The Music of the Universe

Listen to the Universe:

Do you want to listen to the Universe? Just turn on your radio - but have it tuned out of any station. A small fraction of the static that you hear is radiation released by the Universe when it was only 380,000 years old, i.e., about 13.7 billion years ago.  The ingredients, along with the story of the early evolution, of the Universe are literally written  in this cosmic radiation which scientists call the cosmic microwave background radation (CMBR). NASA and ESA, the European Space Agency, have put out two satellites, WMAP and Planck,  to read the history book of the Universe as told by the CMBR. These satellites are the latest   in about a dozen probes - ground-based, upper atmosphere balloons, and satellites - that have studied the CMBR since the mid-1960s. So why is this radiation coming from the Universe's childhood so important to warrant such intensive studies? This blog will answer this question by showing how the ingredients of the Universe can be determined by analyzing the CMBR, only possible with WMAP and Planck.

Origins:

The best way to appreciate the importance of the CMBR is to first understand its origin. In the few years after the Big Bang, 13.73 billions year ago, the Universe was a thick soup of subatomic particles and radiation. Radiation couldn't travel far before hitting a particle and getting scattered. Particles want to stick together whereas radiation being radiation wants to roam free. Radiation and matter interacted strongly with the intense radiation blowing up particles that tended to stick to each other. Remember, all of this tug of war is happening while the Universe continued to expand. The expansion makes the radiation cooler and cooler. The cooler the radiation the less it is able to blow stuff up. 

Now you can see where this is heading. The simplest atom, Hydrogen, which is made up of a single electron orbiting a single proton couldn't form till the Universe became cool enough, “only” 3000K which is about 3273 centrigrade!,  so that radiation wouldn't blow it up. This happened at 380,000 years after the Big Bang when the electrons got locked up in Hydrogen atoms thus leaving radiation free to travel unhindered throughout the Universe to become what we observe today as the CMBR. An interesting point to keep in mind is that hydrogen and the CMBR where born together. The date of birth of the CMBR and hydrogen is called the time of decoupling. The CMBR that we observe today is a lot cooler, about 2.7 Kelvins or -271 centigrade, than when it decoupled from matter. This is due to the expansion of the Universe.

Discovery:

George Gamow, a brilliant Russian astrophysicist living in the USA during the 1940s,  was the first to predict the existence of the CMBR. However, since the wavelength of the CMBR is in the microwave regime (the cousins of the waves that heat your food in a microwave oven), the existing technology at the time didn't allow the detection of the CMBR. Radar and  telecommunication devices were just being developed for military purposes. So Gamow's prediction had to sit in the shadows for 20 years.

In 1964, the CMBR was discovered accidentally when two scientists were tuning up a microwave antenna for a telephone company in New Jersey, USA. The antenna seemed to pick a continuous static regardless of the efforts of the scientists. A group of physicists was working unsuccessfully in nearby Princeton University to design an antenna to detect the CMBR. Eventually, those involved realized that the annoying hiss in the telephone company's antenna is nothing else but the CMBR. Guess who won the Nobel Prize? Gamow? No. The Princeton group? No. It was the telephone scientists, the ones who stumbled upon the CMBR by accident without even knowing it!

Decoding:

After the discovery it was quickly realized the temperature of the CMBR is about 2.7 Kelvins and that it contains a Universe's worth, literally, of information about the Universe. If you still remember from above, prior to the formation of the CMBR, matter and radiation were strongly coupled.  This means that the whatever matter is doing the radiation will always carry the mark. Hence, when matter and radiation parted ways at the moment of decoupling, radiation was left bearing the mark of the matter. The CMBR carries the signature of the matter distribution at the time of decoupling. The distribution depends in subtle ways on the kinds of matter and energy that make up the Universe. The content of the Universe controls  how it evolves. The information about all of this is written in the CMBR.

To decipher the story written in the CMBR one needs better and better detectors, detectors that are sharp enough and sensitive enough to look at the CMBR very closely. It's like needing a lens that allows you to read the inscriptions written on a very small coin to read the year and who was king at the time. The best device to study the CMBR would be an antenna that divides the sky into very small pixels, i.e., has very sharp vision, and be able to measure the temperature of each and every pixel very accurately:  big pixels will give you a blurry view, inaccurate temperature measurements, well let's just say that anything inaccurate is useless. All information lies in the small pixels and their temperature so you want to make your pixels as small as possible and your measurements as accurate as possible, and this depends on the available technology: big enough antennas, the ability to have at small pixels, and a good thermometer to measure accurate temperatures. 

