Why is the universe expanding?

We know that the universe expanded after the Big Bang, but why is it still expanding at an accelerated rate? This question should be: what is dark energy and why is it causing the universe to expand?

First let's explore how we know that the universe is expanding. Expansion of the universe means that the fabric of space is expanding. This strange effect is realized by what is known as the metric (measure of distance) expansion of space, which causes the distance between objects to grow larger. In other words, the space we're immersed in is growing larger all of the time.

How do we know this? The answer is that we know it because the light from distant galaxies is being stretched into the red part of the spectrum. It's known as the Red Shift. It's a Doppler effect, the same kind of effect that a train whistle's sound changes in frequency or pitch as the train approaches us and then when it recedes. This also happens to light. Light from galaxies that move away from us shifts toward the red part of the spectrum. Light from galaxies that move toward us shifts toward the blue end. They actually look red or blue. The fact is that galaxies that are further away show a red shift that's larger than that shown by galaxies nearer to us. In fact, this is the way that astronomers measure distance to these galaxies. At the edge of the observable galaxy this expansion is near the speed of light. But at billions of light years away, this is no big deal.

You have to keep in mind that we're talking unbelievable distances here. On the small scale that we are used to here on Earth, this expansion is impossible to measure because it's literally too small. The problem of scale is difficult for to us to grasp. It's mind-boggling.

If the Big Bang were the only reason that the universe is expanding, the rate of expansion would slow as time goes on. This is not happening. The expansion rate is increasing. The only way to explain this is to suggest that there is some force or energy that is opposite to gravity in play. This energy has to be negative, an idea that is difficult to understand. Astronomers call this negative energy Dark energy.

Dark energy makes up 69% of the universe's stuff, be it matter or energy. It doesn't matter because they're interchangeable. The density of this mysterious energy is very low, much lower than ordinary matter or even dark matter, but it's uniform all throughout space and it's exerting a small negative pressure, essentially acting like anti-gravity.

What the heck is this stuff that causing space to expand? That, my friends, is the most pressing question in all of physics, and there are a lot of wild theories about what it is and where it came from. Basically, dark energy has to be some intrinsic property of space. In other words, it's some energy field that exists in every square meter of space. Some call this Quintessence, but no one has found evidence to support this idea.

Some say that dark energy is the cost of having space, which is just some energy field that is connected to space itself. The cosmologists call this Lambda, a symbol that represents the cosmological constant or equation of state. It's like a negative pressure of space equal to its energy density, and that's what's causing space to expand at an accelerating rate. Most of this sounds like mathematical theory.

Some cosmologists believe that this expansion began 5 billion years ago and that before that the expansion was slowing. For some reason the density of dark matter decreases faster than dark energy so dark energy wins. This idea suggests that dark energy and dark matter were created at the Big Bang but the density of dark matter was so large, dark energy had no real effect until the expansion of the universe reached a point where it began to have effect and the expansion rate began to accelerate.
Obviously, there is a lot more work to be done on the nature of dark energy. Stay tuned!

Thanks for reading.