Introduction to Cosmology

WMAP The field of cosmology attempts to answer very big questions about our universe: What were the earliest moments of the universe like? How did the early universe evolve into the large scale structure we observe today and how will it continue to evolve in the future?

In the universe today, we observe galaxy clusters and superclusters, millions of light-years across, containing massive amounts of matter in the form of galaxies, stars, planets, black holes, neutron stars, brown dwarfs, and other compact bodies. However, in its earliest stages, soon after the Big Bang, the universe was a hot, dense plasma of photons, electrons, and baryons (protons & neutrons). These particles were constantly interacting at tremendously high energies.

The CMB and the Early Universe

Since the earliest time the universe has been expanding and cooling. About 380,000 years after the Big Bang, during an era termed recombination, the primordial photons had cooled enough that they were no longer able to scatter with the surrounding baryons. and could travel freely into space. These photons are what we observe as the Cosmic Microwave Background (CMB) today.

The CMB is almost perfectly uniform afross the sky, but the natter and dark matter distribution at the epoch of recombination "imprinted" it with fain temperature and polarization inhomogeneities. The temperature inhomogeneities can be seen in the map above (courtesy WMAP team). By observing the CMB temperature and polarization pattern we can attempt to answer the big cosmological questions. Indeed, CMB observations have already helped to answer some of these questions.

One of the questions the CMB may help up 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 21cm Emission Line and the Current Universe

About 90% of the interstellar medium is in the form of neutral and ionized hydrogen gas. 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.

The energy of the atom is slightly higher when the spins of the electron and the proton are aligned than when they are opposite. Therefore, if the electron has spin in the opposite direction to the proton, it will eventually change spin direction, releasing energy in the process and thus emitting a photon with frequency 1420 MHz. This frequency corresponds to a wavelength of 21 cm.

Radiation from neutral hydrogen has extensive applications in radio astronomy; in particular, the 21-cm emission line can be used to map the distribution and density of neutral hydrogen in the Galaxy and to find the velocity of hydrogen clouds, which can then be used to track the distribution of mass in the Galaxy. Thus neutral hydrogen gives information about not only the physical shape of the Galaxy (its distribution can be used to estimate distances to the spiral arms of the Galaxy), but also about the amount of matter in the Galaxy. Notably, calculations of the amount of matter and mass in the galaxy have led scientists to predict the existence of dark matter in the universe. Neutral hydrogen is therefore of interest for various subfields of astronomy such as stellar astronomy, galactic astronomy, and cosmology. In addition to dark matter surveys, applications in cosmology of the 21-cm hydrogen line include furthering our understanding of the “dark ages” of the universe, the era between recombination and reionization, and investigation of theories about dark energy.

CMB Overview / Tutorial

Read the Big Bang tutorial in Ukranian!