пятница, 28 мая 2021 г.
А куда еще? Ученые выдвинули теорию о том, что черные дыры ведут в другие Вселенные
Мы не можем физически исследовать черные дыры, из-за чего никто не может утверждать наверняка, что же происходит внутри них. Новая теория предлагает версию, о которой наверняка задумывались многие люди — внутри них существует переход в другие Вселенные. Мы очень мало знаем об устройстве нашего мира. Несмотря на огромное количество исследований, телескопов, моделирований, мы все равно далеко от понимания того, как живет наша Вселенная. Уникальна ли она? Как она появилась? А может до вероятного Большого взрыва и вовсе существовала другая Вселенная? Гипотез очень много, вопросов еще больше, а ответов пока что мало. В свое время Эйнштейн приложил множество усилий для изучения черных дыр. Это одни из самых мощных о в то же время загадочных объектов в космосе. Они буквально искривляют свет, пространство и время. Если мы выясним, что же происходит внутри черных дыр, то мы определенно станем ближе к понимаю многих процессов во Вселенной. Однако здесь есть одна проблема, с которой мы пока что не знаем, как справиться. Все, что приближается к черной дыре, попадает в ее мощное гравитационное поле, из которого выбраться уже невозможно. Что происходит с телом внутри черной дыры мы не знаем, но в нашем мире оно уже точно не появится. Из-за этого физически исследовать эти объекты невозможно, ведь выхода из них не существует — то есть даже теоретически попав внутрь черной дыры, все полученные знания останутся там же.
четверг, 20 мая 2021 г.
Спагеттификацию звезды черной дырой, возможно, впервые запечатлели напрямую
Астрономы намерены предоставить доказательства хорошо известного в теории процесса. Согласно астрофизическим моделям, срок жизни звезды, в зависимости от начальной массы, составляет от нескольких миллионов до десятков триллионов лет. В конечной стадии эволюции, в зависимости от массы, звезда либо сбрасывает внешнюю оболочку, становясь белым карликом, либо становится сверхновой, после чего от нее остается нейтронная звезда или черная дыра. Но есть и иной, насильственный и жестокий способ гибели звезды. Когда светило слишком близко приближается к черной дыре, приливная сила ее гравитационного поля так сильно тянет звезду, что в конечном итоге она разрывается на части. Это явление известно, как событие приливного разрушения (TDE), или, как удачно назвали его астрономы, спагеттификация. Мы можем обнаружить этот процесс, потому что он сопровождается яркой вспышкой света, вызванной гравитационным и фрикционным воздействиями в аккреционном диске вокруг черной дыры. Этот свет постепенно гаснет по мере истощения материала. Именно такое событие было обнаружено в апреле 2019 года камерой Zwicky Transient Facility, которая изучает небо в поисках переходных событий. Рассматриваемое событие, позже названное AT2019dsg, было необычно ярким даже для TDE, вспыхивая в рентгеновских, ультрафиолетовых, оптических и радиоволнах.
Внутренний край аккреционного диска, ближайший к черной дыре, является самой горячей частью диска и поэтому производит наиболее энергичное излучение — рентгеновские лучи. То, что мы вообще можем обнаружить рентгеновские лучи, означает, что мы смотрим на полюс сверхмассивной черной дыры — в противном случае он был бы закрыт внешними областями аккреционного диска.
Ученые и раньше наблюдали рентгеновское излучение в TDE, однако в данном случае оно имеет свои особенности. Международная группа астрономов во главе с Джакомо Канниццаро и Питером Йонкером из Нидерландского института космических исследований изучила электромагнитный выход и обнаружила линии поглощения.
Когда астрономы делают изображения света, если этот свет прошел через что-то, что замедляет или блокирует некоторые длины волн — например, газ или пыль, — это будет отображаться в виде темных линий в спектре. Они появляются в самых разных местах, но полюса сверхмассивных черных дыр среди них обычно нет.
Вариация и ширина этих линий поглощения тоже были странными. Наблюдаемая конфигурация, казалось, предполагала наличие нескольких нитей материала, похожих на клубок веревки. Это, в свою очередь, предполагало, что нечто было обернуто вокруг черной дыры под необычным углом. Ученые сделали вывод, что это спагеттифицированные волокна разрушенной приливом звезды, возможно, отброшенные от основной массы звездного мусора.
