6.3 Towards an experimental detection of fine scale CMB
spectral distortions
There are currently five ongoing experiments aiming to detect the global signal from the epoch of reionization; however, there is only one experiment in the works to detect the cosmological recombination lines APSERa. APSERa—Array of Precision Spectrometers for the Epoch of RecombinAtion—is a venture of the Raman Research Institute, with a dedicated science goal of detecting the spectral ripples from the epoch of recombination. On completion, APSERa will comprise an array of 128 spectral radiometers with cryogenically cooled receivers. APSERa will operate over the frequency range of 2–4 GHz at a radio quiet site possibly close to the poles. I present here some specifications to guide system design for experiments seeking to detect fine-scale CMB spectral distortions such as those from recombination to reionization.
• Antenna designThe antenna is the first element in a radio telescope. It acts as a sensor
of electromagnetic radiation, coupling sky radiation to receiver electronics. It is of prime importance that the antenna does not severely attenuate or modify the shape of the received signal at this first stage since the sky-signal has not yet been amplified by any electronics. A few essential antenna design criteria are frequency independence and smooth & high reflection efficiency and radiation efficiency. When an antenna is frequency dependent, the antenna beam and thus sky coverage varies as a function of frequency. This results in a ‘leakage’ of spatial structures into the frequency domain potentially introducing spectral shapes that may confuse signal detection. Such a leakage is termed mode-mixing in literature (see, for example, Liu & Tegmark, 2011). It is thus desirable to have an antenna that is frequency independent over the entire band of interest. Antennas that are electrically short can potentially be frequency independent. It is also desirable for the antenna have a smooth return loss or equivalently reflection efficiency. The return loss gives the ratio of the power reflected by the antenna to the input power. The antenna return loss determines the coupling between the sky-power and the receiver. Thus, any spectral shape in the return loss will appear as a multiplicative shape in the sky spectrum. Finally, the fraction of sky power coupled by the antenna to the receiver electronics is measured by the total efficiency which is a combination of the antenna return loss (reflection efficiency) and the radiation efficiency. Losses to the ground and ohmic heating are possible mechanisms that deteriorate radiation efficiency. Since both the cosmological signals of interest are inherently weak, poor antenna efficiency results in increased observing time to reach desired sensitivities.
Some examples of antennas that have been custom designed to detect CMB spectral distor- tions from the epoch of recombination and reionization are shown in Figure 6.1. A discussion on some ultra-wideband antennas custom designed to detect the monopole CMB spectral distortions is presented in Subrahmanyan et al. (2016). While the antennas described in Sub- rahmanyan et al. (2016) have some properties that are favourable to detecting CMB spectral distortions, these designs require further improvements to achieve the desired science goal.
(a) (b)
Fig. 6.1 (a) A fat, profiled dipole antenna operating over 87.5–175 MHz above an absorptive ground plane adopted by SARAS. This antenna was custom designed to be frequency independent over the entire bandwidth of operation, a feature that is essential for detection of weak spectral distortions in the CMB. Figure reproduced from from Patra et al. (2013). (b) A frequency independent fat profiled monopole antenna designed for operation over 2–4 GHz. This antenna has a beam that varies less than 10% over the octave bandwidth and has a smooth return loss, favorable to detection of spectral ripples from the epoch of recombination. However the resistive loss in the antenna and dependence on earth properties below the electrical ground plane call for further improvements in antenna design. Figure reproduced from Raghunathan et al. (2015).
Thus antenna design is a critical challenge that requires special attention and possibly several iterations to arrive at a final configuration.
