Gravitational waves shed light on the enigma of the acceleration of the expansion of the universe
The exact determination of the Hubble-Lemaître constant linked to the expansion of the observable universe and the nature of the black energy has always been problematic.The boom in the astronomy of gravitational waves should allow you to see more clearly and the latest results have just fallen on this subject with publications of Ligo and Virgo collaborations.
Vous aimez nos Actualités ?Inscrivez-vous à la lettre d'information La quotidienne pour recevoir nos toutes dernières Actualités une fois par jour.In 1917, Albert Einstein made one stone two blows by introducing the first model of relativistic cosmology from the equations of general relativity as well as his famous cosmological constant.The following year, he also published a founding article on another consequence of these equations, gravitational waves (he had already discussed the subject in 1916).These three discoveries by Albert Einstein were going to make researchers perplex for decades.The father of the theory of relativity will start, the first, to doubt both the existence of these waves in the 1930s but also of the relevance in physics of the cosmological constant.
Everything has changed well since the advent of a new astronomy, strongly rewarded by the 2017 Nobel Prize in Physics, that of gravitational waves.Today, the members of the collaborations behind the detectors Ligo and Virgo, as well as Kagra, have just said that during the third campaign to detect these waves having been held from November 2019 to March 2020, 35 new events havebeen detected with 90 the number of gravitational waves signals observed to date.These are mainly collisions followed by mergers between two black holes of stellar masses being part of binary systems, but also some collisions between two neutron stars and, more rarely, between a black hole and a neutron star.
Retrospectively, we therefore have a little trouble believing that in the 1950s again, as explained in their remarkable work Nathalie Deruelle and Jean-Pierre Lasota, the question of the theoretical existence of gravitational waves in Einstein's theorystill made a debate before being settled from several angles thanks to the work of the French mathematician and physicist Yvonne Choquet-Bruhat and the British physicist Félix Pirani in the first place, and of Richard Feynman second (we generally tend to retain onlyFeynman's contribution, wrongly).
The physicist Bernard Schutz tells us about the gravitational waves of Einstein.To obtain a fairly faithful French translation, click on the white rectangle at the bottom right.Subtitles in English should then appear.Then click on the nut to the right of the rectangle, then on "Subtitles" and finally on "Translate automatically".Choose "French".© Scienceface
The question of the cosmological constant is still wide open.There was indeed the allocation of a Nobel Prize in physics in 2011 for the discovery of the acceleration of the expansion of the universe.The existence of this constant realizes this acceleration (we expected that the speed of expansion is always decreasing since the Big Bang when it has increased for a few billion years), but we do not understandIts nature which is part of different theories on what has been called black energy.However, on its determination most likely depends that of the fate of the observable cosmos.He could just as easily continue his expansion eternally as, ultimately, contract on himself if the black energy changed sign and value in an indefinite future, becoming attractive and no longer repulsive.
To try to know it, cosmologists seek to measure, more and more precisely, the value in time and space of the speed of expansion of the observable universe, hoping to flush a law of betraying variation everythingtimes a cosmological constant in variable reality and finally the law and physics governing its potential variations.
The problem of the cosmic distances scale
The company is not simple because it is necessary to be able to measure distances in the cosmos and this is done using what is called the distances scale in cosmology (cosmic distance ladder, in English) which consists, in someSingle, to use stars to measure distances from other more distant stars, which introduces errors which propagate in the measurements of distances and which are added in a way to each other.It starts with parallax measurements for stars in the Milky Way.They make it possible to determine the distances of the Cépheids, variable stars which serve as standard candles to define the distances of nearby galaxies which then serve to calibrate much brighter standard candles, allowing to probe the cosmos on billions of light years,The supernovae SN ia.
Details a bit of what it is about.
A simple presentation of the combined methods to measure the distances in the universe, that of the parallax to that of Hubble for the SN IA supernovae.© Unisciel
To measure variations in the time of the expansion speed of the observable universe, we must measure the distances and the spectral discrepancies of certain objects that can be considered as standard candles, or little should be, that is-To say objects whose absolute brightness is known and relatively constant.We have good reasons to think that this is the case of the SN ia, these supernovae which are explosions linked to white dwarfs, since these are compact objects whose masses should be of the order of thefamous mass of Chandrasekhar, which limits the energy available for the radiation emitted, whether electromagnetic or gravitational.
