PRATUSH : Probing ReionizATion of the Universe using Signal from HydrogenPRATUSH is a future radiometer in lunar orbit that will reveal the Cosmic Dawn of our Universe. PRATUSH will answer the question of when the first stars formed in our universe, the nature of the first stars, and what was the light from the first stars or, in other words, the colour of the light of Cosmic Dawn. PRATUSH will be the pioneering space telescope that will reveal, for the first time, the history of our infant Universe as it transformed after the Big Bang - from cold gas into stars and galaxies and the universe as we know it today. PRATUSH, as its name denotes, will inform us of the first rays of the first suns in the infant universe.The current best model of the Universe suggests that the early Universe, moments after the ‘beginning’ was a dense and hot place. All matter that we see around us today, existed in the form of constituent particles, primarily protons, electrons, some Helium nuclei and trace amounts of some other elements. As the Universe expanded and cooled, these particles combined to form the very first atoms - Hydrogen and Helium, and the ionized Universe became almost fully neutral. The end of this period marked the beginning of the dark ages where there were no sources of radiation other than the light released in the initial moments of the Universe. We observe this light today at microwave frequencies and is called the Cosmic Microwave Background (CMB). The present day Universe is populated by hundreds of billions of galaxies, each with hundreds of billions of stars. Clearly these stars and galaxies must have ‘switched on’ sometime in the history of the Universe. The period when the first sources of radiation (such as stars and galaxies) first formed is appropriately called the ‘Cosmic Dawn’. The radiation from these sources eventually re-ionized most of the Hydrogen atoms to once again result in mostly protons and electrons. The duration over which this re-ionization is thought to have occurred is called the Epoch of Reionization.The Cosmic Dawn and Epoch of Reionization (collectively EoR henceforth) are among the most poorly understood periods in cosmology. Among the things we do not know are (a) When did the first sources form? (b) What were they? Were they bright hot stars comprised entirely of hydrogen, were they black-holes or something so exotic that we don’t have a word for it yet?! (c) How exactly did reionization proceed? What kind of radiation did these first sources emit and how did this radiation interact with and ionize neutral matter?One of the most promising tools to study and understand EoR is radiation from neutral hydrogen gas. The neutral hydrogen atom naturally emits a signal at a wavelength of 21-cm or 1420 MHz due to statistical processes. Although one hydrogen atom emits this signal once in almost half a million-billion years, there is enough hydrogen in the Universe that 21-cm emission is used to study distant galaxies and the Universe as a whole. By tracing the strength and shape of this 21-cm signal as a function of comic time through Cosmic Dawn and Epoch of Reionization, as the Universe transitions from being mostly neutral to mostly ionized, we can seek to answer many of the above questions that to date are unanswered.There exist many theoretical models, some invoking exotic physics, that predict a different shape and strength of the global (average) 21-cm signal as a function of frequency. The frequency at which the signal appears is directly related to the redshift of the signal, which tells us time in the history of the Universe. The signal as predicted to be observed today is expected to range over 10 MHz < ν < 200 MHz (corresponding to redshift range 150 < z < 6 ) and very weak in strength typically 100 milli-Kelvin in brightness temperature units. For context at 100 MHz, this would be as weak as trying to pick up an FM radio station almost at the distance of our neighboring galaxy Andromeda!Experiments are underway to attempt a ground based detection of this weak signal and these have give us some constraints on the astrophysics and timeline of reionization, including one claimed anomalous detection that awaits confirmation and acceptance in the wider community. With the in-house SARAS series of experiments, the RRI CMB DISTORTION lab is among the leaders in the field. SARAS has already ruled out over 10 percent of theoretical models of reionization. It is also the first experiment to constrain astrophysical parameters of the first sources using the 21-cm signal technique. Work is underway to make a detection of the signal from the ground! However results from ground based experiments will always be riddled with some uncertainty that come from unique observing challenges.Results ground-based experiments will most certainly be plagued and limited by issues unique to a ground-based measurement: man-made radio-frequency interference (RFI), coupling of instrument response to ground characteristics, and atmospheric contaminants in the form of molecular absorption lines & ionospheric refraction, emission, and absorption. Low-level RFI and ionospheric effects, which are more pronounced at low frequencies, are a fundamental limitation to detection of this cosmic signal from ground. The only way to