The news was launched this beginning of week: scientists found gravitational waves!
Different blogs and websites write about this great news, there are underneath some of them…
“In the most anticipated announcement in physics since the discovery of the Higgs Boson, the first detection of a gravitational wave has been reported. If verified, the find will dispel any lingering doubts about Relativity theory, while providing astrophysicists with a new tool to probe the universe. The importance of the detection is hard to overstate.
As part of his General Theory of Relativity, Einstein predicted that acceleration of large masses would cause waves to ripple through space in a manner analogous to ripples on the surface of a pond. Indirect evidence abounds for gravitational waves, but almost a century after Einstein predicted it direct evidence remained elusive – until today’s announcement by the Harvard-Smithsonian Center for Astrophysics. The paper is now available on arXiv.
The IGOs now appear to have been scooped by a team from the Harvard-Smithsoinian Center for Astrophysics using a completely different method. The Cosmic Microwave Background (CMB) is the left over radiation from a four hundred thousand years after the Big Bang stretched by the expansion of the universe to peak in the microwave part of the spectrum. In the mid 1990s astrophysicists proposed that the polarization of the CMB could provide evidence for gravitational waves from the birth of the universe.
Photons can oscillate in different directions as they travel; up or down, side to side or even in a circular manner clockwise or anticlockwise. Hot sources produce photons with random orientations, but certain forces can create a bias where there is a preponderance of photons oscillating in a particular direction as they travel, making the radiation as a whole polarized.
The CMB was found to have a very slight polarization in 2002 as a result of density perturbations in the universe. Gravitational waves however, would be expected to induce a slightly different form of polarization. However, this pattern is so slight, and so vulnerable to false positives caused by other things, that there has been considerable skepticism that we would be able to detect the gravitational wave-induced polarization, at least with existing instruments.
The Plank space observatory has been studying the CMB since 2009, and some astronomers hoped it would be able to provide the evidence, but in the end the results came from an even more remote location, the Background Imaging of Cosmic Extragalactic Polarization (BICEP) detector located at the South Pole, where the cold dry air makes microwave astronomy possible.
“Detecting this signal is one of the most important goals in cosmology today. A lot of work by a lot of people has led up to this point,” said Prof John Kovac of the Harvard-Smithsonian Center for Astrophysics and a leader of the BICEP2 collaboration.
Rumors of the discovery leaked well before the announcement leading to considerable debate online. While some astrophysicists were sceptical as to whether such a subtle signal could be detected with confidence, others not involved in the research were given prior access to the data. “I’ve seen the research; the arguments are persuasive, and the scientists involved are among the most careful and conservative people I know,” Professor Marc Kamionkowski of Johns Hopkins University told BBC News.
Technical papers are available and are being poured over by researchers from teams worldwide.
The discovery of the CMB polarization by gravitational wave, should it stand the test of time, settles one question on its own, the debate over whether the early universe was inflationary. According to the most popular, but not universally accepted, theory of the early universe, 10-34 seconds after it began the universe experienced a period of rapid growth – expanding 100 trillion trillion times to something the size of a marble.
An inflationary period would produce larger gravitational waves than would have been generated without. Nevertheless, even most inflationary models do not predict a gravitational wave large and polarizing enough to be detected by BICEP.
The signal BICEP has found is so strong it makes many of the inflationary models of the early universe untenable, and leaves non-inflationary versions completely on the outer, suggesting the energy in the universe at that moment was well very much at the upper end of what was previously thought possible.
One of the reasons gravitational waves are so keenly sought is the hope that they will provide information about the crucial first moments of the universe in ways other instruments cannot. “People talk about the Square Kilometre Array as enabling us to detect the radiation from the Big Bang, but that is not strictly correct, Professor Jesper Munch of Adelaide University told Australasian Science. For the first 300 million years the universe was opaque to all electromagnetic radiation. However, gravitational waves could propagate through this early universe, and we can thus in principle detect signatures from the time of the Big Bang. It is probably the only way we can get signals from the origin of the universe.
Merely detecting a way is exciting, but we want more information than that it exists. The strength of the wave is expected to vary at different wavelengths. Finding out where it is strongest and weakest will tell us a lot about how the inflation occurred. The most important information of all is how energy dense the universe was during this era, and this could potentially be found by comparing wavelengths.
Gravitational wave perturbations from those first moments are directly dependent on the inflation, unlike density perturbations which are modulated by an unknown potential energy function. Consequently they would give us direct evidence of the energy of inflation in those first moments. ”
I fucking love science, 17th March 2014
“Albert Einstein l’avait prédit. Un siècle plus tard, les chercheurs l’ont mesuré. Des astrophysiciens américains du Harvard-Smithsonian Center for Astrophysics ont détecté des ondes gravitationnelles. En d’autres termes, des fluctuations de l’espace-temps, qui indiquent que l’univers est en expansion. Cette découverte est en fait une preuve directe qu'”au commencement” était le big bang, comme le titre aujourd’hui le quotidien belge De Morgen. Et que l’univers a connu, en une fraction de seconde, une expansion vertigineuse, qui a laissé des traces indélébiles.