If we use the metaphor for sound to describe the spread of gravitational waves in the fabric of space-time, collisions of compact stars, black holes and/neutron stars should therefore be "standard sirens", becauseWe can deduce the amplitude of the gravitational waves emitted from their observed characteristics.The relationship between the amplitude emitted and the amplitude observed then allows us to deduce a distance, as with the SN ia.By analogy, the more a candle is far away, the less it appears to us luminous;The further a mermaid, the less powerful it seems to us.
However, as we have said before the advent of gravitational astronomy, distance measures were done and are still done by relying on a scale of techniques which begins with the measurement of stellar parallaxes in the Milky Way andContinues with the extragalactic cepheids to the supernovae.The errors specific to each technique accumulate and tarnish uncertainties the measurement of distances in the universe, which can be determined the speed of expansion of the observable cosmos.
Bernard F. Schutz is an American physicist whose research focuses on the theory of general relativity of Einstein, more concretely, on the physics of gravitational waves.He was one of the directors and heads of the astrophysics group of the Max-Planck Institute in Gravitational Physics in Potams, Germany.He played a key role in the Foundation of the Numérique Living Reviews in Relativity.© N. Michalke, Aei
Gravitational waves and Hubble constant
However, in 1986, a renowned and now well-known American relativist physicist (especially for his works on general relativity), Bernard Schutz, realized that gravitational waves made it possible to measure the distances in cosmology with more precisionAnd in particular to have more precise values of the expansion speed of the cosmos observable via the famous constant of Hubble-Lemaître.
Instead of measuring the distances and the spectral offset towards the red of a large number of SN IA supernovae to always assess the Hubble-Lemaître constant more precisely and therefore the speed of expansion of the observable universe, Schutz has shown that'He could pay to detect with instruments like Ligo and Virgo the gravitational waves emitted by at least ten collisions of compact stars in binary systems, a few hundred million kilometers away.
According to him, we could then assess this constant at 3 %.
Indeed, it turns out that frequency changes in gravitational waves emitted by the two compact stars starting their collision are directly connected to the amplitude of the waves emitted, and therefore finally to the brightness in the gravitational form of the collision.Thus, knowing the intrinsic brightness of these binaries, we directly obtain a distance without having to go through the previous scale and therefore by cutting short the propagation of measurement errors, hence the gain of precision obtained.Obviously, as for standard candles, the less the amplitude of the waves detected on earth is large, the less the binary system appears to us luminous, which gives us a measure of its distance, knowing its intrinsic brightness.
This possibility is all the more important since we know that in recent years there have been discrepancies between the measures of the Hubble-Lemaître constant made using the SN ia and those using the fossil radiation observed by the Planck mission.
This could point out a new physics but one of the methods used could also simply suffer from a bias that would have escaped the sagacity and rigor of the researchers from the teams committed to these measures.
Futura had devoted a long article to these questions during the editorial of Françoise Combes celebrating the 20th anniversary of Futura.
Astrophysicist Simone Mastrogiovanni.© Simone Mastrogiovanni
As we announced at the start of this article, members of Ligo and Virgo collaborations, joined by their Japanese colleagues from Kagra, detected 90 sources of gravitational waves and they have just put an article on ARXIV.of a new estimate of the value of the Hubble-Lemaître constant.
Astrophysicist Simone Mastrogiovanni, who has just joined the Côte d'Azur Observatory (ANR Cosmerge/Laboratory Lagrange/Artemis) and who is responsible for the publication of which he coordinated the writing, gives explanations on this subject inOne of the press releases in the relativistic astrophysics laboratory, theories, experiences, metrology, instrumentation, signals (Artemis) where his colleague Olivier Minazzoli is also located, which proposed a new theory of gravitation, the theory of introciated relativity.
He explains that: “Our result is quite robust, because we have analyzed the data in two different ways, and the two give completely consistent results.One only uses our data, the other uses a catalog of galaxies.These two methods certainly have biases, especially the second.We tried to minimize them.The result is visible in the figure below.Our results corresponding to different cases are indicated by the different curves.
