One of the most well-known and most viable theories about the origins of the universe is the Big Bang theory – the idea that we came to be in a massive explosion 13.8 billion years ago. But, of course, it's still called a theory for a reason. It doesn't answer all the questions about how we came to be.
Hoping to build on the Big Bang, American physicist Alan Guth proposed the theory of cosmic inflation in 1981. Since then, cosmic inflation has become an important concept in the study of the early universe — to the point that it’s currently integrated into the Standard Model of Cosmology (SMC) that embraces the Big Bang theory.
What is cosmic inflation?
The theory of cosmic inflation proposes a period of extremely rapid exponential expansion of the universe during its first few moments (starting at 10−36 seconds after the Big Bang singularity, to be exact).
Astrophysicist Ethan Siegels explains the term “exponential expansion” as follows:
“If your Universe were filled with radiation, it would expand like the square root of time: the distance between you and this particle scales as ~t1/2.
If your Universe were filled with matter, it would expand like time to the two-thirds power: the distance between you and this particle scales as ~t2/3.
But when your Universe inflates, space expands exponentially: like ~eHt, where H is the expansion rate of the Universe."
Simply put, the universe developed from a tiny speck (hypothetically containing the entirety of space) into something much, much bigger. Cosmic inflation explains how this occurred uniformly in spite of the rapidness of the process.
Because there are no known particles that could be responsible for the inflationary process, physicist Alan Guth proposed a hypothetical scalar field called "the inflaton field". The inflation is a quantum field that permeates all of space and time and contains energy that exists even in a vacuum.
The inflaton quantum field starts off with a large amount of vacuum energy. According to inflaton theory, after the period of expansion, the inflaton decays, transforming into regular matter and radiation.
What problems does cosmic inflation resolve?
As mentioned above, the theory of cosmic inflation came to resolve a few “buts” of the Big Bang cosmology. You can't fully understand the theory of cosmic inflation without taking into account the issues it’s meant to solve —so here they are.
The flatness problem
Alan Guth started developing the theory of cosmic inflation after attending a lecture by Robert Dicke about the flatness problem of the universe.
The flatness problem basically questions why the universe density is so close to the critical density. The critical density is the average density of matter required for gravity to halt the expansion of the universe. A universe with the critical density is said to be flat. However, according to Big Bang cosmology, the curvature of the universe should grow with time.
Between 2001 and 2010, NASA’s unmanned spacecraft Wilkinson Microwave Anisotropy Probe (WMAP) studied the cosmic microwave background and produced data indicating that the universe is, in fact, relatively flat. But the process of cosmic inflation would account for the near flatness that WMAP detected.
The horizon problem
The horizon problem is also referred to as the homogeneity problem because it questions why the universe is uniform in all directions —a property called isotropy.
Many distant regions of space in opposite directions are so far apart that, assuming a standard Big Bang expansion, they could never have been in causal contact with each other. This is because the travel time (for light) between them exceeds the age of the universe. Yet research has demonstrated that the cosmic microwave background temperature is uniform throughout the universe - this tells us that these regions must have been in contact with each other in the past.
How can extremely distant zones in space have their matter and radiation distributed so evenly when they’re too far apart to have been in casual contact with each other?
Again, cosmic inflation is the answer. Its integration into the Big Bang theory implies that there was a stage in the early universe when everything was much closer. For the universe to get “inflated” or expanded, it must’ve been “deflated” or contracted first.
The distant zones of space obtained the same temperature while interacting with each other before the inflationary process began, and kept it after it finished.
The magnetic monopole problem
Big Bang cosmology predicts that a very large number of heavy, stable "magnetic monopoles" (hypothetical particles that consist of an isolated magnet with only one magnetic pole) should have been produced in the early universe. However, magnetic monopoles have never been observed, so if they exist at all, they are much rarer than the Big Bang theory predicts.
Why haven’t magnetic monopoles been observed in the universe as predicted by the Big Bang theory?
Inflation allows for magnetic monopoles to exist, but only if they were produced before the period of inflation. During inflation, the density of monopoles dropped exponentially, so their abundance also dropped to undetectable levels.
Proofs and detractors
The theory of cosmic inflation has been criticized by a number of scientists, including one of its co-founders, American physicist Paul Steinhardt.
In 2013, Steinhardt and his team analyzed data collected by the Planck satellite and concluded that this seemed to rule out the simplest inflationary models. The more complex ones, however, required more parameters, more fine-tuning of those parameters, and more unlikely initial conditions.
Steinhardt called it “the unlikeliness problem.” He even talked about “bad inflation” and “good inflation.” The first is a period of accelerated expansion that leads to a result that contradicts the observations, while the latter is compatible with them. “Not only is bad inflation more likely than good inflation, but the absence of inflation is also more likely than not,” he explained in a paper titled The Inflation Debate.
The unlikeliness is supported by scientists like British mathematician Roger Penrose who, according to Steinhardt,
“applied thermodynamic principles (...) to count the possible starting configurations of the inflaton and gravitational fields. Some of these configurations lead to inflation and thence to a nearly uniform, flat distribution of matter and a geometrically flat shape. Other configurations lead to a uniform, flat universe directly—without inflation. Both sets of configurations are rare, so obtaining a flat universe is unlikely overall. Penrose’s shocking conclusion, though, was that obtaining a flat universe without inflation is much more likely than with inflation."
The theory of cosmic inflation has also been accused of being too flexible, meaning that it can pave the way for so many outcomes that there is no way to disprove it… or to prove it (another criticism aims at the fact that cosmic inflation can’t be evaluated using the scientific method).
But the data seem to indicate that cosmic inflation doesn’t only tie up the loose ends of the Big Bang theory, it may be correct - at least in some form. It has become widely accepted by many physicists because it has also led to new predictions which have been observationally confirmed.
For example, the theory of cosmic inflation predicted a post-inflation temperature of the universe of ~1019 GeV in the Planck scale —a value that was subsequently validated by studies of the cosmic microwave background.
The theory also predicted the existence of super-horizon fluctuations (density variations in the early universe which formed the seeds of all structure in the universe). These fluctuations have been confirmed by polarization data collected by the Planck satellite and NASA’s WMAP.
As Ethan Siegel says, “Inflation has literally met every threshold that science demands, with clever new tests becoming possible with improved observations and instrumentation. Whenever the data has been capable of being collected, inflation's predictions have been verified.”