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Geodes...

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(Just_Super/iStock)

 

Topics: Black Holes, Cosmology, Dark Energy, Einstein, General Relativity, Gravity


A fifty-year-old hypothesis predicting the existence of bodies dubbed Generic Objects of Dark Energy (GEODEs) is getting a second look in light of a proposed correction to assumptions we use to model the way our Universe expands.

If this new version of a classic cosmological model is correct, some black holes could hide cores of pure dark energy, pushing our Universe apart at the seams.

University of Hawaii astrophysicist Kevin Croker and mathematician Joel Weiner teamed up to challenge the broadly accepted notion that when it comes to the Universe's growing waistline, its contents are largely irrelevant.

"For 80 years, we've generally operated under the assumption that the Universe, in broad strokes, was not affected by the particular details of any small region," said Croker.

"It is now clear that general relativity can observably connect collapsed stars – regions the size of Honolulu – to the behavior of the Universe as a whole, over a thousand billion billion times larger."

Not only could this alternative interpretation of fundamental physics change how we understand the Universe's expansion, but we might need to also consider how that growth might affect compact objects like the cores of collapsing stars.

 

Black Holes May Hide Cores of Pure Dark Energy That Keep The Universe Expanding
Mike McCrae, Science Alert

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Easy-Peasy...

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(Image: © Shutterstock)

 

Topics: Black Holes, Cosmology, General Relativity, Wormholes


Everybody wants a wormhole. I mean, who wants to bother traveling the long-and-slow routes throughout the universe, taking tens of thousands of years just to reach yet another boring star? Not when you can pop into the nearest wormhole opening, take a short stroll, and end up in some exotic far-flung corner of the universe.

There's a small technical difficulty, though: Wormholes, which are bends in space-time so extreme that a shortcut tunnel forms, are catastrophically unstable. As in, as soon as you send a single photon down the hole, it collapses faster than the speed of light.

But a recent paper, published to the preprint journal arXiv on July 29, has found a way to build an almost-steady wormhole, one that does collapse but slowly enough to send messages — and potentially even things — down it before it tears itself apart. All you need are a couple of black holes and a few infinitely long cosmic strings.

In principle, building a wormhole is pretty straightforward. According to Einstein's Theory of General Relativity, mass and energy warp the fabric of space-time. And a certain special configuration of matter and energy allows the formation of a tunnel, a shortcut between two otherwise distant portions of the universe.

Unfortunately, even on paper, those wormholes are fantastically unstable. Even a single photon passing through the wormhole triggers a catastrophic cascade that rips the wormhole apart. However, a healthy dose of negative mass — yes, that's matter but with an opposite weight — can counteract the destabilizing effects of regular matter trying to pass through the wormhole, making it traversable.

OK, matter with negative mass doesn't exist, so we need a new plan.

 

Physicists Just Released Step-by-Step Instructions for Building a Wormhole
Paul Sutter, Live Science

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The Gravity of the Matter...

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Testing Einstein: conceptual image showing S0-2 (the blue and green object) as it made its closest approach to the supermassive black hole at the center of the Milky Way. The huge gravitational field of the black hole is illustrated by the distorted grid in space–time. (Courtesy: Nicolle R Fuller/National Science Foundation)

 

Topics: Astrophysics, Black Holes, Cosmology, Einstein, General Relativity


A key aspect of Einstein’s general theory of relativity has passed its most rigorous test so far. An international team led by Tuan Do and Andrea Ghez at the University of California, Los Angeles confirmed the Einstein equivalence principle (EEP) by analyzing the redshift of light from the star S0-2 at its closest approach to Sagittarius A* – the supermassive black hole at the center of the Milky Way. The study combined over 20 years of existing spectroscopic and astrometric measurements of S0-2 with the team’s own observations.

Since Einstein first proposed his general theory of relativity in 1915, the idea has stood up to intense experimental scrutiny by explaining the behaviors of gravitational fields in the solar system, the dynamics of binary pulsars, and gravitational waves emitted by mergers of black holes.

In 2018, the GRAVITY collaboration carried out a particularly rigorous test – observing S0-2 at its closest approach to Sagittarius A* in its 16-year orbit.

As expected, the GRAVITY astronomers observed a characteristic relativistic redshift in light from S0-2. This redshift is a lengthening of the wavelength of the light and arises from both the motion of the star (the Doppler effect) and the EEP. The latter is a consequence of general relativity and predicts a redshift in light from a source that is in a gravitational field such as that of a supermassive black hole.

 

Einstein’s general theory of relativity tested by star orbiting a black hole
Sam Jarman, Physics World

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Wormhole Slow-Mo...

