CLASS is uniquely built to measure the large-angle polarization patterns in the Cosmic Microwave Background (CMB) from Inflation. A key technology that enables this measurement is called “modulation”.
CLASS modulates the polarization signal of the CMB. This modulation is analogous to AM radio. In fact AM stands for amplitude modulation, in which a radio station’s audio signal is modulated at a specific radio carrier frequency to isolate it from noise and the signals of other stations. Analogously, CLASS uses high frequency amplitude modulation to isolate the CMB polarization signal from the lower frequency atmospheric and instrumental noise.
To illustrate how CLASS uses modulation, we have created the following audio-frequency example. In this example the signal is represented by a voice saying “The Cosmic Microwave Background Polarization”. When unmodulated, the signal is difficult isolate from low-frequency noise. With modulation, the signal is encoded at a distinct (and comically higher) frequency. The ear can then distinguish the high-frequency modulated signal from the low-frequency noise.
The Fully Outfitted CLASS Observing Site
The CLASS team, including students, postdocs, and professors, have installed generator power, a telescope control room, and the first telescope mount at the CLASS observing site in the Atacama Desert. The fully outfitted site is shown in Figure 1. The site is ready to accept the CLASS telescopes.
Figure 1: The fully outfitted CLASS observing site in the Atacama Desert at 5200 m elevation. A diesel generator set is in the foreground and the control room and other buildings are within the fenced area. We use the large yellow crane to assemble the telescopes. The tall structure to the right of the crane is the first telescope mount, ready to accept the CLASS telescopes.
Big Bang Theory and Inflation
In the popular imagination, the Big Bang Theory describes an event at the beginning of the universe (sometimes literally imagined as the “bang” of an explosion) from which the universe is born. In physics, the Big Bang Theory describes not the beginning of the universe (much less a “bang”) but rather the evolution of the universe. During the first half of the twentieth century Edwin Hubble and other scientists demonstrated that the universe is expanding. In its basic essence, the Big Bang Theory is the theory of the expanding universe.
But what do we mean by “the expanding universe”? According to the Big Bang Theory, a given volume in the universe is literally growing in size. The distances between objects in the universe that are not bound to each other grow with time. Correspondingly, the matter (e.g., atoms), light and other “stuff” inside that volume are becoming less dense with time. Therefore, the density of universe was higher in the past.
The heat in the universe also grows more diffuse with time causing an ever decreasing temperature. Long ago the universe was very hot — so hot that it glowed with high energy radiation. After much expansion this glow is measured today as microwaves called the Cosmic Microwave Background (CMB) and its existence was a prediction of the Big Bang Theory. Thus the announcement of the discovery of the CMB in 1965 was a pivotal triumph of the Big Bang Theory. The figure below illustrates the history of the expanding universe. The Wilkinson Microwave Anisotropy Probe (WMAP) satellite measured the age of the universe to be 13.77 billion years. The CMB originates just 375,000 years after the beginning.
But what happened at “the beginning”? We don’t know, but an idea called “Inflation” may answer this question. The Big Bang Theory successfully describes the evolution and cooling of the universe over billions of years. Within the Big Bang evolution framework tiny initial “primordial” density fluctuations grew under gravity to become the structure of the universe (e.g., galaxies and their stars) we see today. But the Big Bang Theory does not explain the origin of these primordial fluctuations — it does not explain the beginning. Inflation theory completes the picture by describing how the initial state of the universe was established by a rapid expansion at the very beginning stretching random submicroscopic quantum fluctuations into primordial density fluctuations — the seeds of cosmic structure. To quote physicist Brian Greene, “Galaxies are nothing but quantum mechanics writ large across the sky.”
The Big Bang Theory describes the expansion history of the universe, here illustrated, with a timeline from left to right, by an expanding volume filled with matter and other forms of energy. At the far left, inflation theory describes a rapid expansion of space at the beginning of the universe during which microscopic quantum fluctuations in density and the fabric of space-time grew to macroscopic cosmological size. From these primordial cosmological fluctuations (at the left of the figure) grew galaxies and other cosmic structures (at the right). After Inflation slows, the Big Bang Theory expansion takes over. (Image from the WMAP website)
The CLASS Path to Chile
The CLASS telescopes are assembled and tested at Johns Hopkins University in the Department of Physics and Astronomy high bay (Step 1 in figure). After a telescope passes all the tests, it is carefully dismantled and packed into several shipping containers, which are loaded on trucks and driven to the port of Baltimore (Step 2 in figure). The containers then travel by sea through the Panama Canal to the port of Iquique in northern Chile (Step 3 in figure). Finally trucks take the containers to the site (Step 4 in figure). The total trip takes 5-6 weeks.
How CLASS Measures the CMB Over 70% of the Sky
From the site in northern Chile, the CLASS telescopes measure polarization over 70% of the sky. Astronomers use the concept of the celestial sphere to locate stars, galaxies, and areas of the CMB by means of longitude and latitude in the same way that cities, lakes and mountains are located on the spherical surface of the Earth. Figure 1 shows in orange the section of the celestial sphere over which CLASS measures the CMB.
Figure 1: By observing from Chile, the CLASS telescope can capture 70% of the sky. This figure first appeared in the Summer 2014 issue of Arts & Sciences Magazine.
