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.
Cosmologists are exploring a new method to determine the universe's shape – whether it's flat, spherical, or saddle-shaped – by analyzing pairings of gravitational lenses. Traditional methods rely on the cosmic microwave background, but this new technique uses the subtle distortions of light from distant galaxies bent around massive foreground objects. By examining the statistical correlations in the shapes and orientations of these lensed images, researchers can glean information about the curvature of spacetime, potentially providing an independent confirmation of the currently favored flat universe model, or revealing a surprising deviation. This method offers a potential advantage by probing a different cosmic epoch than the CMB, and could help resolve tensions between existing measurements.
HN commenters discuss the challenges of measuring the universe's shape, questioning the article's clarity on the new method using gravitational waves. Several express skepticism about definitively determining a "shape" at all, given our limited observational vantage point. Some debate the practical implications of a closed universe, with some suggesting it doesn't preclude infinite size. Others highlight the mind-boggling concept of a potentially finite yet unbounded universe, comparing it to the surface of a sphere. A few commenters point out potential issues with relying on specific models or assumptions about the early universe. The discussion also touches upon the limitations of our current understanding of cosmology and the constant evolution of scientific theories.
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.