The UW Observational Cosmology Group

1. The UW Observational Cosmology Group The CMB and the Early Universe The main goal of the Observational Cosmology group at the University of Wisconsin-Madison is to answer fundamental questions about the beginning and evolution of the universe. We specialize in precision measurements of the Cosmic Microwave Background (CMB) radiation. The CMB is the oldest light in the universe. By observing the CMB we hope to learn about the very early universe. We are also developing techniques to observe the redshifted 21-cm line of neutral hydrogen gas in order to study the accelerating expansion of the universe, and dark energy.  Since the earliest time the universe has been expanding and cooling. Around 380,000 years after the Big Bang the photons in the universe had cooled to a point where they could no longer scatter with the surrounding protons and electrons, and could travel freely into space. These photons are what we observe as the CMB today. Courtesy WMAP team The CMB is almost perfectly uniform across the sky with only very small temperature and polarization fluctuations, shown above, courtesy WMAP team. One of the questions the CMB may help us answer concerns the earliest moments of the universe. Inflation theory proposes that the universe underwent a period of accelerated expansion known as cosmic inflation during its first 10-34 seconds. During this cosmic inflation, gravitational waves would have been produced, which would have interacted with the plasma ,and left a specific pattern in the polarization of the CMB. Our lab, and others working in the CMB community, are currently working to develop the technology capable of making this type of observation. The 21-cm Emission Line and the Current Universe The most abundant element in the interstellar medium is neutral hydrogen. Emission of photons at the 21 cm wavelength occurs when these hydrogen atoms, which consist of a single proton and a single electron, undergo a transition at the ground state. In the very early universe, waves (called Bayon Acoustic Oscillations) traveled through the plasma of protons, electrons, and photons, leaving behind an imprint, like ripples. These imprints can be seen in large-scale structure in the universe, such as the clustering of galaxies. BAOs have a characteristic length scale so they make very good ‘standard rulers’ with which we can very accurately measure distances in the universe. Using BAO to measure these distances will help us to learn more about the nature of dark energy. One method of detecting BAOs is by mapping the 21-cm emission line. The 21-cm line results from the hyperfine transition of ground-state neutral hydrogen (HI) and can be used to trace out structure formation and evolution in the universe. From observations of the CMB we have learned that the universe is made up of only 4% atom. Most of the universe is made up of dark energy. This energy, distinct from dark matter, is responsible for the present-day acceleration of the universal expansion, but is still not well understood. Visit us on the web at: cmb.physics.wisc.edu