Multiverse: An Ocean where each Spacetime Bubble has its own Physical Laws

Genesis of bubbles in the quantum vacuum

A primordial phase of exponential expansion

At the very first instant, the Universe experienced a phase of exponential expansion. The quantum vacuum fluctuations, amplified by this inflation, could have generated zones where the energy field locally froze, creating universe bubbles.

Bubbles with unique physical laws

Each of them stops inflating locally and becomes a distinct universe with its own energy density, geometry, and physical constants.

From the local Big Bang to eternal inflation

The decay of the inflaton field released the energy that heated our cosmos, triggering the local Big Bang. Meanwhile, other regions continue to swell beyond our causal horizon. This process of eternal inflation (cosmological model where the inflation process continues to produce new universe bubbles in the quantum vacuum) leads to a global space filled with continuously expanding universe-bubbles.

Expansion described by the Friedmann metric

The growth of our Universe can be described by a mathematical model called the Friedmann-Lemaître-Robertson-Walker metric. This formulation simply translates how space itself stretches over time. The scale factor \(a(t)\) indicates the relative size of the Universe at a given time: the more \(a(t)\) increases, the more the Universe expands.

The main equation is written as:

\(\left( \frac{\dot{a}}{a} \right)^2 = \frac{8 \pi G}{3} \rho - \frac{k c^2}{a^2} + \frac{\Lambda c^2}{3}\)

In this equation, \(\rho\) represents the energy density (matter, radiation…), \(k\) describes the curvature of space, and \(\Lambda\) is the cosmological constant (cosmological constant, representing the vacuum energy that accelerates expansion). Thus, the expansion rate of the Universe depends on its matter content, its shape, and the vacuum energy. This approach allows us to understand why the Universe is growing today and how it grew during inflation.

Comparison between a single universe and an inflationary multiverse

A limited horizon within infinity

Our cosmological horizon is defined by the speed of light and the age of the Universe. Beyond it, other bubbles continue to grow at apparent speeds > c, not because matter moves faster than light, but because space itself expands. Thus, no information can cross the boundaries between universe-bubbles.

Another vision of the multiverse: Everett's interpretation

A revolution in the understanding of quantum mechanics

In 1957, physicist Hugh Everett (1930-1982) proposed a radical interpretation of quantum mechanics, known as the Many-Worlds Interpretation (MWI). Faced with the measurement problem in quantum mechanics, Everett rejected the idea of a "wave packet reduction" (wave function collapse) and postulated that all possible outcomes of a quantum measurement actually occur, each in a distinct branch of reality.

An infinity of bifurcated realities

According to this model, each quantum interaction causes a bifurcation of the Universe. For example, if a particle can take two states, the Universe divides into two branches: in one, the particle is in state A; in the other, it is in state B. These worlds are real in the same way as ours, but they cannot communicate with each other. Unlike the inflationary multiverse, which creates separate bubbles in space, Everett's multiverse is a space of possibilities that branches at every instant, generating an infinity of coexisting realities.

Fundamental distinction from the inflationary multiverse

It is important not to confuse these two approaches. The inflationary multiverse is a consequence of cosmology: it describes distinct regions of spacetime, physically separated by horizons. Everett's multiverse, on the other hand, is a consequence of quantum mechanics: it describes a multiplicity of branches of the same quantum universe, resulting from the deterministic evolution of the wave function. The former is a question of cosmic expansion, the latter a question of quantum superposition. Both, however, extend our conception of reality far beyond the observable Universe.