10 We are so small
9 rogue planets
8 space debris
7 the sounds of space
6 the super void
3 the dark flow
2 Supermassive black holes
1 further speculations and philosophies
THE DATA CAN BEST BE EXPLAINED AS X-RAYS FROM SOURCES OUTSIDE THE SOLAR SYSTEM
X-rays are a form of high-energy, electromagnetic radiation released by extremely hot objects. In the early 20th century, astronomers realized that space should be flooded with X-rays from the sun. Moreover, the sun’s X-ray spectrum would reveal a lot about the processes at work within the star. However, X-ray astronomy was not possible until the advent of rockets and satellites. Despite their energy, X-rays are easily absorbed, which is why they are so good at imaging the body. Water vapor in Earth’s atmosphere effectively blocks X-rays from reaching the surface—a good thing for life,
because high-energy X-rays can cause damage and mutations when they impact on soft, living cells.
The first glimpse of the sun’s X-rays came in the late 1940s, during a US Naval Research Laboratory (NRL) program to study Earth’s upper atmosphere. A team led by US rocket scientist Herbert Friedman fired German V-2 rockets into space equipped with X-ray detectors—essentially modified Geiger counters. These experiments provided the first incontrovertible evidence of X-rays from the sun. By 1960, researchers were using Aerobee sounding rockets to detect X-rays, and the first X-ray photos of the sun were taken from an Aerobee Hi. Two years later, the first cosmic X-ray source was detected.
“Nothing is going to happen unless you work with your life’s blood.” Riccardo Giacconi
Riccardo Giacconi, an Italian astrophysicist then working for American Science and Engineering (AS&E), had successfully petitioned NASA to fund his team’s X-ray experiment. The team’s first rocket misfired in 1960, but by 1961 it had a new, improved experiment ready for launch. This instrument was one hundred times more sensitive than any flown to date. Using a large field of view, the team hoped to observe other X-ray sources in the sky. Success followed a year later: the rocket aimed its camera first at the moon and then away from it. What the camera saw came as a complete surprise to the team. The instrument detected the X-ray “background”—a diffuse signal coming from all directions—and a strong peak of radiation in the direction of the galactic center.
Stars like the sun emit about a million times more photons at visible light frequencies than they do as X-rays. The source of the X-ray signals, by contrast, radiated a thousand times more X-rays than light. Although a small, barely visible point in the sky, the source was pumping out one thousand times more X-rays than the sun. Furthermore, certain physical processes were taking place within the source and these had never been seen in the laboratory. After weeks of analysis, the team concluded that this must be a new class of stellar object.
Search for the source
There was no candidate in the solar system to account for the intense radiation. The most likely source was named Scorpius X-1 (Sco X-1 for short) after the constellation within which it was located. Herb Friedman at the NRL confirmed the result using a detector with a larger area and better resolution than the AS&E instrument. Sco X-1 is now known to be a double star system and is the brightest, most persistent X-ray source in the skies.
Further launches revealed a sky dotted with X-ray sources, both galactic and extra-galactic. In a short space of time, the team had detected a disparate set of celestial oddities emitting X-rays. These included supernova remnants, binary stars, and black holes. Today, more than 100,000 X-ray sources are known.
By the mid-1960s, instruments were becoming ever more sensitive. Detectors were able to record X-rays one thousand times weaker than Sco X-1 just five years after Giacconi’s discovery. Initially proposed by Giacconi in 1963, Uhuru, the first satellite dedicated solely to X-ray astronomy, was launched in 1970. It spent three years mapping X-rays. This all-sky survey located 300 sources, including a bizarre object in the center of the Andromeda galaxy, and it earmarked Cyg X-1 as a potential black hole. Uhuru also found that the gaps in galaxy clusters are strong sources of X-rays. These apparently empty regions are in fact filled by a low-density gas at millions of degrees Kelvin. Although thinly spread, this “intercluster medium” contains more mass than that of all of the cluster’s galaxies combined.
In 1977, NASA launched its High Energy Astronomy Observatory (HEAO) program. HEAO-2, renamed the Einstein Observatory, was equipped with highly sensitive detectors and revolutionized X-ray astronomy. With its fused quartz mirrors, the telescope was a million times more sensitive than that of Giacconi’s 1961 discovery rocket. Einstein observed X-rays emanating from stars and galaxies, and even from planetary aurorae on Jupiter.
