Chapter 13 Questions


Question:

You use the analogy of an expanding gas to explain why the photons of CBR cooled. Why did the individual photons cool? In a gas, the gas molecules do not lose energy, but the system loses energy as it expands, and since Temperature is a measure of Ekinetic, the system cools. I take this to be the same for the expanding universe. As the photons spread out, the energy of the system is lowered, thus the temperature drops. What causes the photons themselves, individually, to lose energy? My confusion stems from the beginning of chapter 13, when it is said that "each photon has lost much of its energy in the overall expansion". Why? Is it because we see them redshifted as they move away, thus we perceive that they have lost energy?

Answer:

Gas cools because it does work on its surroundings; the gas molecules do lose energy.

The analogy with the expanding gas is limited. There are two effects in the universe: expansion causes the photons to spread out (reduction in number of photons per unit volume), and redshift (change in the photon wavelength due to the expansion of space) occurs. This is a real loss of energy which results from the metric equation (i.e. it is a relativistic effect) but it is not due to anything but expansion, and the lost energy doesn't "go" anywhere (at least so far as we are able to define energy). The photons do NOT do any work - they don't push against anything. Everything is homogeneous, so there are no pressure gradients.

Question:

How is primordial gas used to obtain the density of the universe?

Answer:

Mainly through the determination of its deuterium, and to a lesser extent its lithium-7, concentrations. See pp. 370--73 for an explanation of how primordial elemental abundances place limits upon the baryon density of the universe.

Question:

How could the big bang have happened at every point in space? Shouldn't there have been a bunch of little bangs everywhere?

Answer:

This difficulty in visualization is quite common and is probably a result of thinking of the big bang as some kind of explosion, an image reinforced by its name. (A few years ago there was a contest to create a better name, but "big bang" has been around for 50 or 60 years and is well established, and in any case no one was able to come up with a clear improvement.) It's also not easy to imagine a three-dimensional hypersphere (remember that what we are talking about here is the surface of a four-dimensional ball). The big bang is really more like the blowing up of a balloon, starting from some extremely tiny (Planck-sized) entity. In that sense it isn't a "bang" at all, although the expansion is quite rapid in the early universe. When a balloon is blown up, each point on its surface recedes from every other point; it isn't enlarging at each individual point. (This assumes the usual kind of filling of a balloon, excluding pinching and any other manipulations.) This overall expansion is what happened in the big bang, and indeed continues to the present day.

Question:

How do conjectures about the early universe, at such incomprehensible tiny times, get made?

Answer:

Let us confine our attention here to hypotheses that might be regarded as scientific; we shall ignore conjectures with no basis in science. (Obviously, one may make any conjecture one wishes, but not all will have any scientific or logical validity.) Science generally proceeds by first applying what is known. The hot big bang model of the early universe illustrates this; as more was learned in other arenas of physics, such as nuclear and particle physics, the conditions of the early universe became progressively more comprehensible.

The first step was to develop a simple model of the evolution of the universe in terms of scale factor and temperature. This was accomplished by the 1930's. However, at that time little was known about extreme conditions, so speculations about the early universe tended to be wrong, if they were made at all. Most scientists working before World War II avoided hypothesizing about the conditions of the early universe. One of the few who took the plunge was Georges Lemaitre. He made an analogy to the most familiar high-energy process known at the time, and envisioned matter evolution in the early universe as similar to radioactive decay, which of course we now know is incorrect.

Gradually, nuclear physics developed on its own. Fusion and fission became thoroughly understood, partly under the pressures of the Manhattan Project. After WWII, the fundamentals of nuclear physics and nuclear processes were established. Once the conditions (e.g. temperature, density, etc.) under which nuclear processes occurred were understood, it became possible for George Gamow and his collaborators to apply the theory to that epoch of the early universe during which the standard model predicted that the temperature was appropriate, an interval we now call the nucleosynthesis epoch. However, the same standard model predicted the corresponding matter density, revealing that the density was too low for the triple-alpha process to occur; this showed that only a few light elements could have been synthesized during the nucleosynthesis epoch.

The same process of extrapolation has continued. Particle physicists established, among other things, the theory of the electroweak interaction. Understanding of the conditions of the electroweak unification made it possible to push the understanding of the early universe back to the point at which temperatures were appropriate to this phenomenon.

Sometimes new physics must be developed. Quantum gravity is a good example of this; we know that general relativity cannot be extrapolated as it stands to any conditions of the early universe. But none of this occurs in isolation from what is already known. Progress in science is often uneven, but new understanding in one field can often enlighten other, seemingly unrelated, areas.

Question:

Is it possible to create antimatter? Is there any around? Do we see any matter/antimatter reactions going on?

Answer:

Certainly it is possible to create antimatter, it happens all the time at particle accelerators. The most important mechanism is pair production. (See pp. 357--58.) Positrons are relatively easy to create, and some particle laboratories have storage rings to keep a supply of them on hand. (Some labs can also store small quantities of antiprotons for short intervals.) The antimatter storage rings probably represent the greatest concentration of antimatter in our immediate vicinity in the universe. However, antimatter occurs naturally in the present universe wherever interactions, such as collisions, are sufficiently violent.

Several astronomical objects produce the gamma radiation characteristic of electron-positron annihilation. For example, some recent observations detected a significant electron-positron signal from the center of the Milky Way.

Question:

How long will this present state of matter domination last in the universe?

Answer:

Forever in open and flat models; till the big crunch in a spherical model. In the open (hyperbolic) and flat models, eventually matter will become essentially irrelevant and the universe will expand like a de Sitter model, but this occurs asymptotically (i.e. that point is approached ever closer but is never quite reached) so we might as well say that matter domination continues indefinitely in these models.

