# How can radio telescopes pick up the Cosmic Microwave Background? **URL:** \ **Category:** Factual Questions\ **Created:** [May 23, 2013, 10:02pm UTC](https://boards.straightdope.com/t/how-can-radio-telescopes-pick-up-the-cosmic-microwave-background/659167 "2013-05-23T22:02:19Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [May 23, 2013, 10:02pm UTC](https://boards.straightdope.com/t/how-can-radio-telescopes-pick-up-the-cosmic-microwave-background/659167/1 "2013-05-23T22:02:19Z")
If the CMB is leftover blackbody radiation, then why isn’t its signal swamped by thermal noise within the receiver? Shouldn’t it require a cryogenically chilled receiver, like far-infrared detectors? ---
**Author:** ![Jas09](https://avatars.discourse-cdn.com/v4/letter/j/d07c76/32.png) [@Jas09](https://boards.straightdope.com/u/Jas09)\ **Post date:** [May 23, 2013, 10:24pm UTC](https://boards.straightdope.com/t/how-can-radio-telescopes-pick-up-the-cosmic-microwave-background/659167/2 "2013-05-23T22:24:21Z")
As I understand it, cryo-chilled receivers are in fact part of the design for telescopes attempting to measure CMB. As an example, POLARBEAR says it cools its receivers to 0.3K ([http://bolo.berkeley.edu/polarbear/?q=node/7](http://bolo.berkeley.edu/polarbear/?q=node/7)) ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [May 24, 2013, 1:24am UTC](https://boards.straightdope.com/t/how-can-radio-telescopes-pick-up-the-cosmic-microwave-background/659167/3 "2013-05-24T01:24:57Z")
If you believe Wikipedia, [Penzias and Wilson used a detector cooled with liquid helium](http://en.wikipedia.org/wiki/Discovery_of_cosmic_microwave_background_radiation) when they first detected the CMB. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [May 24, 2013, 1:27am UTC](https://boards.straightdope.com/t/how-can-radio-telescopes-pick-up-the-cosmic-microwave-background/659167/4 "2013-05-24T01:27:29Z")
Ah, I’d been under the impression that it was detectable with ordinary microwave receivers. Ignorance fought. ---
**Author:** ![Speaker\_for\_the\_Dead](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@Speaker\_for\_the\_Dead](https://boards.straightdope.com/u/Speaker_for_the_Dead)\ **Post date:** [May 24, 2013, 1:29am UTC](https://boards.straightdope.com/t/how-can-radio-telescopes-pick-up-the-cosmic-microwave-background/659167/5 "2013-05-24T01:29:54Z")
But only after chasing off some pigeons in the receiver. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 24, 2013, 2:46pm UTC](https://boards.straightdope.com/t/how-can-radio-telescopes-pick-up-the-cosmic-microwave-background/659167/6 "2013-05-24T14:46:00Z")
That depends on what you mean by “detectable”. The CMB will represent some percentage of the signal in any detector of the appropriate wavelength. It will just be a relatively small percentage, buried under other noise sources, in a non-cryogenic detector. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [May 24, 2013, 2:58pm UTC](https://boards.straightdope.com/t/how-can-radio-telescopes-pick-up-the-cosmic-microwave-background/659167/7 "2013-05-24T14:58:40Z")
Like on an old rabbit eared TV.