Eager to probe the X-ray background further, Giacconi once again proposed an advanced telescope. In 1999, this became the Chandra X-Ray Observatory, the third of the orbiting Great Observatories. Chandra is the most powerful X-ray telescope ever built, tens of billions of times more sensitive than the early detectors. Its phenomenal performance outstripped all expectations and its mission lifetime was tripled from five to 15 years. As of 2016, however, its mission is ongoing. Chandra’s outstanding technical firsts include detecting sound waves coming from a supermassive black hole. The X-ray data, when combined with optical observations from the Hubble Space Telescope and infrared data from the Spitzer Space Telescope, have provided stunning images of the cosmos.
Realm of the X-rays
X-ray astronomy observes the highest-energy objects in space: colliding galaxies, black holes, neutron stars, and supernovae. The energy source behind this activity is gravity. As matter falls toward a massive concentration of material, particles collide and accumulate. They give up their energy by emitting photons, which at these speeds have X-ray wavelengths (0.01–10 nanometers, or billionths of a meter)—equivalent to temperatures of tens of million of degrees. The same mechanism, is at work in a wide range of dramatic phenomena: active stars more massive than the sun, for example, produce strong solar winds and significant amounts of X-rays. “X-ray binary star” systems, in which mass transfers from one star to its partner, also produce intense radiation.
“The universe is popping all over the place.” Riccardo Giacconi
Seeing black holes
When stars explode at the end of their lives, the blast waves from the supernova compress the interstellar medium, causing the gas to release Xrays. Left within what remains of the supernova, the massive star continues life as a neutron star or a black hole. Turbulence generated by material being torn apart as it is sucked into a black hole will also produce X-rays. The radiation being pumped out causes the outer layers of the supernova remnant to fluoresce in a range of colors.
Certain galaxies have centers that outshine all the billions of stars in the galaxy itself, with emissions that are bright at all wavelengths. The center of such an “active galactic nucleus” is assumed to contain a supermassive black hole. Material falling toward the centers of galaxy clusters—the largest structures in the universe—also shines in X-rays, and is not visible in other light frequencies. Chandra has now taken two “deep field” images of the Xray background—23- and 11-day exposures of the northern and southern hemispheres of the sky. X-ray instruments of the future may help scientists see how black holes are distributed.
Born in Genoa, Italy, in 1931, Riccardo Giacconi lived in Milan with his mother, a mathematics and physics high school teacher. She instilled a love of geometry in the young Riccardo. Giacconi’s first degree was from the University of Milan. With a Fulbright Scholarship, he moved to Indiana University in the US, and then to Princeton, to study astrophysics.
In 1959, Giacconi joined American Science and Engineering, a small firm in Cambridge, Massachusetts. AS&E built rocket-borne monitoring equipment for measuring electrons and artificial gamma-ray bursts from nuclear weapons. Giacconi was tasked with developing instruments for X-ray astronomy. He was at the heart of most of the breakthroughs in X-ray astronomy, and in 2002, he was awarded a share of the Nobel Prize in Physics for his contributions to astrophysics. In 2016, he was still working in his mid-80s, as principal investigator for the Chandra Deep Field-South project.
A DAY WITHOUT YESTERDAY
The idea that the universe originated from a tiny object in the form of an egg appears in The Rigveda, a collection of Hindu hymns from the 12th century BCE. However, there were few scientific clues to the universe’s true origins until Albert Einstein provided a new way of conceiving time and space with his general theory of relativity in 1915. Einstein’s insight led many to revisit the idea that the universe started small, among them the Belgian priest Georges Lemaître, whose 1931 proposal would carry echoes of The Rigveda.
In the 17th century, Johannes Kepler, observing that the night sky is dark, argued that the universe cannot be infinite in both time and space, as otherwise the stars shining from every direction would make the whole sky bright. His argument was restated in 1823 by German astronomer Wilhelm Olbers and became known as Olbers’ paradox. Despite this problem, Isaac Newton stated that the universe was static (not getting any bigger or smaller) and infinite in time and space, with its matter distributed more or less uniformly over a large scale. At the end of the 19th century, this was still the prevailing view, and one that Einstein himself initially held.
An unchanging universe?
