Light and the Age of the Universe - the Cosmic Microwave Background

Our main window to understanding the universe is light and the electromagnetic spectrum. Trapped here on earth, there is very little of the universe that we can actually touch and test with our own hands, but light provides an amazing tool. The Cosmic Microwave Background is perhaps on of the best methods we have of finding the age of the universe.

All objects that are in thermal equilibrium - that is, the matter and EM radiation in the objects are the same temperature - have what is known as a black body spectrum - EM radiation with properties that are a function of the temperature of that object only. That spectrum might be modified a little but atomic absorption and emission lines, but the fundamental black body spectrum will remain. The spectrum looks like the curves on this graph:

Each curve represents a black body emitter with a particular temperature, shown in Kelvin (roughly the temperature in degrees plus 273, where 0 is absolute zero). The Sun, indeed all stars have a black body spectrum. In the case of the sun, the surface, and black body temperature is about 6000K, so it looks not so dissimilar from the 5000K curve. You can see that the black body spectrum continues beyond the visible - indeed the IR part is what is responsible for heat from the sun. The earth has a black body spectrum of about 278K (5.5 Celsius), which peaks in the infra red.

So what does this have to do with the age of the universe? Well when the universe was a mere 400,000 years old, about 13.7 billion years ago, everything was very much closer together, though space was expanding rapidly, and so the universe was much hotter than it is now, so hot in fact that there were no atoms, there was just a sea or plasma of hydrogen and helium nuclei (and a bit of lithium) electrons, Electromagnetic radiation and other subatomic particles (earlier than this there weren't even nuclei, but that's earlier than we are interested in here). The universe was still too hot for the electrons to bind to the nuclei, and so photons were constantly being absorbed and re-emitted by the various charged particles that were around, and the universe was in a state of equilibrium between matter and radiation. This means there was a black body spectrum. Eventually, as the universe expanded electrons no longer had enough energy to constantly escape binding to the nuclei, and they finally bound, becoming hydrogen and helium atoms. There was still substantial interaction between matter and radiation, particularly in the form of scattering, such as Compton Scattering and Thompson Scattering. The universe continued to cool as it expanded further, and eventually cooled down to a temperature of about 4000K at which the scattering dropped off. The radiation at this point became decoupled from the matter in the universe, as the universe became transparent, though the shape of the spectrum remained imprinted on the light that passed on, and continued traveling through the universe.

In the intervening billions of years, space itself continued to stretch. Imagine drawing a wave on a balloon, and blowing up the balloon. You will see the wavelength becomes longer and longer. The same effect occurs to the radiation, but now the the very space of the universe is expanding, so photons that initially had a short wavelength, over time were stretched out so the wavelength was longer and longer, so long in fact, that the BB spectrum which peaked at 4000K now peaks at a temperature of just 2.725K - barely above absolute zero.

This temperature is the same in all directions, though there are tiny fluctuations, which resulted from small changes to the very uniform distribution of matter and energy in the early universe as we can see in the (Wilkinson Microwave Anisotropy Probe ) WMAP satellite image below. It is these tiny imperfections that seeded the collapse of the primordial matter into the stars and galaxies of the universe today.

By knowing the rate at which the universe is expanding, which we can measure from the red shift of other features of the universe such as distant stars, galaxies and quasars, and these initial temperatures (which we know from looking at hydrogen and helium in the lab) we can then deduce the age of the universe (in much the same way as we can determine how long a cup of water has been standing on a table for if we know it was boiling when it was put there)...

...13.7 billion years old.

Lasers

We are all pretty familiar with lasers these days, from laser pens to the sorts of lasers that evil masterminds use to cut British Secret agents in half (an example here being James Bond and Goldfinger), but what are they exactly?

Laser is actually an acronym, and it stands for Light Amplification by Stimulated Emission of Radiation. let's go through the terms. Light... well we know about that. Amplification - makes things brighter - easy so far. Radiation - another word for the electromagnetic spectrum, which includes light, so again, no problem. But what about Stimulated Emission? what does that mean? To explain this, I will start off with spontaneous emission in atoms. This also works with molecules as well, but atoms are a little easier to explain.

Normally atoms rest in their ground state, all the electrons in the atom are as low as they can be. The energy levels in atoms are limited, so if we imagine them as shelves, only two electrons can go on the bottom shelf, eight on the next, eighteen on the next and so on, with as many electrons in the atom as there are protons. Now when an atom becomes excited either by being heated or when it absorbs a photon, one or more of these electrons can jump into a higher shell (I will refer the difference in energy between the lower and upper level as the energy gap) . Left on its own, the atom may only stay in this state for a limited length of time, after which the electron will then drop down again to the ground state, emitting a photon along the way. The energy of the photon is the same as the energy gap. The important thing here though, is that length of time - it is only an average and is quite random. The electron spontaneously decays to the ground state, without any influence, and the emission of the photon in this case is called spontaneous emission.

Now imagine that we have our atom in its excited state, just like before, only this time, the atom is hit by some electromagnetic radiation of the same energy as this energy gap. This can jiggle the atom and force the electron to decay back to its ground state, so now we have two photons which can then go on to hit a couple more excited atoms, and we have four photons and so on. This kind of forced decay of electrons into their ground state is called stimulated emission. The stimulated emission is usually seeded by some spontaneous emission occurring in the laser.

There are limits to this of course; for one we need to have lots of excited atoms, and once we run out of excited atoms then we can't have any more stimulated emission. To get the excited atoms in the first place we have to dump lots of energy into the lasing material, and we do this in a technique known as pumping. There are lots of ways to pump a laser, but they all amount to pretty much the same thing - dumping lots of energy into the laser material, so that when photons pass through the cavity, they can stimulate the emission of more photons. The lasing medium itself has to be something that we can excite relatively easily, and there are lots of different materials such as ruby, special glasses (like Yttrium Aluminium Garnet), dye lasers and even gas lasers like Helium Neon (HeNe) lasers and Argon Ion lasers. Probably the most familiar sorts of lasers to us now are semiconductor lasers, used everywhere from DVD players to laser pointers.

A special feature of this light is that it is coherent. If we remember the previous post about the wave properties of light, coherent means that all the photons of one frequency are oscillating in step with one another. That means they always constructively interfere. This is unlike light from a normal fluorescent or incandescent bulb, which emits incoherent light (by spontaneous emission). It is this coherence that makes lasers so powerful, and why you have to be extremely careful when using a 4W laser like an Argon Ion laser (you have to wear special glasses and even stray reflections can burn your skin), even though the actual energy it emits is far less than a 60W bulb..

Laser Cavities

To make the laser more useful and to form a stronger pulse or beam, we can take our laser pump material and stick mirrors on the ends. One of the mirrors reflects all the light, and one of them reflects almost all of the light - usually something like 99%. Now the light can bounce backwards and forwards in the cavity. These mirrors are often specially shaped to make the beam as stable as possible. You do have to be careful here though, as if the amount of energy in the cavity gets too high - the light gets too bright - then you can start to get odd effects happening in the cavity such as self focussing, which can easily blow a hole in the lasing material. For glasses of course this will break the laser and the pump material will need replacing, and this presents a problem for high powered lasers.

Hopefully this has given you an idea about the basics of lasers. I haven't delved into any of the mathematics here, but you may be unsuprised to know that Einstein was a pivotal figure here once again, as he determined that stimulated emission had to occur.