What is dark energy? Describe two ways that we can infer its presence.

Dark energy is a hypothetical form of energy that is proposed to permeate all spaces throughout the universe, contributing to accelerated expansion. The term 'Dark Energy' refers to this phenomenon because it is not directly observable and is understood principally through its gravitational effects.

The first way we can infer the presence of dark energy is through observing supernovae. Supernovae are exploding stars that, at their peak, shine as brightly as an entire galaxy. By observing Supernova 1a events, which are a specific type of supernova event with a consistent peak brightness, astronomers have discovered that distant supernovae are fainter than expected. This implies these supernovae are further away than their redshift suggests, pointing to the conclusion that the expansion rate of the universe is accelerating, which is indicative of dark energy.

The second way to infer the presence of dark energy is through measuring the cosmic microwave background (CMB) radiation. CMB radiation is the afterglow left over from the Big Bang, and it offers a snapshot of the universe when it was just 380,000 years old. Variations in the temperature of this radiation across the sky reveal subtle density differences in the early universe. The patterns observed suggest a flat geometry to the universe, implying it must contain the critical density of mass-energy. Given the manifested universe (all known atoms, photons, neutrinos, and dark matter) contributes to less than half of this critical density, it leaves room for an energy component needed to make up the difference; this component is assumed to be dark energy.