Sunday, 11 March 2012

Cosmology and the Big Bang

Cosmology and the Big Bang
Cosmology is the study of the universe as a whole -- what happened in its past and what will happen in the future. Cosmology is a fast-moving subject at the moment and often in the news. It has many concepts which are hard to grasp; as such, many of the questions that are sent in to us are about cosmology.
The Big Bang
One of the best know theories in cosmology is the Big Bang. This is the idea that our universe started out much hotter and denser than it is now and has been expanding since then. This theory is based on observations of our universe, among which are:
External galaxies are receding in such a way that their recessional speeds are proportional to the distance they are away from us (this is called Hubble's Law after Edwin Hubble who first noticed it). This observation is explained well by a uniform expansion of the universe. If the universe is expanding, it must have started out very small some time far in the past. It is this point which has been called the beginning of the universe or the "Big Bang."
When we observe the night sky we see an excess of radiation which is called the CMB radiation (cosmic microwave background radiation). It is a perfect black body with a temperature of 3 Kelvin. Taken with the expansion of the universe, this radiation says that the universe must have been much hotter in the past and also opaque to radiation. It turns out that the CMB radiation fits in perfectly with being from the first photons to escape after the universe became transparent. The universe became transparent for the first time when atoms first formed (in an event known inexplicably as recombination).
Large scale structure of the universe

Galaxies are not randomly scattered across the universe, but rather they are preferentially found in groups or clusters, and the groups and clusters are also seen to be arranged in superclusters. Our own galaxy, the Milky Way, is known to be a member of the group of galaxies that we call the Local Group. The group has two large galaxies (the Milky Way and Andromeda) as well as a host of dwarf galaxies. Clusters are just much larger versions of groups. A good analogy is that the groups are like towns, while clusters are like cities. In this analogy the superclusters are like the state (or country for those of us not from the US) that the towns and cities are in.
Superclusters are not relaxed systems - that means that they are still expanding with the universe, while groups and clusters are (in general) believed to be held together by the mutual gravity of the galaxies in them and so are no longer expanding with the universe. Superclusters form a filamentary network in space, sometimes referred to as a "cosmic web".
By studying this large scale structure in the universe astronomers can learn a lot about cosmology. The superclusters are probably the remnants of the first structure formation in our universe (which you can see above in the image of the CMB) and the way structure first formed depends a lot on the cosmology of our universe.
Cosmology in the 21st Century

As the last century came to a close and the first few years of this century passed us by, cosmologists have been beginning to feel like they are getting close to knowing what the basic parameters of the universe are. A recent talk by a cosmologist at Cornell started with the question "Is cosmology complete?" The answer, by the toss of a coin was no! The talk then went on to explain why in some sense the answer is no, and in others it is yes.
Recent observations (in particular from WMAP) have solved several long-standing problems in cosmology. Evidence strongly suggests that the Hubble Constant is around 71 km/s/Mpc (accurate to about 5 percent) and that the universe is geometrically flat, but that it is probably dominated by some weird form of energy called "dark energy." "Ordinary" matter also seems to be dominated by "dark matter," which cannot be the same as the matter (called baryonic matter) that makes up humans. Overall, it's a strange universe, but one that we are beginning to understand in greater detail than ever before!

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