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THE UNIVERSE IS EXPANDING IN ALL DIRECTIONS

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In the early 1920s, American astronomer Edwin Hubble provided proof of the true size of the universe. Working at the Mount Wilson Observatory near Pasadena, California, Hubble used the newly constructed 100-in (2.5-m) Hooker Telescope, at that time the largest in the world, to settle the greatest argument then raging in astronomy. His observations would lead to the startling revelation that the universe is not only far, far larger than previously thought, but is also expanding.

Settling the Great Debate

At the time, the question of whether spiral nebulae were galaxies beyond the Milky Way or a special kind of nebula was the subject of a “Great Debate.” In 1920, a meeting was held at the Smithsonian Museum in Washington, D.C., in an attempt to settle the question. Speaking for the “small universe” was Princeton astronomer Harlow Shapley, who contended that the Milky Way comprised the entire universe. Shapley cited as evidence reports that the spiral nebulae were rotating, reasoning that this must make them relatively small because otherwise the outer regions would be spinning at speeds faster than the speed of light (these reports were later shown to be wrong). Opposed to Shapley was Heber D. Curtis, who supported the idea that each nebula was far beyond the Milky Way. Curtis cited as evidence the discovery by Vesto Slipher that light from most “spiral nebula” galaxies was shifted to the red part of the electromagnetic spectrum, indicating that they were moving away from Earth at enormous speeds—speeds far too high for them to be contained within the Milky Way.

Hubble set out to see whether there was a relationship between the distances of spiral nebulae and their velocity. His strategy was to search for Cepheid variable stars—stars whose luminosity changes predictably—within nebulae and to measure their distances from Earth. This provided Hubble with his first big discovery in the winter of 1923.

Beginning with photographic plates of the closest and clearest nebulae, Hubble spotted a Cepheid variable on one of the first plates he reviewed. The distances he calculated for even relatively nearby nebulae were so vast that it effectively killed the Great Debate immediately: NGC 6822 was 700,000 light-years away, while M33 and M31 were 850,000 light-years away. It was instantly clear that the universe extended beyond the Milky Way. Just as Curtis had maintained, the spiral nebulae were “island universes,” or “extragalactic nebulae” as Hubble termed them. Over time, the term “spiral nebulae” fell into disuse and they are now simply called galaxies.

“Observations indicate that the universe is expanding at an ever-increasing rate. It will expand forever, getting emptier and darker.” Stephen Hawking

Hubble is seen here looking through the lense of the Hooker Telescope at Mount Wilson. It was here that he measured galaxy distances and a value for the expansion of the universe.

In the realm of the nebulae

Hubble pressed ahead with his program of measuring the distances to galaxies beyond the Milky Way. Farther out, however, it became impossible to pick out individual Cepheid variables in such faint and fuzzy distant galaxies. He was compelled to fall back on indirect methods, such as the so called “standard ruler” assumption: reasoning that all galaxies of a similar type are the same size allowed him to estimate the distance to a galaxy by measuring its apparent size and comparing it to the expected “true” size. Thanks to Slipher’s measurements, Hubble already knew that light from most spiral nebulae was redshifted. In addition, the fainter spirals had higher values of redshift, showing that they were moving faster through space. Hubble realized that, if there were indeed a relationship between a galaxy’s distance from Earth and its recession velocity, these redshifts would serve as a cosmic yardstick, enabling the distances of the very farthest and faintest galaxies to be calculated, and a ballpark figure to be put on the size of the universe as a whole. Meanwhile, Milton Humason, the assistant astronomer at Mount Wilson, checked Slipher’s redshifts and collected new spectra from distant galaxies. It was hard, punishing work, and he and Hubble spent many bitterly cold nights in the observer’s cage at the top of the tube telescope on Wilson Mountain in California.

Hubble’s landmark paper, “A relationship between distance and radial velocity among extra-galactic nebulae,” was published in a journal called Proceedings of the National Academy of Science in 1929. It contained a straight-line graph that plotted 46 galaxies from near to far against their redshifts. Although there was a considerable scatter, Hubble managed to fit a straight line through the majority. The graph shows that, with the exception of the nearest galaxies, Andromeda and Triangulum, which are encroaching on the Milky Way, all other galaxies are receding. What is more, the farther away they are, the faster their movement.

Toward an interpretation

If, from Earth’s perspective within the universe, all galaxies are seen flying away, then the potential explanations are that (a) Earth lies at the center of the universe; or (b) the universe itself originated from a single point and is expanding as a whole.

