Observations of the Cosmic Microwave Background


By Emily LevesqueUniversity of Washington

Detailed observations of microwave signals can be extraordinarily difficult. Eliminating every other stray source of noise or interference on the planet in order to detect and study the whispered signal of the cosmic microwave background is an immense challenge. To get a good, close, detailed look at this faint radiation, telescopes needed to be both as powerful as possible and as remote as possible.

Image of QUIET, a cosmic microwave background polarization telescope
To study the cosmic microwave background, telescopes need to be powerful and space-based to be as remote as possible. (Image: Joezuntz/Public domain)

Designing COBE

Space-based telescopes proved perfect for this work, and in 1982, NASA began building the Cosmic Background Explorer, or COBE, a satellite carrying several instruments. One was a microwave detector that would make a map of any tiny anisotropies in the cosmic microwave background. 

The microwave detector’s design and construction was led by George Smoot, who had previously designed detectors that rode aboard stratospheric balloons and high-altitude planes to study the cosmic microwave background.

Another instrument onboard COBE was a spectrograph that detected very long wavelength infrared light. The team that built this was led by John Mather, and it was specifically designed to measure the cosmic microwave background’s all-important blackbody spectrum.

Artistic render of COBE satellite on Earth orbit
COBE was successful in mapping the cosmic microwave background through the course of a year in orbit. (Image: NASA/Public domain)

Change of Plans

COBE was originally designed to be launched by a space shuttle crew. Unfortunately, the Challenger disaster grounded all shuttle flights, and COBE was ultimately redesigned, launched by an uncrewed Delta rocket in 1989, and placed into something called a Sun-synchronous orbit. 

This orbit sent COBE around the Earth very close to the planet’s poles, following the apparent motion of the Sun relative to the Earth and carefully oriented so that the satellite’s instruments were always pointed away from both the Sun and the Earth. One year of observations would allow COBE to slowly scan the entire sky, mapping the cosmic microwave background and measuring its temperature along the way.

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The Map of Cosmic Microwave Background

The experiment’s design worked spectacularly. In 1992, the COBE team announced that they had, in fact, mapped anisotropies in the cosmic microwave background and carefully measured its blackbody temperature. 

Their groundbreaking map of the cosmic microwave background was lauded as an immense scientific discovery, identifying the fluctuations that had been predicted by cosmologists and pointed back to clear evidence of the Big Bang. In 2006, George Smoot and John Mather were awarded the Nobel Prize for COBE’s observations.

The Microwave Anisotropy Probe

COBE’s all-sky image of the cosmic microwave background has continually been improved upon in the decades since. Another NASA spacecraft, originally named MAP, or the Microwave Anisotropy Probe, was launched in 2001 to do exactly what its name suggested, mapping the cosmic microwave background in exquisite detail and building on COBE’s success. 

The mission was renamed WMAP, with the ‘W’ recognizing David Todd Wilkinson: he had been one of the original members of the Princeton team building a Dicke interferometer and hoping to measure the cosmic microwave background back in the 1960s, and he had gone on to work on the MAP mission’s science team before passing away due to cancer just five months before the mission’s first hotly anticipated data map was released.

The Never-ending Search for the Hubble Constant

In the end, the WMAP data revealed the cosmic microwave background in exquisite detail, measuring minute temperature fluctuations smaller than one-thousandth of a degree Fahrenheit. WMAP observed the cosmic microwave background from an even darker and more remote location than COBE’s Sun-synchronous orbit. It was launched well past the Moon, at a distance of 930,000 miles from the Earth. 

Near the end of its mission lifetime, it was joined there by an observatory operated by the European Space Agency known as Planck, named for the same Max Planck who studied blackbody radiation. The Planck observatory improved again on the maps of previous missions, releasing an astonishingly sharp all-sky map of the cosmic microwave background and its subtle fluctuations in 2013.

Images comparing maps of the cosmic microwave background taken by COBE, WMAP, and Plank.
Over the years, the map of the cosmic microwave background has become more detailed. (Image: NASA/ESA/Public domain)

Teams today have continued to analyze the results of these missions and use these data as a linchpin for measuring everything from the age and composition of the universe to the ever-elusive value of H-naught, the Hubble constant—these are the data that give a dramatically low value for the Hubble constant of 68, as compared to the immensely high 72 or so from studies using observations of nearby stars and galaxies.

Why Study the Cosmic Microwave Background?

Current research is still focused on investigating the cosmic microwave background for subtle but crucial signals. For instance, examining the background’s electric field could reveal primordial gravitational signals from just after the Big Bang. Information hidden in the carefully studied anisotropies could help us understand the evolution of the early universe and the obscure physics happening in the vacuum of space.

Finally, the cosmic microwave background has been combined with other groundbreaking discoveries to help us understand the fundamental make-up of our universe. While not all of these discoveries have been recognized with Nobel Prizes, the incredible science behind them, and the heroes that have made them possible, have contributed crucial pieces to our evolving puzzle of explaining the building blocks of our cosmos.

Common Questions about Observations of the Cosmic Microwave Background

Q: What was the purpose of the space telescope called COBE?

COBE or the Cosmic Background Explorer was a space-based telescope built by NASA which carried several instruments. One instrument was a microwave detector and the other was a spectrograph that detected very long wavelength infrared light. The telescope’s mission was to scan and make a map of the cosmic microwave background.

Q: How was COBE launched into space?

COBE was first designed to be launched with a space shuttle crew but after the Challenger disaster, it was re-designed to be launched by an uncrewed Delta rocket. It was sent into a Sun-synchronous orbit. The orbit sent the telescope very close to the Earth’s poles while carefully adjusting the orbit to always have its instruments pointed away from the Earth and the Sun. After a year, COBE was able to produce a map of the cosmic microwave background.

Q: What are some of the benefits of studying the cosmic microwave background?

Examining the background electrical field, which could help reveal details about the primordial gravitational signals right after the Big Bang, or researches on anisotropies, which could help us understand how the universe slowly evolved into what we know it to be today, are just two examples among many in which the cosmic microwave background could be used along with other groundbreaking research to help us understand the building blocks of our universe.

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