The Nobel Prize for Physics in 2011?

But that is long ago, the Nobel Prize 2012 has already been awarded and in October the winners of the 2013 prize will be known!

Yes, this post is long overdue, I know 🙂 Every year I am interested, being a physicist myself, who will get the Nobel Prize for physics and for what . And nowadays often I have no idea what it is about :-(, being out of touch with the modern developments for so long already. So I was quite happy that I understood the importance of the discovery made by Perlmutter, Schmidt and Riess in 1998 that our Universe is expanding at an accelerated rate.

From left to right, Perlmutter, Schmidt and Riess

Of course you have heard about the Big Bang, the primordial explosion that created the Universe, about 14 billion year ago. As a result of this explosion the Universe is expanding and also cooling down. Proof: when we look at faraway galaxies, we observe that they are moving away from us and each other, the farther away the faster they move. And in 1965 the Cosmic Background Radiation was discovered, proof of the cooling down of the Universe.

When I was doing my PhD research, in the seventies, the Big Bang theory was widely accepted. And also that the rate of expansion should decrease with time because of the mutual gravitational attraction between all matter in the Universe. If the Universe contained enough mass, the expansion would finally stop, followed by contraction and ending in what became known as the “Big Crunch” where the whole Universe would again be concentrated in a single point. And might even start again in another Big Bang! An attractive idea in those hippie days!

Here are the possible scenarios. In the coasting scenario there is not enough mass to stop the expansion, in the middle one there is just enough mass to stop it (asymptotically), but not enough to reverse the process (as in the left scenario)

The problem was that when you counted all the visible mass in the Universe, there was just not enough to stop the expansion. It was named the “missing mass problem”.

Would it be possible to determine experimentally which scenarios was the correct one? To measure the rate of expansion, you should measure the velocity of very faraway galaxies. Measuring the velocity is not that difficult, you have to measure the Doppler shift. When an ambulance passes you, you will first hear a higher sound of the siren, and a lower sound when the ambulance is moving away from you. For light it is basically the same, here you will see a difference in colour. When a star or galaxy is approaching is, the colour is a bit bluer, when it moves away it will be redder. Measuring the “redshift” gives us the velocity.

The big problem is how to determine the distance to such a faraway galaxy! The technique used in astronomy is based on the fact that light from a light source becomes more spread out when the distance is larger. Probably every photographer is aware of this “inverse-square law”

So if you know how “strong” the light source itself is, you can determine the distance by measuring the amount of light at that distance. But how do we know how much light a star really produces? In general that is impossible, because you have big bright stars and small, not so bright stars.

What the Nobel Prize winners did was looking at very special events, so-called (type 1A) supernova’s. A supernova is a star that explodes at the end of its life. During a few days/weeks it can produce more light than a whole galaxy. And the intensity of this light is basically the same for each supernova explosion (of type 1A). They are extremely rare events, it is estimated that in our own Milky Way they occur only a few times in a century! But when they occur, they are so bright that they can even be observed in very distant galaxies. And there are so many galaxies.

Finally we can now explain the research done by the (competing) teams of Perlmutter and Schmidt & Riess. They looked for type 1A supernovas in distant galaxies and determined the distance and the velocity. To show you how complicated this kind of research is, here is an image of a recent supernova discovery, SN Wilson. In this image a few bright points are stars, but many are galaxies. The tiny square contains the galaxy with the supernova.

Here are three enlarged images of this tiny square. The galaxy is the round spot in the center. Left image shows the situation before the supernova exploded, in the middle one the supernova has exploded. You don’t see any difference? Let the computer “Subtract” the left image from the middle one and you get the image to the right! Voila, the supernova ..:-)!

These images, taken by the Hubble telescope were taken by the team of Riess in 2010. The distance is 10 billion light year, which makes this galaxy the most distant one, observed until now.

As the light of this galaxy needed 10 billion years to reach us, we observe it now as it was 10 billion years ago! Looking far away means looking in the past. The scientists expected to find that in the past the expansion of the universe would be faster than it is now, as explained in the beginning of this post.

What they actually found, shocked the scientific world: the expansion of the Universe was accelerating . It was so unexpected that it was very fortunate that two research teams came to the same conclusion.

So there had to be a repulsive force, stronger than the attractive force of gravitation. This repulsive force is now named “dark energy” but we still have no clear idea what it is.

It is for this discovery that the two teams shared  the Nobel Prize.

The three scenarios, mentioned above are all wrong. It is the fourth scenario, shown below, that we now believe to be correct. There is even a possibility that this acceleration will increase so dramatically with time, that the Universe would end in a Big Rip, where finally, stars, planets, even atoms would be ripped apart.

Much progress has been made since 1998, especially in the analysis of the Cosmic Background Radiation. It has confirmed that there is a repulsive force, now named “dark energy”. It has also confirmed that there is a lot of invisible matter in the Universe, now called “dark matter”. In both names “dark” describes our ignorance, at the moment we just do not know what they are. I am planning to write a separate post another time about this topic.

Let me end this post with an image that gives the distribution of “normal” matter, dark matter and dark energy in our Universe. I have seen this kind of picture numerous times, and I still find it shocking.

The stars, the planets, humans, everything is made of normal matter: protons, neutrons, electrons. We know a lot about it.  But it is only 4% of our Universe. About the other 22+74 % we know next to nothing at the moment!

If I could start a new life now, I would choose astrophysics and cosmology as my field of study…:-)

Several images above have been taken from this very interesting set of lecture notes.