<body bgcolor="#ffffff" text="#000000" link="#0000ff" vlink="#0000ff"> <center> <table cell border=10 cell padding=10 bgcolor=aqua> <tr><td><img width=600 src="http://hendrix2.uoregon.edu/~imamura/123/lecture-1/ CMB_Timeline150-wmap.jpg"> </td> <td><h2><center><font color=black>History of the Universe</font></center><p> <h4>Reading: Chapter 26, Cosmology and Chapter 27, The Early Universe</td> </tr></table> </center> <p><h3> <i> <font color=green> <ul> <ul> The <i>Big Bang </i> theory for the Universe is in good shape. The Universe is expanding, is bathed in a low temperature sea of background radiation (the CMBR), and has a chemical composition (primrily hydrogen and helium) which can be understood in terms of the Big Bang theory. There are issues for sure, but the <font color=magenta>basic Big Bang is secure</font>. </ul> <p> </i> </font> <p> A potential problem is that the <font color=magenta> Universe is inferred to be 13.7 billion years old</font> based on observations of the Cosmic Microwave Background Radiation (CMBR) and our current best version of the <font color=magenta><i>Big Bang</i> Theory </font>. The ages of the oldest stars in our Galaxy (the Milky Way galaxy) are found from studies of: (i) <font color=magenta>globular cluster stars </font> which are estimated to have ages to lie between <font color=red>11 and 18 billion years with 95 % confidence that they must be older than ~11 billion years</font>. If globular clusters are found to have ages closer to the upper end of their estimated ages, then significant alterations will need to be made to our current thoughts about the Universe. (ii) <font color=magenta>Cold white dwarfs</font> also offer a way to determine the age of the Milky Way galaxy. Hansen et al. (2004) found that the ages of the oldest white dwarfs are <font color=red>12.7 billion years with an uncertainty of 0.7 billion years</font>. This, again, is younger than the age of the Universe, but the outer edge of the estimate does place the ages of the oldest white dwarfs near the age of the Universe which could be problematic as structure, such as stars, likely did not form that early. <p><hr><hr> <p><h2> <center> OVERVIEW OF THE HISTORY OF THE UNIVERSE </center> <p> <center><table cellpadding=4 border=10 bgcolor=aqua><tr> <td bgcolor=white> <center><img width=700 src="radiation-matter-darkenergy.jpg"></center> </td></tr><tr> <td><center><h3><font color=blue>Timeline for the Universe: From Today to the Beginning</font></center> <p><h3>Today, the Universe is bathed in the 2.7 Kelvin CMBR and we have estimates for the amounts of matter (normal and dark) and dark energy in our Universe. Consequently, given ideas about how the Universe evolves, we can determine what the Universe was like when it was younger (and smaller). If we were run the clock backward from today, we would find that the Universe was hotter and denser and we would encounter almost unimaginably extreme conditions at early times. We would find that at the earliest times in the Universe, the Universe was dominated by <i><font color=magenta>radiation</font></i>. As the Universe expanded and cooled, eventually <i><font color=magenta> matter</font></i> dominated the evolution of the Universe. The crossover occured around 50,000 years after the <font color=magenta>Big Bang</font>. As the Universe continued to expand and cool, <i><font color=magenta>Dark Energy</font></i> took over and dominated the evolution of the Universe. This crossover occured around 4 billion years ago. We are currently in the <i><font color=magenta>Dark Energy</font></i> era. At early times, <i><font color=magenta>Dark Energy </font></i> is less important than <i><font color=magenta>matter</font></i> and <font color=magenta>radiation</font></i>, in general, and so may be ignored in the discussion of the <i><font color=magenta>Early Universe.</font> </i>This statement carries a caveat, however, in that some repulsive force did act early in the evolution of the Universe during the <font color=magenta>Era of Inflation. </font> In the sections below, we discuss the evolution of the Universe in more detail.