*The further science advances, the more we begin to understand just how little we know.*

### TL;DR

Physicists have introduced a new theory suggesting our Universe is an expanding 4D bubble at the intersection of two five-dimensional spaces. This bold idea could help explain dark energy, the force driving the Universe’s expansion, which has puzzled scientists for decades. The theory also builds on the concept of branes, developed by Lisa Randall and Raman Sundrum, and proposes that anti-de Sitter spaces can make string theory compatible with our observations.

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Physicists have proposed a daring new theory to explain the ongoing mystery of the Universe’s expansion – we could be living inside a “bubble” that exists between other five-dimensional spaces.

If this concept seems confusing, that’s normal – it’s meant to challenge the mind. However, we can break it down step by step. As we currently understand, our Universe has four dimensions: three dimensions of space (up-down, left-right, front-back) and one dimension of time that moves forward continuously.

The new idea, suggested by physicists from Uppsala University in Sweden, introduces the possibility of five-dimensional spaces. Where two of these spaces meet, they create an expanding bubble. According to a press release from Uppsala University, the entire Universe exists on the surface of this expanding bubble.

Moreover, it’s possible that other bubbles could exist, representing entirely different universes. This concept may sound far-fetched, but it has a crucial purpose – to help us finally understand “dark energy,” the mysterious force driving the Universe’s continuous expansion.

Scientists have been trying to solve the dark energy puzzle for decades, often turning to string theory, but with little success. String theory is a fascinating but challenging framework. It holds promise for uniting different aspects of physics, but it hasn’t been backed by strong evidence so far.

The theory emerged from attempts to extend Einstein’s theory of general relativity by adding more dimensions. It proposes that properties like charge and spin arise from tiny, vibrating strings within multiple dimensions.

Depending on the number of dimensions or constraints, these strings can behave in countless ways, resulting in up to 10^500 possible solutions, of which our Universe’s specific properties are just one.

However, string theory is increasingly being seen as flawed, as it often doesn’t align with what we observe in the Universe. Even when it’s not entirely wrong, string theory is difficult, if not impossible, to test in practical terms. So why do researchers keep pursuing it?

In science, not all mistakes are equally significant. Although string theory has major issues, it’s like an old car that no longer works but still has valuable parts. This brings us back to the concept of expanding space.

Scientists have long attributed the Universe’s expansion to a force called dark energy, which adds more space without affecting objects within it.

This model is known as the “de Sitter Universe,” named after Dutch astrophysicist Willem de Sitter, and it is widely accepted as a good fit for current observations.

Unfortunately, no version of string theory has successfully explained the de Sitter model. The models only work when vacuum energy is either decreasing or steadily negative.

This leaves us with two options: either abandon string theory or discard the widely-accepted de Sitter model. However, the Swedish researchers have presented a new solution that preserves both string theory and the de Sitter Universe model.

Their idea is based on a concept that was introduced around two decades ago. At that time, American physicists Lisa Randall and Raman Sundrum proposed alternative models of the Universe involving five-dimensional spaces, called “branes,” to address the problem of gravity’s relative weakness.

Imagine a space that includes time, up-down, left-right, front-back, and an additional dimension that is difficult to visualize.

Crucially, this five-dimensional space is called an “anti-de Sitter space,” which, unlike our Universe, isn’t dominated by dark energy and has the necessary negative energy properties.

By connecting two of these five-dimensional anti-de Sitter spaces, Randall and Sundrum theorized that different branes could account for different forces.

In this new theory, the Swedish researchers used similar spaces to describe our expanding four-dimensional Universe, complete with particles that transmit the correct amount of gravitational force.

How does string theory fit into this? The “strings” extend out from the extra dimension into our Universe, giving it its unique characteristics. According to the press release, “all matter in the Universe corresponds to the ends of strings that stretch out into the extra dimension.”

Most importantly, this model works because of the inclusion of anti-de Sitter spaces. It describes the Universe as a growing bubble with a five-dimensional space both inside and outside.

Like other string theory models, this one is unlikely to be the final answer. While the idea of living on the surface of an inter-dimensional bubble is poetic, it remains highly speculative. Physics is reaching a point where researchers must propose creative ideas that work in theory and then test them against reality.

Within these ideas of vibrating strings and bubble-like Universes, there may be clues that match strange patterns seen in particle accelerators or discrepancies in astronomical data.

And perhaps, in the future, elements of string theory will play a role in the next big breakthroughs in physics.

This research has been published in *Physical Review Letters*.