Season 17, Episode 8: The “Red Hot” Early Universe
Hey StarTalkers! Following-up on the previous post, Neil’s rant about the idiom “red hot” is about as significant for cosmology as it is for astronomy.
Thing You Thought You Knew – Red Hot, Blue Hot - StarTalk Radio
(The discussion starts at 13:45)
The last post introduced the concept of blackbody radiation – why an iron fresh out of the forge can glow “red hot.” What we didn’t talk about last time is that you can measure the blackbody radiation of the whole universe.
The “Hot” Big Bang and the Surface of Last Scattering
Everyone’s heard of the Big Bang, but not many people know it’s technically called the “hot” Big Bang. The early universe was much hotter and much denser, and for some time, radiation dominated over matter.
This is because the average energy of a photon was much higher than the energy you need to rip an atom apart. As soon as an electron and proton formed a hydrogen nucleus, a photon with over 13.6 electron-volts of energy would come along and rip it apart. Light was everywhere, and the universe was hot and opaque.
As the universe expanded, it cooled. Photons became less energetic and atoms spread out. After a certain point, the interactions became rarer and the universe became transparent.
But there is an “afterglow” (Revisiting cosmic microwave background radiation using blackbody radiation inversion ) leftover from this last moment, which we call the cosmic microwave background radiation (CMB).
The Discovery of the CMB
In July 1965, The Astrophysical Journal’s 142nd volume featured two back-to-back reports that brought the CMB into the mainstream.
Penzias and Wilson (A Measurement of Excess Antenna Temperature at 4080 Mc/s) reported an “excess temperature” at their antenna, which seemed to come from all directions and had a temperature of about 3.5 K. At first, they thought their antenna was just picking up noise. They even wiped the pigeon poop off the equipment. But the finding stayed strong.
Dicke et. al. (Cosmic Black-Body Radiation.) had the explanation in their companion letter. They identified the excess temperature as that afterglow, the final flash of the hot, early universe. They were searching for it too, but Penzias and Wilson beat them to it.
The Temperature of the Universe Today
Over time, the universe continued to expand and cool, stretching the wavelength of the CMB along with it. This remnant of the universe’s “hot” phase now has a temperature of just 2.725 K, a few degrees Celsius above absolute zero.