Objectivity—a kind of foundational law in science—requires that there is no reason to assume that Earth occupies a unique position. Instead, the light from distant nebulae showed that the universe was not static. Many astronomers quickly reached the conclusion that this was due to the expansion of the universe, although Hubble never stated this explicitly. In reality, Vesto Slipher had indicated the trend in 1919, four years before Hubble made his observations, and Georges Lemaître had proposed the expansion of the universe from a “primeval atom” in 1927. However, Hubble’s result provided a simple link between his redshift-measured velocities and distance, and with it, the convincing proof that the scientific community needed. “Hubble’s law,” stating that the redshift of galaxies is proportional to their distance from Earth, was accepted almost unanimously.

“Equipped with his five senses, man explores the universe around him and calls the adventure Science.” Edwin Hubble

Einstein’s blunder

The revelation that the universe might be expanding made news all over the world—not least for the fact that it directly contradicted a theory of Albert Einstein’s. Einstein saw that gravity could eventually cause the universe to collapse under its own weight, so he used a value he called the cosmological constant—a kind of negative pressure—to prevent this from occurring in the field equations of general relativity. He abandoned the idea in the wake of Hubble’s discovery.

Einstein and others assumed that the observed velocities were Doppler effects caused by the galaxies’ speed of recession, but there were some dissenting voices. Swiss astronomer Fritz Zwicky suggested that the redshift might be due to “tired light” reaching Earth—caused by the interaction of photons with the intervening matter. Hubble himself found it hard to believe that the velocities indicated by the redshifts were actually real, and was happy to use them solely as distance indicators. In fact, the velocities of galaxies observed by Hubble are due to the expansion of spacetime itself.

Here, the galaxies of the universe are imagined as dots stuck to an inflating balloon (for visualization, the dimensions are reduced from the three of space to the two of the balloon’s surface). As the balloon doubles in diameter, the distances between dots also double. The farther away a dot is from another dot, the farther, and so more quickly, it moves. C has moved 2 cm from A, but B has moved just 1 cm from A.

K-factor

Hubble showed how fast spacetime is expanding by plotting a straight-line graph—which he grandly called the “K-factor.” The gradient is described mathematically by a value now known as the Hubble Constant (H0). This important number determines not only the size of the observable universe but also its age. The Hubble Constant allowed astronomers to work backward and calculate the moment in time of the Big Bang itself, when the radius of the universe was zero.

The initial calculation of H0 was 300 miles (500 km) per second per megaparsec (one megaparsec is approximately 3.26 million light-years). This presented a problem, since it gave a figure of 2 billion years for the age of the universe, less than half the accepted age of Earth. The discrepancy was found to have been caused by systematic errors in Hubble’s distance measurements. Many were out by a factor of seven due to his method of taking the brightest star in any galaxy—or even the luminosity of the galaxy itself—and assuming it to be a Cepheid variable star. Luckily for Hubble, the inaccuracies were fairly consistent throughout the dataset, allowing him to plot the trend in spite of them.

The ESA’s Planck Observatory operated between 2009 and 2013. It produced data that helped to measure many cosmological parameters, including the Hubble Constant.

Hubble Key Project

Calculating the rate of expansion of the cosmos drove the decision to develop the Hubble Space Telescope from its inception in the 1970s to its 1990 launch. NASA made one of the telescope’s “Key Projects” determining the Hubble Constant to within 10 percent. As a result, the instrument spent years measuring Cepheid light curves. The final result, delivered in 2001, gave an age for the universe of 13.7 billion years. This figure was fine-tuned to 13.799 billion years (with an error of 21 million years either way) by data from the Planck Space Observatory in 2015. The most dramatic revision to Hubble’s law, however, came in 1998 when astronomers discovered that the universe’s expansion is accelerating due to a mysterious and unknown agent known as dark energy, which has led to a renewal of interest in Einstein’s socalled blunder, the cosmological constant.

EDWIN HUBBLE

Born in 1889 in Missouri, Edwin Hubble was a gifted athlete as a youth, leading the University of Chicago’s basketball team. After graduating with a science degree, he studied law at Oxford University. Returning from England dressed in a cape and behaving like an aristocrat, he was described by Harlow Shapley as “absurdly vain and pompous.”

Despite his flair for self-publicity, Hubble was a cautious scientist. He described himself as an observer, and reserved judgment until he had sufficient evidence. He reacted furiously if anyone encroached on his field of research. It is to Hubble’s discredit, therefore, that he did not acknowledge that 41 of the 46 redshifts used to formulate his famous law were measured not by him but by Vesto Slipher. Hubble spent his last years campaigning for a Nobel Prize to be awarded in astronomy. He died in 1953.

