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Dark Energy

Pierre Binétruy explains that the discovery of an acceleration of the expansion of the Universe requires a new form of energy: dark energy.
Our story starts in the dark. We are in this room, dark as the Universe and we would like to identify its size, its geometry, like we try to do with the full Universe. So we have scattered in the room candles, all identical candles. And depending on how far the candle is from my viewpoint, I see it less luminous. And this is precisely what we’re trying to do in the Universe, trying to identify the size, the geometry of the Universe by using standard candles far away from us, which means at very early times.
As so you see that we have identified the geometry and the size of the room just by using those candles when the room was dark. This type of approach has been used in astrophysics in order to measure distances. But how do you obtain candles in the Universe? Well, you have light sources. But you would like standard candles, candles of the same luminosity. That means you want sources which have equal luminosity whether they are close by– so that means young– or very far away. That means very old. One thinks one has found such sources, such standard astrophysical candles, with supernova explosions, supernova explosions of a certain type which is called type 1a.
Now a supernova explosion is an explosion of a star at the end of its life. It’s called a white dwarf. And a white dwarf, in some cases, is eating up the matter, attracting the matter of a companion star– as in this picture– and at some point undergoes a thermonuclear explosion. So the whole star is exploding. This explosion is called a supernova of type 1a. And one believes that these explosions are identical whether they are rather recent or very old. So they provide the standard candles one was looking for.
So how does one identify such explosions in the sky? Well, this composite picture is trying to summarise the procedure one adopts. One takes pictures of the same portion of the sky regularly, for example, every week. And one compares the different pictures. And sometimes, in some cases, one identifies a new source, or a brighter source. For example, in this case, when you compare the picture on the left-hand side, which was taken three weeks ago, to the central picture, which was taken today, and makes a comparison between the two, one realises that there is more light on one of the sources. And the difference tells you that indeed, probably an explosion has occurred in the meantime.
Then one checks this region with a more powerful telescope, for example, the Hubble telescope. That’s what you see on the upper right-hand side. And one identifies the presence of a new source which was not present in the past weeks. Now, before attributing this to explosion of a supernova of type 1a, one has to study this source in more detail, for example, looking for spectroscopic lines that will tell us which elements are present in that source. And in some cases, one concludes, like in this case, that one has witnessed the birth or the explosion of a supernova of type 1a.
In the 1990s, the teams who were studying these supernovae encountered a big surprise. They were measuring the distances of the supernovae in two different ways– one, with the luminosity, the other one, as have seen in preceding chapters, using the spectral red shift. And these two measurements of distance were not coinciding. It seemed that the more distant, the older supernovae were less luminous than the younger ones. So did it mean that they were not standard in the sense that the older ones had a different history, had a different luminosity than the younger ones? Well, after many checks, they realised that the conclusion was different. The older supernovae were less luminous because they were further distance than expected.
So that meant that the expansion of the Universe had accelerated since their explosions. And so this was a sign of the acceleration of the expansion of the Universe– a very great surprise for most of the astrophysicists.
So why was it such a surprise to see that the expansion of the Universe was accelerating? Well, we have seen that the Universe is expanding because of the gravitational force. This is one of the consequences of Einstein’s equations. But we have seen also that, for example, matter in the Universe tends to attract each other. If you have two galaxies, two stars, then gravitation is an attractive force. And so the presence of matter because of this attraction will tend to slow down the expansion, to decelerate expansion. Similarly, when we have been looking at the effect of mass on light rays, it tends to curve light rays, so again, attraction.
And so not only the presence of mass, but the presence of light in the Universe, the presence of electromagnetic radiation, tends to slow down the expansion of the Universe. Now, electromagnetic radiation and matter were supposed to be the largest components in the Universe. And so if one was identifying an acceleration of the expansion, that meant that one had to rely on a new component, a yet unknown component. And that’s why it was such a surprise to see the acceleration of the Universe in recent history of the Universe. Well, this was a surprise. But this was a welcome surprise. And the reason is that we were precisely looking for an extra component in the Universe to account for its flatness.
Remember that the prediction of inflation is a spacially flat Universe, and that this is so if the total energy density in the Universe is 10 to minus 26 kilogrammes per cubic metre. And we’re missing 70% of that number. So now, this new component, new form of energy which would be responsible for the acceleration of the expansion, could be precisely the missing component in the energy budget of the Universe. So this form of energy is called dark energy because it’s dark. And the main property at this point is the fact that it should accelerate the expansion of the Universe.
And the total energy budget of the universe is 70% of dark energy and 30% today of matter, radiation being negligible today, 30% percent of matter, most of it– actually 25%– being dark matter, and the smaller part– 5%– being the luminous matter of the stars. Now, observation has shown that this acceleration of the expansion is a fairly recent event in the history of the whole Universe.
To give you an idea, it took place, it started this expansion, at a time where the observable region of our Universe was twice as small. To give you a comparison, when you think of the recombination of hydrogen that led to the cosmic microwave background the Universe at that time, the observable Universe, was 1,000 times smaller. So you see that compared to the time where the cosmic microwave background was produced, the start of the acceleration of the expansion is much more recent.
So let us summarise the concepts introduced so far. Besides the matter that we observe, what we call the luminous matter, there’s also another form of matter, dark matter, which plays a very important role in the formation of galaxies. Luminous matter and dark matter, as well as radiation, are not sufficient to account for the energy density necessary for space to be flat, which is one of the key predictions of the theory of inflation.
We have also seen that the expansion of the Universe is presently undergoing an acceleration. This was observed for the first time in 1999. And on the time scale of the history of the Universe, this phase of acceleration has started rather late. Matter and radiation slow down the expansion. So that means that we have to introduce a new component, a new form of energy, in order to account for the acceleration of expansion. That new form of energy is called dark energy. And we’ll see in the next sequence what could be the nature of this dark energy.
In 1999, the news that, in the recent evolution of the Universe, the expansion has been accelerating came as a total surprise, all the more because all known forms of energy tend to slow down the expansion. One had to resort to a new form of energy, dark energy, to explain this phenomenon. And one soon realized that dark energy appears to be the dominant form of energy in our Universe! (10:57)
One word of caution before you start. I have already mentioned last week in step 3.4 the concept of “flat” space. Let me warn you that we physicists are using the word in a slightly different way than in everyday life. For us, “flat” means “not curved”. A 2-dimensional space, like the surface of a table, is flat, in the physicist sense as well as in the usual sense. But physicists also say that our 3-dimensional space is flat! What does it mean? It means that it is not curved. In other words, the geometry is just the Euclid geometry that we are taught at school: two parallel lines never meet, the sum of the inner angles of a triangle is egual to 180 degrees, and so on. All these results are not valid in a non-flat i.e. curved space (for example, parallel lines meet!). Sorry for this long digression, but, in the past, this use of the word “flat” has confused many, who associated it with a (2-dimensional) surface.
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