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Cosmology 101: The Big Bang

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Cosmology 101

This page is a beginner's explanation of the Big Bang. I intend it to be accurate. It is my understanding of the matter, however, so I can't guarantee it.


Stand Back!!!


That's how it started, or so they say. I have to admit, even with a science background, I wasn't immediately convinced. Then I learned a discouraging truth: it was easy to be swayed by superficial arguments if I remained ignorant. And it was difficult to accept, but I was ignorant.

How'd I find that out? I started reading. I had a physics degree already, so I didn't think I was ignorant. Then again, my degree was nye onto 30 years old, and I hadn't really been keeping up in my own field. I'd just been doing Newtonian physics for 30 years.

So when I started reading, I found out how ignorant I was, and how compelling the arguments are for the Big Bang. That's right -- compelling.

The Germ Of An Idea, Relatively Speaking

When did the idea of the big bang get started? That's a bit hard to say. Even in Einstein's time, Einstein was intrigued by the story his relativity equations were saying. The debate of that era was whether we lived in a static universe or a changing one. At the time, around 1900, the discovery that our Milky Way galaxy wasn't the entire universe was just being made.

Einstein's equations of general relativity told him that the universe could not be static. His space-warping math said that the universe must be either expanding or contracting, that the forces of gravity between all the stars and their motions simply couldn't lead to a static arrangement. When he first formulated his general relativity equations, the Milky Way galaxy was still considered to be the entire universe, and it appeared to be quite static on the large scale.

So Einstein tried to bring his equations into alignment with generally accepted astronomical principles by adding his famous cosmological constant into the mix as a repulsive force to balance gravity. He hoped in this way to find a stable and static solution that would fit the current view of the universe. But he could not find a static solution. Even with the new cosmological constant, a static universe just didn't seem possible. We either had to be coming or going -- universe-wise.

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Which Is It? Are We Coming, Or Going?

A fellow named Edwin Hubble solved the coming or going part. In 1929 he determined that galaxies external to our own, other than those bound together in the local group, were all receding from us. The further away they were, the faster they were receding. That showed, emphatically, that the universe was expanding.

Einstein promptly removed his ad hoc cosmological constant and declared it to be the biggest mistake he'd ever made.

It seemed logical enough even in that era to wonder what would happen if one could turn the clock backward? If objects in normal time are getting further apart -- wouldn't they be getting closer together if the clock ran backward? That is, in the long past things must have been close together. And if that was so, and it was also true that gravity pulls things together, something spectacular must've happened to push things apart. In fact, push them apart with such force that they are still moving apart today, some billions of years later.

So the Big Bang idea started to take place. But up to this point it was largely a mental exercise. Galaxies that we observe receding must have been closer at one time -- but in the same place? What was the proof?


Shhhhh ... Did You Hear That?

Evidence began to mount when Penzias and Wilson of Bell labs were having trouble getting the noise out of their radio telescope back in 1965. They tried everything from checking all of the electronics, to double checking that they weren't picking up ground based interference, to even making sure the pigeon droppings in the antenna housing weren't bouncing transmissions around.

Their conclusion? They determined that the noise they couldn't get rid of was from outer space. A microwave signal emanating from all directions, like that which would come from a black body a couple degrees above absolute zero.

It occurred to them that just such a result is what should be seen if there had in fact been a massive explosion some billions of years ago. We should see a low temperature radiation (due to the expansion and cooling of the universe) from all directions. What else could explain that?

Now if you are into physics, you may know that it is a vast field of study made up of a number of sub fields. Astronomy is one such sub field. Others are thermodynamics, electricity and magnetism (called E and M), relativity, and quantum physics, to name a few. I've already mentioned that relativity started off the debate.

Is That All You Got?

It turns out that each of these other realms of physics have something to say about the Big Bang. Only one field, you understand, would have to be in disagreement with the evidence to cast doubt on the theory.

E and M is the study of electricity and magnetism, which gets into microwaves, x-rays, light, and all forms of electromagnetic radiation. The statement E and M makes about a Big Bang is that a vast amount of radiation would be given off. That's what Penzias and Wilson discovered with their radio telescope.

Thermodynamics is the study of heat flow, the base of knowledge that originally led to the invention of the steam engine. Thermodynamics predicts the nature of the residual background radiation and it's apparent temperature after the expansion. Again, an observation in agreement with relativity and astronomy (Hubble).

Quantum physics, not entirely understood by anyone, is the study of the interior of the atom. I recall that when I was a young college student and a beginning amateur astronomer, I considered astronomy and quantum physics to be at opposite ends of the physics spectrum of study.

Quantum was the study of the very small, and Astronomy study of the the vary large. Each had their own mathematics and methodologies for solving problems. It was hard to see what relationship the two fields had with one another.

What's This Black Hole Nonsense?

Then the buzz about black holes started. Collapsing stars, infinite density, the singularity, the break down of relativity, etc. That seemed pretty far fetched. Certainly no one had ever seen a black hole. Besides, how do you see one? A black hole against a black background?

It's true, black holes really can't be seen directly. But they exhibit a gravitational attraction that can reveal itself by the actions of objects and atoms around them. Large objects, like stars, can be seen moving around something this is obviously more massive than the star, yet invisible. It has to be either a neutron star or a black hole -- what else?

Atoms in gas getting too close to a black hole get so accelerated that their mutual collisions create high energy emissions, including x-rays. Not much else is known that can cause that. Both of these phenomenon have been observed repeatedly. Black holes are a fact of life.

