That wavelength corresponds to a temperature of 2.725 K. The radiation’s intensity plotted by wavelength makes it obvious that the CMB has a very specific intensity curve, where the strongest signal is at 2 mm. The COBE science team’s first announcement, in 1990, was the result of that measurement. One of its instruments measured the intensity of the microwave glow at wavelengths ranging from 0.1 to 10 millimeters across the entire sky. The Cosmic Background Explorer (COBE) launched in 1989. While the light 380,000 years into the universe’s history would have been visible to human eyes if we were around, cosmic expansion has since stretched the light into the longer wavelengths of microwaves - at least, that’s the wavelength astronomers had predicted. Since the 1960s, telescopes on Earth have captured that glow in every direction of the sky. And so it has traveled, mostly unhindered, in the approximately 13.8 billion years since that time of “last scattering.” These photons carry a snapshot of the 380,000-year-old universe. With fewer individual particles zooming around, light could finally move about freely. The universe became mostly neutral hydrogen, with some heavier elements swirled in. Around 380,000 years after the Big Bang, the temperature dropped to about 3,000 kelvins, cool enough for electrons to latch onto hydrogen nuclei. Over the next thousands of years, the cosmos continued to expand, giving the particles more room to move and allowing the universe’s temperature to cool bit by bit. Electrons and light collided and scattered off of those atomic nuclei. A few minutes later, the universe’s constituent primordial subatomic particles glommed together into an elemental soup of atomic nuclei containing hydrogen, helium, and trace amounts of lithium. The universe began with the Big Bang 13.8 billion years ago as a fiery sea that expanded rapidly. Specifically, three satellites - COBE, WMAP, and Planck - revealed that our current cosmos, which is complex and filled with clusters of galaxies, stars, planets, and black holes, evolved from a surprisingly simple early universe. It would take satellites launched above Earth’s obscuring atmosphere to map that microwave glow to precisions on the order of millionths of a degree. In the mid-1960s, when Arno Penzias and Robert Wilson discovered the CMB’s pervasive microwave static across the sky, it appeared identical everywhere. “And it gives us so much information because all the things that we now see out in space - the galaxies, the clusters of galaxies - the very earliest seeds of those, we see in this CMB light.”Įxtracting these clues from the CMB has taken multiple generations of telescopes on the ground, lofted into the atmosphere, and launched into space. “It’s the earliest view we have of the universe,” says Princeton University cosmologist Joanna Dunkley.
It holds within it an incredible wealth of knowledge that astronomers have been teasing out for the past few decades.
The primordial cosmic microwave background (CMB) radiation has since traveled some 13.8 billion years through the expanding cosmos to our telescopes on Earth and above it.īut the CMB isn’t just light. This light is the remnant signature of the cosmic beginning - a dense, hot fireball that burst forth and created all mass, energy, and time.
A glow undetectable to the human eye permeates the universe.
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