Cobra Effect · Big open questions
What is speeding up the universe?
Why the universe's expansion is speeding up, and the rival ideas about why.
7 cards, read aloud in 2:44, with a test and sources.
In the 1990s, two rival teams set out to measure how fast cosmic expansion was slowing.
Gravity pulls everything together, so the expansion was expected to be gradually slowing down. Both teams measured a kind of exploding star that always shines with about the same true brightness. One team was led by Saul Perlmutter, and the other by Brian Schmidt, working with Adam Riess.
In 1998, both teams found the distant explosions fainter than expected.
That meant they were farther away than a slowing universe would allow. Both teams reached the same startling answer. The expansion is speeding up. In 2011, Perlmutter, Schmidt and Riess shared the Nobel Prize in Physics.
Whatever is driving the speeding up is called dark energy.
The name is a label for something nobody understands. It seems to make up about seventy percent of the energy content of the universe. Other evidence, from the oldest light and the pattern of galaxies, agrees that the speeding up is real.
The simplest explanation is an idea Einstein once added to his equations, and later dropped.
It treats dark energy as a constant energy belonging to empty space itself. It fits most measurements well. But when physicists estimate how much energy empty space should have, they get a number vastly too large.
Recent measurements hint that dark energy may be changing over time.
In 2024 and 2025, the DESI project published maps of millions of galaxies and quasars. Combined with other data, its results lean toward dark energy growing weaker. But the evidence is not yet strong enough to count as a discovery, and hints like this have faded before.
A separate puzzle is how fast the universe is expanding today.
Measurements based on the oldest light give one rate, and measurements based on nearby stars give a faster one. The gap has survived years of careful checking. It is related to dark energy, but it is not the same question.
So when a headline says dark energy has been overturned, ask how strong the evidence really is.
That the expansion is speeding up is well established. What drives it, and whether it changes, is still open. Physicists usually wait for very strong evidence before they call something a discovery.
Sources
- New DESI results strengthen hints that dark energy may evolve, Berkeley Lab. The 2025 results, and why they are still only a hint.
- Dark energy, NASA Science. The 1998 discovery and the leading ideas.
- Hubble tension, Wikipedia. The two measured expansion rates and where the disagreement stands.
Nearby ideas
- The universe that is growing. How shifts in galaxies' light and their distances revealed that the universe is expanding.
- What is dark matter?. What the evidence says about dark matter, and why nobody knows what it is.
- The hiss left over from the Big Bang. How a hiss in a radio antenna turned out to be the afterglow of the Big Bang.
- Why is there more matter than antimatter?. Why the universe is made of matter, when the Big Bang should have made equal antimatter.
- Why do we have experiences at all?. Why brain science can track experience, but not yet explain why it exists.
- Are we alone in the universe?. Why no sign of life beyond Earth has been confirmed, despite planets everywhere.
- Why do we sleep?. Why every animal seems to need sleep, and the rival ideas about what it is for.
- How did life begin?. What is known about how life began, and which parts are still guesses.