Space & Innovation

Did LIGO's Black Holes Come From the Dawn of Time?

The colliding black holes that generated the first detection of gravitational waves may have been primordial.

Where did those black holes come from?

This is the question currently being pondered by physicists studying the revolutionary first detection of gravitational waves in the Summer of 2015. And, in new research, the answer could be primordial.

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Generated by the cataclysmic collision of two black holes some 1.3 billion light-years away, the discovery, made by the Laser Interferometer Gravitational-wave Observatory (LIGO) and announced on Feb. 11, confirmed Albert Einstein's 100-year-old theory of general relativity and heralded a new era of gravitational wave astronomy.

With breathtaking precision, the gravitational wave signal -- called GW150914 -- revealed two black holes, one 29 solar masses and the other 36 solar masses, get trapped in an orbital dance that ultimately resulted in their collision and merging as one. The signal wasn't only the first direct detection of gravitational waves, it was the first time astronomers "saw" the guts of one of the most energetic collisions in our universe. And, best of all, the gravitational waves LIGO detected precisely matched our theoretical predictions about what black holes are and how they act before and after a merger.

On June 14, a second announcement came about another detection of gravitational waves, confirming the first detection wasn't a fluke.

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As momentous as this landmark discovery was, a puzzle quickly presented itself.

Generally, the most common black holes found in the modern universe come in two broad categories: stellar mass black holes and supermassive black holes. Stellar mass black holes are, you guessed it, formed after a massive star explodes as a supernova. Supermassive black holes, which "weigh in" at millions to billions of solar masses, are the monsters that live in the centers of galaxies. (There is currently a lot of work going into understanding how black holes evolve from stellar to supermassive and some studies have found "intermediate mass" black holes may be the "missing link", though we have some ways to go before working out if this is the case.)

In short, the two black holes that generated GW150914 were too big and can't be explained as being two stellar mass black holes colliding and merging as one. So what kind of black holes were they?

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Dark Matter Clues?

In two new papers published in the journal Physical Review Letters, researchers take a stab at identifying the origin of these precursor black holes, leading to the conclusion that they may have been formed when the universe was very young. In other words, LIGO could have been witness to the smashup of two primordial black holes -- the most ancient kind of black holes. What's more, future detections of similar collisions could provide tantalizing clues as to whether primordial black holes make up the majority (if not all) of the universe's missing mass. In other words, could these massive objects be the dark matter astrophysicists are looking for?

Some universal evolution models predict the rapid formation of black holes shortly after the Big Bang. The early universe was full of dense knots of matter that likely collapsed under mutual gravity forming a huge number of massive black holes. Though black holes are thought to evaporate over time (via Hawking radiation), many of these primordial black holes are thought to persist, bulking up the universe's missing mass, creating the strange observations of galaxies and galactic cluster that appear to be more massive than the visible matter they contain.

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There's much debate over the range of masses these primordial black holes could form and how often LIGO (and other gravitational wave detectors in the future) will detect them, but it is clear that this new exciting era of gravitational wave astronomy is already hot on the trail of one of the biggest mysteries of modern astrophysics and cosmology.

Could primordial black holes be the answer? Only time will tell; we need to detect a lot more of these strangely-sized black hole collisions before we'll know where dark matter is hiding.

via Science News

GALLERY: 100 Years of General Relativity:

Exactly 100 years ago on Nov. 25, 2015, physicist Albert Einstein, then 36, presented a fourth and final lecture to the Prussian Academy of Sciences about his new general theory of relativity. The idea not only redefined the concept of gravity, but also ended up reshaping humanity’s perspective on reality. Here’s a look at the theory in thought and action.

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Einstein was famous for his thought experiments, which often played out for years in his imagination. From the gedankenexperiment, as it is known in German, Einstein grasped fundamental concepts about the physical world that could be verified by observation and experiments. One of his most famous ones began in 1907 when Einstein pondered if a person inside a windowless elevator could tell if he was in a gravitational free-fall, or if the elevator was being hauled up by a constant acceleration. Einstein decided the laws of physics must be the same in both cases. The mathematical equation he derived to explain this so-called principle of equivalence, which equated the effects of gravitation with acceleration in zero-gravity, became the basis for general relativity.

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A total solar eclipse on May 29, 1919, gave astronomers an opportunity to verify Einstein’s general theory of relativity by proving that the sun’s gravitational field was bending the light of background stars. The effect was only observable during time when the sun’s light was dim enough for stars to become visible. British astronomer Arthur Eddington led an expedition to the island of Principe, off the West Coast of Africa, to photograph the eclipse, which lasted nearly seven minutes. The images of stars in the region around the sun proved that Einstein’s interpretation of gravity trumped the 200-year old Newtonian model, which interpreted gravity as a force between two bodies. Einstein saw gravity as warps and curves in space and time.

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In 1917, Einstein amended his general relativity theory to introduce what he called the “cosmological constant,” a mathematical way to counter the force of gravity on a cosmological scale and stave off the collapse of the universe, which the general relativity theory posited. At the time, astronomers believed that the Milky Way was surrounded by an infinite and static void. In 1923, Edwin Hubble and other astronomers find the first stars beyond the galaxy and by 1929 Hubble provides evidence that space is expanding. Einstein realized the cosmological constant was a blunder. Or perhaps not. In 1998, scientists made the startling discovery that the expansion of the universe is speeding up, driven by an anti-gravity force called dark energy, which in many ways acts like Einstein’s cosmological constant. Pictured here is the Hubble Space Telescope’s extreme deep field view, which contains about 5,500 galaxies. The telescope is named after Edwin Hubble.

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One of the first implications of the general relativity theory was the realization that if an object is compressed enough, the dimple it generates in the fabric of space and time will be too strong for even photons of light to escape. Thus, the idea of black holes was born. Though they can’t be directly observed, astronomers have found black holes of all sizes by measuring how they affect nearby stars and gas. Pictured here is an artist’s rendering of a black hole named Cygnus X-1, siphoning matter from a nearby star.

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Like ripples in a pond, scientists believe that gravity transmits in waves, deforming space and time across the universe. It is similar to the movement of electromagnetic radiation, which propagates in waves, except that gravitational waves are moving the fabric of space and time itself. So far, attempts to find gravitational waves, such as those caused by two black holes colliding for example, have been unsuccessful. Next week, the European Space Agency plans to launch a prototype space-based observatory called the evolved Laser Interferometer Space Antenna (eLISA) to test a technology to find gravitational waves. Pictured above is an artist's rendering of two merging galaxies rippling space and time.

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