The cosmic microwave background (CMB) is leftover radiation from the Big Bang redshifted (stretched) by the universe's expansion. In this CMB map, NASA's Wilkinson Microwave Anisotropy Probe (WMAP) records minuscule temperature fluctuations in the "billboard" as different colors. In principle, an advanced civilization could create a universe and encode information in the CMB that would let civilizations in the offspring universe know that their universe had been intentionally created.

If our universe were created on purpose - perhaps by a deity or an advanced civilization in another universe - could the creator have left a calling card?
The idea may not be as crazy as it seems. Renowned cosmologists such as Andrei Linde (Stanford University) and Alan Guth (MIT) have speculated that an advanced civilization could, in principle, cook up a new universe in a laboratory by concentrating huge quantities of energy into a tiny volume of space. And even the religious skeptic Carl Sagan concocted a story at the end of his science-fiction novel Contact in which scientists discover a message from a creator embedded deep within the number pi.
In a paper recently posted on the Internet site Astro-ph, physicists Stephen Hsu (University of Oregon) and Anthony Zee (University of California, Santa Barbara) come up with an alternative idea: astronomers could look for a message in the cosmic microwave background (CMB) - the afterglow of the Big Bang.
"Our work does not support the Intelligent Design movement in any way whatsoever, but asks, and attempts to answer, the entirely scientific question of what the medium and message might be IF there was actually a message", write the authors.
The trick, say Hsu and Zee, is for the creator to fine-tune the inflaton field - the field responsible for inflating the very early universe (S&T: November 2005, page 32) - to encode a binary message in the subtle warm and cool spots of the CMB. These spots vary in temperature only by about 0.00001°C. Because there are certain regions of the universe so far apart that they are not causally connected, only a cosmic creator outside our space-time could place a message in the CMB that all civilizations could detect. As the authors note, the CMB is a "giant billboard on the sky" visible to all civilizations in all galaxies.
Signs of an artificial message, argue Hsu and Zee, would show up as deviations from normalcy in the graph plotting the magnitude of the temperature fluctuations versus their angular sizes. Given the limited number of distinct regions of the sky of any fixed size, Hsu and Zee derive a rigorous upper bound of 100,000 bits on the information containable in the message. Such a message might, for example, reveal fundamental laws of physics. While current experiments like NASA's WMAP satellite do not have sufficient angular resolution or sensitivity to detect the extremely small-scale temperature fluctuations that would encode the message, future missions might do so. The authors urge that scientists analyze subsequent CMB data for possible unnatural patterns.
"This may be even more fun that SETI," they conclude (SETI is the search for extraterrestrial intelligence).
In a response paper also posted on Astro-ph, Douglas Scott and James Zibin (University of British Columbia, Canada) counter that because there are inherent limitations in the ability of any civilization to measure such small-scale temperature fluctuations, and because the CMB probability distribution does not directly reflect the behaviour of the inflaton field, Hsu and Zee overestimate the amount of information that can be encoded in the CMB by perhaps two or three orders of magnitude. Zibin comments, "While Hsu and Zee may say we've made a pessimistic assessment, we say they were very optimistic." Hsu responds, "Both groups agree that one can encode a universal message in the CMB. But we disagree as to its maximal information content."