Dark energy is the name astronomers give to whatever is causing the expansion of the universe to speed up. Galaxies far from us are not mostly flying through space like shrapnel, because space itself is stretching between galaxy clusters. This matters because dark energy appears to make up most of the total energy content of the universe.
Understanding it is one of the biggest unsolved problems in modern astronomy and physics.
The main evidence for dark energy came from observations of distant Type Ia supernovae, which appeared dimmer than expected because the universe had expanded more than a slowing-expansion model predicted. In the standard model of cosmology, dark energy acts like a nearly uniform energy of space with negative pressure. Its effect is tiny on the scale of stars, planets, and galaxies, but it dominates across billions of light-years.
If dark energy remains constant, distant galaxies will move beyond our observable reach as cosmic expansion continues accelerating.
Key Facts
- Dark energy is estimated to be about 68% of the total energy content of the universe.
- Hubble's law describes cosmic expansion: v = H0d, where v is recession speed and d is distance.
- Accelerating expansion means the expansion rate changes so that distant galaxies recede faster over time.
- A common model treats dark energy as the cosmological constant: Λ.
- Dark energy has negative pressure, which can make the expansion of space accelerate.
- Dark energy affects very large cosmic scales, but it does not pull apart atoms, planets, solar systems, or galaxies.
Vocabulary
- Dark energy
- Dark energy is the unknown form of energy that appears to drive the accelerating expansion of the universe.
- Cosmic expansion
- Cosmic expansion is the stretching of space itself, causing distant galaxies to become farther apart over time.
- Hubble constant
- The Hubble constant is the present-day value of the expansion rate of the universe.
- Type Ia supernova
- A Type Ia supernova is a stellar explosion with a predictable brightness that astronomers use to measure large cosmic distances.
- Cosmological constant
- The cosmological constant is a term in Einstein's equations that represents a constant energy density of empty space.
Common Mistakes to Avoid
- Thinking dark energy is the same as dark matter is wrong because dark matter adds gravity that helps pull matter together, while dark energy is linked to accelerated expansion.
- Saying galaxies move away from us because Earth is at the center is wrong because expansion happens everywhere, so most distant galaxies recede from every observer.
- Using Hubble's law for nearby bound objects is wrong because gravity holds systems like the Solar System, the Milky Way, and galaxy clusters together against cosmic expansion.
- Calling dark energy a normal force like magnetism is misleading because it is modeled as a property of space on cosmic scales, not as a contact force pushing on objects locally.
Practice Questions
- 1 Use Hubble's law v = H0d with H0 = 70 km/s/Mpc. What is the recession speed of a galaxy 200 Mpc away?
- 2 A Type Ia supernova is observed to be dimmer than expected in a universe with slowing expansion. Explain in one or two sentences how this supports an accelerating universe.
- 3 Two galaxies in the same galaxy cluster do not move apart with the general expansion of the universe. Explain why dark energy does not noticeably stretch the space between them.