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THE KEY TO A DISTANCE SCALE OF THE UNIVERSE

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Some of the most important, but often most challenging, measurements for astronomers to make have been the distances to extremely remote objects— which includes most celestial objects aside from the moon, sun, and other planets of the inner solar system. Nothing in the light coming from distant stars and galaxies gives any direct indication of how far that light has traveled through space to reach Earth.

For several hundred years, scientists realized that it should be possible to measure the distances to relatively nearby stars by a method called parallax. This is based on comparing the position of a nearby star against the background of more distant stars from two perspectives—usually Earth’s different positions in space six months apart in its orbit around the sun. Although many others had tried (and failed) before him, the first astronomer to measure a star’s distance accurately using this method was Friedrich Bessel, in 1838. However, even with increasingly powerful telescopes, measuring star distances by parallax proved difficult and, by the year 1900, the distances to only about 60 stars had been measured. Furthermore, the parallax method could be applied only to nearby stars. The difference in perspective for more distant stars over the course of a year was too small to be accurately determined. New methods were therefore needed to measure large distances in space.

“A remarkable relation between the brightness of these (Cepheid) variables and the length of their periods will be noticed.” Henrietta Swan Leavitt

Measuring brightness

In the 1890s and early 1900s, the Harvard College Observatory in Massachusetts was one of the world’s leading astronomical research institutions. Under the supervision of its director, Edward C. Pickering, the Observatory employed many men to build equipment and take photographs of the night sky, and several women to examine photographic plates taken from telescopes throughout the world, measure their brightness, and perform computations based on their assessment of the plates. These women had little chance to do theoretical work at the Observatory, but several of them, including Williamina Fleming, Henrietta Swan Leavitt, Antonia Maury, and Annie Jump Cannon, nevertheless left a lasting legacy.

Henrietta Swan Leavitt, who had originally joined the Observatory as an unpaid volunteer in 1894, eventually became the head of the photographic photometry department. This mainly involved measuring the brightness of stars, but a specific aspect of Leavitt’s work was to identify stars that fluctuate in brightness—known as variable stars. To do this, she would do a comparison of photographic plates of the same part of the sky, made on different dates. Occasionally she would find a star that was brighter on different dates, indicating that it was a variable.

“One of the most striking accomplishments of Miss Leavitt was the discovery of 1,777 variable stars in the Magellanic Clouds.” Solon I. Bailey

The Cepheid variable stars that Leavitt studied are in the Magellanic Clouds, known today to be galaxies outside the Milky Way. The Large Magellanic Cloud is about 160,000 light-years away; the Small Magellanic Cloud is about 200,000 light-years away. Both are part of the Local Group galaxy cluster that includes the Milky Way.

Cluster variables

A specific task that Leavitt took on was to examine some of the photographic plates of stars in the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). At the time, the SMC and LMC were thought to be very large star clusters within the Milky Way, which itself was assumed to comprise the entire universe. Today, they are known to be relatively small, separate galaxies that lie outside the Milky Way. The Magellanic Clouds are visible to the naked eye in the night sky of the southern hemisphere, but are never visible from Massachusetts, where Leavitt lived and worked. Therefore, although she examined numerous photographic plates of the LMC and SMC obtained by astronomers at an observatory in Peru, it is highly unlikely that she ever physically observed them in the sky.

After several years’ work, Leavitt had found 1,777 variables in the SMC and LMC. One particular kind that caught Leavitt’s attention, representing a small fraction of all the variables she had found (47 out of 1,777), was of a type called a Cepheid variable. Leavitt called them “cluster variables”—the term Cepheid variable was introduced later. These are stars that regularly vary in brightness with a period (cycle length) that could be anything from one to more than 120 days. Cepheid variables are reasonably easy to recognize because they are among the brightest variable stars, and they have a characteristic light curve, showing fairly rapid increases in brightness followed by a slower tailing off. Today, they are known to be giant yellow stars that “pulsate”—varying in diameter as well as brightness over their cycles—and are very rare. As a class of stars, they also have an exceptionally high average brightness, which means they stand out even in other galaxies. In examining her records of Cepheid variables in either the LMC or SMC, Leavitt noticed something that seemed significant. Cepheids with longer periods seemed to be brighter on average than those with shorter periods. In other words, there was a relationship between the rate at which Cepheids “blinked” and their brightness. Furthermore, Leavitt correctly inferred that, since the Cepheids she was comparing were all in the same distant nebula (either the LMC or the SMC), they were all at much the same distance from Earth. It followed that any difference in their brightness as viewed from Earth (their apparent magnitude) was directly related to differences in their true or intrinsic brightness (their absolute magnitude). This meant there was a definite relationship between the periods of Cepheid variables and their average intrinsic brightness or their optical luminosity (the rate at which they emit light energy).

