Dark Energy: The Universe's Invisible Fuel
Imagine throwing a ball into the air, but instead of slowing down, it speeds up forever. That’s what’s happening to our universe, and the mysterious force behind it is called dark energy.
Put simply, dark energy is the unknown phenomenon driving the accelerating expansion of our universe. According to calculations, it makes up roughly 68-72% of the universe’s total energy and matter (also known as its cosmic energy budget). In fact, the things you see every day, like stars and galaxies and even your kitchen table, make up less than 5% of this total budget.
You can consider dark energy as an “anti-gravity” force driving cosmic objects like galaxies and stars apart at an increasingly rapid rate, instead of pulling them together the way gravity does.
Despite decades of research into dark energy, we still don’t know what exactly it is. If we can solve the mystery of dark energy, we might be able to understand how the universe will end.
I. The Expanding Universe
In 1912, the American astronomer Henrietta Swan Leavitt was studying a special class of stars called Cepheid variable stars. A unique characteristic of these stars is that their brightness is constantly fluctuating.
Leavitt discovered that there is a relationship between their regular period of brightness (the time taken for the star to go from bright, to dim, to bright again) and their luminosity (the amount of light they radiate). In other words, the longer the period of the star, the more light it puts out, and vice versa.
This was a very important discovery in the field of astronomy, since this made it possible for astronomers to calculate the distances between Earth and Cepheid stars that are very far away, allowing us to measure the distance between us and far-off galaxies.
At the same time, astronomer Vesto Slipher observed spiral galaxies using a piece of relatively new technology at the time, known as a spectrograph. This device splits light into the colours it’s made up of, similar to the way a prism splits white light into the colours of a rainbow.
He used the spectrograph to observe the wavelengths of light coming from the galaxies in different spectral lines. He realised that the spectral lines emitted from the galaxies were shifted slightly to the red side, compared to Earth.
To understand what this means, one must first understand the Doppler Effect:
When a source, e.g. a speaker, is stationary, it sounds the same from all directions. This is because the waves are equally spaced from all directions, and so the frequency is the same.
When the source is moving, however, it sounds higher-pitched in the direction it’s moving towards, because the waves get bunched together and so the wavelength gets shorter. This makes the perceived frequency higher, so it sounds higher-pitched. In the same way, in the direction the source is moving away from, the waves are further apart, making the perceived frequency lower.
In the case of light, when the wavelength gets shorter, the colour shifts towards the blue end of the spectrum, and when it gets longer, the colour shifts towards the red end of the spectrum. This means that when a source of light is moving away from us, its spectral lines should be shifted towards the red side.
In other words, Slipher had become the first observer of a phenomenon known as ‘redshift’, showing that the galaxy was moving away from us.
This would be essential to the discovery of dark energy.
In 1929, Hubble was able to use Leavitt’s discovery of the period-luminosity relation of Cepheid variable stars, as well as Slipher’s discovery of galactic redshift, to make possibly one of the most fundamental discoveries in cosmology of all time.
Based on observations made by his associate, astronomer Milton Humason, he measured the redshift of spiral galaxies and then studied the Cepheid stars in those galaxies to determine their distance. By comparing the distances of these galaxies to the redshift, he was able to find that cosmic objects, like galaxies, are moving away from Earth faster the further away they are. This is now known as Hubble’s Law.
In other words, the universe must be expanding.
This breakthrough led scientists to the development of the Big Bang theory, and later on, the discovery of dark energy.
II. The Accelerating Universe
Prior to the discovery of dark energy, scientists thought that gravity would gradually slow down the universe’s expansion over time, based on Einstein’s theory of general relativity. Gravity pulls matter together, the universe should recollapse.
In 1998, two independent research teams led by astronomers Adam Reiss, Saul Perlmutter, and Brian Schmidt, were observing Type 1a supernovae. They noticed that, at a specific redshift, the supernovae were dimmer than expected. This specific type of supernova is known to have a certain level of luminosity, meaning there must have been another factor making the objects dimmer.
Using their brightness, the researchers were able to calculate the distance of these supernovae and how fast they were moving away from us, and found that they had travelled farther away from us faster than expected. In other words, the universe’s expansion is actually speeding up.
There are two ways to explain these observations: either Einstein’s theory of general relativity is wrong, or there is some kind of repulsive force counteracting gravity — namely, dark energy.
