As I prepare for my fourth semester of teaching introductory geology, I remember that the first couple of lectures usually throw me. I decided to prepare in advance for the moments I look at my notes and have no idea where I was going.
Yes, kids, it happens. I am not an expert in everything that might ought to be covered in an introductory geology classes. Sometimes I get flummoxed when I’m lecturing about things that are not within my realm of expertise.
The problem I have is that it’s the first major topic of the class that’s really not my cup of tea. The topic is the origin of the universe, including measuring distances between astronomical objects. I almost always flounder during this lecture, which kinda wrecks my credibility for the rest of the semester.
Gotta keep up my cred, right?
So. I’m going to review my notes and make sure I know what I’m talking about in advance.
And in the meantime, you might learn something too. Or you can correct me if I’m wrong, ‘cause it’s possible.
Origin and Age of the Universe
The leading theory is that the universe began with all matter and energy concentrated at one infinitessimally small point, and then this point started to expand, marking the beginning of the universe. This expansion is thought to have originated around 14 billion years ago. I know this is not a precise number. I think I’ve heard 17 billion years as a figure as well.
Where does this number come from? Well, we do know that the universe is expanding. We can calculate how fast objects are moving and how far away they are. From this, we can calculate at what point in time all of the objects in the known universe.
So, how do we estimate speed and distance. And how do we know the universe is expanding?
Let’s look at the second question first.
It’s about the Doppler effect. You know it. An ambulance races by. The wailing sirens are high pitched as the ambulance approaches, and abruptly drops in pitch the moment it passes. This is because the sound waves are compressed in front of the ambulance as it approaches, decreasing the wavelength and raising the pitch. Once the ambulance is past, the sound waves are stretched out, increasing the wavelength and decreasing the pitch. The same thing works with light. When objects approach, light off of the objects is compressed causing it to move toward the blue end of the spectrum. This is called blue-shift. When objects move away, the light is stretched out, causing it to shift to the red end of the spectrum, the so-called red-shift.
Objects observed throughout the universe tend to have colors shifted toward the red end of the spectrum, thus they are moving away. All observed galaxies in the universe are moving away from us. The only way to explain this is with an expanding universe.
The red shift is evident by looking at the complete light spectrum coming from stars. Different elements cause absorptions within the complete spectrum. These are basically missing colors. We can look at the absorptions from our own sun, which isn’t moving relative to us and compare them to absorptions from different stars and see that the same absorptions are there, but shifted toward the red end of the spectrum. The amount of this shift is dependent upon the speed of the object: faster objects have greater shifts.
So we have an expanding universe and we know how fast things are moving. How can we determine how far away something is?
This is a little more complex and usually where I falter. Firstly, we do actually know with great precision the size of the Earth’s orbit. We’ve determined this with various spacecraft, plus radar. Then, for objects fairly close to the Earth, we can use the parallax method. With parallax we can measure the angle to an object when the Earth is on opposite sides of its orbit. In doing so, we can estimate the distance away that the object is. Unfortunately, this works best for objects that are relatively close.
For objects much further away, different methods are needed. These methods make my brain hurt a bit. Some involve a measurement of the amount of light observed from an object compared to how much light is known to actually be emitted. The loss of light is related to the distance of the object from the Earth and can thus be used to calculate distance. You can read more about it here.