Leavitt published her initial findings in a paper that first appeared in the Annals of the Astronomical Observatory of Harvard College in 1908. Then, in 1912, after further study, which included plotting graphs of the periods of Cepheid variables in the SMC against values for their minimum and maximum brightness, she confirmed her discovery in more detail. It became known as the “period−luminosity” relationship. Formally, it stated that the logarithm of the period of a Cepheid variable is linearly (i.e., directly) related to the star’s average measured brightness.

“A straight line can readily be drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods.” Henrietta Swan Leavitt

A Cepheid variable belongs to a class of star called a pulsating variable. These stars expand and contract over a regular cycle, at the same time regularly varying in brightness. They are hottest and brightest shortly after reaching their most contracted phase. The graph of the star’s luminosity (light output) against time is called its light curve.

Building on Leavitt’s work

Although it is possible that Leavitt did not realize the full implications right away, she had discovered an extremely valuable tool for measuring distances in the universe, far beyond the limitations of parallax measurements. Cepheid variables were to become the first “standard candles”—a class of celestial objects that have a known luminosity, allowing them to be used as tools to measure vast distances in space.

One of the first people to appreciate the significance of Leavitt’s discovery was Danish astronomer Ejnar Hertzsprung. Due to the period−luminosity relationship discovered by Leavitt, Hertzsprung realized that by measuring the period of any Cepheid variable it should be possible to determine its luminosity and intrinsic brightness. Then by comparing its intrinsic brightness to its apparent magnitude (measured average brightness from Earth), it should be possible to calculate the distance to the Cepheid variable. In this way, it should also be possible to determine the distance to any object that contained one or more Cepheid variable star.

However, there was still a problem to be solved: although Leavitt had established the important period−luminosity relationship, initially all this promised was a system for measuring the distance to remote objects relative to the distance to the SMC. The reason for this is that Leavitt had no accurate information about the distance to the SMC, nor indeed any accurate data about the intrinsic brightness of any Cepheid variable.

“I should be willing to pay thirty cents an hour in view of the quality of your work, although our usual price, in such cases, is twenty five cents an hour.” Edward C. Pickering

Calibrating the variables

To turn Leavitt’s finding into a system that could be used to determine absolute distances, not just relative distances, it needed calibrating in some way. In order to do this, it would be necessary to measure accurately the distances to and intrinsic brightness of a few Cepheid variables. Hertzsprung therefore set about determining the distances to a handful of Cepheids in the Milky Way galaxy, using an alternative complex method called statistical parallax, which involves calculating the average movement of a set of stars assumed to be at a similar distance from the sun.

Having obtained the stars’ distances, it was a straightforward step to figure out the intrinsic brightness of each of the nearby Cepheids. Hertzsprung used these values to calibrate a scale, which allowed him to calculate the distance to the SMC and the intrinsic brightness of each of Leavitt’s Cepheids in the SMC. Following these calibrations, Hertzsprung was able to establish a system for determining the distance to any Cepheid variable from just two items of data—its period and its apparent magnitude.

