From ESOblog: “A Nobel Achievement (part II)” Bruno Leibundgut

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From ESOblog


Bruno Leibundgut

8 February 2019

In 2011, the High-z Supernova Search Team won the Nobel Prize in Physics for the discovery that the expansion of the Universe is accelerating. Bruno Leibundgut, ESO’s Very Large Telescope Programme Scientist, was one of two ESO scientists who contributed to this extraordinary discovery, with the other being Jason Spyromilio. Bruno tells us the story of this game-changing piece of astronomical research in the second post in a two-part series about this prize-winning discovery.

Also see: A Nobel Achievement (part I)

At the beginning of the 1990s, the biggest question in astronomy was probably: what is the future of the Universe? Is it going to collapse? Or will it expand forever? Nobody knew.

At the time many astronomers were looking at type Ia supernovae, which are the extremely bright explosions that occur when two stars in a binary system merge. This type of supernova always produces a similar amount of light, so we know how far away they are by how bright they look from Earth. And because they are so bright, we can often see supernovae even when they are really distant. This all means that we can use type Ia supernovae to find out about the past and future of the Universe; by comparing the apparent distance predicted by their brightness to their actual distance, it is possible to determine whether the expansion of the Universe has decelerated since the explosion occurred.

Artist´s impression of a binary system before and after merging to create a supernova. Credit: ESO

I was working as part of a team that was trying to do just that. In 1995 and 1996, team member Adam Riess collected our observations on the brightness of ten distant supernovae, and the and team leader Brian Schmidt compared their distances and their brightnesses.

They came up with the result that the expansion of the Universe was not decelerating. This was very surprising as we had expected that all the matter in the Universe is pulled together by gravity, leading to a decelerating expansion. But then in December 1997, Adam said to us: “Those distant supernovae are too far away. It’s like something has pushed them away from us. Could it be that the expansion of the Universe is actually accelerating?”

This sparked a heated discussion — via email as the team was distributed all over the world! Brian was in Australia, Adam was on the west coast of the US, we had people on the east coast, we had people in Hawaii. And I was in Germany working with the data from the ESO telescopes. We would send an email in the evening and get up in the morning to find out about a number of other issues. But in the end, Adam and Brian could prove that there was no obvious mistake in the analysis. [Saul Perlmutter heads the Supernova Cosmology Project at Lawrence Berkeley National Laboratory at the same time with the same goal. He shared the Nobel prize with Adam Riess and Brian Schmidt.]

In this much sped-up artist´s impression showing a collection of distant galaxies, the occasional supernova can be seen. Each of these exploding stars briefly rivals the brightness of its host galaxy. Credit: ESO/L. Calçada

So, we decided we would have to submit a paper presenting our results, and we were sure that someone else would tell us what was wrong. But this didn’t happen. There were some people who didn’t believe us, but they were in the minority, and they couldn’t prove we were wrong.

I’m not sure we were fully aware at the time what a big deal this discovery was. The fact that the expansion of the Universe is accelerating means that there must be some invisible “thing” in the Universe driving the expansion, causing the objects in it to flow apart faster than we would expect even for a universe without matter. The calculations tell us that this “thing” must be about three quarters of the energy content of the Universe. In a way, this was like discovering three quarters of the Universe that people had no idea existed.

We were helped by another discovery made around the same time, related to the Cosmic Microwave Background (CMB).

CMB per ESA/Planck

ESA/Planck 2009 to 2013

People were using the CMB to study the geometry of the Universe. The tiny temperature fluctuations in the CMB indicated that the geometry of spacetime is flat, which requires a specific amount of matter and energy. Einstein’s famous equation E=mc2 tells us that mass (matter) and energy are equivalent. But determinations of the amount of matter and energy known to exist in the Universe made up just 25% of the amount required by a flat Universe. In other words, 75% of the matter and energy was missing.

This value matched perfectly with our discovery of the extra energy component that makes up three quarters of the matter/energy content of the Universe. It was the combination of the two discoveries at almost the same time that convinced most people.

This new component is now called dark energy. But more than twenty years later, we still have no idea what dark energy actually is! There is no physical explanation for it, but astronomers all over the world are working to find one.

This discovery certainly affected my career. All of a sudden, I became one of the best-known observational cosmologists in Europe, which came with its pros and cons.

As one of only two Europeans on the High-z Supernova Search Team, I got invited to many, many conferences here in Europe to present the result, and was asked to write major review papers on it. This took a lot of time out of my research.

And then in 2011, Adam Riess and Brian Schmidt won the Nobel Prize in Physics for this research — they each won a quarter, and Saul Perlmutter of the Supernova Cosmology Project won the other half. We all went along for the Nobel Prize celebrations, which was an amazing experience.

The High-z Supernova Search Team just after the Nobel Prize award ceremony. Bruno stands on the right and Jason Spyromilio on the left of the back row.
Credit: Nicolas Suntzeff

But in the long run, I decided that I didn’t want to be part of large collaborations any more. The High-z Supernova Search Team wasn’t that big — around 25 people — but there were still so many teleconferences and meetings. I just felt tired of all that. I wanted to do things that I could be recognised for directly, rather than being a member of a team. I wanted to create something that people could recognise as coming from me.

I’m still doing cosmology, though not the same type any more. That kind of research now requires large teams of hundreds of people. I’ve started to pick smaller problems again — things that I can do with students, to solve some of the smaller questions that we have about supernovae. It’s interesting to come from a big stage, from a place where the whole world pays attention to you, to go back to smaller problems that are not necessarily seen by everybody, and maybe not even seen as interesting by a lot of people. But that’s OK.

See the full article here .


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ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening star).
elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo,

ESO LaSilla
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ESO VLT 4 lasers on Yepun

ESO Vista Telescope
ESO/Vista Telescope at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level.

ESO/NTT at Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres.

ESO VLT Survey telescope
VLT Survey Telescope at Cerro Paranal with an elevation of 2,635 metres (8,645 ft) above sea level.

ALMA Array
ALMA on the Chajnantor plateau at 5,000 metres.

ESO/E-ELT,to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

APEX Atacama Pathfinder 5,100 meters above sea level, at the Llano de Chajnantor Observatory in the Atacama desert.

Leiden MASCARA instrument, La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

Leiden MASCARA cabinet at ESO Cerro la Silla located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

ESO Next Generation Transit Survey at Cerro Paranel, 2,635 metres (8,645 ft) above sea level

SPECULOOS four 1m-diameter robotic telescopes 2016 in the ESO Paranal Observatory, 2,635 metres (8,645 ft) above sea level

ESO TAROT telescope at Paranal, 2,635 metres (8,645 ft) above sea level

ESO ExTrA telescopes at Cerro LaSilla at an altitude of 2400 metres