The Mystery of Dark Energy

The idea of ‘dark’ energy is closely entwined with the observation that the universe is expanding. The first person to strike the concept of the universe’s expansion was Albert Einstein. It was a few years after he published his General Theory of Relativity in 1915, that he first hit the idea. He decided he would try and apply the theory to the entire universe, and in doing so, found something that is still, to this day, not understood.

The theory predicted that the universe cannot be static, but rather, it must be expanding or contracting — a very strange discovery. Theories and suggestions were made to decipher between the two, or in some cases attempt to destroy the entire concept. However, it was not until 1929, when Edwin Hubble entered the discussion, that it was solved. Hubble measured the velocities of many distant galaxies. Given the Earth is not in a ‘special’ location within the universe, his expectation was for about the same number of galaxies to be moving toward us as the number moving away from us. But instead, he found that no matter where he looked, galaxies were moving away from us… the universe seemed to be ‘expanding’. Between then and now, millions of galaxies have been observed using better equipment, and every single observation (except for a few galaxies close to us) agrees with Hubble’s theory.

This theory remained untouched until the 1990s. It was believed that gravity would cause the expansion to slow down over time. Although such a ‘decelerating expansion’ had not been observed, people knew that gravity is an attractive force that pulls matter together, and so they saw it as the only logical explanation. However, this changed in 1998 after the Hubble Space Telescope made observations of extremely distant supernovae.

Light travels at a finite speed (~300 million metres per second), and thus, light from objects a large distance away takes longer to reach us than light from closer objects. As a result, as you look further away, you ‘look back in time’. The distant supernovae the HST observed seemed to be expanding more slowly than closer supernovae. This means that in the past, the universe was actually expanding slower than it is today. Therefore, the expansion of the universe is accelerating, not decelerating!

Many physicists have tried to come up with explanations for this, ranging from Einstein’s theory of gravity (general relativity) being wrong to some strange invisible ‘fluid’ that fills space. The main point is that we still don’t have an agreed explanation, but we do know that there is some ‘dark’ force that is causing the universe to expand at an increasing rate, and we have a name for it: dark energy.

So what is dark energy?

Unfortunately, no one has answered that question. We know it must be there (and how much of it there is), but that’s about it. Physicists use the known amount of matter in the universe, along with the rate of expansion to calculate how much dark energy there ‘should be’ to account for the observed expansion. It turns out that roughly 68% of the universe must be dark energy, while dark matter makes up about 27%. This leaves less than 5% for all ‘normal’ matter (i.e. you, me, the Earth, and literally anything we’ve ever observed). I find it absolutely fascinating that the part which makes up the vast majority of the universe is a complete mystery to us.

The cosmological constant

Perhaps the most popular theory for dark energy is that it’s simply a property of space. Deriving initially from Einstein, it was realised that empty space may not be completely empty. But rather, it is predicted by one version of Einstein’s theory that ‘empty’ space can contain energy. This is a strange result and is referred to as the cosmological constant of Einstein’s theory. Another discovery of Einstein’s is that is is possible for more space to come into existence. Combining that with the previous idea that empty space can contain energy, it can be seen that as more space does come into existence, more energy would appear. Thus, this additional ‘energy of space’ may be what causes the universe to expand at an increasing rate. The problem, however, is that no one can explain why the cosmological constant should even be there!

Quantum theory

A second explanation for this strange energy of space comes from the quantum theory of matter. The proposal is that empty space is indeed not empty. Instead, it is full of temporary ‘virtual’ particle-antiparticle pairs that continually form, annihilate one another, and then disappear. Unfortunately, when physicists attempted to calculate an answer for how much additional energy empty space would acquire when taking this into account, their answer came out 1⁰¹²⁰ times too big. That’s 10 x 10 x 10 x 10… 120 times times. This answer is so incredibly far off, that the theory has essentially been disregarded.

Mysterious fluid

Another potential explanation is that dark energy is a strange, dynamic fluid. The explanation describes that this fluid fills all of space, but as a distinct property which has an opposing effect to matter or ‘normal’ energy: rather than causing things to attract, it causes them to repel. It has actually been given the name ‘quintessence’ (after the fifth element of Greek philosophers). However, there is yet to be any evidence of this, and so we don’t know what it would be like or whether it even exists.

Einstein is just wrong?

The final possibility I’ll include here is quite a crude one: perhaps Einstein’s theory of gravity is simply… wrong? The development of a new theory may be able to incorporate and thus explain the expansion of the universe. However, there have (to date) been no new successful theories. Furthermore, Einstein’s theory is so famous because it correctly describes so many of our observations. As a result, it’s been verified repeatedly and is an ‘accepted’ theory.

Perhaps Einstein’s theory is just a small part of an overall picture? A new theory would still need to agree with Einstein when looking at situations such as our Solar System, but would need to provide a different prediction when looking at the universe as a whole.

And what is dark matter?

As stated earlier, scientists have successfully calculated the (rough) composition of the universe: 68% dark energy, 27% dark matter, and 5% normal matter. Dark matter is like dark energy, where being ‘dark’ essentially means that we can’t see it. Observations of the universe show that there’s far too small an amount of visible matter to make up the calculated 27% of dark matter. In other words, dark matter is not in the form of stars and planets that we can see. Not only is dark matter not like normal matter, but it’s not like antimatter either. If it were antimatter, we would notice the effects of it annihilating with the normal matter in the universe, but we have not seen anything.

So, just like with dark energy, we really don’t know much about dark matter either. I wonder if the answer to these questions will be discovered in the near future, or perhaps they form part of a bigger picture of the univere that humanity simply cannot comprehend. Either way, we will do our best to come up with ideas!

Originally published at http://thephysicsfootprint.com on January 30, 2022.