Einstein’s general theory of relativity explains how gravity works at the largest scales. He realized that it could be used to test whether the Newtonian model of the universe could exist long-term without becoming unstable, and to explore which other types of universe might be feasible. The exact relationship between mass, space, and time was explained in a series of 10 complex equations. These were called Einstein’s field equations. Einstein found an initial solution to his equations that suggested the universe is contracting. Since he could not believe this, he introduced a “fix”—an expansion-inducing factor called the cosmological constant—to balance the inward pull of gravity. This allowed for a static universe.
In 1922, Russian mathematician Alexander Friedmann attempted to find solutions to Einstein’s field equations. Starting with the assumption that the universe is homogenous (made of more or less the same material everywhere) and spread out evenly in every direction, he found several solutions. These allowed for models in which the universe could be expanding, contracting, or static. Friedmann was probably the first person to use the expression “expanding universe.” Einstein first called his work “suspicious,” but six months later acknowledged that his results were correct. However, this was Friedmann’s final contribution as he died two years later. In 1924, Edwin Hubble showed that many nebulae were galaxies outside the Milky Way. The universe had suddenly become a lot bigger.
The expanding universe
Later in the 1920s, Lemaître entered the debate about the large-scale organization of the universe. He had worked at institutions in the United States, becoming aware of Vesto Slipher’s work on receding galaxies and Hubble’s measurements of galaxy distances. A competent mathematician, he had also studied Einstein’s field equations and found a possible solution to the equations that allowed for an expanding universe. Putting these various threads together, in 1927, Lemaître published a paper that proposed that the whole universe is expanding and carrying galaxies away from each other and from Earth. He also predicted that galaxies that are more distant from us would be found to be receding at a faster rate than closer ones.
Lemaître’s paper was published in an obscure Belgian journal, and as a result, his hypothesis failed to attract much attention at the time. He did, however, communicate his findings to Einstein, telling him of the solution he had found to the field equations allowing for a universe that expands. Einstein introduced Lemaître to Friedmann’s work, but remained ambivalent about Lemaître’s idea. Famously, Einstein is said to have said: “Your calculations are correct, but your grasp of physics is abominable.” However, the British astronomer Arthur Eddington later published a long commentary on Lemaître’s 1927 paper, describing it as a “brilliant solution.”
In 1929, Hubble released findings showing that there was indeed a relationship between the remoteness of a galaxy and how fast it was receding, confirming for many astronomers that the universe was expanding, and that Lemaître’s paper had been correct. For many years the credit for the discovery of the expansion of the universe was given to Hubble, but today most agree it should be shared with Lemaître and possibly also with Alexander Friedmann.
The primeval atom
Lemaître reasoned that, if the universe is expanding and the clock is run backward, then far back in time, all the matter in the universe must have been much closer. In 1931, he suggested that the universe was initially a single, extremely dense particle containing all its matter and energy—a “primeval atom” as he called it, about 30 times the size of the sun. This disintegrated in an explosion, giving rise to space and time on “a day without yesterday.” Lemaître described the beginning of the universe as a burst of fireworks, comparing galaxies to the burning embers spreading out from the center of the blast.
The proposal initially met with scepticism. Einstein found it suspect but was not altogether dismissive. In January 1933, however, Lemaître and Einstein traveled together to California for a series of seminars. By this time, Einstein (who had removed the cosmological constant from his general theory of relativity because it was no longer needed) was in full agreement with Lemaître’s theory, calling it “the most beautiful and satisfactory explanation of creation to which I have ever listened.”
Lemaître’s model also provided a solution to the long-standing problem of Olbers’ paradox. In his model, the universe has a finite age, and because the speed of light is also finite, that means that only a finite number of stars can be observed within the given volume of space visible from Earth. The density of stars within this volume is low enough that any line of sight from Earth is unlikely to reach a star.
“The radius of space began at zero, and the first stages of the expansion consisted of a rapid expansion determined by the mass of the initial atom.” Georges Lemaître
Refining the idea
Compressed into a tiny point, the universe would be extremely hot. During the 1940s, Russian-American physicist George Gamow and colleagues worked out details of what might have happened during the exceedingly hot first few moments of a Lemaître-style universe. The work showed that a hot early universe, evolving into what is observed today, was theoretically feasible. In a 1949 radio interview, the British astronomer Fred Hoyle coined the term “Big Bang” for the model of the universe Lemaître and Gamow had been developing. Lemaître’s hypothesis now had a name.