«Когда звезда разрушается, обломки могут образовывать самогравитирующие потоки. То, что мы можем видеть, – это линии поглощения, вызванные такими потоками, где различные орбитальные движения и прогнозируемые скорости этих разных потоков вызывают изменение ширины линий. Чтобы получить подобное, нам нужно, чтобы некоторые из самогравитирующих потоков отклонялись на большие углы, в то время как основная часть разрушенного материала циркулирует в аккреционный диск», – пишут исследователи.
Если эта интерпретация окажется верной, то ученые в первый раз получат прямые доказательства существования процесса спагеттификации.
Когда астрономы делают изображения света, если этот свет прошел через что-то, что замедляет или блокирует некоторые длины волн — например, газ или пыль, — это будет отображаться в виде темных линий в спектре. Они появляются в самых разных местах, но полюса сверхмассивных черных дыр среди них обычно нет.
Вариация и ширина этих линий поглощения тоже были странными. Наблюдаемая конфигурация, казалось, предполагала наличие нескольких нитей материала, похожих на клубок веревки. Это, в свою очередь, предполагало, что нечто было обернуто вокруг черной дыры под необычным углом. Ученые сделали вывод, что это спагеттифицированные волокна разрушенной приливом звезды, возможно, отброшенные от основной массы звездного мусора.
«Когда звезда разрушается, обломки могут образовывать самогравитирующие потоки. То, что мы можем видеть, – это линии поглощения, вызванные такими потоками, где различные орбитальные движения и прогнозируемые скорости этих разных потоков вызывают изменение ширины линий. Чтобы получить подобное, нам нужно, чтобы некоторые из самогравитирующих потоков отклонялись на большие углы, в то время как основная часть разрушенного материала циркулирует в аккреционный диск», – пишут исследователи.
Если эта интерпретация окажется верной, то ученые в первый раз получат прямые доказательства существования процесса спагеттификации.
суббота, 8 мая 2021 г.
Watch a black hole tear a star to bits in epic new animation
The awesome spectacle of a black hole ripping a star to shreds can be seen in this striking new visualization from the Deutsches Elektronen-Synchrotron (DESY), a particle accelerator lab in Hamburg, Germany. Such events are known as stellar tidal disruptors, and they are fairly rare, occurring just once every 10,000 years in a typical galaxy, according to NASA. Stars are typically flung toward a ravenous black hole after interacting gravitationally with another star or massive object, only to become stretched and devoured should they come too close to the black hole's maw in a process called spaghettification. Gravitational tidal forces, similar to the ones that cause the moon to raise tides on Earth, are responsible for most of the destruction. At first, the star's outer atmospheric layers will get pulled toward the black hole, spinning around its edge like water going down a drain and forming what's known as an accretion disk, as the video depicts. Surprisingly, the black hole only consumes about 1% of a star's mass, according to NASA. The majority will actually get catapulted back out into space in the form of enormous jets of energy and matter that shoot from the black hole's central region. These jets can sometimes light up the cosmos, allowing astronomers on Earth to catch glimpses of distant black holes, which are otherwise mostly invisible. Tiny, ghostly particles called neutrinos will also be flung from the black hole, occasionally giving researchers insights into processes occurring during the consumption event.
Some of the star's material does fall past the event horizon, the point after which nothing, including light, can escape. The visualization shows some of the strange optical effects that the event horizon produces, such as bending light so much that regions at the back of the accretion disk can be seen from its front.
Witnessing how swiftly the black hole dismembers and dispatches the star is an excellent reminder that no one should want to get anywhere near such a powerful object any time soon.
Witnessing how swiftly the black hole dismembers and dispatches the star is an excellent reminder that no one should want to get anywhere near such a powerful object any time soon.
понедельник, 3 мая 2021 г.
