For centuries, the question of the origin of the dark night fascinates, and refers to that of the origin of the Universe. We all probably think that the night is dark, making it a daily experience. However, why is the night black? By questioning some children, students, or friends about it, the answers obtained - usually full of common sense - often invoke the fact that the Sun, passing under the horizon and becoming invisible for a few hours, no longer lights up the sky. , which becomes de facto black.
This story of Sun giving way to night is the correct answer, but another question, which could be "why is the sky less bright at night?". But not to the question of why the deep and dark darkness of the night. And this, despite the incredible number of stars. The dark night seems so obvious to us that we forget its extraordinary character which allows us to probe nothing less than the origin of the Universe. The night makes us enjoy each evening the spectacle resulting from a fascinating and eventful history which began 13,7 billion years ago.
Known as the Olbers paradox, the problem can be summarized as follows: given the large amount of stars in the Universe, any direction in the sky should at one point cross a star. A fairly simple calculation of license level shows that the brightness of the sky should be, in every point, equal to that of the surface of the Sun. Daily observation shows that it is not so.
This paradox, which has sparked many reflections since at least the sixteenth century, including Thomas Digges, later Philippe Jean de Chéseaux, Edmond Halley and finally Heinrich Olbers in the nineteenth century, was resolved intuitively by the poet Edgar Allan Poe in 1848 who sensed that the stars had a finite age, and independently by the physicist Lord William Kelvin at the end of the nineteenth century and published in 1901.
If the light of these stars did not reach us all, the problem would be solved. To do this, two conditions are required: first, the stars have a finite age; then, the light propagates at a finite speed. In this way, some of the starlight will not be emitted or will not reach us all.
So, a question as simple as "why is the night dark?" allows to invoke the finitude of the speed of light and the history of the stars making up the Universe, and touches on the questioning of the origins of the Universe. This response, correct in its time, is currently no longer considered sufficient. Since 1901, our conception of the Universe has radically evolved.
With the origin of the Universe and its constituents, such as stars and galaxies, the origin of the dark night is a fundamental question which fascinates, and which can be approached in a mythological, theological, philosophical (and metaphysical) way. , artistic or scientific. The scientific approach to the origin of the Universe differs from others in that it attempts to answer the question "how did it happen?", But by no means "why did it happen?" he passed? " which refers to a possible quest for meaning.
The separation is clear between the how and the why, and everyone is free to invoke the why while hearing the how. It then becomes clear that the mixing of genres, as sometimes seen in the media about the metaphysical origin of the Universe compared to its scientific reality, blurs, if not listening, at least the message of distinction between objective scientific approach and counter speculation orchestrated by modern charlatans.
In order to understand how, the scientific approach is to develop a theory based on fundamental principles and then confront it with the data. These are analyzed in the chosen model, and allow (or not) to conclude to its confirmation or denial, with a degree of confidence quantifiable.
Additionally, theories evolve as new data is obtained. We sometimes ask the question: "Do you believe in the Big Bang?". The question is badly posed since the scientific approach does not give reason to believe, but does show the agreement (or not) of the data acquired with models. It is not a question of belief in this approach, but of theories, observations, confrontation, questions, doubts, debates, questioning.
The question could have been: "Do you think that the model of the Big Bang is the one which agrees best with the totality of the existing observations?". Although simplistic in its proposition, this question admits as a simplistic answer "yes" by most of the scientific community; this answer would deserve to be detailed to illustrate the indisputable successes of the predictions and agreements with the observations, without obscuring the open questions and acute problems which remain.
Physical cosmology is the science that seeks to understand the universe as a whole, its structure and evolution. Our Universe is understood today as having had a turbulent history: initially very hot and dense, it cools under the effect of the expansion. A little like the fluid in the refrigerator, before it is recompressed in a sweet sound of purring our kitchens. In this warm phase, the light could not propagate freely: the Universe was opaque, a little like inside the Sun now. In a brief episode of about 380 000 years after the Big Bang (still under the standard model), the Universe became transparent, and the radiation that bathed the Universe could spread, until today.
It is the fossil radiation, or cosmic microwave background, a kind of luminous echo of the Big Bang, that scientists are tracking down to the smallest detail, especially with the European Planck satellite. The latter will provide us, in a few months, the most detailed view ever obtained of the fossil radiation.
At the end of this brief but important episode, the material could begin to aggregate effectively and then form the large structures of the Universe: clusters of galaxies and galaxies, and form stars within them, then planetary systems around certain stars.
In this modern context, the paradox of the dark night is today solved with three conditions: finiteness of the speed of light; the constituents of the Universe have a finite age; and finally the Universe is expanding. What intellectual path traveled, from the childish question to the physical history of our Universe as a whole!
It remains true that our Universe is bathed in radiation, in the first place of which the fossil radiation. There are other, much less intense radiations, such as extragalactic radiation, which is due to the light emitted by all generations of stars in galaxies.
It is therefore permissible to write, almost without irony, that the night is not dark, since, if our eyes were sensitive to infrared and microwave radiation, they would see a brilliant night of cosmological radiation. The paradox is only apparent, because there are many areas of wavelength of light for which the night is bright, but our eyes do not see it.
The night inspires us, fascinates us, amazes us. Whatever our approach, scientific or artistic, philosophical or theological, the part of humanity that is in each of us always enters, at one time or another of our existence, in resonance with the immensity of the night. Keeping this link between men and the nocturnal beauty seems more important than it seems.
However, can we still observe the night, if only for a few minutes at night, in urban areas that are illuminated by dazzling street lights? More and more rarely. It would be enough to reduce the intensity of the public lighting, to better direct it to the ground, or even to extinguish it in the middle of the night to generate substantial energy savings, for the happiness of our reflection - and the nocturnal flora and fauna.
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Astrophysicist, lecturer at the Institute of Space Astrophysics at Orsay, University Paris-Sud, and CNRS, and member of the Institut Universitaire de France.
A galaxy specialist, he worked for NASA before becoming a teacher-researcher at Orsay in 2004. Co-author of "L'Observation en astronomie" (Ellipses, 2009), he is a member of the Planck collaboration,
the satellite of the European Space Agency,
and coordinates a hundred researchers
During the night of June 23 to 24, the abbey of Fontevraud (Maine-et-Loire) becomes "the ideal city at night", with a series of intellectual and artistic proposals that will help you get through this sleepless night. Meet at 22:30 p.m., astrophysicist Hervé Dole who will be doing a conversation under the stars. Meetings in partnership with "Le Monde" animated by Nicolas Truong
Hervé Dole
Source: http://www.lemonde.fr/idees/article/201 ... _3232.html
I like this expression, bravo!
(dede being on vacation ...)
since the propagation is like a "wave")
