Two questions that do come up quite frequently are those about Dark Matter and Dark Energy, what they are and what we know about them. So today I would like to take the time to delve deeper into the mysterious darkness of the universe! As this is a fairly extensive question I think it would be prudent to split it into two parts.. Here we go!
Part 2: Dark Energy
In part 1 we looked a little at dark matter but now we're going to have a look at something even more fascinating and mysterious, dark energy! We deduced that dark matter is an invisible mass in the universe, so dark energy must be an invisible energy.
As some random information that I came across while channel surfing a while back, I stumbled across a documentary on George Lucas. One comment that was made on the program was that, when the original Star Wars films came out, many scientists at the time thought that it was ludicrous to suggest that there was an invisible energy that permeated the whole universe, George called this "The Force"! Nowadays, however, we just accept that there is an invisible energy out there (granted, there isn't anyone out there that can lift cars and throw them around by channeling Dark Energy...but still). I just love how un-fictional science fiction can really be 🙂
So how do we know that dark energy exists? Well, there are a couple of ways that it has been hinted at. Edwin Hubble discovered that the farther an object was away from us the faster it was receding from us. In simplest terms, this means that the universe is indeed expanding. Hubble was able to realize this with the use of a 100 inch telescope and by looking at a type of star called a "Cepheid Variable." For those who don't know what that is, it's a type of star that gets brighter and dimmer over a very short period of time (this can range from about a day to about two months). There is a correlation between brightness and period times, so Hubble was able to use stars like this to work out redshifts and he found that the further away the stars were the more they were red shifted.
Our biggest aid to understanding the existence of dark energy, however, comes from the WMAP data. In some ways it's sort of the holy grail of astrophysics--it is because of the WMAP data that we know that the universe is made up of 4.6% Atoms, 23% Dark Matter and 72% Dark Energy. But what is dark energy? Of that, we're actually not quite sure. But we know that because of dark energy the universe is currently expanding at a rate of about 72 km/s/Mpc, some of you may look at that and think "Huh?".
Well, for every 3.26 million lightyears (1 Mega Parsec) everything moves way from us at 72 km/s. To give an example, the Andromeda Galaxy is 2.57 million lightyears away or 0.788 Mpc away. This means that it is moving away from us at nearly 57 km/s due to dark energy. Andromeda is however moving towards us in excess of 200 km/s due to gravity! So what we can assume is that, as Andromeda gets closer to us over the next 1-2 billion years, it will accelerate because the gap is slowly being closed and dark energy stretches the universe less. All the while, galaxies that are a long way from us keep accelerating farther away.
It is because of this that dark energy is sometimes called a "negative gravity" or a "negative pressure." BUT it's not quite true. Dark energy causes the space between two objects to expand, not push away from one another, but physically increase on a level that we are yet to fully understand or even comprehend. To conclude, dark energy is truly mysterious and something that we are struggling to even begin to understand. From observation, we can deduce that it exists, WMAP data tells us how much of the universe is made of the stuff, and it even tells us how much of an expansion we get. Ultimately though.... May The Force be with you!