Adam Brown — Bubble universes, space elevators, & AdS/CFT

Adam Brown begins with a warning about confidence: the universe's ultimate fate lies so far in the future that our current answer may not be reliable. A century ago, scientists pictured a static universe. Hubble's observations then established expansion, prompting a debate over whether gravity would eventually reverse it in a Big Crunch or merely slow it. In the 1990s, observations instead indicated that expansion is accelerating, commonly described through dark energy or a cosmological constant.

That shift matters for more than cosmic scenery. If the acceleration continues, distant galaxies recede beyond reach. Civilizations might otherwise turn their matter into destinations, computation, or other sources of value, but Brown says that a positive, truly constant cosmological term could put much of this material permanently out of reach. Even probes sent close to light speed could fail to arrive or return. The result would be a finite supply of accessible free energy and, eventually, heat death.

Brown stresses that this is not settled physics. The fact that cosmological models have changed before is a reason for caution, and the cosmological constant may not be constant. If it can naturally decay toward zero, the universe's future could differ. More ambitiously, descendants might learn to change the effective vacuum themselves. This possibility remains speculative, but Brown argues that some quantum-gravity theories allow transitions among regions with different effective laws and cosmological constants.

The word vacuum here does not mean empty space in the everyday sense. Brown describes it as a possible set of effective physical laws, corresponding to a minimum in a higher-dimensional landscape of states. The vacuum we inhabit may be only a local minimum, with lower-energy alternatives reachable through spontaneous transitions or deliberate intervention. His analogy is water: steam and liquid water are different phases, and a barrier can keep a system temporarily in a less stable state. Cloud seeding illustrates how an intervention might help a transition occur.

The existence of multiple minima is well established in physics; deliberately engineering a controlled transition between them is the speculative step. Brown imagines something akin to a highly controlled particle accelerator that could collapse a field and create a bubble of a new vacuum large enough to expand rather than collapse under surface tension. The engineering challenge includes avoiding a black hole and, more seriously, avoiding a transition to a vacuum with hostile values for other constants, such as the electromagnetic coupling or strong nuclear force.

This would not literally rewrite the fundamental laws, Brown says. The deeper rules of the theory would stay fixed; what changes is the effective low-energy physics experienced in a region, much as water's density or viscosity can vary without changing the underlying laws of physics. He places the intervention beyond familiar megastructures such as Dyson spheres in the technological tree, while emphasizing that it is not magic and follows as a possibility from some leading quantum-gravity ideas.

The conversation asks whether a process that intelligent beings could trigger might also occur naturally. Brown notes that nature can produce phenomena humans can also engineer, sometimes in surprising forms: Africa's natural fission reactors are an example of a nuclear chain reaction occurring without a bomb. In his view, if vacuum decay is physically possible, quantum fluctuations should eventually produce it in the future, although the waiting time could be exponentially long.

A civilization might therefore choose to act rather than wait. Brown gives two reasons: spontaneous decay could take an impractically long time, and an uncontrolled transition could land in a vacuum inhospitable to life. A deliberate attempt might aim to lower the cosmological constant while preserving other useful features, although that remains conjectural. He also points to speculative models in which our own universe began as a bubble produced by an earlier vacuum transition, with the Big Bang marking a local decay from a higher-energy state.

The discussion turns from whether vacuum transitions can happen to where their energy would come from. Brown distinguishes local energy conservation, which remains valid in general relativity, from global conservation across an expanding universe. General relativity does not require a single conserved total energy for the whole expanding cosmos. A laboratory cannot create free energy through an ordinary local process, but a process involving the expansion of the universe is not governed by that same global accounting rule. On this account, a bubble universe could contain more matter and energy without a compensating global energy debit.

That distinction may radically change the long-term energy outlook. If the cosmological constant is truly fixed and positive, the accessible free energy is finite. If it can be manipulated or decays naturally, descendants could in principle avoid heat death and gain access to an unbounded supply of free energy. Brown notes that a bubble nucleated by a larger parent universe would be associated with a higher cosmological constant in that parent. The hierarchy permits transitions from higher to lower values more readily than the reverse, offering a limited sense in which the cascade still has a direction.

Brown sketches a nested picture: a rapidly expanding parent universe can produce bubbles with lower cosmological constants. A bubble with a nonnegative value can continue expanding, while a negative value eventually leads to collapse. Descendants might create further lower-constant regions, naturally or deliberately. He emphasizes that the engineering details are unknown; his narrower claim is that this scenario is not inconsistent with the laws of physics as currently understood.

When asked how likely it is that our own Big Bang came from a bubble in a metastable parent state, Brown gives roughly 50 percent, then explains why he will not go higher: combining quantum mechanics with expanding universes remains an active research problem, with major unresolved questions. The idea has an intuitive pull because quantum mechanics may populate regions that can expand, but Brown rounds down out of epistemic humility. He contrasts that uncertainty with a separate conditional claim: if the relevant physics is right and humanity survives, far-future descendants may eventually confront the cosmological-constant problem themselves.

A bubble is not a contained laboratory or a power source that can be used from outside. Such bubbles tend to expand at light speed and would soon engulf nearby observers and their future light cones. Brown therefore frames deliberate vacuum change as choosing a new environment to inhabit, not as building a device whose output can be harvested while remaining safely outside. Any attempt would have to land in a vacuum compatible with life.

This possibility also has a governance consequence. If one person or AI could trigger a transition that destroys everyone's future light cone, the action would impose a vast negative externality. Brown argues that unrestricted individual freedom would be hard to defend in such a world; institutions would need to prevent unilateral catastrophe. The risk could be worse if dangerous physics created incentives to build a high-output power plant near a vacuum instability, tempting each operator to accept a tiny chance of universal destruction for a private benefit. Whether such physics exists is unknown, but the example shows how basic physical limits could shape political institutions.