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Space

RIPPLES THROUGH SPACETIME

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In 1916, as he worked on his theory of relativity, Albert Einstein predicted that, as a mass moved, its gravity would cause ripples in the fabric of spacetime. Every mass would do this, although larger masses would make bigger waves, in the same way that a pebble dropped in a pond makes an ever-increasing circle of ripples, while a meteor impacting the ocean creates tsunami-sized waves.

In 2016, 100 years after Einstein’s predictions, a collaboration between scientists working under the name LIGO announced that they had discovered these ripples, or gravitational waves. Their decades-long search had revealed the gravitational equivalent of tsunamis created by two black holes spiraling around each other and then colliding.

It is hoped that the discovery of gravitational waves will provide a new way of observing the universe. Instead of using light or other electromagnetic radiation, astronomers are hoping to map the universe by the gravitational effects of its contents. While radiation is obscured in many ways, including by the opaque plasma of the early universe up to 380,000 years after the Big Bang, gravitational waves pass through everything. This means that gravitational astronomy could see back to the very beginning of time, a trillionth of a second after the Big Bang.

Within a period of 20 milliseconds, the two black holes LIGO had detected increased their orbital speed from 30 times a second to 250 times a second before colliding.

Wave behaviors

LIGO stands for Laser Interferometer Gravitational-Wave Observatory. It is a remarkable set of instruments for measuring expansions and contractions in space itself. This is no easy task. A ruler cannot do it because, as space changes in size, so does the ruler, so the observer measures no change at all. LIGO succeeded using the benchmark that remains constant whatever space is doing: the speed of light. Light behaves like a wave, but it does not require a medium through which to travel. Instead, light (and any kind of electromagnetic radiation) is an oscillation of an electromagnetic field: in other words, light is a disturbance in a field permeating all of space.

Gravitational waves can be understood as disturbances in the gravitational field that permeates the universe. Einstein described how these disturbances are caused by the mass of objects curving the space around them. What is understood as the “pull of gravity” is a small mass appearing to alter its motion and “fall” toward a larger mass as it encounters a region of warped space.

All masses are in motion—planets, stars, even galaxies—and as they move, they leave a trail of gravitational disturbances in their wake. Gravity waves propagate in a comparable way to sound waves, by distorting the medium through which they travel. In the case of sound waves, that medium is made of molecules, which are made to oscillate. In the case of gravity, the medium is spacetime, the very fabric of the universe. Einstein predicted that the speed of gravity would be the same as the speed of light, and that the ripples in spacetime would move outward in all directions. The intensity of these ripples diminishes rapidly with distance (by a square of the distance), so detecting a distinct gravity wave from a known object far out in space would require a very powerful source of waves and a very sensitive instrument.

Laser-guided

As its name suggests, LIGO employs a technique called laser interferometry. This makes use of a property of waves called interference. When two waves meet, they interfere with one another to create a single wave. How they do this depends on their phase—the relative timing of their oscillations. If the waves are exactly in phase—rising and falling perfectly in sync—they will interfere constructively, merging to create a wave with double the intensity. By contrast, if the waves are exactly out of phase—one rising as the other falls—the interference will be destructive. The two waves will merge and cancel one another out, disappearing completely.

LIGO’s source of waves is a laser, which is a light beam that contains a single color, or wavelength, of light. In addition, the light in a laser beam is coherent, which means that its oscillations are all perfectly in time. Such beams can be made to interfere with one another in very precise ways.

The laser beam is split in two and the resulting beams are sent off perpendicular to one another. They both hit a mirror and bounce straight back to the starting point. The distance traveled by each beam is very precisely controlled so that one has to travel exactly half a wavelength farther than the other (a difference of a few hundred billionths of a meter). When the beams meet each other again, they are exactly out of phase as they interfere, and promptly disappear—unless a gravitational wave has passed through space while the beams were traveling. If present, a gravitational wave would stretch one of the laser tracks and compress the other, so the beams would end up traveling slightly altered distances.

With no gravitational waves, LIGO’s light waves cancel one another out when they are recombined. Gravitational waves stretch one tube while compressing the other, so that the waves are no longer perfectly aligned, and a signal is produced

Noise filter

The laser beams are split and sent on a 695-mile (1,120-km) journey up and down LIGO’s 2.5-mile (4-km) long arms before being recombined. This gives LIGO the sensitivity to detect minute perturbations in space that add up to a few thousandths of the width of a proton. With the distances put very slightly out of sync, the interfering beams would no longer cancel each other out. Instead, they would create a flickering pattern of light, perhaps indicating a gravity wave passing through LIGO’s corner of space.

