Why do Galaxies in the Young Universe Appear so Mature?
October 8, 2004
Notice to Journalists: Media Embargo Until January 5th, 2004, 9:20am EDT/4:20am HST
- Peter Michaud
- Roberto Abraham
University of Toronto
Until now, astronomers have been nearly blind when looking back in time to survey an era when most stars in the Universe were expected to have formed. This critical cosmological blind-spot has been removed by a team using the Frederick C. Gillett Gemini North Telescope, showing that many galaxies in the young Universe are not behaving as expected some 8-11 billion years ago.
The surprise: these galaxies appear to be more fully formed and mature than expected at this early stage in the evolution of the Universe. This finding is similar to a teacher walking into a classroom expecting to greet a room full of unruly teenagers and finding well-groomed young adults.
"Theory tells us that this epoch should be dominated by little galaxies crashing together," said Dr. Roberto Abraham (University of Toronto) who is a Co-Principal Investigator of the team conducting the observations at Gemini. "We are seeing that a large fraction of the stars in the Universe are already in place when the Universe was quite young, which should not be the case. This glimpse back in time shows pretty clearly that we need to re-think what happened during this early epoch in galactic evolution. The theoreticians will definitely have something to gnaw on!"
The results were announced today at the 203rd meeting of the American Astronomical Society in Atlanta, Georgia. The data will soon be released to the entire astronomical community for further analysis, and three papers have been submitted for publication in Nature, The Astrophysical Journal, and The Astronomical Journal.
These observations are from a multinational investigation, called the Gemini Deep Deep Survey (GDDS), which used a special technique to capture the faintest galactic light ever dissected into the rainbow of colors called a spectrum. In all, spectra from over 300 galaxies were collected, most of which are within what is called the "Redshift Desert," a relatively unexplored period of the Universe seen by telescopes looking back to an era when the universe was only 3-6 billion years old. These spectra represent the most complete sample ever obtained of galaxies in the Redshift Desert. By obtaining large amounts of data from four widely separated fields, this survey provides the statistical basis for drawing conclusions that have been suspected by past observations done by the Hubble Space Telescope, Keck Observatory, Subaru Telescope and the Very Large Telescope over the past decade.
Studying the faint galaxies at this epoch when the Universe was only 20-40% of its current age presents a daunting challenge to astronomers, even when using the light-gathering capacity of a very large telescope like Gemini North with its 8-meter mirror. All previous galaxy surveys in this realm have focused on galaxies where intense star formation is occurring, which makes it easier to obtain spectra but produces a biased sample. The GDDS was able to select a more representative sample including those galaxies which hold the most stars–normal, dimmer, and more massive galaxies–that demand special techniques to coax a spectrum from their dim light.
"The Gemini data is the most comprehensive survey ever done covering the bulk of the galaxies that represent conditions in the early Universe. These are the massive galaxies that are actually more difficult to study because of their lack of energetic light from star formation. These highly developed galaxies, whose star-forming youth is in fact long gone, just shouldn't be there, but are," said Co-Principal Investigator Dr. Karl Glazebrook (Johns Hopkins University).
Astronomers trying to understand this issue might have to put everything on the table. "It is unclear if we need to tweak the existing models or develop a new one in order to understand this finding," said the survey's third Co-Principal Investigator, Dr. Patrick McCarthy (Observatories of the Carnegie Institution). "It is quite obvious from the Gemini spectra that these are indeed very mature galaxies, and we are not seeing the effects of obscuring dust. Obviously there are some major aspects about the early lives of galaxies that we just don't understand. It is even possible that black holes might have been much more ubiquitous than we thought in the early Universe and played a larger role in seeding early galaxy formation."
What is arguably the dominant galactic evolution theory postulates that the population of galaxies at this early stage should have been dominated by evolutionary building blocks. Aptly called the Hierarchical Model, it predicts that normal to large galaxies, like those studied in this work, would not yet exist and would instead be forming from local beehives of activity where big galaxies grew. The GDDS reveals that this might not be the case.
The spectra from this survey were also used to determine the pollution of the interstellar gas by heavy elements (called "metals") produced by stars. This is a key indicator of the history of stellar evolution in galaxies. Sandra Savaglio (Johns Hopkins University), who studied this aspect of the research said, "Our interpretation of the Universe is strongly affected by the way we observe it. Because the GDDS observed very faint galaxies, we could detect the interstellar gas even if partly obscured by the presence of dust. Studying the chemical composition of the interstellar gas, we discovered that the galaxies in our survey are more metal-rich than expected."
