How Fast Is the Universe Really Expanding? | Quanta Magazine
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The Joy of Why
How Fast Is the Universe Really Expanding?
By
Steven Strogatz
and
Janna Levin
July 23, 2026
One of the most contested issues in cosmology is our understanding of the Universe’s expansion. Adam Riess discusses why there is disagreement and what might resolve it.
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Chanelle Nibbelink for Quanta Magazine
Introduction
Authors
Steven Strogatz
Podcast Host
Janna Levin
Contributing Columnist
July 23, 2026
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astronomy
astrophysics
cosmology
dark energy
dark matter
fundamental physics
Hubble constant
physics
The Joy of Why
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One of the biggest mysteries in cosmology seems to keep getting bigger. Astronomers have known since the 1930s that the universe is expanding, but in the 1990s, the discovery that this expansion is accelerating rather than slowing down came as a huge shock to the field. Something had to be driving that acceleration, and dark energy was proposed as the cause. What began as a seismic shock in cosmology eventually led to a Nobel prize.
But this story has a sequel, and it comes with another major plot twist. The two main methods for estimating the universe’s present-day expansion rate are producing significantly different answers. As a result, how fast the universe is really expanding has become a matter of considerable debate — with no small amount of angst — and the discrepancy has become known as the Hubble tension. Perhaps most surprising of all is that one of the loudest voices raising concern is Adam Riess, the astrophysicist whose Nobel Prize-winning work helped ignite the acceleration debate in the first place.
Riess joined co-host Steven Strogatz on The Joy of Why to explain how we got to this point, what the Hubble tension might be telling us, and what may happen next.
Listen on Apple Podcasts, Spotify, TuneIn or your favorite podcasting app, or you can stream it from Quanta.
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Transcript
[Music plays]
STEVE STROGATZ: I’m Steve Strogatz.
JANNA LEVIN: And I’m Janna Levin.
STROGATZ: And this is The Joy of Why.
LEVIN: A podcast from Quanta Magazine, where we explore some of the biggest unanswered questions in math and science today.
STROGATZ: Well, great to see you, Janna. How’s it going?
LEVIN: Hi, Steve. Yeah, great to see you. I’m excited to hear what you have to talk about today.
STROGATZ: Yeah, good, because I could barely contain myself. This was an especially fun conversation I had with Adam Riess, who I guess is someone you would know from the circuit.
LEVIN: Yeah. Definitely. I was speaking with him not too long ago.
STROGATZ: Uh-huh. Well, right, I would think that in the astrophysical world you would have crossed paths, so I’m not super-surprised to hear it. But actually, I myself had not spoken to Adam or seen him for many, many decades. It turns out he was a, uh, a student when I was just beginning as a professor.
LEVIN: Oh, wow. In your class?
STROGATZ: He was literally one of my students at MIT in my first job.
LEVIN: Wow, how amazing to think these young kids in your class, one of them’s gonna be a Nobel prize winner.
STROGATZ: Well, exactly. Yeah, very memorable student. Really a great guy. But, I would actually like to pick your astrophysics brain if we could for a second because one of the things that I talked to Adam about is the issue of the universe expanding and also its expansion accelerating.
LEVIN: Right.
STROGATZ: You know, like you were there back in 1998 when we got the news about the acceleration. Do you, um, remember anything about that time? Like what it felt like in your community?
LEVIN: Oh, yeah. Well, I was at Berkeley, and that’s where Adam was. And also Saul Perlmutter, the other group, right? These were two different groups working. So I absolutely remember, I was at the Center for Particle Astrophysics, and Saul was kind of up in the hill in Berkeley, and he was coming down with these results, and I was like, “Come on, man.” Then there was a lot of discussion, honestly, about how there’s huge sensitivity to the temperature of the supernova, you know, very high power.
STROGATZ: And so the reason you’re bringing up this thing about the power and the sensitivity is that the results were so shocking that everybody wanted to be very,...