I’ve already received a couple of very interesting questions about the lecture from some students. Since these were relatively early I wrote up some answers. In the interest of giving everyone a chance to look at this, I’ve decided to create posts on the blog that will involve some of your questions and my responses. You can feel free to respond in the comments and I will periodically monitor what is written. That said, those of you who are my students should still e-mail me your lecture questions!
One student wrote me with the following:
“So we can measure the distance between us and another cosmic object by determining the period of nearby cepheids and then the corresponding luminosity. I want to know why the longer the measured period is, the brighter the luminosity. What is the physical mechanism behind that?”
There is a quick answer and a more in depth one. The quick one is that the higher the peak luminosity, the more time it takes to go from the peak luminosity to the minimum luminosity and back.
But why? Well, what makes a star shine? It’s the nuclear fuel that is fused due to the enormous gravitational pressures from having all that mass. Now, throughout most of a star’s life, the fusion involves abundant hydrogen and a little helium, which are in relatively steady supply, but as a star approaches the end of its life cycle, it can get into a situation where it has to fuse even heavier elements (more helium than hydrogen, lithium, etcetera). Well, the bang for your buck in fusing heavier elements is less, so the star is less able to withstand the gravitational pressures pulling it in on itself. However, as the star shrinks, it burns more rapidly (greater pressures) which causes it to actually expand out again. So it pulsates in this way (again, this is more toward the end of the star’s life cycle). A more massive star will have more extreme swings which take more time to work themselves out. When it is on the more crushed down side, it will be relatively more crushed down than a star that has less mass, this creates more pressure causing more energy to be released, but there is a quirk: the outmost helium becomes ionized, blocking the radiation from escaping. Instead, the helium is driven outward–the star expands, the helium cools, becomes electrically neutral again, and the star’s light comes through. The bigger the star, the more time this whole cycle takes and the higher the energies involved and hence, the more maximally luminous.
The student asked another question which is quite thought provoking:
“our conclusion from the observation of galaxies moving away is that the space itself is stretching. Which fundamental force is actually stretching the space? What is the physical definition of space? If we go back in time until the universe itself is a point singularity, how would be describe the region that is outside that singularity?”
So, last week I mentioned that there are four fundamental forces: gravity, electromagnetism, and the weak and strong nuclear forces. Now, one of these forces is a sort of odd man out–it’s gravity. It turns out that you can think of gravity as a force, but alternatively, you can think about it as the actual shape of space itself–that is, imagine that space is like a rubber sheet. When you have a mass sitting on the sheet, it distorts it–this is what mass does to space, it changes its shape. When another much smaller mass rolls by on the rubber sheet its path gets curved. When a small mass passes by a big one in space, the big one’s gravity pulls on it causing its path to curve. So there is a nice analogy between gravity and the bending of space. So gravity is somehow related to the expansion of space–but in fact, it works AGAINST the expansion. Masses attract each other, which is sort of like the space between them getting squeezed. So what is responsible for the expansion? Well, if you run the expansion backwards, it implies that everything was all together at some point in time. This seems to imply that everything exploded into existence–what we call the big bang. It’s the initial impulse of this event that continues to drive expansion…although there are some caveats to this statement.
Okay okay, so what is space? Where is this “singularity” we call the big bang situated? Space can be thought of as the arena in which matter and energy interact. However, it is not passive, as I mentioned before, it actively participates in the interactions since it is distorted by mass and energy.
There are lots of ideas regarding the notion behind this singularity, but the most straightforward to state and yet very difficult to grasp is that there isn’t any “outside” of this singularity. The big bang happened–there is no before and there is no elsewhere since the big bang was the origin of space and time.
Now, an alternative view is that what we call the observable universe is embedded in a much bigger object–the “entire” universe let’s call it. Perhaps our region of the entire universe was produced by some singular process, but the entire universe that it is embedded in existed forever? This is a viable idea that theorists work with. It gets rid of this pesky notion of a beginning to time and suggests the possibility that there might be a “history” to trace back before the big bang. That said, these ideas are purely speculation (with a lot of mathematics nonetheless). The simplest idea is that the universe (the entire universe) began with a big bang almost fourteen billion years ago–counter-intuitive as that may be…