The primordial Universe was opaque, so that means that light could not travel through the Universe. Remember that the Universe, at that time, it was made of particles, elementary particles,
and most of them were charged: protons, electrons. Now light is emitted by charged particles, but it’s immediately caught by these particles, as is in this diagram here, where you see that light is travelling very short paths between two particles. As the Universe cooled down, the charge sort of gathered together and started making structures, like atoms. And as the structures developed, there were less and less charged particles, and the Universe became more and more transparent.
One important structure to form is the hydrogen atom. The hydrogen atom is just made of a proton and an electron. And it is neutral. Since most of the matter in those early times is made of hydrogen, so that means that most of the matter when hydrogen combined, so when electrons and protons form an atom of hydrogen, most of the matter becomes neutral, and suddenly, the Universe becomes transparent. This happens– this is a stage which is called recombination, recombination of hydrogen. We should say, combination of hydrogen because hydrogen was never formed before. But physicists tend to call this the recombination phase. This occurs when the temperature of the Universe was 3,000 Kelvin.
The average energy, 0.26 electronvolt and this happened 380,000 years after the Big Bang. So at that time, the Universe became transparent to light, but which was the light available at this time? This is what we are going to discuss now.
So let us return to the observer, us, today, observing the sky. We have seen that the further we look, the more distant we look, the earlier in time we observe. And so let’s just imagine that we could look all the way to 14 billion light years, which corresponds to the age of the Universe. In principle, we would see the first times of the Universe. But of course, because the Universe was opaque at that time, at some point, we’ll see an opaque wall which corresponds to the period of recombination 380,000 years after the Big Bang.
And so it’s a bit like in this room, where we look and it stops at a certain point, which is this black body, which corresponds to a black wall. It’s exactly the same in the Universe. When we observe, we should be stopped by a black wall which corresponds to a black body, which corresponds to the very early Universe. And we will see that this black body, which is heated at a temperature of 3,000 Kelvin, as I said, is emitting a radiation which is characteristic of its temperature and characteristic of the condition of the Universe at those very early times.
It had been known for some time that a hot black body is emitting a radiation, which is characteristic of the temperature of the body. You have here the spectrum of a black body at different temperatures. By spectrum, we mean that we represent the intensity of radiation versus the wavelength. And you see on the left-hand side, wavelength, which correspond to visible light, so very hot black bodies emit in the visible range. But you have other emission outside the visible range of electromagnetic radiation. Now, it is only at the beginning of the 20th century, a young physicist with the name of Max Planck, who is represented here precisely at the time of his discovery. It is Max Planck who discovered that…
, or who understood the spectrum of this radiation by making the assumption that the emission was not continuous. It was not continuous light. It was the emission of grains of light, what one called immediately quanta of light, we now would call them photons. So he explained the spectrum by assuming that there was a discontinuous emission of these grains of light. And the energy of these grains was proportional to the frequency of the light emitted. And this constant is now known as the Planck constant. It has a very small value, which is, as we said, with the microscopic nature of these grains of light, a value of the order of 10 to minus 34.
So you see a very small value in the units of the international system. Now, the assumption by Planck, he assumed this was a mathematical trick. And one had to wait until Einstein came and explained to us that these grains of light were not just mathematical devices, they were reality. And light was, at the same time, waves and quanta (and particles), waves and photons. And so this was the birth of what we know now as quantum mechanics. And the constant introduced by Planck, the Planck constant, turns out to be the constant that describes all the quantum processes.
So what about the cosmological black body? That black wall that we see when we look at the furthest possible distance 14 billion light years, exactly like when you try to see around this room. We stop at the absorbing wall. Well, we have seen that this black body is very hot, a temperature of something like 3,000 Kelvin. And so it emits radiation, which is specific of that black body radiation which we expect to detect here. In a range of a frequency which has been red shifted because of the expansion of the Universe. Now it was discovered indeed by chance by two engineers of Bell Labs, Penzias and Wilson in 1964.
Penzias and Wilson were working on a big antenna, a big antenna like you see here, which was supposed to detect electromagnetic radiation in the radio and microwave frequencies. And they identified in the antenna a noise, so it means something which was not an electromagnetic signal , that seemed to be coming from maybe the apparatus. So they checked that it was not an electronic noise. They even checked that… there were some pigeons living in this big antenna. They scrapped all the remains of the pigeons in order to make sure that this was not the cause of this noise.
And so they had to conclude that this noise was a signal, and it turned out, this was a signal of the cosmological black body. And in order to prove that, they realised, or they checked that this radiation was identical from all directions, so that means it was homogeneous and isotropic. It had exactly the same property coming from this region of the sky, or this region of the sky, or this one. And so that sort of showed the cosmological origin, the cosmic origin of this radiation. And so they had discovered what we now call the Cosmological Microwave Background, since this is now, as I said, red shifted in the microwave frequency domain of the electromagnetic spectrum.
Of course, it’s not enough to have radiation coming identical from all parts of the sky to conclude that this is coming from the very early Universe, the first 100,000 years after the Big Bang. One had to check the exact spectrum of this black body. And so one had to wait until the 1990s and the satellite, launched by NASA, the COBE satellite, in order to measure the exact spectrum of this radiation. So the COBE satellite indeed identified the full spectrum– you’ll recognise the spectrum of a black body– in an extremely precise way. And identified also the temperature of this light, which corresponds to a black body of 2.73 Kelvin.
So I mentioned that when it was emitted, the temperature was 3,000 Kelvin, but that light has been red shifted because of the expansion of the Universe. And so now it comes to us at a much lower temperature, and so what you see here is the spectrum characteristic of a black body at 2.73 Kelvin. And you see the precision. So the different dots of the experimental points. The curve is the theoretical curve, and so you see the precision at which one measured this black body spectrum.
And so it’s rather remarkable that the discovery of Max Planck, which gave the birth to quantum mechanics and to the physics of the microscopic world of the 20th century, can be seen at the same time in the largest possible object you know, the sort of black wall that surrounds us, at 14 billion light years. And so for the discovery of that spectrum and the identification, a very precise identification of this spectrum, John Mather got the Nobel Prize in 2006. Let us now summarise. The Universe is opaque during the first 380,000 years. The reason is that light is trapped by the charges of the elementary particles that formed the Universe at that time.
380,000 years after the Big Bang, the last charges combine. Electrons and protons form neutral hydrogen, and the Universe becomes transparent to light. The earlier Universe appears to the observer today as a shell which absorbs light as a very hot black body. And according to the laws of quantum mechanics, this back body emits quanta of light with a spectrum determined only by its temperature. It was first observed, this emission, by Penzias and Wilson in 1964. And it has been called since the cosmological microwave background, or CMB if we just use the first letters. And the spectrum of this cosmological microwave background was first detected and measured by the COBE satellite in 1990.