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Glossary · Galaxies & Cosmology
Dark energy
The name for whatever is making the expansion of the universe speed up.
In 1998 two teams measuring distant supernovae found the expansion of the universe is accelerating, which nothing in the inventory of matter and radiation can do. "Dark energy" is the placeholder for the cause, and it accounts for roughly 68% of the total energy content of the universe. The simplest candidate is a cosmological constant — an intrinsic energy of empty space, unchanging in time. Recent results from DESI hint that it might instead be weakening, which would be a far more interesting outcome and is not yet established. The honest position is that we have measured its effects precisely and understand its nature not at all.
Where it has come up lately
NASA Science
Universe Today
ScienceDaily
Phys.org
Phys.org
Related terms
Fast radio burst
A millisecond flash of radio energy from another galaxy, cause still unsettled.
Redshift
The stretching of light from distant objects, and the universe's odometer.
Hubble tension
Two good measurements of the universe's expansion rate that stubbornly disagree.
Dark matter
Matter that has gravity but does not interact with light, and outweighs the visible sort five to one.
Questions
What is Dark energy?
The name for whatever is making the expansion of the universe speed up.
Why does it matter?
Recent results from DESI hint that it might instead be weakening, which would be a far more interesting outcome and is not yet established. The honest position is that we have measured its effects precisely and understand its nature not at all.