Digital Universe Guide - Hayden Planetarium

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150 4. THE EXTRAGALACTIC ATLAS theory suggests that origins of space and time, matter, and energy come from a time when the Universe was extremely hot and dense. About 13.7 billion years ago, the Universe exploded, forming space itself, which would later be populated with the galaxies we see today. Very soon after the Big Bang, matter in the Universe was so hot that it formed a plasma, a state in which atoms are separated into nuclei and free electrons, similar to the Sun’s interior. These free electrons prohibit the free flow of light in the Universe. The Universe was opaque to light in these early times. Later, as the Universe expanded and cooled, the electrons combined with the nuclei and formed hydrogen atoms. With few free electrons, light was now free to travel the Universe. This transition took place about 379,000 years after the Big Bang, ushering in a new era in the Universe. The Universe was then transparent to light as photons, which traveled freely across the Universe, encountering few collisions that would absorb or scatter the light. If we could have seen it, the Universe would have been about 3,000 Kelvin and would have glowed orange in visible light. In fact, the peak intensity of the Universe at that time was around 1 micron (in the infrared). Since that time, the Universe has expanded 1,000-fold and the light waves have expanded along with space. The light, then, appears redshifted as space itself expands. Today, the peak intensity of the Universe is 1,000 times lower than it was then, so the wavelength is 1,000 times greater and is close to 1 millimeter (1 micron × 1,000 = 1 mm). This peak intensity corresponds to a gas temperature 1,000 times less, or 3 Kelvin (3,000 K ÷ 1,000 = 3 K). Light at 1 mm is in the microwave portion of the EM spectrum, and this is where we see the remnant light from the Big Bang, called the Cosmic Microwave Background radiation, or CMBR. The CMBR pervades the Universe. It is an imprint of the time of recombination in the Universe, when protons and electrons combined to form hydrogen, when light began to travel in space unimpeded. When we look at that light, we are looking back in time to that event, 379,000 years after the Big Bang. Before that time, the Universe was opaque, like looking through a fog, so we will never see the Big Bang event itself. The CMBR, then, defines that part of the Universe that we can see, what we call our Observable Universe. While the CMBR is everywhere in the Universe, we can also think of it as our outer limit, that which we cannot see beyond. The best observations of the CMBR come from the Wilkinson Microwave Anisotropy Probe (learn more about this space telescope in the “Microwave (WMAP) All-Sky Survey” section). The image is a snapshot, a baby picture if you will, of the early Universe, tracing out the farthest reaches of the
4.1. EXTRAGALACTIC ATLAS OVERVIEW 151 observable Universe. It shows the minute variations in temperature that were present in the Universe during that early era. These temperature variations are on the order of millionths of degrees around the average temperature of 2.725 Kelvin. In the next section, we will see just how important these minute fluctuations are in determining the structure and evolution of the Universe. 4.1.4 Cosmological Structure Formation The minute temperature fluctuations in the CMBR are directly responsible for the complex large-scale structure in today’s Universe. Scientists are still on the quest toward understanding how these small fluctuations evolve into the galaxy clusters and filaments we see today. The temperature fluctuations in the WMAP may be thought of as density fluctuations in the early Universe. These slight differences in density become amplified by gravity. Regions that are more dense, even by tiny amounts, will gradually collapse because of this excess gravity. This process, called gravitational instability, worked slowly over billions of years until galaxies formed. Those galaxies are now hundreds of times denser than the nearly empty space that surrounds them. In addition to galaxy-sized objects, this process gave rise to clusters and filamentary strands of galaxies. The structure of the Universe is akin to human settlement in the world. We settle in towns of various sizes (a galaxy) and these towns often cluster around each other (a galaxy cluster). Connecting these towns are trails, rivers, and highways, along which smaller towns or groups of towns are found (galaxy filaments). While galaxy groups and clusters are gravitationally bound systems such that no galaxy can escape, superclusters have not yet settled into this state. In our analogy, this would be similar to the major cities in the Northeast United States. From Washington, D.C., to New York, one can travel from town to town almost without leaving an urban area. Interstate 95 connects them all as one area. But this urban region has not yet merged into one large megalopolis, just as a supercluster has not yet become gravitationally bound. 4.1.5 The Distance Scale The Universe has been expanding continuously, although not linearly, since the Big Bang, about 13.7 billion years ago. Its expansion gradually slowed because of the total gravity of matter in the Universe. However, it was recently discovered that during the past few billion years the expansion has
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The Digital Universe Guide Brian Ab
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Contents Contents 3 List of Tables
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List of Tables 2.1 Log and Linear F
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1.2. GETTING HELP 7 1.2 Getting Hel
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1.5. ACKNOWLEDGMENTS 9 1.5 Acknowle
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2.2. LAUNCHING THE MILKY WAY ATLAS
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2.3. LEARNING TO FLY PARTIVIEW 13 T
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2.3. LEARNING TO FLY PARTIVIEW 15 M
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2.4. PARTIVIEW’S USER INTERFACE 1
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2.5. DIGITAL UNIVERSE FILES 23 File
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3.1. MILKY WAY ATLAS OVERVIEW 25 3.
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3.2. MILKY WAY ATLAS TUTORIAL 27 3.
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3.3. MILKY WAY DATA GROUPS 67 Earth
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3.3. MILKY WAY DATA GROUPS 69 Data
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3.3. MILKY WAY DATA GROUPS 71 The l
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3.3. MILKY WAY DATA GROUPS 73 3.3.3
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3.3. MILKY WAY DATA GROUPS 93 inclu
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4.4. OPTIMIZING THE EXTRAGALACTIC A
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Appendix A Light-Travel Time and Di
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Appendix B Parallax and Distance On
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absolute magnitude M, can be calcul
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discovered that there was a linear
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Appendix D Electromagnetic Spectrum
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Appendix E Constants and Units In a
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Metric Prefixes These prefixes are
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INDEX 221 center command, 50, 61 cl
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INDEX 223 Earth’s radio signals,
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INDEX 225 irregular galaxy, 147, 17
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INDEX 227 entering, 21 every, 206 j
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INDEX 229 stellar distance uncertai
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Digital Universe Guide - Hayden Planetarium

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