Science is full of theories, especially in astronomy. The bubble universe is one of them; it’s the hypothetical region of spacetime that could form during a process called eternal inflation. According to the bubble universe theory, our observable universe is just one bubble among many others. Many movies, TV shows, and books have discussed this and told this as science fiction stories, and it mainly is. However, the idea is actually coming from real models in cosmology. Some models of eternal inflation allow regions of an inflating space to stop inflating and form separate bubble regions. Those bubbles could then evolve into universes with their own histories and, depending on the model, potentially different physical properties.
The idea that “models support” is a very common theme in astrophysics and, more generally, in astronomy. Models are useful because they let scientists make predictions that can be tested against observations. Some models become strongly supported when observations repeatedly match their predictions. Others remain speculative when their predictions are difficult to test or when observations do not support them.
Bubble universe models are in the second group. They arise from real mathematical models of cosmology, but we have not found evidence confirming the existence of other bubble universes. What exactly is a bubble universe theory? How could one form? Could our universe be one? And is there any way we could detect another bubble?
Note: Images in this article have been generated with the help of AI, publicly available LLMs.
What is a Bubble Universe?
A bubble universe is a hypothetical universe that forms as a separate region within a larger inflating spacetime. Like, imagine just balloons floating in space, but each balloon is a universe in its own right. Our universe is massive; the observable universe is 93 billion light-years, with millions, if not trillions, of galaxies, stars, planets, and even life. According to the bubble universe theory, there are bubble universes like ours, some of them as big as ours, some of them are smaller, and so on. Most of this thinking is closely linked to the idea of eternal inflation.
In some models, under the eternal inflation idea, inflation continues in parts of space even after it ends in other regions. When inflation ends in one of these regions, it can form a bubble-like region that evolves into a universe. Like, the universe is a bubble, and it may expand on all sides, but in one region, some part of that bubble’s end, the inflation ends. That creates a separate bubble with conditions developing into the hot, expanding universe described by standard cosmology. Our entire observable universe could, in some eternal inflation models, be inside one such bubble.
That does not mean scientists have found other bubbles. It is a theoretical possibility that comes from particular models of inflation.
How Could a Bubble Universe Form?
Bubble universe theory is hypothetical, and we don’t even know if they exist, but we still think about how bubble universes may form if they are real. It comes down to cosmic inflation. Shortly, cosmic inflation is a (proposed) period of extremely rapid expansion in the very early universe, a tiny fraction of a second after the Big Bang. During this brief phase, space itself expanded far faster than the speed of light (not because objects moved through space that fast, but because space itself was stretching).
This rapid expansion is why the universe looks so uniform on large scales today, including the nearly even temperature of the cosmic microwave background and the overall flatness of space. NASA describes inflation as a very early period of rapid expansion that occurred around 13.8 billion years ago. This is different than normal inflation in the universe because the universe is still expanding, but at a slower rate.
In some models, inflation does not end everywhere at the same time. An inflating environment continues expanding while other regions’ inflation stoops. These regions can be bubbles within the larger inflating space. They form small “pockets” in areas where inflation has stopped while the space continues to inflate.
Bubble Universe vs Multiverse: What’s the Difference?
While you are reading this, it may look extremely similar to the multiverse theory. I thought the same when I was learning about the subject, so I did my own research and took a deep dive into it. To keep it short, the bubble universe and multiverse theory are not the same. The ideas are, in essence, closely related, but they are scientifically quite different.
A bubble universe is a specific mechanism. It comes from certain models of eternal inflation, where regions of space stop inflating and form isolated “bubbles.” Each bubble can evolve into a universe with its own history, structure, and possibly different physical properties. In the bubble universe theory, the multiverse is structured. A physical process generates it: inflation continues in some regions while ending in others. This creates a potentially infinite “foam-like” structure of bubble universes inside a larger inflating spacetime. Also, under the bubble theory, if what we think is correct, there may be signatures in the cosmic microwave background that we can trace back to bubble collisions. Technically, they may be observable.
A multiverse, on the other hand, is a broader theory. It simply means “many universes exist,” in different forms and in different shapes. There are also many theories under the multiverse theory. Each has it’s own way how they form, behave, and look like. Not all of these multiverse theories involve bubbles or inflation. In other multiverse theories, the universes may not be connected at all. For example, in the Many-Worlds Interpretation of quantum mechanics, universes branch from quantum events rather than forming from cosmic inflation. These universes do not exist in a shared inflating space. They are separate outcomes of quantum evolution.
Could Our Universe be a Bubble Universe?
Possibly, but we do not know. Some eternal inflation models suggest that an observer inside a bubble would see a universe very similar to ours, meaning our observable universe could be one bubble among many. However, this remains purely theoretical. We have strong evidence for the expansion of the universe, the hot early universe, and the cosmic microwave background, but none of this confirms a multiverse or bubble structure. So while the idea is possible in theory, we cannot currently say our universe is a bubble universe.
What is Eternal Inflation?
Inflation is a quite broad term in astronomy. You have to define what you are talking about if you want to understand the concepts. There is cosmic inflation and normal inflation, and then we have the eternal inflation theory.