</td> </tr></table></center> <p><hr><hr><p> <h3> <font color=magenta><center>Friedman Models and the Future of the Universe </font></center><p> </h3> The principal distinction between the models is the manner in which the Universe eventually winds-up its evolution. The early times are fairly similar. This simplifies our discussion. Before I continue, let me define some quantities which have appeared in some earlier plots, but which were not discussed in detail. <p> The ultimate fate of an <font color=blue>expanding Universe</font> is determined by the interplay of;<ul> <li><font color=magenta> the mutual gravitational attraction of mass in the Universe</font> <li><font color=magenta> the repulsive nature of Dark Energy</font> </ul> <p><hr><p> <center><table cellpadding=4 border=10><tr> <td><center><img src="http://hendrix2.uoregon.edu/~imamura/123/lecture-6/ scale.gif"></center></td></tr><tr><td><h3> The above two plots show the behavior of the Friedman models in two forms. The left figure shows how universes look in terms of how the redshifts of distant <i>galaxies</i> depend on their distances (this is the Hubble Law for small redshift, <i>z</i>). The right plot shows how the scale factor R(t) changes with time for the different universes.<p> Initially, the Universe was driven to expand by some unknown impetus. The current rate at which the Universe expands is measured by the <font color=magenta>Hubble constant H<sub>o</sub></font>. The expansion is slowed by the gravitational attraction of material contained in the Universe. If the amount of material is large enough then the expansion of the Universe will be halted and the Universe will eventually stop expanding and start to contract! In this sense, one can define an <font color=red><i>escape speed</i></font> for the Universe. If the Universe exceeds this <font color=red><i>escape speed</i></font>, it will expand forever. </ul><p> <ul><font color=blue>There is then some critical amount of material in the Universe (which determines how strongly gravity slows the expansion). For this precise amount of mass, the Universe assumes a critical form where it is <font color=magenta>neither</font> open nor closed, but is <font color=magenta>flat</font>. This amount of mass leads to the definition of the <font color=magenta>Critical Density</font>.<p></font> </ul> </td></tr></table></center> <p><hr><p> The critical density is given by <center><img width=150 src="critdensity.gif"></center> <p> where <b>H</b> (=H<sub>o</sub>) is the Hubble constant and <b>G</b> is the gravitational constant. Note that the faster the expansion (the greater the Hubble constant H<sub>o</sub>, the larger the critical density.) For the currently accepted Hubble constant value, H<sub>o</sub> ~ 22 km per second per million light years, the critical density is around 9x10<sup>-30</sup> grams per cubic centimeter. This critical density which controls the evolution of the Universe is very rarefied; the density of air in this room is around 10<sup>-6</sup>-10<sup>-5</sup> grams per cubic centimeter. <p><hr><p> We define the quantity known as <font color=magenta>Omega</font> as <center> <img src="Omega0.gif"> </center> For a <font color=magenta> flat</font> Universe, the total material in the Universe takes the form<p><center> <img width=220 src="total_omega.png"></center><p> where the material density is composed of two parts, the matter density composed of the <font color=magenta>dark matter and normal matter</font>, and the <font color=magenta>Dark Energy</font> component. Here, the Omegas are the fractions of the critical density contained in the matter component and Dark Energy component, respectively. For an <font color=magenta>open</font> universe, the measured density is less than the critical density and the sum of the Omegas is less than l and for a <font color=magenta>closed</font> universe, the measured density is greater than the critical density and the Omegas are greater than 1.<p><center><font color=magenta> The determination of H<sub>o</sub> and the Omegas are crucial for the determination of the ultimate fate of the Universe</font></center> and will be addressed in the next <i>Topic</i>. We now discuss how the Universe evolved from the time of the <i>Big Bang</i> to the current time. <p><hr><hr><p><center><table cellpadding=4 border=10><tr><td><h3> <center><font color=blue>TIMELINE FOR THE UNIVERSE</font></center> <p> In the following, we discuss each <font color=magenta>era</font> in the evolution of the Universe in more detail. The history is uncertain in the beginning, but after 10<sup>-11</sup> seconds, things are on very solid ground as by this time conditions in the Universe are such that much of the relevant physics may be probed in Terrestrial laboratories. </td></tr><tr> <td><center><img width=800 src="bigbang_eras.jpg"> </center></td></tr></table></center> <p><hr><hr><p> <ul> <li>0 ===> 10<sup>-43</sup> seconds, the <font color=red>Planck Era</font> <p> The <a