The difficulty was that such a sensitive detector was prone to distortions from the frequent seismic waves that run through Earth’s surface. To be sure that a laser flicker was not an earth tremor, two identical detectors were built at opposite ends of the United States: one in Louisiana, the other in Washington state. Only signals registering on both detectors were gravitational waves (the signals are in fact 10 milliseconds apart—the time it takes for light, and gravitational waves, to travel from Louisiana to Washington). Ligo operated from 2002–2010 with no success, then started up again in 2015 with enhanced sensitivity.

LIGO’s precision instruments must be kept completely clean. Maintaining the purity of the laser beams is one of the project’s biggest challenges.

Colliding black holes

On September 14, 2015, at 9:50:45 GMT, two black holes a billion lightyears away collided and unleashed huge warps in the fabric of space. In fact, this event occurred a billion years ago but it had taken that long for the ripples they had released to reach Earth—where they were detected by both LIGO detectors. The researchers took another few months to check their result and went public in February 2016.

The search is now on for more gravitational waves, and the best place to do it is from space. In December 2015, the spacecraft LISA Pathfinder was launched. It is headed to an orbit at L1, which is a gravitationally stable position between the sun and Earth. There, the spacecraft will test laser interferometry instruments in space, in the hope that they can be used in an ambitious experiment called eLISA (evolved Laser Interferometer Space Antenna). Provisionally scheduled for 2034, eLISA will use three spacecraft triangulated around the sun. Lasers will be fired between the spacecraft,
making a laser track 2 million miles (3 million km) long that is many times more sensitive to gravitational waves than LIGO.

The discovery of gravitational waves has the potential to transform astronomers’ view of the universe. The patterns in the fluctuations in the light signals from LIGO and future projects will produce new information, providing a detailed map of mass across the universe.

“Gravitational waves will bring us exquisitely accurate maps of black holes—maps of their spacetime.” Kip Thorne

LIGO splits one beam of laser light and sends beams down two tubes at 90° to each other. To prevent unwanted interference, the tubes are vacuums at one trillionth of the pressure of Earth’s atmosphere. LIGO also has to make adjustments to allow for the tidal pull of the sun and the moon

RAI WEISS AND KIP THORN

LIGO is a collaboration between Caltech and MIT, and also shares its data with a similar experiment called Virgo, which is running in France and Poland. Hundreds of researchers have contributed to the discovery of gravitational waves. However, there are two people, both Americans, who stand out among them all: Rainer “Rai” Weiss (1932–) and Kip Thorne (1940–). In 1967, while at MIT, Weiss developed the laser interferometry technique used by LIGO, working from the initial ideas of Joseph Weber, one of the inventors of the laser. In 1984, Weiss cofounded LIGO with Thorne, a counterpart at Caltech, who is a leading expert on the theory of relativity. LIGO is the most expensive science project ever funded by the US government, with a current cost of $1.1 billion. After 32 years of trying, in 2016 Weiss and Thorne announced their discovery of gravitational waves at a news conference in Washington, D.C.

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Space

MOST STARS ARE ORBITER BY PLANETS

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In 1995, two Swiss astronomers, Michel Mayor and Didier Queloz, researching at the Observatoire de Haute-Provence near Marseille, found a planet orbiting 51 Pegasi, a sunlike star 60 light-years away in the constellation of Pegasus. This was the first confirmed observation of a true extrasolar planet, or exoplanet—a planet beyond the solar system. It was
orbiting a main sequence star, and was therefore assumed to have formed by the same process as that which created the solar system.

Mayor and Queloz named the new planet 51 Pegasi b, but it is unofficially known as Bellerophon after the hero who rode Pegasus, the winged horse of ancient Greek myth. Its discovery prompted a major hunt to find more exoplanets. Since 1995, several thousand exoplanets have been found, many in multiple star systems. Astronomers now estimate that there is an average of one planet around every star in the galaxy, although this is probably a very conservative figure. Some stars have no planets, but most, like the sun, have several. 4The discovery of 51 Pegasi b marked the final milestone in a process that has forced astronomers to abandon any lingering notion that Earth occupies a privileged place in the universe.

“For more than 2,000 years, people have dreamed of finding other habitable worlds.” Michel Mayor

Copernican principle In the 1950s, the Anglo−Austrian astronomer Hermann Bondi had described a new way for humans to think about themselves, which he called the Copernican principle. According to Bondi, humankind could no longer regard itself as a unique phenomenon of central importance to the universe. On the contrary, humans should now understand that their existence is insignificant in the context of the universe.

The principle is named after Nicolaus Copernicus, who changed the way humankind saw itself by relegating Earth from the center of the solar system to one of several planets that orbited the sun. By the late 20th century, successive discoveries had moved the solar system from the center of the universe to a quiet wing at the edge of a galaxy containing 200 billion other stars. The galaxy was not special either, simply one of at least 100 billion arranged in vast filaments that extended for hundreds of millions of lightyears. Nevertheless, planet Earth and the solar system were still regarded as very special—since there was no evidence that any other stars had planets, let alone planets capable of supporting life. Since Mayor’s and Queloz’s discovery, however, this idea has also succumbed to the Copernican principle.

