A milder relative of the Big Rip, the expansion rate blows up, but only after infinite time.
Little Sibling of the Big Rip
A future where the Hubble rate diverges but only at infinite time, while the cosmic acceleration stays finite, giving a gentler dissolution than the Big Rip without a finite-time singularity.
Looping ambient scene for Rip Scenarios. The rip futures all follow from phantom dark energy: a dark energy whose density increases as the universe expands, which happens when its equation of state w sits below -1. A growing repulsion does not just accelerate the expansion, it strengthens without limit, so structures held together by gravity, then by electromagnetism, then by the nuclear forces, are progressively unbound. The variants differ in how the energy density grows and therefore in whether the end comes at a finite time (the Big Rip), only as time runs to infinity (the Little Rip), or in an intermediate way that spares some structures (the Pseudo-Rip and the Little Sibling of the Big Rip). All of them rest on the premise that w is below -1, which current data neither confirm nor exclude.
§1 · The claim, in one sentence
The Little Sibling of the Big Rip is another way to soften the Big Rip. Here the Hubble rate diverges, but only as time runs to infinity, and the cosmic acceleration stays finite throughout. Bouhmadi-López and collaborators 2015 introduced it as an abrupt-event-free future where the universe still unbinds its structures but avoids both the finite-time singularity of the Big Rip and the bounded force of the Pseudo-Rip.
§2 · Why it might be true
Cosmologists have catalogued a family of possible future singularities, classified by which quantities (the scale factor, the Hubble rate, its derivatives) blow up and when. The Little Sibling of the Big Rip is one such case, defined by a Hubble rate that diverges at infinite cosmic time while the derivative that sets the acceleration remains finite.
The physical effect is a dissolution gentler than the Big Rip. Because the Hubble rate grows without bound, bound structures are still eventually torn apart, but because it takes infinite time and the acceleration stays finite, there is no sudden doomsday and no instant where the equations break down at a finite date.
The variant matters less as a likely future than as a mapping of the possibilities. Once you allow dark energy to depart from a cosmological constant, a whole landscape of endings opens up, and careful classification (which this variant belongs to) is how cosmologists keep track of what a given dark-energy behaviour would actually do to the universe.
The family stance
The universe ends by being torn apart. A dark energy that grows stronger over time eventually overwhelms gravity and every other force, dismantling bound structures from the largest scales down to atoms. Whether this happens at a finite future moment or only asymptotically depends on exactly how the dark energy density grows.
§2.5 · Evidence
- It is a mathematically well-defined member of the classified set of future cosmological behaviours, not an ad hoc construction
- It avoids the finite-time singularity, addressing the main theoretical objection to the Big Rip
- It illustrates that the boundary between tearing-apart and fading-away futures is a continuum, which is editorially useful for orienting readers
§3 · What you'd need to test it
- The Hubble rate diverges only at infinite time, with finite cosmic acceleration throughout
- Bound structures are eventually unbound, but with no finite-time singularity and no abrupt event
- The future is distinguished from the Big and Little Rips by the specific combination of which expansion quantities diverge
- The scenario corresponds to a particular dark-energy equation of state evolution that, in principle, observations of w(z) could constrain
§4 · Where it breaks
- It is a minor and recent entry with limited independent follow-up, so its standing as a distinct fate is weaker than the Big or Little Rip
- Like all rip variants it requires dark energy beyond a cosmological constant, with no positive evidence
- Its observational signature is extremely subtle and essentially indistinguishable from neighbours with current data
Go deeper
Bouhmadi-López, Errahmani, Martín-Moruno, Ouali, and Tavakoli 2015 (Int. J. Mod. Phys. D 24, 1550078) define the Little Sibling of the Big Rip by a Hubble rate H that diverges at infinite time while its cosmic-time derivative stays finite, so the scale factor and H blow up but the acceleration does not run away abruptly. They place it within the broader taxonomy of dark-energy-driven future events, alongside the Big Rip, Little Rip, and the sudden and Type II to IV singularities.
This taxonomy is the real content. Nojiri, Odintsov, and Tsujikawa 2005 set out the classification of finite-time future singularities by which of the scale factor, energy density, and pressure diverge; the rip variants and the exotic singularities in this chapter are entries within or adjacent to that scheme. Reading them together shows that a single question, how does the dark energy density behave at late times, fans out into a structured set of possible endings.
Cross-references: see the Exotic Future Singularities family in this chapter for the closely related sudden and Type II to IV singularities, and the Big Rip and Little Rip variants here for the better-known members of the rip sequence.
Variants in this family
Compare variants▸§5 · Who built it, and when(1 source, 1 established)
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