At any given time, only the part of the celestial sphere that is above the Earth’s horizon is visible to CLASS. As the Earth rotates, other parts of the celestial sphere rise over the horizon and become accessible to CLASS. In practice, a CLASS telescope points at a fixed elevation above the horizon and rotate about the zenith axis (i.e., in azimuth). In this way the telescope’s field of view sweeps out large circles on the celestial sphere. The CLASS Survey Movie shows an animation of how, over a 24-hour period, CLASS is able to measure the CMB over 70% of the celestial sphere. Every direction of the sky that is observed by CLASS is seen many times a day by different detectors and then again every day. CLASS thus provides tremendous statistical power to distinguish the CMB from other spurious sources of signals.
CLASS Survey Movie: Located at the center of the animation, a CLASS telescope (not shown) points with its hexagonal field of view at the celestial sphere. The telescope scans the sky in large circles, which, due to the rotation of the earth, map out the full section (70%) of the celestial sphere targeted by CLASS.
CLASS 38 GHz Focal Plane Assembly Timelapse
CLASS maps 70% of the sky at a frequency of 38 GHz, corresponding to a wavelength of 7.9 mm, in the microwave region of the electromagnetic spectrum to provide the most sensitive map of the sky ever made at these frequencies. To achieve this, the CLASS team has developed novel technologies like wide-bandwidth, smooth-walled copper feedhorns (patent pending) and the first transition-edge-sensor (TES) bolometers at such a low frequency. Low-temperature bolometers operating at a temperature just 0.1 degree Celsius above absolute zero, where atoms stop moving altogether, are the most sensitive microwave detectors available.
The video above shows the CLASS 38 GHz focal plane as it was built in stages. The video shows the 36 individual detector chips, microfabricated on silicon wafers at NASA’s Goddard Space Flight Center, as they were assembled into the focal plane, along with associated readout electronics around the edges. Each detector chip and its associate feedhorn form a pixel for the 38 GHz camera and measure the polarization of the cosmic microwave background.
The CLASS Multifrequency Survey
On August 4, 2015 we released a paper entitled “Measuring the Largest Angular Scale CMB B-mode Polarization with Galactic Foregrounds on a Cut Sky”. The paper is a theoretical study involving computer simulations to predict how well CLASS can detect the signal in the Cosmic Microwave Background (CMB) caused by primordial gravitational waves from Inflation. One of the main challenges of any measurement of the CMB is that our own galaxy is also “in the picture”, and we need to distinguish the true primordial CMB signal from the false signal from our Galaxy. We do this by imaging the sky at different microwave frequencies (or, equivalently, at different wavelengths).
A simple analogy
Telling the difference between the primordial CMB signal and the Galaxy using an image of the sky at only one frequency is analogous to distinguishing lemons, limes and oranges in a black and white picture of citrus fruit (Figure 1). It is much easier to distinguish between the fruit if one has a color image: limes are green, lemons are yellow and oranges are, well, orange. To make a color image of the fruit you need measurements at multiple optical wavelengths (red, green, and blue, say) that combine to show colors.
Figure 1: Multiple colors (corresponding to multiple frequencies of light) are needed to distinguish between different objects, such as these fruit, in an image.
The same is true for the CMB. Though you can’t see the CMB with your eyes, it is still light and obeys the same physical laws as visible light. (More generally, visible light and microwaves are electromagnetic radiation.) Microwaves from dust in our galaxy are brightest at short wavelengths (high frequencies) while microwaves from fast-moving electrons (called synchrotron radiation) are brightest at long wavelengths (low frequencies). We need images of the sky at multiple microwave frequencies to tell apart the dust and synchrotron emission from the CMB.
The CLASS Multifrequency Measurements
CLASS will make images of the sky at four microwave frequencies: 40 GHz (7 mm wavelength), 90 GHz (3 mm wavelength), 150 GHz (2 mm wavelength), and 220 GHz (1.4 mm wavelength). In the study described in our paper, we found that images at these four different frequencies, when combined, should allow us to distinguish the CMB signal from the Galactic synchrotron and dust emission (Figure 2).
Figure 2: The CLASS multifrequency measurements allow us to distinguish the primordial CMB signal from microwave-bright matter in our own galaxy, the Milky Way. This figure first appeared in the Summer 2014 issue of Arts & Sciences Magazine.
The CLASS High and Dry Site
CLASS will make observations from a site located high in the Andes of Northern Chile at an elevation of 5,200 m (17,000 ft). The latitude and longitude of the site are 22°57′35″S and 67°47′14″W. It is in the Atacama Desert, one of the driest places on Earth. Locating CLASS at a high and dry site is important for limiting the negative impacts of microwave emission from water and oxygen molecules in the atmosphere.
The CLASS site is 40 km from the nearest town, San Pedro de Atacama. Therefore the site is fully self-sufficient, with its own diesel generators, conduit for power and data, a control room, a laboratory, and a machine shop.
In 2014 geotechnical and civil engineering studies were carried out to design the telescope foundations. Pits were dug for the foundations and concrete poured on to a reinforcing mesh of iron bars. (See photos.) An archaeologist oversaw the site construction to ensure that no items of paleontological or cultural value were affected.
While scientists and engineers work at the high and dry site during the day for maintenance, the team sleeps in the town of San Pedro de Atacama (elevation 2,400 m or 8,000 ft). Data is transmitted from the telescope to our office in San Pedro by a 40 km radio link. This link is also used for sending commands to the telescope.
CLASS is located in the Atacama Desert of northern Chile, near the borders of Argentina and Bolivia.
Ground is broken for the CLASS telescope foundations (February 2015) – click to enlarge.
The finished CLASS telescope foundations. The iron ring of bolts in the foundation secures the telescope base (June 2015).