Some of the most basic questions that people have been pondering since people started pondering things are: What is the Universe like? How old is the Universe? How was the Universe created? How will the Universe end - if it does end? Where are we in the Universe? Why is there never anything good to watch on tv? Maybe not that last question, but the rest of those questions are sort of basic philosophical questions that have been thought about over time. Now I think we need to start tackling them. First we'll try to figure out what the Universe is like. The study of the nature of the Universe is known as Cosmology.
How big is the Universe? That's a pretty straightforward question. What's the answer? The first person to tackle this question with some real scientific gusto was Isaac Newton. Of course, he had an advantage over the people who thought about it before he did, since he had the Law of Gravity to help him in his quest for an answer. First of all, what if the Universe had a given size, so that it extended only out to a certain distance - that is to say, what if the Universe was finite? Newton thought that this can't be the case. Why? When he looked up in the sky beyond the Moon, the Sun and the Planets, he did not detect any motions of objects. He thought that the Universe was static. This doesn't mean that it had lousy reception, but that the Universe did not appear to move on large scales. What does that tell you? Newton knew that the Universe had stuff in it - stars, fuzzy things, etc., and he knew that these things had mass. All of the stuff in the Universe was attracted to all of the other stuff in the Universe (just basic gravity at work here). If that is the case, then the stuff on the edge of a finite Universe would be attracted by all of the other mass in the Universe and would be pulled toward the center of the Universe. If that happens, the stuff on the edge of the Universe would be moving toward the middle. That sort of motion wasn't seen by Newton or anyone else during his time - the saw no large scale motions. The stuff at the edge (if there is an edge) isn't moving inwards. What could prevent that? - probably more stuff further out. What is preventing the other material from being pulled inwards? There'll have to be even more stuff out there to prevent that stuff from being pulled in. What about the stuff further out; won't it be pulled in? - not if there is even more stuff. I think you can see where this is going. If there were an edge to the Universe, the stuff at the edge would not sit still, and since Newton thought that stuff was sitting still (since he saw no motion), that meant there was no edge.
Figure 1. Newton's argument against a finite Universe. If the Universe started out having a certain size, then the gravitational pull of stuff near the center of the Universe would cause the stuff near the edge of the Universe to be pulled inward. Eventually, stuff on the edge of the Universe would be moving inward to the center and there would be obvious large scale motion toward the center.
What do I mean no edge to the Universe? - just what I said. Newton thought that the Universe was infinite in size. This was the only way that he could explain the lack of motion of objects on the largest scales. He of course didn't know about all of the motions we see today, since his technology wasn't able to detect it. Newton wasn't really happy with the idea of the Universe being infinite. If that were the case, then that would mean there must be an infinite amount of mass as well. If there was an infinite amount of mass, what does that say about the gravity? - you got it - infinite! Actually, saying the Universe is infinite doesn't really make sense when you look at the gravity of the whole Universe, or the Universe as Newton saw it, but that was the best he could make of it.
Figure 2. Why having an infinite Universe may be a bad thing. When astronomers look through their telescopes at the night sky, they see galaxies scattered around and a lot of dark space (view on the left). If the Universe was infinite, then there would be an infinite amount of starlight and the telescopic view would be what is shown on the right - a blazing white sky.
While an infinite Universe may seem reasonable, there are problems with this. One of the problems that comes from having an infinite Universe is known as Olbers's Paradox. This one is a bit confusing, so bear with me. If the Universe is infinitely large, infinitely old, and filled with stuff in every direction (so that gravity is in a perfect balance and nothing moves), then when you look in any direction in the sky you should see something. You may have to look a great distance, but eventually you will see something. You should see a galaxy, a star or something in all possible directions in the sky. When you go outside at night, you don't see something in every single direction of the sky; there are a lot of places where nothing is seen. Another way of stating Olbers's Paradox is to ask the question, "If the Universe is infinite, why is the night sky dark?" An infinite Universe would have an infinite number of objects in it, which are giving off various types of light depending on their temperatures, so there should be an amount of light in the Universe. If this is case, there would be constant brightness everywhere and at all times - day or night.
