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Why did the signal detected by Penzias and Wilson seem to come from every direction?

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Oval map of the cosmic microwave background radiation from NASA's WMAP mission, showing a mottled pattern of blue, green, yellow, and red representing temperature variations. — Cosmic microwave background A full-sky map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) showing cosmic background radiation, a very uniform glow of microwaves emitted by the infant universe more than 13 billion years ago. Color differences indicate tiny fluctuations in the intensity of the radiation, a result of tiny variations in the density of matter in the early universe. According to inflation theory, these irregularities were the “seeds” that became the galaxies. WMAP's data support the big bang and inflation models, and cosmic microwave background is at the farthest limits of the observable universe.

The signal detected by Arno Penzias and Robert Wilson appeared to come from every direction because the cosmic microwave background (CMB) is a pervasive, uniform field of electromagnetic radiation that fills the entire universe.12

This radiation is a residual effect of the big bang, representing the "vanished brilliance" of the early universe. Its high degree of isotropy—the fact that it appears the same in every direction—is explained by the theory of inflation. Before the universe underwent rapid, exponential expansion, the matter and radiation that make up the observable universe were in thermal contact, allowing them to reach the same temperature. As the universe expanded and cooled, this radiation became decoupled from matter, creating a uniform background that we observe today as a blackbody spectrum at approximately 2.735 K.123