slow-roll-inflation-versus-tunneling-between-two-vacua)) So initially when temperature was high, vacuum energy was "stuck" at a higher value, thus causing exponential inflation. But then due to quantum fluctuations, this energy found a lower value (which is the natural tendency), ending the inflation and giving us the steady pace of expansion of space we see today. #### Problems with Inflation and eternally inflating the universe One problem of inflation is that this transition from higher vacuum to ~0 vacuum energy has to be very precise in order to give us what we see in the universe. Critics of inflation argue that we have taken one type of problem identified with the Big Bang and simply shifted into another one of finetuning to obtained desired inflationary outcomes . Hence, inflation doesn't really solve anything unless we can explain **why the inflation started and ended .** One explanation to answer the finetuning is that **inflation never ended**. This scenario is called [Eternal Inflation](https://en.wikipedia.org/wiki/Eternal_inflation) and in it our larger universe is continuously exponentially expanding. Whenever the vacuum energy in a region of this larger space fluctuates to close to zero, this region of space effectively nucleates and steady expansion starts within it. Such nucleation will look like the Big Bang to us in our region of space. Eternal inflation also explains many other apparently finetuned properties of the universe (such as why did the fundamental forces in our universe split exactly like it did). It does this by suggesting that each nucleated universe will see _different_ laws of physics (as far as fundamental forces are concerned) depending on how randomly quantum mechanical fluctuations happened for that universe. ### Did inflation really happen? Right now, whether inflation happened or not is [one of the greatest unsolved problems in physics](https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_astronomy). If inflation really did happen, it would leave a specific type of pattern in CMB and in 2014, it looked like we had found the evidence for it. But later analysis revealed that it wasn't due to inflation but due to some other mundane source. So, in a nutshell, as beautiful an idea that inflation is, we don't yet know for sure it happened and hence the three mysteries of the Big Bang still remain unsolved. And with those mysteries, we can say with confidence that our knowledge of the past of our universe blurs from 0 to about $10^{-10}$ seconds of the Big Bang (when the electromagnetic and weak forces separated). <iframe class="signup-iframe" src="https://invertedpassion.com/signup-collector" title="Signup collector"></iframe>