Up to 2016, the biggest telescopes were composed of an array of antennas located in a particular site, like the one at Atacama Desert, in Chile. called ALMA (Atacama Large Millimeter/ Submillimeter Array). Its construction dates from 2004 and consists of sixty-six 8-to-12-meter diameter antennas aiming to receive millimeter wavelengths. Among others things, ALMA is capable of performing deep space detections, what allows to obtain information about the first stars and galaxies that emerged billions of years ago at huge distances from us. Due to the universe expansion, most of these objects’ emissions have stretched out to the millimeter and submillimeter wavelengths.

Using a technique known as interferometry – or interference pattern between the signals received by each antenna, to recompose a complete unique image- the 66 antennas at ALMA work together as though they were a single telescope. These antennas capture electromagnetic and radio waves; the last ones penetrate clouds of dust and other material. Since supermassive black holes (SMBH) are surrounded by gas and dust, the radio waves are better suited, but then a huge limitation arises. Radio waves require a large region of detection, and the SMBH as the one located at the center of our galaxy – the milky way – is too far, and hence, too small to be detected by a single site on earth.

The Event Horizon Telescope (EHT) initiative has been created for such an endeavor. EHT is an international collaboration that aims to obtain the first image of the event horizon of a black hole, using a virtual telescope the size of the earth.  Different telescopes scattered across the planet collect the data of the same object, which is then combined and processed by a supercomputer to produce a final image through the interferometry technique, just as the one used in ALMA, but this time at a global scale.  By triangulating the data from the nine telescope arrays – (ARO/SMT), (APEX), (IRAM), (JCMT), (LMT), (SMA), (ALMA), (SPT) -, the EHT works like one enormous antenna with a radio dish thousands of kilometers across.

Since April 2017 the EHT team have been collecting data on Sagittarius A* and comparing it to models of what GRT predicts the black hole to look like. Results are expected sometime between 2018 and 2019, which is pretty soon. The EHT will not only capture the image of the SMBH event horizon, but it will also serve to test general relativity theory (GRT). Following GRT, which describes gravitation, the event horizon should be spherical. If a different shape is observed, the theory would have to be revised. Either way, these are excited times to witness!

By: Ines Urdaneta, RSF research Scientist

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