NASA’s Nancy Grace Roman space telescope, which will be launched in the mid-1920s, will use general relativity to track down exoplanets. However, the same techniques can also be used to search for much darker objects: stellar mass black holes
On May 29, 1919, the theory of general relativity received one of its first experimental confirmations: observing, during an eclipse of the Sun, a distant star whose line of sight passed near the solar disk, it was observed that its position was deviated. In fact, general relativity predicts that the presence of a mass deforms space-time and that the light rays that pass through this deformation have a deviated trajectory compared to the original one. A star whose light passes in the deformation seems to be in a different position from the real one, as in the case of the eclipse of 1919, because it is the direction from which its light seems to come that determines its position in the celestial vault.
This process is at the origin of the so-called gravitational lensing , i.e. the fact that a mass can behave like a lens for distant light sources , as long as it is in the right line of sight, because the masses deflect light rays just like lenses do. . This process can be exploited to find, for example, brown dwarfs, stars that do not shine with their own light are not easily observable, but which can behave like gravitational lenses. Even an exoplanet, if it is in the line of sight of another more distant star, can barely deviate its light in the so-called microlensing phenomenon and by measuring this deviation we can therefore discover its existence.
Nancy Grace Roman is the name of the space telescope that NASA will launch in the mid-20s dedicated to the search for exoplanets by exploiting this phenomenon. But there’s more, because the same method can also be used to track down stellar-mass black holes , those miniature monsters formed when stars with a mass greater than twenty solar masses run out of stellar fuel. While their larger companions – or super-massive ones such as those found at the center of galaxies – can be found thanks to the material that surrounds them, those of stellar mass risk being absolutely invisible . The microlensinggravitational can therefore give the possibility to discover their existence, when the light of distant stars passes in the deformation of space-time induced by their mass .
“Roman will revolutionize our search for black holes because it will allow us to find them even when there is nothing nearby,” says Kailash Sahu , an astronomer at the Space Telescope Science Institute in Baltimore. “The galaxy should be littered with these objects.”
Between saying and doing, you know, however, the vastness of the cosmos is involved: microlensing measurements require extreme precision and incredibly sophisticated tools , as the deviation of the apparent position of a star can be less than one thousandth of a second. degree of sky (try to take a degree on the protractor, divide it by 3600 and then again by 1000). As if this were not enough, we do not know if and when this deviation will take place, because we need the combination of conditions that bring the star to be right in the line of sight of the black hole. For this Roman will observe many different stars at the same time: he will look in a huge field of the sky monitoring every microscopic variation of the position of the stars.
“The stellar-mass black holes that we have so far discovered in binary systems have strange properties compared to our expectations,” continues Sahu. «They are all at least 10 times more massive than the Sun, but we think they should exist with masses between 3 and 80 solar masses. By taking a census of these objects, Roman will help us understand more about these objects resulting from the death of the stars ».
Featured image: The negative of the photographic plate used by Sir Arthur Eddington during the solar eclipse of 1919. It was the first experimental proof of general relativity © INAF
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