Wednesday, March 3, 2010

Light and the Age of the Universe - The Discovery and Analysis of the CMB

Discovery of the Cosmic Microwave Background

The Comic Microwave Background was discovered pretty much by accident by Arno Penzias and Robert Wilson who were working for Bell Laboratories, looking for signals from radio waves reflected from balloons. In the course of their experiments, they had to eliminate all noise sources such as radio broadcasts, and even a "white dielectric substance" left on the inside of the detector horn by a family of pigeons who had taken nest there. Once they had got rid of and accounted for every bit of noise they could, they noticed that there was a constant microwave hiss, from every direction, day and night - they had discovered the Cosmic Microwave Background.

The antenna where they made this discovery is now a national monument in the US:

They still did not know what they had found however, but when a friend of theirs told them about a still unpublished paper by Jim Peebles talking about the possibility of finding a signal like theirs, and what it would mean, they began to realise the significance of their discovery. The papers by Peebles and his colleagues, and the paper published by Penzias and Wilson were published together in Astrophysical Journal Letters. Penzias and Wilson won the 1978 Nobel Prize for their work.

The Cosmic Background Explorer

It was thought from early on after the discovery, that there would be small anisotropies (differences depending on direction) in the CMB, but ground based measurements were not good enough to measure them. It was not until the COsmic Background Explorer (COBE) was launched in 1989 that these anisotropies were first observed.

These fluctuations were very small, just one part in 10,000 of the average temperature. The resolution was still relatively low however, and so there was still much detail to be found. One additional important piece of evidence however came out of this - the match between the theoretically predicted Black Body curve based on the Big Bang model, and the experimental curve. The two matched precisely:

These results earned another Nobel Prize, but this time for the principal investigators on the COBE project; George Smoot and John Mather. The CMB wasn't the only thing that COBE was analysing however, and there were other important experiments and discoveries made. A good outline of the COBE satellite's other results can be found here.

The Wilkinson Microwave Anisotropy Probe (WMAP)

The next satellite to look at the CMB was WMAP. This time dedicated to the analysis of the CMB. After the success of COBE, WMAP was designed to not only view the CMB at higher resolution and sensitivity, but also to look at other features of the CMB such as polarization in order to give a better understanding of the early universe. There are a number of interesting results from WMAP, which will continue to operate until (currently) September 2010, and more details can be found here.

A brief summary of some of the WMAP results
  • The universe is 13.73 billion years old (the most accurate figure we have))
  • The universe is very flat (Euclidean)
  • Around 23% of the universe is dark matter.
  • The anisotropies appear to be random (though there are some hints of deviations from simple randomness which could give further clues into the early nature of the universe)

The Future Exploration of the CMB

This article has provided only a brief outline of the discovery and analysis of the Cosmic Microwave background. There are a number of features that have not been discussed, such as doppler shift, polarization and so on, and there is still much work to be done in understanding the details of the CMB. Although WMAP only has a few months of life left, the European Planck observatory, which started to take measurements in 2009, and is expected to begin to release results in 2012.

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.

Monday, March 1, 2010

Quickfire Question: How do incandescent (filament) bulbs work?

We are all familiar with incandescent bulbs, which have been until relatively recently been the most popular sort of bulb.

A voltage is placed across a metal filament held in an inert gas like argon, neon or nitrogen, in order to stop the gas from reacting with the filament and allowing the bulb to live longer. The filament has a high resistance to the current flowing through it, and this heats the filament, causing the atoms in the filament to vibrate. As the atoms vibrate, they then radiate energy in the form of light.

An important point that can be made here, is that all vibrating atoms and molecules will radiate somewhere on the electromagnetic spectrum. The hotter they are, the faster they vibrate, and thus the higher the energy (and frequency) the photons are that they emit.

A problem with this sort of lamp, is that they are very inefficient. Because the light is generated by heating, large amounts of energy is lost in unwanted heat. Also as the filament is heated, it slowly evaporates over time and eventually breaks, leading to a relatively short lifetime. It is mainly for these reasons that there is a move to using energy saving bulbs, which both have a longer life, and also produce the same amount of light for less energy input.