The 21-cm absorption profile is detected in the sky-averaged radio spectrum, but is much stronger than predicted, suggesting that the primordial gas might have been cooler than predicted. As the first stars heated hydrogen in the early Universe, the 21-cm hyperfine line—an astronomical standard that represents the spin-flip transition in the ground state of atomic hydrogen—was altered, causing the hydrogen gas to absorb photons from the microwave background. This should produce an observable absorption signal at frequencies of less than 200 megahertz (MHz). Judd Bowman and colleagues report the observation of an absorption profile centred at a frequency of 78 MHz that is about 19 MHz wide and 0.5 kelvin deep. The profile is generally in line with expectations, although it is deeper than predicted. An accompanying paper by Rennan Barkana suggests that baryons were interacting with cold dark-matter particles in the early Universe, cooling the gas more than had been expected. After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz1. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars; however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions2. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude3. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.

An absorption profile centred at 78 megahertz in the sky-averaged spectrum

The techniques and technologies currently being investigated to detect weapons and contraband concealed on persons under clothing are reviewed. The basic phenomenology of the atmosphere and materials that must be understood in order to realize such a system are discussed. The component issues and architectural designs needed to realize systems are outlined. Some conclusions with respect to further technology developments are presented.

Standoff Detection of Weapons and Contraband in the 100 GHz to 1 THz Region

Author(s): DeBoer, David R; Parsons, Aaron R; Aguirre, James E; Alexander, Paul; Ali, Zaki S; Beardsley, Adam P; Bernardi, Gianni; Bowman, Judd D; Bradley, Richard F; Carilli, Chris L; Cheng, Carina; Acedo, Eloy de Lera; Dillon, Joshua S; Ewall-Wice, Aaron; Fadana, Gcobisa; Fagnoni, Nicolas; Fritz, Randall; Furlanetto, Steve R; Glendenning, Brian; Greig, Bradley; Grobbelaar, Jasper; Hazelton, Bryna J; Hewitt, Jacqueline N; Hickish, Jack; Jacobs, Daniel C; Julius, Austin; Kariseb, MacCalvin; Kohn, Saul A; Lekalake, Telalo; Liu, Adrian; Loots, Anita; MacMahon, David; Malan, Lourence; Malgas, Cresshim; Maree, Matthys; Martinot, Zachary; Mathison, Nathan; Matsetela, Eunice; Mesinger, Andrei; Morales, Miguel F; Neben, Abraham R; Patra, Nipanjana; Pieterse, Samantha; Pober, Jonathan C; Razavi-Ghods, Nima; Ringuette, Jon; Robnett, James; Rosie, Kathryn; Sell, Raddwine; Smith, Craig; Syce, Angelo; Tegmark, Max; Thyagarajan, Nithyanandan; Williams, Peter K. G; Zheng, Haoxuan

https://iopscience.iop.org/article/10.1088/1538-3873/129/974/045001/pdf

Hydrogen epoch of reionization array (HERA)

The Murchison Widefield Array is a dipole-based aperture array synthesis telescope designed to operate in the 80-300 MHz frequency range. It is capable of a wide range of science investigations but is initially focused on three key science projects: detection and characterization of three-dimensional brightness temperature fluctuations in the 21 cm line of neutral hydrogen during the epoch of reionization (EoR) at redshifts from six to ten; solar imaging and remote sensing of the inner heliosphere via propagation effects on signals from distant background sources; and high-sensitivity exploration of the variable radio sky. The array design features 8192 dual-polarization broadband active dipoles, arranged into 512 ldquotilesrdquo comprising 16 dipoles each. The tiles are quasi-randomly distributed over an aperture 1.5 km in diameter, with a small number of outliers extending to 3 km. All tile-tile baselines are correlated in custom field-programmable gate array based hardware, yielding a Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point spread function quality. The correlated data are calibrated in real time using novel position-dependent self-calibration algorithms. The array is located in the Murchison region of outback Western Australia. This region is characterized by extremely low population density and a superbly radio-quiet environment, allowing full exploitation of the instrumental capabilities.

