Cosmic Dawn

by George Rhee

  Fig. 6.3The merging of two spiral galaxies like our own Milky Way. As the two galaxies approach (upper left), on their first close pass, they sideswipe each other, throwing out long tails of stars and gas (upper right). They then move apart before gravity pulls them back together again and their centers merge, when the stars at greater distances from the center are tossed into random orbits (lower left). Eventually the merged spiral galaxies form an elliptical galaxy (lower right) (Credit: Patrik Jonsson, Greg Novak and Joel Primack, University of California, Santa Cruz, 2008)

  The Formation of the Cosmic Web

  Galaxy maps reveal galaxies located inside filaments and clusters at the meeting points of these filaments. There are also walls or sheets of galaxies located within this web. The largest of these structures span hundreds of millions of light years. A similar cosmic web forms in the simulations of the formation of galaxies. Unlike star formation, the web is not put in by hand in the simulations, it emerges naturally from the very small variations in density from one location to the next that were discovered by COBE and WMAP satellite images of the early universe.

  One can think mathematically of the density variations as being waves on an ocean. To get the true shape of the surface of the ocean, we add up waves of differing wavelengths. The waves are in fact specified by two numbers, their wavelengths, the distance from crest to crest, and their amplitude, the height of the waves. We can estimate the amplitude of the waves from observations of the cosmic background radiation. Unlike the two dimensional surface of the ocean, we are dealing with a three dimensional density distribution. So, we add up the waves in three dimensions. In this way we produce a model that gives us the starting value of the density of dark matter at any location in space. We can then use our computers to calculate the density at any time in the future. We run our simulation and stop it when we think we have the best match with the observations. Figure 6.4 illustrates the formation of the cosmic web in a simulation using a series of snapshots of the particles. It is remarkable that this simple procedure produces results that match the observations. Starting with observations of light from a few hundred thousand years after the big bang we can explain key features of the galaxy maps that we have made if not the details of the galaxies themselves.

  Fig. 6.4The cosmic web in the Millenium-II simulation. Time increases from top to bottom in each of the three columns. Column 2 figures are a zoom in of column 1. The third column is a further zoom in. The z numbers label the redshifts corresponding to the time in the simulation when the snapshot was taken. The top row is the dark matter distribution about 1 billion years after the Big Bang. The bottom row is the dark matter distribution today. The image box size is expanding at the same rate as the universe (Credit: Michael Boylan-Kolchin, Volker Springel, Simon D. M. White, Adrian Jenkins, and Gerard Lemson (2009) Monthly Notices of the Royal Astronomical Society, 398 1150B, by permission of Oxford University Press on behalf of the Royal Astronomical Society)

  Review: What the Theorists Taught Us

  The calculations show that a region that is slightly denser than its surroundings will grow increasingly overdense with time until it finally collapses to form an object such as a galaxy. We can follow this process initially with simple equations that we can solve with pencil and paper. To create a more realistic picture with many dark matter halos of various masses forming and interacting we require powerful computers. The results reveal in detail the structures that form through the action of gravity. We can measure how their density varies with distance from the center of the object. We can study the dark matter halos as they form. We can determine the number and shape of these halos. We can also study how the halos are affected by collisions and mergers. Finally, we can measure distribution of these halos in space.

  The results depend on the very small, density variations that we start with. We describe these density variations mathematically using waves of varying wavelength and amplitude. The density variations of large wavelength had smaller amplitude than the smaller wavelength density variations. One could use a sound recording as an analogy. The density variations on large scales correspond to the bass frequencies and the small scale variations correspond to high frequencies. The idea is that low frequencies are less prominent in the recording than the high frequencies. The role of gravity is then equivalent to turning up the volume of all the frequencies. This simple idea is valid until the variations in density are comparable with the density itself at which point this simple picture breaks down and the cosmic web and many collapsed halos start to form.

  For many years, there were no data to support this picture. But, in the early 1990s, NASA’s Cosmic Background Explorer Satellite followed in 2001 by NASA’s WMAP satellite provided evidence of variations in the cosmic background radiation intensity from one place to the next. This is a tremendously important finding for cosmology. It reveals how large the matter density variations were at a finite time in the very distant past. This confirms that our simple view of galaxy formation is at least partly correct and enables us to put observed numbers into our theoretical speculation. In the next chapter we present the results of observations of the cosmic background radiation that are fundamental to cosmology.

  Further Reading

  Introduction to Cosmology. B. Ryden, San Francisco, Addison Wesley, 2003.

  How Did the First Stars and Galaxies Form? A. Loeb, Princeton, Princeton University Press, 2010.

  Theoretical challenges in understanding galaxy evolution J.P. Ostriker and T. Naab, Physics Today, volume 65, p43, 2012.