“We are getting much closer to seeing solar systems like our own.” Didier Queloz

When a large Jupiter-like planet orbits its star, it exerts a gravitational pull on the star. Both star and planet revolve around a common center of gravity. The “wobble” in the star’s orbit allows the planet to be detected

Wobbling light

Queloz and Mayor found 51 Pegasi b using a system called Doppler spectroscopy. Also known as the radial velocity or “wobble” method, Doppler spectroscopy can detect an exoplanet by its gravitational effects on its host star. The star’s gravity is far greater than that of the planet, and this is what keeps the planet in orbit. However, the planet’s gravity also has a small effect on the star, making it wobble back and forth as the planet moves around it. The effect is tiny: Jupiter changes the sun’s speed by about 12 miles/s (7.4 km/s) over a period of 11 years, while Earth’s effect is only 0.1 miles/s (0.16 km/s) each year.

In 1952, US astronomer Otto Struve had suggested that this kind of star wobble could be detected as small fluctuations in a star’s spectrum. As the star moved away from Earth, its emissions would be slightly redshifted from the norm. When it wobbled back again toward the observer, the light would be blueshifted. The theory was solid but detecting the wobble required an ultrasensitive detector. That detector was a spectrograph named ELODIE developed by Mayor in 1993. ELODIE was about 30 times more sensitive than any previous instrument. Even then, it was only capable of measuring velocity changes of 7 miles/s (11 km/s), which meant it was limited to detecting planets about the size of Jupiter.

The Kepler observatory looked outward from the plane of the ecliptic, so that Earth, the moon, and the sun did not obscure the view.

Improving the search

In 1998, an even more sensitive spectrograph, named CORALIE, was installed at La Silla Observatory in Chile, which again was searching for planets using the radial velocity technique. In 2002, Michel Mayor began overseeing HARPS (High Accuracy Radial velocity Planet Searcher) at the same site, using a spectrograph capable of detecting exoplanets about the size of Earth. The wobble method of detection was very slow, so new techniques of spotting exoplanets were developed. The most successful method was the transit method, which looked for periodic changes in the brightness of a star. These changes were very small and happened when a planet transited the star, passing between the star and the observer, and causing it to dim very slightly. The best place to look for exoplanets by the transit method was out in space and so, in 2009, the Kepler observatory, named after the man who first described planetary orbits, was launched to do just that.

Staring at one place

Kepler was placed in a heliocentric orbit, trailing behind Earth as it circled the sun. The craft was designed to keep its aperture firmly fixed on a single patch of space, called the Kepler field. This made up only about 0.25 percent of the whole sky, but the spacecraft could see 150,000 stars in that area. To find exoplanets, Kelper would have to concentrate on this single field of view for years on end. It was unable to see individual exoplanets, but could identify stars that were likely to have them.

Kepler could only detect the transits of exoplanets with orbital paths that crossed the spacecraft’s line of sight. Many exoplanets would be orbiting at the wrong angle for that. Those that were correctly oriented would only transit their star once every orbital period (the planet’s year), so Kepler’s method was better at finding planets that orbited close to their star, taking a few years and months (or even weeks and days) to complete each revolution.

“We were not expecting to find a planet with a 4-day [orbital] period. No one was expecting this.” Michel Mayor

Candidate stars

By the start of 2013, Kepler had identified about 4,300 candidate stars that might have extrasolar planetary systems. Unfortunately, the guidance system used to keep Kepler locked on target then failed, bringing its planet hunt to an end about three years sooner than expected. However, the data it had collected was enough to keep researchers busy for years to come. Kepler’s candidate stars could only be confirmed as planetary systems using radial velocity measurements from ground-based observatories, such as HARPS in Chile and the Keck Telescope in Hawaii. (Radial velocity is the velocity of the star in the direction of Earth.) So far, about a tenth of Kepler’s candidate stars have proved to be false positives but, after three years of analysis, the program had identified 1,284 exoplanets, with more than 3,000 stars left to examine. The statistics for the exoplanets in the Kepler field are striking— most stars are part of a planetary system. This means that the number of planets in the universe is likely to exceed the number of stars.

