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Ripples in Space, the Origin of Structures in the Universe
TopHat in Antarctica

One of the fundamental mysteries that we encounter in modern cosmology is the origin of the structures we observe in the sky today. By this I mean the larger astronomical objects that we can see --- galaxies and clusters of galaxies, the large 'voids' in which galaxies seem to be lacking, the 'walls' with a higher than average density of galaxies.

While more traditional astronomy clarifies the detailed nature of these objects, the goal of cosmology is to understand the physical origin and evolution of the Universe on the largest of spatial scales, and how these objects came into existence. Einsteinian gravitation provides the theoretical basis for describing currently accepted cosmological scenarios. Coupled with certain key observations over the past 50 years (such as the Hubble expansion), we have used this general theory to create an outline of the major events in the history of the Universe. In particular, there is an initial event in which all the matter in the Universe, including space itself, was compressed into a very small size.

From this region, the Universe expanded to become what it is today. This initial event can reasonably be defined as the 'birth' of the Universe since, at that time, the Universe was so hot and dense that all information from any previous existence would have been erased. These physical conditions, and the subsequent rapid expansion, have led us to dub the initial event the 'Big Bang'. The existence of the Big Bang also allows for defining the age of the Universe as the elapsed time since the event.

Depending on various details such as the total mass of the Universe, current estimates for this age run between 10 to 16 billion years. Current estimates of the age of stars run from 11 to 15 billion years, almost as old or older than the age of the Universe. How could stars form almost immediately after the Big Bang? Is there a way to explain the distribution of mass in the Universe in terms of more fundamental physics? These are some of the challenges of modern cosmology.

The Cosmic Microwave Background Radiation (CMBR), discovered in the early 1960's, is a glow in the sky in the microwave and far-infrared region of the electromagnetic spectrum (wavelengths of about 1 to 10 mm) which has a distribution of energy with wavelength that is typical of a system in thermal equilibrium. It was quickly established that the intensity of the CMBR with position in the sky is extremely uniform, exceeding the sensitivity of the early instruments which probed this uniformity at the 1% and 0.1% levels. Since that time, cosmologists have realized that measuring the spatial variations, or anisotropy, in this radiation would provide important clues to why matter is distributed the way it is today. 

Imagine an extremely hot and dense Universe right at the initial Big Bang. The physical conditions are so far beyond currently understood physics that we cannot really describe the details of physical processes at that time. As the Universe expands and thus cools off (within the first few seconds), the density and temperature decrease sufficiently that we start to enter the regime of high energy physics theory. This framework provides extrapolations based on measurements by the largest particle accelerators. Although the Universe is so hot that most common forms of matter are still in an undifferentiated state that resembles a 'soup' of pure thermal energy, some elementary particles (e.g., protons, electrons) eventually condense out of this soup and become stable. As the cooling continues, atomic nuclei become stable until, eventually, hydrogen atoms can form. This happens when the Universe is approximately 300,000 years old. The critical event for this epoch is that photons (light), the major constituent of the primordial soup, start to behave independently from the matter in the Universe. During earlier times, the free electrons and protons interact strongly with the photons so they are, in a sense, all moving together as a well blended cosmic soup. After the electrons are bound up with the protons to make hydrogen, the Universe becomes transparent to light, allowing the photons to propagate freely. For this reason, we name the event the era of decoupling (of matter from radiation). When we look deeply into the sky, this is the light that we see as the CMBR. 

Because all the constituents (particles and radiation) in the early Universe interact strongly among themselves, they are in a special state called thermal equilibrium for which the amount of energy at each wavelength (spectrum) is predictable. As the Universe cools, the shape of this spectrum changes in a special way which is characteristic of cooler and cooler temperatures. We observe that the photons in the CMBR today have a distribution characteristic of 3 Kelvins, and, knowing that hydrogen atoms are stable at temperatures below about 3000 Kelvins, we can predict that the epoch of decoupling occurred when the Universe was only 3/3000 or 1/1000'th of its current size. 

When we study the distribution of intensity of the CMBR in the sky, we are actually looking at the distribution of matter during the era of decoupling. This is like looking through a transparent Universe back to the time of decoupling, before which the Universe was opaque like a dense fog. By studying the bright and dark spots of the radiation, we infer the pattern on the surface of the fog. In the case of the CMBR, the pattern is the distribution of matter. The regions which are slightly more dense than others gravitationally attract the photons a bit more and, in the expansion of the Universe, ultimately cause them to lose some energy and appear to be at a lower temperature. Those regions which are less dense have less of this effect, and the radiation appears to be at a relatively higher temperature. These small differences in matter distribution are the ripples in space which ultimately develop into the stars and galaxies we observe today.