The Dark Energy Survey (DES) is a ground-based cosmology experiment led by astronomers from the US, Brazil and Europe. It has begun its trip to Chile where it is scheduled to begin observations in November, 2011 using the 4 meter Victor M. Blanco telescope in the Atacama desert. It uses a new highly sensitive camera design called DECam, with resolution totaling 570 Megapixels and employing very large pixels, and it emphasizes sensitivity in the red and infrared portions of the spectrum, in order to measure galaxies out to redshifts of 1 and beyond. Galaxies in the early universe are far away from us and have high redshift values. Light which they would have originally emitted in the blue or yellow portions of the optical spectrum has shifted toward the red or infrared, thus the emphasis on detection of infrared photons for this work.
The DES uses a 4-pronged attack to improve the measurement of the dark energy and other cosmological parameters. These 4 tests are:
- Supernovae – Type 1a supernovae are thought to occur when a white dwarf in a binary stellar system accretes mass from its companion. Once enough mass is accreted, the white dwarf is pushed over the Chandrasekhar limit of 1.4 solar masses, the gravity of the star’s mass overwhelms the pressure support from its ‘degenerate electron’ matter, and the white dwarf undergoes core collapse and becomes a supernova. It is temporarily as bright as an entire galaxy. Such a supernova can be detected at large distances (high redshifts) and very importantly, since the mass of the supernova is always the same, the absolute brightness of this type of supernova is essentially expected to be the same as well. This allows us to use them as standard candles for distance measurement and thus for cosmological tests.
- Baryon acoustic oscillations – This test looks at the statistics of galaxy separations at very large scales. In the early universe, sound waves were established in the hot dense plasma, reflecting pressure generated by the interaction of photons and ordinary matter. Dark matter does not participate except gravitationally. A “sound horizon” is expected with a present size of about 500 million light years, and this acts as a standard ruler as the universe expands. A bump in the correlation function, which measures the probability of one galaxy being near another, is expected at this characteristic distance.
- Galaxy cluster counts – This test of how many galaxy clusters are detectable versus redshift was apparently first proposed by myself in 1980, in the context of X-ray emission from the very hot diffuse gas found between galaxies in galaxy clusters. This approach offers certain advantages in comparison to simple galaxy counts versus redshift. In this case it will be performed in the infrared and red, observing the galaxies themselves. Galaxy clusters contain up to 1000 or more galaxies within a single cluster. The number of clusters that can be seen at a given redshift is dependent on the cosmological model and the mass of the cluster, since dark matter promotes cluster formation through gravitational attraction. Dark energy inhibits cluster formation, so this helps to measure the relative strength of dark energy at earlier times. The team expects to detect over 100,000 galaxy clusters, out to redshifts of 1.5.
- Weak lensing – This refers to gravitational lensing. This occurs when a source galaxy is behind an intervening galaxy cluster and the gravity of the cluster bends the light from the source galaxy in accordance with general relativity. By surveying a very large number of galaxies, a strong statistical measure of this bending, also known as cosmic shear, can be taken. The amount of shear will be measured as a function of redshift (distance). This shear is sensitive to both the shape of the universe and the way in which structure develops over time.