Ce sont des traces directes du Saint-Graal de l’astrophysique que les scientifiques ont mesuré au pôle Sud, rappelle le journal flamand. D’autres sont plus enthousiastes encore. “Les arguments sont convaincants et les scientifiques impliqués sont parmi les personnes les plus prudentes que je connaisse”, a ainsi déclaré le Pr Marc Kamionkowski, de l’Université Johns Hopkins, à la BBC, qui parle déjà d’un Nobel.”
Courrier international, 18 mars 2014
“Sans être hyper calé en science, le concept de Big Bang vous dit probablement quelque chose. L’idée qu’au commencement (ou du moins à un certain instant qu’on assimile à un «commencement»), il y a quelques 14 milliards d’années, il s’est produit quelque chose, qu’on imagine (faute de mieux et donc à tort) similaire à une explosion et qui a créé l’univers. Et que «dans la première fraction de seconde, pour reprendre les termes du communiqué officiel, l’univers a vécu une expansion exponentielle, s’étendant bien au-delà de la vue de nos meilleurs télescopes.» Vous avez certainement, même vaguement, tout cela en tête. Très bien: c’est le concept de l’inflation cosmique. Sauf que tout cela «n’était qu’une théorie». Jusqu’à maintenant.
Concrètement, les scientifiques viennent en fait d’observer la trace de ces premiers instants de l’univers. Le «marqueur résiduel de l’inflation»,résume encore la BBC. Ça fait en effet un bail que les scientifiques, Albert Einstein en tête, soupçonnent que tout cela n’a pu se produire sans laisser des indices derrière lui. Et qu’il y aurait des espèces«d’“échos” de l’explosion», pour reprendre la métaphore de Terry Pratchett dans La Science du Disque-Monde:
«Comme si Dieu avait crié “Bonjour!” à la création et que nous entendions encore un léger“onjouronjouonjouonjour…” venant des montagnes lointaines.»
Ces traces, on les appelle «ondes gravitationnelles primordiales». Dans sa théorie de la relativité générale, Einstein avait ainsi déjà envisagé la propagation de ces ondes dans l’univers, «tout comme les ondes sismiques se propagent dans la croûte terrestre», écrit Nature dans un très bon résumé (en anglais) sur les apports de cette découverte. Sauf qu’il n’imaginait pas qu’on pourrait un jour les détecter.
C’est désormais chose faite, à en croire les chercheurs américains qui auraient observé «la forme d’une torsion distinctive dans la plus vieille lumière détectable [le rayonnement fossile, ndlr] avec les télescopes»,raconte la BBC.
Il faudra néanmoins s’en assurer, car «une découverte aussi révolutionnaire demande confirmation par d’autres expériences pour qu’on y croit vraiment», précise le site Scientific Americain.
A ce titre, elle fait donc énormément penser à une autre révolution scientifique récente: l’identification du boson de Higgs, particule présumée, observée puis confirmée, qui a valu à ses premiers concepteurs le Prix Nobel en 2013. C’est peut-être d’ailleurs un autre point commun: puisque il est ici aussi (déjà!) question de Nobel.
Car l’observation de ces ondes primordiales ne permet pas seulement de confirmer un peu davantage encore la théorie du Big Bang et, par extension, d’éclairer nos origines, elle permet aussi de faire le lien entre deux grands ensembles théoriques que les chercheurs essaient en vain de relier depuis des années: la mécanique quantique et la théorie de la relativité générale. Ou, pour le dire plus simplement, les règles qui définissent le très petit à celles qui s’appliquent au gigantesque.
Pourquoi? Parce que «l’inflation est un phénomène quantique» (échelle microscopique, atomique) quand «les ondes gravitationnelles font partie de la physique classique» (échelle macroscopique), explique encore Nature. Relier les deux permettrait donc de combler ce trou théorique qui turlupine les scientifiques.
Reste à savoir comment: certains avancent l’hypothèse de l’existence de particules particulières permettant d’expliquer tout ça, telles que les«gravitons». Des idées qui restent aujourd’hui au stade d’hypothèse. Les chercheurs ont du boulot sur la planche mais savent au moins, comme un passionné de sciences nous le faisait remarquer sur Twitter, «enfin d’où partir!»“.png?resize=800%2C565)
Slate, 17 mars 2014
And, to refresh knowledge about the Big Bang theory:
“The Big Bang theory describes how the Universe began in a rapid expansion about 13.7 billion years ago and has evolved since that time. It is thought that all of space was created in this first moment.
Since the 1940s, when the modern form of the theory took shape, scientists have detected radiation from the early Universe with radio telescopes and satellites and named it cosmic microwave background radiation (CMB). The CMB, which is formed of microwaves and radio waves, is considered important evidence in support of the Big Bang because it matches theorists’ predictions.”
More on BBC Science
