Experimental Cosmology Group Oxford Astrophysics

OverviewCLOVER is a UK-led experiment to detect the B-mode polarisation of the Cosmic Microwave Background (CMB). Sited at Llano de Chajnantor in the Atacama Desert, Chile, CLOVER will comprise three telescopes measuring three frequencies: 97, 150 and 225 GHz. The aim is to detect the B-mode polarisation with sufficient accuracy to probe primordial gravitational waves from the very start of the universe. This will in turn constrain models of the early universe, since gravitational waves are a prediction of inflation. CLOVER is a collaboration between Oxford, Cardiff, Cambridge and Manchester Universities

Chris North - Astrophysics, Oxford University

Above Left: The angular power spectra of the Cosmic Microwave Background, showing the Temperature (TT), primordial E-mode (EE) and B-mode (BB) polarisation and the B-modes from weak lensing. Above right: The mount, optical assembly and cryostat of the 97 GHz instrument.

Polarisation of the CMBLinear polarisation is an inherent consequence of Thomson scattering in a quadrupolar radiation field. Since the usual Stokes parameters, Q and U, are coordinate-specific, polarisation fields are decomposed into grad-like and curl-like components, which are commonly referred to as “E-mode” and “B-mode”. E-modes are caused primarily by Thomson scattering at reionisation. B-modes can be caused by primordial gravitational waves, but are also produced by weak lensing of E-modes. Since the primordial B-mode signal is at least an order of magnitude lower, this “Lensing B-mode” is dominant on small angular scales (high multipoles).

The InstrumentsThe focal plane of the optical assembly of each telescope contains a close packed array of horns behind a 30cm diameter window. There will be 256 horns at 150 and 225 GHz and 160 at 97 GHz. Each horn is connected to an Orthomode Transducer (OMT) which splits the polarisations and outputs them to separate detectors.

The detectors are Transition Edge Sensors (TES). These are superconducting chips which are heated by the incident photons. The TESs are biased to sit in the middle of their superconducting transition, so their resistance is a very strong power of temperature (see below). The resistance is measured using very precise current detectors called SQUIDs.

To aid in the measurement and control of systematic errors, polarisation modulation will be used. Each telescope will be able to rotate around its optical axis, which will rotate the polarisation coordinate system. Other methods of modulating the polarisation involve

ForegroundsOne of the major challenges to measuring the CMB polarisation are polarised foregrounds. The most dominant of these are galactic diffuse emission and point sources (which are mainly extragalactic). The galactic diffuse emission consists of both synchrotron and thermal dust emission. The spectral indices are negative and positive respectively, so the Synchrotron is dominant at lower frequencies and Dust at higher frequencies.

Even in the quietest parts of the sky (i.e. out of the Galactic plane) the levels of this emission are on the micro-K scale, and so dwarf the required, sub-μK, science signal. As with the temperature anisotropies, removal techniques can be used to extract the CMB. Even with removal, only the areas of the sky with the lowest foreground contamination can be used. This factor has governed the observing strategy of CLOVER (see below).

Unpolarised, Quadrupolar Incident Radiation Field

Thomson Scattering off electron

Linearly polarised Scattered Radiation

The SiteCLOVER will be sited at Llano de Chajnantor, which is a plateau in the Atacama Dessert high in the Chilean Andes. At 5080 m above sea level, the site is high and dry. Other existing or planned telescopes in the area are APEX (Atacama Pathfinder EXperiment), CBI (Cosmic Background Imager) and ALMA (Atacama Large Millimetre Array). The area is far from dull, with frequent small Earthquakes, nearby active Volcanoes and other seismic activity. The altitude means that the atmospheric pressure is about half that at sea level, so portable oxygen packs are needed when working in areas which are not artificially oxygenated.

The site’s latitude of 23°S means that around two thirds of the sky can be seen, and since the galactic plane runs nearly North-South there is less foreground contamination than at other latitudes. The problems posed by having to observe around the Sun are made up for by the availability of planets as calibrators.

Top left: Polarisation of light through Thomson scattering. Top Right: radiation pattern from scalar fluctuations (credit: Wayne Hu). Above right: Radiation pattern from tensor fluctuations (credit: Wayne Hu). Above left: Correlation of E-mode and B-mode polarisation direction and amplitude (credit: Wayne Hu). Left: Predicted error bars for CLOVER on the E-mode and B-mode signals.

Left: Schematic of the detector chip, showing the finline transition from microstrip to empty waveguide on the left and the square TES island on the right. Right: Measured R—T curve for a TES.

Modulation and CalibrationBecause the required signal is so small, CLOVER needs very good control and measurement of systematic errors and noise. As well as the galactic foregrounds, the atmosphere will be a problem, but since it is unpolarised this can be controlled by careful design of the observation and modulation strategies. Nodding the telescope in elevation would modulate the atmospheric signal in at a given frequency, allowing the determination of its effect.

Instrumental errors, such as cross-polarisation, can be measured and controlled by polarisation modulation and calibration. Simply rotating the telescope would modulate them in a known way.

If polarisation modulation is included in the detector assembly, then differencing the two detector outputs would remove any unpolarised signals such as the atmosphere.

Above: The galactic foregrounds at 97 GHz. The solid black circles are the selected CLOVER fields, the black dotted line is the Ecliptic plane, and the black-white dashed line is the latitude of the Llano de Chajnantor site.

inserting a phase delay into one of the two polarisations. This can be achieved by putting a birefringent half wave plate in front of the focal plane array, or by applying the delay to one of the polarisations after the OMT, using rotating waveguides, Faraday rotators or planar circuitry.

The horns, OMTs, polarisation modulators and the detectors all sit in a cryostat, with temperature stages ranging from 4 K for the horns to 70 mK for the detectors.

Calibration of the telescope will be through both astrophysical and artificial means. Signals from known, non-variable point sources, such as extragalactic objects and the Planets, will allow total gain calibration. While there is currently little information on the population of extragalactic sources at 97 GHz, observations in the near-future will investigate this. A well characterised polarised artificial source will allow the polarisation response of the telescope to be measured at different angles.

Left: The Llano de Chajnantor site, showing the dome of CBI in the centre. Right: Lascar – the most active volcano in the Andes – erupting just miles from the site.

Left: The atmospheric transmission over the CLOVER frequencies. The CLOVER bands are shown as shaded boxes.