The amount of dimming during a transit gives an indication of how big an exoplanet might be, but the study of an exoplanet’s size and characteristics is still in its early stages. The light reflected from a planet is about 10 billion times fainter than the star it orbits. Astronomers are waiting for the James Webb Space Telescope in 2018 and the European Extremely Large Telescope in 2024 to image this light directly and analyze the chemistry of exoplanets. Until then, they have to speculate using very little data: an approximate mass of the planet, its radius, the orbital distance, and the temperature of the star. This tells them what the planet is probably made of and allows them to conjecture what the surface conditions are likely to be.

“Red dwarfs with rocky planets could be ubiquitous in the universe.” Phil Muirhead

The “super-Jupiter” Kappa Andromedae b, shown here in an artist’s render, has a mass of 13 times Jupiter’s. It glows a reddish color, and may yet be reclassed as a brown dwarf

Hot and super Jupiters

The exoplanets discovered so far have added a host of weird worlds to the neat family portrait that is the sun’s planetary system. For example, 51 Pegasi b was the first of many “hot Jupiters.” These have a mass similar to Jupiter’s and a large size that shows that they are mostly made of gas. 51 Pegasi b is half as massive as Jupiter, but is slightly larger. This gas giant orbits its sunlike star every four days. That means it is much closer to its star than Mercury is to the sun. Such proximity means it is tidally locked to the star— one side always faces the scorching stellar surface, and the other always faces away. Many hot Jupiters have been found. They have confounded scientists, who are trying to understand how gas planets can exist so close to a star without evaporating. Some exoplanets are dozens of times more massive than Jupiter, and are known as “super-Jupiters.” These super-Jupiter planets do not appear to grow in size as their mass increases. For instance, Corot-3b is a super-Jupiter that is 22 times as heavy as Jupiter but more or less the same size, due to its gravity holding its gaseous contents together. Astronomers have calculated that the density of Corot-3b is greater than that of gold and even osmium, the densest element on Earth.

Kepler 10b in the Draco constellation is shown transiting its star in an artist’s impression. Its extremely hot surface temperature and dizzying orbit mean life there is improbable.

Brown dwarfs and rogues

When a super-Jupiter reaches 60 Jupiter masses, it is no longer regarded as a planet, but as a brown dwarf. A brown dwarf is essentially a failed star—a ball of gas that is too small to burn brightly through nuclear fusion. The brown dwarf and its star are seen as a binary star system, not a planetary one. Some super-Jupiters and small brown dwarfs have broken free of their star to become free-floating rogue planets. One, named MOA-2011-BLG-262, is thought to have a satellite, and could be the first exoplanet found with an exomoon.

Another class of planet are called the super-Earths. These have a mass 10 times that of Earth but less than that of an ice giant like Neptune. SuperEarths are not rocky but made from gas and ice: alternative names for them are mini-Neptunes or gas dwarfs.

The size of the habitable zone (green) depends on the size of the star. The red zone is too hot, while the blue zone is too cold. The habitable zone is closer to cooler stars than it is to hotter stars. The size of a planet, the shape of its orbit, and the speed of its rotation between night and day also affect its habitability.

Living planets

Earth’s solar system has terrestrial planets (planets with a rocky surface), of which Earth is the largest. So far, exoplanet searches have struggled to find many terrestrial planets, because they are generally small and beyond the sensitivity of the planet detectors. The first confirmed terrestrial exoplanet was Kepler-10b, which is three times the mass of Earth and is so close to its star that it orbits once an Earth day and has a surface temperature that would melt iron. Life seems highly unlikely there, but the hunt continues for rocky planets that might be more hospitable.

Astrobiologists—scientists who search for alien life—focus on the particular conditions that all life needs. When choosing likely places to look, they assume that alien life-forms will require liquid water and carbon-based chemicals, just like life on Earth. Living planets would also need an atmosphere to shield the surface from damaging cosmic rays and to act as a blanket that retains some of the planet’s heat during the night.

The region around a star where the temperatures would allow planets to have liquid water, carbon chemistry, and an atmosphere, is known as its habitable zone, also called the “Goldilocks zone”—like Baby Bear’s porridge in the fairy tale, “not too hot, not too cold.” The size and locations of habitable zones depend on the activity of the host star. For example, if Earth were orbiting a K-type star, an orange dwarf that is considerably cooler than the sun (the sun is a G-type, or yellow dwarf), it would need to orbit at about one-third its current distance to receive the same amount of warmth.