Question:

Has all the antimatter that existed in the universe annihilated? Why is there more matter than antimatter?

Answer:

We believe that the antimatter annihilated early in the history of the universe. The leftover matter (about one part in a billion) is the result of some fundamental asymmetry between matter and antimatter. This may provide an important clue to the nature of the grand unified theory.

Question:

What is antimatter like? What research is done into antimatter?

Answer:

Antimatter is just like matter but with opposite values of certain quantum properties, electrical charge being the most obvious example. Antimatter can be, and is, created in high energy particle accelerators where it is studied for clues to high energy physics theories.

Question:

Why can't we think of our galaxy as an antimatter galaxy? What is so special about matter?

Answer:

We call the stuff of which we and our galaxy are made "matter," but if you want to call it antimatter, fine. If things weren't perfectly symmetric in the GUT symmetry-breaking transition, then one or the other (positively-charged protons, or negatively-charged protons) would be favored and whichever was favored would end up being called "matter" by the creatures made of it.

Question:

How far back do cosmologist know what happened after the big bang?

Answer:

Conditions are quite well understood back to 1 sec after the big bang. Our knowledge is increasingly less secure to about 10-43 sec after the big bang. Earlier than that, we have no established theory at all.

Question:

It seems like the laws of physics were different early on in the big bang. Doesn't that violate the cosmological principle?

Answer:

The laws of physics can change with time and still be consistent with the CP so long as they remain the same everywhere in space at that time. (After all, the conditions become hotter and denser going backward in time, but that does not violate the CP.) Of course, if the laws of physics change with time that violates the Perfect CP, but there are other reasons to doubt that the PCP describes the universe.

Question:

How do we know what occurred at the beginning of the universe? It seems impossible to make enough correct assumptions to really know.

Answer:

We have been remarkably successful applying known physical theories to regimes (e.g., the early universe) that seem otherwise inaccessible. Remember that the nucleosynthesis calculations predicted the cosmic background radiation well before the CBR was discovered. It is perhaps more difficult to probe backwards further in time and expect similar successes, but this may not be impossible if we are able to develop improved theories of Grand Unification and quantum gravity.

Question:

How can we even make predictions at time intervals we could never picture like 10-43 seconds after the big bang started? Is it just whoever's idea sounds the best wins because it could never be proven?

Answer:

Any number of ideas can be proposed, and I think it is important to remember, as you say, that unless you can test an idea we must be reluctant to give any speculation too much credence. However, theories applicable to such earlier times must be able to make predictions about phenomena that we can observe in order to qualify as scientific. Hence we can say with some confidence that eventually a theory will be developed that will give us a good idea of conditions at or near the Planck time, even though we cannot directly observe the extremely early universe.

Question:

If galaxies of the past are vastly different than they are now, isn't it possible that when we look back across time to the quasars we could be seeing the Milky Way at its birth?

Answer:

We could not see the Milky Way literally, but with improved space-based telescopes we should eventually be able to observe the formation of spiral galaxies such as our Milky Way. The quasar phenomenon does seem to be tied into early galaxy formation and evolution.

Question:

With the continual cooling and expansion of the universe is it likely that the four fundamental forces could break down any further?

Answer:

That's an interesting question. Is there some new physics waiting to be discovered at ultralow temperatures? We tend to think not, since we can study low temperatures in the laboratory. But perhaps there are aspects that we haven't considered at the extremely low temperature state in the distant future when even massive black holes will seem "hot" compared to the universe's temperature.

Question:

Can we eventually "see" the big bang (not the temperature as in the CMB but as an actual visual image)?

Answer:

Any light from then will definitely be redshifted below the visible, so we would have to observe it with some instrument and construct a viewable image from the raw data, as was done with COBE. We might imagine observing the neutrino background radiation (assuming the neutrino is massless) in order to "see" the big bang at less than one second after t=0, or, more exotically, observing the gravitational radiation background to see the cosmos right after the Planck epoch. At least we can imagine doing such things; but it is difficult to conceive of a technology that would permit us to make such observations.

Question:

If atomic particles consist of subparticles, are subparticles made up of smaller particles and so on to an infinite degree?

Answer:

We believe that there are fundamental indivisible particles and that the subdivision does not go on indefinitely. The photon is one such fundamental particle. The electron is believed to be the fundamental lepton. Quarks are believed to be the fundamental particle for hadrons.

Question:

Why weren't elements other than hydrogen and helium created in the big bang?

Answer:

An isotope of lithium was also created. But to create elements heavier than that you must first make carbon. The only way to create carbon is through a three helium reaction, schematically He + He + He -> C, which requires higher densities than are found during the nucleosynthesis epoch of the big bang.

Question:

Based on the current rate of progress, when do you think we will know for certain what happened at the beginning of the universe?

Answer:

April 23, 2008.

But seriously, its impossible to say. It is very hard to make progress in these areas because it is impossible to do experiments involving the physics of the extremely tiny or the very high energies associated with the beginnings of the universe. Perhaps someone will make a breakthrough and it will all fall into place. Otherwise we will keep working very slowly at it.

Question:

Is it possible that the reason for the imbalance of matter and antimatter in the universe is because antimatter collapsed into a bunch of black holes early on?

Answer:

Why would just the antimatter collapse? What favors the antimatter collapse over matter collapse?

Question:

In the big bang theory how do they explain how something came from nothing? What exactly is the big bang theory?

Answer:

Big bang theory in its current manifestation describes the evolution of something after it came from nothing. We don't have a theory to explain where the something came from. It is unclear to me whether such a theory is even possible.

Question:

Have any theories of quantum gravity been proposed and failed? Is there any speculation?

Answer:

Yes several have been proposed. At present the superstring theory (or some version of it) is the most promising speculation.


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Copyright © 1998 John F. Hawley