/pdf/the-murchison-widefield-array-design-overview-1op0axbp6l.pdf

The Murchison Widefield Array: Design Overview

Cooperating Organizations American Astronomical Society (United States) • Netherlands Institute for Radio Astronomy (ASTRON) (Netherlands) • Ball Aerospace & Technologies Corporation (United States) Canadian Astronomical Society (CASCA) (Canada) • European Astronomical Society (Switzerland) • ESO—European Southern Observatory (Germany) • International Astronomical Union • Korea Astronomy and Space Science Institute (KASI) (Republic of Korea) • National Radio Astronomy Observatory • POPSud (France) • TNO (Netherlands)

/pdf/ground-based-and-airborne-telescopes-iii-2e6sjio423.pdf

Ground-based and Airborne Telescopes III

In this letter, the design and measurement of the first SiGe integrated-circuit LNA specifically designed for operation at cryogenic temperatures is presented. At room temperature, the circuit provides greater than 25.8 dB of gain with an average noise temperature (Te) of 76 K (NF = 1 dB) and S11 of -9 dB for frequencies in the 0.1-5 GHz band. At 15 K, the amplifier has greater than 29.6 dB of gain with an average Te of 4.3 K and S11 of -14.6 dB for frequencies in the 0.1-5 GHz range. To the authors' knowledge, this is the lowest noise ever reported for a silicon integrated circuit operating in the low microwave range and the first matched wideband cryogenic integrated circuit LNA that covers frequencies as low as 0.1 GHz.

/pdf/a-0-1-5-ghz-cryogenic-sige-mmic-lna-3juj6xlagn.pdf

A 0.1–5 GHz Cryogenic SiGe MMIC LNA

The noise models of InP and GaAs HEMTs are compared with measurements at both 300 and 20 K The critical parameter, Tdrain, in the Pospieszalski noise model is determined as a function of drain current by measurements of the 1-GHz noise of discrete transistors with 50- Ω generator impedance The dc I-V for the transistors under test are presented and effects of impact-ionization are noted InP devices with both 100% and 75% indium mole fraction in channel are included Examples of the design and measurement of very wideband low-noise amplifiers (LNAs) using the tested transistors are presented At 20-K physical temperature the GaAs LNA achieves 10-K noise over the 07-16-GHz range with 16 mW of power and an InP LNA measures 20-K noise over the 6-50-GHz range with 30 mW of power

Noise Measurements of Discrete HEMT Transistors and Application to Wideband Very Low-Noise Amplifiers

Design and test data for a full waveguide band MMIC tripler using anti-parallel Schottky diodes are reported in this paper. The circuit outputs between -3.7 dBm and +2.0 dBm from 75 to 110 GHz. When tuned for power flatness, the output is between -4.6 dBm and -1.3 dBm across the band. The conversion efficiency is about 1.5% in both cases. To the authors' knowledge, this is the first reported MMIC frequency tripler to cover the entire W-band.

/pdf/a-full-waveguide-band-mmic-tripler-for-75-110-ghz-1e7dfa4tw2.pdf

A full waveguide band MMIC tripler for 75-110 GHz

The combination of the traveling wave OMT device and the ultra-low-noise MMIC amplifiers has allowed us to develop a broadband 6-8 GHz receiver with a noise temperature of around 10 K. The combination of receiver noise and the additional noise contributions by the telescope optics gives an overall receiver temperature of around 28 K and 34 K in the two polarizations. The large collecting area of the telescope gives rise to a system equivalent flux density of around 4.5 Jy at 7 GHz

Low-noise 6-8 GHz receiver

A 0.1-mum InP HEMT Ka-band LNA with high and flat gain, very low noise figure and low VSWR has been developed. Across the entire Ka-band, of 26 GHz to 40 GHz, the MMIC LNA demonstrated associated gain of 21.9 plusmn 0.9 dB and an average noise figure of 1.5 dB with a minimum of 1.3 dB at 34 GHz. The LNA chip was cryogenically cooled to 12 K where it exhibited an associated gain of 23.0 plusmn 1.1 dB and an average noise temperature of 15.5 K, i.e. 0.23-dB noise figure. Two LNA chips were cascaded and assembled into a module. At room temperature, the module achieved an associated gain of 37.6 dB plusmn 1.8 dB and an average noise figure of 1.3 dB. At 15 K, the average noise temperature was improved to 11.4 K with 41.0 plusmn 2.4 dB associated gain

/pdf/full-ka-band-high-performance-inp-mmic-lna-module-5dalz0jlbc.pdf

Sander Weinreb

Papers

A 0.1–5 GHz Cryogenic SiGe MMIC LNA

Noise Measurements of Discrete HEMT Transistors and Application to Wideband Very Low-Noise Amplifiers

A full waveguide band MMIC tripler for 75-110 GHz

Low-noise 6-8 GHz receiver

Full Ka-band High Performance InP MMIC LNA Module