Dark Energy: The Mysterious Driver of the Universe

FROM THE LECTURE SERIES: Redefining Reality: The Intellectual Implications of Modern Science

By Steven Gimbel, Ph.D., Gettysburg College

The turn of the 21st century saw a series of cosmological developments that were pivotal for scientific progress. The conception of the Big Bang theory inadvertently led to the discovery of dark matter. This, in turn, suggested the existence of dark energy, which drives the inexplicable speeding up of the expanding universe.

Computerized depiction of the universe
Our universe is expanding at a speeding rate, but the driving force behind this phenomenon is unknown, and is referred to as dark energy. (Image: Sergey Nivens/Shutterstock)

From the Big Bang Theory to Dark Matter to Dark Energy

When the Big Bang theory was conceived, it answered a lot of questions, but it also opened astrophysicists and particle physicists to a slew of newer questions. There were some questions that were answered by the inflationary hypothesis of the Big Bang theory. But the scientific movement of sorts that had started because of the Big Bang theory then stumbled upon a discovery that was completely inexplicable.

Learn more about Big Bang cosmology.

While studying the gravitational effects of galaxies, it was discovered that the mass of a galaxy was actually six times the mass of the visible stars in that galaxy. The rest of the mass, the majority of the mass in space, is actually invisible to us; its gravitational effects, however, are measurable. This invisible matter was called dark matter.

Scientists, quite humorously, started calling the particles of this matter WIMPs, or Weakly Interacting Massive Particles. They must be massive, as their gravitational effects suggest. The ‘weakly interacting’ in the name comes from the lack of their other interactive forces. 

WIMPs are theoretically needed to completely reconcile our existing theories of matter, the standard model of matter. For a long time, physicists have experimented with quarks and leptons to find how dark matter can exist in the universe, but no answer has been found. 

Astrophysicists have been unable to see them in the real world, despite the fact that some of the best minds have been put to finding them. 

The search has not, however, reached a dead end. For example, extending the standard model of matter with supersymmetry could explain the existence of dark matter. 

This theory was so perfect that it achieved scientific consensus in the 1990s.  Yet, there was another contradiction at work here, that was baffling the scientific community of the time. 

Learn more about dark matter.

The Expansion Anomaly: Discovering Dark Energy

Gravity is one of the four physical forces and is the only one that works over long distances. It is also an attractive force. 

So, all the matter in the universe must be pulling on all the other matter in the universe by the force of gravity, thereby warping space-time to oppose the expansion which has been ongoing ever since the Big Bang. This left two possible alternatives for the universe in the long term.

The first is that the gravitational pull in the universe is strong enough, by virtue of there being enough matter, to stop this expansion in its tracks, and reverse it to cause the universe to start contracting. This contraction could be sufficient to make the energy density so large that it could give rise to another Big Bang, and the universe would continue in this series of oscillations. 

The other possibility is that there is not enough matter, and therefore not enough gravitational energy, to fully stop the expansion of the universe, though it will slow down. Instead of contracting, the expansion will continue until the energy density becomes so small that it will collapse upon itself, creating a dead, cold universe. 

Which of the two possibilities will come to fruition, depends on the energy and matter contained in the universe. 

But there was an anomaly, an outcome that was not even dreamt of. There were only these two options considered so far: the contraction, subsequent Big Bang, and continued oscillation of the universe, or slowing expansion, finally leading to a cold, dead universe. 

Photo of Saul Perlmutter.
Nobel Laureate Saul Perlmutter was one of the scientists who discovered that the expansion of the universe is speeding up.
(Image: Holger Motzkau, Wikipedia/CC BY-SA 3.0/Public domain)

In 1998, American scientists Saul Perlmutter, Brian Schmidt, and Adam Reiss discovered that the case, in reality, is quite different: the expansion of the universe is not slowing down at all, it is, on the contrary, speeding up. As a result, the universe is expanding at an increasing rate. 

How could this be? The only known force capable of acting over interstellar distances is gravitation, and that is an attractive force. If anything, it would have led to a reduction in the speed of expansion. Yet, the fact that the expansion of the universe is speeding up has been corroborated many times since 1998.

The only logical explanation would be that there must be some other kind of energy that is behind this expansion. What this energy is is not known to us. But just as the invisible matter increasing the mass of the universe was termed as dark matter, the invisible energy speeding up the expansion of the universe was termed dark energy. 

This is a transcript from the video series Redefining Reality: The Intellectual Implications of Modern Science. Watch it now, on The Great Courses Plus.

What Do We Know about Dark Energy?

The similarities between dark matter and dark energy do not end with the name. Similar to the case of dark matter, not much is known for sure about dark energy. We do not know where dark energy really comes from and what it is like.

What we do know is how much of it there must be, given the rate of acceleration in the universe. And that is a lot: the normal matter and energy, the stuff which was originally thought to be all there is in the universe, makes up only 5% of the real universe. Dark matter makes up a whole quarter, 25% of the furniture of the universe. And dark energy, which makes up the rest of the 70%, is the majority of the content of the universe. What this essentially means is that an overwhelming majority, in the vicinity of 95%, of our universe is invisible to us.

But there are still a lot of questions on this topic. If the dark energy of the universe was evenly distributed like any other energy field, it would have been diluted by expansion, leading to a decrease in the speed of expansion over time. But this has not been noticed so far, implying that the density of dark energy stays constant over time.

Learn more about grand unified theories.

Moving Beyond Dark Energy

Dark energy is so hard to find because it doesn’t interact with normal matter, and the fact that we do not know anything about it is what makes it such an anomaly. There is a set paradigm, a picture of reality that is governed by the standard model and by the general theory of relativity. This paradigm contains matter, dark matter, and dark energy, though we do not know much about the majority of the contents of this paradigm. 

Both the standard model and the general theory of relativity have flawlessly predicted outcomes that have also been proven experimentally. Yet, there is a dire need for a theory that encompasses both of them, which reconciles their differences, and perhaps answers the questions of the building blocks of the universe better. This creates the need for the theory of everything, a unified theory, which, if conceived, could explain the functioning of the universe in a way that has not been thought of before.

Common Questions about Dark Energy

Q: What is dark energy?

Dark energy refers to the mysterious energy which is responsible for the speeding up of the expansion of the universe.

Q: What would a unified theory do?

A unified theory is needed to bring together the understandings of the standard model and the general theory of relativity, to perhaps provide a better understanding of dark matter and dark energy as well.

Q: What is the composition of the universe?

Only 5% of the universe is made up of normal matter and energy, while 25% is made up of dark matter, and the majority, 70%, is made up of dark energy.

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