III. What is Dark Energy?
At the moment, dark energy is very much still a mystery. We don’t actually know what it is or what it’s made from. It can’t be directly detected, and we can only infer it from the effect it has on visible matter. What we do know is that it must have a negative pressure (pushing space outward), and that it seems uniform (it doesn’t clump the same way visible matter does).
Scientists have come up with three main explanations for what dark energy could be, which I’ll outline below.
When Einstein came up with his theory of relativity, he originally introduced a “cosmological constant” counterbalancing gravity into his equations to allow for a static, unchanging universe. When Hubble confirmed that the universe was actually expanding, as Einstein’s equations had suggested, he removed the constant.
After Hubble’s discovery of the expanding universe, some scientists suggested that there could actually be a cosmological constant that would instead accelerate the expansion of the universe. This could arise from “vacuum energy”, or a theoretical background energy throughout space.
However, based on quantum field theory, scientists have shown that either:
there is such a huge amount of vacuum energy that the early universe would have expanded extremely quickly and with so much force that no stars or galaxies could have formed;
or there is no vacuum energy at all.
The amount of vacuum energy in the cosmos must be much smaller than it is in these predictions for the theory to hold. This problem is known as the “cosmological constant problem”.
Nicknamed after the theoretical fifth element discussed by ancient Greek philosophers, quintessence is a hypothetical type of energy fluid or field that fills space and behaves oppositely to visible matter. Unlike vacuum energy, the amount and distribution of quintessence can change over time and space, and could behave differently in the early and late universe.
Another argument is that dark energy is a defect in the fabric of universe. Dark energy could exist as “wrinkles” or one-dimensional strings formed in the early universe, causing the apparent acceleration.
IV. Alternative Cosmological Models
Alternatively, it is possible that dark energy doesn’t exist at all. It might not be something physical that we can discover. The observations detailed earlier may instead have resulted from a flaw in the theory of general relativity. It could be possible to modify our theories of gravity in a way that explains the observations of the universe, without needing dark energy.
Here are some examples of alternative explanations.
One argument is that the universe’s expansion is actually just an illusion caused by the uneven matter distribution in our universe. Low density regions in our universe expand faster than denser regions, which could be affecting our observations.
Modified Newtonian Dynamics (MoND) adjusts Newton’s laws at very low accelerations. It was originally developed to explain galaxy rotation curves without needing dark matter, but it has been extended to cosmology to account for dark energy in a similar way.
Relativistic MoND incorporates Einstein’s relativity to improve MoND’s predictive power. It can explain cosmic microwave background radiation adn large-scale structure formation.
Another option is retarded gravity, which suggests that gravitational effects propagate with a delay. Itclaims to match observations without needing dark energy.
Another argument is that local cosmic structures, like galaxies and voids, affect large-scale comsic expansion. These effects could mimic dark energy without requiring a new substance or force.
These theories are still under investigation, and none have yet displaced the dominant Lambda-CDM model, but they challenge our assumptions about the very fabric of space, time, and gravity.
V. Dark Matter vs Dark Energy
Though dark matter and dark energy sound similar and share the same prefix, they are separate and very different concepts.
The role of dark matter is to hold galaxies together, while the role of dark energy pushes the universe apart. Dark matter doesn’t interact with light in the way baryonic matter (visible matter) does. This means dark matter is literally “dark”, unlike dark energy, which is just referring to a mysterious nature.
Understanding the nature of dark energy is key to building an accurate model of the evolution of the universe over time. This includes how cosmic structure formed, the shape the universe takes, and the eventual fate of the universe.
Essentially, the way in which the universe will end depends upon a “critical density”. If the density of matter and energy in the universe is below this, then the force of gravity won’t be enough to hold back the force of dark energy, and the expansion of the universe will continue forever until energy spreads too thin for new stars or life, leading to Heat Death.
Alternatively, the fate of the universe could be a Big Rip, in which dark energy would overpower all of the universe’s fundamental forces, ripping apart everything currently bound by those forces, from galaxies and planets to the protons and neutrons that make up atoms.
If, on the other hand, the density is greater than the critical density, then the force of gravity will overpower dark energy, causing the universe to recollapse into a dense point, known as the Big Crunch.
The more we know about dark energy, the better we can evaluate how the universe changes over time, which could lead to new theories about the nature of our universe.
Dark Energy and Dark Matter | Center for Astrophysics | Harvard & Smithsonian
What is Dark Energy? Inside Our Accelerating, Expanding Universe - NASA Science
What is dark energy? | Space
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Source: Dark Energy: The Universe's Invisible Fuel
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