Rather, it seems that our result agree with the measurements made by Planck with the microwave cosmological background (CMB).But the uncertainties are still quite large, and in fact they are compatible with the two most precise measures CMB and supernovae IA/Cepheids.A greater number of detections of binary sources of compact objects will allow us to reduce uncertainties on our measurement of the Hubble constant.»»
Distributions de probabilité postérieures pour la constante de Hubble (H0) correspondant à différentes analyses. Chaque distribution de probabilité est une courbe représentant la meilleure estimation de la valeur de H0 après avoir effectué une analyse donnée. La ligne noire continue exprime le résultat obtenu en utilisant uniquement la source d'ondes gravitationnelles GW170817 issue de la collision entre une étoile à neutrons et un trou noir, et son homologue électromagnétique sous forme d'une kilonova, ce qui a permis d'obtenir directement un décalage spectral selon la méthode habituelle en astronomie. La ligne bleue en pointillé montre le résultat de l'analyse n'utilisant aucune information du catalogue de galaxies. En orange continu et en vert pointillé, ce sont les résultats des analyses qui prennent en compte le catalogue de galaxies avec et sans l'événement de type kilonova également inclus respectivement. Enfin, les deux bandes verticales (magenta et vert foncé) montrent les contraintes sur H0 obtenues respectivement à partir du CMB (Planck) et des Supernovae + Céphéides (SH0ES). © Observatoire de la Côte d’Azur (ANR COSMERGE/laboratoire Lagrange/Artemis)
Les astrophysiciens relativistes ont utilisé deux méthodes pour estimer la valeur de la constante de Hubble-Lemaître (H0) et nous allons exposer surtout le principe de l'une d'entre elles.
To determine this constant, it is necessary to measure the distance of an object and the spectral discrepancy of this object.The study of the gravitational waves signal makes it possible to determine a distance and also the mass of compact stars which have collided.
On the other hand, in the case of black holes mergers, one does not expect a counterpart in the field of electromagnetic waves as is the case with mergers of neutron stars, in particular at the origin of the kilonovae.
A tip allows you to get around the problem.The calculations show that there is also a spectral offset effect of gravitational waves due to the expansion of the observable cosmos.If we draw up a curve representing the masses of the stellar black holes involved in the emissions of gravitational waves, the effect of the spectral lag will manifest itself in the form of a gap of the distribution curve of the masses.However, it turns out that the theory of stellar structure and the occurrence of a black hole by collapse of a star exploding in supernova implies that there must be a maximum mass for stellar black holes.Indeed, when a star is too massive, it explodes according to the model of the supernovae with instability of pairs which leads the whole star to be blown, without leaving residues in the form of a compact star.
Le nombre de sources avec des trous noirs binaires déjà disponibles permet alors de tracer une courbe de population pour les masses des trous noirs et d'exhiber l'effet du décalage sur le pic de cette population (pic lié aux limitations de la masse d'un trou noir stellaire), conduisant à une estimation de la constante H0.
The second method is also statistical, using the Galaxies Glade+ catalog containing distances and spectral discrepancies.As we can assess the chances that the gravitational wave sources detected are located in such or such galaxies, we can also indirectly estimate a value of the Hubble constant.
À ce stade, l'astronomie gravitationnelle n'a pas encore tranché le débat en ce qui concerne le conflit entre les déterminations précédentes de H0 mais l'avenir semble brillant car le nombre (l'Inde construit une version de Ligo) et la sensibilité des détecteurs d'ondes gravitationnelles vont grimper dans un futur proche. On peut s'en convaincre avec le communiqué de la collaboration Virgo sur son site : « Une nouvelle phase d'améliorations est en cours pour les détecteurs Ligo et Virgo. La prochaine prise de données commune (O4) est prévue pour le second semestre 2022 et devrait se caractériser par une sensibilité encore accrue, laquelle permettrait d'observer un volume d'Univers près de 10 fois plus grand que précédemment et donc de détecter encore plus de signaux d'ondes gravitationnelles ».
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Liens externesConstraints on the cosmic expansion history from GWTC-3Quelle est la vitesse de l'expansion cosmique ?Définitions associéesUne galaxie : définition simpleDéfinition de "trou noir"Qu'est-ce qu'un trou noir stellaire ?Ce que "Énergie noire" veut direUne Émission : définition simple
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