Multiverse: An Ocean of Expanding Space-Time Bubbles

One Universe Among Many

The concept of multiverse comes from inflationary cosmology. In this framework, our observable Universe may be just one expanding space-time bubble among many, born from a primordial quantum field. This idea stems from the work of physicists Alan Guth (1947-) and Andrei Linde (1948-), who proposed that cosmic inflation never completely stops, continuously producing new universe bubbles.

Note:
The inflationary field is a special form of energy proposed to explain why the Universe expanded so rapidly just after its birth. This field, also called a scalar field, filled all space at the very beginning of time. Its energy exerted negative pressure, causing the Universe to inflate like a bubble at tremendous speed. When this energy converted into particles and radiation, inflation ended and the hot Big Bang began, giving rise to matter and galaxies.

Birth of Bubbles in the Quantum Vacuum

At the very first moments, the Universe underwent a phase of exponential expansion. Quantum vacuum fluctuations, amplified by this inflation, created regions where the energy field locally stabilized, forming universe bubbles. Each stops inflating locally and becomes a distinct universe with its own energy density, geometry, and physical constants.

The decay of the inflaton field released energy that heated our cosmos, triggering a local Big Bang. Meanwhile, other regions continue inflating beyond our causal horizon. This process of eternal inflation produces 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. It simply shows how space itself stretches over time. The scale factor \(a(t)\) indicates the relative size of the Universe at a given moment: the larger \(a(t)\), the more the Universe expands.

The main equation is:

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

Here \(\rho\) is the energy density (matter, radiation…), \(k\) describes space curvature, and \(\Lambda\) is the cosmological constant. The expansion rate of the Universe depends on its matter content, shape, and vacuum energy. This approach explains both the current growth and the rapid inflation phase.

Comparison Between a Single Universe and an Inflationary Multiverse

References: Alan Guth (1947-), Andrei Linde (1948-), Alexander Vilenkin (1949-), Max Tegmark (1967-), Sean Carroll (1966-).
Sources: Physical Review D, Journal of Cosmology and Astroparticle Physics, Cambridge University Press, 2017.

A Limited Horizon in an Infinite Space

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

Conclusion: A Bubble in the Cosmic Foam

If eternal inflation is correct, our Universe is just one bubble in an infinite ocean of universes. This model greatly expands our view of reality: it is no longer only galaxies in one space, but sets of isolated universes within a meta-space quantum domain.

Note:
According to the many-worlds interpretation by physicist Hugh Everett (1930-1982), every quantum interaction could create a branching of possible universes, offering another purely quantum vision of the multiverse.