  George RheeAstronomers' UniverseCosmic Dawn2013The Search for the First Stars and Galaxies10.1007/978-1-4614-7813-3_7© Springer Science+Business Media, LLC 2013

  7. The Weight, Shape, and Fate of the Universe

  George Rhee1

  (1)Department of Physics & Astronomy, University of Nevada, Las Vegas, Nevada, USA

  Abstract

  In order to estimate the size, luminosity and mass of galaxies we need to know the Hubble constant and the density of the universe. In recent years, these numbers have been measured to better than 1% accuracy. This is a remarkable improvement on the situation 20 years ago when the Hubble constant was not known to better than a factor of 2.

  Three observations taken together have made this improvement possible. The first is the observation of the variations in brightness of the cosmic background radiation. The second is the observation of distant supernova explosions. The third is the observation of the clustering of galaxies. The attempt to better constrain the density of the universe using supernovas led to the remarkable discovery that instead of slowing down as we might, expect the expansion of the universe is in fact accelerating. This discovery implies the existence of a new force in the universe that astronomers call dark energy.

  Madame Dieterlen gave me coffee in her caravan. I asked her what traces the cattle herders of the Sahel would leave for an archaeologist. She thought for a moment and answered, “They scatter the ashes of their fires. No, Your archaeologist would not find those. But the women do weave little chaplets from grass stems, and hang them from the branch of their shade trees.”

  Bruce Chatwin, The Songlines

  In order to estimate the size, luminosity and mass of galaxies we need to know the Hubble constant and the density of the universe. In recent years, these numbers have been measured to better than 1% accuracy. This is a remarkable improvement on the situation 20 years ago when the Hubble constant was not known to better than a factor of 2.

  Three observations taken together have made this improvement possible. The first is the observation of the variations in brightness of the cosmic background radiation. The second is the observation of distant supernova explosions. The third is the observation of the clustering of galaxies. The attempt to better constrain the density of the universe using supernovas led to the remarkable discovery that instead of slowing down as we might, expect the expansion of the universe is in fact accelerating. This discovery implies the existen
ce of a new force in the universe that astronomers call dark energy.

  Echoes of Creation: Discovery of the Cosmic Microwave Background

  Remarkably most of the light in the universe was not emitted by stars and galaxies but comes from the cosmic background radiation, the afterglow of the Big Bang (see Chap.  2). This background radiation was discovered serendipitously by Arno Penzias and Robert Wilson, two researchers working at Bell Laboratories. The antenna they used had been constructed for the purpose of satellite communication. When astronomers look at data they distinguish between signal (such as light from a galaxy) and noise (random signals from other sources such as detector electronics and background light from the night sky). One man’s noise can be another man’s signal. If you are looking for the best radio frequencies for satellite communications, any diffuse radiation from the night sky is an annoyance and is classified as noise. If you are an astrophysicist looking for the light that has been traveling 13.7 billion years since the Big Bang, that diffuse radiation is most definitely signal.

  In the early 1960s astronomers thought that there was only one significant source of background radiation, the Milky Way galaxy. Penzias and Wilson were thus setting out to measure to high accuracy the background radiation of our galaxy at radio wavelengths. They started their observations at a wavelength of 7 cm where they expected to see no signal. This would be a way for them to measure the noise in their antenna and also noise from the Earth’s atmosphere. They were surprised to detect a significant amount of radiation at 7 cm and checked the antenna and the instruments to make sure no mistake had been made. In the end they had to conclude that it looked as if they were detecting background radiation emitted by matter at a temperature of 3. 5 ∘  K. We call it background radiation because it is diffuse, it seemed to come from all over the sky. We see this effect when we look at the night sky from a big city. It is much harder to see faint stars because of all the city light that makes the sky very bright. Penzias and Wilson did not know the origin of the radiation they had found, they had in fact detected the afterglow of creation.

  They found this out by talking to astronomers. Penzias called up Bernard Burke at MIT, and mentioned the issue of the background radiation he had detected. Burke had heard rumors of a talk by Princeton University astrophysicist Jim Peebles concerning the anticipated existence of background radiation left over from the Big Bang. Penzias and Wilson published their observations in a paper entitled “A measurement of Excess Antenna Temperature at 4,080 Mc/s”. This work earned them the Nobel prize for physics in 1978. An accompanying paper by Dicke, Peebles, Roll and Wilkinson gave the explanation for the origin of the background radiation.

  Steven Weinberg in his book The First Three Minutes points out that long before the year of discovery (1965) it would have been possible to both predict and detect the existence of this radiation. In fact, both the prediction of the radiation and the measurements proving its existence existed since the 1940s but no one put two and two together. The chain of argument is quite simple once one knows the helium abundance in nature. Ten percent of the atoms in the universe today are helium atoms. This number is determined by the neutron to proton ratio at the time of nucleosynthesis. This neutron to proton ratio can be used to infer the approximate temperature and density at which nucleosynthesis took place. By observing the density of baryons today we can determine by how much the universe has expanded since nucleosynthesis. The known amount of expansion enables us to determine the temperature of the radiation today. This rough argument would enable us to predict the temperature of the background radiation to lie between 1 and 10 K.