Of the thousands of exoplanets that have been identified, only a tiny proportion are candidates orbiting in their star’s habitable zones, with Earthlike conditions for life—rocky surface with liquid water. Typically, they are larger than Earth, and very few have good prospects for being Earth-like. If and when Earth-like planets are found, astrobiologists will look at the atmospheric chemistry for signs of life, such as the presence of elevated levels of oxygen, produced by photosynthesizing life-forms. How life evolved from nonliving material on Earth is still a mystery but the study of Earth-like planets may throw light on that process. Even if life is found, it is likely that most extraterrestrial natural histories will not have moved beyond microorganisms. As every step toward evolving more complex life-forms becomes ever more unlikely, so alien civilizations that match humankind’s will be a lot less common. However, if only G-type stars, like the sun, are counted, there are about 50 billion in the galaxy. It is estimated that 22 percent of them have an Earth-like planet in their habitable zones, which equals 11 billion possible Earths. Adding in other types of stars such as orange and red dwarfs, that number rises to 40 billion. Even if the probability of civilizations evolving is one in a billion, the chances are that humankind is not alone.

“If we keep working as well and we keep being as enthusiastic … the issue about life on other planets will be solved.” Didier Queloz

MICHEL MAYOR

Michel Mayor was born in Lausanne, Switzerland, and has spent most of his career working at the University of Geneva. His interest in exoplanets arose from his earlier study of the proper motion of stars in the Milky Way. To measure this motion more accurately, he developed a series of spectrographs, which eventually culminated in ELODIE. The ELODIE project with Didier Queloz was initially intended to search for brown dwarfs—objects that were bigger than planets but not quite large enough to be stars. However, the system was sensitive enough to spot giant planets as well, and, following their 1995 discovery, Mayor is currently the chief investigator at the HARPS program for the European Southern Observatory in Chile. His team has found about half of all the exoplanets discovered to date. In 2004, Mayor was awarded the Albert Einstein medal.

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History

WRINKLES IN TIME

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The Cosmic Microwave Background, or CMB, was discovered in 1964. This is the afterglow of the Big Bang and it is as near as scientists can get to observing the event that brought the universe into existence, 13.8 billion years ago. Linking the structures observed in the universe to the features discerned in the CMB remains a key challenge for cosmologists.

“I always think of space-time as being the real substance of space, and the galaxies and the stars just like the foam on the ocean.” George Smoot

Wrinkled time

The first great breakthrough came from the Cosmic Microwave Background Explorer, known as COBE, a NASA satellite launched in 1989. The detectors on COBE, designed and run by George Smoot, John Mather, and Mike Hauser, were able to find the oldest structures in the visible universe, described by Smoot as “wrinkles in time.” These wrinkles in otherwise uniform space were once dense regions containing the matter that would form stars and galaxies. They correspond to the large-scale galaxy superclusters and great walls seen in the universe today, and add weight to the inflationary model of the early universe proposed by American Alan Guth.

The CMB is a flash of radiation that was released about 380,000 years after the Big Bang, at the time the first atoms formed. The expanding universe had cooled enough for stable ions (positively charged nuclei) of hydrogen and helium to form, and then, after a little more cooling, the ions began to collect electrons to make neutral atoms. The removal of free electrons from space led to the release of photons (particles of radiation).

Those photons are visible now as the CMB. The CMB comes from the whole sky, without exception. It has redshifted (the wavelengths have stretched), and it now has wavelengths of a few millimeters, while the original radiation’s wavelengths would be measured in nanometers (billionths of a meter). One of the key observations of the CMB came in the 1970s, and removed any doubt that it was an echo of the Big Bang. This was the discovery that the thermal spectrum of radiation from the CMB tallied very closely with that of a theoretical black body.

The Cosmic Microwave Background Explorer (COBE) spent four years in space collecting information about the CMB, scanning the celestial sphere every six months

Black bodies

Black bodies do not really exist—they cannot be made and no object observed in the universe functions as black bodies do in theory. However, the CMB is the closest match that has ever been found.

A black body absorbs all radiation that hits it. Nothing is reflected. However, the absorbed radiation adds to the thermal energy of the object, and this is released as radiation. In 1900, German Max Planck, the founding figure of quantum physics, showed that the spectrum of radiation released by a black body is entirely dependent on temperature.

In an everyday example of radiation varying with temperature, an iron bar glows red when first heated. Heating it more makes it orange, and eventually the bar will glow “blue hot.” Metalworkers learn to roughly judge the temperature of iron by its color. The metal is not particularly close to a black body in the theoretical sense, but stars and other astronomical objects are a much closer match to a black body, and so the color, or wavelengths of their emissions, can be compared to the thermal spectrum of a theoretical black body to give a relatively precise temperature.

The temperature of the CMB today is a chilly 2.7 K. The thermal spectrum at that temperature contains no visible light, which is why space looks black to human eyes. However, the spectrum has redshifted (stretched) over time as the universe has expanded. Extrapolating back to the moment the CMB was emitted gives an original temperature of about 3,000 K. The color of radiation at this temperature is orange, so the CMB started out as a flash of orange light that shone out from every point in space.