That's a fine pickle. How do we get out of this predicament? Answer: the Universe could be infinite in size, but it is not infinitely old. It is not infinitely old, so the stuff that is in certain directions isn't visible to us yet, because it takes time for light to travel from those distances to us. The light from the most distant objects hasn't reached us yet. We will only be able to see these distant objects if we wait long enough for their light to get to us. This is the problem with having light travel at a certain speed - you can't see everything right away. As you'll see later, the Universe is expanding, and this affects the light from distant objects - stretches it out into wavelengths that we can't see with our eyes. Light can leave a distant galaxy at visible wavelengths, but the light will get so stretched out into longer wavelengths by the expanding Universe that our eyes won't see it - another reason why the sky isn't full of bright lights at night.
So is the Universe infinite in size? It could very well be, however we'll never be able to see it as such. Why? It has to do with that fun little concept from Special Relativity, the speed of light is constant. We can only see parts of the Universe whose light has gotten here. In other words, if the Universe is 5 billion years old, we could see out to a distance of, at most, 5 billion lightyears. If it is 10 billion years old, we could only see out to a distance of 10 billion lightyears. We can only see out to a distance from which light has gotten to us. Actually the distances I just used are a bit incorrect, since I didn't account for the expansion of the Universe, but even that would not help us to see areas outside of the observable Universe. To see an entire, infinite Universe, you'd have to wait until the Universe were infinitely old.
It seems that the idea of having an infinite Universe is okay, so long as the Universe isn't infinitely old. What else is known about the Universe? One thing we can look at is how the Universe looks to us - what are some general things about it that we can describe? First of all, when you look out over great distances you see stuff everywhere on large scales. This distribution of material is known as Homogeneity. Now we know that galaxies aren't uniformly spread around and that clusters aren't uniformly spread around, but when you look at the largest scales, distances in the billions of parsecs or so, then things look pretty smoothed out. That's sort of like describing the ocean. When you are on a boat in the ocean, you may notice how non-uniform it is, that there are waves that are high or low. If you look at the ocean from a great distance, like in the space shuttle, it looks pretty smooth. On large scales (or the largest scales) the Universe is pretty homogeneous (uniform).
Another feature of the Universe is known as Isotropy. This concept says that the Universe looks the same no matter where you are looking and no matter where you are located in the Universe. This sort of prevents there from being a center to the Universe, since everyone sees (again on the largest scales) the same stuff. There is no preferred location; all the seats are good.
When the concepts of homogeneity and isotropy are combined together, they make up the Cosmological Principle. This is sort of a rule that needs to be followed when people think about making models of the Universe. Another way of looking at the cosmological principle is to say that the Universe is boring! Think about it - both parts of the C.P. refer to how uniform, same, non-unique the Universe is - dullsville, man; bland as bland can be.
The Cosmological Principle helps us describe the Universe, but there is a much more important rule that we use when we try to figure out how the Universe works. That is the concept of Universality, which basically says the laws of physics are the same everywhere, which means that if gravity behaves a certain way here, it should behave the same way elsewhere in the Universe. Atoms should behave the same way, light should be the same, and all the rules should be the same no matter where in the Universe you are. We really need this rule to be true or else it would be impossible to understand the Universe, if the laws of physics were random or screwed up all over the place.
We've seen what Newton thought of the Universe, but what did modern astronomers think of it? By modern I mean astronomers working over the past 100 years. In the early part of the 1900s the technology available to astronomers was just getting sophisticated enough to cause some major disruptions in the theories of the Universe. One set of observations was of galaxy spectra by a fellow named Vesto Slipher (1875-1969). He obtained many spectra of spiral nebulae (remember, at this time people still didn't know what the spiral nebulae were). He noticed that practically all of the spiral nebulae showed redshifts - that they were moving away from us.
Slipher's observations led to Edwin Hubble's discovery that the Universe was expanding (in 1929), as is illustrated in the Hubble Law. One aspect of this expansion is that the more distant galaxies are moving away from us the fastest. What does this tell us about the expansion? The type of expansion observed by Hubble is a Uniform Expansion. So what? Is this really all that important?
