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iqueScience et Vie Mag<\/p>\nDear readers, faithful companions of the lantern and the basketball,<\/p>\n
There are days when the storm rumbles, when words clash, when passions set pages ablaze. The last outburst I published unleashed many reactions, and I understood, with a touch of sadness, that certain subjects require a circumspection that only those who live them from within can truly possess. I am only a Swiss astronomer, a watcher of the stars. I am not American. And to speak of America, of its wounds, of its angers, one must be American, or at least steeped in them.<\/p>\n
But I want you to know this, from the bottom of my heart: I love the United States. I love them for what they have been, for what they still are in the hearts of millions of their citizens, for that dream of freedom and justice they carried so high, for so long. I love their music, their literature, their cinema, their boldness, their generosity. And I sincerely hope they will recover. That they will pass through this night, as they have passed through so many others, and find their way back to the light.<\/p>\n
If my outburst hurt anyone, I apologise. That was not my intention. I did not want to add pain to pain, or anger to anger. I only wanted to say, in my own way, that things could be different, and that truth deserved to be defended. But I know that words sometimes overshoot thought, and that passion can wound. So, forgive me.<\/p>\n
For after the rain comes fair weather. After the noise, silence. After the tearing, contemplation. Astronomy is not an escape. It is a return. A return to beauty, to truth, to what brings us together instead of dividing us. It is that lantern we light when the house grows too dark. And tonight, it leads us toward the oldest thing we can see. A light that has travelled for thirteen billion eight hundred million years before striking our detectors. A light born in a universe still burning, still opaque, still without stars, without galaxies, without planets. A fossil light, reaching us from every direction at once, like a murmur from the beginning.<\/p>\n
Tonight, we speak of the cosmic microwave background.<\/p>\n
1. The Noise That Came from the Sky<\/p>\n
The story begins in 1964, in New Jersey, on a quiet hill. Two engineers, Arno Penzias and Robert Wilson, were working for Bell Laboratories. They had in their hands a horn-shaped antenna, designed for satellite communications. But there was a problem: the antenna was picking up a background noise, a low and persistent hiss, no matter which direction it pointed. A noise coming from everywhere, not from a single point in the sky.<\/p>\n
They tried everything to get rid of it. They checked the cables, tightened the connections, cooled the circuits. Nothing. The noise remained. They even found a pair of pigeons nesting in the antenna, and they cleaned the droppings with great care, thinking they had found the source. Nothing. The noise remained, stubborn, cosmic.<\/p>\n
A few dozen kilometres away, in Princeton, a group of theoretical physicists led by Robert Dicke was searching for exactly that signal. They had predicted that if the Big Bang had occurred, there should remain a residual radiation, a fossil heat, cooled by the expansion of the universe. Penzias and Wilson had just discovered it without knowing.<\/p>\n
That noise was the cosmic microwave background. The first light of the universe. In 1978, they received the Nobel Prize for this discovery.<\/p>\n
2. A Light Born in Opacity<\/p>\n
To understand this light, we must travel back in time. At the very beginning, the universe was a burning plasma, an ocean of charged particles, electrons and protons, so dense and so hot that light could not move through it. Photons bounced endlessly off free electrons, like marbles in a fog. The universe was opaque. Light was imprisoned.<\/p>\n
Then, about 380,000 years after the Big Bang, the temperature dropped below 3,000 kelvins. Electrons combined with protons to form the first hydrogen atoms. This is what we call recombination. Suddenly, matter became neutral, and light could travel freely. The universe became transparent.<\/p>\n
That first released light is the cosmic microwave background. It departed from everywhere at once, and it is still travelling. At the time, it shone in the visible range, with an orange glow, like a cosmic sunset. But the expansion of the universe has stretched it, cooled it, shifted it. Today, it is no longer in the visible range: it lies in the microwave domain, at a temperature of 2.7 kelvins above absolute zero. It is the oldest photograph of the universe, taken by the universe itself.<\/p>\n
3. How Do We Know It Was 380,000 Years After the Big Bang?<\/p>\n
I am often asked this question, and it is a fundamental one. How can we claim that this light was emitted 380,000 years after the Big Bang?<\/p>\n
The answer can be summed up in one phrase: the physics of hydrogen. We know, from laboratory experiments, at what temperature a hydrogen plasma combines into neutral atoms. That is the recombination temperature: about 3,000 kelvins. Above that temperature, electrons are free, and light is trapped. Below it, electrons bind to protons, and light escapes.<\/p>\n
Now, we measure today the temperature of the cosmic microwave background: 2.7 kelvins. We also know that the universe is expanding, and that the wavelength of light stretches in proportion to that expansion. The ratio between the recombination temperature (3,000 K) and the current temperature (2.7 K) is about 1,100. That means the universe has expanded by a factor of 1,100 since that light was emitted.<\/p>\n
