Discordance: The Troubled History of the Hubble Constant

by Jim Baggott

The troubled history of the Hubble constant told in an authoritative, comprehensible, and entertaining manner

In 1927 Georges Lemaître argued that our universe is expanding, a conclusion rendered more startling by the astronomical data that backed it up, presented two years later by Edwin Hubble. The speed of this expansion is governed by Hubble's constant, and Discordance tells its troubled history.

This unpredictable and fascinating story begins with the first tentative steps to measure the distances to nearby stars and galaxies. It traces the extraordinary interplay between cosmological theory and astronomical observation which has given us the standard Big Bang theory. It was not all plain sailing, and the narrative takes us through the discovery of dark matter, the Hubble Wars of the 1970s, the invention of cosmic inflation, and other crucial scientific moments. Further satellite missions were expected to add to the clarity of our measurements. But from about 2009 onward, the results began to diverge. This is the Hubble tension and perhaps even a crisis.

Jim Baggott clearly and entertainingly guides the reader through this gripping scientific voyage--one littered with crises of confidence, astonishing discoveries, and extraordinary personalities--which still continues today.

Genres: Science

336 pages, Hardcover

Published January 22, 2026

About the author

Jim Baggott completed his doctorate in physical chemistry at the University of Oxford and his postgraduate research at Stanford University.

Reviews

[Leo · Rating 3 out of 5]

I got audiobook arc for review on netgally.

I found the book very interesting and I enjoyed but I had hard time following it and felt like a lot went over my head. I still enjoyed my time listening to it but do think it's easier understood if you already know a bit about space and such.

[Brian Clegg · Rating 4 out of 5]

Jim Baggott is one of the most reliable hands in the business when it comes to exploring complex physics and cosmology issues in an approachable but never over-simplified fashion, so a new Baggott on the shelves is always a treat. For reasons I'll go into in a moment, while the explanation here is as lucid and interesting as possible, I found the topic a little underwhelming.

The topic Discordance covers is the Hubble tension - the reality that there are two ways of measuring the Hubble constant that describes the rate of expansion of the universe, both of which are based on solid science, but which don't agree. Admittedly, the distinction is only seven per cent, but in theory they should converge, and as Baggott makes clear, although both of the measurement methods have potential issues, if the tension proves genuine, it puts the most widely accepted version of the Big Bang theory in jeopardy.

The book starts with historical material on how we measure distances in space, the theory behind and discovery of the expanding universe, and the various errors and disagreements between scientists along the way, going right back to whether the Milky Way was just one of many galaxies, or the whole universe. As things develop, we also get introduced to the cosmic microwave background radiation, which provides the second way of measuring the Hubble constant after the red-shifting of galaxies at known distances (where 'known' always has a degree of uncertainty). There's also coverage of the dark energy thought to be causing the acceleration of expansion, and even dark matter that has a more indirect involvement. Finally, we look to the future of where we go from here and the potential for new physics to explain what is happening.

My agent always used to say 'Is it a book or is it a magazine article?' This topic is definitely more than an article, but I felt it might have been better as a couple of chapters of a book. The elements are interesting and the ongoing split between different ways of measuring the constant is intriguing. But too much of what was involved was getting more or less accurate measurements for absolute stellar magnitude, establishing safe standard candles, reaching accurate numbers on expansion rate/acceleration and the like. It's true that lots of scientific work is, frankly, boring repetitive slog. And we perhaps don't see enough of this in popular science. But despite all of Baggott's skill, it's hard to make this too engaging.

Don't get me wrong, this is a really good book - far better than much of the popular science I read. I'm glad I read it and while the historical material and that on dark matter/energy was very familiar, some of the more recent attempts to explain away the Hubble tension and its implications for the canonic Big Bang theory was new in the detail and of genuine interest. But the overall subject, for me, simply wasn't ideal for book-length treatment.

[Holly Taggart · Rating 4 out of 5]

Many thanks to Oxford university Press and Recorded books for an advanced Listener copy of Discordance by Jim Baggott!







This is my year to read about outer space and when I saw this audiobook on offer at net Galley, I jumped at it!

As I've already noted, I am not a math or physics genius, but I do have a bit of an obsession with outer space, and while I know I won't ever be an arm chair Cosmologist, I know I certainly LOVE to learn more as I toddle along.

Now, with no spoilers--- can one really spoil a non-fiction book? We are given the story of the Main main character, the Universe. Making cameo appearances are the Big Bang Theory, Afterglow radiation or Cosmic Microwave background radiation, Cosmic inflation and the Lambda-CDM model. Side Characters of note include: The Hubble Telescope, multiple scientists, The Space Shuttle, The ALMA Observatory (Atacama Desert), Apache Point observatory, (and more), the Supernova Cosmology project, the JPL Cosmology Group, and so many more. Supporting roles go to Dark matter and Dark energy (yes, we can't see you, but it seems you might be there, maybe...) Redshift, the strange and charm quarks (and yes the other top and bottom ones too) and gravitational lensing. There's more, but as any good Academy award winner will note, we are grateful to all players, too numerous to mention.

In this account of humans moving closer and closer to deducing the full nature of the universe and all that it encompasses, Dr Baggott introduces readers to the basic physics of Cosmology. Readers learn about models and ideas, generally accepted concepts of the universe, and key phenomena and building blocks (equations/theories and computations) that hold our concepts of the universe together.

As a complete lay person, I will note that there is quite a bit of math, and a few times, I did experience a few flashbacks to my ill fated Astronomy 101 college class. Baggott intersperses some amusing thoughts, comments or stories in between all the math so I did remain engaged. He is particularly adept at describing the "space drama" regarding a possible "new physics" without going completely off the rails.

I did enjoy this listen. I can't say I recommend it for my Library Book Club - as we are diving into Remarkably Bright Creatures this month, and this is definitely not in the same book universe if you will.

I can recommend it for space enthusiasts, students and readers who want to be challenged as they read. For me, it was definitely worth my time. I feel it answered some of my questions as to why humans aren't just travelling the universe as Bester's Demolished Man predicted back in 1953!

Dr. Baggott does his own narration, and it was clear, and easy to follow. He is an excellent speaker and I found it an easy listen (I mean the math was hard... )

[Cindy · Rating 4 out of 5]

I was more familiar with the Hubble Telescope than the Hubble Constant, so I learned a lot listening to this audiobook! It's mostly about the fascinating methods early astronomers used to measure the distance of planets, stars, and galaxies. By measuring these distances over time, it was clear to the scientists that our universe is expanding.. The Hubble Constant is an important part of such equations. Edwin Hubble's father wanted him to pursue a career in law or business. Aren't we glad that he followed his heart and eventually became an astronomer? Lots of academic wrangling and posturing is also discussed. So, too, is the extremely difficult road for women with an interest in and aptitude for astronomy. There's (unsurprisingly) a lot of math. I didn't pay that much attention since I've no present use for Hubble's Constant, but it's there if you want it.
The author @JimBaggott does an excellent job narrating the audiobook and throwing in the occasional joke or wry remark to keep things amusing and interesting. That's quite a feat for such math-heavy material.
My thanks to the author, publisher, @RecordedBooks, and #NetGalley for early access to the audiobook of #Discordance for review purposes. Publication date: 22 January 2026.