Dark Energy and the Expanding Universe

Dark energy... sounds like something out of a Star Wars movie, right? Well, it's a real thing, and there's a lot more of it than we can even fathom. As we understand it, dark energy makes up 70% of our universe. Dark energy is defined as unknown energy that seems to be the source of a repulsive force causing expansion of the universe to accelerate. Kind of sounds made up, but we know that it has to exist.

How do we know this? Let's think -- the Big Bang provided the energy for initial expansion, but in theory, gravity should bring all things back together. However, what actually happens is quite the opposite. We've observed that the universe is actually accelerating in its expansion.

How do we know that the universe's expansion rate is accelerating? For this, we look at Hubble's Law. Hubble's Law states that:

d = (v/H៰)

where d represents distance from the observer, v represents the velocity of the object, and H៰ represents the Hubble's constant. This number is about 70 km/s/lightyear. These units seems slightly strange, but they are correct. We know that things that are further away from us are traveling faster with respect to us than object closer in. Because of that, we know that as we look further back in time, the expansion rate of the universe is actually slower back then than it is right now. This implies that the universe is expanding at an accelerated rate!

Image courtesy of https://www.e-education.psu.edu/astro801/book/export/html/1964

Let's back up for a second and think about our theories. The universe is made up of matter, dark matter, normal energy, and dark energy. All matter has a gravitational attraction, so we would think that all matter in the universe would eventually recombine into what it was at the moment of the big bang. We also know that there isn't enough normal energy in the universe to prevent this recombination, so what can it be that's accelerating this expansion? The answer to this question is dark energy. Einstein originally theorized dark energy and called it the "cosmological constant". He thought about what we consider to be empty space, and said that this seemingly void vacuum actually has its own energy. He also noted that this energetic empty space can be created, so "as more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster" [1].

Below is a model of the actual expansion of our universe.

 

The important feature in this is the portion of time in which the universe was not expanding. We theorize that dark energy wasn't dominant in our universe until fairly recently. Now that dark energy is dominant, we can see that the universe has begun expanding again.

So, dark energy isn't something out of Star Wars, but it's actually something that dominates our entire universe. It's wild that something that most people aren't even aware exists is actually the most prevalent thing we know of.


[1] https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy