String theory has revolutionized cosmology, introducing models like brane cosmology that leverage its core concepts—branes—to explain the universe's origin and evolution.
Physicists including Nima Arkani-Hamed, Raman Sundrum, Lisa Randall, Paul Steinhardt, and Neil Turok pioneered brane cosmology in the late 1990s. Their work addresses the hierarchy problem: why gravity is vastly weaker than other fundamental forces.
To solve this, they draw on string theory's extra dimensions. Our observable universe spans four dimensions (three spatial plus time), but others exist beyond our perception.
Electromagnetism, weak and strong nuclear forces remain confined to these four dimensions, while gravity propagates through all, diluting its intensity in our space-time.
These ideas, grounded in string theory, inspired models where our four-dimensional universe resides on a dynamic brane within a higher-dimensional 'bulk' or hyperspace.
Central to brane cosmology is the brane (short for membrane), a dynamic, extended object with properties like energy, mass, and charge, evolving in space-time.
Specifically, a p-brane has p spatial dimensions. Our universe is modeled as a 3-brane (three spatial dimensions plus time), precisely a D3-brane—'D' for Dirichlet, honoring mathematician Johann Dirichlet.
In this framework, particles and forces (except gravity) arise from open strings with ends anchored to the brane, per Dirichlet boundary conditions.
Strings are one-dimensional objects defined by length, tension, vibration, and energy. Vibration modes dictate particle types—electrons, quarks, and more. Strings can be open segments or closed loops.
Gravity stands apart as closed strings, unbound to the brane, allowing propagation into the bulk's extra dimensions. This naturally resolves the hierarchy problem.
Beyond that, brane cosmology illuminates the universe's origin and fate.
In the early 2000s, Neil Turok, Paul Steinhardt, and Burt Ovrut proposed the ekpyrotic model, named after the Greek 'ekpyrosis'—a cyclic conflagration, rebirth, and renewal envisioned by ancient thinkers like Plutarch.
Here, our D3-brane drifts in a higher-dimensional bulk alongside other branes, some empty, others harboring universes.
The Big Bang? A collision between two branes in the bulk. Their impact converts kinetic energy into a quark-gluon plasma hot soup, fueling expansion—honoring Big Bang observations while alternating to inflation.
Such collisions could recur randomly, suggesting a cyclic cosmos where our universe may not be the first or last. Some variants predict Big Bounces, alternating Bangs and Crunches.
Testable predictions distinguish it: unlike inflation, it forecasts no B-mode polarization in primordial gravitational waves. Confirming this absence would bolster the model.
