Ekpyrotic Cyclic Universe

2001Paul Steinhardt, Neil Turok, Justin Khoury, Burt OvrutSpeculativeAlso answers, Ch.02 The Origin of Our UniverseAlso answers, Ch.07 The Fate of the Universe8 primary sources, 8 established

Two parallel branes collide periodically, each collision is a Big Bang.

Visualisation · Cyclic & Bouncing Cosmologies

brane: A higher-dimensional surface on which strings can end. Our universe may be a 3-brane embedded in a larger space.
bulk: The higher-dimensional space in which lower-dimensional branes are embedded.
dark energy: The unknown component driving the accelerating expansion of the universe, making up about 68 percent of its energy.
inflation: A brief burst of accelerating expansion in the very early universe, smoothing out and stretching whatever existed before.
string theory: A framework proposing that fundamental particles are tiny vibrating strings rather than points, requiring extra dimensions of space.
scalar field: A field with one value at each point in space, like a temperature map. The inflaton is one example.
quantum fluctuation: A spontaneous, unpredictable variation in a quantum field. Tiny in normal conditions, decisive at the smallest scales.

§1 · The claim, in one sentence

Our universe was born from a collision between two parallel branes, and the collision repeats every trillion years.

§2 · Why it might be true

In the ekpyrotic model, our universe sits on a 3-brane (a three-dimensional sheet) floating in a higher-dimensional space called the bulk. A parallel "hidden" sheet lurks just nearby in the extra dimension. Slowly, over enormous timescales, the two branes are drawn together. When they collide, the released energy fills our brane with hot matter and radiation, that is our Big Bang.

After the collision, the branes separate, the universe expands and cools, and eventually dark energy dilutes it. Then the slow attraction begins again. The cycle repeats forever.

The family stance

A previous cycle, aeon, contracting phase, or alternate-brane state existed before our universe. The "before" is a physically connected predecessor, not nothing or another arena.

§2.5 · Evidence

Resolves horizon and flatness problems without inflation
Low tensor-to-scalar ratio prediction consistent with current data

§3 · What you'd need to test it

Almost no primordial gravitational waves: the tensor-to-scalar ratio r, the strength of gravitational-wave ripples relative to density ripples, sits near zero (r ≈ 0)
The spectrum of primordial density ripples is slightly stronger at large scales than small, a so-called red tilt
Each cycle ~10¹² years or longer

§4 · Where it breaks

Requires string theory and extra dimensions
Brane-collision mechanism not fully derived
Has its own initial-conditions problem

Go deeper

In the cyclic model, the scalar field (a brane modulus) drives slow ekpyrotic contraction. The equation of state w = p/ρ greatly exceeds 1, suppressing anisotropies and curvature exponentially.

Quantum fluctuations during contraction generate a nearly scale-invariant spectrum of density perturbations, similar to inflation but with much lower tensor-to-scalar ratio.

▸§5 · Who built it, and when(8 sources, 8 established)

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Currently #2 in this family · #6 in Ch.1

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