Presentation Speech by Professor Olga Botner, Member of the Royal Swedish Academy of Sciences; Member of the Nobel Committee for Physics, 10 December 2011

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

Allow me to start in English by citing a short poem by the Danish scientist, poet and designer Piet Hein, called Nothing is indispensable – grook to warn the Universe against megalomania:

The Universe may
be as big as they say.
But … it wouldn’t be missed
if it didn’t exist!

Well      …         If the universe didn’t exist, we would definitely not be sitting here today!

Humans are a part of the universe – just like the planets, the stars and the galaxies. The building blocks of every cell in our bodies – carbon, oxygen and other atoms – were formed inside ancient stars that exploded in the Milky Way perhaps 10 billion years ago, long before the solar system came into existence, just like the elements that the planets were formed from. Galaxies, stars and planets are affected by the same forces of nature as we are – especially gravity, which determines the orbits of planets and affects the life cycles of stars – and makes sure we have our feet on the ground. In order to understand ourselves, we must try to understand the universe!

This year’s Nobel Prize in Physics is about star explosions and about gravity – indeed, about the whole universe.

By observing distant galaxies with the help of telescopes, nearly 100 years ago scientists discovered that our universe is getting bigger and bigger. The distances between the galaxies are constantly increasing, like the spaces between raisins in a raisin cake swelling in the oven. If we extrapolate backwards in time, we realise that this expansion must have begun about 14 billion years ago – in a primordial  explosion that British astrophysicist Fred Hoyle called the Big Bang. During the past five billion years, roughly since the formation of our solar system, the universe has doubled in size, and it is continuing to expand. But – all matter in the universe is attracted to all other matter because of gravitation. The distances between the galaxies should not be able to continue increasing as rapidly for all eternity – the expansion should eventually slow down. It can be calculated that if the universe contains more than six atoms per cubic metre, this expansion must stop. The universe should start shrinking and eventually end with a Big Crunch, the opposite of the Big Bang. Is that the fate of the universe?

In order to answer this question, we must study whether the expansion rate of the universe is changing over time. Luckily we have access to a “time machine”: The light reaching our telescopes from distant stars has travelled through space for millions and even billions of years and has gradually become redder, since space itself has expanded. If we compare the redshift that is measured for many different objects using the models for the expanding universe permitted by Einstein’s General Theory of Relativity, we can discover the model that describes the real universe. The redshifts can be interpreted as distances, but this interpretation depends on various assumptions in the models about the balance between different forms of energy, for instance the quantity of radiation and matter. If we can compare these model-based distances with other, independent distance measurements of the same objects, we can determine the energy balance of the universe. But then we need objects that are visible across enormous distances – sources of light billions of light years away.

The scientists who are being awarded the Nobel Prize in Physics today have studied distant star explosions – supernovae. These events release enormous quantities of energy: during a few weeks, a single supernova can outshine all the hundreds of billions of stars in a galaxy. Stars can be ripped apart in many different ways, but it turns out that in special cases, explosions occur that always emit the same amount of light. These explosions can be recognised by carefully studying observed starlight. Since the quantity of light released is always the same, the distance can be determined from the intensity of observed starlight. The further away the supernova – the fainter it seems!
 
But the universe is big, and finding the right kind of supernovae is a challenge to scientists. The Laureates and their research teams used digital technology and invented efficient methods for repeatedly searching patches of sky and comparing thousands of images. Dozens of supernovae were discovered, and with the help of the world’s largest telescopes, their type and brightness could be determined, as well as the distance indicated by their redshift.

To everyone’s utter amazement, the supernovae seemed to be much too dim – weaker than expected if the expansion of the universe had slowed down. Instead, the universe has been expanding faster and faster.

What, then, is causing this expansion rate to increase? The answer might be: a special form of energy, called dark energy, which is pushing the universe “outward”. A similar form of energy – the cosmological constant – was discussed by Einstein as early as 1917 but was later removed from his theory. Dark energy seems to make up about 73 per cent of all energy in the universe, which has been confirmed during the past decade by the distribution of galaxies across large distances and by studies of cosmic background radiation.

The discovery of accelerating expansion through studies of distant supernovae has thus changed our image of the universe in an unexpected, dramatic way. We have realised that we live in a universe which largely consists of components that are unknown to us. Understanding dark energy is a challenge for scientists all over the world – in keeping with Piet Hein’s motto:

Problems worthy of attack
prove their worth by hitting back.

Professor Perlmutter, Professor Schmidt, Professor Riess,

You have been awarded the Nobel Prize in Physics for the discovery of the accelerating expansion of the Universe through observations of distant supernovae. On behalf of the Royal Swedish Academy of Sciences it is my honour and my pleasure to convey to you the warmest congratulations.  I now ask you to step forward to receive your Nobel Prize from the hands of His Majesty the King.