March 17, 2010

Big Bang for beginners-4: The speed of cosmic evolution

(My latest book God vs. Darwin: The War Between Evolution and Creationism in the Classroom has just been released and is now available through the usual outlets. You can order it from Amazon, Barnes and Noble, the publishers Rowman & Littlefield, and also through your local bookstores. For more on the book, see here. You can also listen to the podcast of the interview on WCPN 90.3 about the book.)

For previous posts in this series, see here.

What may surprise people is how rapidly the universe went from a very hot initial state to one in which it was cool enough for atoms and molecules to form. If we push our theories back as far as we dare, bearing in mind that we have stretched them to the limits and that we may well be wrong in some aspects, the earliest time that we can speak of is 10-43 seconds after the Big Bang (called the Planck time). i.e., this is 0.0000… 0001 seconds (43 zeros in all, including the one before the decimal) after the Big Bang. In other words, it is a really tiny time. It is estimated that the temperature of the universe at that time was about 1030 degrees. That is 10 followed by 30 zeros, a really huge number.

It is now believed (according to the inflationary model of the universe), that roughly around 10-36 seconds after the Big Bang, the universe began undergoing an extraordinarily rapid expansion. This expansion lasted for a very short time (a tiny fraction of a second) but it was sufficient to increase the linear size of the universe by a factor of around 1026, which is such a staggeringly rapid expansion that it boggles the mind. After that tiny period of very rapid inflation, the universe settled into the steady and slow expansion that we currently experience.

It took just one millionth of a second for the universe to cool from its extremely hot initial state to a cooler (but still very hot) 1013 degrees, the temperature at which the photons' energy became low enough that protons and neutrons could form out of quarks and gluons without being immediately blasted apart by them. So quarks and gluons existed in the free state for just about a millionth of a second after the Big Bang and ever since then have been confined in protons and neutrons. At this stage, the universe was now about the size of our Solar System.

About two to three minutes after the Big Bang, its temperature had dropped to about one billion (109) degrees, and now nuclei could also exist without being blown apart by photons. The size of our universe was now about 50 light-years (a light-year is the distance traveled by light in one year which works out to roughly 1014 miles), which is still pretty small when you consider that the radius of just our own Milky Way galaxy is about 50,000 light years.

After about 300,000 years, atoms began forming. The only atoms that could form were the very smallest ones (hydrogen, helium, and lithium) because only their nuclei existed at this time. The temperature of the universe is now about 3,500 degrees.

After about 100 million years all the matter that initially existed as a more or less uniformly spread out gas that occupied the entire universe, start forming into clumps. The atoms in the clumps attract each other because of gravity and they coalesce to form all the galaxies, stars, and planets that the universe now contains. It is estimated that the first stars began to be formed about 150 million years after the Big Bang. Galaxies started forming after about 1 billion years, when the temperature was about 20 K or -253 degrees Celsius. (The temperature scale called Kelvin (K) is used in physics and its value is obtained by adding 273 to the Celsius temperature. For very high temperatures, we can ignore this difference as well as the difference between Celsius and Fahrenheit scales, which is why I have not been too specific about the temperature scale so far.)

The stars that were formed initially consisted of mostly hydrogen and helium. The energy that makes the stars so bright and hot is primarily caused by nuclear reactions in which hydrogen nuclei fuse together to form helium nuclei. It is only within stars that are above a certain size that all the other heavier elements we now have were created. At the end of their lives, these massive stars first collapse and then explode in what we call a supernova, spewing all the heavier elements that they created into space, where they end up in planets like ours. The mass of our own Sun is not large enough to explode in this way. Its life will end with a whimper, not a bang.

So that is the basic story of the Big Bang: Starting with a highly dense, uniform, and hot gas consisting of quarks, gluons, electrons and photons that was compressed into a tiny amount of space that was smaller than a golf ball, it rapidly expanded and cooled to become the vast universe we now have with all the basic elements.

Our universe began 13.7 billion years ago, stars came into being about 150 million years later, galaxies started forming after about one billion years, and our Solar System with the Sun, Earth, and other planets was formed about 4.5 billion years ago.

This graphic gives a summary of the time evolution of the universe.


If we want to run the clock further to see what happened on Earth alone, bacteria appeared about 3.5 billion years ago, followed by green plants and algae (1.3 billion years ago), the first wormlike animals (600 million years ago), fish (550 million years ago), amphibians (400 million years ago), reptiles (350 million years ago), mammals (250 million years ago), birds (180 million years ago), and humans (6 million years ago), bearing in mind that these numbers are approximate. The New Scientist magazine gives a more detailed timeline for evolution.

In subsequent posts I will examine some subtleties of the Big Bang theory and the evidence in support of it.

POST SCRIPT: Richard Dawkins on Australian TV

Richard Dawkins appeared on an Australian TV panel program called Q/A that takes questions from the audience. It is interesting that Dawkins is the person who most directly answers questions while the others tend to waffle and hedge. Calls for 'respect for religion' make their predictable appearance when Dawkins's points against it hit too close to home. Well worth watching. (Thanks to Pharyngula.)