href="http://hendrix2.uoregon.edu/~imamura/123cs/lecture-6/planck.html"> Planck era</a> when the <font color=magenta>four forces of the Universe -- the gravitational, electro-magnetic, strong (nuclear), and weak forces -- may have been unified</font>. This is the era of quantum gravity -- the time when a theory which encompasses both quantum mechanics and gravity must be used. <font color=blue>The size of the current Universe at this time is less than 10<sup>-50</sup> centimeters</font>. <p> <li>10<sup>-43</sup> ===> 10<sup>-32</sup> seconds, <font color=red> Era of the GUTs and On to Inflation</font> <p> <table cellpadding=10 border=10><tr> <td> <img src="http://hendrix2.uoregon.edu/~imamura/123/lecture-6/guts.jpg"> </td> <td><h3>Era of the <font color=magenta> Grand Unified Theories (GUTs)</font> when gravity separates from the other three forces (i.e., gravity becomes an interaction distinct from the other forces). This ended after around 10<sup>-32</sup> seconds had elapsed after the Big Bang. At this time the Universe had a temperature of 10<sup>27</sup>-10<sup>28</sup> Kelvin. <i><font color=magenta>Inflation</font></i> begins at the end of this stage -- <i><font color=magenta>inflation</font></i> is driven by some as of now, unknown repulsive force. We describe <font color=magenta><i>inflation</i></font> in the next section. </td></tr></table> <p> <li>10<sup>-32</sup> ===> 1 second, <font color=red>Inflation, the Quark Epoch, and On to Element Production, Nucleosynthesis</font> <p> The strong (nuclear) force becomes a distinct interaction at the beginning of this stage and <a href="http://hendrix2.uoregon.edu/~imamura/123cs/lecture-7/lecture-8.html"> inflation</a> stops at the beginning of this stage. During inflation, the Universe increases in size by a huge factor -- perhaps by as much as a factor of 10<sup>10<sup>12</sup></sup>!!! Some models say that the size of the current Universe increased from 10<sup>-50</sup> centimeters to roughly the size of a grapefruit during inflation. <font color=magenta>A region as small as the Planck length (~ 10<sup>-33</sup> cm) would expand enough to encompass the currently observable Universe.</font> <p><center><table cellpadding=4 border=5><tr> <td><img width=400 src="pair-production.jpg"> <p><center><img width=200 src="quark_2.jpg"></center></td> <td><h3><center>Pair Production and Annihilation</center><p> After inflation, the Universe continues to expand. The Universe is still exceedingly hot, T > 10<sup>15</sup> Kelvin and very energetic. The high-energy radiation carries very large energies. This has important consequences. In his <i>Special Theory of Relativity</i>, Einstein showed that there is an equivalence between mass and energy, <center><img src="emc.gif"></center> This means that high-energy radiation can transform into massive particles, if the radiation is energetic enough. Interestingly, the production always produces pairs of massive particles, one an ordinary particle and the other its anti-matter twin. In the inverse process, matter particles and their anti-matter twins will <font color=red><i>annihilate</i></font> to produce energy (radiation)! When the Universe is very hot, <font color=red><i>pair production</i></font> of <font color=magenta>quarks</font> and <font color=magenta>leptons</font> occurs.<center> <table><tr> <td><img width=500 src="quarks_leptons.jpg"></td> </tr></table></center> Something odd happens during this time, however, because for an unknown reason more matter quarks and leptons are produced than anti-matter quarks and anti-matter leptons, even though they should always be created in matter/anti-matter pairs. The slight excess of matter over anti-matter leads to the outcome that there is matter in our Universe today. </td> </tr></table></center><p><table cellpadding=20><tr> <td><h3> After <font color=magenta>inflation</font> the Universe is dominated by a <font color=magenta>quark-gluon</font> plasma. What are <font color=magenta> quarks</font> and <font color=magenta>gluons</font>? To the right is a schematic picture of what is known as a <font color=magenta>baryon</font>. <font color=magenta>Baryons</font> are a class of fundamental particles which includes <font color=magenta>protons</font> and <font color=magenta>neutrons</font>; protons and neutrons are composed of <font color=magenta>three quarks</font> held together by the intermediary particles known as <font color=magenta>gluons</font>. When the Universe is very hot and energetic, the quarks and gluons are free and are not tied up in <font color=magenta> baryons</font>. </td><td> <img