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Does the universe have a shape?

John Healy

This Quanta Magazine article (below) is a helpful visual recap of whether the cosmos is “flat” or not. Does the cosmic landscape of stars and galaxies extend / expand in all directions like an endless piece of paper? Or another “flat” geometry – “by cutting a chunk out of Euclidean space and gluing it together.”

Or does space curve in some way in higher dimensions? (“By gluing up a suitable chunk” of a 3-sphere, for example.)

What’s the evidence against a flat universe? The paths of “straight” lines and angles in triangles might tell.

• Quanta Magazine > “What Is the Geometry of the Universe?” by Erica Klarreich (March 16, 2020)

There was a time, after all, when everyone thought the Earth was flat, because our planet’s curvature was too subtle to detect and a spherical Earth was unfathomable.

We can ask two separate but interrelated questions about the shape of the universe. One is about its geometry: the fine-grained local measurements of things like angles and areas. The other is about its topology: how these local pieces are stitched together into an overarching shape.

3 thoughts on “Does the universe have a shape?”

  1. Olbers’ paradox

    The darkness of the night sky is one of the pieces of evidence for a dynamic universe, such as the Big Bang model. That model explains the observed non-uniformity of brightness by invoking spacetime’s expansion, which lengthens the light originating from the Big Bang to microwave levels via a process known as redshift; this microwave radiation background has wavelengths much longer than those of visible light, so appears dark to the naked eye. Other explanations for the paradox have been offered, but none have wide acceptance in cosmology.

  2. Here’s Ethan Siegel’s take on Olbers’ paradox.

    Medium > “Why Is The Sky Dark At Night?” by Ethan Siegel (Aug 27, 2019) – The darkness of the night sky was a mystery for generations of humans. Here’s the reason why.

    … you might expect to see light in every direction and at every location you were capable of looking in. After all, if the Universe is truly infinite, then the void of deep space goes on forever. In any direction you can imagine, eventually your line-of-sight will run into a shining point of light.

    Back in the 19th century, Olbers used this line of reasoning to conclude that the observable Universe could not be infinite, but he couldn’t be sure.

    But if the Universe were truly infinite, the [dust] problem of Olbers’ Paradox would show up for every dust grain out there: each grain would have to absorb an infinite amount of starlight, until it, too was radiating at the same temperature of all the light it absorbed!

    It takes two facts, together, to explain why the night sky is dark. The first one is that the Universe has only been around for a finite amount of time, which limits the extent and amount of the radiation that’s presently observable to us. The second is that we can only see light in a limited part of the electromagnetic spectrum: the optical portion [not the microwave portion].

  3. Progress in mapping the observable universe … more galaxies, more detail … more supercomputers …

    • Space.com > “Scientists just mapped 1 million new galaxies, in 300 hours” by Brandon Specktor (Dec 4, 2020) – All-sky surveys usually take years. This one took weeks.

    This new sky survey, which Australia’s national science agency (CSIRO) described in a statement as a “Google map of the universe” , marks the completion of a big test for the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope –- a network of 36 antennas rooted in the remote Western Australia Outback. While astronomers have been using ASKAP to scour the sky for radio signatures (including mysterious fast radio bursts) since 2012, the telescope’s full array of antennas has never been used in a single sky survey –- until now.

    See also:

    • Science Alert > “Astronomers Just Mapped 1 Million Previously Unknown Galaxies, And You Can Take a Tour” by Aidan Hotan, The Conversation (1 December 2020)

    The Rapid ASKAP Continuum Survey (or RACS) has placed the CSIRO’s Australian SKA Pathfinder radio telescope (ASKAP) firmly on the international astronomy map.

    Modern astronomy is a multi-wavelength enterprise. What do we mean by this?

    Well, most objects in the Universe (including humans) emit radiation over a broad spectrum, called the electromagnetic spectrum. This includes both visible and invisible light such as X-rays, ultraviolet light, infrared light and radio waves.

    To understand the Universe, we need to observe the entire electromagnetic spectrum as each wavelength carries different information.

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