The first light in the Universe

The WMAP satellite. After three phasing loops around m a

e Earth, WMAP flew just behind T e

c the orbit of the Moon, three n e i c weeks after launch S P A M W / A S ience Team A c BE S N CO f / o A AS y N s f e o t r sy

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Ana Lopes and Henri An artist’s impression of the COBE spacecraft, Boffin take us on a launched by NASA into an Earth orbit in 1989 to trip back in time – make a full-sky map of the cosmic microwave back - probing the history of ground radiation left over from the Big Bang. The first the Universe. results were released in 1992

Photographs of the Universe ave you ever wondered when Historians often use photographs the first lights in the Universe It was not until about 400 000 years and other pictures to tell them about began to shine? Most of us have after the Big Bang that light was able the past, and in this respect, astro- Hwatched the rising of the Sun in the to travel freely in the Universe. Ever nomers are no different. Using the morning, the dawn of a new day. since the Big Bang, the Universe COBE (COsmic Background Explorer) Astronomers go a step further and has been expanding and cooling (for and Wilkinson Microwave Anisotropy w1 look for the first sources of light – a description, see Boffin & Pierce- Probe (WMAP ) satellites to map the peering into the history of the Price, 2007 ), stretching that primeval cosmic microwave background radia - Universe using powerful telescopes. light from its initial high frequency, tion, astronomers have created a ‘pho - Their ultimate aspiration is even more until it can be detected today as tograph’ of the Universe as it was ambitious: to trace the entire history photons in the microwave range: approximately 400 000 years after the of the Universe, from its birth – the cosmic microwave background Big Bang. The data from COBE won Big Bang – to the present day, almost radiation, coming from all over the John Mather and George Smoot the w2 14 000 million (14 billion) years later. Universe. 2006 Nobel Prize in Physics.

48 Science in School Issue 13 : Autumn 2009 www.scienceinschool.org Science topics

Original image courtesy of NASA

This article offers interest - ing and detailed informa - tion about modern research into the history of the Universe and its evo - lution. It could be used for interdisciplinary teaching, for example in physics, , or philosophy. As well as providing valuable back - W ground reading, teachers E

I could use it to develop

V educational materials. E Vangelis Koltsakis, Greece

R The electromagnetic spectrum

Original image courtesy of SG Djorgovski and Digital Media Center, Caltech The standard cosmological model of how the Universe evolved tells us An outline of cosmic history that by about 400 000 years after the

Big Bang, the Universe had cooled to Time since the Big Bang The Big Bang about 3000 degrees Kelvin, a tempera - (years) the Universe filled with ionised gas ture low enough for all the electrons ~300 thousand The Universe becomes and protons to combine, forming neu - neutral and opaque tral hydrogen from the ionised gas. The ‘dark ages’ start Electrons in neutral hydrogen (as in other atoms or molecules) absorb photons very efficiently , so a universe and begin full of neutral hydrogen is opaque. In to form contrast, when protons and electrons The reionisation starts ~500 million are separate, they cannot capture pho - tons so a universe full of ionised gas – as was the case until about 400 000 years after the Big Bang, and is again the case today – is relatively transpar - The ‘dark ages’ end ent. The COBE and WMAP maps show us the Universe during its Reionisation complete, the ~1 billion Universe becomes transpar - opaque stage, at the beginning of the ent again ‘dark ages’ of the Universe. This peri - od came to an end when the Universe Galaxies evolve became ionised again (see diagram to the right) .

We also have ‘photographs’ of a ~9 billion Solar System forms much more recent Universe: galaxies full of stars, as they were 1000 million Today: Astronomers are years after the Big Bang – once the ~13 billion figuring it out Universe had become transparent again. Because of the finite speed of -1 light (300 000 km s ), the light from

www.scienceinschool.org Science in School Issue 13 : Autumn 2009 49 Image courtesy of NASA / WMAP Science Team

WMAP observes the first light of the Universe – the afterglow of the Big Bang. This light emerged approximately 400 000 years after the Big Bang. Patterns imprinted on this light encode the events that hap - pened only a tiny fraction of a second after distant objects takes much longer to the Big Bang. In turn, the patterns are the reach us than that from neighbouring seeds of the development of the structures of galaxies we see now, billions of years objects; we therefore see these distant after the Big Bang objects as they were a very long time ago. By looking at very distant objects, astronomers were able to see the light that has travelled for almost 13 000 million years – that is, they saw those objects as they had been less than 1000 million years after the Big Bang. But what happened between these two photographs, between the release of the cosmic microwave background radiation 400 000 years after the Big Bang and the light emitted by these very distant galaxies, nearly 1000 mil - f Image courtesy o NASA / WMAP Science lion years later? When and how did Team the cosmic fog lift? What changed a sea of particles almost devoid of structure into a Universe lit by numerous stars in young galaxies ? As Harvard University astronomer Abraham Loeb puts it: “The situation that astronomers face is similar to having a photo album containing the first ultrasound image of an unborn baby and some additional photos of that same person as a teenager and an The detailed, all-sky picture of the infant universe created from five years of adult ” ( Loeb , 2006). What scientists WMAP data. The image reveals 13.7 billion-year-old temperature fluctuations (red do not know – but are trying to figure regions are warmer and blue are cooler) that correspond to the seeds that grew to out – is when and how the very first become the galaxies stars and galaxies were born. Loeb continues: “Astronomers are currently

