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Quintessence: dark energy that changes
1988· Theory
SpeculativeA serious idea with no experimental proof yet.
The idea that dark energy is a slowly changing field filling space, not a fixed constant, so its push can fade or grow.
See it on the Science map →How it works
Picture a field, like the one thought to drive inflation, resting on a very gentle slope of its energy hill. While it barely moves, it pushes space apart almost exactly like a cosmological constant. As it rolls, its push weakens. Bharat Ratra and Jim Peebles, and separately Christof Wetterich, proposed such fields in 1988. Robert Caldwell, Rahul Dave and Paul Steinhardt named it in 1998 after quintessence, the 'fifth element' that ancient Greek thinkers placed in the heavens.
Why it matters
If dark energy changes, the future is open: expansion could slow, or in some versions even reverse into collapse. It would also mean a new, extremely light field exists.
How we know
No confirmation. Data matched a pure constant until 2024, when DESI hinted at fading dark energy, which quintessence would naturally produce. The hint is below discovery strength.
Still a mystery
Such a field must be fantastically light and barely touch ordinary matter, or it would add a detectable fifth force. Some string theorists argue a true constant is impossible, favoring quintessence; this is disputed.
Connections (2)
▸Dark energy may instead be Quintessence: dark energy that changesSpeculative
The relationship
A field's energy has two parts: motion, and position on its energy hill. When the field barely moves, position energy dominates, and that gives negative pressure, the same trick that powered inflation, at a vastly lower energy. Dark energy would then be a field still creeping downhill. Its push weakens as it picks up speed, so its strength would differ between eras.
How the link was found
Bharat Ratra and Jim Peebles, and Christof Wetterich, independently in 1988. Ivaylo Zlatev, Limin Wang and Paul Steinhardt (1999) showed some fields could 'track' matter, perhaps explaining why dark energy took over so recently.
Is it proven?
Not proven. To tell it from a constant, surveys must measure the expansion history to about 1% at many eras. Only DESI's hint so far favors change.
Open questions and mysteries
Even if dark energy rolls, why is its energy so small? Quintessence explains change, not size; the vacuum-energy puzzle remains untouched.
▸DESI (2024–25): is dark energy weakening? hints at Quintessence: dark energy that changesSpeculative
The relationship
DESI's distances, combined with the afterglow and supernovae, fit best with a dark energy whose pressure is drifting: once more negative than a constant's, now less. A weakening push today is what a quintessence field gently rolling downhill would give. So the data point toward a changing field rather than a fixed property of space, though not yet decisively.
How the link was found
The DESI collaboration, more than 900 researchers led from Lawrence Berkeley National Laboratory, in its April 2024 and March 2025 analyses.
Is it proven?
Not proven. A significance of 2.8–4.2σ is suggestive, not conclusive, and depends on the supernova sample used. Hints at this level have vanished before.
Open questions and mysteries
The best fit's earlier phase, with pressure more negative than a constant's, is something simple quintessence cannot produce. Is it a more exotic field, dark energy trading energy with dark matter, modified gravity, or error?