From Dr. Becky.

Everything we know about the shape of the Universe could be completely wrong.

This is one of the most fascinating unsolved problems in cosmology, and it almost never gets talked about outside of research papers. It’s called the curvature tension, and it links in to the "crisis in cosmology". Astrophysicists have always thought the geometry of the Universe is flat, not its shape, its geometry: which is a very different thing.

But back in 2019, a careful analysis of 6 years worth of data from the Planck telescope suggested the geometry of the Universe wasn’t flat, and instead was closed. And since then, the debate has gone back and forth in the scientific literature, with some data pointing to a curved Universe and some to a flat one. This video will dive into the details of the curvature tension!

Papers mentioned
Di Valentino et al. (2019) – https://arxiv.org/pdf/1911.02087
Handley (2019) – https://arxiv.org/pdf/1908.09139
Wu & Xhang (2025) – https://arxiv.org/pdf/2411.06356
Calabrese et al. (2026) – https://arxiv.org/abs/2503.14454
Louis et al. (2025) – https://iopscience.iop.org/article/10.1088/1475-7516/2025/11/062/pdf
Addison et al. (2016) – https://iopscience.iop.org/article/10.3847/0004-637X/818/2/132
Rosenberg et al. (2022) – https://academic.oup.com/mnras/article/517/3/4620/6717656
Tristram et al. (2024) – https://www.aanda.org/articles/aa/full_html/2024/02/aa48015-23/aa48015-23.html
Bernardis (2000; – behind paywall) – https://www.nature.com/articles/35010035
Spergel et al. (2003) – https://arxiv.org/pdf/astro-ph/0302209
Hinshaw et al. (2013) – https://arxiv.org/pdf/1212.5226
Planck 2015 results – https://arxiv.org/pdf/1502.01589
Planck 2015 constraints on inflation – https://arxiv.org/pdf/1502.02114
Planck 2018 results – https://arxiv.org/pdf/1807.06209

00:00 – Introduction
01:05 – I – What is the difference between the shape and geometry of the Universe?
03:22 – II – Why we care about whether the Universe is flat or not
05:15 – III – How we measure if the Universe is flat
08:33 – IV – The curvature tension and what different data is telling us
15:01 – Bloopers

Video filmed on a Sony ⍺7 IV
Video edited by Martino Gasparrini: martino.freelance@gmail.com
Video produced by Marina Hui & Dr Becky Smethurst

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👩🏽‍💻 I’m Dr. Becky Smethurst, an astrophysicist at the University of Oxford. I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don’t know. If you’ve ever wondered about something in space and couldn’t find an answer online – you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.

http://drbecky.uk.com