Uploaded April 2026 | Updated September 2026, 2 weeks ago
What’s going on with the expansion of the Universe? And is it jerking or not?! Because back in the 1920s when Edwin Hubble first looked at the speed galaxies appear to be moving away from us, he found that the further away a galaxy was, the faster it appeared to be moving away. With a direct correlation between the two. That along with many other observations made at the time all came together to give us the hypothesis that the space between galaxies growing, i.e. the universe itself is expanding. And because Hubble had observed this direct correlation, that expansion rate was assumed to be constant: the universe was and always had been expanding at the same speed. And that’s we thought right up until the 1990s, when Riess, Perlmutter and Schmidt then did the same experiment Hubble did but pushed it out to greater distances, and found more distant galaxies were moving away faster than they should be. The Universe wasn’t expanding at a constant speed, it was accelerating in its expansion. We still don’t know what’s causing this expansion, although we call it dark energy, but since the 90s we’ve assumed that acceleration rate is constant - the Universe hasn’t changed how hard its pushing down on the gas pedal. But now recent results from the Dark Energy Spectroscopic Instrument, or DESI, mapping out the positions and speed of galaxies away from us across a huge chunk of the Universe are now suggesting that the acceleration rate also doesn’t appear to be a constant, it too is changing. The correct physics term for this is that the Universe is JERKING. And now the question is do we assume once again that the jerk happening at a constant rate? Or is the rate of jerk also changing? Is this just an endless cycle that physics can’t escape from where we assume the speed of expansion is constant, but it’s not, then assume the acceleration is constant but maybe it’s not, then assume the jerk is constant, but what if it’s not?!
Hubble (1929) - pnas.org/doi/epdf/10.1073/pnas.15.3.168
Riess et al. (1998) - iopscience.iop.org/article/10.1086/300499/pdf
Perlmutter et al. (1999) - iopscience.iop.org/article/10.1086/307221/pdf
DESI collaboration (2024) - iopscience.iop.org/article/10.1088/1475-7516/2025/02/021/pdf
Video filmed on a Sony ⍺7 IV
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📚 My book, "A Brief History of Black Holes", out NOW in hardback, paperback, e-book and audiobook (which I narrated myself!): lnk.to/DrBecky
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👕 My merch, including JWST designs, are available here (with worldwide shipping!): dr-becky.teemill.com
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🎧 Royal Astronomical Society Podcast that I co-host: podfollow.com/supermassive
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🔔 Don't forget to subscribe and click the little bell icon to be notified when I post a new video!
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👩🏽💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford (Christ Church). 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.
drbecky.uk.com
What’s going on with the expansion of the Universe? And is it jerking or not?! Because back in the 1920s when Edwin Hubble first looked at the speed galaxies appear to be moving away from us, he found that the further away a galaxy was, the faster it appeared to be moving away. With a direct correlation between the two. That along with many other observations made at the time all came together to give us the hypothesis that the space between galaxies growing, i.e. the universe itself is expanding. And because Hubble had observed this direct correlation, that expansion rate was assumed to be constant: the universe was and always had been expanding at the same speed. And that’s we thought right up until the 1990s, when Riess, Perlmutter and Schmidt then did the same experiment Hubble did but pushed it out to greater distances, and found more distant galaxies were moving away faster than they should be. The Universe wasn’t expanding at a constant speed, it was accelerating in its expansion. We still don’t know what’s causing this expansion, although we call it dark energy, but since the 90s we’ve assumed that acceleration rate is constant - the Universe hasn’t changed how hard its pushing down on the gas pedal. But now recent results from the Dark Energy Spectroscopic Instrument, or DESI, mapping out the positions and speed of galaxies away from us across a huge chunk of the Universe are now suggesting that the acceleration rate also doesn’t appear to be a constant, it too is changing. The correct physics term for this is that the Universe is JERKING. And now the question is do we assume once again that the jerk happening at a constant rate? Or is the rate of jerk also changing? Is this just an endless cycle that physics can’t escape from where we assume the speed of expansion is constant, but it’s not, then assume the acceleration is constant but maybe it’s not, then assume the jerk is constant, but what if it’s not?!
Hubble (1929) - pnas.org/doi/epdf/10.1073/pnas.15.3.168
Riess et al. (1998) - iopscience.iop.org/article/10.1086/300499/pdf
Perlmutter et al. (1999) - iopscience.iop.org/article/10.1086/307221/pdf
DESI collaboration (2024) - iopscience.iop.org/article/10.1088/1475-7516/2025/02/021/pdf
Video filmed on a Sony ⍺7 IV
---
📚 My book, "A Brief History of Black Holes", out NOW in hardback, paperback, e-book and audiobook (which I narrated myself!): lnk.to/DrBecky
---
👕 My merch, including JWST designs, are available here (with worldwide shipping!): dr-becky.teemill.com
---
🎧 Royal Astronomical Society Podcast that I co-host: podfollow.com/supermassive
---
🔔 Don't forget to subscribe and click the little bell icon to be notified when I post a new video!
---
👩🏽💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford (Christ Church). 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.
drbecky.uk.com