“Leavitt left behind a legacy of a great astronomical discovery.” Solon I. Bailey

Further applications

It was not long before Leavitt’s findings, tuned by the work of Hertzsprung, led to further important results in terms of helping to understand the scale of the universe. From 1914 to 1918, the American astronomer Harlow Shapley (who was also the first person to show that Cepheid variables are pulsating stars) was one of the first to use the newly developed concept that the distances of variable stars could be found from knowing their periods and apparent brightness. Shapley found that objects called globular star clusters— all part of the Milky Way—were distributed roughly in a sphere whose center lay in the direction of the constellation of Sagittarius. He was able to conclude from this that the center of the Milky Way galaxy is at a considerable distance (tens of thousands of light-years) in the direction of Sagittarius and that the sun is not, as had previously been supposed, at the center of the galaxy. Shapley’s work, which led to the first realistic estimate of the true size of the Milky Way, was an important milestone in galactic astronomy.

Right up to the 1920s, many scientists (including Harlow Shapley) maintained that the Milky Way galaxy was the whole universe. Although there were those that believed otherwise, neither side could conclusively prove their argument one way or another. In 1923, however, the American astronomer Edwin Hubble, using the latest in telescopic technology, found a Cepheid variable in the Andromeda nebula, allowing its distance to be measured. This led directly to the confirmation that the Andromeda nebula is a separate large galaxy (and is now called the Andromeda galaxy) outside the Milky Way. Later, Cepheids were similarly used to show that the Milky Way is just one of a vast number of galaxies in the universe. The study of Cepheids was also employed by Hubble in his discovery of the relationship between the distance and recessional velocity of galaxies, leading to confirmation that the universe is expanding.

The star RS Puppis is one of the brightest Cepheid variables in the Milky Way. It is about 6,500 light-years from Earth and has a cycle of variability lasting 41.4 days

Revising the scale

In the 1940s, the German astronomer Walter Baade was working at the Mount Wilson Observatory in California. Baade made observations of the stars at the center of the Andromeda galaxy during the enhanced viewing conditions afforded by the wartime blackout. He distinguished two separate populations, or groups, of Cepheid variables that have different period– luminosity relationships. This led to a dramatic revision in the extragalactic distance scale—for example, the Andromeda galaxy was found to be double the distance from the Milky Way that Hubble had calculated. Baade announced his findings at the International Astronomical Union in 1952. The two groups of Cepheids became known as classical and Type II Cepheids, and started to be used for different purposes in distance measuring.

Today, classical Cepheids are used to measure the distance of galaxies out to about 100 million light-years—well beyond the local group of galaxies. Classical Cepheids have also been used to clarify many characteristics of the Milky Way galaxy, such as its local spiral structure and the sun’s distance from the plane of the galaxy. Type II Cepheids have been used to measure distances to the galactic center and globular clusters.

The measurement of distances to Cepheid variables for more accurate calibration of period–luminosity relationships is still considered extremely important, and it was one of the primary missions of the Hubble Space Telescope project when it was launched in 1990. A better calibration is crucial, among other things, to calculate the age of the universe. Leavitt’s findings from over a century ago are still having significant repercussions in terms of truly understanding the scale of the cosmos.

“Hubble’s underwhelming acknowledgment of Leavitt is an example of the ongoing denial and lack of professional and public recognition that she suffers from, despite her landmark discovery.” Pangratios Papacosta

A simplified version of the mechanisms that cause Cepheid variables to fluctuate in size is shown here. The pressure forces inside a star include gas pressure, maintained by heat output from the star’s core, and radiation pressure. Another mechanism that may be involved is a cyclical change in the opacity (resistance to the transmission of radiation) in gas within the star’s outer layers.

HENRIETTA SWAN LEAVITT

Henrietta Swan Leavitt developed an interest in astronomy while studying at Radcliffe College, Cambridge, Massachusetts. After graduation, she suffered a serious illness that caused her to become increasingly deaf for the rest of her life. From 1894 to 1896 and then again from 1902, she worked at Harvard College Observatory. Leavitt discovered more than 2,400 variable stars and four novae. In addition to her work on Cepheid variables, Leavitt also developed a standard of photographic measurements, now called the Harvard Standard.

Due to the prejudices of the day, Leavitt did not have the chance to use her intellect to the fullest, but she was described by a colleague as “possessing the best mind at the Observatory.” She was remembered as hardworking and serious-minded, “little given to frivolous pursuits.” Leavitt worked at the Observatory until her death from cancer in 1921.

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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.

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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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