Expansion of Universe: Fifth force as "Dark Energy"?

Continuation of the previous post - "Expansion of the Universe: Is there a limit?"

What do we know about "Dark Energy" so far? 

and 

How is it accelerating the expansion of the universe?

The entire concept of mysterious energy, dubbed as "dark energy", which has been driving our universe to an accelerated expansion is not even that old. This hypothesis has only been out for the last two decades when compared to the dark matter theory, which is about half a century old. 

Apart from the strong resemblance in their names, they're both entirely different entities for sure. For one, dark matter is a matter, that possesses mass and has properties like gravitational attraction, while dark energy, is a kind of energy that has a repulsive gravitational-like force. 

From the previous data collected from space telescopes and calculations, we know that our universe consists of about 5% of the observable universe, while the rest 95% is unknown to us, which includes 27% of dark matter and 68% of dark energy! This technically makes dark energy the fifth fundamental force in the universe.

In this post, we'll be looking into how the existence of dark energy was realized initially in cosmology and its consistency with basic cosmological models. We'll also be looking into some other interesting theories that are in line with the dark energy hypothesis, revolving around the expansion of the universe. 

History

The idea that the universe was expanding at an accelerated rate was first discovered by Edwin Hubble, and to measure the rate of this expansion, scientists came up with the concept of using Type I A supernova light, as a scale to measure light from great distances (Type I A supernovae are the ones that occur in binary star systems where one of them is a white dwarf). 

They chose this specific type of supernovae light as a standard scale for comparison because every Type I A explosion emitted a similar amount of light so that we can measure the redshift of light emitted from them and thus, figure out the expansion rate. They found from the data collected from their wavelengths, that the rate of expansion of the universe is increasing. 

Now, many giant theories explain the existence of dark energy which are consistent with earlier observations, like the notion of the critical density of the universe and Einstein's GR equation, as the "Cosmological Constant," and we'll see more of it in another post. 

Critical Density of Universe

We saw earlier that dark energy or the repulsive force makes up about 70% of the total universe. You'll see within a few minutes why I just brought it up when you reach the end of this section, so just bear with me :) 

Anyways, when we talk about cosmology, we mainly deal with the origin, the evolution and possibly the future of our universe as a whole. There are mainly three types of scenarios in which our universe might be in, in terms of its progression after the Big Bang:

1. Spherical space:  This particular space refers to a closed universe, where the gravitational attraction of all the matter in the universe overcomes the expansion until it finally re-collapses also known as the "Big Crunch." This happens when there's an overwhelming amount of matter to contribute to the attractive force. In this scenario, if you shine two light rays separately, parallel to each other, then they'll eventually converge at a distant point in the universe, as it's a closed universe.

   

   Closed universe

   
   
2. Flat space:  The flat space refers to a universe where both the gravitational attraction (amount of matter present) and the expansion are balanced out equally. Here, the expansion of the universe will go on till infinite time. Also, if we flash two light rays parallel to each other, they'll remain parallel throughout the universe. In other words, the geometry of space, in this case, would be flat. The mass density that's required for a flat universe, is called the critical density of the universe.

   

   Flat universe

   
   
3. Hyperbolic space:  In this case, the expansion of the universe overtakes the net gravitational attraction exerted by matter present in the universe, which means there's only little mass present. This kind of universe will have an accelerated expansion forever and is referred to as an open space or geometry. The two light rays which were emitted parallel to each other initially will eventually diverge apart from each other, as they travel through the universe.

   

   Open universe

   
   

A flat universe

Now, up until the introduction of the dark energy hypothesis, the cosmological models that existed at the time predicted that the universe was flat in all its four dimensions, analogical to the 2nd scenario which we saw above. This means that our universe is in its critical mass density state, where the expansion and the self-gravitational attraction are balanced out. 

But when scientists observed the universe's total observable mass (matter) and dark matter, they found that it only made up to 30% of what was required to reach the critical density! This means that there might be some unknown entity that makes up the rest 70% of the universe. This was a mystery until dark energy came in. Scientists realized that the remaining unknown 70% was comprised of dark energy, which was realized after the notion of dark energy was first introduced and also shows that dark energy theory agrees with the cosmological models.

The notion of dark energy is also linked with Relativity and quantum mechanics, as the energy emerging from the quantum fluctuations in a vacuum, but let's just save the details for a later post.

Apart from dark energy, there have been other theories that were developed to explain the expansion of the universe, the Quintessence field theory being one of them. But unlike dark energy, quintessence is thought of as a dynamic energy field in the universe, which varies in space and time. This theory was first noticed when Aristotle introduced the fifth force as 'essence' which is present throughout the universe. But when compared to dark energy, this theory is less popular, although not completely off the table. 

There hasn't been any development on what kind of particles the dark energy may consist of, although active studies are going on from different parts of the world, with the help of theoretical models and powerful particle accelerators which we have in the present. 

We can gladly say that the future of physics is exciting, yet nerve-wracking, as we don't know what observation and science might bring us next!  

Click here to check the previous post on Universe Expansion.