Caltech astronomer, Dr. Richard Ellis commented, "The Gemini Deep Deep Survey represents a very significant achievement, both technically and scientifically. The survey has provided a new and valuable census of galaxies during a key period in cosmic history, one that has been difficult to study until now, particularly for the quiescent component of the galaxy population."
Making observations in the Redshift Desert has frustrated modern astronomers for the last decade. While astronomers have known that plenty of galaxies must exist in the Redshift Desert, it is only a "desert" because we couldn't get good spectra from many of them. The problem lies in the fact that key spectroscopic features used to study these galaxies have been redshifted–due to the expansion of the Universe–into a part of the optical spectrum that corresponds to a faint, natural, obscuring glow in the Earth's nighttime atmosphere.
To overcome this problem, a sophisticated technique called "Nod and Shuffle" was used on the Gemini telescope. "The Nod and Shuffle technique enables us to skim off the faint natural glow of the night sky to reveal the tenuous spectra of galaxies beneath it. These galaxies are over 300 times fainter than this sky glow," explains Dr. Kathy Roth, an astronomer at Gemini who was also part of the team and obtained much of the data. "It has proven to be an extremely effective way to radically reduce the "noise" or contamination levels that are found in the signal from an electronic light detector."
Each observation lasted the equivalent of about 30 hours and produced nearly 100 spectra simultaneously. The entire project required over 120 total hours of telescope time. "This is a lot of valuable time on the sky, but when you consider that it has allowed us to help fill in a crucial 20% gap in our understanding of the Universe, it was time well spent," adds Dr. Glazebrook who developed the use of Nod and Shuffle with Joss Hawthorn for faint galaxy observations while at the Anglo-Australian Observatory a few years ago. A more complete history and explanation of the technique, including its original development in the mid 90's can be found on the Nod and Shuffle background page.
Previous studies in the Redshift Desert have concentrated on galaxies that were not necessarily representative of mainstream systems. For this study, galaxies were carefully selected based upon data from the Las Campanas Infrared Survey in order to assure that strong ultraviolet emitting starburst galaxies were not oversampled. "This study is unique in that we were able to study the red end of the spectrum, and this tells us about the ages of old stars," says Dr. Abraham. "We undertook incredibly long observations with Gemini–about ten times as long as typical exposures. This let us look at much fainter galaxies than is usually the case, and let us focus on the bulk of the stars, instead of just the flashy young ones. This makes it a lot easier for us to work out how the galaxies are evolving. We are no longer guessing at it by studying young objects and assuming the old objects were not contributing much to the story of galaxy evolution. It turns out that there are lots of old galaxies out there, but they're really hard to find."
The GDDS was supported by a grant from the Packard Foundation and institutional support from the United State's National Science Foundation, Canada's National Research Council and the Natural Sciences and Engineering Research Council of Canada, the United Kingdom's Particle Physics and Research Council and the GDDS team-member institutions consisting of:
Karl Glazebrook & Sandra Savaglio Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD
Patrick J. McCarthy Observatories of the Carnegie Institution of Washington, Pasadena, CA
David Crampton & Richard Murowinski Herzberg Institute of Astrophysics, National Research Council, Victoria, British Columbia, Canada
Inger Jørgensen & Kathy Roth Gemini Observatory, Hilo, HI
Isobel M. Hook Department of Astrophysics, Nuclear & Astrophysics Laboratory, Oxford University, England
Hsiao-Wen Chen Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA
Ronald O. Marzke Dept. of Physics and Astronomy, San Francisco State University, San Francisco, CA
The Gemini Multi-Object Spectrograph used on the Frederick C. Gillett Gemini Telescope on Mauna Kea to make the GDDS observations is one of two identical instruments, which are used on both Gemini telescopes. The GMOS instruments were built as a joint partnership between Gemini, the National Research Council of Canada's Herzberg Institute of Astrophysics, the UK Astronomy Technology Centre and Durham University, UK. Separately, the U.S. National Optical Astronomy Observatory provided the detector subsystem and related software. GMOS is primarily designed for spectroscopic studies where several hundred simultaneous spectra are required, such as when observing star and galaxy clusters. GMOS also has the ability to focus astronomical images on its array of over 28 million pixels.
