End of the cosmic dark ages: a novel radio array may be first instrument to detect the elusive signal from the Epoch of Reionization.
LET'S FACE IT: it doesn't look too impressive at first sight. A casual tourist, strolling or cycling through the scenic landscape of the Dutch province of Drenthe, would hardly notice the central antenna fields of the Low Frequency Array (LOFAR). Unlike the 305-meter Arecibo dish, this novel radio observatory, inaugurated by Her Majesty Queen Beatrix on June 12, 2010, will never be featured in a James Bond movie. The array's inconspicuous omnidirectional antennas are arranged in clusters that are concentrated near the rural village of Exloo. But they are spread out all over northwestern Europe, and they lack high-tech sex appeal.
Still, LOFAR might soon give us our first glimpse of an important phase in the universe's very early history: the Epoch of Reionization.
The early universe went through a period of frigid gloom--the Dark Ages. During the subsequent Epoch of Reionization (EoR), high-energy radiation from the first cosmic beacons turned the murky clouds of neutral hydrogen and helium into the hot, ionized plasma that fills intergalactic space today. But no one knows exactly how and when this happened. LOFAR might soon provide the first hint of an answer. "It would be a major scientific result," says Steve Rawlings (Oxford University, England), "but it's also a big technological challenge." Adds LOFAR Director Mike Garrett of the Netherlands Institute for Radio Astronomy (ASTRON): "This is Nobel Prize stuff"."
Our universe was born in a hot Big Bang some 13.7 billion years ago. For hundreds of thousands of years it was filled with seething and glowing plasma--a gas consisting of positively charged hydrogen and helium nuclei and free negatively charged electrons. In other words, the normal baryonic matter in the expanding universe was fully ionized, because the electrons were not attached to the atomic nuclei. (The universe also contained huge amounts of mysterious, nonbaryonic dark matter.)
But 380,000 years after the Big Bang, temperatures had dropped to below 3,000 Kelvins, low enough for nuclei and electrons to combine into neutral atoms, which do not affect photons of light. Without free electrons to impede their progress, photons from the cosmic background radiation--the "afterglow" of the Big Bang--could propagate freely through space. The universe became transparent. From then on, the expanding universe only became emptier and darker. Moreover, it cooled down, eventually to just a few tens of degrees above absolute zero--cold enough for molecular hydrogen ([H.sub.2]) "snowflakes" to occasionally form. The universe plunged into the Dark Ages.
But just as Europe's uninspiring Middle Ages were overcome by the Renaissance (literally "rebirth"), the cosmic Dark Ages were supplanted by the Epoch of Reionization, where the prefix 're' acknowledges the fact that the universe had gone through an earlier ionized phase. Ever since, interstellar and intergalactic space has contained a hot plasma of nuclei and free electrons--just like in the early days, albeit much more tenuous. Neutral atoms can now be found only in dense, molecular clouds, where they can't be ripped apart by high-energy photons.
Analyses of the cosmic microwave background, which is slightly affected by free electrons, show that the Dark Ages lasted at least 400 million years. And observations of absorption lines in the spectra of very remote quasars indicate that reionization was almost complete when the universe was 1 billion years old. In other words, the EoR occurred somewhere between 13.3 and 12.7 billion years ago, corresponding to redshifts between 12 and 6.
According to Rawlings, theorists have known for a long time that the universe emerged from a dark, neutral phase. But only since the mid-1990s did the EoR start to catch the attention of observational astronomers, as the universe's early youth slowly came within reach of new instruments and technologies. Finally, it looked like astronomical observations might solve the riddle of what exactly reionized the universe. High-energy photons certainly did the job, but what was their source? When did the EoR start, how long did it last, and how did it proceed?
ASTRON astronomer Ger de Bruyn, who heads LOFAR's EoR key science project, began preliminary observations this past winter. "In the late 1990s, we first discussed the possibility of detecting the signal," he says. If the EoR occurred when the universe was just 100 million years old, as some theorists suggested a decade ago, LOFAR would never be able to succeed. But, says de Bruyn, there's now enough circumstantial evidence that the Dark Ages lasted some 800 million years. "Later this year we should have a good idea of our chances of success."