Cosmic inflation is a proposed period of extremely rapid expansion in the very early universe. It is different from the expansion we observe today. Our universe is still expanding, but cosmologists do not call this present-day expansion “inflation.” The cosmic inflation happened right after the Big Bang.
Eternal inflation is a class of models in which inflation continues indefinitely in some regions of spacetime. While inflation ends in some regions, it can continue in others. In certain models, the regions where inflation ends can develop into bubble universes.
A simplified picture:
- A region of spacetime undergoes inflation
- Inflation ends in some areas
- Those areas become bubble universes
- Inflation continues elsewhere
- More bubbles form over time
In some eternal inflation models, inflation continues in parts of the larger inflating region while other regions exit inflation and form bubble universes. This process can continue indefinitely in the inflating regions, potentially producing a very large number of bubble regions. However, keep in mind that both the eternal inflation and the bubble universe theories have no direct or indirect evidence. Our mathematical understanding says that this is possible.
Quantum Tunneling in the Bubble Universe Theory
Quantum tunneling is used in theoretical cosmology to describe how regions of spacetime could transition between different physical states. In quantum physics, systems can transition between states even when classical physics would say they lack the energy to overcome a barrier, because particles and fields are described probabilistically by wavefunctions rather than by fixed paths.
In cosmology, similar quantum field effects can allow transitions between vacuum states, which are the lowest-energy configurations of a field yet still contain structure that determines particle and force properties. Some of these inflationary models say that transitions like this can shift a region of space into a new vacuum state. In return, that potentially triggers rapid expansion and forms a bubble universe. This does not mean a particle literally creates a universe, though. Instead, quantum field theory introduces probabilistic vacuum transitions that can produce expanding regions that evolve by themselves (in some models). The “bubble” is a conceptual way to describe a region of spacetime that separates from the surrounding inflating background and develops its own evolution.
There is no single confirmed mechanism for bubble universe formation. Different models propose different types of vacuum transitions, including false vacuum decay and fluctuations during eternal inflation, while others remain purely theoretical. As a result, bubble universes remain a possible but unverified outcome of certain cosmological frameworks.
Does Each Bubble Universe Have Its Own Laws of Physics?
In some models, yes, but not in all cases. In certain versions of eternal inflation, different bubble universes can settle into different vacuum states after they form. If that happens, the underlying vacuum energy can determine the behavior of fundamental particles and forces, meaning that physics inside one bubble could, in principle, look different from physics inside another.
However, this outcome is highly model-dependent and is not a required feature of bubble universe theories. Many inflationary models do not necessarily predict large variations in physical laws between bubbles. For example, in multiverses, though, that is not the case. There are universes where the laws of physics are fundamentally different.
Bubble Universe vs the Big Bang
These are not competing ideas; they are different layers of the same cosmological framework that describe different stages or aspects of how a universe like ours could evolve. The Big Bang refers specifically to the early hot, dense expansion phase of our observable universe, which sets the initial conditions for everything we see today. Inflation is a proposed even earlier period of extremely rapid expansion that helps explain why the universe appears so uniform and flat on large scales. Eternal inflation takes this idea further and says that inflation may not have ended everywhere at once, but instead continues in some regions of spacetime, creating a constantly evolving, multiverse-like structure. Within that framework, bubble universes may form when localized regions stop inflating and transition into their own independent expanding spacetime regions.
The Role of Observational Evidence in Bubble Universe Hypothesis
There is currently no confirmed observational evidence that bubble universes exist. However, scientists have proposed ways to test the idea. One possibility is to search the cosmic microwave background for unusual patterns that could have been produced if another bubble collided with ours. We have already searched for possible bubble collision signatures in CMB data. So far, these searches have not produced a confirmed detection.
This is an important distinction. A proposed way to test a theory is not the same as evidence that the theory is correct. If future observations found a pattern that could not be explained by standard cosmology but matched a predicted bubble collision signature, that would give scientists something important to investigate.
All of this is extremely hypothetical, and personally, I think we are far away in our journey to find a real answer to whether the bubble universe theory is real or not. Probably millions of years later, there will be (probably humans will not survive that long, but the universe in its general sense) some sort of evidence if the bubble universe is real.
FAQ
What is a Bubble Universe?
A bubble universe is a hypothetical region of spacetime that could form in some models of eternal inflation. Inflation ends in that region while continuing elsewhere, potentially creating a separate expanding universe.
What is the Theory of Eternal Inflation?
The Theory of Eternal Inflation is about space growing forever. It causes new bubble universes to form. This happens through a process where space changes to create hot, dense areas, like the young universe.
What are the differences between Bubble Universe Theory and traditional cosmological theories?
Bubble Universe Theory looks past the Big Bang. It suggests another way the universe could work. It imagines many worlds inside a bigger universe, different from old theories.
What is the cosmic landscape of a Bubble Multiverse?
The Bubble Multiverse is like a huge quilt of different universes. Each one has its own rules and makeup. This makes a world of endless possibilities.
How does observational evidence support the Bubble Universe hypothesis?
There is no observational evidence that supports the Bubble Universe theory. We have indirect observational evidence through our other theories or what we know about the cosmos.