mage courtesy of NASA, ESA, searching for the missing pages of the I S Beckw ith (S pace Tele cosmic photo album, which will show sco pe Sc how the Universe evolved during its ien ce In s infancy and made the building blocks ti tu te ) of galaxies like our own Milky Way.” a n d Before any stars were formed , the

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m of energy level corresponding to pho - tons in the ultraviolet (UV) domain. The Hubble Ultra Deep Field is the deepest visible-light image of the cosmos. This Therefore whatever reionised the -studded view represents a ‘deep’ core sample of the Universe, cutting across Universe must have released signifi - billions of light-years. The snapshot includes galaxies of various ages, sizes, shapes, cant amounts of UV radiation . and colours. The smallest, reddest galaxies may be among the most distant known, Although astronomers are still uncer - existing when the Universe was just 800 million years old. The nearest galaxies – the tain what could have released this larger, brighter, well-defined spirals and ellipses – thrived about 1 billion years ago, when the cosmos was 13 billion years old ionising UV radiation, they speculate

50 Science in School Issue 13 : Autumn 2009 www.scienceinschool.org Science topics

Peterson’s prediction: they detected The effect of reionisation on spectra an unambiguous trough in the spec -

Reionised, transparent Ionized bubbles in a still Opaque neutral gas in trum of a very distant quasar discov - Universe largely neutral universe the early universe (before reionisation) ered during the Sloan Digital Sky w3 Survey , a huge astronomical survey

Line of sight to The quasar that examined the spectra of about a the quasar hundred thousand quasars. The trough was in the infra -red part of the spectrum, because the quasar is so far away : its light started the journey h c e t l towards Earth only about 900 million a C , r years after the Big Bang, and so has e t n e

C taken almost 13 000 million years to a y i t d i

e reach us , during which time its initial - s M n l e a t t ly UV light has been stretched (red - i g n i I D shifted ) into the infra -red by the d n a i expansion of the Universe. Quasars k s v o

g slightly closer to Earth did not show r o j

D such a trough. This indicated that the

G Wavelength or redshift S f last patches of neutral hydrogen in o y s e t the Universe were ionised about 900 r u o Isolated transmission spikes correspond to the Dark regions correspond to the still opaque, c million years after the Big Bang.

e ionised bubbles along the line of sight neutral gas along the line of sight g a m I The microwave footprint The cosmic microwave background that it was either the first, very hot, hydrogen to impede it (see diagram radiation released soon after the Big stars, or early black holes, releasing above) . (Note that while the neutral Bang is another source of information huge amounts of UV radiation as hydrogen atoms absorb all wave - about the reionisation epoch. material fell into them. If this is the lengths of light, most wavelengths are As the Universe started to be case, stars must have formed before released again. UV light, in contrast, reionised, the electrons that were the reionisation epoch – thus if we ionises the atoms and is completely released affected the polarity of light. can date the reionisation, we have at absorbed.) A free electron can interact with a least a latest date for the emergence of If even a tiny portion of the inter - photon in a process called Thomson the first stars. galactic medium (as little as one part scattering : the electron is accelerated The ultraviolet footprint in a million ) had been neutral when and the incident light is polarised the quasar released the light we now along the direction of motion of the In 1965, American astronomers see from Earth, this would have left a electron. This effect was most pro - James Gunn and Bruce Peterson pre - noticeable imprint on the spectrum – nounced during and just after the dicted that the spectra of quasars a suppression of light in the UV reionisation ; later, because the could be used to date the final stages range, known as the Gunn-Peterson Universe continued to expand, the of the reionisation epoch. Quasars are trough. Therefore James Gunn and density of free electrons decreased, very distant and ancient galaxies of Bruce Peterson predicted that quasars reducing their polarising effect. extreme brilliance , thought to be pow - beyond a certain distance from Earth, Between 2001 and 2006, the WMAP w1 ered by material falling into giant for which we observe light that was satellite was used to study the black holes at their centres . If the released before the reionisation had degree of polarisation of the cosmic quasar is so distant that the light we finished, would show a ‘trough’ in microwave background photons. By observe from it escaped during the their spectra. Quasars closer than this looking at different frequencies of ‘dark ages ’, its UV light will have would not – they released the light light, astronomers could look at dif - been absorbed by the neutral hydro - we observe from Earth only after the ferent periods in the Universe’s histo - gen present at the time; if the quasar reionisation had finished . ry – and the degree of polarity gave is closer and the light we observe was In 2001, a team of scientists led by an indication of the density of free emitted only after the reionisation, Robert Becker from the University of electrons around at that time (the there will have been no neutral California, USA, confirmed Gunn and greater the polarity, the higher the

