Scientists at the Raman Research Institute, Bangalore, have come up with a novel antenna design which can perform sky measurements 2.5 - 4 Gigahertz (GHz), a frequency range with the best possibility of detecting the faint Cosmological Recombination Radiation (CRR) signals. These elusive and undetected signals hold vital clues capable of improving our understanding of the thermal and ionization history of the Universe. 

Our universe is about 13.8 billion years old. Soon after the Big Bang, the infant Universe was an extremely hot and dense place. So hot, that matter could not exist as atoms. It instead broke down into electrons, protons, and other light nuclei (Helium, Lithium). Also, co-existing with matter in the early Universe is radiation. Today we observe this radiation as the Cosmic Microwave Background (CMB). This CMB is capable of retaining crucial information about the cosmological and intervening astrophysical processes by means of distortions in its spectral shape.

One such distortion comes from the process of the formation of the first atoms in the early Universe over the Epoch of Recombination. This period is characterised by the expansion and gradual cooling of the Universe which resulted in ordinary matter (Baryonic matter) to enter into a transition phase -- from a fully ionized primordial plasma into mostly neutral atomic hydrogen and helium atoms. This process is accompanied by the emission of photons or radiation, which is termed as Cosmological Recombination Radiation (CRR). This forms an additive distortion to the underlying CMB spectrum.

The detection of the never-before-detected CRR, which is nine orders of magnitude (1 part in a billion) fainter than the CMB which measures about 3-degree Kelvin (-270 degree Celsius, the temperature of deep space), will be an important confirmation of our understanding of the thermal and ionization history of the Universe. A detailed measurement of the CRR will provide the only way to experimentally measure the abundance of helium in the Universe before more helium starts forming in the cores of the stars.

Due to the weak and elusive nature of CRR, the challenge before the scientific community is to design highly sensitive instruments that can aid in their detection.

As a first step towards detection of such a signal, a group of researchers from Bengaluru have designed a unique ground-based broadband antenna capable of detecting signals as faint as one part in 10,000.

Researchers Mayuri Rao and Keerthipriya Sathish from Raman Research Institute (RRI), an autonomous institute of the Department of Science and Technology (DST), Government of India, and their collaborator Debdeep Sarkar from the Indian Institute of Science (IISc), have come up with an antenna design which can perform sky measurements between 2.5 - 4 Gigahertz (GHz), the frequency range identified to be best suited for CRR detection.

“For the sky measurements we plan to perform, the broadband antenna offered us the highest sensitivity when compared to other antennas designed for the same bandwidth. The metric of being frequency-independent over the wideband and ensuring smooth frequency performance is unconventional, something only a custom design, such as ours, could achieve. An off-the-shelf wideband antenna just won’t work,” said Keerthipriya Sathish, lead author of the paper and Research Scientist at RRI.

A fantail antenna has been proposed as it has a radiation pattern with the same shape across frequencies with just a +/- 1% variation in its characteristics. This antenna is a dual polarised dipole antenna with four arms and each arm shaped in the form of a fantail. What makes a fantail antenna unique is that using its custom design, the antenna stares perfectly at the same patch of the sky over its full operational bandwidth of 1.5 GHz (2.5 to 4 GHz) which is important in being able to separate spectral distortions from galactic foregrounds.

Weighing 150 grams, the square box-shaped antenna measures 14cm x 14cm. The top flat substrate is a low loss dielectric on which the antenna is etched in copper and the bottom is an aluminium ground-plate. Sandwiched in between these two plates is a radio-transparent, thick foam layer that houses the antenna’s connectors with the receiver base.

“The antenna has a sensitivity of around 30 millikelvin (mK) across the 2.5-4 GHz frequency range, enabling it to detect very small temperature variations in the sky. Even before scaling it to an array, this antenna will enable exciting first science results once integrated with its custom receiver. We plan to study a reported excess radiation in the sky from a previous experiment at 3.3 GHz, which has been attributed to exotic physics including Dark Matter annihilation. Such experiments with this antenna will help inform improvements in the antenna and experiment design to go all the way to the sensitivity needed for a CRR detection” said Mayuri Rao, faculty, RRI.

The researchers said that an antenna array will be deployed in radio-quiet locations, that is, where there is minimal or no radio frequency interference. The design of this planar antenna is such that it is easily fabricated using methods similar to those used in Printed Circuit Board (PCB) printing. Thus, this design offers high machining accuracy and consistency during replication for multiple-element arrays, is portable and easily deployable.   

Using techniques adopted in this antenna design, the trio are already planning improvements that can take them closer to achieving their formidable goal of 1 part per billion sensitivities.   

 

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