  Interestingly in 1948, there was a prediction made of the existence of cosmic background radiation with a temperature of 5 ∘  K. This work was done by George Gamow, Ralph Alpher and Robert Herman. George Gamow was born in the Ukraine in 1904. Prior to settling in the US, he moved from one renowned center of European physics to the next. He worked at the University of Gottingen, the Cavendish Laboratory in Cambridge and the Institute of Theoretical Physics in Copenhagen. Gamow was named Professor of Physics at Leningrad University in 1931. He escaped Stalin’s oppressive regime in 1933 and moved to the United States, working first at George Washington University and later at the University of Colorado at Boulder.

  It would have been possible to detect the cosmic background radiation as early as the mid 1940s. Why did this not happen? Weinberg lists three reasons. First, the Big Bang theory of the origin of the elements ran into a number of problems as it was first formulated. It purported to explain the abundances of all the elements. But, we know today that elements such as carbon and heavier elements are made in stars and supernova explosions. It was thus clear at the time that the Big Bang theory could not explain the origin of all the elements. As Weinberg puts it;

  The Big Bang theory of nucleosynthesis, by trying to do too much, had lost the plausibility that it really deserved as a theory of helium synthesis.

  The second reason the background was not detected earlier was that theorists did not realize it could be detected. The predicted temperatures for the background placed most of the emission in the radio part of the spectrum and radio astronomy was still a young science at the time. The third reason stated by Weinberg is that the Big Bang theory was not taken seriously at the time;

  This is often the way it is in physics – our mistake is not that we take our theories too seriously, but that we do not take them seriously enough. It is always hard to realize that these numbers and equations we play with at our desks have something to do with the real world …The most important thing accomplished by the ultimate discovery of the 3 K radiation background in 1965 was to force us all to take seriously the idea that there was an early universe.

  In fact, the cosmic background radiation had been detected in 1941, but not recognized for what it was. Adams and McKellar studied the absorption of light by cyanogen molecules in gas clouds and concluded that the molecules were interacting with light having an effective temperature of 2.3 K.

  When we teach science we emphasize its successes. As much if not more can be learned from its failures. No doubt historians of science will look back on the present era and ask how astronomers could have failed to ask the questions that seem ‘obvious in retrospect’.

  Detecting the Cosmic Background

  The cosmic background radiation is light that is emitted by matter at a uniform temperature. An oven heated to a given temperature will emit radiation. We could make a small hole in the oven and observe the radiation that is emitted. Kitchen ovens do not emit visible light but they do emit light in the form of infrared radiation. We are in some sense living in an oven at 3 ∘  above zero.

  The spectrum of the cosmic background radiation varies smoothly from one wavelength to another (Fig. 7.1). The correct mathematical formula describing how the intensity changes with wavelength was found by Max Planck about 100 years ago. The detailed study of this problem led to the birth of quantum physics. The intensity peaks at a wavelength of about a tenth of a centimeter. This is because the radiation has been greatly cooled by the expansion of the universe. The light is detected as microwaves and millimeter waves, similar in wavelength to the radiation picked up by a television. In fact, about 1% of the static on a TV tuned between stations has come directly from the Big Bang.

  Fig. 7.1The prediction of the Big Bang theory for the energy spectrum of the cosmic microwave background radiation compared to the observed energy spectrum. The FIRAS experiment on NASA’s COBE satellite measured the spectrum at 34 equally spaced points along the blackbody curve. The error bars on the data points are smaller than the thickness of the line used to draw the curve

  Penzias and Wilson made their measurement at a wavelength of about 7 cm. Soon after, Roll and Wilkinson made a measurement at 3.2 cm. The intensities of the radiation measured at these two wavelengths suggested a radiation temperature of about 3 K. The intensity measured by Roll and Wilkinson was higher than that fo
und by Penzias and Wilson by just the right amount. Of course two measurements are not enough to confirm that the radiation follows a curve. The idea was then to make more measurements but how and at what wavelengths?

  From the ground, one can in fact study the intensity of the radiation from wavelengths of about 1 m down to a third of a centimeter. The cosmic background radiation peaks at wavelengths shorter than this. Why not make measurements where the cosmic radiation intensity is highest? The problem is that the atmosphere becomes increasingly opaque at wavelengths shorter than 0.3 cm. For this reason, prior to 1989, the short wavelength part of the spectrum was studied using rockets and balloon experiments.

  In addition to the problem of atmospheric absorption there is the fact that our galaxy is a strong emitter of radiation at both radio and infrared wavelengths. This foreground emission interferes with measurements of the cosmic background. We are really interested in light coming from behind the galaxy. It is as if you are trying to look at some faint star in the sky but the city lights make the night sky too bright. The nearby bright light makes it harder to see the distant faint light. At radio wavelengths the emission from our galaxy comes from electrons emitting radio waves by spiraling in magnetic fields. At infrared wavelengths the emission from our galaxy comes mostly from dust. There is a window at a wavelength of 0.4 cm between the long radio wavelengths and the shorter infrared wavelengths where the cosmic background radiation is dominant.