Lemaître’s idea about the original size of the universe is now considered incorrect. Today, cosmologists believe it started from an infinitesimally small point of infinite density called a singularity.
“A parallel exists between the Big Bang and the Christian notion of creation from nothing.” George Smoot
Georges Lemaître was born in 1894 in Charleroi, Belgium. Following distinguished service in World War I, in 1920 he was awarded a doctoral degree in engineering. He subsequently entered a seminary, where, in his leisure time, he studied mathematics and science.
After his ordination in 1923, Lemaître studied mathematics and solar physics at Cambridge University, studying under Arthur Eddington. In 1927, he was appointed professor of astrophysics at the University of Leuven, Belgium, and published his first major paper on the expanding universe. In 1931, Lemaître put forward his theory of the primeval atom in a report in the journal Nature, and his fame soon spread. He died in 1966, shortly after learning of the discovery of cosmic microwave background radiation, which provided evidence for the Big Bang.
GRAVITY EXPLAINS THE MOTIONS OF THE PLANETS
Gravity is the name given to the force of attraction between any two masses. It is the force that attracts all objects to Earth, giving them weight. It draws objects downward, toward the center of Earth. If the object were on the moon, a much smaller mass than Earth, the force would be six times less and its weight would be one sixth of its weight on Earth. English physicist, astronomer, and mathematician Isaac Newton was the first person to realize that gravity is a universal force, acting on all objects, and that it explains the movement of planets.
“To myself I am only a child playing on the beach, while vast oceans of truth lie undiscovered before me.” Isaac Newton
The shapes of the orbits of the planets were already well-known in Newton’s time, based on the three laws of planetary motion introduced by Johannes Kepler. Kepler’s first law stated that these orbits were ellipses, with the sun at one focus of each ellipse. The second law described the way that planets moved along their orbits more quickly when they were close to the sun than when they were farther away. The third law described the relation between the time taken to complete one orbit and the distance from the sun: the time taken for one orbit, squared, was equal to the cube of the average distance between the planet and the sun. For instance, Earth goes around the sun in one year, while Jupiter is 5.2 times farther away from the sun than Earth. 5.2 cubed equals 140, and the square root of 140 gives the correct figure for one Jupiter year: 11.86 Earth years.
However, although Kepler had correctly discovered the shapes and speeds of planetary orbits, he did not know why the planets moved as they did. In his 1609 book Astronomia Nova, he suggested that Mars was being carried around its orbit by an angel in a chariot. A year later, he had changed his mind, suggesting that the planets were magnets and were being driven around by magnetic “arms” extending from the spinning sun.
Before Newton, several scientists, including Englishman Robert Hooke and Italian Giovanni Alfonso Borelli, suggested that there was a force of attraction between the sun and the individual planets. They also stated that the force decreased with distance. On December 9, 1679, Hooke wrote to Newton saying that he thought the force might decrease as the inverse square of distance.
However, Hooke did not publish the idea and did not possess the mathematical skills to fully demonstrate his proposition. By contrast, Newton was able to prove rigorously that an inverse square law of attractive force would result in an elliptical planetary orbit.
Newton used mathematics to demonstrate that, if the force of attraction (F) between the sun and the planets varied precisely as an inverse square of the distance (r) between them, this fully explained the planetary orbits and why they follow Kepler’s three laws. This is written mathematically as F ∝ 1/r2. It means that doubling the distance between the objects reduces the strength of the attractive force to a quarter of the original force.
The Great Comet
Newton was a shy, reclusive man, and reluctant to publish his breakthrough. Two things forced his hand. The first was the Great Comet of 1680, and the second was the astronomer Edmond Halley.
The Great Comet of 1680 was the brightest comet of the 17th century—so bright that for a short time it was visible in the daytime. Two comets were seen: one that was approaching the sun in November and December 1680; and another that was moving away from the sun between late December 1680 and March 1681. As with all comets at the time, its orbit was a mystery, and the two sightings were at first not widely recognized as the same object. Astronomer John Flamsteed suggested that the two sightings might be of the same comet, which had come from the outer edge of the solar system, swung around the sun (where it was too close to the sun to be seen), and moved out again.