During the Dark Ages, gravity started to draw clumps of dark matter together, establishing the skeleton of the universe's large-scale structure. Neutral hydrogen and helium atoms were pulled along with the dark matter, and accumulated in the highest-density regions. But the details of this primordial gravitational clumping are poorly known, and we don't know which came first, individual stars (probably extremely hot, massive, and luminous) or giant black holes that would later act as the seeds for the formation of full-blown galaxies.
Both stars and black holes produce high-energy photons. Hot, luminous stars emit prodigious amounts of ultraviolet radiation. Matter falls toward black holes, which accumulates in whirling accretion disks that become hot enough to produce X rays (see page 20). No
LOFAR FIRST LIGHT In January 2011, the entire LOFAR network, with the newly completed station in Hampshire, England, took this "first light" image of the quasar 3C 196 (arrowed). With the new station, the array is nearly 1,000 kilometers wide, making it one of the largest telescope networks in the world. one knows what kind of objects first started to shine brilliantly, but one thing's for sure: after hundreds of millions of years, the universe's lights were switched on. And slowly but surely the neutral hydrogen and helium gas surrounding these first beacons became ionized again.
According to computer models, the universe's reionization did not occur in one fell swoop at one particular moment, but in fits and starts, slowly at first and then accelerating, more or less like water in a heated pan as it starts to boil. The EoR probably lasted hundreds of millions of years. Most likely, both giant black holes and individual stars contributed to the process, at different places and different times, and by uncovering the details of the reionization, cosmologists hope to shed more light on the universe's early clumping history.
Whatever the true nature of the first sources of radiation, theorists think they were surrounded by expanding bubbles of ionized matter. And because of the difference in energy output, the bubbles surrounding black holes grew faster and become larger than the bubbles surrounding individual stars. Eventually, ionized bubbles started to overlap, and in the end, the entire universe was reionized. Computer simulations support this simple view, although the details are a bit messier because of environmental effects. For example, stars and black holes are born in high-density regions, and the interiors of dense clouds of cool, neutral gas are difficult to ionize.
But in general, the pattern in which the ionized bubbles grew with time will tell astronomers what caused them in the first place. In other words, charting the universe's reionization history answers the question of which came first--stars or black holes. And that's where low-frequency radio telescopes such as LOFAR come in. Before neutral hydrogen gets ionized, it emits characteristic radio waves at a wavelength of 21.1 centimeters (a frequency of 1420.4 megahertz). By the time these waves reach Earth--some 13 billion years later--cosmic expansion has redshifted these waves to longer wavelengths and lower frequencies (see "Tuning into the Past" on the left).
By observing at different wavelengths, LOFAR will pick up the hydrogen emission from different epochs in the universe's early history. At the very longest wavelengths (the very lowest frequencies), astronomers look back to an era before reionization set in, and the hydrogen signal from this epoch can't be observed, mainly because such a weak, "smooth" signal can't be extracted from the contribution of foreground sources. But at slightly shorter wavelengths (higher frequencies), LOFAR looks back to a time where the first bubbles began to appear, and the radio sky should attain a "Swiss cheese" appearance, which makes it detectable for an interferometer such as LOFAR. At even shorter wavelengths and higher frequencies (but still very much in the low-frequency range), the ionized bubbles should be larger, and the radio signal should start to diminish.
So by tuning the frequency dial, LOFAR effectively carries out tomography on the early universe. Given the array's very large field of view, such observations should, in principle, reveal the chronology of the EoR, thus answering fundamental questions about the very earliest stages of the universe's history. But de Bruyn cautions, "The signal is incredibly weak. Only a giant future radio observatory such as the SKA (Square Kilometre Array) will be able to produce nice maps and images of this epoch." Still, the statistical evidence that LOFAR may provide for structure in the universe from hydrogen emission at various redshifts and lookback times will be very important for theorists.
Both de Bruyn and Rawlings, who heads the United Kingdom involvement in LOFAR and who chairs the SKA's Science Advisory Committee, point to similarities between the current hunt for the EoR signal and the hunt for structure in the cosmic microwave background (CMB). "The LOFAR observations might be comparable to the initial discovery of the CMB in the 1960s and the detection of the ripples by NASA's Cosmic Background Explorer (COBE) satellite," says de Bruyn. According to Rawlings, "A first detection by LOFAR would tell us what kind of instruments we need to build for better imaging." Thus, SKA would be to the EoR what the WMAP and Planck satellites--the successors of COBE--are to the CMB.
