Tuesday, August 11, 2009

What Is The Big Bang?

Dear science community,

Now that I have had a very kind introduction, I think it's time to answer the first question: What is the Big Bang? Given that early universe physics and the accompanying high energy physics is my level of expertise, I will be answering many questions relating to the Big Bang. (This will take several weeks and you will all be experts.) If anyone has any questions at all just ask, and I will write a specific blog post for your question. (Unless it is inappropriate.) We will then move on to other science.

The Big Bang was proposed by Catholic Priest named Georges Lemaître who discovered it as a solution to Einstein's Equations of General Relativity. (I will blog about many details of this later so stay tuned.) He discovered, with some very reasonable assumptions, that General Relativity predicted the universe had a finite age and began in a very hot and dense state before expanding into the majestic structure we see today.

Like all good scientific theories, the early Big Bang model cleared up many known inconsistencies in physics and at the same time made some distinct predictions that scientists had never before considered. (Again, I will blog more about each inconsistency and prediction later.) The three main "new" predictions the original big bang models made were:
  1. The universe was expanding.
  2. The percentages of the various elements in the universe had to be very strict ratios. (Ie. how much hydrogen versus helium versus Lithium, versus etc... had to be exact.)
  3. That there should be some relic photons or light left over from the early universe with microwave size wavelengths and an average temperature of ~3 degrees Kelvin. This is called the Cosmic Microwave Background Radiation or CMB.
Needless to say, all three of these predictions were discovered within the first few decades of Big Bang research. In modern times we get all sorts of other phenomena that is surprisingly explained by the Big Bang and it's more modern additions like inflation.

Now, do not let anybody confuse you. The physical effects predicted by the Big Bang are real and the Big Bang is the only known physical theory that predicts these phenomena. Moreover, the Big Bang is consistent with every experiment we throw at it, something very few theories live to boast about. The Big Bang is on very solid ground scientifically.

It should also be said that the Big Bang was met with much skepticism by early scientists. In fact, the name "Big Bang" was coined by Fred Hoyle, who opposed the theory, to scare and or shock people. Nevertheless, because of the above reasons, the Big Bang seems here to stay as a pillar of modern physics.

Joseph Smidt

PS: Click on picture to see a timeline.


  1. This is wonderful thanks! I have some questions though. While I can understand the big bang as the beginning of the universe, I am not sure I understand it as the beginning of time and space. Can you elaborate on that portion? What was there before the big bang if not time and space, and as Mormons, are we not theologically tied to a God within the universe, and thus the universe must be of an infinite length in time?

    Thanks again.

  2. Matt W.

    Thanks for your questions. I will blog on all the above. Just to let the cat out of the bag a little bit, no modern cosmologist actually believes everything sprang into being at the instant of the big bang. Almost all current "theoretical models builders" are trying to create scenarios where something did exist before the big bang(with space and time presumably) from which our universe sprang from. The problem with these models is so far it is hard to figure out a way to test them. (How do you test for physics before the big bang. Possible but hard.)

    I will explain why physicists are bent on creating such scenarios and why, like the name of the blog, science will probably in fact live in an "Eternal Universe". Let's just say there is good physical motivation to believe this.

  3. Awesome, I will definitely follow along.

  4. Not to disagree with what Joe said - I'm sure he's right on the money with the physics - but in my mind what happened before the Big Bang becomes a philosophical question more than a scientific one. I am not a cosmologist, but from my understanding we are causally separated from anything that may have happened before the Big Bang (the same way we are causally separated from stuff inside of black holes), so if we can never observe anything from before the Big Bang, doesn't the question then become philosophical?

  5. Nick, it would be a philosophical question if we were causally separated as you say so it is an important point. But weather we are causally separated is not 100% clear.

    For example, two photons from the CMB coming from different directions in the sky should be causally separated, if inflation didn't occur. This is the horizon problem and I will get to this later so if some readers are confused things will clear up.

    But because of inflation, two regions that seem to be causally disconnected become causally connected. (It's a miracle!)

    My point is cosmologists are not certain what exactly is causally connected and what isn't. If we are lucky maybe there will be some pre-Big Bang physics that can be detected. Nobody is 100% certain and so the literature on pre-Big Bang physics is ever growing.

    But if nothing else, I will give reasons why if this question is purely philosophical, at least it is well motivated by established physics.

  6. Also, as you brought up black holes, there is a continual debate over whether information can escape a black hole. Currently, the quantum theoreticians have good arguments for why information may be able to escape a black hole. Again, it isn't established, but if true, then "what's inside a black hole" may not be a philosophical question.

    I guess I should eventually write a post on this filling in the details.

    This also illustrates how many theorists work. Even though for all practical purposes, these questions may be beyond science, they are determined to find a loop hole, even if they have to construct things crazy like... string theory. :)

    This also illustrates why some physicists get so angry at theorists.

  7. As we lay on our backs last night looking for Perseids, I was explaining to my 11 year old daughter the constellations, the mythology and eventually the life cycle of stars. When I told her that her body was made of elements formed in the cores of stars, her immediate response was that she thought God had made her body. =:)

  8. When I get into a discussion about the spiritual side of astrophysics, I love to do the same thing Stan describes except with my wedding ring, which is made of gold. As with all elements heavier than iron, gold is primarily made in supernovae. That means that it took the deaths of some of the rarest, most massive stars in the universe to make a little piece of jewelry I wear on my finger everyday.

  9. The only problem is that these days men's wedding rings are often made from other metals like titanium, silver, tungsten, or platinum. All of those can be made in the atmospheres of asymptotic giant branch (AGB) stars and titanium can even be made in the cores of massive stars via fusion processes. Gold is generally regarded as the lightest element that can be made only in supernovae and even then something like 1% of the universe's gold may come from other exotic sources (like black hole accretion disks or gamma ray bursts).

    But the simplified version does nicely illustrate the point that it took an awful lot of work to get us here.

  10. Maybe you want to explain black holes first, but how is the singularity of a black hole similar/different from the singularity of the Big Bang? In other words, can the pre-Big Bang universe be considered one big black hole?

  11. Jared*, you have hit on an important question and one I was going to blog about in the future. The sigularities of black holes are similar to the singularity of the Big Bang, but in both instances we think they only exist in the math because General Relativity is not a quantum theory.

    I will explain more about this later. Just know the signgularities are similar, some have speculated that the universe in some ways is like a giant black hole, but ultimatley we are convinced the signugalrities are not really there and I will untimatly explain why over the coming weeks.


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