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BUBBLE SPEED
In “The Quantum Bubble That Could Destroy the Universe,” Matthew von Hippel discusses vacuum decay, in which a change in the Higgs field would create an expanding quantum bubble that would transform the laws of physics within it. He describes an essay proposing that “humanity could survive vacuum decay by riding the expansion of space itself” and states that in this scenario, “as space pulls apart faster and faster, distant places will be carried apart faster than the speed of light.” Please explain this in relation to Albert Einstein’s theory that the speed of light cannot be exceeded.
KEN T. KERN MINNEAPOLIS
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Do theorists envision a multiverse of quantum bubbles with the edge of each “universe bubble” being bordered by numerous other universe bubbles?
KEVIN ROBERTSON BEACHWOOD, OHIO
VON HIPPEL REPLIES: Regarding Kern’s query, although Einstein’s special theory of relativity does not allow an object within spacetime to go faster than light, his general theory of relativity does allow spacetime itself to expand faster than light. Our universe appears to be doing so now, which means that over time more and more stars are disappearing outside of our view. All particles and fields within spacetime, including the Higgs field, are still limited to traveling no faster than the speed of light.
To answer Robertson: The Higgs field doesn’t appear to have more valleys to explore, according to our current understanding. But more mysterious fields that govern the values of the fundamental constants or the expansion rate of the universe might. So this idea is related to some theorists’ speculations about the existence of a multiverse.
SUNNY TALE
My first reaction to Rowan Jacobsen’s “Can Sunlight Cure Disease?” was a big sigh. Had my most trusted source for scientific literature sunk this low? Two things changed my mind, however: reading the article and recalling folktales about the healing powers of sunlight that my dad told me while I was growing up.
Dad grew up on a farm in Norway. People in his family were exposed to and passed down stories from generation to generation. One was about the healing power of sunlight: a severely injured person or animal could often recover by sitting in the sunlight for days on end. I can imagine that story was useful during the Viking Age, when medicine was very rudimentary. My wife trained as a nurse. She tells me that in the 1930s, before antibiotics to treat the disease were available, a very common therapy for tuberculosis patients was to spend time in the sunlight every day.
GLENN ANDERSEN BUENA PARK, CALIF.
FIRST LIGHT
“Cosmic Dawn,” by Rebecca Boyle, says that about 380,000 years after the big bang, things cooled down enough to let hydrogen and helium nuclei grab free-flying electrons and form electrically neutral atoms. This process of recombination allowed photons to flow through the universe and formed the cosmic microwave background (CMB). But Boyle then says the neutral atoms absorbed the photons that remained, and darkness persisted for the next 50 million years. Why did these atoms not absorb the CMB?
The article also states that the reionization process, which occurred later and stripped electrons off the neutral atoms, made the universe transparent to light again. So it seems that recombination formed neutral atoms, which allowed light to flow, whereas reionization eliminated such atoms, which also allowed light to flow. That is rather mysterious.
DICK WORSHAM BALTIMORE
Why do we see the original pattern of the CMB? Shouldn’t it have changed as the universe evolved between the cosmic dawn and now? Also, the relatively newer photons that were generated by stars must still be around, presumably stretched to various wavelengths via redshift. So why can’t we see the evolving universe at all times since reionization?
BUD SIMRIN VIA E-MAIL
BOYLE REPLIES: To answer Worsham: During recombination, photons were created by the big bang. During reionization, the source of flowing light was instead the first population of stars.
In the epoch of recombination, everything began cooling down enough to form atoms, starting around 380,000 years after the big bang. Neutral hydrogen atoms began to pervade the universe, meaning photons could travel freely. The light of the CMB represents the first energetic photons that shone through the hot early universe.
After the CMB light, darkness fell because there were no more photons—until the first stars were born. As bright objects began emitting light, meaning radiation, the universe once again completed its transition from neutral to ionized. Stars and galaxies grew apace, and the universe has been transparent ever since.
Regarding Simrin’s questions: You can think of the primordial universe in its first 380,000 years as a kind of fog: electrons, protons and neutrons were densely packed together, so there was no “room” for light to flow. During recombination, the fog lifted. We still can’t see through the fog, but we can see where it was by tracing the CMB’s signal. We can see the end of the fog, and then there’s a very long period after it ends but before the first stars and galaxies ignite that is still invisible to us: the cosmic dark ages. For now the CMB is the oldest light we can access, and a huge gap remains between it and the light that followed.
These echoes of the big bang did change and evolve as the universe grew and expanded. The CMB photons have a blackbody spectrum; they are opaque and nonreflecting. Today they are extremely cold, barely above absolute zero, which means they are visible only in the microwave-frequency range of the electromagnetic spectrum. Blackbody photons have “mixed” with other free-floating photons and electrons (and nuclei), but sensitive radio instruments allow us to suss out the background CMB signal, which is like a cosmic wallpaper, underlying everything.
ERRATA
“The Social Lives of Mitochondria,” by Martin Picard, should have said that about 1.5 billion years ago, atmospheric oxygen was already abundant thanks to cyanobacteria. Additionally, the article could have better clarified that mitochondria catalyze the first step in making steroid hormones.
“We Probably Aren’t Alone,” by Sarah Scoles [September], should have said that the Mariner 4 spacecraft flew by Mars in 1965.
