Astronomy & Astrophysics 101: Dark Energy

The universe’s expansion rate is accelerating. The unknown energy source causing this acceleration is called Dark Energy. Credit: Visualization by Frank Summers, Space Telescope Science Institute. Simulation by Martin White, UC Berkeley and Lars Hernquist, Harvard University

What Is Dark Energy?

Dark energy is the name given to the as yet unknown energy source that is causing our Universe’s expansion to accelerate.

Our Universe is expanding. One of the great mysteries of modern astronomy is that the expansion rate appears to be accelerating. The as yet unknown energy source causing this expansion is what astrophysicists refer to as dark energy.

Energy and mass are profoundly related to one another, as expressed by the well-known equation E = mc2. Therefore the composition of the Universe is often described by astronomers in terms of a quantity called the mass-energy density, as opposed to just mass alone. Incredibly, dark energy is believed to make up about 70% of the mass-energy density of the entire Universe.

By studying dark energy, astronomers hope to gain a better understanding of both what it is and how it might affect the future of the Universe: whether the cosmos will continue expanding forever; whether it will reach a steady state; or if it will reverse course and head for a ‘Big Crunch’.

Hubble’s early observations helped astronomers to determine how fast the Universe is expanding over time. To their surprise, the data indicated that the Universe had not been expanding at a constant rate, but instead is accelerating. Many scientists now believe this discovery can be explained by the existence of dark energy.


Dark energy is the name given to the unknown energy source that is causing our Universe’s expansion to accelerate. Credit: ESA/Hubble, Visualization by Frank Summers, Space Telescope Science Institute. Simulation by Martin White, UC Berkeley and Lars Hernquist, Harvard University.

In 2016 astronomers used Hubble to measure the distances to stars in nineteen galaxies more accurately than previously possible. They found that the Universe is currently expanding faster than the rate derived from measurements of the Universe shortly after the Big Bang. Research is still ongoing, as this apparent inconsistency may be an important clue to understanding three of the Universe’s most elusive components: dark matter, dark energy and neutrinos. The team made this discovery by refining the measurement of how fast the Universe is expanding, a value called the Hubble constant, to unprecedented accuracy, reducing the uncertainty to only 2.4%. This is significant because before Hubble was launched in 1990, estimates of the Hubble constant varied by a factor of two. In the late 1990s, researchers refined the value of the Hubble constant to within 10%, accomplishing one of the telescope’s key goals. This latest research has reduced the uncertainty in the value of the Hubble constant to an unprecedented 1.9%.

Credit: Visualization by Frank Summers, Space Telescope Science Institute. Simulation by Martin White, UC Berkeley and Lars Hernquist, Harvard University

By analyzing the COSMOS survey – the largest ever survey undertaken with Hubble – an international team of scientists assembled one of the most important results in cosmology: a three-dimensional map that offers a first look at the web-like large-scale distribution of dark matter in the Universe. Tracing the growth of clustering in the dark matter in this way may also eventually shed light on dark energy.

In 2010 it was announced that an international team of astronomers had used gravitational lensing observations from Hubble to make an important step forward in the quest to solve the riddle of dark energy. Looking at the distorted images allows astronomers to reconstruct the path that light from distant galaxies takes on its long journey to Earth. It also lets them study the effect of dark energy on the geometry of space in the light path from distant objects to the lensing cluster and then from the cluster to us. As dark energy pushes the Universe to expand ever faster, the precise path that the light beams follow as they travel through space and are bent by the lens is subtly altered. This means that the distorted images from the lens encapsulate information about the underlying cosmology, as well as about the lens itself.

For another explanation of Dark Energy, see: What is Cosmic Acceleration and Dark Energy?

AstronomyAstrophysicsDark EnergyEuropean Space AgencyHubble Space Telescope
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  • Howard Jeffrey Bender, Ph.D.

    Another way to explain Dark Energy is suggested by String Theory. All matter and energy, including photons (light), have vibrating strings as their basis.

    String and anti-string pairs are speculated to be created in the quantum foam, a roiling energy field suggested by quantum mechanics, and they immediately annihilate each other. If light passes near these string/anti-string annihilations, perhaps some of that annihilation energy is absorbed by the string in the light. Then the Fraunhofer lines in that light will move a bit towards the blue and away from the red shift. As this continues in an expanding universe we get the same curve displayed by Perlmutter and colleagues at their Nobel Prize lecture, without the need for Dark Energy.

    This speculation has the universe behaving in a much more direct way. Specifics on this can be found by searching YouTube for “Dark Energy – a String Theory Way”

    • Charles G. Shaver

      With no discouragement, disrespect or offense intended to anyone outside the US Government, in my as still unwritten/unpublished model of the universe (e.g. FDA imposed chronic illness of forty-one years and counting) it is pulsing coiling and spiraling outward lines of locally generated/induced gravity force which causes photons, electrons and some molecules to ‘vibrate,’ and make those and other tiny particles/quanta of energy to appear as both particles and waves; ‘duality.’ String theory appears to have arisen from the observation of that ‘pulsing’ in conjunction with the long dogmatic misinterpretation of how gravity actually works. Despite my senior lay efforts to ‘share the wealth’ since first (now on Odysee dot com) posting my first video of low budget at-home “1Gravity” experimentation with ‘wheels’ in 2012, on a now deleted (due to censorship about the Covid-19 ‘scamdemic’) YouTube video channel, it appears I still have all of that wealth to myself.

      Focusing now on ‘Dark Energy’ and a postulated ‘acceleration of the expansion’ of the universe, my still foggy and noisy brain makes it difficult for me to visualize and describe a convincing picture of what appears to be occurring. As small and light as they are, I can see photons accelerating among expanding lines of radiating gravity force as they are emitted from their sources; blue shift from earth; red shift from their source, until they cross the gravity neutral point in that area of deep space then blue shift from their sources and red shift from the earth. If I got that right, this time, then the universe is probably somewhere mid-cycle and not expanding nearly as fast as it’s being interpreted to expand. The conflict presents with assuming the speed of light is constant across the universe. Dual slit experiments tell me it’s relative to location and respective of lines of gravity force (minimally, crystal structure?). With the math being way beyond my ability to calculate using all of the factors/variables, it begs the question for me: “What is the actual age of the universe?”

  • Mike Pollock

    Dark energy is easily explainable. It is merely momentum the galaxies were given when they were created as shrapnel when two object collided 13.8 billion years ago in an already existing, static universe. The “acceleration of the universe” is also easily explained. The collision created a non-isotropic expansion of matter just like any car crash would create. Some pieces are going to be going faster than the average expansion rate and some are going slower. If it is assumed that the entire universe is expanding, it will look like the galaxies are accelerating in their expansion.

    What laws are followed with the big bang theory? How is the first law reconciled with a universe that fit into a “singularity”? How is the second law reconciled when the universe was “born” at trillions of degrees? How is Newton’s third law reconciled when the universe expanded with no experience how? All the laws are physics are ignored in the first second of the “Big Bang”. How is anything ever going to make sense to scientists when everything they base everything on was ignored at the very beginning of the theory?

    The Theory of Everything is here. Science believes it is impossible to reconcile general relativity with gravity but that is only because of the laws being ignored. Gravity is a manipulation of space just like Einstein claimed but when a cloud of gas and dust uses its own gravity to turn itself into a star, the gravity is forced to be caused by the normal matter itself completely ignoring any significance dark matter has in the situation. That is why gravity remains unexplained and will stay that way until the Big Bang theory is eradicated.