Then we must go back in time. The equations of cosmology, based on general relativity and the measured density of the universe, tell us how the expansion has evolved. We can calculate at what moment the universe was 1,100 times smaller than it is today. The answer emerges: about 380,000 years after the Big Bang. This calculation is cross-checked by other methods, notably the abundance of light elements produced by primordial nucleosynthesis.<\/p>\n
That is how we date the oldest light in the universe, with remarkable precision.<\/p>\n
4. What Does That Image Represent? The Inner Volume<\/p>\n
I am often asked: \u201cWhat does that oval image represent, with its blue and orange patches?\u201d I will try to explain it simply.<\/p>\n
iqueScience et Vie Mag<\/p>\n
Imagine an immense hollow sphere, with us at its centre. That sphere is the surface of last scattering. It lies 13.8 billion light-years from us, in every direction. The photons of the cosmic microwave background reaching us today were emitted on that sphere, 13.8 billion years ago, when the universe became transparent. That sphere is not a physical wall. It is a boundary of visibility: beyond it, light cannot reach us, because the universe was opaque. It is the horizon of our observable universe.<\/p>\n
The image of the cosmic microwave background is therefore a map of that sphere, projected onto a flat plane, just as a globe is flattened into a world map. The patches of colour represent tiny temperature differences, on the order of a millionth of a degree. The slightly warmer regions, in red-orange, were a little denser; the slightly colder regions, in blue, a little less dense. These fluctuations are the seeds of all the structures of the universe.<\/p>\n
But be careful: the surface of last scattering does not contain the entire universe. It only encloses the observable universe, that inner volume at whose centre we stand. And that volume is immense, but finite. The universe itself may be infinite, far beyond that horizon.<\/p>\n
So when you look at the image of the cosmic microwave background, you are not seeing a photograph of the universe. You are seeing the inner wall of a bubble of fossil light, at whose centre we stand. A bubble that carries within it all the ripples of the newborn universe, murmuring to us where we come from.<\/p>\n
5. Primordial Ripples: The Seeds of Galaxies<\/p>\n
That light is not perfectly uniform. It carries within it tiny temperature variations, ripples on the order of a millionth of a degree. These fluctuations are the imprints of density differences in the primordial plasma. Some regions were a little denser, others a little less. And those clumps, those seeds, became, under the influence of gravity, the galaxies and galaxy clusters we see today.<\/p>\n
The satellites COBE, WMAP, and then Planck, mapped these ripples with incredible precision. The map of the cosmic microwave background is one of the most important images in the history of science. It shows the baby universe, aged 380,000 years, with its flaws, its fluctuations, its promises. It is our family portrait.<\/p>\n
By analysing the sizes of these fluctuations, cosmologists have measured the curvature of the universe, the amount of ordinary matter, dark matter, dark energy, and even the age of the universe: 13.8 billion years. The cosmic microwave background is a treasure trove of information. It is an open book on the origin of everything.<\/p>\n
6. The Hubble Tension: The Mystery Lying in Wait<\/p>\n
And here our story rejoins that of the Hubble tension. Data from the Planck satellite, which measures the cosmic microwave background, yield a value of the Hubble constant of about 67 kilometres per second per megaparsec. But measurements made with Cepheids and supernovae, in the nearby universe, give about 73. Two incompatible numbers, two methods, two epochs.<\/p>\n
Why this disagreement? Perhaps our cosmological model is incomplete. Perhaps dark energy is evolving. Perhaps new physics exists. The cosmic microwave background, that light born at the dawn of time, whispers to us that we do not know everything. It is both our best evidence for the Big Bang and our greatest source of questions.<\/p>\n
7. The Watchman\u2019s Lesson<\/p>\n
Dear readers, the cosmic microwave background is far more than background noise. It is the trace of the beginning. It is proof that the universe had a childhood, hot, dense, luminous, and that this childhood is still visible today, if we know how to listen.<\/p>\n
When you turn on your television to an empty channel, part of the static you see on the screen is the cosmic microwave background. That ant-like noise, that hiss, is the Big Bang knocking at your door. You cannot hear it, but you can see it. The universe is speaking to you, from thirteen billion eight hundred million years away. It tells you: \u201cI was there. And I am still here.\u201d<\/p>\n
So after the rain, fair weather. After the anger, wonder. After the noise of the world, the silence of the stars. And in that silence, a promise: the light of the beginning has never stopped reaching us. It is there, around us, within us, like a memory of the dawn.<\/p>\n
I set down my lantern. It flickers gently, like the fossil light filling the night. The basketball is in its place. And I think of Penzias and Wilson, of their pigeons, of that antenna that caught the beginning. Sometimes, the greatest discoveries come to those who are not searching, but who know how to listen.<\/p>\n
Thank you, dear friends, for bringing me back to the stars. It is here, in the fossil light, that we are most at home. And whatever happens, the sky is always waiting for us.<\/p>\n
Friendly<\/p>\n
Fran\u00e7ois<\/p>\n