Season 17, Episode 19: Is Dark Matter Just Regular Matter in Another Universe?

Hey StarTalkians! Neil and Chuck got a treat this week, sitting down with Professor Brian Greene for a chat about hidden dimensions, string theory and Hilbert spaces. In the discussion, Professor Greene explained a little about “brane worlds” and how they interact with gravity:

Exploring Hidden Dimensions with Brian Greene - StarTalk Radio

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There is a tantalizing question that comes out of all of this: what if dark matter was just regular matter in another brane?

The Basics of String Theory

The “basics” of string theory are actually easy enough to understand if you don’t delve into the math. It posits that, instead of multiple fundamental particles, the fundamental objects of the universe are actually one-dimensional “strings.”

In the same way guitar strings can produce multiple stable pitches, these cosmic strings can also vibrate in several different ways. These different vibrational “modes” lead to the particles in our universe. So like you can pluck out a whole scale along a guitar string, the cosmic strings create the symphony of matter we see around us.

Brane Worlds Explained

A “brane” is like a cosmic “membrane” that occupies some number of dimensions. A 0-dimensional brane is a particle, a 1-dimensional brane is a string, a flat 2D membrane is a 2-brane and so on.

These branes can have more dimensions, and crucially, get much bigger. It’s possible that a multi-dimensional brane got a huge boost of energy and inflated, ultimately forming our universe. There could equally be other brane worlds out there, alongside ours in a higher-dimensional space.  

Strings interact with these higher-dimensional branes. Strings are either closed (connected to itself) or open. The ends of open strings attach to branes, confining them to that brane world. Most of the forces (electromagnetism and the strong and weak nuclear forces) are confined in this way. Gravity, on the other hand, comes from closed strings, which can freely travel through the inter-brane space, leaking out of our universe.  

The “Dark Brane” and Dark Matter

That explains why gravity is so weak, as Brian pointed out. But it might also explain dark matter. Some physicists propose a hypothetical “dark brane”, which is filled with ordinary matter.

Electromagnetism is confined to the brane, so we can’t see this other world. But gravity is not, so maybe we can feel it, as what we call “dark matter.”