www.scienceinschool.org Science in School Issue 13 : Autumn 2009 51 density of free electrons). From these www.scienceinschool.org/2007/ If you enjoyed this article, you studies, they concluded that reionisa - issue5/fusion might like to browse all science tion started about 400 million years Web references topics previously published after the Big Bang, and was complet - in Science in School . See: ed 400 to 500 million years later. This w1 – For more information about the www.scienceinschool.org/ is in agreement with the findings of WMAP satellite, see: sciencetopics http://map.gsfc.nasa.gov the quasar studies : 900 million years You may also enjoy the following after the Big Bang. w2 – An overview of John Mather articles: and George Smoot’s work on cos - Larson RB, Bromm V (2001) The Future research mic microwave background radia - first stars in the Universe . Scientific tion with COBE and links to further On 14 May 2009, the European American Dec : 64-71. This article is w4 information are given in the press Space Agency launched the Planck available for download from w5 release announcing their Nobel satellite to provide us with a photo - http://www.astro.yale.edu/larson/ Prize: http://nobelprize.org/ graph of the cosmic background radi - papers/SciAm01.pdf ation with even more sensitivity and nobel_prizes /physics/laureates/ Madau P (2006) Astronomy: trouble angular resolution than the WMAP 2006/press.html at first light . Nature 440 : 1002-1003. achieved. It will certainly help w3 – The is doi:10.1038/4401002a. Download astronomers to answer with more the most ambitious astronomical the article free of charge on the details the questions of how the survey ever undertaken. When Science in School website Universe evolved from a glowing completed, it will provide detailed (www.scienceinschool.org/2009/ soup to what we see today. optical images covering more than a issue13/light), or subscribe Although the time when reionisa - quarter of the sky, and a three- to Nature today: tion occurred has been successfully dimensional map of about a million www.nature.com/subscribe identified, a photograph of the galaxies and quasars. As the survey Universe from that time is still lack - progresses, the data are released to Scannapieco E, Petitjean P, ing, as current telescopes are unable the scientific community and the Broadhurst T (2002) The emptiest to image it. The good news, however, general public in annual incre - places . Scientific American Oct : 56-63. is that the European Southern ments. See: www.sdss.org This article is available to Observatory (ESO), together with w4 – For more information about download from astronomers and engineers from the European Space Agency, see: http://scannapieco.asu.edu/papers across Europe, is now engaged in the www.esa.int /sciam.pdf design of the European Extremely w5 – To learn more about the Planck w6 Large Telescope , 42 m in diameter , satellite, see: which will allow us to look this far www.esa.int/esaSC/120398_index_ Henri Boffin is an astronomer and back in time and possibly see the first 0_m.html journalist with extensive international starlight. research experience. As the public w6 – For more information information officer for ESO’s Very References about ESO’s Extremely Large Large Telescope and European Telescope, see: Extremely Large Telescope, he Boffin H, Pierce-Price D (2007) Fusion www.eso.org/public/astronomy/ devotes himself to both science com - in the Universe: we are all stardust. teles-instr/e-elt.html Science in School 4 munication and research. He is author : 61-63. Resources www.scienceinschool.org/2007/ of a number of popular Science in School articles. issue4/fusion The WMAP section of the NASA Ana Lopes is an associate editor at Loeb A (2006) The dark ages of the website provides some teacher Nature , the international weekly jour - Universe . Scientific American Nov : resources, including a brief over- nal of science. She has a degree in 46-53. This article is available to view of the WMAP project and an physics from the Technical University download from inflatable model of the Universe. of Lisbon, Portugal, and a PhD in http://www.cfa.harvard.edu/ See: http://map.gsfc.nasa.gov/ astrophysics from the University of ~loeb/sciam.pdf resources/edresources1.html Oxford, UK. Ana was formerly a sci - Rebusco P , Boffin H, Pierce-Price D For a full list of Science in School ence journalism intern at ESO. (2007) Fusion in the Universe: articles about fusion and the evolu - where your jewellery comes from. tion of the Universe, see: Science in School 5: 52-56. www.scienceinschool.org/fusion 52 Science in School Issue 13 : Autumn 2009 www.scienceinschool.org