The South Pole Telescope, which CMB-S4 would have used to measure variations in the temperature and polarisation 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 roads diverged in a yellow wood,/ And sorry I could not travel both”.
These lines come to mind when I consider the US government’s public letter of 9 July stating that it would no longer support the CMB-S4 project. CMB-S4, short for Cosmic Microwave Background-Stage 4, was meant to be a next-generation, multi-continental telescope facility that would have given all of humanity unprecedented insight into the earliest light to ever fly free through the cosmos.
In the beginning, the universe was full of a dense particle-plasma stew. The plasma was so dense that particles of light, photons, couldn’t go very far without running into something. As well as being thick, the stew was very hot, which prevented the formation of phenomena like atoms. Only after the universe had experienced cosmic inflation, where space-time expanded rapidly for an almost negligible fraction of a second, did it cool off enough for the first hydrogen atoms to form. Once this kind of clumping became possible, photons had room to manoeuvre and began to fly across space-time.
It was 61 years ago that humanity first learned we were awash in these photons, which came to be known as the cosmic microwave background radiation. What seemed like a bit of background noise in a radio signal turned out to be a messenger from the early cosmos. For decades, we have scrutinised these photons: their wavelength (and associated temperature), their intensity and their variation across space.
The CMB is almost a literal gold mine, in the sense that it provides a lot of information about where everything we can see came from, including the stars whose explosions make gold. If we scan the entire sky and look at the temperature associated with the photons, we see small variations in the temperature. Their locations are random, but the size of the variation is consistent across them all.
Our best cosmological theory tells us that these fluctuations are the result of little quantum variations in how much stuff there was at any given location at the moment the photons went free. Places where there was a little bit more were essentially the starting point of gas that gravitationally accumulated into protostars, which became stars that clustered together into what eventually became galaxies. So, those little variations in the CMB are the beginning of us.
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The pullback from the project is part of the US’s reckless retreat from global science collaboration
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Perhaps the single most important measurement we have done of the CMB is characterising how these temperature variations correlate with physical scale. We can ask how many of the variations are due to effects on larger scales or smaller scales, knowing that certain physical phenomena happen at longer distances and others at shorter ones. In other words, different moments in cosmological history are imprinted on the CMB.
For example, we can “see” when the universe became transparent to matter – that first moment when hydrogen formed, an instant known as recombination. We can also “see” how much dark matter and dark energy there is in the universe, even though they are invisible to us. Their existence is imprinted on the CMB.
CMB-S4 was supposed to be the next step in uncovering all of the lessons that the CMB has to teach us. One major goal was to look for evidence of primordial gravitational waves – ripples in space-time caused by cosmic inflation. “Inflation” is really a class of models, and we know that, broadly, they all give the right physics for our universe. But we are still unsure about the details. Because inflation happened in the universe’s earliest moments, imprints of gravitational waves on the CMB are likely to be the best way to distinguish between inflationary models.
The end of governmental support for CMB-S4 is like putting a stick in your own bicycle wheel: we were flying along, joyfully studying the cosmos, and now we have been thrown entirely. The impact will be felt globally too. Historically, the US has invested more into cosmological science than most nations, which is one reason why students from around the world come to the US to study. The data from US-funded experiments has also often become a global resource, so the pullback from this project, which already seemed likely under the previous presidential administration, is now part of the US’s reckless retreat from global collaboration.
Frost ends his poem by talking about his choice of road: “I took the one less traveled by,/ And that has made all the difference.” It is so unfortunate that, when it comes to CMB science, the US decided not to take the road less travelled. It will certainly make a difference, but not for the better.
Chanda’s week
What I’m reading
I have been enjoying Niayesh Afshordi and Phil Halper’s Battle of the Big Bang: The new tales of our cosmic origins.
What I’m watching
I have been rewatching DC Universe films that feature Harley Quinn, a personal favourite.
What I’m working on
I have been trying to capture good images of the Andromeda galaxy from my backyard.
Topics:
- cosmology/
- astrophysics/
- space
