Just what is "The Big Bang"?

Notice that I didn't title this post "Just what was The Big Bang?".

I know I've written about this before, but there is this misconception that the Big Bang, the cosmological not the tv show, is something that happened in the deep distant past of the Universe. This leads to no end of arguments about the creation of the Universe, the mystery of what came "before", and the question of a creator (tip of the hat to the nice young man from the Church of Latter Day Saints who had a 10 minute chat with me at the train station first thing yesterday morning).

So, I am here to teach you what the Big Bang *is*. You may think that this might make this an extremely long post, with lots of jargon, equations, difficult concepts and bizzarre grants. Well, Dear Reader, it won't. You won't have time to even make a cup of tea.

Ready? OK, here it is
"The Big Bang tells us that the Universe was hotter and denser in the past"
Done! Now that was painless wasn't it.

Hang on, I hear you shouting. "I've been short changed". But you haven't. That's it.

Before you leave dejected, you need to look at this fantastic cartoon from xkcd
But replace "String Theory" with "Big Bang" and the string theory wibble-wobble with "Suppose that the Universe was hotter and denser in the past". The difference would be that instead of "I dunno" we would have a wealth of observational tests, all which has been passed with flying colours".

I don't have time to go through all of the evidence today, so I am going to focus on a single thing, something that I think is very cool.

Before continuing, you have to remember that the predictions we make are mathematical, made within the framework of Einstein's General Relativity. It's not like we sit around waving our hands and waffling predictions (unlike a certain area of human endeavour).

So, let's start with an observation (which was originally predicted back in the 1940s), namely the Cosmic Microwave Background. If we have microwave sensitive eyes, our sky would look like this;
Unfortunately, I don't have enough time to explain the bumps and wiggles, but I will one day.

Essentially, we sit in a bath of this radiation, which has a temperature of about 2.7K. This is chilly.

In the past, however, the Universe was hotter and denser (think of what happens when you pump up a bike tyre - when you squeeze the air in the pump, it gets denser and hotter). So this Cosmic Radiation was hotter in the past.

But how do we test this? Well, it turns out that we can measure the temperature. How, I hear you ask?

Well, as well as the radiation out there, we also have atoms and molecules, and the two interact. The radiation actually warms molecules, making them spin a little faster. This faster rotation changes the way that the molecules absorb light from more distant objects, and by working with the spectra of distant quasars and galaxies that pass though more nearby gas, we can measure the temperature of the sea of radiation in which they sit.

I now this sounds a little "out there", but it's all standard chemistry in action. In fact, what is often forgotten that the first measurement of the Cosmic Microwave Background was undertaken in 1940 when Canadian Andrew McKellar noticed that molecules consisting of just Carbon and Nitrogen in the Milky Way appeared to be sitting in a radiation bath with a temperature of 2.4K.

This was before the predictions of the existence of the left over radiation from the extremely hot stages of the very early epochs of the universe, so what it was doing there was a bit of a mystery. Nobel Prize winner, Gerhard Herzberg in his book Spectra of Diatomic Molecules a few years later commented that  this radiation bath has "only a very restricted meaning".

So, now we can do this in the distant universe, which bring us to a very recent press release which tells us that CSIRO Telescope Takes the Temperature of the Universe. Now, this is not the most distant that we have measured the temperature, but it is extremely accurate.

I'm going to let the data speak for itself, so here's a copy of a figure from the paper:
The red dot is the new measurement, the green are more nearby measurements, while the black and blue are in the distant universe. What we have here is the temperature of the Cosmic Microwave Background "out there" on the y-axis, whereas the x-axis is the redshift (which we measure directly, but is related to the expansion of the universe between then and now.

The main point, however, is the dotted-line. This is not a fit to the date, but is the prediction we make from our cosmological equations. Essentially, the temperature out there, T, at a redshift, z, is related to the temperature now, To, by
T = To ( 1 + z )
How simple is that? And it works! Extremely well.

So, just what is the Big Bang? It is the most accurate tool we have for making predictions about the Universe. Nothing else comes close.

Science is awesome (well, cosmology is :)


  1. I've often discussed with people that mention that The Big Bang happened nearly 14 Billion years ago...My answer has always been that The Big Bang is alive and well, and continuing on it's way...Thanks for the simplified explanation.

  2. 1. T=To(1+z) is maths that even I can understand! I plugged in the figures and it worked.

    2. The result was predicted by other means and verified in this experiment. What would have been the significance of this result if the evidence for the prediction had not been made yet? Would it have detected the expansion of the Universe?

    1. The result comes straight from the relativistic equations for the expansion of the universe, more specifically just what happens to a blackbody spectrum in an expanding cosmos.

      If we had the observation, but not the prediction, we would eventually have concluded that we live in an expanding universe. These kind of observations show that other cosmologies (such as tired light) just don't cut it.


  3. 1. Evidence for an expanding Universe is what I suspected. Thanks, Geraint it puts the result in context for me. Do you think it is feasible for astronomers to eventually use this method with upcoming larger radio telescopes to also detect the acceleration of the expanding Universe?

    2. I assume that for calculating the CMB temperaure at a future epoch (instead of the past), one would simply plug in a negative redshift into the nifty little formula and the temperature would move progressively towards zero as redshift increased?

  4. No, wait. That doesn't work, does it? Need to invert the formula so To=T/(1+z)to determine future redshifted temperatures. Is that right?

    1. Nope, you use the same formula - as you correctly note, "future" redshifts are negative, and the maximum future redshift is -1 (as time goes to infinity), and plonking that into the formula, the temperature of the CMB goes to zero.

      A little bit on future redshifts can be read here

  5. You set a very tough homework task, Geraint...


    1. You don't learn anything by just doing the simple stuff! :)


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