from "Outside In"
The Big Bang
Over the past century our knowledge of the Universe has increased greatly. Up until around 100 years ago it was thought that the Universe was static and unchanging. Then the discoveries of Edwin Hubble led to the conclusion that the Universe was expanding. However, not everyone agreed with this idea. It took around another 50 years for the idea to become more widely accepted, and it wasn't until the discovery of the Cosmic Microwave Background in the 1960s that the idea of the Big Bang was accepted by the whole community.
Things changed even more during the 1960s and 1970s, with the revelation that galaxies and clusters of galaxies were not behaving as they should. Everything was moving too quickly, indicating that there was more to the galaxies than met the eyes of the astronomers. The concept of "Dark Matter" was born: a mysterious type of matter that is completely transparent to light, but which interacts with the galaxies and stars purely through its gravitational pull. There has been much work over the decades to detect this Dark Matter, and to map out where it is in the Universe. Nowadays, its existence is accepted by the majority of the astronomical community -- but not by all. One current stumbling block is that we don't know what it is. Theories exists as to what it might be, and these may be proven in the coming years and decades by experiments such as the Large Hadron Collider.
All was still not well with the Universe on its largest scales. The discovery and subsequent measurement of the CMB had thrown up several questions. For one, the Universe is incredibly uniform on the largest scales, but it wasn't clear how it had become so uniform. The proposal of a theory of "Inflation" answered many of these problems. Inflation refers to a very brief period of extremely rapid expansion in the first moments of the Universe's existence. While inflation seems to answer a lot of questions, the details of the theory have yet to be answered.
The most recent conundrum in cosmology was the discovery that the Universe seems to be expanding faster and faster. Before the 1990s it was expected that the expansion of the Universe would slow over time, as the gravitational pull of all the galaxies tried to pull everything together again. However, measurements of distant exploding stars, or supernovae, have indicated that the expansion is in fact now accelerating. This result is in agreement with other measurements, such as the Cosmic Microwave Background and the distribution of galaxies throughout the Universe. One possible explanation is something called "Dark Energy", an energy field pervading the Universe which acts to counteract gravity and push the galaxies apart.
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