Fingerprints:

Let's fast-forward to 1993 when NASA sent the Cosmic Microwave Background Explorer, COBE, into orbit. COBE measured the temperature of the CMBR to be on average about 2.71 Kelvin. The temperature anisotropies, i.e., the pixel-to-pixel temperature variation or the difference between hot and cold pixels, is 1 part per 100,000. This might seem very small, which it truly is. But such minute temperature differences are the origin of today's galaxies! The Universe is almost completely uniform on cosmological scales, scales larger than galaxies and groups of galaxies (billions of light years). It is almost so but not quite. Lucky for us. If it were completely uniform then no galaxies, and stars and planets, could form. It is this 1 part in 100,000 anisotropy that makes us possible.

The question that presents itself now is from where does this very, very small anisotropy come from. Well, if you still remember, before decoupling matter wanted to cling together and the radiation kept on blowing it up. This tug of war sets up acoustic oscillations, sort of sound waves,  in the whole Universe. These cosmic sound waves lead to compression, regions of high matter density, and rarefaction, regions of low matter density. That's the advantage that gravity wants; regions of high matter density will attract more matter due to gravity which increases their mass and thus their gravity that in turn attracts more matter. The positive feedback, when it operates over billions of years, leads to the growth of structure: galaxies and cluster of galaxies. The low density regions become the empty spaces in between. The really amazing part of the story is that the pixel-to-pixel temperature variations in the CMBR, the anisotropies, are the progenitors of the galaxies and galaxy clusters of today! By studying them really close, scientists can determine the types and amounts of matter and energy present in the Universe. The CMBR carries the fingerprints of the Universe and you really need excellent instruments to read them.

WMAP and Planck:

WMAP and Planck are very sophisticated satellites to study the CMBR. One can describe them as  highly trained musical ears, or a DJ's sound analyzers, that can pick out individual musical notes in a rich intricate tune. The rationale behind this analogy will become clear shortly. WMAP was launched by NASA in 2001 and has been operating successfully since then. Planck was launched in May 2009.  

Let's recall it one more time. Before decoupling, before matter and radiation split, matter in the Universe underwent acoustic oscillations due to the tug of war between matter that wants to clump and radiation that wants to break free. Since matter and radiation were strongly coupled, the acoustic oscillations are imprinted on the radiation. These fingerprints are in the form of cold and hot spots in the CMBR, i.e. the observed temperature anisotropies. CMBR analysis relies on studying the correlations between the cold and hot spots on the sky, since these anisotropies are signatures of the acoustic oscillations. So by studying the CMBR cold and hot spots on the sky, scientists are able to calculate how much of each oscillation is there. It is like listening to a symphony and then saying how loud is a particular musical note compared to another different note. WMAP & Planck do exactly this.

The Ingredients:

OK. So how can these acoustic oscillations, that reveal their presence as anisotropies in the CMBR,  tell us about the matter/energy content of the Universe? Let's consider the following analogy. If you pluck a guitar string then the string will start to vibrate at many frequencies or notes. The dominant note with which it will vibrate is related to the length of the string. The volume of the dominant note is related to how hard you plucked the string. Now think of the Universe as a guitar string whose length is increasing.  Gravity makes matter compress and radiation blows it up, so that's your initial pluck that sets the Universe ringing.

CMBR analysis made possible with WMAP and Planck amounts to studying the vibration pattern of the Universe by looking at the pattern of hot and cold pixels in the CMBR and determining the notes and their strength. The dominant note is related to the total matter/energy content of the Universe at the time of decoupling, since the total matter/energy content controls the expansion rate of the Universe and hence its size at the time of decoupling. The strength of the dominant note is related to the amount of gravitating matter since that what is lead to the compression in the first place. After the first compression, radiation has it way and blows up the matter leading to a secondary note. The strength of the secondary note is related to amount of ordinary matter, stuff like protons, neutrons, electron, etc., present since ordinary matter wants to cling.

So now we have measured three quantities: total matter/energy content, amount of gravitating matter, and amount of ordinary matter. If we express things as percentages, then the percentage of total matter/energy is 100%. The percentage of gravitating matter, including ordinary matter, is 30%. The percentage of ordinary matter alone is 4%. Therefore, 26% of the gravitating matter in the Universe is what scientists call Dark Matter. Dark Matter neither emits nor absorbs any radiation. It is a mysterious form of matter that scientists know nothing about except that it follows the known laws of Newton when it comes to gravitational interaction, just like the ordinary matter. Is there something missing? Yes, what is the remaining 70%? It is Dark Energy. A very mysterious form of energy that permeates all space and leads the expansion of the Universe to speed up. It gave an additional kick to the Universe and affected the secondary note. All of these are very reliable measurements and what is more amazing is that they are corroborated by other independent observations of distant supernovae and how galaxies cluster, may be the subject of a future article ...

The CMBR is an amazing musical concert. Listening to its music allows scientist to answer very deep and profound questions about what is the Universe made up of. But what is even more fantastic than the music is the answer. The answer shows us that there is more to the Universe than what we are familiar with, 96% of the Universe is Dark Matter and Dark Energy, of which we still know nothing about. Solving their riddles will be the science of the 21st century and make the discovers Nobel prize winners. So get to work!