The story of the big bang is a great story--it purports to explain everything that is today with a compelling narrative--a "just-so" story. For example, Rudyard Kipling's short story "The Elephant's Child" from his Just So Stories for Little Children explains that the crocodile gave the elephant a long trunk by stretching it from an originally shorter snout, and that is why all elephants have long trunks today. A tall tale, indeed, because there is no convincing evidence this could have ever happened...
...or could it have? (Don't worry, the baby elephant helped to rescue the mother elephant from the crocodile.) While this photograph of a single remarkable event shouldn't be taken as evidence that Rudyard Kipling's just-so story is true, we are going to explore various pieces of evidence that taken together support the big bang just-so story, elevating it from a compelling narrative taken only for its entertainment value to a compelling theory based on observable facts.
This presentation will explore three important pieces of evidence that provide clues for the existence of the big bang. A subsequent presentation will delve into key events in the early universe, when the first matter, atoms, photons, and stars were created to become what they are today.
First, the finite speed of light. It turns out that we have a time-machine of sorts to actually see the universe as it was in the past.
Consider Deneb, the farthest star in the night sky that can be seen from San Luis Obispo, CA with the naked eye. It is 1,400 light years away, and from the definition of the light year, light from Deneb takes 1,400 years to travel to us. This means that when we look at Deneb in the night sky tonight (season permitting), we are seeing Deneb not as it is now, but as it was in the past, 1,400 years ago.
In a dark sky location, you can observe with the naked eye a globular star cluster just outside the plane of the Milky Way disk, out in the halo, as a faint smudge: Messier 13, or the Great Globular Cluster in Hercules. The stars in this cluster are 25,100 light years away, so light from M13 takes 25,100 years to travel to us. This means that when we look at M13 in the night sky tonight (again, season permitting), we are seeing the stars in that cluster not as they are now, but as they were in the past 25,100 years ago.
The farthest object that you can see in a dark sky location with the naked eye is the Andromeda Galaxy (Messier 31), our nearest large neighbor galaxy, 25.4 million light years away. When we look at the Andromeda Galaxy in the night sky tonight (season permitting), we are seeing the stars that galaxy not as they are now, but as they were in the past 25.4 million years ago.
Because of the finite speed of light, looking at objects further and further out away from us lets us see those objects further and further back in the past. Using telescopes with lots of light-gathering power allows us to see very faint, very distant objects as they were a long, long time ago. Here the Hubble Space Telescope was used to collect light from one region of the sky for many months, and observed extremely distant galaxies as they were an extremely long time ago. So we do not need to merely conjecture how galaxies changed over time to become as they are today, we can directly see how galaxies have changed over time to become as they are today, by observing them at different distances from us.
Today, distant galaxies have __________ metals than nearby galaxies. (Assume that all galaxies starting forming at the same time, nearly 14 billion years ago.)
(B) the same amount of.
As observed from Earth, distant galaxies appear to have __________ metals than nearby galaxies. (Assume that all galaxies starting forming at the same time, nearly 14 billion years ago.)
(B) the same amount of.
What will we see if we keeping looking further and further out, further and further back in time?
Second, Olbers' paradox, which we'll treat as Olbers' question, as it is a question with an actual answer.
So if the universe has not been around forever, for how long has it been around?
Third, the Hubble law.
This is the actual data for nearby galaxies taken by Edwin Hubble, looking at their blueshifts and redshifts, or their velocities towards us or away from us (as discussed in a previous presentation). Note that the nearest galaxies are blueshifted, which means they are headed towards us, due to gravitational interaction (which is okay, if you would like the Milky Way to continue to have spiral arms), but as you look further out, galaxies are more and more redshifted, which means they are moving faster and faster away from us. This is the Hubble law, which suggests some sort of cosmic HateradeTM, as except for our immediate vicinity, every other galaxy in the universe is fleeing away from the Milky Way.
Keep in mind these key pieces of evidence that support the big bang--not a "just-so" story, but a "because-so" theory.