Stars That Race through Space at Nearly the Speed of Light
Most people probably know that the universe is full of elementary particles—but not everyone knows that it is also full of extremely fast stars moving freely through space like barracuda through the ocean. These stars are ejected by gravitational slingshots located at the focal point of galaxy mergers—where a pair of supermassive black holes coalesce while kicking stars out of the host galaxy, like a batter hitting a series of home runs out of the park. As the black hole pair tightens, its orbital speed rises, providing an even more powerful swing. Eventually this process launches some stars up to the speed of light in accordance with Albert Einstein’s special theory of relativity, making them what astrophysicists call “relativistic.” In 2014 I and my former postdoc James Guillochon calculated the abundance of free-fall relativistic stars in the vast space between galaxies and the difficulties involved in detecting them at the large distances. It must be thrilling to live on a planet orbiting one of these ejected stars and to witness its trip through space. The journey starts at the center of the parent galaxy, continues through many points of interest, out to the edge of the galaxy’s halo within one million years, and culminates in intergalactic space, passing by cosmological destinations over billions of years—all of which we can barely see through telescopes. These relativistic stars represent the most attractive travel packages that intergalactic tourist agencies can offer, and they also provide health benefits. Traveling close to the speed of light entitles you to the perk of time dilation—slowing down the natural aging process of all travelers relative to those they left behind.
Even in the absence of a galaxy merger, stars tugged by the strong gravity near a black hole at the center of a galaxy could also reach the speed of light. Half of the 2020 Nobel Prize in Physics was jointly awarded to Reinhard Genzel and Andrea Ghez for their program, which monitors stars moving at a few percent of the speed of light near the Milky Way’s supermassive black hole, Sagittarius A*. Relativistic stars are expected to be gravitationally bound to black holes at the centers of many other galaxies.
If relativistic stars in a galactic nucleus run into each other, the resulting head-on collision can produce a blast much more energetic than a typical supernova—an explosion from the collapse of a massive star after its nuclear fuel is exhausted. In order for the two-star collision to occur at nearly the speed of light, the central black hole must weigh more than 100 million suns. At lower masses, as is the case with black holes like Sagittarius A*, which weighs “only” four million suns, the strong tidal force of the black hole spaghettifies stars when they come close to it. The disrupted stars are then spread into a stream of gas long before they can get close enough to the black hole’s horizon to reach the speed of light, as shown in the Ph.D. thesis of my former student, Nick Stone.
At higher masses and at its event horizon, the gravitational tide—which scales inversely with the square of the black hole mass—is sufficiently weak so as not to disrupt a passing star. Stars that orbit at large distances from either type of black hole move at lower speeds, and their collisions result in weak explosions, as I showed in a preprint paper with my former graduate student Doug Rubin and in a follow-up preprint paper with Shmuel Balberg and Re’em Sari, both at the Hebrew University of Jerusalem.
What happens close to the most massive black holes, where stars can orbit at nearly the speed of light without being tidally disrupted? In a new paper,my current graduate student Betty Hu and I show that collisions of stars near these large black holes trigger the most energetic explosions in the universe, releasing up to thousands of times more energy than normal supernova explosions. These superluminous explosions in galactic nuclei would be detectable at the edge of the universe by the Legacy Survey of Space and Time (LSST) camera at the Vera C. Rubin Observatory, which is scheduled to start its operation within a couple of years.
There is yet another way to launch stars from galactic centers at high speeds. A pair of bound stars passing close to a supermassive black hole can be separated by its gravitational tide. One of the stars is kicked closer to the black hole while the other is ejected at a high speed, as predicted theoretically by Jack Hills in 1988. The kick that one star gets toward the black hole could account for the closest stars to Sagittarius A*, which was discovered by Genzel and Ghez. The ejection of their companions is the likely origin of the hypervelocity stars discovered in 2005 by Warren Brown and his collaborators in the halo of the Milky Way. These hypervelocity stars move at up to 2 percent of the speed of light and potentially carry planets with them. Planets that are freed by the ejection process constitute a population of hypervelocity planets, as theorized in a 2012 paper I wrote with my former student Idan Ginsburg.
All in all, galactic nuclei offer launch sites for the fastest habitable platforms that nature offers for free. It would not be surprising if advanced technological civilizations choose to migrate toward galactic centers for the same reason that astronauts and spectators flock to Florida’s Cape Canaveral during rocket launches. With that perspective in mind, searches for extraterrestrial intelligence should check for radio signals coming from riders of hypervelocity stars. We might also notice celebratory fireworks from their relatives at the galactic center whenever a high-speed star is shot out of there.
