This post is basically a summary of me going down a rabbit hole. It all started with final preparation for my astronomy final. As you agree with me, astronomy is pretty freaking cool. Just for fun, I started reading some parts of the textbook which aren’t actually in the portion. Mistake numero uno.
Now, in high school I’d learned about the density parameter but I wanted a refresher. With this, the textbook talked about the “flatness problem”. The density parameter is defined as “the ratio of the average density of matter and energy in the Universe to the critical density. The “critical density” of the Universe is the value of the density of all the mass in the universe at which its expansion stops after infinite time. It has a long derivation; I won’t go into the specifics. The value comes out to be:
(All the non-physics experienced people scoffing at how I called this a value. ) H here is the Hubble constant which some smart guy some time ago calculated. Another kind of intelligent guy named Isaac found the other constant G, or the gravitational constant. Now, calculating the average density of the Universe and dividing it by this value gives the value of the density parameter. This is where it gets goofy.
The density parameter comes out to around 0.3. A value between 0 and 1 means the Universe is considered “open” and will forever keep expanding to infinite size. This checks out for our Universe. Anyone knows that the Universe is currently expanding at an expanding rate; this is due to dark energy (I’ll come back to this). A value of 1 means a “flat” Universe. This means the Universe will expand forever at a decreasing rate. Anything greater than one means a “closed” Universe. It will expand till a certain moment in time, stop, and collapse again.
Credits: wikipedia
All well and good right? Well, what I just found out was that the Universe at some moment in time in our past was actually slowing down. Now it’s expanding??? If this isn’t the definition of confusion I don’t know what is.
Initially, the Universe started slowing down because of gravity. Then, over a couple billion years, somehow dark energy became prevalent throughout the Universe and started pushing it outwards at an ever-increasing rate. Let’s go a little deeper into dark energy.
Normally, any type of pressure you see in real life is positive, or exerts a force outwards. Dark energy behaves in a counter-intuitive, exactly opposite way. It behaves like negative pressure; instead of pushing outwards, it pulls in. This sounds like the direct inverse of what I’m saying but keep with me.
Credits: wikipedia
The above is the acceleration of the universe’s expansion equation determined by many scientists, with the great help of Einstein’s field equations from his theory of general relativity. The “p” term in the parentheses is the pressure term. If ‘3p’ becomes greater than rho, then the acceleration becomes positive, indicating an ever-expanding universe. Dark energy, as mentioned above, behaves with characteristics of negative pressure, thus allowing this equation to spit out a positive value and off we go forever, and ever, and ever, and ever…
What happened in that time frame between slowing down and the expansion now? Which phenomena govern our Universe and make it behave this way? If only I had a dollar for every time I asked myself that…
TheDiscoverer. 