Aviso para los periodistas: Embargo hasta el 5 de enero, 2004, 9:20 a.m EDT
Hasta ahora, los astrónomos han estado casi a ciegas cuando retroceden en el tiempo para censar una era en la cual se suponía que la mayoría de las estrellas del Universo habían sido formadas. Este crítico punto ciego cosmológico ha sido resuelto por un equipo, que utilizando el Telescopio Frederick C. Gillett de Gemini Norte, pudo comprobar que las galaxias en el universo joven no se estarían comportando como se esperaba desde hace 8-11 mil millones de años.
¡ Qué sorpresa !. Aparentemente estas galaxias estaban mayormente formadas en su totalidad y eran más masivas de lo estimado en esta temprana época de la evolución del universo. Este descubrimiento es similar al de un profesor que entra en una sala esperando saludar a un montón de adolescentes inquietos y encuentra a un grupo de bien presentados y más bien corpulentos adultos jóvenes.
Según el Dr. Roberto Abraham de la Universidad de Toronto, co-investigador del equipo en las observaciones en Gemini : "La teoría nos dice que esta época debiera estar dominada por pequeñas galaxias que chocan entre sí. Sin embargo estamos viendo que una fracción importante de estrellas estaban ya en su lugar cuando el universo era bastante joven, lo cual no debiera ser. Esta mirada hacia atrás en el tiempo muestra claramente que necesitamos replantearnos sobre lo que ocurrió durante esta temprana era en la evolución de las galaxias. ¡Definitivamente, los teóricos tendrán ahora algo para roer! ".
Los resultados fueron anunciados hoy en la reunión 203 de la
Sociedad Astronómica Estadounidense en Atlanta,
Georgia. Los datos obtenidos serán pronto puestos a
disposición de toda la comunidad astronómica para futuros
análisis y 4 artículos científicos se encuentran
preparados para su publicación en el Astrophysical y
Estas observaciones provienen de una investigación multinacional, denominada el Gemini Deep Deep Survey (GDDS), quienes utilizaron una técnica conocida , llamada espectro, la cual captura la luz proveniente de las galaxias más tenues que se haya analizado en el arcoiris de colores. En total, se recogieron espectros de más de 300 galaxias, representando así la muestra más completa que alguna vez se haya obtenido de las galaxias de cuando el Universo tenía sólo un 20-40% de su edad actual. El hecho de obtener grandes cantidades de datos de 4 campos suficientemente separados permite a este muestreo proveer la estadística básica para obtener conclusiones sobre lo que sólo se sospechaba a través de observaciones aisladas durante la década pasada.
"A traves de los datos provenientes de Gemini tenemos uno de los muestreos (survey) más completos que se haya hecho cubriendo la mayor parte de las galaxias que representan condiciones en el universo temprano. Estas son las galaxias masivas, las cuales, de hecho, son más difíciles de estudiar por su falta de energía luminosa proveniente de la formación estelar. Estas galaxias altamente desarrolladas, cuya juventud de formación estelar se ha acabado hace mucho tiempo, simplemente no deberían estar allí, pero están ", señaló el Dr. Karl Glazebrook (Investigador co-principal) de la Universidad Johns Hopkins.
Los astrónomos que tratan de comprender este tema quizas
deberán poner todo sobre la mesa. "No es claro si
necesitamos ajustar los modelos existentes o desarrollar uno nuevo para
poder entender este descubrimiento," agregó el tercer
investigador co-principal, el Dr. Patrick McCarthy de la
Institución de Observatorios Carnegie. " Del
espectro obtenido en Gemini, resulta más bien obvio que
éstas son galaxias muy maduras, y que no estamos viendo los
efectos debido a un posible polvo oscurecedor. Obviamente existen
otros aspectos importantes acerca de la vida temprana de las galaxias
que nosotros simplemente no entendemos. Incluso es posible, que
hayan habido muchos más agujeros negros de lo que nosotros
pensamos en el universo joven y éstos hayan jugado un rol
más importante posibilitando así la formación de
Conocida como el "Desierto de corrimientos al rojo" (Redshift Desert) la región del espacio vista por telescopios que miran hacia el pasado, en la era cuando el universo tenía 3-6 billones de años, es vastamente inexplorada. El estudiar las galaxias tenues de esta época presenta un duro desafío para los astrónomos, incluso si se utilizan telescopios grandes como el de Gemini Norte con un espejo de 8 metros. Todos los muestreos de galaxias anteriores en esta región se han focalizado donde está ocurriendo una formación estelar intensa, lo cual facilita la obtención de espectros (ya que son más brillantes) pero produce una muestra totalmente sesgada. El GDDS fue capaz de seleccionar una muestra más representativa que incluye la mayoría galáctica - galaxias normales, débiles, y galaxias más masivas - las cuales demandan técnicas especiales para obtener un espectro de luz débil.