• Bandpass CalibrationThe ripples from cosmological recombination and the global EoR
signal are to be measured as distortions in the sky spectrum and the importance of a clean bandpass cannot be overemphasised. A clean bandpass is essential to measure weak features in the sky spectrum and to be able to confidently attribute these to the cosmological recombination or reionization signal. It is of utmost importance to design elements and assemble the same in the receiver such that the calibrated bandpass is flat. Several additive and multiplicative effects can contaminate the bandpass by introducing features that confuse signal detection. Of importance among these are reflections in connectors and cables between discrete components in the receiver. Reflections create standing wave patterns in the bandpass that are by definition non-flat. Such standing waves can be avoided/minimized by using extremely short cable lengths. While good bandpass calibration typically ensures that multiplicative terms are removed, additive terms introduced by interference of multi-path reflections present a non-trivial challenge.
A design that serves to reduce some receiver systematics is the correlation spectrometer scheme shown in Figure 6.2 as adopted in SARAS (Patra et al., 2013). Noise injection at
6.4 Final remarks 141 the antenna using a directional coupler provides bandpass calibration. Splitting the signal into two separate receiver chains that are later correlated serves to minimise receiver noise that are uncorrelated between the two chains. A switched noise injection scheme using a crossover switch provides absolute calibration and allows for cancelling internal common- mode additive noise. System design for detecting the signal from the epoch of recombination can be guided by the above scheme adopted in SARAS. However some design principles would have to be modified because of the weak sky signals in the 2–4 GHz band compared to the extremely bright galactic synchrotron emission in the SARAS band of 87.5–175 MHz.
• Receiver noiseOver the frequencies of 40–200 MHz observed by EoR signal detection
experiments the sky noise dominates receiver noise. However, over the recommended frequency range of 2–6 GHz for detecting the cosmological recombination signal, receiver noise is likely to dominate the relatively ‘cold’ sky. This necessitates reducing overall system noise by appropriate choice of components with low individual noise-temperature contributions. Further, since the sky signal is weak, it is important to amplify the signal at the earliest possible stage to avoid any degradation of signal-to-noise ratio by receiver electronics. This can be achieved by using a low-noise amplifier connected directly to the antenna. Cryogenically cooling the LNA can serve to reduce LNA and thus system noise temperature.
6.4 Final remarks
I have discussed the feasibility of detection of spectral distortions in the CMB from cosmolog- ical helium and hydrogen recombination. I have also presented a method of separating smooth foregrounds from the intrinsically non-smooth CMB spectral distortion, using Maximally Smooth functions. I estimate that it is in principle possible to detect the CMB spectral distortion from the epoch of recombination with 95% confidence using an array of 128 radio telescopes with cryogenically cooled receivers in 255 days effective integration time.
I extend the application of MS functions to separate foregrounds from the global EoR signal in
mock sky spectra. As the exact nature of the EoR signal isa prioriunknown and because it is
expected to span over multiple octaves in frequency where Galactic and extragalactic emission dominate, a realistic model of the radio sky is necessary to make any useful deductions. I present such a physically motivated model of the radio sky in GMOSS: Global Model for the Radio Sky Spectrum. With GMOSS as the sky model and using MS functions for foreground separation, I find that the global 21-cm signal from EoR can be detected with 90% confidence in 10 minutes integration time with a single-element, uncooled, ideal radio telescope.
Designing and building radio telescopes that meet specifications required to detect CMB spectral distortions is non-trivial. However, identifying these specifications helps lay the path ahead. In this thesis I have provided some methods and tools that will bring us one step closer to detecting fine-scale spectral distortions in the CMB from recombination to reionization.
Fig. 6.2 A correlation spectrometer scheme adopted in SARAS (Patra et al., 2013). Noise source injection provides bandpass and absolute calibration. A crossover switch alternately connects the antenna and the reference noise source to different ports of a 180 power splitter/combiner. While the correlation spectrometer naturally reduces response to uncor- related noise in the two signal chains downstream, the switching allows for cancellation of common-mode noise. The measurement set of this scheme is a complex spectrum where the sky power is expected to appear only in the real part and the spurious systematics from reflections would appear in both the real and imaginary parts. This property can be used to separate systematics from sky power. Figure reproduced from (Patra et al., 2013); refer to the paper for more details.
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