Deceleration of Universe Expansion
The search for the answer began here on the Berkely campus of the University of California. It’s a distinctive outpost in the landscape of science signposted with some of its greatest names. There is even a car park reserved for Nobel Laureates. Nine prize winners in a row – with five in physics alone. And it was here, in 1988, that Saul Perlmutter set out to map the deceleration of the universe.
Saul Perlmutter “There’s nothing you like more than a really good mystery. I wondered if you could actually measure, how much the universe was slowing down. I thought it was a very exciting possibility that you could, make a measurement, and find out what the fate of the universe was.”
Saul Perlmutter was the leading light behind an international team of physicists and astronomers. Under his guidance, they embarked on a ten year voyage of exploration far across the observable universe. The key was to measure how fast universe was expanding in the past, compared to now. They planned to map ancient galaxies – 10.8 billion light years from Earth. But it would take a whole decade to find and analyse what they were looking for. A candle.
Saul Perlmutter “If you want to measure distances across the universe you would like to be able to use an object that’s of known brightness. We call anything that we know the brightness of, a Standard Candle.”
A standard candle always has the same brightness – so you can use it to measure distance very precisely. The further away it is, the dimmer it will appear in our telescopes. But candles are elusive objects.
Saul Perlmutter “We hunt, but what astronomical object could you possibly use, that will behave in this very regular way, so that you can actually compare the distances.”
The galaxies themselves are no good. They come in many different shapes and sizes and at this distance, they are so dim we can barely see them, no good to measure the deceleration.
Saul Perlmutter “We’re talking about distances that are even more vast than usual for astronomy. Now we need to look at some of the most distant objects in the universe so these had to be very bright objects.”
A Supernova – the Standard Candle
Saul had a very bright idea. He would find his way by the light of a dying star. A Supernova.
Saul Perlmutter “When one of these supernovae explode, that one star can be as bright as the entire galaxy of a hundred billion other stars. so this is a remarkably bright, single event.”
Saul Perlmutter had a special kind of supernova in mind. A type 1A is triggered when a dying star draws in mass from its neighbour.
Saul Perlmutter “Just at the point where there is a critical mass, there will be a runaway thermonuclear explosion. So that means that it is triggered at the same mass every time.”
Same mass every time means same brightness every time. They are perfect standard candles. But Saul had to find them first.
Saul Perlmutter “If you could work with anything else in the world besides a supernova to do your research you would. They are just a real pain in the neck to work with. They’re rare, they’re random and they’re rapid.”
A supernova only burns brightly for three weeks. And in any given galaxy, they explode without warning roughly once every 300 years. With those odds, you can’t book valuable time on the world’s best telescopes.
Saul Perlmutter “It makes a terrible proposal, if you were to say that, sometime in the next several hundred years, a type IA supernova, might explode, somewhere in this galaxy. I would like the night of March 3, just in case.”
But Saul had a plan to get the odds working in his favour. With billions of galaxies in the observable universe – there are dozens of supernovae every night. Saul’s team spent six years perfecting a new system for supernovae on demand. They took snapshots of thousands of galaxies at once, then repeated them two and half weeks later. First you don’t see a supernova. Now you do.
Saul Perlmutter “That’s very important, that two and a half weeks, because that guarantees, that everything you find, that’s brighter, on the second night than the first, is on the way up. We can now guarantee that there would not just be one Type 1A supernova, but there would be half a dozen.”
Saul Perlmutter now knew exactly where to point one of the world’s most powerful telescopes – the Keck Observatory in Hawaii. He was finally ready to measure the deceleration of the universe. But by late in 1997, the team was getting some very weird results.
Saul Perlmutter “The points were not showing up where you would expect. This was exciting.”
The supernovae distance measurements didn’t match the predicted deceleration.
Saul Perlmutter “We were then faced with the question, okay, what else could be going wrong?”
Saul and his team spent five more anxious months, eliminating all possible sources of error. But by January 1998 they were finally ready to go public.
Saul Perlmutter “The more we checked, the more we, fine tuned every little step of the calibration, the more the weird results didn’t go away.”
The Continued and Continual Expansion of the Universe
The weird result has reverberated throughout the world of science ever since. In January 2012, Saul Perlmutter won the Nobel Prize for physics and booked a parking space for life.
Saul Perlmutter “At the end, we concluded that actually, the universe really isn’t slowing down, it’s actually speeding up in its expansion. And that was a big shock.”
It’s been described as one of the biggest shocks in modern cosmology. This is a runaway universe and everyone’s on board – whether we like it or not.
Janna Levin “We find out that the universe is not just expanding, but that is getting faster and faster.”
Anthony Aguirre “The cosmological community, when this result came out, was completely incredulous.”
Janna Levin “I didn’t believe it when I first heard about it.”
Anthony Aguirre “There had to be something wrong with these observations, the deceleration was not there.”
The expansion of the Universe was NOT decelerating nor was it heading for implosion – The expansion was accelerating more and more.
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