First, remember that there is no preferred location in the Universe - no center, and everyone will see the same uniform expansion regardless of their location (remember the Cosmological Principle). Let's see what happens when we expand the Universe uniformly. Each part expands in the same way. To illustrate this simply, let's pretend that the Universe is a straight line with a bunch of galaxies along the line. If the Universe were to expand uniformly, then we could have the space between each galaxy double - all would follow the same expansion rate. What do we see?
Figure 3. The uniform expansion of the Universe. When the galaxies start out, they are equally spaced apart. After a time, the Universe expands uniformly; in this case, the distance between each galaxy doubles. We see the nearby galaxies moving only a little way (slower velocity), while the more distant galaxies travel a greater distance (higher velocity).
We see, after the expansion, that the galaxies close to us didn't go very far, so they didn't move very fast. Galaxies that are much further away went a very great distance in the same amount of time. To do this, they must be moving very fast. This is exactly what the Hubble Law shows. It doesn't matter if you are located on galaxy A, B or X, you would see the same thing. This is in part due to the tendency to not imagine ourselves moving, but perceive everyone else as moving. This also means that there is no center to the Universe; the Universe isn't expanding away from just us, but from everyone. It is hard to visualize the expansion using a one or two dimensional model, since you are seeing that model relative to a background, but without anything to compare it to there is no fixed center of the expansion. You could also go the entirely opposite direction and say that everywhere in the Universe is the center of the Universe, since the Universe is expanding away from every point - ouch, my head hurts...
Figure 4. The uniform expansion of the Universe seen from other locations. Here an alien in what we call galaxy D is observing the expansion. In this case, they also see all other galaxies (including our own) moving away from them. If they didn't see such a thing, then there would be no isotropy in the Universe, and some parts of the Universe would see different things - that's not possible. No matter where you are in the Universe, it looks like everything else is moving away from you. Don't take it personally.
When we say that the Universe is expanding do we really mean that or do we mean to say that stuff is moving outwards? Actually, here's where things get a little hairy. The expansion of the Universe is just that - the Universe actually expanding. What exactly is expanding? Space is expanding. Huh? Space is expanding? Yes, that's what I said. Space is expanding, and since things like galaxies are located within this space, they get carried along by the expansion. The expansion of the Universe doesn't just mean that galaxies are moving away from one another on their own, it means that the space that those galaxies are located in is stretching out, causing the galaxies to move away from one another. Galaxies have their own velocities around one another, in clusters and such, so that motion has to be separated from the expansion velocity, but when you get to very great distances the random motions of galaxies are so much smaller than the expansion velocities. Good grief, this is silly, isn't it?
No, it isn't silly, because we have already messed up space with the concepts outlined in General Relativity. If we can warp space, can't we also stretch it? Sure we can, but how are we stretching it? In which directions is it getting stretched, how can we see that and just how does space get screwed up by this stretching? These questions sort of depend upon the curvature of the Universe. Uh-oh, it looks like we're going to be looking at some more weird graphs of warped space! Remember, with the concepts of General Relativity, space can be expanding into another direction, like into a fourth spatial dimension. Can we see this? No, not directly, since we are three dimensional creatures. To understand the possible shapes or the curvature of the Universe we live in, we need to use some lower-dimensional analogies. Instead of describing space as a three-dimensional object, we'll use a two-dimensional analogy which can be distorted (at times) into three spatial dimensions. Now you can pretend that you are ants crawling around in two-dimensional space, where you can perceive only two directions (you can't see up-down). What do you see when you look around in your two-dimensional Universe?
Figure 5. A two-dimensional flat Universe. Creatures in this Universe only know of two dimensions and things here would be similar to what you find on a flat sheet of paper. Triangles have angles that add up to 180 degrees and parallel lines would always stay parallel - never getting any closer or further away. Even though this image has an edge, there would be no edge in the flat Universe and it could easily expand uniformly.
