Rewindwill the universe contract? Is dark energy changing?
Does dark energy weaken over time? DESI data hint at it but are not conclusive. The real science behind the Cosmos Era ending “Rewind” and the Big Crunch.
Could the universe one day stop expanding and start to contract? That depends on dark energy. DESI's 2025 data favor "dark energy weakening over time" at 2.8 to 4.2 standard deviations (sigma), but that is short of the 5 sigma needed for confirmation. The more sigma, the less likely the data are a coincidence, and 5 sigma is the threshold for confirmation. So this is not a conclusion yet, only a signal worth watching. The game's ending “Rewind” pushes this possibility to the extreme: dark energy turns negative, the universe contracts, and even "remembers" the previous run.
This page first explains how to unlock it, then covers what the "Big Crunch" really means, the history of discovering expansion and dark energy, and which parts are fiction.
How to unlock it
- Hint: What if dark energy is not a constant?
- Condition: Unlock “Big Crunch”, then witness the end of the universe.
- Reward: In the next universe, “Primordial universe” output is ×20 until that era ends (Echo).
The ending belongs to the Physics category. The reward is called “Echo”: in the next universe, “Primordial universe” output becomes 20 times higher, lasting only until that era ends. The key to unlocking it is the “Big Crunch” node, the branch where dark energy turns negative.
What the Big Crunch is
If dark energy turned negative, expansion would stop and then reverse. The cosmic microwave background would heat up again, galaxies would move closer together, and in the end everything would return to a single point. This is the scenario the “Big Crunch” node describes.
How expansion was discovered
- Theory first: Alexander Friedmann derived from general relativity that the universe can expand or contract, seven years before observational evidence appeared (Friedmann, Zeitschrift für Physik, 1922).
- The primeval atom: Georges Lemaître independently proposed that the universe is expanding, and later imagined that it began from a "primeval atom", an early form of Big Bang theory (Lemaître, Annales de la Société Scientifique de Bruxelles, 1927).
- Observational confirmation: The farther a galaxy is from us, the faster it recedes. The universe is not static (Hubble, PNAS, 1929).
How dark energy entered the picture
Using distant supernovae as "standard candles", astronomers found that the expansion of the universe is accelerating, and dark energy arrived on the scene. The work won the 2011 Nobel Prize (Riess et al., The Astronomical Journal, 1998). Another team independently reached the same conclusion: there is some form of energy in the universe driving the accelerating expansion (Perlmutter et al., The Astrophysical Journal, 1999).
The Planck satellite measured tiny fluctuations in the cosmic microwave background and gave the most precise cosmological parameters so far: an age of about 13.8 billion years, ordinary matter at only about 5%, dark matter at about 27%, and the rest dark energy (Planck Collaboration, Astronomy & Astrophysics, 2020).
The far future of the universe
Another study worked out the fate of the universe trillions of years from now and beyond: stars burn out, protons may decay, and black holes eventually evaporate (Adams and Laughlin, Reviews of Modern Physics, 1997). This is one far-future picture that has been seriously calculated. The game's “Rewind” pushes a different possibility to the extreme.
Game lore, not real
Dark energy turning negative and the universe contracting and "remembering" the previous run are all fiction. Liu Cixin's Contraction (1999) is about an astronomer who predicts that the universe will switch from expansion to contraction at one exact moment. People gather in an observatory to wait for that moment and for the reversal of time that follows. Its scientific background is this: whether the universe will recollapse depends on dark energy, and there is no physical basis for time running backward during contraction.
Related real research
The studies on expansion and dark energy above are related real science. They cannot prove that the universe will contract. They are only the physics background this ending touches.
Want to read the originals? Search these titles
- Friedmann (1922): "Über die Krümmung des Raumes", roughly "On the curvature of space"
- Lemaître (1927): "Un Univers homogène de masse constante et de rayon croissant", roughly "A homogeneous universe of constant mass and growing radius"
- Hubble (1929): "A relation between distance and radial velocity among extra-galactic nebulae"
- Riess et al. (1998): "Observational evidence from supernovae for an accelerating universe and a cosmological constant"
- Perlmutter et al. (1999): "Measurements of Omega and Lambda from 42 high-redshift supernovae"
- Planck Collaboration (2020): "Planck 2018 results. VI. Cosmological parameters"
- Adams and Laughlin (1997): "A dying universe: the long-term fate and evolution of astrophysical objects"
Real basis
DESI's 2025 data favor “dark energy weakening over time” at 2.8 to 4.2 standard deviations, but have not reached the 5 standard deviations needed for confirmation.
Big Crunch: If dark energy turns negative, expansion will stop and then reverse. The cosmic microwave background will heat up again, galaxies will move closer together, and in the end everything returns to a single point.
Contraction (Liu Cixin, 1999), the science: whether the universe will contract again depends on dark energy; there is no physical basis for time running backward during contraction.
Related research
These studies relate to this topic but are not direct proof of this page.
The universe is expanding: Hubble found that the farther a galaxy is, the faster it recedes. The universe isn't static; it is expanding.
Source: Hubble, PNAS 1929, "A relation between distance and radial velocity among extra-galactic nebulae"
Friedmann's universe: Friedmann derived from general relativity that the universe can expand or contract, seven years before the observational evidence.
Source: Friedmann, Zeitschrift für Physik 1922, "Über die Krümmung des Raumes"
The primeval atom: Lemaître independently proposed that the universe is expanding, and later imagined it began as a “primeval atom,” the forerunner of Big Bang theory.
Source: Lemaître, Annales de la Société Scientifique de Bruxelles 1927, "Un Univers homogène de masse constante et de rayon croissant"
Accelerating expansion: Using distant supernovae as “standard candles,” they found the expansion of the universe is accelerating. Dark energy entered the picture, and the work won the 2011 Nobel Prize.
Source: Riess et al., The Astronomical Journal 1998, "Observational evidence from supernovae for an accelerating universe and a cosmological constant"
42 supernovae: Another team independently reached the same conclusion: some energy in the universe is driving the accelerating expansion.
Source: Perlmutter et al., The Astrophysical Journal 1999, "Measurements of Omega and Lambda from 42 high-redshift supernovae"
Recipe of the universe: The Planck satellite gave the most precise cosmological parameters yet: an age of about 13.8 billion years, with ordinary matter only about 5%.
Source: Planck Collaboration, Astronomy & Astrophysics 2020, "Planck 2018 results. VI. Cosmological parameters"
The last chapter of the universe: Worked out the fate of the universe over trillions of years and beyond: stars go out, protons perhaps decay, and black holes finally evaporate.
Source: Adams & Laughlin, Reviews of Modern Physics 1997, "A dying universe: the long-term fate and evolution of astrophysical objects"
Recipe of the universe: The Planck satellite measured the tiny fluctuations in the cosmic microwave background: ordinary matter makes up about 5% of the Universe's total energy, dark matter about 27%, and the rest is dark energy.
Source: Planck Collaboration, Astronomy & Astrophysics 2020, "Planck 2018 results. VI. Cosmological parameters"
Game lore (fictional)
- Dark energy turning negative, and the universe contracting and “remembering” the previous round, are both fiction.
- Summary of the novel Contraction: An astronomer predicts that the universe will switch from expansion to contraction at one exact moment. People gather at the observatory to wait for that moment, and for the time reversal it will bring.