So black holes are the super massive objects of incredible gravity. That seems very much an astronomy object, on the big scale. But they are collapsed, and actually very small, singularities in fact. Within the vicinity of the singularity, or during the collapse, all that mass gets crunched into a smaller and smaller space. Eventually down to (and beyond) the size of nuclear particles.

So Black Holes Are Really Small,

I Thought We Were Talking Universe Here

The nuclear force is incredibly strong. Under normal conditions gravity is extremely weak in comparison. So weak, that trying to measure any relationship that might exist between the nuclear force and gravity is impossible. But something happened in the 19th century that started a revolution in physics.

A fellow named James Clerk Maxwell figured out that the electrical force and magnetic force, both discovered long before his time, were related. In fact, one might say that electricity and magnetism are two aspects of the same force. He created the equations that explain that relationship. Physicists say that Maxwell unified electricity and magnetism. I think a better layman's terminology is that Maxwell discovered the relationship between the forces.

That discovery led to a big question. There were four know forces known by the end of the 19th century: gravity, electromagnetic, weak nuclear, and strong nuclear. If electricity and magnetism were considered to be separate forces and then were found actually to be aspects of a single force, were the other forces related in some way also?

Ah, you think I digress. I warn you, I've had too many courses in wholism to think straight anymore. I assure you I will get back on track.

Not too long after the start of the 20th century, the question began to get answered. In 1967, Abdus Salam and Steven Weinberg managed to describe the mechanism that unified the weak nuclear force with the electromagnetic force. Thus, the electromagnetic and weak nuclear were aspects of a single force, observed within different realms of nature. At higher temperature (and I mean really high), the forces became one. Sort of like ice and water are really the same material, but under different physical environments. Heat ice up and the relationship to water is revealed.

While still a work in progress, theories are now being explored that will unify (explain the relationship between) the strong nuclear force and what is now called the electroweak force (unified version of electromagnetism and weak nuclear). So it appears that soon three of the four forces known in nature will be unified. Gravity is the odd force out. Why? As I mentioned earlier, it's because compared to the other forces, gravity is so extremely weak. It is not possible to measure any relationship with any device man has been able to contrive.

That is, until you start talking about black holes. When you squeeze the mass of a star (or millions of stars) into a singularity, you have in effect raised the value of gravity to the level of the other forces. Obviously, to understand what goes on in a black hole, it is necessary to understand the relationship between gravity and nuclear forces, now that they are on a more equal footing.

The problem is, in the non-extreme areas of the universe (far away from black holes), the nuclear force/gravity relationship is not measurable. But in the extreme areas of the universe, inside the event horizons of black holes, the events are not observable. How, then, does one figure out the relationship between gravity and the other forces?


Let's Get Back On Track, Shall We?

This relationship (to bring things back together), is the clash point of the macro world of stars, galaxies, and relativity -- and the micro world of electrons, protons, quarks, and nuclear force. What was convenient to observe as two completely different extremes, each with its own mathematics, comes crunching together and must require a common mathematical description.

In the effort to study this micro/macro clash, a lot has been learned about the nuclear world. The functioning mechanisms of stellar interiors, for example. And some pretty good models of the Big Bang, as it relates to quantum matters. Involved in that understanding is what atoms would be produced in a Big Bang.

It's called Big Bang neculeosynthesis. Based on knowledge of stellar interiors (known pretty well now) and particle accelerator studies, the neculeosynthesis model predicts the ratios of the production of atoms, particularly hydrogen, helium, lithium, and beryllium. Heavier elements were not produced in the Big Bang (they're produced when stars go nova).

Since neculeosynthesis predicts element ratios, astronomers have been able to locate primordial gas dispersions in space and with spectral analysis, examine the mix of elements. The findings are consistent with the predictions of neculeosynthesis.

So What's It All Mean?

Ok, it was a long, drawn out argument. The point is, astronomy, relativity, E and M, thermodynamics, and quantum physics all have something to say about the evidences one can observe about the Big Bang. The conclusion is that the findings in all of these areas of physics are consistent with the Big Bang model. There is no other known model that brings all of these fields together.

So you see, by only speculating on perhaps one of these fields and being ignorant of the others, could one say "it's only a theory." When you dig around and see all that physics has to offer, the data leaves very little wiggle room.

So the Big Bang answers just about everything. Well, just about. There is that little problem about galaxy formation. Classic models about star and galaxy formation suggest that stars and galaxies formed much earlier in the hot, expanding universe than possible. I suggest you read the Dark Matter page when it comes available to put that problem to rest.




I made the image with the The Gimp. The Gimp is freely available for both Linux and Windows (I happen to use the Linux version). The star burst pattern was already available, I only had to mess with size, color, etc.

There are a number of good books out there that can fill in blanks in the story of the Big Bang. One that I like is written by Nobel prize winner Steven Weinberg, and titled The First Three Minutes. Though written by a Nobel prize winner, this is a book for the layman, and is a great introduction to the concept of the Big Bang.

Another book that is a sort of case study is that written by George Smoot (another Nobel prize winner) and Keay Davidson. It's named Wrinkles in Time, and describes the incredible and difficult quest to discover and measure the quantum fluctuations in the microwave background radiation.

Quantum considerations of the Big Bang say the fluctuations had to be in the microwave background, but up until George Smoot and his team got the COBE space probe finally off the ground, there was no way to see them. Had the fluctuations not been there, the entire Big Bang idea would have busted. In the book, George gives a hair-raising account of a follow up mission to Antarctica, necessary to validate the COBE measurements.