La teoría de evolución de galaxias más aceptada
actualmente postula que la población de galaxias en esa etapa
temprana era dominada por bloques de construcción evolutivos.
Apropiadamente llamada el Modelo Jerárquico, este predice que
las galaxias normales a grandes, como las estudiadas en este trabajo,
no habrían existido todavía y en cambio estarían
creciendo como panales de abejas locales.
Los espectros de este estudio fueron a su vez utilizados para determinar la contaminación del gas interestelar debida a elementos pesados (llamados 'metales') producidos por las estrellas. Este es un indicador clave de la historia de la evolución estelar en las galaxias. Sandra Savaglio de la Universidad Johns Hopkins quien estudió este aspecto de la investigación señaló, "Nuestra interpretación del Universo es altamente afectada por la forma en cómo lo observamos. Ya que el GDDS observó galaxias muy débiles, nosotros pudimos detectar el gas interestelar incluso si se oscurecía parcialmente por la presencia de polvo. Al estudiar la composición química del gas interestelar, pudimos descubrir que las galaxias en nuestro censo son más ricas en metales de lo que esperábamos.¡ Precisamente de esto es lo que trata la ciencia, haciendo que las largas horas de análisis de información hayan valido el esfuerzo! "
El no poder hacer observaciones en el Desierto de corrimientos al rojo ha frustrado a los astrónomos modernos durante la ultima década, y sabiendo que muchas galaxias deben existir en él, y que es solo un "desierto" porque no podíamos obtener buenos y fiables espectros de ellos. El problema recae en que las características claves espectroscópicas utilizadas para estudiar las galaxias han sido desplazadas - debido a la expansión del universo - hacia una parte del espectro óptico que corresponde a un pálido, natural y obscurecedor destello en la atmósfera nocturna de la Tierra.
Para solucionar este problemase utilizó, una sofisticada técnica denominada "Nod and Shuffle" en los telescopios Gemini. "Esta técnica de Nod and Shuffle nos permite eliminar el destello naturalmente pálido del cielo nocturno para revelar los espectros tenues de galaxias bajo éste, estas galaxias son hasta 300 veces menos brillantes que el cielo," explica la Dra. Kathy Roth, astrónoma de Gemini quien también integró el equipo y observó gran parte de los datos. " Se ha comprobado que es una manera extremadamente efectiva para reducir radicalmente el ruido o los niveles de contaminación que son encontrados en la señal de un detector de luz electrónico." Cada observación duró el equivalente a 30 horas y produjo cerca de 100 espectros simultáneamente. El proyecto entero requirió cerca de 120 horas de telescopio en total. "Esto significa utilizar mucho tiempo valioso de telescopio, pero si se considera que nos ha permitido completar un vacío que equivale al 20% en nuestro entendimiento del universo, entonces es un tiempo muy bien utilizado," agregó el Dr. Glazebrook quien desarrolló el uso del Nod & Shuffle para observaciones de galaxias débiles cuando trabajaba en el Observatorio Anglo-Australiano hace un par de años. Para una mejor y mas completa explicación más completa de esta técnica, incluyendo su desarrollo original en la mitad de los 90 a cargo de Bernard Fort y Jean-Charles Cuillandre, se puede buscar en: www.gemini.edu?
Estudios anteriores en el Desierto de corrimientos al rojo se han concentrado en
galaxias que no necesariamente representaban la totalidad de los
sistemas. Para este estudio, las galaxias fueron cuidadosamente
seleccionadas basadas en informaciones del Censo Infrarojo del
observatorio de Las Campanas para poder asegurar que las galaxias de
brotes estelares que son fuertes emisores en el UV no estuviesen
sobremuestreadas. "Este estudio es único ya que podemos estudiar
el extremo rojo del espectro, lo cual nos permite saber acerca de las
edades de las estrellas viejas", dice el Dr. Abraham. "Nosotros
tomamos exposiciones extremadamente largas con Gemini - alrededor de
diez veces más tiempo que las exposiciones tradicionales. Esto
nos permitió observar muchas galaxias más
débiles de lo normal y a su vez enfocarnos en todas las
estrellas y no sólo en las mas jóvenes y vistosas.
