What Happens At The Edge Of The Universe? | Space Time | PBS Digital Studios

Summary

This episode of 'SpaceTime' delves into the concept of the universe's edge, exploring the observable universe, particle horizon, and cosmic event horizon. It discusses the limitations of what we can observe and interact with due to the universe's expansion. The script also questions whether the universe is infinite or finite, and what lies beyond our current understanding, including possibilities like a multiverse. Through a mix of humor and science, the video encourages curiosity about the nature of space, time, and the mysteries that await discovery.

Takeaways

• 😀 The concept of the 'edge' of the universe is debated, especially since the universe is considered infinite by general relativity.
• 😀 The observable universe has a defined radius of 46 billion light years, referring to the current distance to the most distant part of the universe we can see.
• 😀 The 'particle horizon' is the furthest distance from which we can receive information, and it defines the known universe.
• 😀 In cosmology, distances are affected by the expansion of space, meaning light and other objects travel through an ever-changing spacetime.
• 😀 Traveling to the particle horizon would require infinite distance, even if a spaceship could travel at the speed of light.
• 😀 The cosmic event horizon is different from the particle horizon, being closer and marking the limit beyond which we can never receive information.
• 😀 Galaxies beyond the cosmic event horizon are visible to us but will never be reachable or communicable with, as they are receding faster than light.
• 😀 If we break the cosmic speed limit, we could theoretically reach the particle horizon, but we'd just encounter more universe without reaching a final boundary.
• 😀 The geometry of the universe is likely flat, with measurements suggesting the universe could be infinite, meaning traveling past the particle horizon could lead to an endless journey.
• 😀 There is speculation that the universe could have a small, undetectable curvature, potentially allowing a spaceship to eventually loop back to its starting point if it were a hypersphere.
• 😀 Some theories suggest that our universe could be part of a multiverse, where our universe could have a true edge, and laws of physics or dimensions may differ outside it.

Q & A

What is the edge of the universe and how can we conceptualize it?

The 'edge' of the universe is often described in terms of the 'particle horizon,' which marks the furthest distance from which light and signals can travel to us due to the expansion of the universe. However, it's not a literal edge in space, but rather a boundary defined by the observable universe, which keeps expanding as the universe itself does.

Why do people say the universe is infinite, and how do we know it's flat?

The universe is often described as infinite because general relativity suggests it is 'flat' on large scales, meaning space itself doesn't curve back on itself. Observations of galaxy distributions and the Cosmic Microwave Background (CMB) support this flatness, though with high precision, but not infinite accuracy, leaving open the possibility of small curvature.

What is the difference between the 'particle horizon' and the 'cosmic event horizon'?

The 'particle horizon' refers to the limit of the observable universe, the furthest point from which we can receive any information. The 'cosmic event horizon,' on the other hand, is the boundary beyond which we can never receive any signals emitted in the future, as space will expand faster than the speed of light, preventing any communication or travel past it.

Can we ever reach the 'particle horizon' in a spaceship?

No, even if we could travel at the speed of light, we would never reach the particle horizon. As we approach it, the space between us and the horizon expands faster than we can move, effectively making the distance infinite.

What is the significance of the 'cosmic event horizon' being closer to us than the particle horizon?

The cosmic event horizon is closer to us because it marks the boundary beyond which we cannot receive any new signals. Galaxies within this region are receding from us so quickly that even light emitted today won't be able to reach us, making them effectively unreachable, even though they may still be observable in the short term.

What would happen if we could travel faster than the speed of light, say with a warp drive?

If we could exceed the cosmic speed limit, like with a theoretical warp drive, we might be able to reach regions beyond the cosmic event horizon. However, it's likely that we'd still encounter more of the universe, with no significant edge to cross, just more of the same infinite expanse.

What is the difference between the observable universe and the 'true' universe?

The observable universe refers to the portion of the universe we can see and measure, limited by the distance light can travel since the Big Bang. The true universe might extend beyond our observational limits, possibly infinitely, but we cannot directly perceive or interact with it due to the constraints of spacetime and the universe’s expansion.

What would we encounter if we could cross the particle horizon?

If we could cross the particle horizon, we would encounter a similar distribution of galaxies and cosmic structures as we see today. Since the universe is vast and possibly infinite, it would be essentially the same, with no 'end,' just more of the universe.

Could the universe have a true edge or boundary?

If the universe is curved, there could theoretically be a true edge, but this would depend on the overall geometry of the universe. Some hypotheses suggest the universe could be the 3D surface of a 4D hypersphere, in which case it would be finite but unbounded, with no 'edge' in the traditional sense.

Is the universe perfectly flat, or is there some curvature we haven’t detected yet?

The universe is considered nearly flat based on current measurements, but it's possible that the curvature is so small that we haven’t detected it yet. If there is curvature, it could mean the universe is finite and could be a 'hypersphere,' in which case traveling far enough could bring you back to your starting point.