Information about these topics is encoded in the Cosmic Microwave Background, thermal radiation from the hot plasma which filled the whole Universe when it was young. Measuring and understanding this radiation will allow us to address these key cosmological questions, and many other astrophysical issues as well.
What is the Cosmic Microwave Background?
The early Universe was very hot. For the first several hundred thousand years the Universe was hotter than the surface of the Sun. At those temperatures, electrons do not remain bound to nucleii in neutral atoms. The ionized plasma that they form is opaque, as is the interior of the Sun, and that plasma, like the Sun, was filled with trapped hot radiation. Roughly 300,000 years after the Big Bang the expanding Universe had cooled enough to form neutral hydrogen gas, which is transparent. The radiation which was present at that time has travelled freely through the Universe since then, and we see it today as the Cosmic Microwave Background. By observing this radiation today, we have a view of conditions in the very early Universe - because of the time it takes light to travel large distances, we see astrophysical objects as they were some time ago. For example, we see the Sun as it was eight minutes ago. Most of the Universe is transparent now, and in most directions we look through the recent Universe and see the primordial plasma, roughly 13 billion light years away, as it was almost 13 billion years ago, just after the time of the Big Bang.
Planck will determine what sort of Universe we live in
There are precise and testable models of cosmology, consistent with General Relativity and with an expanding Universe, which predict how the Cosmic Microwave Background should look. In these models the shapes of large angular temperature variations depend on the precise initial conditions of the Big Bang. These itintial conditions laid down the seed fluctuations which grew to form all the objects we see today. Smaller angular scale variations just under one degree depend, among other things, on the expansion rate (Hubble's constant), the overall density, the density of baryons ("ordinary matter"), the cosmological constant and the characteristics of dark matter (if present). If these models are correct we can infer the values of these quantities with a precision of a few percent from the maps of the Cosmic Background which Planck will produce. This will be a very major step forward in understanding the Universe. If these models are not confirmed the results will be even more exciting!
Planck will make all sky maps at nine wavelengths
In addition, Planck will detect the characteristic microwave signatures produced by hot gas in clusters of galaxies, allowing detailed studies of how and when galaxies clustered together. Planck will be sensitive enough to detect all of the clusters in the Universe which are hot enough to produce detectable X-ray flux. In addition an all-sky survey of bright galaxies at long wavelengths will be produced. Planck will also provide an incredible bank of data for investigating "foreground" material in our own galaxy, through the production of all-sky images of unparalleled accuracy and sensitivity at nine frequencies from 30 to 875 GHz. On top of that polarization of the sky will also be measured, providing additional cosmological information, and aiding in the understanding of galactic signals.
Planck is the most ambitious CMB experiment currently conceived
Planck consists of a passively cooled 1.5m gregorian telescope with exceptional rejection of stray light and diffracted contributions, coupled to actively cooled HEMT and bolometer based radiometers. In order to perform such delicate measurements, the mission must be run much more like an experiment than like a classical observatory. The telescope will spin at a constant rate, observing the brightness distribution on a large circle simultaneously in all channels. This circle traces out the whole sky each six months as the earth orbits the Sun. Stable operating conditions are crucial, and no dedicated pointing at specific objects in the sky is contemplated. A huge data analysis effort is required to turn this stream of data into useful maps of the sky with gain and pointing variations and foreground sources removed. Scientists participate in this mission by helping to design and build the instrument and by involvement in the overall analysis tasks, rather than via the conventional avenue of applying for observing time.
COBE 4-year All-Sky Map