Esto nos hace más fácil poder entender cómo van
evolucionando las galaxias. Ya no estamos adivinándolo con
sólo estudiar los objetos jóvenes y asumir que los
más viejos no contribuían mucho en la historia de la
evolución de las galaxias. Al final, resulta que hay muchas
galaxias viejas allá afuera, pero son muy difíciles de
Los Espectrógrafos Multi Objeto de Gemini (GMOS) que han sido usados para hacer estas observaciones son instrumentos gemelos construidos gracias a una asociación conjunta entre Gemini, el Instituto de AstrofÌsica Herzberg dependiente del Consejo de Investigación Nacional de Canadá (NRC), el Centro Tecnológico de Astronomía y la Universidad de Durham en el Reino Unido. Independientemente, el Observatorio Nacional de Astronomía Optica de Estados Unidos brindó el detector y el software correspondiente. GMOS está diseñado prioritariamente para estudios espectroscopía donde se requieren varios cientos de espectros simultáneos, como cuando se observan cúmulos de estrellas y de galaxias. GMOS también tiene la habilidad de tomar imágenes astronómicas en su detector de más de 28 millones de pixeles.
Observatorio Internacional Gemini es una colaboración
multinacional que ha construido dos telescopios de 8 metros
idénticos en Mauna Kea, Hawai`i (Gemini Norte) y Cerro
Pachón en Chile (Gemini Sur) que están abiertos para la
comunidad mundial de astrónomos. Ambos telescopios incorporan
nuevas tecnologías que permiten que espejos grandes y
relativamente delgados recojan y enfoquen tanto la radiación
óptica como la infrarroja proveniente del espacio.
Observatorio Gemini es dirigido por la Asociación de
Universidades para la Investigación en Astronomía (AURA)
bajo un acuerdo de cooperación con la Fundación Nacional
de la Ciencia de Estados Unidos (NSF). La NSF también participa
como agencia ejecutiva para la asociación internacional.
Las otras agencias de investigación de la asociación de Gemini incluyen: el Consejo de Investigación en Astronomía y Física del Reino Unido (PPARC), el Consejo de Investigación Nacional de Canada (NRC), la Comisión Nacional de Investigación Científica y Tecnológica de Chile (CONICYT), el Consejo de Investigación de Australia (ARC), el Consejo Nacional Argentino de Investigaciones Científicas y Técnicas (CONICET) y el Conselho Nacional de Brasil de Pesquisas Científicas e Tecnológicas (CNPq).
This timeline shows some pivotal events in the history of the Universe which is assumed to have begun with the Big Bang some 13.7 billion years ago (background article on age of the Universe here). The “Redshift Desert” is a region where the light from galaxies has been redshifted (stretched by the expansion of the Universe) into a region of the spectrum where a natural glow in the Earth's atmosphere interferes with key spectroscopic features of many of these galaxies. This interference is especially problematic when trying to study dimmer galaxies in the early Universe. Using a sophisticated observing technique that overcomes this problem, the Gemini Deep Deep Survey revealed that a large number of galaxies from this period of cosmic history were fully formed and more massive than the widely accepted Hierarchical Model of galaxy formation predicts.
This timeline also illustrates the concept of “look-back time,” which is what happens when astronomers look at more and more distant objects in space. Because light travels at a finite speed (about 300,000 km/s or 186,000 miles/s), it takes time for the light to reach our telescopes to be studied. This results in a cosmic “time-machine” because the light that we see from distant galaxies has traveled for billions of years. Thus, we see the galaxies as they were long ago when that light began its journey to our telescopes.
Nearby Galaxies Illustrate the Power of the Gemini Deep Deep SurveyThis Canada-France-Hawaii Telescope image shows a small section of the nearby Virgo cluster of galaxies dominated by two giant elliptical galaxies on the left side of the image. The Gemini Deep Deep Survey (GDDS) studied much more distant galaxies than those shown in this image. However, the nearby elliptical galaxies in this image are thought to be older and even more massive, yet similar to some of the larger distant galaxies studied in the GDDS.
