20120729

Presentation: big bang clues

Consider the big bang, where the universe began from an infinitely dense, high energy state, expanding and cooling off, while stars form in galaxies, which are moving apart from each other. "Cool story, bro." (Video link: "The Big Bang.")

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.)
(A) less.
(B) the same amount of.
(C) more.
(D) (Unsure/lost/guessing/help!)

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.)
(A) less.
(B) the same amount of.
(C) more.
(D) (Unsure/lost/guessing/help!)

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.

Suppose you were lost in a forest, with no marked trail, and having no directional clues (sunlight, moss on trees) nor navigational devices (compass, GPS tracker). And it will be dark soon. And there are werewolves. So it is important to be able to get out of this forest in the most expedient manner. If you look around in all directions, the quickest way out is to look for gaps between trees. If in a certain direction you see a clearing in the gaps between trees, then head in that direction, that is the edge of the forest. If in a certain direction there are more trees beyond in the gaps between trees, then the forest continues further on in that direction. If in every direction there are more trees beyond in the gaps between trees, then you're doomed, as the forest would be infinitely large...or at least, the forest is large enough that the edges cannot be readily seen. (Video link: "100806-1180532.")

Heinrich Olbers asked this same question of the universe. Is there an edge to the universe? Or does it continue on forever, or at least, is large enough that the edges cannot be readily seen? Let's answer this with the view from the Hubble Space Telescope, considering that every galaxy in the universe that can ever be seen in these directions will show up in this extremely long time-exposure. Since we don't keep seeing more galaxies beyond in the gaps between galaxies, Olbers' question is answered: the universe is not infinite; it is actually finite, and has an edge in all directions. (Video link: "Pan across the Hubble's Advance Camera for Surveys Ultra Deep Field.")

However, remember that the farther we look out in space, the farther back in time we are looking. Here is a three-dimensional representation of the galaxies observed by the Hubble Space Telescope. As we look further out in space, we run out of galaxies to see, which actually does not mean that we've reached an edge in space--we are going back in time, and have run out of time (or more precisely, a time before galaxies existed). So the edge we see is an edge in time, not space, and the actual answer to Olbers' question is that universe is not infinitely old, but has a finite age. (Video link: "Hubble Ultra Deep Field 3-D Fly-Thru.")

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.

The interpretation for the Hubble law is not that all galaxies in the universe are running away from the Milky Way, but that the universe is expanding, and in a strange sort of manner. In this animation note that the size of the galaxies remain constant (due to gravitational forces within), but that the space between galaxies is increasing. Note that this is different than merely continuously "zooming in." Why the space between galaxies increases is not readily explainable, but the expansion of space between galaxies is consistent with the observations behind Hubble's law. (Video link: "Continued Consistent Expansion.")

If the space between galaxies is expanding today, let's think about going backwards in time, and considering how long ago was the space between galaxies zero? This is the Hubble age of the universe, and we find that at the current rate space between galaxies is expanding, there was no space between galaxies approximately 14 billion years ago (give or take), giving us an estimate for the finite age of the universe. (Video link: "The Big Bang.")

Keep in mind these key pieces of evidence that support the big bang--not a "just-so" story, but a "because-so" theory.

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