The whole history

The order, and why it is this one

Join anywhere. Each stop carries the route with it, so the foot of every question here offers the next one and the way back to this page.

1. 01What happened in the first three minutes?For a few minutes the whole universe was hot and dense enough to fuse nuclei. The theory predicts how much helium, deuterium and lithium should have come out, using one free number. The helium and the deuterium match observation to remarkable precision. The lithium does not.Shallowsdepth, meaning: Interesting questions with substantial evidence and real uncertainty remaining.Known. Strongly supported by evidence and broadly established.Read
2. 02What was the universe like before there were stars?Between the release of the cosmic microwave background and the first stars there is a long stretch with nothing to see. The signal that could open it has been claimed once and contradicted once, which is a useful demonstration of how this kind of result actually gets settled.Abyssdepth, meaning: Fundamental questions where our understanding is extremely limited.Unknown. Humanity does not currently have a satisfactory answer.Read
3. 03What is left when a star dies?A star’s ending is set mostly by its mass. White dwarfs and neutron stars are observed by the thousand. The line between a neutron star and a black hole is where nuclear physics is least certain.Shallowsdepth, meaning: Interesting questions with substantial evidence and real uncertainty remaining.Known. Strongly supported by evidence and broadly established.Read
4. 04Where did gold come from?The lightest elements were made in the Big Bang and most of the rest inside stars, but about half of those heavier than iron, gold among them, need conditions no ordinary star provides. One event in 2017 was seen in gravitational waves and in light, and showed those conditions producing heavy elements directly. How much of the total comes from that source is still argued, on this page as in the field.Surfacedepth, meaning: Things we understand reasonably well.Known. Strongly supported by evidence and broadly established.Read
5. 05Why do two good measurements of the universe disagree?One method reads the expansion rate off the infant universe, the other measures it directly in nearby galaxies. Both have shrunk their error bars. The gap between them has not closed, and that is either a mistake nobody has found or physics nobody has written down.Deepdepth, meaning: Major unanswered questions.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
6. 06How will the universe end?Expansion is accelerating, which was discovered in 1998 and was not expected. If the cause is a constant, the universe thins and cools forever. If it is a field that changes, several other endings become available, and recent survey data mildly prefer a changing one.Deepdepth, meaning: Major unanswered questions.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read