Halley was fascinated by the mysterious form of cometary orbits, and traveled to Cambridge to discuss the problem with his friend Newton. Using his law that related force to acceleration and his insistence that the strength of the force varied as the inverse square of distance, Newton calculated the parameters of the comet’s orbit as it passed through the inner solar system. This breakthrough intrigued Halley so much that he went on to calculate the orbits of 24 other comets, and to prove that one comet (Halley’s comet) returned to the sun around every 76 years. Perhaps more importantly, Halley was so impressed by Newton’s work that he strongly encouraged him to publish his findings. This resulted in the book Philosophiae Naturalis Principia Mathematica, published in Latin on July 5, 1687, in which Newton describes his laws of motion, his gravitational theory, the proof of Kepler’s three laws, and the method he used to calculate a comet’s orbit.
In his book, Newton stressed that his law was universal—gravity affects everything in the universe, regardless of distance. It explained how an apple fell on his head in the orchard of Woolsthorpe where his mother lived, the tides in the seas, the moon orbiting Earth, Jupiter orbiting the sun, and even the elliptical orbit of a comet. The physical law that made the apple fall in his yard was exactly the same as the one that shaped the solar system, and would later be discovered at work between stars and distant galaxies. Evidence was all around that Newton’s law of gravitation worked. It not only explained where planets had been, but also made it possible to predict where they would go in the future.
Constant of proportionality
Newton’s law of gravitation states that the size of the gravitational force is proportional to the masses of the two bodies (m1 and m2) multiplied together and divided by the square of the distance, r, between them. It always draws masses together and acts along a straight line between them. If the object in question is spherically symmetrical, like Earth, then its gravitational pull can be treated as if it were coming from a point at its center. One final value is needed to calculate the force—the constant of proportionality, a number that gives the strength of the force: the gravitational constant (G).
Newton’s law of universal gravitation shows how the force produced depends on the mass of the two objects and the square of the distance between them.
Gravity is a weak force, and this means that the gravitational constant is rather difficult to measure accurately. The first laboratory test of Newton’s theory was made by the English aristocrat scientist Henry Cavendish in 1798, 71 years after Newton’s death. He copied an experimental system proposed by the geophysicist John Michell and successfully measured the gravitational force between two lead balls, of diameters 2 and 12 in (5.1 and 30 cm). Many have tried to refine and repeat the experiment since. This has led to a slow improvement in the accuracy of G. Some scientists suggested that G changed with time. However, recent analysis of type 1a supernovae has shown that, over the last nine billion years, G has changed by less than one part in 10 billion, if at all. The light from distant supernovae was emitted nine billion years ago, allowing scientists to study the laws of physics as they were in the distant past.
“Nature and Nature’s laws lay hid in night: God said, “Let Newton be!” and all was light.” Alexander Pope
Like many of the scientists of his time, Newton was deeply pious and sought a religious meaning behind his observations and laws. The solar system was not regarded as a random collection of planets, and the sizes of the specific orbits were thought to have some specific meaning. For example, Kepler had sought meaning with his notion of “the music of the spheres.” Building on ideas first put forward by Pythagoras and Ptolemy, Kepler suggested that each planet was responsible for an inaudible musical note that had a frequency proportional to the velocity of the planet along its orbit. The slower a planet moved, the lower the note that it emitted. The difference between the notes produced by adjacent planets turned out to be well-known musical intervals such as major thirds.
There is some scientific merit behind Kepler’s idea. The solar system is about 4.6 billion years old. During its lifetime, the planets and their satellites have exerted gravitational influences on each other and have fallen into resonant intervals, similar to the way musical notes resonate. Looking at three of the moons of Jupiter, for every once that Ganymede orbits the planet, Europa goes around twice and Io four times. Over time, they have been gravitationally locked into this resonance.
The three-body problem
The solar system as a whole has fallen into similar resonant proportions to Jupiter’s moons. On average, each planet has an orbit that is about 73 percent larger than the planet immediately closer to the sun. Here, however, there appears a difficult mathematical problem, and one that Newton had grappled with. The movement of a low-mass body under the gravitational influence of a large-mass body can be understood, and predicted. But when three bodies are involved, the mathematical problem becomes exceedingly difficult.An example of a three-body system is the moon-Earth-sun. Newton thought about this system but the mathematical difficulties were insurmountable, and human knowledge of where the moon will be in the distant future is still very limited. Variations in the orbit of Halley’s comet are another indicator of the influence of the gravitational fields of the planets operating in addition to the gravitation of the sun. Recent orbits have taken 76.0, 76.1, 76.3, 76.9, 77.4, 76.1, 76.5, 77.1, 77.8, and 79.1 years respectively due to the combined gravitational influence of the sun, Jupiter, Saturn, and other planets on the comet.
