Eli5 there is this famous map of the universe. How did they make it of there are no signals we can send that far?

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Eli5 there is this famous map of the universe. How did they make it of there are no signals we can send that far?

In: Physics

But these stars and galaxies produce alot of light, so we can see them.
Distance can be measured by observation of the brightness of ‘standard candles’, by ‘parallax measurement’, and combination of those for calibration.
Relative velocity can be measured by ‘redshift’ (like doppler shift but for light) of ‘hydrogen emission lines’
Other ways of measuring are studying the ‘cosmic microwave background’.
Concentration of elements are estimated using ‘spectral emission’

Look up the quoted phrases if you want to know more.

It’s not that we sent a signal out there to be reflected back at us. It’s that we’re receiving signals that are already out there — electromagnetic radiation (light, infrared, x-rays, etc.) being radiated from cosmic objects that are hitting the Earth all the time. Light and stuff from the farthest objects took billions of years to travel to our detectors and telescopes.

In a way, it’s kind of like active sonar vs passive sonar. Our telescopes and such are the passive sonar — listening for signals that have been put out already by the object out there.