The South Pole Telescope, which CMB-S4 could use to measure changes in temperature and polarization of microwave light across much of the sky.
Brad Benson, University of Chicago, Fermilab
Poem by Robert Frost The road has not been taken begins with the words “Two roads diverged in the yellow forest, / And, unfortunately, I could not drive both.”
These lines come to mind when I consider the US government's July 9 public letter stating that will no longer support the CMB-S4 project. CMB-S4, short for Cosmic Microwave Background-Stage 4, was intended to be a next-generation multicontinental telescope that would give all of humanity unprecedented insight into the earliest light ever to fly freely through space.
In the beginning, the Universe was filled with a dense mixture of particles and plasma. The plasma was so dense that particles of light, photons, could not travel very far without bumping into something. In addition to being thick, the stew was very hot, which prevented things like atoms from forming. It was only after the universe experienced cosmic inflation, in which spacetime rapidly expanded for almost a tiny fraction of a second, that it cooled enough to form the first hydrogen atoms. Once such a cluster became possible, the photons had room to maneuver and began to fly through space-time.
61 years ago, humanity first learned that we were inundated with these photons, which became known as cosmic microwave background radiation. What appeared to be little background noise in the radio signal turned out to be a messenger from early space. For decades, we have carefully studied these photons: their wavelength (and associated temperature), their intensity, and how they vary across space.
The CMB is literally a goldmine, in the sense that it provides a lot of information about where everything we see comes from, including the stars whose explosions produce gold. If we scan the entire sky and look at the temperature associated with photons, we see small changes in temperature. Their placement is random, but the size of the variations is the same for everyone.
Our best cosmological theory tells us that these fluctuations are the result of small quantum changes in the amount of matter present at any given location at the moment the photons were released. Places where there was a little more of it were essentially the starting point of gas that gravitationally accumulated into protostars, which became stars, which clustered into what eventually became galaxies. So these small changes in CMB are the start of us.
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The abandonment of the project is part of a reckless US abandonment of global scientific cooperation.
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Perhaps the most important measurement of the CMB we have made is characterizing how these temperature changes correlate with the physical scale. We can ask how much of the change is due to effects on larger or smaller scales, knowing that some physical phenomena occur over long distances and others over shorter ones. In other words, various moments of cosmological history are imprinted on the cosmic microwave background radiation.
For example, we can “see” when the Universe became transparent to matter – at that first moment of hydrogen formation, a moment 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 recorded in SMB.
CMB-S4 was supposed to be the next step in unlocking all the lessons CMB can teach us. One of the main goals was to search for evidence of the existence of primordial gravitational waves – ripples in space-time caused by cosmic inflation. “Inflation” is actually a class of models, and we know that in general they all give the correct physics for our Universe. But we're still not sure about the details. Since inflation occurred in the earliest moments of the universe, gravitational wave imprints on the CMB are likely to be the best way to distinguish inflationary patterns.
The end of government support for CMB-S4 is like sticking a stick in the wheel of your own bicycle: we were flying, joyfully exploring space, and now we have been thrown out completely. The impact will be felt throughout the world. Historically, the United States has invested more in cosmological science than most countries, and this is one of the reasons why students from all over the world come to the United States to study. Data from US-funded experiments also often become a global resource, so the abandonment of this project, which already seemed likely under the previous presidential administration, is now part of the US's reckless abandonment of global cooperation.
Frost ends his poem by recounting his choice of road: “I took the one least traveled, / And that made all the difference.” It is unfortunate that when it came to CMB science, the US chose not to take the path less traveled. This, of course, will change the situation, but not for the better.
Chanda week
What am I reading
I really enjoyed the songs by Niayesh Afshordi and Phil Halper. The Battle of the Big Bang: New Tales of Our Cosmic Origins.
What am I watching
I've been re-watching the DC Universe movies starring Harley Quinn, which are my favorites.
What am I working on
I was trying to get good pictures of the Andromeda Galaxy from my backyard.
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