The Story of Warm Little Inflatons and Cold Dark Matter

A fraction of a second after birth, before his eyes were even open, my son was pooping. Now a six-year-old, he tells the story of his first act proudly whenever the subject of babies comes up. We laugh at the memory, and the event is documented in photographs and highlighted in his baby book. It’s part of his story.

Without witnesses, photos, and written records, the universe’s first few fractions of a second are much harder to unravel. Scientists look at the current state of this 13.8 billion-year-old and try to work backwards, creating models that begin somewhere, somehow, and bring us here. The successful models make predictions that match astronomical observations, indirect records like the cosmic microwave background, and the physical laws of the universe. Modern cosmology is a well-established field, but many unknowns remain.

A representation of the evolution of the universe over 13.77 billion years. The cosmic microwave background (detected by the WMAP space probe, far right) has traversed the universe largely unimpeded since it was emitted around 300,000 years after the big bang.     If the warm little inflation scenario is correct, this background should contain temperature fluctuations that can be observed with next-generation detectors. Image credit: NASA / WMAP Science Team.

Two of the biggest puzzles in modern cosmology are dark matter and inflation, according to Luís Ventura, a cosmologist at the University of Aveiro (UA) in Portugal. Dark matter is a hypothetical kind of mass that is abundant in the universe but doesn’t interact with light. Inflation is a well-motivated hypothetical phase in the evolution of the universe during which the universe rapidly expands. Dark matter and inflation are consistent with observations and help explain the universe as it appears now, but neither has been confirmed and fundamental questions remain: What is dark matter and where did it come from? Why did the universe rapidly expand?

Last month Ventura and João G. Rosa, also from UA, published theoretical research in the American Physical Society’s journal Physical Review Letters that offers a unified story of dark matter and inflation. They show that dark matter naturally emerges if the universe experienced a “warm little inflaton scenario” just after formation, instead of the more well-accepted inflation scenario in which the universe rapidly cools as it expands.

The story of inflation goes something like this. The infant universe was full of particles called inflatons (don’t worry, no one really knows what they are). Like excited electrons, the inflatons were in a high-energy state, which led to a net repulsive force that caused the universe to rapidly expand and cool. After inflation, the inflatons decayed into the matter and radiation particles we know today, causing the universe to warm up again.

The warm inflaton scenario, proposed by Arjun Berera, has the same characters but a different twist. This time, the inflatons release a fraction of their energy as heat. They still cause the universe to rapidly expand, but this heat keeps it from supercooling. Warm inflation is appealing, because it doesn’t require a reheating period after inflation to explain why the universe looks the way it does.

Warm inflation is in line with cosmological observations, but for several years the details were problematic. In order for the scenario to work, inflatons had to have released heat slowly. This constraint caused the theory to get really complicated, so the simpler cold inflation scenario gained traction. However, three years ago, Rosa, Berera, and a couple of colleagues published an article in Physical Review Letters showing that warm inflation does work well and simply if the inflatons follow certain symmetry principles (here’s the Physics Magazine synopsis), and they called this the “warm little inflaton scenario.”

In this latest research, Ventura and Rosa go a step further and show that dark matter is a natural result of the warm little inflaton scenario. They explain it this way, “What we have now realized is that the same symmetries that guarantee a warm inflation period also prevent the complete decay of the inflatons into ordinary matter particles and radiation after inflation.” In other words, in the warm little inflaton scenario, inflatons can decay into matter and radiation only during inflation, not after. Any that decayed during inflation would give us the matter and radiation we see today. And—here’s the new result—any that didn’t decay would still be around today!

According to the warm little inflaton scenario, inflatons emit heat during inflation (red dot) and become cold dark matter over time (blue, then gray dot). Image credit: Galaxy Cluster MACS J0717 image (far right) by NASA, ESA, CXC, C. Ma, H. Ebeling and E. Barrett (University of Hawaii/IfA), et al. and STScI; illustration by Luís Ventura.

Inflatons only interact with matter and light when the temperature of the universe is much higher than it is now, says Ventura, so they are “ideal candidates for the mysterious dark matter that permeates all known galaxies and galaxy clusters.”

Of course, just because a scenario is possible doesn’t mean it actually happened. To investigate the likelihood that it did, we need physical evidence. Luckily, that might be available soon. According to the researchers, warm inflation would lead to distinctive temperature fluctuations in the cosmic microwave background and influence the abundance of elements like helium and deuterium in the universe. These predictions could be tested against data from a number of upcoming projects, including the Large Synaptic Survey Telescope (LSST) (under construction, should be fully operational in 2022), the next-generation cosmic microwave background experiment CMB-S4 (in the planning stages), and the Euclid space telescope (under construction, launching in 2022).

Cold or warm, why does it matter? To me, finding out the real story of inflation matters for the same reason my son’s birth story matters. Not because it affects daily life or lower the price of iPhones, but because it is part of my story, his story, humanity’s story. It will give us a more complete understanding of the early stages of the universe, shedding light on where we came from, who we are, and maybe even where we’re going.

To answer another way, I’ll offer a quip Ben Franklin is often credited with saying to a cynic who, while watching one of the world’s first hot air balloons rise, wondered what it was good for. “What is the good of a newborn baby?”

Kendra Redmond

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