What do we hope to accomplish? The goals of the ACT project are to study how the universe began, what it is made of, and how it evolved to its current state. This pursuit is part of the field of scientific cosmology in one which asks questions about the Universe on the largest and grandest scales. Over the past two decades, there has been a tremendous flourishing of the field, driven by many excellent astronomical measurements. This has led to the development of a precise and elegant understanding of cosmology.
Perhaps the most amazing development is that we have now a scientific model of the universe that is based on measurement and well-tested physics. The model agrees with virtually all observations of the cosmos.
ACT experiment has two main goals to further our knowledge of cosmology:
- To improve on the measurements of parameters which describe the very early universe
- To measure distant, large clusters of galaxies and their environments.
- The first goal is important because the conditions in the very early universe determine how it evolves later on. It also helps us to accept or reject models of how the universe behaved in its infancy. For example, from observations to date, the "Inflationary" model of the Big Bang gives an excellent explanation of the early universe. But other theories, such as ones where the universe goes through many cycles of expansion and contraction, are still possible. We can narrow the possibilities by looking at what happened very early on.
- The second goal, that of looking at distant galaxies, will help us to understand how structure (a cosmologist's catch-phrase for any grouping of matter in the universe — stars, gas clouds, planets — but usually galaxies or groups of galaxies) evolved. We will be able to observe very distant galaxies. Since light travels at a finite speed, looking at far-away galaxies means looking at old galaxies. In this way, we will be observing the history of the growth of galaxies.
How will the ACT fulfill these science goals? The short answer is by observing the cosmic microwave background — usually abbreviated as the CMB — and by comparing the ACT measurements with those done at other wavelengths.
What is the CMB? Let's look at each word in turn. Cosmic simply indicates that it comes from outer space, from the cosmos. Microwave refers to the kind of light it is. Of all the wavelengths of light that exist, the human eye can only detect a narrow band from about 4 to 7 ten-thousandths of a millimeter in wavelength. Microwave light has a much longer wavelength. Our telescope will see light which is about a millimeter in wavelength. (For reference, a microwave oven uses light which is several centimeters long. At these wavelengths the water molecules in food are easily excited and heat up.) Finally, background means that the light comes from all parts of the sky and is not produced by any source in the "foreground" (by stars, for example).
When the CMB was discovered in 1965 by Arno Penzias (link is external) and Robert Wilson (link is external) (for which they shared the Nobel Prize), it was soon realized that it must be light emitted by hot gases soon after the Big Bang. Initially, the light would have had a much shorter wavelength, but as the universe expanded, it stretched out and became microwave light.
Since the CMB was released so early in the Universe's history, it contains information about the early universe (our first science goal). But it has also travelled a long way since then and has had time to interact with galaxy clusters along the way. Such interactions leave a mark on the CMB which we can use to see those galaxies (our second science goal).