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Dark Energy Definition
Dark energy is the name given to the undiscovered phenomenon that is responsible for the accelerating expansion of the universe.

Dark energy is one of the most enigmatic and intriguing phenomena in the realm of modern astrophysics. Making up approximately 68% of the universe, it is the phenomenon that is responsible for the accelerating expansion of the universe. Despite its significant role in the shaping of the cosmos, its nature remains largely elusive and mysterious.

  • Dark energy is an unknown form of energy that is thought to be responsible for the accelerating expansion of the universe.
  • Theories about dark energy include the cosmological constant, quintessence, modified gravity theories, and additional dimensions.


The story of dark energy begins with the discovery of the expansion of the universe, which was first proposed by American astronomer Edwin Hubble in 1929. Hubble observed that galaxies were moving away from each other, indicating that the universe was not static as previously thought.

The Cosmological Constant

Before Hubble’s discovery, Albert Einstein had introduced a “cosmological constant” (Λ) into his equations of general relativity to allow for a static universe, which was the prevailing view at the time. Once it became clear that the universe was expanding, Einstein discarded the cosmological constant, famously calling it his “greatest blunder.”

Supernovae Observations

The pivotal moment for the inception of the dark energy theory occurred in the late 1990s. Two independent teams of astronomers were studying Type Ia supernovae and made an astonishing discovery. They found that not only is the universe expanding, but the rate of its expansion is also accelerating. This observation couldn’t be explained by gravity alone, thus leading to the proposal of dark energy as a force driving this acceleration.

What Is Dark Energy? Definition

Dark energy is an unknown form of energy that permeates all of space and appears to exert a repulsive force that causes the universe to expand at an accelerating rate. Unlike matter, it does not clump under gravity and is homogeneously distributed across the universe.

Dark Energy vs Dark Matter vs Baryonic Matter

Dark energy should not be confused with dark matter, which makes up about 27% of the universe. Unlike dark energy, dark matter has mass and clumps under gravity, and it primarily affects the formation and motion of galaxies. While they are both mysterious and poorly understood, they serve very different roles in the cosmos.

Both dark energy and dark matter differ from baryonic matter, which accounts for only about 5% of the universe. Baryonic matter is all the ordinary matter that we are familiar with: protons, neutrons, electrons, antimatter, etc. Baryonic matter is the stuff that makes up the visible universe, from the smallest particles to atoms and molecules to celestial bodies.

Dark Energy Theories and Explanations

There are several theories that describe dark energy:

  • Cosmological Constant: A constant, unchanging force that exists in the vacuum of space.
  • Quintessence: A dynamic field whose energy density can vary over time.
  • Modified Gravity Theories: Alternatives to dark energy that propose modifications to general relativity.
  • Extra Dimensions: Theories suggesting that interactions with higher-dimensional space could cause cosmic acceleration.

The Cosmological Constant Revisited

One of the simplest explanations for dark energy is the reintroduction of Einstein’s cosmological constant. In this model, the constant acts as a vacuum energy that exists in space itself, providing a constant, unchanging force that drives cosmic acceleration.


Quintessence posits that dark energy is a dynamic field, unlike the cosmological constant, which remains constant. The energy density of quintessence can change over time, possibly interacting with matter and other forms of energy in the universe.

Modified Gravity Theories

Some theories suggest that dark energy doesn’t exist at all. Instead, modifications to general relativity, such as f(R) gravity, could explain the observed acceleration without requiring an unknown form of energy.

Extra Dimensions

Theories involving extra dimensions, often seen in string theory, also propose alternative explanations. In these models, our universe might be a 3-dimensional “brane” embedded in higher-dimensional space, and interactions with this larger “bulk” could cause the observed acceleration.

How Scientists Study Dark Energy

Various experiments and missions study dark energy and provide a better understanding of its properties. These efforts often focus on measuring the geometry and expansion rate of the universe with high precision. Here are some key experiments and missions:

Type Ia Supernovae Observations

Continued observations of Type Ia supernovae are one of the keys to understanding the universe’s expansion. The Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) is an example of a project aimed at discovering and characterizing these celestial events.

Cosmic Microwave Background (CMB) Studies

The Planck satellite provided a very detailed map of the CMB, which is radiation leftover from the Big Bang. The distribution of temperature fluctuations in the CMB provides insights into the universe’s geometry, including the role of dark energy.

Baryon Acoustic Oscillations (BAO)

Projects like the Sloan Digital Sky Survey (SDSS) and its successors use the BAO method to map the distribution of galaxies in the universe. BAO serves as a “cosmic ruler” for measuring the universe’s size at various epochs, thus providing insights into its expansion history.

Dark Energy Survey (DES)

This is an astronomical survey for understanding the dynamics of the expansion of the Universe. It uses a 570-megapixel camera mounted on a telescope in Chile to map an eighth of the sky in great detail.

Large Synoptic Survey Telescope (LSST)

Now renamed the Vera C. Rubin Observatory, this facility surveys of the sky to help in understanding the structure and evolution of the universe, including the mysterious dark energy.

Euclid Mission

The European Space Agency’s Euclid mission aims to map the geometry of the dark Universe with high precision. Euclid will observe billions of galaxies and vast cosmic structures to model the universe’s expansion more precisely.

WFIRST (Nancy Grace Roman Space Telescope)

This NASA mission, named after one of NASA’s pioneering astronomers, aims to study dark energy using techniques like weak gravitational lensing and galaxy clustering to understand the distribution and properties of dark energy.

DESI (Dark Energy Spectroscopic Instrument)

Mounted on the Mayall Telescope at Kitt Peak National Observatory in Arizona, DESI creates a detailed three-dimensional map of the universe with a focus on understanding dark energy. It collects spectra for tens of millions of galaxies and quasars.

Quantum Experiments

Although more speculative, some theories suggest that understanding the properties of vacuum fluctuations at the quantum level could shed light on dark energy. Experiments in quantum mechanics and studies in string theory aim to explore this possibility.


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