A la recherche du temps perdu
We seem to have lost about a billion years of our history. Recent careful measurements of the age of the universe have produced a puzzle. We were pretty sure that the Universe was 13.8 billion years old, and we had two kinds of evidence for that, both stemming from measurement of the expansion of the Universe. The Planck space telescope's measurements of certain standard rulers, the so-called baryon acoustic oscillations, provided standard rulers and measurements of distant supernovae and galaxies provided so-called standard candles.
Now, however, a very careful new analysis combined with new data seems to show a Universe about a billion years younger than previously thought, or, rather, two conflicting estimates of that age. Unless there's some major error, and the measurements were very carefully done, we have a problem with our basic model, the Lambda CDM model, which combines dark energy and cold dark matter.
Some details in this news story: https://www.nbcnews.com/mach/science/universe-may-be-billion-years-younger-we-thought-scientists-are-ncna1005541?utm_source=pocket-newtab
Technical details here: https://arxiv.org/abs/1903.07603 (abstract, contains link to full paper)