After I had written about how we think the Universe got started I realized that we had left off with a Universe consisting only of gas and radiation. That it came into being at all is pretty amazing but to be honest, a ball of hot gas and energy isn’t very useful, especially if you have ambitions of becoming a complex organism that can explore space, investigate the mysteries of creation and write blogs! So this piece will be about what happended next, how we went from a ball of energy and gas to stars, galaxies and planets. Oh yes, and blog writers.
We left off with a universe that had suddenly grown to many times its original size and as we know, when something expands it cools, ok nearly everything, my waist line has expanded over the years but I wouldn’t claim it has got cooler! So as the Universe expanded and cooled it reached a point where matter as we know it could condense out and form a swarm of protons and electrons and some neutrons. A proton on its own is the nucleus of a Hydrogen atom. Some protons and neutrons got squeezed togther to form Deuterium and some Helium and tiny amounts of Lithium and Berylium but nothing heavier was created until the first stars got to work. At this point our baby universe is between 10 and 20 seconds old.
There is also a little mystery here. In fact quite a big mystery. When matter appeared something else appeared too, antimatter, particles with that are identicle but with opposite charges and when the two meet they annihilate each other completely. Now as the Universe was tiny the matter and antimatter would have had no chance of avoiding each other so everything we see today is the left overs from all that destruction. What puzzles physisists is why there was a very tiny difference that allowed a small fraction of our kind of matter to survive.
Until the point where electrons joined protons, the ‘cosmic soup’, was too thick with particles for light to get through it. The photons in there couldn’t travel far before they hit something. As the Universe expanded and cooled even more the protons and electrons were able to join to create the first atoms, one proton and one electron giving a hydrogen atom. When the first atoms formed the ‘soup’ thinned and suddenly the photons could travel almost unhidered through the Universe. This era is called ‘Recombination’ which is a bit of a silly name as up til then nothing had been combined before but the old names seem to stick so that’s what we call it. By now the little universe is 380,000 years old, still an infant.
Now at this point the tempreature is still about 3000 degrees Celcius so that equates to a lot of energy which equates to very high frequency light and we will come back to that and why it is significant shortly. What happened next is, well nothing much really for quite a while. In fact this period is known as ‘The Dark Ages’ because so far there are no stars to light up the skies. It wasn’t until about 150 million to about a billion years after the Big Bang, that the first stars began to form but first we need to back track a bit, well a lot actually. Right at the very beginning when matter first started to form there were two kinds, that which we call ‘matter’ and an as yet unidentified kind of matter that does not interact with photons at all so we can’t detect it directly. This is what we call ‘Dark Matter’ and although it doesn’t interact with photons, it does interact with gravity and this is very important. It means that it can clump together but unlike ‘ordinary’ matter it can’t get blown apart in the high energy enviroment of the early universe by all those photons whizzing about which means that all the clumps of Dark Matter that formed right at the very begining are still around as the Universe expands and and this creates a kind of skeleton for all the stuff that forms to cling to.
The light that started travelling across the Universe when the ‘soup’ thinned was at very short wavelengths, but because spacetime is expanding the wavelength of this light has become stretched. Now wavelength and temperature are related so as the wavelength stretches it corresponds to a lower temperature and we can measure that temperature, 2.75 Kelvin and this is what we call the Cosmic Background Microwave radiation and it marks the furthest back we can see. Before that point light couldn’t go far so we can’t see anything from before this point.
At about this time the first stars are forming and these are big, I mean really big, giants many times the size and mass of our Sun and when a star is very big it acts like many human stars, they burn brightly and then go out with a bang. Live fast, die young! These giant stars form heavier elements which are blown into space when the stars go supernova and this forms the basis of the next generation of stars which can then make even heavier elements that can make small rocky planets…
As the first stars are forming they are also gathering together in groups. First dozens, then hundreds, then thousands, millions etc, the first galaxies and groups of galaxies were built up around the scaffolding of the original clumps of Dark Matter and these first galaxies also grouped together into groups and these groups in turn into laŕger groups. From the original, microscopic, clumps of Dark Matter in the very early unnivere we can now observe clusters of a few hundred galaxies that are part of super clusters containing millions of galaxies, spirals like our own Milky Way, elipticals, irregulars. There are tiny dwarf galaxies of a few hundred stars up to giant galaxies that make our Milky Way look puny.
So this is where we are today, with a universe full of billions of galaxies and this is what it looks like if you happen to own a large telescope orbiting your home planet-