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.