\n<\/p>\n
For a fraction of a second after the big bang occurred 13.8 billion years ago, most physicists believe, the newborn universe dramatically ballooned in size, jumping from being smaller than a proton to being bigger than a softball. Such an exponential expansion may seem minor, but it is equivalent to a grape in the palm of your hand swelling to become tens of thousands of times larger than the observable universe. Known as cosmic inflation, this strange, fleeting period is usually considered to have been an expansion of near nothingness because, at the time, most of the universe\u2019s elementary particles had yet to blink into existence. In other words, the standard view of cosmic inflation suggests the universe didn\u2019t really begin as a hot, dense fireball but rather as a cold void that only later reheated into a plasmatic soup of particles by some poorly understood process.<\/p>\n
But a new theoretical study published in the journal Physical Review Letters suggests that inflation may have been warm from the start. In fact, the researchers find, a warm period of inflation that began to populate the universe with matter could have naturally arisen from interactions within physics\u2019 Standard Model, the theory that describes the fundamental forces and elementary particles in the universe.<\/p>\n
\u201cWhat we have shown with this paper is that actually being warm during inflation is extremely generic and extremely simple,\u201d says its lead author Kim Berghaus, a postdoctoral scholar in theoretical physics at the California Institute of Technology. The solution to the problem of cold inflation requires only one unconfirmed type of particle, she says. \u201cIt takes us to this footing of \u2018This may have actually occurred in nature, and we can go look for it,\u2019\u201d Berghaus adds.<\/p>\n
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Inflation itself is a big mystery. Most physicists think it happened in the first 0.00000000000000000000000000000001 second (10\u201332 second) of cosmic time, by which point the universe had expanded by a factor of as much as 1050. The reason anyone takes this mind-boggling idea seriously is that it would explain what the big bang alone can\u2019t\u2014namely, why the universe appears so extremely uniform at very large scales. Studies of the big bang\u2019s afterglow\u2014an all-sky whisper of radiation called the cosmic microwave background, or CMB\u2014show it to be basically the same everywhere. The most straightforward way to account for this preternatural smoothness is a period of inflation that provides time for the baby universe to reach a uniform temperature.<\/p>\n
Meanwhile inflation would also have magnified tiny random fluctuations in the early universe, creating density variations in the primordial plasma. These acted as seeds for cosmic structures; as their gravity glommed together more matter, denser regions would eventually grow to become star-filled galaxies and galaxy clusters.<\/p>\n
As of yet, there is no direct observational evidence for inflation, so sorting out its details is a task for theorists. Initially, physicists envisioned inflation as a cold process that involved energy fields that permeated all of space. It was powered by a field with high potential energy, called the inflaton field, which behaved a bit like a ball rolling down a hill and transforming its potential energy into kinetic energy as it descends. In the beginning, this \u201chill\u201d was gentle, almost flat, and as the inflaton \u201cball\u201d rolled down, the exponentially expanding universe rapidly became less dense. The \u201chill\u201d eventually bottomed out when the expanding universe was diluted to nearly a vacuum. At this point, the rolling inflaton \u201cball\u201d could essentially carom around the resulting \u201cvalley,\u201d unleashing its kinetic energy to create many elementary particles and reheat the universe. Only then did the \u201cnewborn universe as fireball\u201d scenario proceed.<\/p>\n
But precisely how this reheating step would have occurred wasn\u2019t well-understood, says Vahid Kamali, a visiting professor at McGill University and an associate professor at Bu-Ali Sina University in Iran, who studies early-universe cosmology and wasn\u2019t involved in the new research. Physicists wondered if this step was needed or if there was a way for the whole inflationary process to stay warm.<\/p>\n
Arjun Berera, a theoretical physicist at the University of Edinburgh, who was also not involved in the new study, was the first to propose warm inflation in 1995. Cold inflation was in some ways too simple, Berera says. \u201cWhen systems interact, we expect there to be friction and particle production,\u201d he says. \u201cAnd inflation, in the standard picture, didn\u2019t have that.\u201d<\/p>\n
Berera\u2019s first model was initially dismissed. Critics argued that warm inflation would have effectively burned itself out, prematurely churning out interacting particles that would have sapped its potential energy. In the hill analogy, the inflaton \u201cball\u201d would have suddenly plunged down a too-steep slope, bringing the whole process to an abrupt end.<\/p>\n
\u201cThe challenge has always been how to find the model that produces the particles but doesn\u2019t make such a steep hill,\u201d Berera says.<\/p>\n
Berera and his colleagues published a paper in 2016 that found such a model using interactions and fields similar to those known in the Standard Model. Berghaus and her co-authors Marco Drewes of the Catholic University of Leuven in Belgium and Sebastian Zell of Ludwig Maximilian University of Munich take this a step further in their new paper, firmly situating warm inflation in the Standard Model itself. Their calculations show that a feeble interaction between the inflaton field and elementary particles called gluons would be sufficient to warm up inflation. Gluons carry the strong nuclear force, which glues together fundamental particles called quarks to make protons and neutrons.<\/p>\n
\u201cWhat they have done is to make this connection that you can have warm inflation with Standard Model interactions,\u201d says Rudnei Ramos, a theoretical physicist at Rio de Janeiro State University in Brazil, who co-authored the 2016 paper but was not involved in the new study.<\/p>\n
These Standard Model interactions would have heated the inflating universe, sidestepping the complication of needing a subsequent reheating phase. In cold inflation, the initial fluctuations are all inconceivably small and quantum, Berera says, and must later transition to larger, so-called classical interactions during the reheating. The trouble is that no one really understands how that process unfolds. But in warm inflation, \u201cit\u2019s not a big issue,\u201d he says, \u201cbecause they\u2019re already classical.\u201d<\/p>\n
The new model has one key caveat: the particle that creates the inflaton field is not yet known to exist. It would be a very light, chargeless particle called an axion, Berghaus says. Physicists have been on the quest for axions for decades because some possible variants of these particles might constitute most or all of the universe\u2019s dark matter. There are hints that axions might exist, including a faint background glow in space that was detected by the New Horizons spacecraft in 2022. If they do exist, trillions of them should be afloat in every cubic centimeter of the solar system. Projects such as the Axion Dark Matter Experiment, a collaboration of the University of Washington, Lawrence Livermore National Laboratory and other institutions, are currently hunting these particles by using intense magnetic fields to convert them to detectable microwave photons.<\/p>\n
The new model\u2019s reliance on axions suggests that there are two avenues for eventual experimental validation\u2014one via future surveys of the CMB to test some of the model\u2019s predictions and the other via ongoing laboratory-based searches for these elusive particles.<\/p>\n
While the prospect of testability makes this new model very exciting, Kamali says, there is still much more to be done to reconcile it with other theories in cosmology. One example, he says, is that the size of the inflaton field in the new model doesn\u2019t match predictions from string theory. Even so, the allure of thoroughly explaining cosmic inflation within the Standard Model\u2019s well-known tenets is likely to prove irresistible for eager theorists and experimentalists alike.<\/p>\n
\u201cIn our work, there is an opportunity for a discovery that can probe the connection between particle physics and the big bang,\u201d Berghaus says. \u201cBecause our proposed theory connects intimately with the Standard Model, it is testable.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"
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