The first type of curvature we'll look at is the simplest - no curvature, or a flat Universe. Stuff in this type of Universe acts like it would when things are drawn on a big flat piece of stretchy material. When it is stretched out uniformly, things keep their relative shapes and dimensions. Things behave like you expect them to - straight lines will always be straight and triangle corners always add up to 180 degrees. You might wonder why I mention straight lines and triangles, but just stay tuned because we're going to go to the strange Universes next.
Figure 6. A spherical (positve curvature) two-dimensional Universe is shown. It should be remembered that the sphere itself is not the Universe, but rather the surface of the sphere is the Universe. Creatures in this Universe would only know of the two directions, like in the flat Universe, and would not be able to see into the sphere. In this Universe, you could make triangles with angles greater than 180 degrees - even though they have straight lines that make up their edges. Also, parallel lines will converge (come together) eventually.
The next type of surface has what is called positive curvature. This is best represented in two dimensions by the surface of a sphere. Remember, the 2-D creatures in this Universe don't see up or down, so they don't know that they are on a sphere. This is sort of what you experience every day in your life when you look around (you live in Iowa, after all). The Earth isn't really flat, but it looks pretty flat since it is so big. Now in this Universe things can be really strange. One thing that can happen in this Universe is that if you have two parallel straight lines, those lines will eventually converge (come together). This is like how the lines of longitude on a globe all come together at the poles - these lines are straight, but because they are located on a curved surface, they come together. Also, you can draw a triangle on this surface made up of three straight lines, but with angles that add up to greater than 180 degrees! Egad, this is crazy! Another nifty aspect of this type of Universe is that if you get into a rocket and blast off, you'll eventually end up at the same place you started from! That makes it sort of like a trapped system; you can't get out of this Universe.
Figure 7. A saddle (negative curvature) shaped Universe. Actually, it looks rather like an infinitely large Pringles potato chip. In this two-dimensional Universe, the curvature is such that triangles have angles that add up to less than 180 degrees and parallel lines diverge (get further apart).
I suppose if there is positive curvature, there must also be negative curvature. This Universe is rather strange in that it is shaped like a saddle or a Pringles potato chip. The way things act in this Universe are sort of the opposite as to the positive curvature case. Straight lines that start out as parallel will diverge as you go further out into space - they'll get further apart. Also, triangles can be drawn in this Universe that have angles adding up to less than 180 degrees - strange.
What type of curvature does our Universe have? I'll tell you right now that it is really hard to determine that, since it is sort of like how the Earth sort of seems flat. You know that the Earth is a sphere, yet you are so small compared to it that you see it as being very flat. The Universe is so darn huge that the curvature, if it exists, is so small that we can't easily tell what type of curvature there is. In the cases of the positive and negative curvature, you can blow those surfaces up to such huge sizes that the little ants in them would think that the surfaces are flat or really close to flat.
I don't want to tell you what astronomers think the shape (curvature) of the Universe is right now, so I'll save it until later, since it is sort of important in our current studies of the Universe, and I love to build up the suspense.
Before we go into more about the Universe, in particular its origin, I want to mention briefly what Einstein thought about the Universe. When he did his work on General Relativity in 1917, he really didn't know what astronomers knew about the Universe, so he asked a few of them what the Universe was like. They told him that it was homogeneous and isotropic. They also told him that it was rather static (even though Slipher had detected all of those spiral nebulae redshifts; this was before the Curtis-Shapley Debate and when Hubble did all of his stuff, so no one really knew what galaxies were any ways). Einstein did his calculations, but his formulas always came out wrong - they showed that the Universe wasn't static, that it is probably moving. No matter how hard he tried, the darn formulas never came out the way he thought they should - or at least following what he was told, that the Universe wasn't moving in any organized way. He, well, he basically, um, cheated. Yes, the genius was so intent at getting what he thought was the right answer that he sort of fudged the formula by adding an extra term to it, what is called the Cosmological Constant. It is usually represented by the Greek letter . The Cosmological Constant actually represents anti-gravity, just what Einstein needed to counteract the effects of gravity and keep the Universe static. Later, when he learned that the Universe wasn't static but expanding, he thought that he had made the biggest mistake of his life - but had he? We'll have to keep Einstein's blunder in mind later on.