Note the bright, smaller and bluer spiral galaxies on the right (center and bottom). These nearby spiral galaxies have active star formation occurring that makes them appear brighter and bluer. The limited number of galaxies observed spectroscopically in the redshift desert prior to the GDDS were mostly of this type. The GDDS allowed astronomers to thoroughly survey the more massive, redder yet dimmer galaxies as well.
Light from galaxies in the nearby Virgo cluster has traveled for approximately 45 million years, whereas light from the galaxies studied in the GDDS has been traveling between 8-11 billion years to reach us.
Credit: “Canada-France-Hawaii Telescope/J.-C. Cuillandre/Coelum”
Why are some galaxies brighter than others?
Bright galaxies where stars are forming may outshine dimmer, more massive galaxies. In all previous surveys of the “Redshift Desert,” the brighter star-forming galaxies were the only type bright enough to be sampled. The Gemini Deep Deep Survey was able to sample the dimmer, more massive galaxies and provide a better representation of the galaxies at this epoch of the Universe. The GDDS results revealed that there is a greater abundance of these more massive and older galaxies than expected from current models when the Universe was only 20-40% of its current age.
Gemini Illustration by Jon Lomberg
What do apartment buildings and distant galaxies have in common?Observing older, massive galaxies in the “Redshift Desert” is similar to trying to determine the number of residents in an apartment building by counting the number of lit windows. In past surveys of this period of our Universe, mainly the brighter galaxies where stars are forming were bright enough to be studied. Prior to the Gemini Deep Deep Survey (GDDS), the more massive, older and fainter galaxies were not well represented in surveys of this epoch in the Universe––like the dark windows in the buildings above. The Gemini spectra from the GDDS allows us to study these dimmer galaxies and understand their properties such as mass, age and heavy element abundances.
Gemini Illustration by Jon Lomberg
The Hierarchical TheoryArguably, the most widely accepted theory of galactic formation says that galaxies formed from “collisions” of smaller structures, which then evolved over the past 8-11 billion years into the galaxies we see today. This is called the “Hierarchical Theory”. The Gemini Deep Deep Survey brings into question key predictions of the Hierarchical Theory.
Gemini Illustration by Jon Lomberg
Gemini North – Cloaked DomeThis “invisible dome” image of the Gemini North telescope was made by digitally “stacking” images of the open Gemini dome as it rotated. Approximately 40 individual 5-second exposures were stacked to produce the transparent dome effect, and one additional oneminute exposure was obtained of the sky following the dome rotation. Technical Notes: All images used a Nikon D1X camera with a 14mm f/2.8 Nikkor lens at an ISO setting of 800. Additional lighting was used inside the dome to illuminate the telescope and inside of the dome. (Note to photo editor: This image is also available in a landscape crop, but the sky is much less striking due to lack of bright Milky Way.)
Gemini Observatory Photo by Peter Michaud & Kirk Pu`uohau-Pummill
Gemini North Interior by MoonlightA one-minute exposure of the interior of the Gemini North telescope by moonlight showing the 7story high telescope structure as well as the large wind vents for keeping the air inside the dome thermally stable.
Gemini Observatory Photo by Peter Michaud
Gemini North with Southern Star-trailsApproximately 2 hours of stacked exposures of the summer sky over Gemini North. The setting moon provided light on right of dome and twilight provides a glow to the left side of dome, a small red light provides highlight on center of dome. . A star field has been offset by about 30 minutes to show individual stars separated from trails revealing Scorpius and Sagittarius over the Gemini dome. Technical Data: Each exposure was 50 seconds using a Nikon D1X camera and a 14mm f/2.8 Nikkor lens at an ISO setting of 800 and “stacked” in Photoshop to create the single image from the over 100 individual images.
Gemini Observatory Photo by Peter Michaud & Kirk Pu`uohau-Pummill
Making a statement about the age of the Universe implies the fundamental assumption that the Universe had a beginning.
Although we can easily determine that most constituents of our visible Universe, such as the islands of Hawai‘i, our Sun, the Earth, the Moon and the stars that we see at night, had a beginning, it is much more difficult to determine if the ensemble of all those things had a starting point in the past. One could imagine that the Universe itself has always been there, but that all of its components––the atoms, the planets and the stars––go through a continuous cycle of birth, life and death.
Until recently, this view of an eternal universe has dominated the history of philosophy. Today, we talk much about the Big Bang theory, which is a very solid and consistent physical theory of the origin and the evolution of the universe as we now observe it. We forget that for most of human history, an eternal universe was accepted as the most economical way of explaining the Cosmos.