Credit: CC0 Public Domain

 

Topics: Black Holes, Einstein, General Relativity, Science Fiction, Wormholes


“Sometimes people don't want to hear the truth because they don't want their illusions destroyed.” Friedrich Nietzsche, Good Reads

A Harvard physicist has shown that wormholes can exist: tunnels in curved space-time, connecting two distant places, through which travel is possible.

But don't pack your bags for a trip to other side of the galaxy yet; although it's theoretically possible, it's not useful for humans to travel through, said the author of the study, Daniel Jafferis, from Harvard University, written in collaboration with Ping Gao, also from Harvard and Aron Wall from Stanford University.

"It takes longer to get through these wormholes than to go directly, so they are not very useful for space travel," Jafferis said. He will present his findings at the 2019 American Physical Society April Meeting in Denver.

Despite his pessimism for pan-galactic travel, he said that finding a way to construct a wormhole through which light could travel was a boost in the quest to develop a theory of quantum gravity.

 

Travel through wormholes is possible, but slow, American Institute of Physics, Phys.org

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Event Horizon...

Scientists have obtained the first-ever image of a black hole — at center of the galaxy M87. Credit: Event Horizon Telescope collaboration et al.

 

Topics: Astrophysics, Black Holes, Cosmology, Einstein


(Yesterday) At six simultaneous press conferences around the globe, astronomers on Wednesday announced they had accomplished the seemingly impossible: taking a picture of a black hole, a cosmic monster so voracious that light itself cannot escape its clutches.

This historic feat, performed by the Event Horizon Telescope (EHT)—a planet-spanning network of radio observatories—required more than a decade of effort. The project’s name refers to a black hole’s most defining characteristic, an “event horizon” set by the object’s mass and spin beyond which no infalling material, including light, can ever return.

“We have taken the first picture of a black hole,” the EHT project’s director, Sheperd Doeleman, said in a news release. “This is an extraordinary scientific feat accomplished by a team of more than 200 researchers.”

The image unveils the shadowy face of a 6.5-billion-solar-mass supermassive black hole at the core of Messier 87 (M87), a large galaxy some 55 million light-years from Earth in the Virgo galaxy cluster. Such objects are a reflection of Einstein’s theory of general relativity, which predicts that only so much material can be squeezed into any given volume before the overwhelming force of its accumulated gravity causes a collapse—a warp in the fabric of spacetime that swallows itself. Left behind is an almost featureless nothingness that, for lack of better terms, scientists simply call a black hole.

"Gargantua," special effects from the movie, Interstellar, 2014 (Kip Thorne et al guessed right):
Image Source: HDQ Walls dot com

 

At Last, a Black Hole’s Image Revealed, Lee Billings, Scientific American

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Sagittarius A...

Getty Images


Topics: Astronomy, Astrophysics, Black Holes, Cosmology, Einstein


They've captured our imaginations for decades, but we've never actually photographed a black hole before – until now.

Next Wednesday, at several press briefings around the world, scientists will apparently unveil humanity's first-ever photo of a black hole, the European Space Agency said in a statement. Specifically, the photo will be of "Sagittarius A," the supermassive black hole that's at the center of our Milky Way galaxy.

But aren't black holes, well, black, and thus invisible, so none of our telescopes can "see" them? Yes – therefore the image we're likely to see will be of the "event horizon," the edge of the black hole where light can't escape. [1]

*****


Next week, a collection of countries around the world are going to make a big announcement, and no one is sure exactly what it’s going to be. However, there are some possibilities, and the most exciting one is that they are about to reveal the first-ever photograph of the event horizon of a black hole.

Taking a photo of a black hole is not an easy task. Not only are black holes famous for not letting any light escape, even the nearest known black holes are very far away. The specific black hole astronomers wanted to photograph, Sagittarius A*, lies at the center of our galaxy 25,000 light-years away.

The international Event Horizon Telescope project announced its plan to photograph Sagittarius A* back in 2017, and they enlisted some of the world’s biggest telescopes to help out. The researchers used half a dozen radio telescopes, including the ALMA telescope in Chile and the James Clerk Maxwell telescope in Hawaii, to stare at Sagittarius A* over the past two years.

And while a picture of the black hole itself is impossible, the EHT astronomers were really aiming at the next best thing: the event horizon, the border of the black hole beyond which not even light can escape. At the event horizon, gravity is so strong that light will orbit the black hole like planets orbit stars, and our telescopes should be able to pick that up. [2]
 

1. 'Something no human has seen before': The first-ever photograph of a black hole will likely be unveiled next week, Doyle Rice, USA Today
2. We Might Be About to See the First Ever Photo of a Black Hole, Avery Thomson, Popular Mechanics

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