It's time to recap what we know about the Universe at this point. First of all, it is expanding (Hubble's Law shows this). To be precise, we should actually say that space is expanding and the stuff in it is just along for the ride. It also has some sort of shape (curvature), though the Universe is too large for us to see what that shape is easily - while it may look flat, it may actually be something else. We know that galaxies evolve, since very distant galaxies look quite different from nearby galaxies, and we also have the concepts of the Cosmological Principle and Universality to keep in mind. We know that the Universe is expanding, so it must have been a lot smaller in the past. That makes sense; just hit the rewind button so that the Universe goes back to a smaller and smaller scale - how small? As small as we can get it; as small as it was when it first started to expand - back to the time when it was created, back to the dawn of time, to the event known as the Big Bang!
As mentioned previously, the value of the Hubble Constant (Ho) is related to the age of the Universe - and that's just simply a matter of algebra. So if we know the value of the Hubble Constant, then that's the age of the Universe? Oh if only it were that easy! Unfortunately, the Hubble Constant isn't. Isn't what? CONSTANT! The value has changed over time as the rate of the Universe's expansion has changed. Think of what happens when you throw a ball up into the air. When it leaves your hand it is moving at its highest speed, but over time it slows down and slows down and slows down. So the rate of its motion changes due to the effects of gravity. The Universe has to also obey the effects of gravity, and therefore must also have changed the pace of its expansion over time. Gravity would have slowed it down in the past, which tells us the Hubble Constant would have changed over time. That's sort of why we have the little "o" on it - that means the value it has now. So the value of the Hubble Constant doesn't exactly give you the age of the Universe, but it does give an estimate to its value.
What else can be used to determine the age of the Universe? We could use the fact that the speed of light is a constant to look for the most distant galaxy. For example, if astronomers find a galaxy that is 5 billion light years away, that means that the galaxy has to be at least 5 billion year old (since the light had to be emitted and had to travel for 5 billion years to reach us). And if the galaxy is 5 billion years old, then the Universe has to be at least that old. So if we find the most distant galaxy, then we should be able to determine a lower limit for the age of the Universe, right? Not quite. You'd certainly determine how far away the galaxy is from us, but again, this is a lower limit for the age. It would have taken time for the galaxy to form. Also, how do you go about finding the most distant galaxy? Wouldn't it be very, very, very, faint and therefore very difficult to even see? Certainly astronomers try to find objects that are further away than any other objects, but to just try to randomly search for such objects is very difficult. Astronomers search for not only the most distant galaxy but the most distant supernova as well as gamma-ray bursts. These objects can be seen over great distances, but again they only give us a glimpse of an object that is close to the furthest distances we can see.
Recently astronomers announced the discovery of a very old star, one that is estimated to be 13.2 billion years old! This star, HE 1523-0901, was found to have an unusual composition, one that showed a decrease in the amounts of various radioactive elements which helped astronomers to determine the age. You'll learn more about the use of radioactive elements for age determination in the Planets part of this course. But radioactive decay is one of the most reliable methods of determining how old something is, even things like stars.
The old star mentioned above isn't alone. Many stars are very old, particularly the stars found in globular clusters (remember those?). These stars would have been formed at the same time, but they die at different rates depending upon their mass. This allows astronomers to measure the age of a globular cluster by looking for the largest Main Sequence stars in the clusters. This method requires the use of computer models that predict how stars evolve, and these give fairly consistent results, again with ages around 12-13 billion for the oldest stars in globular clusters.
Of course the best way to figure out how old something is would be to just look at it and see. And in a way that's what many cosmologists do. They look at all aspects of the Universe and try to piece together from the various observations a consistent model to explain not only why it is the way it is today, but what it was like in the past, and just how far back that past goes. This includes looking at the structure of the Universe (clusters of clusters of clusters), the composition of the stars and galaxies (metals form over time), and the current rate of expansion (Hubble's Law).
The age of the Universe is pretty old, but like all things, it had a beginning. That's what we'll tackle in the next section.