So, last week I dealt with dark matter. This week we'll be talking about dark energy.
Let's just have a recap on dark matter and visible matter.
The visible universe is made up of ordinary matter - that includes our Earth, the Sun, stars and other galaxies. It makes up less than 5% of mass in the universe.
The rest of the universe is made up of dark matter (25% of the universe's mass) that's difficult (some say impossible) to detect.
And there's also a force present in our universe that repels gravity - this is known as dark energy (and accounts for 70% of the universe's mass).
And our universe is expanding - that's an important point too. But first...
What's the difference between dark matter and dark energy?
Dark matter produces the attractive force of gravity, but dark energy produces a repulsive force - commonly referred to as anti-gravity. That's the difference. Dark matter attracts, but dark energy repels.
Remember I said that the universe is expanding? Well, two teams of scientists tried to measure the rate of that expansion in the 1990s. They were shocked to discover that the universe isn't just expanding - it's accelerating.
Scientists had previously assumed that gravity would eventually slow down the expansion of the universe. They were wrong. More baffling still, this repulsive force gets stronger as the universe expands.
It's a bit like Ironman throwing his helmet in the air and, instead of it falling back to Earth, it accelerates away from him.
What's causing this accelerated expansion?
No one really knows. The term "dark energy" was coined for this mysterious force and that's all dark energy really is at this stage - a name.
But theories do exist. One is that dark energy is a fifth fundamental and unknown force called quintessence that only exists when the universe is a certain size. Under quintessence, dark energy could eventually disappear and our universe will stop accelerating.
Another idea is that dark energy is consistent with a cosmological constant. In other words, dark energy is constant and our universe will continue to accelerate in its expansion.
Alternatively, dark energy could result from the weird behaviour seen for the physics of the incredibly small - aka quantum mechanics. On such tiny scales, energy and matter can appear out of nothing for the tiniest of instants. The constant appearance and disappearance of matter could produce dark energy.
But every possible explanation still does not explain why dark energy exists in the first place.
Extra reading and watching
If you Google "dark energy", there are a myriad of articles and videos out there. It's a big topic.
The most up-to-date and accessible article I could find is by Adrian Cho - Is dark energy an illusion? It discusses some of the challenges in the dark energy space right now (no pun intended).
And this video is a great round-up of dark energy:
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