The blog post "Open and Closed Universes" explores the concept of universe curvature and its implications for the universe's ultimate fate. It explains how a "closed" universe, with positive curvature like a sphere, would eventually collapse back on itself in a "Big Crunch," while an "open" universe, with negative curvature like a saddle, would expand indefinitely. A "flat" universe, with zero curvature, represents a critical point between these two scenarios, also expanding forever but at a decelerating rate. The post uses the analogy of a ball thrown upwards to illustrate these concepts, where the ball's trajectory depends on its initial velocity relative to escape velocity. It concludes by mentioning the current scientific consensus, based on observations, which favors a flat or very slightly open universe, destined for continuous expansion and eventual heat death.
The blog post "Open and Closed Universes (2021)" by Sunil Pai explores the fascinating cosmological question of the universe's ultimate fate, focusing on the concepts of open, closed, and flat universes. It begins by establishing the context of the Big Bang theory, explaining that the universe originated from a singularity and has been expanding ever since. The post then delves into the crucial role of gravity in determining the universe's long-term trajectory. Gravity, as an attractive force, acts as a brake on the expansion, potentially halting it entirely or even reversing it.
The post introduces the concept of the critical density, a pivotal value that determines the universe's geometry and destiny. If the universe's actual density is greater than this critical density, the gravitational pull will eventually overcome the expansion, leading to a closed universe. This scenario is analogous to a ball thrown upwards; it eventually falls back down due to gravity. In a closed universe, the expansion would reverse in a "Big Crunch," potentially leading to another singularity.
Conversely, if the universe's density is less than the critical density, gravity will be insufficient to halt the expansion. This results in an open universe, akin to a rocket escaping Earth's gravity. The expansion continues indefinitely, albeit at a decelerating rate.
The third possibility is a flat universe, where the density precisely equals the critical density. In this case, the expansion continues forever, but the rate of expansion approaches zero asymptotically. The post uses the analogy of a ball rolling on a perfectly flat, frictionless surface, gradually slowing down but never completely stopping.
The post then discusses the observational evidence supporting the idea of a flat universe, citing measurements of the cosmic microwave background radiation, a relic from the early universe. These measurements suggest a geometry very close to flat, although the post acknowledges the possibility of slight deviations that could be difficult to detect.
Furthermore, the post introduces the concept of dark energy, a mysterious force counteracting gravity and accelerating the universe's expansion. The presence of dark energy complicates the simple picture of open, closed, and flat universes based solely on matter density. Even if the universe is geometrically flat, the accelerating expansion due to dark energy implies a future where distant galaxies become increasingly isolated, eventually disappearing beyond our observable horizon – a "Big Freeze" scenario.
The post concludes by emphasizing the ongoing nature of cosmological research and the uncertainties surrounding the universe's ultimate fate. While current evidence points towards a flat or very nearly flat universe dominated by dark energy and headed for a Big Freeze, further observations and theoretical advancements are necessary to refine our understanding of this profound question. The interplay between gravity, dark energy, and the universe's geometry remains a central theme in modern cosmology, and the quest to determine the universe's ultimate destiny continues.
Summary of Comments ( 2 )
https://news.ycombinator.com/item?id=43087884
HN commenters largely discuss the difficulty of truly comprehending the vastness and complexity of the universe, with some pointing out the limitations of human intuition and the challenges of visualizing higher dimensions. Several express fascination with the concept of a closed universe and its implications for the finite yet unbounded nature of space. Some debated the philosophical implications, touching upon the potential for simulated universes and questioning the nature of reality if our universe is indeed closed. A few comments also delve into more technical aspects, like the role of dark energy and the expansion of the universe in determining its ultimate fate. One commenter suggests looking at the problem through the lens of information theory and entropy, proposing that the universe might be both open and closed simultaneously depending on the observer's perspective.
The Hacker News post titled "Open and closed universes (2021)" with the URL https://news.ycombinator.com/item?id=43087884 has a moderate number of comments discussing various aspects related to the linked article about open and closed universes.
Several commenters discuss the difficulty of truly comprehending the concept of an expanding or closed universe, with one highlighting the challenge of visualizing something that expands "into" something else when, by definition, the universe encompasses everything. Another commenter points out the limitations of analogies used to explain these concepts, suggesting they can sometimes oversimplify and even mislead. There's a brief exchange about the "shape" of the universe, clarifying that the commonly used terms like "flat," "spherical," and "hyperbolic" refer to the curvature of spacetime and not the universe's physical shape in the way we typically think about it.
A couple of comments touch upon the philosophical implications of different universe models. One commenter wonders about the implications of a closed universe for the concept of infinity, questioning whether a finite yet unbounded universe could still contain infinite possibilities. Another thread explores the idea of simulations and whether it would be possible to detect if our universe were simulated, connecting it back to the nature of a closed universe.
One commenter draws a parallel between the expansion of the universe and the expansion of software complexity, jokingly suggesting that software might be evidence of an open universe. Another comment pivots to discussing the measurement of the Hubble constant and the discrepancies between different measurement methods. This leads to a short discussion about the limitations of current cosmological models and the possibility of unknown factors influencing the expansion rate.
A few comments delve into more technical aspects, such as the mathematical framework used to describe the curvature of spacetime. One commenter mentions the Friedmann equations, which are used to model the expansion of the universe. Another refers to the concept of a Poincaré dodecahedral space as a possible topology for a closed universe.
Overall, the comments on Hacker News demonstrate a range of engagement with the topic, from casual musings on the nature of the universe to more in-depth discussions about cosmology and theoretical physics. While not containing groundbreaking new insights, they offer a diverse set of perspectives and reflections on the complexities of understanding the universe and its potential structures.