In an intriguing development, scientists at the Raman Research Institute (RRI) in Bangalore, India, have developed a novel antenna design that could revolutionise our ability to detect the faint signals of Cosmological Recombination Radiation (CRR).
These signals, which are crucial for understanding the thermal and ionization history of the Universe, have so far remained undetected due to their elusive nature. The newly designed antenna is capable of measuring signals in the 2.5 to 4 Gigahertz (GHz) frequency range, which is optimal for detecting CRR, a signal that is approximately one billion times fainter than the Cosmic Microwave Background (CMB).
As per available sources, the universe is approximately 13.8 billion years old, and in its earliest stages, it was extremely hot and dense. During this time, the Universe was composed of a plasma of free electrons, protons, and light nuclei such as helium and lithium. The radiation coexisting with this matter has been detected today as the CMB, which holds vital information about the early cosmological and astrophysical processes.
One such process, known as the Epoch of Recombination, marks the transition from a fully ionized primordial plasma to mostly neutral hydrogen and helium atoms. This transition emitted photons, creating the Cosmological Recombination Radiation (CRR), which distorts the underlying CMB spectrum. Detecting these faint CRR signals would provide a wealth of information about the Universe’s early ionization and thermal history and could even offer the first experimental measurements of helium abundance before it was synthesized in the cores of stars.
However, detecting CRR is a significant challenge because these signals are extremely weak—about nine orders of magnitude fainter than the CMB. To address this, scientists need highly sensitive instruments that can isolate these signals from the vast cosmic noise surrounding them.
To this end, researchers from RRI, including Mayuri Rao and Keerthipriya Sathish, along with Debdeep Sarkar from the Indian Institute of Science (IISc), have developed an innovative ground-based broadband antenna designed to detect signals as faint as one part in 10,000. Their design is capable of making sky measurements in the 2.5 to 4 GHz range, the frequency band most suitable for CRR detection.
According to Keerthipriya Sathish, the lead author of the study, “For the sky measurements we plan to perform, this broadband antenna offers the highest sensitivity compared to other antennas designed for the same bandwidth. The antenna’s frequency-independent performance across a wide range and its smooth frequency response are features that set it apart from conventional designs.”
The antenna is compact and lightweight, weighing just 150 grams, with a square shape measuring 14 cm by 14 cm.
The proposed antenna is a dual-polarized dipole antenna with a unique four-arm structure shaped like a fantail. This design ensures that the antenna maintains the same radiation pattern across its entire operational bandwidth, with a mere 1% variation in its characteristics. This is crucial for distinguishing spectral distortions from galactic foregrounds. The antenna’s custom design allows it to “stare” at the same patch of sky throughout its full operational range of 1.5 GHz (from 2.5 to 4 GHz), which is key to separating the CRR signals from other cosmic noise.
The antenna is compact and lightweight, weighing just 150 grams, with a square shape measuring 14 cm by 14 cm. It is made using a low-loss dielectric flat substrate on which the antenna is etched in copper, while the bottom features an aluminum ground plate. Between these plates lies a radio-transparent foam layer that houses the antenna’s connectors and receiver base.
With a sensitivity of around 30 millikelvin (mK) across the 2.5-4 GHz frequency range, the antenna is capable of detecting tiny temperature variations in the sky. Even before being scaled to a full array, this antenna design is expected to provide valuable first scientific results when integrated with a custom receiver. One of the anticipated experiments is to study an excess radiation reported at 3.3 GHz, which has been speculated to result from exotic phenomena, including dark matter annihilation. These early tests will help refine the antenna’s performance and guide future design improvements aimed at achieving the sensitivity required for CRR detection.
The researchers plan to deploy an array of these antennas in radio-quiet areas, where radio frequency interference is minimal or absent. The antenna’s design is straightforward and can be easily fabricated using methods similar to those employed in Printed Circuit Board (PCB) manufacturing, ensuring high machining accuracy and consistency for scaling up to multiple-element arrays. The antenna is portable, making it easy to deploy in remote locations for scientific observations.
The team is already looking ahead, planning further improvements to achieve even greater sensitivity, with a long-term goal of detecting CRR signals at sensitivities as low as one part per billion. With this innovative antenna design, the team hopes to make significant strides toward uncovering the secrets of the early Universe and its formation.