Is the Universe Infinite? What Physics Says About Its Size

Key takeaways

• The observable universe is a sphere with a radius of about 46 billion light-years and an estimated 200 billion to two trillion galaxies.
• A hypersphere model curves space back on itself, so a traveler moving in a straight line would eventually return to the starting point.
• In the hyperdonut model, distant galaxies could appear in multiple positions across the sky, at different stages of their life cycle.
• An infinite universe made of a finite set of fundamental particles should contain infinite copies of every configuration, including replicas of Earth.
• Looking deeper into space means looking back in time, so the finite age of the universe and the speed of light set a horizon.

Nobody knows yet. The observable universe is about 93 billion light-years across, but the whole universe could be much larger or infinite. Planck satellite data combined with galaxy-survey (BAO) measurements put the curvature of space within about 0.2 percent of flat at 68 percent confidence (Ωk = 0.0007 ± 0.0019), a result that fits both a finite universe and an infinite one.

We have always looked up at the night sky and wondered – is the universe infinite? Does it stretch on forever, or does it have an edge, a limit to its vast expanse? This question has captivated humanity for centuries, and even with our modern scientific knowledge, it remains a profound mystery.

In this ScienceShot, we will investigate the most recent theories and hypotheses about the nature of the universe’s boundaries. We will look at the evidence supporting both finite and infinite models, as well as the mind-bending implications of each.

The Observable Universe

To begin, we must understand the concept of the observable universe. Due to the finite speed of light, we can only see a spherical region of the cosmos whose light has had time to reach us in the 13.8 billion years since the Big Bang. This sphere, centered on our position, has a radius of about 46 billion light-years and contains an estimated 200 billion to two trillion galaxies.

While this observable universe represents the limit of our current visibility, it does not necessarily define the true extent of the cosmos. The real universe could be much larger, or even infinite.

Finite Universe Models

One possibility is that the universe is finite, but without a traditional edge or border. Imagine an ant crawling on the surface of an orange – from the ant’s perspective, the orange’s skin appears flat, yet it is ultimately a finite, curved surface without edges.

In a similar way, our three-dimensional space could be curved back on itself, forming a hypersphere. If this were the case, a traveler venturing in a straight line would eventually return to their starting point, having circumnavigated the entire universe.

This hyperspherical model solves the paradox of a finite universe having an “outside” – there is no outside, just as there is no true edge to the orange’s skin. However, for this to work from our perspective, current observational constraints only place a lower bound on how much larger than the observable universe the hypersphere would need to be – without a precise multiplier established – and the true size could be far larger still.

Another intriguing finite model is the hyperdonut universe. In this scenario, space-time has the topology of a higher-dimensional donut shape. This would result in fascinating phenomena, such as seeing distant galaxies in multiple positions across the sky, and observing the same celestial objects at different stages of their life cycle.

While these finite models may seem outlandish, they are mathematically consistent and offer potential solutions to the conundrum of a universe without edges.

Infinite Universe

The alternative to a finite universe is one that is truly infinite, stretching endlessly in all directions. Whether the universe is finite or infinite remains genuinely undetermined by current data: measurements from missions like the Planck satellite show the universe is consistent with being spatially flat within measurement uncertainty, a result compatible with both possibilities.

In an infinite universe, the possibilities are mind-blowing. As you travel outward, you will encounter an infinite number of new galaxies, stars, and celestial phenomena, with no end in sight. However, this infinity generates perplexing paradoxes.

For instance, if the universe is infinite and made up of a finite set of fundamental particles, then by pure chance, there should be infinite copies of every possible configuration of matter and energy, including identical replicas of Earth, the solar system, and even yourself.

While the probability of encountering such an exact duplicate may be infinitesimally small, in an infinite universe, even the most improbable events become inevitable over vast distances and timescales.

The Edge of Time

Though these theories are fascinating, we have to face a sobering fact: from our human perspective, time itself is the universe’s definitive edge.

As we peer deeper into the cosmos, we are effectively looking back in time, observing the universe as it was billions of years ago. Eventually, we reach a point where there is no further past to observe, a cosmic horizon beyond which we cannot see.

This temporal boundary is not a true edge in the physical sense, but rather a limitation imposed by the finite age of the universe and the finite speed of light. It is a horizon that we may never be able to surpass, no matter how far our scientific capabilities advance.

Conclusion

As we have explored, the question of whether the universe is infinite or finite remains unanswered, and perhaps unanswerable with our current scientific understanding. Both models present profound mysteries and paradoxes that challenge our intuitive notions of space, time, and reality itself.

The vastness of an infinite universe, filled with an endless array of wonders and possibilities, is a concept that ignites the imagination and speaks to the boundless potential of existence.

Yet, even if the universe is truly infinite, our human experience will always be confined to a finite realm – the observable universe, the edge of time. This realisation serves as a reminder that our understanding of the universe, no matter how advanced, will always be constrained by our physical limitations and the finite nature of our being.

Related reading: the cosmology and astrophysics section and the Great Filter theory, which asks what such a large universe should have produced.

@online{scienceshot-the-cosmic-paradox-is-2024,
  author       = {Turner, Alex},
  title        = {{Is the Universe Infinite? What Physics Says About Its Size}},
  organization = {ScienceShot},
  date         = {2024-04-05},
  year         = {2024},
  url          = {https://scienceshot.com/post/the-cosmic-paradox-is-the-universe-finite-or-infinite}
}