completely mitigate contamination from RFI, eliminate these impediments, and confidently detect this cosmic signal, is via a space-based mission observing from the far side of the Moon. PRATUSH: Probing ReionizATion of the Universe using Signal from Hydrogen, seeks to do just that.PRATUSH will carry a wideband frequency-independent antenna, operating over the frequency band 30-250 MHz, a self-calibratable analog receiver, and a digital correlator with high spectral resolution. Each of these components will be custom designed such that the calibrated instrument response is spectrally smooth, devoid of spurious shapes that can confuse a signal detection. The target instrument sensitivity is at the level of few millikelvin without being limited by any systematic features. This requires a dynamic-range in bandpass calibration of one part in a million, which will be achieved with a multi-step calibration strategy involving bandpass calibration by in-situ injection of well characterized broadband noise, and on-sky calibration to characterize antenna properties and coupling of sky-power to receiver. The observing strategy of PRATUSH will be to continually observe large sky regions, and recording spectra of the beam-averaged radio emission with a high spectral resolution of 100 kHz. The nominal lifetime of the payload will be two years for achieving high signal-to-noise ratio with sufficient sky-coverage. The preferred orbit for the payload will be a circumlunar orbit to enable a measurement of radio sky spectrum from the dark and far side of the Moon. Such an orbit would avoid the ionosphere, and also solar and terrestrial radio frequency interference, which are acknowledged as the major show stoppers from the ground. PRATUSH is currently funded for pre-project studies by the Indian Space Research Organization.
Plans afoot to place radio telescope on far side of Moon BENGALURU: Raman Research Institute (RRI) and the Indian Space Research Organisation (ISRO) are mulling over a plan to put a sophisticated radio telescope on the far side of the Moon to detect the elusive radio signal from the age of the universe when the first stars and galaxies were born. Dr Saurabh Singh, Research Scientist, RRI, said the far side of the Moon is known to be the “quietest” location in the solar system for radio telescopes to function to detect cosmic radio signals without any interference from terrestrial-based radio frequencies. ISRO has funded a study conducted by the RRI to work out the feasibility of placing a radio telescope named Pratush, which is presently being built by RRI. Dr Singh said the biggest problem faced by radio signals-hunting astronomers around the world is that the cosmic signals are in the radio frequency wavelength band used by several communications equipment, TV and FM radio station, which interfere with finding genuine cosmic radio signals. This will not be the case with a radio telescope functioning in the “quiet” environs of the moon’s far side which will be shielded from terrestrial radio frequency interference.The project which is still in its nascent phase is being planned to be carried out in two phases. The first phase will see the radio telescope being tested in earth’s own orbit. This will be more like a laboratory model, and will be brought down to earth after the tests to determine its functions in space. The second phase will actually see the radio telescope being launched onboard ISRO’s satellite launching rockets to take Pratush into the lunar orbit and then sent down on the lunar surface on the far side of the moon, which remains hidden from the earth as the moon’s rotation and revolution being the same, ensure that only one side always faces the earth. “This is only a plan at this stage, but it is being worked out. We are looking at it being launched on one of the next few ISRO missions to the moon,” Dr Singh said.
Having the telescope in lunar orbit instead of on the surface would make operation a lot easier:- direct communications with Earth, instead of having to rely on a relay satellite- no need to have a power system that can power the telescope through the lunar night- no need to build a lander
PRATUSH : Epoch of ReionizationPRATUSH -- Probing ReionizATion of the Universe using Signal from Hydrogen -- is a proposed cosmology experiment to detect the global red-shifted 21-cm signal from the Cosmic Dawn and Epoch of Reionization (CD/EoR). PRATUSH will seek to precisely measure the low-frequency radio sky-spectrum over the frequency range of 40 to 250 MHz in an orbit around the moon. In September 2021, an ISRO appointed review committee recommended that PRATUSH move to project mode. Phase-I of PRATUSH will seek to fly in a low earth orbit for technology demonstration, early risk mitigation, and preliminary science measurements. This will be followed by the PRATUSH phase-II in lunar orbit in the future. The PRATUSH baseline design has been realised and it operates over the crucial frequency range of 55-110 MHz and a ground-based concept model is under development. Individual hardware subsystems of the concept model using ground based components are ready and under testing. The system integration and field testing to demonstrate the concept with sky measurements and their comparison to simulations are planned.
