In this theory, called "eternal inflation," proposed by Paul Steinhardt, other bubble universes are constantly popping up in other parts of the inflationary sea, with the whole ensemble making up a "multiverse." Some scientists think that when our universe dropped out of this inflationary phase, it was just one tiny bubble in a vast sea of inflating space.
In the conventional view of the Big Bang, in order to explain the uniformity of the CMB, it's necessary to postulate an early spurt of superfast expansion known as inflation.
Is our universe just one bubble in a vast multiverse? (Image credit: VICTOR DE SCHWANBERG/SCIENCE PHOTO LIBRARY via Getty Images) (opens in new tab) In the case of the Randall-Sundrum model, such tests could involve measuring gravitational waves emitted by black holes linking one brane to another. But it's not enough for a theory to explain facts we already know - it has to make new predictions that can be tested experimentally. For example, theoretical physicists Lisa Randall, of Harvard University, and Raman Sundrum, of the University of Maryland, proposed a version of the braneworld that explains an asymmetry in subatomic forces by suggesting the existence of other branes parallel to our own.
This higher dimensional space is referred to as "the bulk," while our universe is a three-dimensional membrane - or "brane" - floating inside the bulk.Īs complicated as it sounds, the braneworld picture solves several problems in physics. Some theories, however, suggest another spatial dimension - which we can't perceive directly - in another perpendicular direction. (Image credit: Shutterstock) (opens in new tab)Īn aspect of the universe we take for granted is that it's three dimensional - there are three perpendicular directions you can move in.