How did it happen?
While the cause of inflation is uncertain, the leading theory postulates it occurred because of an extremely high-energy state in the early universe. This state was left over after the separation of the fundamental forces (gravity, electromagnetism, and the weak and strong nuclear forces), which were unified when the universe began. The extreme amount of energy left the universe unstable, and as it settled to a lower, more stable state, the surge of released energy caused the remarkable expansion.
Inflation also explains why the universe is so uniform. If you gaze skyward with a sensitive radio telescope, you’ll encounter the soft glow of the first light emitted — the cosmic microwave background (CMB) radiation — spread almost evenly across the sky in every direction. The fact that this radiation could be nearly uniform in such disparate regions long puzzled cosmologists because the temperature of the universe — already 90 million light-years across — couldn’t have equalized to a uniform value by the time the light was emitted. Guth’s theory neatly showed that a virtually uniform universe, post-inflation, would create the signature of the CMB almost exactly.
Now, the CMB isn’t perfectly smooth. Across the universe, there are small deviations, measured as slight differences in temperature, typically just a millionth of a degree. These changes originated with tiny quantum fluctuations in the early universe and would eventually become the backbone of galactic structure in the cosmos.
Back in the brief moments before inflation, the universe was hot and dense. While energy was evenly distributed, on a microscopic scale, quantum mechanics dictated that random spots have slightly higher or lower densities. As inflation shook the universe, those tiny over- and under-densities supersized into larger regions. And as the universe evolved, the denser regions became galaxy clusters as they slowly accumulated more and more mass.
“If we take a galaxy survey, where we can map out the positions of where galaxies are and how they’re clustered together across the sky, there’s a remarkable agreement in the distribution of galaxies after billions of years and the prediction that you get right at the beginning of the universe,” says Jo Dunkley, a cosmologist at Princeton University. “We can trace from the early times to the later times, and we’re seeing a very consistent story.”
Today, this consistency between the variation in the CMB and the large-scale structures of galaxies across the universe is one of the few pieces of observational evidence for inflation found so far.
“With inflation, the number of observational quantities we have is limited,” says Marc Kamionkowski, a professor and theoretical physicist at Johns Hopkins University. “Therefore, it limits the level of detail at which the model or theory can be specified.”