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CALTECH AY 21 - MISSING PAGES IN OUR PHOTO ALBUM OF THE INFANT UNIVERSE

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arXiv:astro-ph/0702298v1 11 Feb 2007Draft version February 11, 2007Preprint typeset using LATEX style emulateapj v. 10/09/06MISSING PAGES IN OUR PHOTO ALBUM OF THE INFANT UNIVERSEAbraham Loeb1Draft version February 11, 2007ABSTRACTExisting data sets include an image of the Universe when it was 0.4 million years old (in the formof the cosmic microwave background), as well as images of individual galaxies when the Universewas older than a billion years. But there is a serious challenge: in between these two epochs was aperiod when the Universe was dark, stars had not yet formed, and the cosmic microwave backgroundno longer traced the distribution of matter. And this is precisely the most interesting period, whenthe primordial soup evolved into the rich zoo of objects we now see. In this popular-level overview Idescribe how astronomers plan to observe this nearly-invisible yet crucial period.Subject headings: cosmology1. FIRST LIGHTWhen I loo k up into the sky at night, I o ften wonderwhether we humans are too preoccupied with ourselves.There is much more to the universe than meets the eyeon earth. As an astrophysicist I have the privilege ofbeing paid to think about it, a nd it puts things in per-sp e c tive for me. There are things that I would otherwisebe bothered by - my own death, for example. Everyonewill die sometime, but when I see the universe as a whole,it gives me a sense of longevity. I do not care so muchabout myself as I would otherwise, because of the bigpicture.Cosmologists are addressing some of the fundamentalquestions that p e ople attempted to resolve over the cen-turies through philosophical thinking, but we are doingso based on systema tic observation and a quantitativemethodology. Perhaps the greatest triumph of the pastcentury has been a model of the universe that is sup-ported by a large body of data. The value o f such a modelto our society is sometimes underappreciated. When Iopen the daily newspaper as part of my morning rou-tine, I often see lengthy descriptions of conflicts betweenpeople about borders, pos sessions or liberties. Today’snews is often forgotten a few days later. But when oneopens ancient texts that have appealed to a broad audi-ence over a longer period of time, such as the Bible, wha tdoes one often find in the opening chapter? A discussionof how the constituents of the universe – light, stars, andlife – were created. Although humans are often caughtup with mundane problems, they are curious about thebig picture. As citizens of the universe we cannot helpbut wonder how the first sources of light formed, howlife came into exis tence and whether we are alone as in-telligent beings in this vast space. Astronomers in the21st century are uniquely positioned to answer these bigquestions with scientific instruments and a quantitativemethodology.It is sometimes argued that science takes away thesense of mystery about our origins. However, currentscientific circumstances appear to only enhance the mys-tery. Consider what astrono mers have learned so farabout the early Universe. We have an image of the Uni-1Astronomy Department, Harvard University, 60 Garden Street,Cambridge, M A 02138, USA; E-mail: [email protected] at the moment tha t hydrogen atoms first formed init – namely the cosmic microwave background radiation– and we have pictures of individual galaxies more than abillion years later. The intervening epoch, though, was aperiod that started when the Univer se was dark, starshad not yet formed, and the cosmic microwave back-ground no longer traced the distribution of matter. Andthis is precisely the most inte resting period, when theprimordial soup evolved into the rich zoo of objects wenow see.The situation is similar to having a photo album of aperson that contains the first ultrasound image of him orher as an unborn baby and some additional photos as ateenager and an adult. If you tried to guess from thesepictures what happe ned in the interim, you could be se-riously wrong. A child is not simply a scaled-up fetusor scaled-down adult. T he same is with galaxies. Theprimordial hydrogen gas was composed of atoms, so youmight suppose that the Universe traced out a straight-forward and rather boring path towar ds the assemblyof atoms into galaxies. It did not. Observations of thesp e c tra of early galaxies and quasars, which reveal theconditions in their environments, indicate that the cos-mic gas ac tua lly underwent a wrenching transition fromatoms back to their constituent protons and electrons –a process known as reionization. In fact, although theworld around us is composed of ato ms , the bulk of theUnive rse’s ordinar y matter today is still in the form offree electrons and protons, located deep in intergalacticspace.How the Universe underwent this transition is one ofthe most exciting questions in cosmology today. Mostresearchers associate the transition with the firs t gener-ation of stars, whose ultraviolet radiation s treamed intointergalactic space and broke atoms apart. Others con-jecture that material plummeting into black holes gaveoff radiation on its death plunge. But as often is thecase in s c ience , new observational data is required to testwhich of these scenarios describes reality better.The timing of reionization depends on astrophysicalparameters such as the efficiency of making stars or blackholes in galaxies, but most importantly it depends on thenature and the initial inhomogeneities of the cos mic mat-ter. Galaxies form by a process known as gravitationalinstability which is seeded by the initial inhomogeneities.2Fig. 1.— Cosmology is like archeology. The deeper one looks,the older is the layer that one is revealing, owing to the finitepropagation speed of light.A region that starts slightly denser than the average den-sity of the Universe will tend to pull itself together by itsown gravity. Although initially the region expands likethe rest of the Universe, its extra gravity slows its ex-pansion down, turns it around, and make the region col-lapse upon itself to make a bound object like a galaxy.Most of the c osmic matter is known to be dark, i.e. havea very weak interaction (aside fr om gravity) with or di-nary matter and radiation. To get the process of g alaxyformation started, one needs inhomogeneities in the cos -mic matter distribution on the sma ll scale of galaxies.Such inhomogeneities exist and can see d the formationof dwarf galaxies at early times only if the dark ma


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CALTECH AY 21 - MISSING PAGES IN OUR PHOTO ALBUM OF THE INFANT UNIVERSE

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