Even today there is an independent physical theory of the Universe called the "Steady State Universe." This theory, supported by a small minority of astronomers, defends a model of an eternal universe where the well-known expansion of the Universe is countered by the continuous creation of one hydrogen atom per 10,000 cubic meters per year (this is a very small amount of matter) to maintain its average density constant. Hence the Universe should appear the same in all directions (this is isotropy) and at all distances and epochs (this is homogeneity).
The idea that the Universe had a beginning is relatively new and quite revolutionary.
Drilling into the History of the Universe
Admitting a beginning is philosophically awkward, and the nagging question of what was before naturally arises. What the modern physicist or astronomer has to accept is that the concept of a beginning, such as the Big Bang, also acknowledges the impossibility of looking further back in time than this point using the laws of physics. The intellectual positions of present day scientists and philosophers are more modest than that of Dr. John Lightfoot (1601-1675), Vice-Chancellor of the University of Cambridge, who declared that heaven, Earth and man were created on October 23, 4004 BC, at nine o’clock in the morning!
Did our Universe have a beginning? If so, how old is it? How do we determine its age? These questions are among the most difficult and also the most exciting addressed by modern astrophysics. The question of the Universe's age is a basic driver of contemporary cosmology. This quest has motivated the funding of many sophisticated ground-based and space experiments and is mobilizing the efforts of hundreds of scientists and engineers.
The cosmological questions are not new, but several millennia of exploration and the modern tools of physics and astronomy allow us a new perspective, and hopefully provide some credible answers. We can now give outstandingly precise answers to questions about the age of many of the Universe's components, such as the ages of cosmic bodies like the Earth and the Sun and their chemical constituents. It follows that the Universe itself should be at least older that any of its oldest components.
Light travels at about 300,000 kilometers per second in the vacuum of space. The powerful telescopes on Mauna Kea allow us to observe galaxies at distances of billions of light-years. This means that we capture particles (photons) of light that were emitted long ago and have traveled through expanding space eventually hitting the detectors mounted on our telescopes. These momentous tiny collisions end an incredibly long journey, but give us important clues on the Universe afar.
Thus, looking at distant galaxies is like drilling into the past and seeing the ancient history of the Universe.
A critical question that astronomers ask when determining the age of the Universe is how nearby galaxies compare to far-off galaxies, which we are seeing as they were at a time closer to their suspected birth. If the Universe began 13.7 billion years ago, as the latest data suggest, should galaxies at 6, 8 or even 10 billion light-years away look different? Theoretically, the answer is yes. Hubble Space Telescope images of the distant Universe tend to support this view that galaxies that are half or a third of the presently accepted age of the universe are different. However, we know that the collection of objects looked at by Hubble is limited.
What is the Age of Things?
Even the fundamental building blocks of matter like the proton and the electron are not eternal. We have known since the early 1970s that protons have colossal lifetimes, on the order of 1031 or 1032 years. We can state for sure that the constituents of the Universe are at least younger than 1031 years, which is a stunningly large number, but not a very useful conclusion.
A simple, but more robust way to infer an approximate age of the Universe is to calculate the time it takes for large structures, like galaxies and clusters of galaxies, to stabilize due to gravitational attraction. Just as flying sand particles slowly come to rest after a strong wind, disturbed systems in space take a period of time to come to a state of rest. Straightforward physics shows that under the attraction of gravity, primordial clouds of gas, star systems and galaxy clusters collapse to form well-defined structures that can be understood and over timescales that can be calculated.
The laws of physics also tend to constrain cosmic objects into flattened features (like the solar system, or the disks of spiral galaxies) or highly symmetrical structures (like clusters of stars and of galaxies). For example, we see millions of stable, well-settled galaxies throughout the nearby Universe. We can infer from their regular spiral or football shapes that their ages are much greater than 100 million years. Therefore, we know that the Universe must be much older than this.
On the other hand, we observe many clusters of galaxies that are still in the process of assembly. It is calculated that it can take as long as 40 billion years for these large systems to stabilize, which suggests that the Universe is not yet old enough for them to have reached equilibrium. Hence, one can conclude that the age of the Universe must be less than 40 billion years.
Radioactive Elements and the Evolving Colours of Stars as Cosmic Clocks
Although useful, the arguments above give us just a gross approximation of the possible ages of the Universe and of its constituents. We need better tools.