“I have not been able to discover the cause of these properties of gravity from phenomena, and I frame no hypotheses.” Isaac Newton
Shaping the planets
While Newton searched for religious meaning in his scientific work, he could find none behind his theory of gravity. He did not discover the hand of God setting the planets in motion, but he had found a formula that shaped the universe.
The action of gravity is key to understanding why the universe looks as it does. For instance, gravity is responsible for the spherical shapes of the planets. If a body has sufficient mass, the gravitational force that it exerts exceeds the strength of the material of the body and it is pulled into a spherical shape. Astronomical rocky bodies, such as the asteroids between the orbits of Mars and Jupiter, are irregular in shape if they have a diameter of less than about 240 miles (380 km) (the Hughes-Cole limit).
Gravitation is also responsible for the size of the deviations from a sphere that can occur on a planet. There are no mountains on Earth higher than the 5.5 miles (8.8 km) of Mount Everest because the gravitational weight of a taller mountain would exceed the strength of the underlying mantle rock, and sink. On planets with lower mass, the weight of objects is less, and so mountains can be bigger. The highest mountain on Mars, for instance, Olympus Mons, is nearly three times as high as Everest. The mass of Mars is about one-tenth that of Earth, and its diameter is about half Earth’s. Putting these numbers into Newton’s formula for gravitation, this gives a weight on the surface of Mars of just over one-third that on Earth, which explains the size of Olympus Mons.
Gravity thus also shapes life on Earth by limiting the size of animals. The largest land animals ever were dinosaurs weighing up to 40 tons. The largest animals of all, whales, are found in the oceans, where the water supports their weight. Gravity is also responsible for the tides, which are produced because water bulges toward the sun and moon on the side of Earth nearer to them, and also bulges away from them on the other side where their gravitational pull is weaker. When the sun and moon are aligned, there is a high spring tide; when they are at right angles, there is a low neap tide.
“The motions of the comets are exceedingly regular, and they observe the same laws as the motions of the planets.” Isaac Newton
Gravity profoundly affects human mobility. The height a person can jump is determined by the gravitational field at ground level. Newton realized that the strength of gravity would affect the ease of travel beyond the atmosphere. To break free from Earth’s gravitational pull, it is necessary to travel at 25,020 mph (40,270 km/h). It is much easier to get away from less massive bodies
such as the moon and Mars. Turning the problem around, this escape velocity is also the minimum velocity that an incoming asteroid or comet can have when it hits Earth’s surface, and this affects the size of the resulting crater. Today, gravity is held to be most accurately described by the general theory of relativity proposed by Albert Einstein in 1915. This does not describe gravity as a force, but instead as a consequence of the curvature of the continuum of spacetime due to the uneven distribution of mass inside it. This said, Newton’s concept of a gravitational force is an excellent approximation in the vast majority of cases. General relativity only needs to be invoked in cases requiring extreme precision or where the gravitational field is very strong, such as close to the sun or in the vicinity of a massive black hole. Massive bodies that are accelerating can produce waves in spacetime, and these propagate out at the speed of light. The first detection of one of these gravitational waves was announced in February 2016.
Isaac Newton was born on a farm in Woolsthorpe, Lincolnshire, on December 25, 1642. After school in Grantham, he attended Trinity College Cambridge, where he became a Fellow and taught physics and astronomy. His book Principia set out the principle of gravity and celestial mechanics. Newton invented the reflecting telescope; wrote theses on optics, the prism, and the spectrum of white light; was one of the founders of calculus; and studied the cooling of bodies. He also explained why Earth was oblate (a squashed sphere) in shape and why the equinox moved, and formalized the physics of the speed of sound. He spent much time on biblical chronology and alchemy. Newton was at various times President of the Royal Society, Warden and Master of the Royal Mint, and member of parliament for Cambridge University. He died in 1727.
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