If relativistic stars in a galactic nucleus run into each other, the resulting head-on collision can produce a blast much more energetic than a typical supernova—an explosion from the collapse of a massive star after its nuclear fuel is exhausted. In order for the two-star collision to occur at nearly the speed of light, the central black hole must weigh more than 100 million suns. At lower masses, as is the case with black holes like Sagittarius A*, which weighs “only” four million suns, the strong tidal force of the black hole spaghettifies stars when they come close to it. The disrupted stars are then spread into a stream of gas long before they can get close enough to the black hole’s horizon to reach the speed of light, as shown in the Ph.D. thesis of my former student, Nick Stone.
At higher masses and at its event horizon, the gravitational tide—which scales inversely with the square of the black hole mass—is sufficiently weak so as not to disrupt a passing star. Stars that orbit at large distances from either type of black hole move at lower speeds, and their collisions result in weak explosions, as I showed in a preprint paper with my former graduate student Doug Rubin and in a follow-up preprint paper with Shmuel Balberg and Re’em Sari, both at the Hebrew University of Jerusalem.
What happens close to the most massive black holes, where stars can orbit at nearly the speed of light without being tidally disrupted? In a new paper,my current graduate student Betty Hu and I show that collisions of stars near these large black holes trigger the most energetic explosions in the universe, releasing up to thousands of times more energy than normal supernova explosions. These superluminous explosions in galactic nuclei would be detectable at the edge of the universe by the Legacy Survey of Space and Time (LSST) camera at the Vera C. Rubin Observatory, which is scheduled to start its operation within a couple of years.
There is yet another way to launch stars from galactic centers at high speeds. A pair of bound stars passing close to a supermassive black hole can be separated by its gravitational tide. One of the stars is kicked closer to the black hole while the other is ejected at a high speed, as predicted theoretically by Jack Hills in 1988. The kick that one star gets toward the black hole could account for the closest stars to Sagittarius A*, which was discovered by Genzel and Ghez. The ejection of their companions is the likely origin of the hypervelocity stars discovered in 2005 by Warren Brown and his collaborators in the halo of the Milky Way. These hypervelocity stars move at up to 2 percent of the speed of light and potentially carry planets with them. Planets that are freed by the ejection process constitute a population of hypervelocity planets, as theorized in a 2012 paper I wrote with my former student Idan Ginsburg.
пятница, 30 апреля 2021 г.
See 2 black holes in cosmic dance
NASA has released a cool new visualization showing two massive black holes orbiting each other in a stunning display of light. The intense gravity and other phenomena all play a part in producing this mesmerizing display. Black holes are one of the most fascinating and mind-bending phenomena in the universe. The gravity of a black hole is so powerful that not even light can escape it. There are many varieties of black holes, and, in April 2021, NASA released a new visualization of binary (double) black holes, two black holes orbiting a common center of gravity, circling each other in a cosmic dance. The black holes depicted in this visualization are millions of times more massive than our sun. Each is surrounded by a disk of hot gas called an accretion disk. In the visualization, the gravity of the black holes distorts the light coming from their respective disks, producing a mesmerizing light show. Various other relativistic phenomena also contribute to the display.
Visualization of two massive black holes orbiting each other. The one in the foreground (with a red accretion disk) has a mass two hundred million times that of the sun, and its gravity bends the light coming from the accretion disk surrounding the smaller black hole (blue) behind it. The colors correspond to temperature, where the blue accretion disk is hotter and the red one is cooler. Image via NASA/ Goddard Space Flight Center/ Jeremy Schnittman/ Brian P. Powell.
Face-on overhead view of the two black holes and their accretion disks. A small and distorted edge-on view of the larger black hole appears near the smaller black hole, and a similar small image of the smaller black hole appears near the inner ring of light of the larger black hole. In effect, we are seeing both black holes from the side and from above at the same time. Image via NASA/ Goddard Space Flight Center/ Jeremy Schnittman/ Brian P. Powell.
