Introduction to Cosmology
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
medium is in the form of neutral and ionized
hydrogen gas. Emission of photons at the 21 cm wavelength occurs when
hydrogen atoms, which consist of a single proton and a single electron,
a transition at the ground state.
is slightly higher when the spins of the electron
the proton are aligned than when they are opposite. Therefore, if the
has spin in the opposite direction to the proton, it will eventually
spin direction, releasing energy in the process and thus emitting a
frequency 1420 MHz. This frequency corresponds to a wavelength of 21 cm.
neutral hydrogen has extensive applications
in radio astronomy; in particular, the 21-cm emission line can be used
the distribution and density of neutral hydrogen in the Galaxy and to
velocity of hydrogen clouds, which can then be used to track the
of mass in the Galaxy. Thus neutral hydrogen gives information about
the physical shape of the Galaxy (its distribution can be used to
distances to the spiral arms of the Galaxy), but also about the amount
matter in the Galaxy. Notably, calculations of the amount of matter and
the galaxy have led scientists to predict the existence of dark matter
universe. Neutral hydrogen is therefore of interest for various
astronomy such as stellar astronomy, galactic astronomy, and cosmology.
addition to dark matter surveys, applications in cosmology of the 21-cm
hydrogen line include furthering our understanding of the “dark ages”
universe, the era between recombination and reionization, and
theories about dark energy.
Read the Big Bang tutorial in Ukranian!