mountain profile Institute for Astronomy University of Hawaii

Cosmology

Maintained by LG

Cosmology is the study of the Universe as a whole. Clues to the history of the Universe and to what it is made up can be obtained by

Here are some of the cosmology projects underway at the Institute for Astronomy

 

 

The COSMOS Deep Field

David Sanders and Guenther Hasinger are the principal investigators for the far-infrared (Spitzer) and X-Ray (XMM-Newton) surveys, respectively, of the Hubble Space Telescope Cosmic Evolution Survey, which is the largest contiguous deep-field survey ever carried out with the Advanced Camera for Surveys (ACS) on HST .The far-infrared and  X-ray data have been used to determine the evolution of the luminosity functions of luminous infrared galaxies and powerful AGN, respectively, over a large range of look-back times, corresponding to redshifts, z ~ 0–5. IfA researchers are carrying out multiwavellength observations with telescopes on Mauna Kea, in combination with the vast amount of data currently available within the international COSMOS collabo­ration, to better understand the origin and evolution of powerful infrared galaxies and X-rayselected AGN, and the overall role these ex­otic objects have played in the evolution of all galaxies throughout cosmic time.

Optical image of a small part of the Cosmos field obtained using the Subaru telescope on Mauna Kea.

 

 

 

Type Ia supernovae

Distant supernovae are brighter than predicted, implying that the universe
expanded more slowly in the past.

John Tonry continues a long-standing collaboration to exploit the "standard candle" property of Type Ia supernovae to probe cosmology. Comparing distance with redshift we have discovered that the Universe is accelerating under the influence of "Dark Energy". We continue to use facilities such as Pan-STARRS and telescopes on Mauna Kea to refine our understanding of the properties of Dark Energy, for example whether it has varied over cosmic time.

Tonry is embarking on a new use of SNIa, this time to determine the properties of "Dark Matter". A new project called ATLAS will find 10 new SNIa every day to redshifts of 0.1, and it is possible to infer the distribution of Dark Matter from its gravitational effect on these SNIa. SNIa can offer the cleanest, most bias-free measurement of this type, but they are so rare that it will require some years for even a massive discovery machine such as ATLAS to find enough of them.

 

 

Lyman-alpha Escape Fraction

The ultraviolet Lyman-alpha spectrum line is the strongest hydrogen emission line, and it is therefore widely used to find high-redshift galaxies. For the highest redshift z > 6 galaxies, this line is the only spectroscopic signature that can be used to confirm the redshift of a galaxy selected on the basis of its color properties. However, Lman-alpha is a difficult line to interpret because its photons are scattered by neutral hydrogen.

To understand how the line escapes from galaxies, Len Cowie and his colleagues have been observing nearby examples of Lyman-alpha emitters using HST and GALEX observations. These samples can be used to study how the escape depends on the mass, metallicity, age, and size of the galaxy. We have recently shown that Lyman-alpha emitters with luminosities comparable to those of the highest-redshift objects are first seen at an age corresponding to a redshift of z = 1.

 

The Chandra Deep Field South

In 2010 the Chandra X-ray Observatory obtained the deepest image yet of the X-ray sky: a 160 hour image of the Chandra Deep Field South. In such an ultradeep image, a strong active galactic nucleus can be detected out to a redshift near five. It is also possible to use the image to stack objects to explore the average X-ray properties of galaxies to much fainter levels.

Len Cowie, Guenther Hasinger, and their colleagues used this technique to examine the average X-ray properties of galaxies out to redshifts near eight. Below a redshift of five, they were able to determine the average X-ray properties of the galaxies and found them to be consistent with expectations for the contributions from high-mass X-ray binaries in the galaxies. At higher redshifts, there is no longer a significant signal from any of the known galaxy populations.

 

Reionization

The mean free path of ionizing photons at high redshift is a crucial input for determining the ionizing history of the intergalactic medium and thus of the formation and evolution of the sources presumed to ionize it, widely assumed to be galaxies at z > 3.

Antoinette Songaila Cowie and Len Cowie have used observations of high-redshift quasars with the Keck telescope to determine how the mean free path of ionizing photons varies with wavelength.

 

The figure on the right shows that there is a smooth connection between mean free path and redshift out to z ~ 6, but it probably wise not to extrapolate the mfp beyond the measured range.

 

The Integrated Sachs-Wolfe Effect

The cosmic microwave background is a snapshot of the early Universe; however, the light we observe has been processed by large-scale structure at low redshift, in part through the late-time integrated Sachs-Wolfe (ISW) effect.

As photons travel through time-varying gravitational potentials, they are slightly heated or cooled. In a Universe dominated by dark energy, the gravitational potential decays with time even in linear theory, heating photons traveling through crests and cooling photons in troughs of large-scale matter density fluctuations.

Istvan Szapudi and his colleagues measured hot and cold spots in the cosmic microwave background associated with supercluster and supervoid structure and found a mean temperature deviation is 9.6±2.2 μK. They interpret this as a detection of the late-time ISW effect, in which cosmic acceleration from dark energy causes gravitational potentials to decay, heating or cooling photons passing through density crests or troughs. In a flat Universe, the linear ISW effect is a direct signal of dark energy.