The big bang is a model that describes the circumstances that lead to the creation of the universe. The term “the hot big bang” was coined by astronomer Fred Hoyle out of frustration over not being able to believe that there was a single instant in time when the universe was unimaginably small and then just a fraction of a second later (actually less than 10-43 seconds) the universe was unimaginably large and that it has been expanding ever since. The name “Big Bang” and its associated concepts have found there way into contemporary society and are now widely know as the beginning of the universe.
The big bang model explains that the universe began approximately 15 billion years ago as a hot dense soup sometimes referred to as “quark soup.” This “soup” was so hot and dense that only the smallest pieces of atoms were present, thus the name quark soup. The universe at this time was extremely hot and dense, and only two of the four fundamental physical forces were present, namely, the strong and weak forces. These forces only act on extremely small particles such as quarks, gluons, and leptons – the things atoms are made of. The universe at this time experiences what some cosmologist believe was a very rapid inflation that created the immense size of the universe and also amplified any small difference that existed at this very early stage. The amplification of these differences are not amplifications of their size, but rather amplifications of the scale that separate these differences. It is important to note that this rapid inflation is only another model that modern cosmologists have devised to explain the observational consequences that they now see, and as such has not yet been proven. None the less, one very important piece of this puzzle to understand is that at this very early time when the universe is only fractions of a second old the proton to neutron ratios that we now see today have been established and the path to present day is set. Later we will see that present day cosmologist are studying the amplifications of these small differences by studying the cosmic microwave background (CMB) and that this field of cosmology is very active.
A very short time later (3-minutes) the universe is still extremely hot (about 109 degrees Kelvin) and still very dense but it has cooled sufficiently that the nuclei of light elements such as hydrogen, helium, and lithium are forming along with a special type of hydrogen called deuterium. This formation of nuclei is made possible by the third of the four fundamental physical forces the electromagnetic force. The electromagnetic force is responsible for electrically charged particles being attracted to each other. The nuclei of these light elements now comprise the majority of all visible mater in the universe. However, the universe is still too hot and dense at this point in time for photons to escape and begin their travel through the universe. Photons are still prisoners captured in a maze of radiation bouncing off protons, neutrons, and electrons.
A short time later in astronomical terms about 300,000 years the universe has continued to cool and is approximately 3000 degrees Kelvin. The electromagnetic force has had time to allow nuclei to capture electrons and now most of the universe is in atomic form. With the capture of electrons by nuclei photons are able to stream through the universe in a straight path from their release. This point in time is referred to by cosmologist as decoupling or the first scattering of light and is the furthest back in time that we can see the visible universe. The Cosmic Microwave Background is the distant glow of the very distant past universe. This glow is seen as an almost uniform distribution of temperature at approximately 2.73 degrees Kelvin, and has been used by cosmologist to demonstrate that the universe is flat, and confirm that the universe is expanding at an accelerated rate. Once decoupling has occurred and the universe is in atomic form the fourth fundamental force of physics, gravity becomes dominant and slowly pulls together the universe, as we know it today.
Gravities role in the universe is responsible for what astronomers refer to as large-scale structure formation. Such formation is responsible for star and galaxy formation and ultimately heavy element formation due to the death of stars. Large-scale structure formation took many billions of years and we when we look out at the night sky or study images from the Hubble space telescope (HSS) we are looking back in time to the beginning of this structure formation.
The question of what is the Universe like now is closely related two central questions in cosmology. At what rate is the universe expanding, and what forces are responsible for the formation of the universe as we see it? Edwin Hubble during the 1920’s played a key role in establishing what galaxies are all moving way from us very quickly. In 1924 Hubble measured the distance to the Andromeda galaxy (our closest neighbor galaxy) and showed that it must be a separate galaxy because of its great distance from us. Hubble went on to measure the distance to many other galaxies and soon realized that he could measure the speed with which the galaxies are moving toward or away from us by calculating their Doppler shift. Edwin Hubble looked at galaxies and the type of light they emit to calculate their Doppler shifts.
When you look at a visible astronomical object through a prism or gradient you can split the light it emits into a rainbow of colors. Visible light can be easily split into a spectrum of colors (red-orange-yellow-green-blue-violet). With highly specialized equipment called a spectrograph Hubble was able to split the light from distant galaxies and study their Doppler shift. When he did so he noticed that the light from galaxies that are further away from us appear to be stretched so that it appears redder than normal. This shifting is caused by the Doppler shift. The Doppler shift describes what happens to light (or sound when it travels toward or away from an individual). When light travels away from an individual it is stretched so that it appears redder. When it is traveling toward an individual the light will appear bluer. Hubble was able to determine the Doppler shift or what astronomers commonly refer to as the red shift by calculating the ratio of change in the wavelength of the light. By doing so he was able to then calculate the speed that the galaxy must be traveling away from us to account for this shift. Hubble graphed his findings and this graphical representation is now known as the “Hubble diagram”. By plotting his data Hubble learned that every galaxy he looked at was traveling away from us, that galaxies which are further away from us are traveling faster than those that are closer and furthermore that they are all traveling away from each other! However, Hubble was not able to survey the universe as completely as we are today, and today we have been able to refine Hubble’s original estimates and now believe that the universe is composed of between 80 and 120 billion galaxies all moving away from us at approximately 50 kilometers per second per megaparsec (50 km/s/Mpc). This constant is commonly referred to and the Hubble constant (Ho) and it is still being refined. Before we continue with this story it is important to understand the “measuring stick” that astronomers use to measure very large distances. Two units of measure that astronomers use are the light year (ly) and the parsec (pc). One light year is the distance light travels in one year at a velocity of approximately 300,000 kilometers per second (km/s). At this rate light travels approximately 9.4 trillion kilometers in one year. Another, larger unit of measure used is the parsec. One parsec is approximately equal to 3.26 ly and a megaparsec (Mpc) is equal to one million parsecs. So in the case of the Hubble constant a galaxy that is 1 megaparsec away will be traveling away from us at a velocity of 50km/s and a galaxy that is 10 megaparsec away will be traveling away at 10 times this speed!