src="three_quarks.jpg"></td> </tr></table> As the Universe expands it cools. After around around ~ 10<sup>-11</sup> seconds hve elapsed, the temperature is around ~ 10<sup>15</sup> Kelvin and the weak nuclear force and the electromagnetic force become distinct (a further <font color=magenta>symmetry breaking</font>. We now have a universe dominated by the <font color=magenta>four known forces</font> of the Universe, the gravitational force, the strong nuclear force, the weak nuclear force, and the electromagnetic force. From this time on, we are rather secure in our understanding of the Universe. The conditions in the Universe from here onward are attainable in Terrestrial laboratories and so amenable to detailed experimental study. <p> Around 10<sup>-6</sup> seconds after the birth of the Universe, quarks and gluons combine to form <i>baryons</i> (neutrons and protons). This marks the end to the <font color=magenta>Quark Epoch</font>. The production of baryons continues until the temperature drops below 10<sup>13</sup> Kelvin where the Universe is no longer energetic enough to produce baryonic matter/anti-matter pairs. Unfortunately, there is no barrier to annihilations of matter/anti-matter pairs and the annihilation process continues quite efficiently. As noted earlier, we are <i>lucky</i> in that there was a slight asymmetry between matter and anti-mattter quarks and leptons which allows some matter baryons to survive the annihilation! The production of the matter baryons is referred to as <font color=magenta>baryogenesis</font> while the existence of the slight excess of matter over anti-matter referred to as the <a href="http://hendrix2.uoregon.edu/~imamura/123cs/lecture-7/asymmetry.html"> Matter-Anti-Matter Asymmetry Problem</a>. <p> Because electrons are less massive than baryons, the similar event for electrons and their anti-matter twins, the positrons, occurs later, around ~1 second after the birth of the Universe when the temperature has dropped below 1 billion Kelvin.<p> <li>1 second ===> 100 seconds, <font color=red>Nucleosynthesis and the Nuclear Epoch</font> <p><center><table cellpadding=10 border=10><tr><td> <a href="4h-he.jpg"><img width=400 src="4h-he.jpg"></a><p><hr> <a href="Carbon-formation.gif"><img width=400 src="Carbon-formation.gif"> </a><p><hr> <a href="heavier_unstable.gif"><img width=400 src="heavier_unstable.gif"></a><p> </td><td><h3><center>Big Bang Nucleosynthesis</center><p> The final symmetry breaking has already occured by this time (at time 10<sup>-11</sup>seconds); quarks and gluons have combined to form protons; protons and electrons combined to form neutrons; and the stage is set for the formation of the elements in a process referred to as <font color=magenta> nucleosynthesis</font>. In <font color=magenta>nucleosynthesis</font>, <font color=red>hydrogen, helium</font>, a little <font color=red>lithium</font> and the <font color=red> hydrogen isotope deuterium</font> are produced in the few minutes after 1 second has elapsed into evolution of the Universe. The <a href="pp-bb.html">top panel</a> shows how helium nuclei are produced from four hydrogen nuclei (protons). The middle panel shows how the larger, more massive element <font color=magenta>Carbon</font> may be produced. This chain almost never completes in the early Universe because the intermediate step where Beryllium forms leads to a bottleneck. Beryllium decays (breaks apart into helium nuclei) after 6.710<sup>-17</sup> seconds, way too fast for it capture another helium nucleus. The bottom panel shows paths to form some lighweight elements through the capture of hydrogen nuclei (protons). </td></tr></table></center> <p><hr><p> <center><table cellpadding=10 border=10><tr><td> <img width=400 src="BB_nucleosynthesis.gif"></td><td><h3> In <font color=magenta>Big Bang Nucleosynthesis</font>, only <font color=red>hydrogen, helium, lithium and the hydrogen isotope deuterium</font> are produced in significant amounts (see the figure to the left). The amount of <font color=magenta>helium produced in the Big Bang is insensitive</font> to the mass contained in baryons in the Universe, but the amount of <font color=magenta>deuterium</font> is very sensitive to the mass in baryons in the Universe. <font color=magenta> Based on current measurements of <font color=red>the primordial abundances of hydrogen, helium, and deuterium</font>, we infer that the amount of <i>normal</i> matter which can be contained in the Universe is 3-4 % of the critical density (the gray bar in the figure to the left)!