How does this happen?
When you view the disks almost edge-on, from the orbital plane, they have a double-bulged look to them, reminiscent of seeing Saturn from the edge, slightly above or below the ring plane; a flat circular disk with a huge round bulge on top and bottom.
When one of the black holes passes in front of the other, however, as seen from our vantage point, the intense gravity of the black hole closer to us distorts the light coming from the black hole behind it. This creates a rapidly changing series of colorful arcs as they move. The light from both black holes is being modified by the distorted fabric of space-time near the black holes.
In the visualization, the light from the disks is shown as brilliant blue or red. This is partly to make them easier to distinguish, but also depicts the different temperatures of each disk. Hotter gas is shown as blue and cooler gas as red. Most of the light in both disks is emitted in the ultraviolet (UV) part of the electromagnetic spectrum instead of in visible light.
Stronger gravitational effects also produce higher temperatures so the disks look brighter on one side (the side closest to their respective holes). This is due to gravity distorting the light coming from different parts of the disks. The Doppler boosting effect – also known as relativistic beaming – also plays a part in these changes in brightness: the luminosity of the disk is affected by the gas moving faster near the black hole so that the side that is rotating toward the viewer appears brighter, while the side rotating away looks dimmer.
Another phenomenon that contributes to this visualization is relativistic aberration, where the black holes look smaller when they are moving closer to the viewer, but larger when moving away. That sounds counter-intuitive, but nature can be weird.
What happens if you look at the black holes from above, instead of from the side?
Those odd visual effects go away, but bizarre new ones take their place. Each black hole produces a small visual “copy” of the other black hole that orbits around it, sort of like watching a planet orbit a star from directly above. But those small copied images show the partner black hole in an edge-on view (the way we saw them before, from the side) instead of an above view. How does that happen? The light from the black hole disks is being bent by gravity at 90 degrees. This means that we can see both the primary black hole disk face-on from above, and the smaller image of the companion black hole disk edge-on, at the same time. How bizarre is that? Schnittman said:
A striking aspect of this new visualization is the self-similar nature of the images produced by gravitational lensing. Zooming into each black hole reveals multiple, increasingly distorted images of its partner.
The researchers used the Discover supercomputer at the NASA Center for Climate Simulation to create the visualization. It would have taken about a decade to make using just a regular desktop computer, but with Discover, while using only 2 percent of its 129,000 core processors, it took only a day.
The result is the beautiful movie of two black holes in their cosmic dance that you see here.
How does this happen?
When you view the disks almost edge-on, from the orbital plane, they have a double-bulged look to them, reminiscent of seeing Saturn from the edge, slightly above or below the ring plane; a flat circular disk with a huge round bulge on top and bottom.
In the visualization, the light from the disks is shown as brilliant blue or red. This is partly to make them easier to distinguish, but also depicts the different temperatures of each disk. Hotter gas is shown as blue and cooler gas as red. Most of the light in both disks is emitted in the ultraviolet (UV) part of the electromagnetic spectrum instead of in visible light.
Stronger gravitational effects also produce higher temperatures so the disks look brighter on one side (the side closest to their respective holes). This is due to gravity distorting the light coming from different parts of the disks. The Doppler boosting effect – also known as relativistic beaming – also plays a part in these changes in brightness: the luminosity of the disk is affected by the gas moving faster near the black hole so that the side that is rotating toward the viewer appears brighter, while the side rotating away looks dimmer.
What happens if you look at the black holes from above, instead of from the side?
Those odd visual effects go away, but bizarre new ones take their place. Each black hole produces a small visual “copy” of the other black hole that orbits around it, sort of like watching a planet orbit a star from directly above. But those small copied images show the partner black hole in an edge-on view (the way we saw them before, from the side) instead of an above view. How does that happen? The light from the black hole disks is being bent by gravity at 90 degrees. This means that we can see both the primary black hole disk face-on from above, and the smaller image of the companion black hole disk edge-on, at the same time. How bizarre is that? Schnittman said:
A striking aspect of this new visualization is the self-similar nature of the images produced by gravitational lensing. Zooming into each black hole reveals multiple, increasingly distorted images of its partner.
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