The program is a little under an hour and can be viewed in six parts. The last two segments deal mostly with a specific Australian issue about how they treat refugees who arrive by boat, an issue that has created a huge controversy in that country.

The very last question gets back to religion with a question about the afterlife. Again compare the wishful thinking of the other panelists with Dawkins's directness.

Part 1:

(part 2, part 3, part 4, part 5, part 6)


Trackback URL for this entry is:


"About two to three minutes after the Big Bang, its temperature had dropped to about one billion (109) degrees, and now nuclei could also exist without being blown apart by photons. The size of our universe was now about 50 light-years"

Does that mean that the universe expanded faster than the speed of light? How was that possible? Or was that the only occasion when it was possible?

Posted by Scott on March 17, 2010 04:02 PM


The speed of light limitation applies to objects in space and to information transfer. So no particle can travel in space faster than the speed of light.

But here it is space itself that is expanding and no such restriction applies to it.

There are other cases (such as the collapse of the wave function and the phase velocity of wave packets) which are believed to be faster than the speed of light but no information is conveyed by either of these effects faster than the speed of light.

Posted by Mano on March 17, 2010 05:35 PM

Hi Mano - I was going to ask the same question. Your answer raises a more fundamental question - what is space? Also, its corollary, what does the absence of space look like?

It is very hard to visualise all of the matter in the current universe existing in a 'space' smaller than a 'golf ball' (I actually understood it to be a dimensionless point, perhaps you were trying to help with the mental picture?) It just seems to be...impossible (and I am a 'believer', that is, I have no reason to doubt the scientific picture you are painting).

But point, or golf ball - what was around it before it exploded, or even as it expanded? What is beyond the boundary of our Universe (not space)?

I guess that is always the difficulty with trying to fully comprehend relativity - I can visualise gravity warping space time and slowing time and bending light etc, but I still don't know what space is. Certainly not nothing...



Posted by Bill on March 18, 2010 05:42 AM

Bill: I don't want to speak for Mano (since he obviously can explain this better) but what I always understood from multiple viewings of the series Cosmos is that a three-dimensional universe expanded (and continues to expand) into a fourth-dimensional space. Since we exist in three dimensions, it's difficult for us (myself included) to conceive of a fourth dimension. Did I get that right, Mano?

Posted by Scott on March 18, 2010 03:21 PM


The idea of our 3D space curving into a 4D space is often invoked to try and give us some visual clue as to what might be going on. It is usually done as an extension of the visual image of a 2D flat surface being curved into a 3D sphere.

But we must always bear in mind that the 3D embedding in 4D space is an analogy and should not be taken to mean that there IS really a fourth dimension that the universe is expanding into. What we do know is that everything that we measure is within our 3D space and within our universe, and we see no edge or boundary or center to the universe. It is in order to explain that that the analogy is used.

Posted by Mano on March 18, 2010 04:26 PM

Thanks Scott and Mano - the concepts are very hard to get a handle on. For a start, visualising the Big Bang, it is necessary to be on the outside looking in, so the speak. From what I am hearing, there is no outside to look in from. Very hard then to visualise a fifty light year universe because I automatically stand outside of it to 'see' that fifty light years.

I don't really get the 4D analogy either; though I do try to liken it to our 3D world looking 2D (that is, we can move in a plane but in doing so, travel in a single direction long enough and, voila, you are back where you started). Is this how to think of it? If the Universe is x lightyears across, and I move x light years in any given direction, I will end up where I am?

Posted by Bill on March 18, 2010 08:34 PM

Mano, thank you for this beautiful post!
I gulped down "God vs. Darwin: The War Between Evolution and Creationism in the Classroom", can't wait for your next masterpiece. I love how you explain things simply but always compellingly. I'd love to see your presentations (I personally think presentations are the some of the nicest ways to explain the big bang theory, like so:,The_Big_Bang_theory), any chance we'll see some of that from you in the near future?

Posted by Edna Gershwin on March 29, 2010 07:09 AM


Thanks for that link. It was nice and concise!

Posted by Mano on March 29, 2010 08:58 AM

You say space could expand in this fraction of time but not matter. But then you say "all the matter that initially existed as a more or less uniformly spread out gas that occupied the entire universe". Since gas is matter, how did it get all over the universe of billions of light years in 100 million years? I am having difficulty understanding this.

Posted by EDWARD EMORY on July 4, 2010 02:32 PM


The really difficult thing to understand is the period known as 'inflation' that happened right after the beginning and caused all the matter to appear and expand extremely rapidly at mind-boggling rates. This is the part that is not easy to understand.

What happened at 100 million years was not so dramatic, just that the matter started clumping at that time.

Posted by Mano on July 4, 2010 06:28 PM