The visible universe that we observe is made up of large-scale structure. At the local level our solar system is composed of one star (our sun) and nine planets. We live at the edge of the Milky Way galaxy, which contains approximately 200 billion stars and is approximately 100,000 light years in diameter. The Milky Way galaxy belongs to a local group of about 30 galaxies and this local group is referred to as a cluster. When astronomers look out at the universe they see other clusters of galaxies and that these clusters form super clusters containing about 12 clusters each. With all of these clusters and super clusters one would think that the universe is so completely dense that galaxies are literally on top of one another. However, if you look at the universe and even more closely at our solar system you will quickly realize that the vast majority of space is empty. Cosmologists call these apparently empty spaces voids and bubbles, but these voids and bubbles are deceiving, as we will find out.
Surprisingly, all of the visible matter in the universe only accounts for only approximately five percent (5%) of all the matter. That’s correct, everything we see in the universe accounts for only 5% of the mass of the universe. Another way of saying this is that ninety-five percent (95%) of the universe is made of unknown matter and energy! Cosmologists refer to all of the normal matter that they can see in the universe as baryonic. There are however other forms of matter and energy that must exist if the total density of the universe is equal to one hundred percent (100%), and they are called dark matter and dark energy. Cosmologist have determined that dark matter accounts for approximately twenty-nine percent (29%) of the total density of the universe, and that dark energy accounts for approximately sixty-five percent (65%) of the density of the universe. If you add up these totals you will quickly notice that we have not accounted for one percent (1%) of the density, but cosmologist have determined that the missing 1% is accounted for by counting a relatively exotic particle called the neutrino. You should at this point be asking yourself how do they know all of this? The answer has to do with gravity!
Gravity since very early in the universe (when the universe was only about 300,000 years old) has been pulling together all of the large-scale structure that we have been talking about. The problem that cosmologist quickly realized was that if the universe is expanding at an accelerated rate and if gravity is doing the pulling, then there must be some form of unseen matter and energy that is doing the pulling because the portion of the universe that can be seen cannot explain the accelerated expansion that is also observed. Additionally, cosmologists for a long time have wrestled with three important questions regarding the faith of the universe. Is the universe open and will it expand forever, is the universe closed and will expansion eventually stop and cause the universe collapse upon itself, or is the universe flat, and precisely balanced? To determine which one of these questions is correct cosmologist need to precisely determine the density of the universe. When talking about the density of the universe cosmologist use the term Omega (W) and talk about Omega being less than one, greater than one or exactly equal to one. Omega is commonly referred to as the critical density factor of the universe, because depending on the value of Omega the universe will end up expanding forever, collapsing, or teetering perfectly balanced forever. When astronomers look at distant galaxies they ask themselves the question, “If galaxies are all moving away from each other at such tremendous velocities then why don’t they simply fly apart, and why are they attracted to each other? “The only answer they come up with is that there must be some “Great Attractor” that is moving galaxies together and causing their accelerated expansion. The most likely candidate as to the nature of the Great Attractor is dark matter and dark energy! While the precise nature of dark matter and dark energy are still not known recent observations of the Cosmic Microwave Background have confirmed that the universe is indeed flat, that the universe looks the same in all directions which is referred to as being isotropic and that the universe is homogeneous or that at very large scale the volume of space that you look in the universe at is like any other volume of space that you look at in the universe. The principle that the universe is both isotropic and homogeneous is called the cosmological principle. The implication of these findings points directly to some unseen matter and energy working in the universe to ultimately determine its faith. The study of the cosmic microwave background and the clues that have lead cosmologist to proposing the possibility of dark matter and dark energy began in 1989 with the launch of a satellite called COBE (COsmic Background Explorer) which carried with it three specialized instruments to measure the cosmic microwave background. Most recently an earth-based telescope called DASI (Degree Angular Scale Interferometer) confirmed and improved upon the measurements of COBE and settled the argument that the universe is flat and strongly suggest the existence of dark matter and dark energy.
The question of “Where is the universe going” is highly speculative, but there are some general inferences that can be made based on the current observations. One such inferences is that the universe expanding and many astronomers believe that it is doing so at an accelerated rate, and that dark matter and dark energy play a vital role in understanding this expansion. Another inferences is if accelerated expansion is true and galaxies will continue to move away from each other then as a consequence the universe will become darker and colder than it is today as more stars reach the end of their life cycle and burnout. To study these questions there are several current and proposed investigations that hopefully will shed light on the future of the universe. They are:
As you can see the ultimate answer to the questions of how did the universe begin, where is the universe now and where is the universe going? depend upon further investigation and solving the mystery of the universe. This primer is intended to start you along with your students on the adventure of discovering the facts that have lead to our understanding of the universe. There are many additional resources listed at the end of this booklet for those interested in pursuing cosmology in more detail.