And just like measuring minuscule temperature
fluctuations in the CMB, it won't be easy to detect the EoR signal. In fact, the faint, redshifted radiation of primordial neutral hydrogen will be swamped by much stronger foreground emissions, both inside and outside our Milky Way Galaxy. These foreground signals--mainly synchrotron radiation from electrons spiraling along magnetic field lines--can be filtered away because they show a different dependence on frequency. "Still," says de Bruyn, "this is probably the most difficult project ever undertaken in radio astronomy."
Five years from now, new observatories such as the Atacama Large Millimeter Array (ALMA) in northern Chile and NASA's James Webb Space Telescope may actually reveal the very first generation of stars, black holes, and galaxies in the universe. Although these observatories have a very small field of view compared to radio observatories such as the future Square Kilometre Array, they could provide important supporting evidence for the preliminary findings on the Epoch of Reionization that LOFAR may provide. Says de Bruyn: "I remain confident that we will succeed."
TUNING INTO THE PAST
Neutral hydrogen emits characteristic radio waves at a frequency of 1420.4 megahertz, which corresponds to a wavelength of 21.1 centimeters. But hydrogen in the early universe had its radio waves stretched out by cosmic expansion during the long journey to Earth. As a result, we observe the hydrogen signal at a longer wavelength and a correspondingly lower frequency. Because of the light travel time, astronomers look back into the universe's past by observing these redshifted signals. For example, the emission of primordial neutral hydrogen at a redshift of 7 is observed at a wavelength of about 1.7 meters (corresponding to a frequency of 178 MHz), and at this redshift, astronomers are observing the universe as it was 780 million years after the Big Bang.
LOFAR and Its Competitors
LOFAR is a novel radio telescope under construction in northwestern Europe. Eventually, it will consist of many thousands of small, simple antennas of two types, for two frequency ranges. The unmovable, omindirectional antennas are clustered in 44 stations, half of which lie in the core region, close to the village of Exloo in the northern part of the Netherlands. Others are spread out over the Netherlands, with outlying stations in Germany, France, the United Kingdom, and Sweden (see the map on page 27). A fast fiberoptic network connects all antenna stations to a powerful central supercomputer in Groningen, the Netherlands.
Unlike traditional radio telescopes, the LOFAR antennas cannot be aimed at a particular point in the sky. They pick up low-frequency radiation from all directions simultaneously. "Pointing" the telescope is done by sophisticated software that takes into account tiny differences in photon arrival time for the individual antennas. As a result, LOFAR can "look" in as many as eight different directions simultaneously. Even though the observatory is not yet completed, it has already obtained promising observations of radio galaxies, pulsars, cosmicray showers, and solar flares.
LOFAR is not the only radio observatory trying to detect signals from the Epoch of Reionization. In western India, the Giant Metrewave Radio Telescope (GMRT) is also in the race to observe the faint signal. In western Australia, the Murchison Widefield Array (MWA) is trying to do the same. Like LOFAR, MWA is a precursor instrument for the future Square Kilometre Array (SKA), a giant network of radio antennas that will be built by an international consortium either in western Australia or in southern Africa.
JOINING THE HUNT Several other radio arrays will join the hunt to detect radio signals from the Epoch of Reionization. Left: This 45-meter antenna is part of the Giant Metrewave Radio Telescope, near Pune, India. The 30-telescope array can detect frequencies from about 50 to 1420 megahertz. Above: The small white structures are dipole antennas belonging to the Murchison Widefield Array in Western Australia, which is still under construction. The MWA picks up signals from 80 to 300 MHz. Right: An artist depicts the future Square Kilometre Array. Construction of this network of radio antennas is scheduled to begin in 2016 in Western Australia or southern Africa. SKA should be able to study the EoR signal in considerable detail.
Like tens of millions of other Europeans, S&T contributing editor Govert Schilling lives inside the LOFAR telescope, some 140 kilometers southwest of the array's core region.
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|Title Annotation:||Phase Transition in Early Universe|
|Publication:||Sky & Telescope|
|Date:||May 1, 2011|
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