The disintegration of radioactive elements is one of the most powerful and accurate ways to derive the ages of astronomical systems. Like we use radiocarbon dating to infer the age of recent archaeological artefacts of 50,000 years or less, geophysicists and astronomers have used the properties of uranium isotopes and other radioactive elements to deduce an age of about 4.5 billion years for the Earth.
We can even determine that uranium and most of the heavy elements themselves found on the Earth and the Moon were produced about 8.8 billion years ago. They were probably "cooked" in a powerful supernova that exploded somewhere in the Milky Way and polluted the primordial interstellar cloud of gas and dust that would later be used in the formation of our solar system. Hence, the Universe must be older than 8.8 billion years, the age of the supernova “mother star” that produced the heaviest elements found on Earth.
One other powerful technique can be used to infer the age of the Universe to a finer precision. It employs the tracking of the colour and the luminosity of stars as they evolve during their long lifetimes. Images of stellar clusters taken through several filters allow astronomers to display of the systematic patterns of luminosity versus colour that betray ages and other properties of the stars. Globular or open clusters each contain thousands and even million of stars giving instantaneous “portraits” for given ages. By looking at different clusters, astronomers can plot colour-magnitude diagrams for many clusters. Each cluster will give a different pattern depending on its age, whether it is only a few million or 10 billion years old. These changing patterns of star luminosity and colour can be well reproduced through sophisticated computer modeling of stellar evolution. Astronomers have used this approach for half a century to infer the age of stellar clusters and their stars.
The oldest ages, inferred from studies of star clusters, are between 12 and 15 billion years, with uncertainties of a few billions years. Nevertheless these numbers tell us something reasonably certain about the oldest stars––they have to be younger than the age of the Universe. However, the venerable age of the oldest star clusters leaves an uncomfortably short amount of time between the current estimates for the birth of the Universe at about 13.7 billion years and the formation of the first stars.
Running the Movie of the Universe Backward
All of the previous examples indicate to us that the constituents of the Universe have a finite age. However that does not preclude the fact that the Universe could be eternal. The strongest evidence that the whole Universe had a beginning is the fact that it is expanding, which was discovered by Edwin Hubble (1889-1953) and Milton Humason (1891-1972) in the 1930’s.
The expansion of the Universe puts tight limitations on everything. The fact that on a scale greater than a few million light-years all galaxies seem to be flying apart, surfing in expanding space-time, indicating that all things were together at some time in the past. The expansion of the Universe, once we accurately know the density of matter and energy, can be turned around to infer a very accurate birthday for the whole process. Like running a movie backward, astrophysicists can crunch everything existing in the present Universe into some sort of incredibly small and hyperdense nugget of space-time 13.7 billion years ago at the instant of the Big Bang––the time when everything, including space and time, began.
This prediction has implications on how things should look as our telescopes pierce further and further into the Universe. For example, when we observe distant galaxies, shining as they were 8 to 10 billions years ago, these “infant” galaxies should look different, very different, from the nearby more “mature” galaxies we see today.
The Gemini Deep Deep Survey of the Distant Universe
The recently completed Gemini Deep Deep Survey took the deepest spectra ever of very distant galaxies. Using data obtained with the Frederick C. Gillett Gemini North Telescope on Mauna Kea, a team of USA/Canada/UK astronomers have completed the analysis of the images and spectra representing several hundreds of galaxies corresponding to a time window when the Universe was between a 20% to 40% of its present age. They find a very puzzling “landscape.” The galaxy populations they encounter look the same, with surprisingly no sign of evolution during this crucial epoch that was thought to be a period of most significant changes in the assembly of galaxies. Even more intriguing: massive and fully formed galaxies are found at the largest distances, or youngest epochs of the Gemini survey. The big massive ones should not be there, but they are.
This finding leaves very little time between the Big Bang and that epoch for forming these Gemini Deep Deep Survey galaxies. Either something is wrong with our present models of collapsing large structures right after the Big Bang, or we need to revisit the way galaxies formed. For instance, massive black holes could be much more ubiquitous than we thought in the early Universe and may act as numerous and efficient seeds to form the first galaxies.
Although many indicators lead to an age of about 13 billions years for the universe and its constituents, there is an uncomfortably short period between the beginning of space-time-matter and the appearance of the first objects in the known Universe.