Gregory: Is there any other point to which you would wish to draw my attention?
Holmes: To the curious incident of the dog in the night-time.
Gregory: The dog did nothing in the night-time.
Holmes: That was the curious incident.
– From "Silver Blaze", Arthur Conan Doyle
Over the biggest science stories of the last year is the on-again, off-again discovery of gravitational waves left over from the Big Bang. But the biggest story in cosmology – one that has been building for 15 years – almost always flies under the radar. Since the year 2000, we have increased our stock of knowledge on the microwave background – fossil light from the Big Bang – by a factor of maybe 10,000. Likewise, our data on the distribution of galaxies in space has grown by between 10 and 100. Despite these advances, the "big picture" concordance cosmology describing the evolving universe has hardly changed at all.
Let me absolutely clear: this is good news for cosmologists. It means we can dig into the detailed history of the universe and test the two huge hypotheses which underpin the concordance cosmology – dark matter and dark energy. These are ad hoc assumptions (and profound challenges for theoretical physicists) but the predictions of the concordance model have survived a vast increase in our ability to test them.
When the first microwave background data from the Planck spacecraft was released in 2013 this pair of piecharts was part of the media package:
The spin from the European Space Agency media team was that the estimated amounts of dark matter and "ordinary matter" in the universe had gone up, while the fraction of dark energy had dropped. To a cosmologist, this is interesting news. But the bigger story is that the two pies are very similar, with no slices added or subtracted, even though Planck had made huge strides in measuring the cosmos.
The next batch of Planck data was released yesterday (very early in the morning here in New Zealand), and this time the big news is that Planck has accurately measured how long it took for the first stars to light up after the Big Bang. The headline story is that the dark ages – the time before stars – lasted roughly 550 million years, 100 million years longer than Planck's previous estimate. (This is a long time, but a fraction of the 13.8 billion years since the Big Bang itself.)
Planck probes the dark ages because the first stars burn so brightly they ionise most of the gas in the primordial universe leaving a tell-tale imprint on the microwave background. However, this news adds confidence to the concordance cosmology as observations of distant galaxies are a better fit with this result than the previous estimate. If the gap hadn't narrowed, it might have looked as if some exotic process (decaying particles left over from the big bang?) helped to ionise the primordial universe – and many of these theories are now ruled out.
Sooner or later, there are bound to be surprises, even if we are back to square one when it comes to primordial gravitational waves. But just as Sherlock Holmes solved a mystery because a dog did not bark in the night, the biggest news today is what Planck does not say. To quote Holmes again, once once you eliminated the impossible, whatever remains, however improbable, must be the truth. So the concordance cosmology, with its dark energy and dark matter, is looking more and more like the truth, however improbable it might seem.
Postscript 1: Cosmologists could argue for ages over just how to measure the amount of information at our disposal and the numbers here are guesstimates. For the microwave background, the spectacular growth reflects our near-complete ignorance of its properties as little as 25 years ago, whereas 3D galaxy positions have been mapped for much longer. Either way, though, you can make a case that the storehouse of data used to test models of the evolving universe has a grown a million times bigger since the year 2000.
Postscript 2: The new Planck dataset has any number of interesting hints, and the key paper on inflation is still in preparation. I am still gathering my thoughts on that.