Smooth signal

The early observations of the CMB suggested that it was isotropic, which means that its spectrum is the same everywhere. In cosmology, the terms density, energy, and temperature are somewhat synonymous when discussing the early universe. So the isotropic nature of the CMB suggested that, in those early days, space had a uniform density, or spread of energy. However, this did not tally with the developing theories of the Big Bang, which demanded that matter and energy were not evenly spread through the young universe, but had been concentrated together in places. These denser areas, or anisotropies, were where the stars and galaxies formed. COBE was sent into space to take a close look at the CMB to see if it could find any anisotropies, to find out whether the CMB changed, however slightly, depending on where it looked.

The full-sky map produced by WMAP in 2011 showed many fine details of the isotropy of the CMB. Colder spots are blue, while hotter spots are yellow and red.

COBE’s mission

A mission to study the CMB from space had been in the planning stages since the mid-1970s. Construction of COBE began in 1981. It was initially designed to enter polar orbit (its orbit passing over both poles). However, the Challenger disaster of 1986 grounded the shuttle fleet, and the COBE team had to look for another launch system. In 1989, the satellite was launched using a Delta rocket, and it was placed in a sun-synchronous geocentric orbit —orbiting in a way that saw it pass over each place on Earth at the same time of day. This worked just as well as a polar orbit in that it allowed COBE to point away from Earth and scan the entire celestial sphere, strip by strip.

The spacecraft carried three instruments, all protected from the sun’s heat and light by a cone-shaped shield, and chilled to 2 K (colder than space itself) using 100 gallons (650 liters) of liquid helium. George Smoot ran the
Differential Microwave Radiometer (DMR), which mapped the precise wavelengths of the CMB, while John Mather was in charge of FIRAS, the Far-InfraRed Absolute Spectrophotometer, which collected data on the spectra of the CMB. These two experiments were looking for anisotropies. The third detector on COBE had a slightly different goal. The Diffuse Infrared Background Experiment, run by Mike Hauser, found galaxies that were so ancient and far away that they are only visible by their heat radiation (or infrared).

COBE’s instruments created the most accurate map of the CMB to date. However, it was not a simple surveying job. Smoot and Mather were interested in primary anisotropies—that is, the density differences that were present at the time the CMB formed. To find these, they needed to filter out the secondary fluctuations caused by obstacles that lay between COBE and the edge of the universe. Dust clouds and the effects of gravity had interfered with the radiation on its long journey to Earth. The data from the three instruments were used to detect and correct these so-called secondary anisotropies.

“[COBE has made] the greatest discovery of the century, if not of all time.” Stephen Hawking

In addition to mapping the CMB, WMAP measured the age of the universe as 13.77 billion years, dark matter as 24.0 percent of the universe, and dark energy as 71.4 percent.

Tiny fluctuations

After 10 months in space, COBE’s helium ran out, which limited the function of the two infrared detectors, but the DMR continued working until 1993. By 1992, the COBE team’s analysis had shown what they were looking for. The CMB, and thus the early universe, was not a uniform blob of energy. Instead it was riddled with tiny but significant fluctuations. The differences were minute, with density variations of about 0.001 percent. However, the pattern was enough to explain why the contents of the universe are clustered together, while the rest of space is made from vast empty voids.

Since COBE, two subsequent missions have added detail to the picture of the CMB. Between 2001 and 2010, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) mapped the CMB to a higher resolution than COBE. Then, from 2009–2013, the ESA’s Planck Observatory produced the most accurate map to date.

Every wrinkle on the map is the seed from which an entire galaxy formed about 13 billion years ago. However, no known galaxy can be seen forming
in the CMB. The CMB radiation detected today has traveled from near the edge of the observable universe over the course of most of the age of the universe. Astronomers can only see 13.8 billion light-years away, but most of the universe now lies farther away than that. The galaxies forming in the CMB are now far beyond what can be observed, and are receding faster than the speed of light.

GEORGE SMOOT

After a childhood in Florida and Ohio, Smoot began his career as a particle physicist working at MIT. His interests switched to cosmology and he moved across the country to the Lawrence Berkeley National Laboratory. It was there that Smoot studied the CMB and developed ways of measuring its radiation.

Smoot’s early work involved fitting detectors to high-altitude U2 spyplanes, but in the late 1970s, he became involved in the COBE project to take his detector into space. After his success with COBE, Smoot cowrote Wrinkles in Time with Keay Davidson to explain the discovery. Smoot won the Nobel Prize in 2006, along with John Mather, for his work on COBE. He reportedly gave his prize money to charity. However, three years later, Smoot won an even greater sum when he bagged the $1 million jackpot on the TV game show Are You Smarter Than a 5th Grader?

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