Federation of Galaxy Explorers Space Science Once Upon A Big Bang
Learning Objectives: 1. Explain how the universe was created using the Big Bang theory. 2. Understand how the existence of Cosmic Background Microwave Radiation supports the Big Bang theory. 3. Identify possible sources of dark matter and how it may affect the fate of the universe. 4. Define the Hubble Constant and how it relates to the age of the universe. One of the fundamental questions throughout history has been “How did it all start?” While scientists still haven’t been able to prove a theory conclusively, the Big Bang theory is the best explanation available for the origins of the Universe. The Basics The Big Bang theory suggests that the known universe started when all the matter and energy that is currently in the universe expanded explosively from a single hot, dense, and infinitely small point. Everything that we now see in the universe was squished into an incredibly small area. For some reason (scientists still don’t know why) that clump of mass began to expand extremely quickly in all directions. This was known as ‘the Big Bang’. That expansion continues today at a much slower rate. All of the matter that combined to form the stars, planets, comets, and everything else in the universe came from that initial big bang.
In the beginning… When the big bang occurred, all of the matter in the universe expanded rapidly in all directions. This matter was extraordinarily hot and roughly uniform. In the first few seconds after the big bang, the matter and energy was exceedingly hot: trillions of Kelvin. By about 400,000 years after the big bang the universe had cooled to around 4000K - enough for the free protons, electrons, and neutrons to combine to form hydrogen. As the universe continued to cool, other types of atoms formed, eventually coalescing into stars. The Leftovers When the big bang happened, matter and energy were scattered across the universe in a basically uniform fashion. As the universe cooled, some matter combined to form larger objects. Some of the energy from the big bang still exists – this can be thought of as the leftover heat from the big bang. This uniform distribution of heat is called the Cosmic Microwave Background Radiation (CMB). The fact that CMB radiation is mostly uniform supports the big bang theory in that matter expanded from the big bang in all directions, so there is no reason for one area to have significantly more radiation than any other area. The figure below is from the MAP project and shows a false color image of the CMB radiation across the universe. Note that any variations are completely indistinguishable. The COBE telescope measured the CMB radiation even more precisely. COBE found that while it is primarily uniform, there are some small variations. Again, this characteristic supports the big bang. Many scientists believe that there must have been small variations in the initial distribution of matter and energy at the big bang. These minor variations prompted some matter to start combining to form atoms and eventually evolve into stars. The other important characteristic of CMB radiation is its temperature. Because the CMB has been continually cooling since the Big Bang, it is now extremely cool: 2.726K. Remember that it took over 400,000 years just to cool to 4000K!
How old are we? Scientists are still debating the age of the universe. Most scientists think that it is between 10 billion and 20 billion years old. There are two ways scientists try to determine the age of the universe. The first method involves determining how old the oldest stars are. It makes sense that the universe is at least as old as the oldest star. The other method is based on the expansion rate of the universe. They judge this based on the red shift that is apparent in stars and galaxies. Edwin Hubble discovered that the further away an object is, the more the apparent red-shift. This suggests that objects farther from us are moving away more quickly than near-by objects. The relationship between distance and speed is characterized by the Hubble Constant. Once you know the speed of an object, and its distance away from a point, you can calculate when it was at the initial point. (If you know a car travels at 55miles per hour (its speed) and that it has traveled 55 miles (its distance), than you know that it has been traveling for an hour, right?) In the case of the universe, since everything is expanding away from everything else, if scientists can calculate the speed an object is traveling and the distance of an object away from Earth, they can determine when it was with the Earth – theoretically at the big bang. Measuring the distance of stars is difficult, but scientists are getting better and better at it every day. Since the Hubble Constant determines the relationship between an object’s distance and its velocity, scientists can figure out how fast a star is moving as long as they know the Hubble Constant and its distance. Once they know the distance and velocity of an object, it is easy to calculate the time that object has been traveling away from us, and hence the age of the universe. Sound easy, right? Unfortunately, measuring the distance to stars is a difficult task and figuring out the Hubble Constant is even harder. Scientists and astronomers are still trying to accurately measure the Hubble Constant. Projects like the Hubble Space Telescope are continuing to refine stars’ distances and investigating the relationship between distance and velocity. The search for the Hubble Constant is one of the most important issues in Cosmology today.
