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100 years since Slipher observed our expandin -

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Robyn Williams: Who is VM Slipher, and what is the centenary they're celebrating in Flagstaff, Arizona? Let's join Diane Hope.

Diane Hope: Flagstaff, Arizona, is home to the historic Lowell Observatory where Perceval Lowell thought he'd found those infamous canals on Mars, and Clyde Tombaugh the mysterious ninth planet Pluto, subsequently demoted to minor status. But there's one universe-changing discovery made here 100 years ago which has stood the test of time. And the audible Doppler shift in each passing freight train's horn is a clue to what that was.

David DeVorkin: It started out as something that was just extraordinary, astounding. I mean, we hear all of these superlatives when you go back and read the original literature in 1912, coming up with these enormous motions, faster than anything else. Mainstream astronomers at the time when they received the news of what Slipher was finding, they weren't saying it was impossible, they were saying it was astounding. Astronomy at that time thought it was working in a universe of stars, not galaxies. It was relatively static, it had always been that way.

Diane Hope: David DeVorkin is senior curator of science history at the Smithsonian. He was just one of a group of science historians and astronomers who gathered here recently to celebrate the discovery of the expanding universe. The astronomer who made the ground breaking observation, on our neighbouring Andromeda galaxy, was Vesto Melvin Slipher, and modern astronomers are still hugely influenced and impressed by Slipher's work.

Karl Glazebrook: My name is Karl Glazebrook, I'm an astronomer at Swinburne University in Melbourne. Slipher's measurement of the first redshift was one of the most significant cosmological measurements ever. That was the start of the process of understanding that nebulae galaxies were receding from our galaxy at thousands of kilometres per second, and that was the start of the whole idea of the expanding universe, that was the basic observational factor where it all began.

I should say that Andromeda itself is actually a negative redshift, it's coming towards us. By the time Slipher had measured 25 galaxies he'd found around 23 redshifts and only two blueshifts, so it's clear that most galaxies in the universe are rushing away from us.

Michael Way: It took Slipher eight hours of observing to make a single image of the spectrum of this Andromeda galaxy. My name is Michael Way, I work at the NASA Goddard Institute for Space Studies. Astronomers don't do observations like that any more. There are not people that sit at a telescope for eight hours straight. Today you can make the same observation in a couple of minutes with an auto-guiding telescope, with a night assistant in a nice cosy room. Slipher was sitting out here in the elements. This takes a very robust personality, and people ascribe his ability to do this because he grew up on a farm, so he knew what it was like to do hard work.

But the person that became the standardbearer for these discoveries was Edwin Hubble, who we have a space telescope named after. So while Slipher's discoveries are extremely important and opened up a completely new field of science, the irony is that he's almost completely forgotten by most people.

Diane Hope: And the impact of Slipher's discovery still dominates a large chunk of modern astronomy. Chris Impey is an observational cosmologist at the University of Arizona.

Chris Impey: Slipher kicked off a huge field that now chews up probably half of the time on all the world's big telescopes. He measured a couple of dozen redshifts of galaxies and now we're up to tens of millions of galaxy redshifts, which might just seem greedy, why do we need so many? Well, it's because we're trying to trace out the large-scale structure of the universe and the evolution of all matter over most of the history of the universe. Pretty much 13 billion years out of the 13.7 billion years can be mapped with galaxies now.

The discovery of when the first galaxies in the universe formed will be a natural bookend to Slipher starting off measuring galaxy redshifts. Because at that point you've seen so far back in time that it is the very first stars and galaxies. There's a pretty active chase to try and find the highest redshift galaxy. Of course you never know when you found the highest redshift, there might always be one a little bit earlier or a little bit further away.

Diane Hope: But continuing that chase to the universe's origins is making the astronomers' task a lot more challenging.

Chris Impey: Galaxies that far away have been shifted, their light stretched by travelling through space and a long time, where actually the work gets a lot harder. But galaxies don't put out any optical light when they are that far away, so your only choice is to try and measure them in the infrared, and that's a hundred times harder. It pretty much has to be 6 metres or more, so it's the 10-metre telescopes in Mauna Kea, it's the four 8-metres in Chile, it's our 6.5-metres and a few other big telescopes around the world, all with very good instruments that can measure and not just one galaxy but dozens or maybe hundreds at a time.

Diane Hope: So with all this data and the search for the oldest galaxies almost over, you'd think astronomers would be feeling pretty confident about their understanding of the universe, right?

Ken Freeman: We know so little. I'm Ken Freeman, I'm a professor at the Australian National University at Mount Stromlo Observatory, and I'm particularly interested in the problem of dark matter. We think the universe is made up of about 23% of gravitating matter altogether, about 4% of that is in the form of ordinary matter made of protons and neutrons, and all the rest of it is in this form of dark energy. So you've got something like 97% of the universe made up of these two things; dark energy and dark matter, and we don't know what either of them is. We don't really know if they are related. So it's a very uncomfortable position to be in.

Diane Hope: Ken Freeman's current research is focused on decoding some of the mystery surrounding dark matter.

Ken Freeman: One of the things that we've discovered fairly recently is that the bigger the galaxy, the fluffier the dark matter distribution. The galaxy that we live in is quite a big galaxy, and it has a very fluffy, very extended dark matter distribution. But when you go to some of these much smaller galaxies, galaxies that are perhaps a million times smaller than ours, you find that the dark matter is actually very dense and very compact, and the density is perhaps 1,000 times bigger in these little galaxies than it is in the big galaxies like ours. And this is something that would have been set up very, very early in the life of the universe, just when galaxies were starting to come together. So we may be getting closer to finding out what dark matter is. The Large Hadron Collider may help us understand this.

Diane Hope: One big advance that has been made in recent years is that cosmic expansion isn't constant, a discovery with profound and somewhat disturbing implications for the long-term future.

Ken Freeman: The exciting thing that Brian Schmidt and his colleagues and competitors found in the late '90s was that the universe is in fact accelerating, and Brian and his colleagues used a particular kind of exploding star, it's called a supernova 1A, as a distance indicator. They showed that in fact the universe is now in an accelerating phase. This is in some ways a rather dismal prospect because on very long timescales, the universe will basically go out.

Fred Adams: My name is Fred Adams, I'm a professor of physics at the University of Michigan in Ann Arbor. The immediate future, if the universe continues to accelerate like we think it is now, each bound structure in the universe today, so each galaxy halo, will become its own isolated island universe-like entity. So in the future of the universe, once the expansion has accelerated everything outside our field of view, we won't be able to see any external galaxies, so it will be difficult or impossible to see the cosmic expansion.

The microwave background photons will be redshifted to the point where they will be difficult or perhaps impossible to see. And finally, because of the continued stellar evolution over the next millions and millions of years, it will be increasingly hard to see the signature of Big Bang nucleosynthesis. So in that sense it becomes harder and harder to do cosmology as the cosmos itself ages.

Diane Hope: It's a mark of how mysterious the universe still is, that around 96% of what forms the galaxies Slipher measured 100 years ago still isn't understood. But as Adams points out, Sllipher's first redshift measurement remains a giant leap.

Fred Adams: That's the measurement that has stood the test of time. He did it 10 years earlier than everyone else, he was ahead of the game and he did it right. So that's huge. All of us should be so lucky as to make a contribution that important.

Robyn Williams: Diane Hope in Flagstaff, Arizona, and you heard Dr Ken Freeman there from the ANU, winner of last week's PM’s Prize for Science.