The question of what the universe is made of has been a driving force for astronomers for centuries. Scientists have long believed that only 5% of the universe is made up of “normal” matter like atoms and molecules. Another 25% is believed to be “dark matter,” an invisible substance that can only be detected through its gravitational effects on normal matter. The remaining 70% is made up of “dark energy,” an unknown form of energy that was discovered in 1998 and is thought to be causing the universe to expand at an accelerating rate.
In a new study set to be published in the Astronomical Journal, researchers have measured the properties of dark energy in more detail than ever before. Their findings suggest that dark energy may be a hypothetical vacuum energy proposed by Einstein, or it may be something even more complex and dynamic.
Dark energy was first introduced by Einstein when he realized that his equations showed the universe should either be expanding or shrinking, which seemed contradictory. To resolve this, he added a “cosmological constant” to balance out the force of gravity and keep the universe static. However, when it was later discovered that the universe was indeed expanding, Einstein abandoned the cosmological constant, considering it his “greatest mistake.”
In 1998, researchers found evidence that the expansion of the universe was actually accelerating, indicating the possible existence of something similar to Einstein’s cosmological constant, which is now referred to as dark energy. Since then, scientists have been using various methods, including studying supernovae, to measure and understand dark energy. Previous observations have suggested that the density of dark energy remains constant as the universe expands.
To measure the properties of dark energy, astronomers use objects in space whose brightness they know, referred to as “standard candles.” One common standard candle used is a Type Ia supernova, which is a white dwarf star that explodes after accumulating enough mass. By measuring the brightness and fading rate of these supernovae, astronomers can calculate their distance from Earth.
The Dark Energy Survey is the largest effort to measure dark energy to date. It involves over 400 scientists from multiple continents who have been observing parts of the southern sky for nearly a decade. By repeatedly observing these regions, they can detect changes, such as new supernovae, and gather more data for analysis.
The latest results from the Dark Energy Survey, which used around 1,500 Type Ia supernovae, have provided a more precise measurement of dark energy. The researchers found that the value of w, which represents dark energy, is approximately -0.80 ± 0.18. This is close to, but not exactly, -1, which would indicate that dark energy is equivalent to the cosmological constant or the energy of empty space.
These findings suggest that a more complex model of dark energy may be necessary, one in which its properties have changed over the course of the universe’s existence.