The Fate of the Universe Scientists generally agree that the universe is expanding and that it is expanding at a slower rate now than it used to be. That makes sense, because while objects are moving away from each other due to the big bang, they are also being attracted to each other because of gravity. Depending on how strong gravity is, the rate of expansion will slow down faster or more slowly. One force – the result of the big bang – pushes the objects apart from each other. The other – gravity – pulls the objects closer together. The stronger the pull of gravity, the more slowly objects will move apart. If gravity gets strong enough, the objects will start to move together instead of apart. The strength of gravity depends on how much matter is in the universe. Scientists are confused right now about how much matter is out there. Velocity and the shape of orbits can be used to determine the mass of an object. However, using these methods, the mass of galaxies is about ten times larger than the mass that is measured when scientists add up the mass of all the visible stars in a galaxy. The remaining mass comes from what scientists call “dark matter”. They are not sure exactly what dark matter is, but there are three primary possibilities. Dark matter may be from brown dwarfs, stars whose cores are not hot enough to burn hydrogen or deuterium, so they aren’t visible like other stars. Another idea is that dark matter is made up of super-massive black holes. Both brown dwarfs and black holes are made up of the same type of matter that form the planets and stars. Another theory, though, is that dark matter is actually a different kind of matter entirely. Scientists have designed supercolliders to speed up two particles of normal matter and smash them together. They call this new form of matter WIMPs (Weakly Interacting Massive Particles). It is important to know how much matter is in the universe, because that will determine the fate of the universe. If there is enough mass so that the force of gravity becomes stronger than the force from the big bang, the universe will start moving together, instead of expanding. This will eventually lead to “the big crunch”. The closer objects get to each other, the stronger the force of gravity will be, and the faster the objects will move toward each other. Eventually all of the matter will be crunched together – maybe in the same way it was right before the big bang. If, on the other hand, there is not a lot of mass (relatively speaking) the universe will continue expanding forever. The final option is that the universe is perfectly balanced between the motion from the big bang and the force of gravity. In this case, the universe has critical density. The universe will continue expanding forever, and the rate of expansion will continue to slow down, approaching zero, but never stopping completely. All current measurements of the density of the universe indicate that it is close to critical density. Scientists are doing a lot of research to answer the questions about the density of the universe – and to determine its fate. Says Who? Arno Penzias and Robert Bell discovered CMB radiation
A version of the Big Bang theory was first suggested in 1927 by a Belgian priest named Georges Lemaitre. He suggested that the universe began with the explosion of a “primeval atom”. While his description of the mass as an atom was incorrect, the basic
concept has been supported. In the early 20th century, astronomers noted that most stars and galaxies are red shifted. Then, in 1929, Edwin Hubble established what is now known as the Hubble Law: the farther away a galaxy is, the larger the red shift. This suggests that all of the objects in the universe are moving away from us, and that the further away they are, the faster they are moving. As the big bang theory developed, George Gamow predicted the existence of the cosmic microwave background radiation in 1948. The CMB radiation was accidentally discovered in 1965 by Arno Penzias and Robert Wilson at the Bell Telephone Labs. They were building a radio receiver, and were stumped by the excessive background noise the receiver was picking up. When researchers at Princeton University heard about the dilemma, they quickly identified the source of the noise: CMB radiation. In the last several decades, telescopes have measured the CMB temperature and distribution as well as more accurately measuring the distance of stars. Results from the Hubble Space Telescope, Chandra Telescope, the Microwave Anisotropy Probe (MAP), and the Next Generation Space Telescope will continue to refine and expand the big bang theory.
Activities: 1. Get a balloon and a dark permanent marker. Before you blow up the balloon, draw dots all around the balloon. As you blow up the balloon, notice the dots move away from one another. This can help you understand how the universe is expanding in all directions. From the point of view of any one dot, all of the other dots are moving away from it, and the surface of the balloon in general is expanding. 2. Draw a timeline of the universe starting at the big bang. Include the evolution of atoms, stars, planets, solar systems, and any other events you think are important. 3. Write a “Biography of a Proton” book for Third Class Galaxy Explorers. Explain how the proton developed from its place in the initial dense clump of mass through the big bang to wherever that proton is now.
4. Research some other concepts for the origins of the universe. You may want to focus on historical trends, religious ideas, or contrary scientific hypothesis. As a group, discuss the merits of these other concepts based on our current observations of the universe. 5. When discussing temperatures on the universal scale, scientists use the terms “Kelvin” and “Absolute Zero” instead of Celsius or Fahrenheit. Do some research to learn what Absolute Zero is and how temperatures in Kelvin are related to Fahrenheit measurements. Convert 4000K (the temperature when atoms began to form) and 2.726K (the temperature of CMB radiation right now) to Fahrenheit. How do those values compare to “hot” and “cold” temperature we experience here on Earth? 6. The Big Bang theory is constantly being revised and refined. Look for current news articles about the big bang. What is the latest development of this theory? What is the current best guess for the age of the universe? What is the current estimate of the Hubble Constant? What recent data has been collected to support or disprove the big bang theory? Note: All images courtesy of NASA