Recently it occurred to me that my mental image of the CMB radiation, gleaned from a range of popular science books, was probably completely wrong. I set about some thinking. The following notes outline my thoughts so far, and I would appreciate the comments of others more knowledgeable than I. Thinking about the CMB What is the cosmic microwave background? Put simply; it is the remnant radiation from the Big Bang. The theory is that the Universe, up to almost 400,000 years after the Big Bang, was so hot and dense that it was effectively a plasma within which photons were prevented from travelling any distance, light could not travel through it, so it was opaque. At a little less than 400,000 years (some references give this as 300,000 years), at what is referred to as the photon decoupling event (also known as the period of last scattering), the Universe had cooled sufficiently for protons and electrons to combine into atoms. This changed the state of the Universe such that photons were able to move freely through the it. It became transparent. These photons have been travelling ever since, but are no longer in the wavelengths of visible light. There energy has decreased, and, correspondingly, their wavelength has increased, so that they now appear as microwaves, with a temperature of about three degrees above absolute zero (almost 3K). Some questions must arise from this, for instance: 1. Where is the radiation coming from? 2. Where is it going? 3. It is travelling at the speed of light, so why had it not passed us long ago? 4. If its origin is in a small spot that must be central to the Universe, does this give a preferred direction to the Universe? Attempting to answer these questions challenges the lay person's intuitive image of the Big Bang, the expanding Universe and the CMB radiation. 1. Where is the radiation coming from? This implies another question: Where did the Big Bang happen? The answer to this is that it happened everywhere in the Universe. At the first instant, the Big Bang was the Universe; the Universe was the Big Bang. There is no part of the Universe today, nor will there ever be, however long it continues to expand, in which the Big Bang did not occur. The Big Bang was everywhere; so the radiation must be coming from everywhere. 2. Where is it going? If it originated everywhere in the Universe, it follows that it must be going everywhere in the Universe. It is moving at the speed of light from every point in the Universe to every other point in the Universe, without exception. 3. It is travelling at the speed of light, so why had it not passed us long ago? To some extent, this question has already been answered, but if by "us" we mean the point in the Universe at which the Earth is situated, we must accept that it has been passing us ever since the original radiation was able to move freely through the Universe, and it will continue to pass us, and every other point in the Universe, as long as any energy remains in the waves. 4. If its origin is in a small spot that must be central to the Universe, does this give a preferred direction to the Universe? As mentioned earlier, we have to abandon the image of the Big Bang happening at the centre of the Universe, and the radiation emanating from there and moving outward. Viewed from the Earth, the radiation would be seen to be coming from every direction. This does not mean that the Earth is at the centre of the Universe, because, if that image were right, the radiation would appear to be moving away from Earth, and would, therefore, not be visible. Also, the Earth's position is not special. Whatever viewpoint in the Universe an observation might be made from, the radiation would still be observed to be coming from every direction towards that point. Therefore, no preferred direction can be identified by observation of the CMB radiation. I said that radiation that is receding would not be visible, yet we are all familiar with doing something like shining a powerful flashlight into the sky and seeing a beam of light that is obviously moving away. This is due to the fact that some of the light is reflected back from water molecules, dust particles etc in the atmosphere. The clearer the air, the less visible would be the beam of light. In space, we would not see it at all unless it were shining straight towards our eyes. For this reason, only the CMB radiation moving directly towards a detector can be detected. One might also ask if the CMB, because it fills the Universe, can be regarded as a "fixed" reference against which motion in the Universe can be measured. If this were the case, motion would no longer have to be regarded as solely relative. Absolute motion could be demonstrated. The first thing to recognise is that the CMB is electromagnetic radiation, and as such it is observed as moving at the speed of light, irrespective of the position and (relative) motion of the observer. It seems unlikely, therefore, that the CMB could be regarded as an absolute frame of reference, any more than can any other electromagnetic radiation, including visible light.