Dark Energy May Not Be the Cosmological Constant as Theorized by Einstein

Dark Energy


In 1917, Albert Einstein originally included a cosmological constant in his field equations of general relativity in order to achieve a steady-state static Universe. Shortly after Edwin Hubble’s discovery of the expanding Universe in 1929, Einstein removed the cosmological constant, calling it the “greatest blunder of my life”. In the late 1990s, astronomers measured the distances to extremely distant exploding supernovae and discovered that the Universe is actually accelerating (not just expanding), and therefore reintroduced the extra dark energy vacuum term in the cosmological equations.

Most scientists have assumed that dark energy is in fact Einstein’s cosmological constant, but recent theories on a special form of dark energy, called quintessence, have suggested that the strength of dark energy itself may change with time.

Quintessence is a more general form of dark energy whose pressure and energy density change with time.

A trio of astrophysicists, two from Princeton, the other from New York University, has calculated how soon the universe could collapse if theories regarding dark energy as having quintessence are correct. In their paper published in Proceedings of the National Academy of Sciences, Cosmin Andrei, Anna Ijjas, and Paul Steinhardt suggest it could be as soon as 100 million years from now.


In this new effort, the research trio wondered how long it might take the universe to slow down, stop, begin contracting, and eventually reach a single point if dark energy has quintessence. To envision such an idea, they built a model of the universe—one that used actual data describing features of the known universe. It showed them that if the idea of quintessence is true, then the universe could already be slowing its acceleration. It also showed that it could slow all the way to a standstill in approximately 65 million years — and could start contracting as soon as 100 million years from now. The theory, like the one that suggests dark energy is a constant, cannot be proven as there is no way to test it yet. Astrophysicists have to rely on signals coming from light years away, which suggests that if the universe is currently contracting, we will not be able to measure it for millions of years.

The Last Word –Paul Steinhardt, Albert Einstein Professor in Science. Professor of Physics; Director,  Princeton Center for Theoretical Science

“The cyclic universe is motivated, in part, by the discovery that the universe is entering an epoch of accelerated expansion,” writes Steinhardt, best known for his development of new theories of the origin, evolution and future of the universe. “Since the mid-1990s, my group has been playing a leading role establishing the experimental case for accelerated expansion and exploring the possibility that the acceleration is driven by a dynamical energy component with negative pressure, called “quintessence.” More recently, the effort has turned to combine the idea of a slowly time-varying cosmological constant and a cyclic universe that can naturally explain the small, positive value observed today.” 

In an email to The Daily Galaxy Paul Steinhardt wrote: “Dark energy refers to any form of energy with sufficiently negative pressure that it causes the expansion rate  to speed up.  Vacuum energy, also known as cosmological constant, is a special form of dark energy whose pressure and density do not change with time, causing the accelerated expansion to last forever.  Quintessence is a more general form of dark energy whose pressure and energy density do change with time.  In particular, as the quintessence evolves, its pressure can change  from negative to positive such that it causes expansion to stop altogether and switch to contraction**.  Our article points out that quintessence of this type where expansion ends in less than 100 million years is consistent with all current observations – so a realistic possibility — as well as consistent with recent ideas about the evolution of the universe (so-called bouncing cosmology) and about the nature of quantum gravity.”

[**Note: Contraction – not collapse.   Contraction is not the same as what we call gravitational collapse because the latter leads to black holes and here the universe may undergo a bounce to a new phase of expansion instead.]

More information: Cosmin Andrei et al, Rapidly descending dark energy and the end of cosmic expansion, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2200539119

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Paul Steinhardt and Proceedings of the National Academy of Sciences

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