Type Ia Supernovae and the Cosmological Constant

Type Ia Supernovae
     Stars explode as supernovae in basically two ways: When a supermassive star runs out of nuclear fuel in its core, it collapses, rebounds and explodes. The result is a type II supernova. Many stars appear in binary systems. If a white dwarf (a compact dense star) draws in material from its companion, the extra weight can cause the star to collapse. The nuclear chemistry of the star is upset and it explodes. This is a type I supernova. A type Ia supernova is distinguished from other type I supernovae by certain features in its spectrum.
     Since supernovae are very bright, often as bright as an entire galaxy of stars, very faraway supernovae can be seen from telescopes on Earth. It turns out that, from the shape of the light curve, one can determine the intrinsic brightness of a type Ia supernova. By comparing the intrinsic brightness to the apparent (or Earth-based observed) brightness, the distance to the supernova can be established. In short, type Ia supernovae are standard candles that can be used as distance measurers.
     The speed of a type Ia supernovae is also easily deduced from its spectrum. The speed and distance measurements are then used to measure the expansion rate of the Universe. From observations in the late 1990's of distant type Ia supernovae, astronomers concluded that the expansion rate of the Universe was increases – the Universe was accelerating!

The Cosmological Constant Problem
      The cosmological constant Λ, which is not usually part of the Friedmann-Robertson-Walker model but can be included, produces a rather unusual effect on cosmology. It yields a "negative pressure" causing a gravitational repulsion that drives matter apart at increasing rates. The data from type Ia supernova observations suggest the existence of a cosmological constant.
     Theorists have been reluctant to introduce a cosmological constant in the theory because of a fine-tuning problem. It turns out that if Λ is non-zero then it should have a natural value that would cause a very rapid expansion and very dramatic cooling of the Universe. The current measured temperature of the Universe is 2.725 degrees Kelvin. Although extremely cold by human standards (just a few degrees above absolute zero), if Λ is present, it must be about 10122 times smaller than the natural value to agree with this temperature measurement. How could Λ be set to such a small value with such precision? This is known as the cosmological constant problem. Its solution had been to assume that Λ was exactly zero. Type Ia supernova measurements thus created a fine-tuning problem for cosmology.

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