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Discordance - Jim Baggott ****

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. 

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