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The most abundant particles.

What are the most common kind of photons in the universe?

--Fred Ringwald, Fresno, Calif.

Those would be the radio photons of the cosmic microwave background radiation.

When it comes to electromagnetic radiation--visible light, X rays, infrared, radio, or whatever--the longer the wavelength, the lower the energy of each photon (particle of electromagnetic energy). It takes about 1,000 photons of the microwave background radiation to match the energy of one photon of starlight.

One way to get at the answer is to realize that the entire sky appears to emit blackbody microwave background radiation, as if it were a solid wall. No other blackbody emitter covers the entire surface of the sky.

This is why, for instance, an ultrahigh- energy cosmic ray is more likely to be destroyed by hitting a photon of the microwave background (S&T: March 2008, page 24) than by hitting any other particle of any kind.

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Title Annotation:Astro Q&A
Author:MacRobert, Alan
Publication:Sky & Telescope
Article Type:Brief article
Geographic Code:1USA
Date:Feb 1, 2009
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