</font> <p> After the finish of the <font color=magenta>era of nucleosynthesis</font>, the Universe continues to expand and cool but no new chemical elements are created until stars form and die. </td></tr></table></center> <p> <li>380,000 years ===> <font color=magenta>Formation of CMBR</font> <p><cente><table cellpadding=4 border=6><tr><td><h3> <center>Recombination and the CMBR</center><p> The Universe becomes cool and rarefied enough for electrons and protons to form neutral hydrogen atoms. This occurs around when the temperature of the Universe has dropped to around 3,000 Kelvin (<i>z</i> ~ 1,100). <p><center><table cellpadding=5 border=0> <tr><td><img width=300 src="hydtube.jpg"></td><td><h3>Hydrogen gas is not an efficient absorber of light and so the Universe becomes transparent. To understand look at the figure to the left which shows a tube that contains hydrogen gas and the spectrum of the tube at the lower right. An electric current is run through the tube exciting the hydrogen atoms causing them to radiate. Notice that the hydrogen atoms produce only certain colors (wavelengths) of light. Absorption being the inverse of emission means that the same hydrogen atoms will absorb only the same colors of light which it emits. Most of the light impinging on hydrogen gas is unaffected by the presence of the gas!</td></tr><center><table cellpadding=6 border=0><tr> <td><h3>Before the <i><font color=magenta>Era of Recombination </font></i>, the Universe was opaque to radiation, that is, the Universe was "foggy" in that light could not travel very far which meant that our view of the Universe was limited. After <font color=magenta> Recombination</font>, the Universe cleared up, it became transparent and our view of the Universe was unimpeded. <font color=magenta>What are the consequences of this?</font></td> <td><img width=600 src="recombination.jpg"> </td></tr></table></center><p> <table cellpadding=10 border=10><tr><td> <center><img src="cmb_scatter.jpg"></center></td></tr> <tr><td><h3>After <font color=magenta>Recombination</font>, we can get unimpeded view of the distant Universe. So, as the Universe gets older, we can see further and further into the past. However, we cannot see forever because at some point the Universe becomes opaque; we can only see up to <font color=magenta>Recombination</font>, that is, we can see the Universe as it was 378,000 years after the Big Bang. This is what we see when we observe the Cosmic Microwave Background Radiation, the <i>CMBR</i>. This ia analogous to the situation we encounter when we see the light from a cloud in the sky. We see the clear space between the cloud and us, but we cannot see into the dense, opaque part of the cloud. </td></tr></table> <p><hr><p> <li>10<sup>6</sup> ===> 10<sup>9</sup> years, <font color=red>Stars, Galaxies, and Large-Scale Structure Formation</font> <p><center><table cellpadding=10 border=10><tr><td> <img width=600 src="http://hendrix2.uoregon.edu/~imamura/123cs/lecture-6/GalaxEvC.gif"> </td><td><h3><font color=magenta> Protogalaxies</font> begin to form. This may be annoying since the clumps of matter and the clumps of the clumps of overdense matter should distort the CMBR. The reason is that if these clumps are formed by the attraction of gravity, then such large overdensities take a fairly long time to form (recall the discussion of the free-fall time). If the Universe was only <font color= magenta>normal matter</font>, the observed variations in the CMBR are too small to explain the observed structure in the Universe. We can reconcile this problem by the existence of large amounts of <font color=magenta>Dark Matter</font>. Significantly, this allows the overdense regions to start growing early in the evolution of the Universe; the origin of the seeds which eventually lead to the current structure in the Universe have their origins at <font color=magenta>Inflation</font>.<p>Currently, the largest structures in the Universe are consistent with the imperfections seen in the CMBR. If we see much larger structures, then there will be trouble. </td></tr></table></center> <p><hr><p> <li>1 billion years ===> 3 billion years <p> Quasars and some radio galaxies become detectable <p><hr><p> <li>3 billion years ===> 13.7 billion years and <font color=red>Dark Energy Era</font> <p> Solar System, planets, life, ... <p><hr><p> <li>13.7 billion years ===> <a href="http://en.wikipedia.org/wiki/Future_of_an_expanding_universe"> End</a> </ul> <p><hr><p> <center> <a href="http://hendrix2.uoregon.edu/~imamura/123cs/astro.123cs.html"> Return to Home Page</a>