The end of US support for the CMB-S4 telescope is devastating
The South Pole Telescope, which CMB-S4 would have used to measure variations in temperature and polarization of microwave light across most of the sky.
Brad Benson, University of Chicago, Fermilab
Robert Frost’s poem The road not taken opens with “Two diverging roads in a yellow wood,/ And sorry I couldn’t travel both.”
These lines come to mind when I think of the US government’s July 9 public letter stating that it would no longer support the CMB-S4 project. CMB-S4, short for Cosmic Microwave Background-Stage 4, was supposed to be a next-generation multicontinental telescope that would have given all of humanity an unprecedented glimpse of the first light to ever fly freely through the cosmos.
At first, the universe was filled with a dense stew of particles and plasma. The plasma was so dense that the particles of light, photons, couldn’t travel very far without hitting something. In addition to being thick, the stew was very hot, which prevented the formation of phenomena like atoms. Only after the universe experienced cosmic inflation, in which space-time expanded rapidly for an almost negligible fraction of a second, did it cool enough for the first hydrogen atoms to form. Once this type of aggregation became possible, photons were given room to maneuver and began flying through space-time.
61 years ago, humanity first learned that we were flooded with these photons, known as cosmic microwave background radiation. What looked like background noise in a radio signal turned out to be a messenger from the early cosmos. For decades, we have been examining these photons: their wavelength (and associated temperature), their intensity and their variation in space.
The CMB is almost a gold mine, in the sense that it provides a lot of information about the origin of everything we can see, including stars whose explosions produce gold. If we scan the entire sky and look at the temperature associated with the photons, we see small variations in temperature. Their locations are random, but the magnitude of variation is the same for all.
Our best cosmological theory tells us that these fluctuations are the result of small quantum variations in the amount of matter present at a given location at the time the photons were released. The places where there was a little more were essentially the starting point for gas that gravitationally accumulated into protostars, which became stars that clustered together to form what eventually became galaxies. So, these small variations in the CMB are our start.
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The withdrawal of the project is part of the United States’ reckless retreat from global scientific collaboration.
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Perhaps the most important measurement we have made of the CMB is the characterization of the correlation between these temperature variations and the physical scale. It is questionable how much variation is due to effects at larger or smaller scales, given that some physical phenomena occur at longer distances and others at shorter distances. In other words, different moments of cosmological history are imprinted on the CMB.
For example, we can “see” when the universe became transparent to matter – that first moment when hydrogen was formed, an instant known as recombination. We can also “see” the amount of dark matter and dark energy present in the universe, even though they are invisible to us. Their existence is printed on the CMB.
CMB-S4 was supposed to be the next step in discovering all the lessons that CMB has to teach us. One major goal was to search for evidence of primordial gravitational waves – ripples in space-time caused by cosmic inflation. “Inflation” is actually a class of models, and we know that, generally speaking, they all give the right physics for our universe. But we’re still not sure of the details. Since inflation occurred from the earliest moments of the universe, gravitational wave imprints on the CMB will likely be the best way to distinguish inflationary patterns.
The end of government support for CMB-S4 is like putting a spoke in the wheel of your own bicycle: we were flying, happily studying the cosmos, and now we are completely turned upside down. The impact will also be felt globally. Historically, the United States has invested more in cosmological science than most countries, which is part of the reason why students from around the world come to study in the United States. Data from U.S.-funded experiments have also often become a global resource, so withdrawal from this project, which already seemed likely under the previous presidential administration, is now part of the United States’ reckless withdrawal from global collaboration.
Frost ends his poem by talking about his choice of road: “I took the one less traveled,/And that made all the difference.” » It is truly unfortunate that when it comes to CMB science, the United States has decided not to take the road less traveled. It will definitely make a difference, but not for the better.
Chanda’s week
What I read
I enjoyed Niayesh Afshordi and Phil Halper Battle of the Big Bang: The new stories of our cosmic origins.
What I watch
I’ve been re-watching the DC Universe movies featuring Harley Quinn, a personal favorite.
What I’m working on
I tried to capture some good images of the Andromeda Galaxy from my backyard.
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