What discovery in 1998 led to the concept of dark energy?

Question

Accepted Answer

The observation that the expansion of the universe is accelerating. In 1998, two independent research teams studying distant Type Ia supernovae discovered that the expansion of the universe is not just continuing but actually accelerating. This was completely unexpected, as gravitational attraction from all the matter in the universe was expected to slow the expansion. The mysterious force driving this acceleration was named dark energy, and this discovery earned Saul Perlmutter, Brian Schmidt, and Adam Riess the 2011 Nobel Prize in Physics.

The detection of gravitational waves was first achieved by LIGO in 2015, which confirmed a prediction of general relativity but is unrelated to dark energy. The Higgs boson was discovered at CERN in 2012 and relates to particle physics and the origin of mass, not cosmic expansion. Mapping dark matter in galaxy clusters is done through gravitational lensing and provides evidence for dark matter, not dark energy, which is a separate phenomenon.

Dark energy is estimated to constitute about 68% of the total energy content of the universe and is one of the greatest mysteries in modern physics. On the DSST Astronomy exam, students should understand the distinction between dark matter (which causes gravitational attraction and holds galaxies together) and dark energy (which drives the accelerating expansion of space), as well as the observational evidence for each.

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