Mysteries in Science: When did Inflation Occur?

Jun'ichi Yokoyama

(Professor, Research Center for the Early Universe (RESCEU))

The universe we live in today is globally uniform over an observable span of 97 billion light years and is essentially flat with a very large radius of curvature. Such uniformity and flatness are curious properties that cannot be reconciled with the Big Bang cosmology, which states that the universe began in a small, hot state and has continued to expand ever since, albeit at a slowing rate under the influence of universal gravitation.

Cosmic microwave background radiation ― a kind of living fossil that allows us to directly visualize the state of the universe after it became transparent (380,000 years after its creation), filled with electrically-neutral hydrogen atoms formed from the components of an ionized plasma ― is observed to be isotropic to an accuracy of 4 orders of magnitude on the present horizon, which is far beyond the horizon (the upper limit of the distance that information can be transmitted at any given time) at that time. This indicates that the Big Bang cosmology is inconsistent with causality, a major theoretical flaw.

The solution to this problem is an inflationary cosmology, which hypothesizes in the young universe a period of swiftly accelerating expansion, thereby inflating the horizon rapidly. This also explains the finding that the universe becomes a flat space, because the sudden inflationary expansion stretches out any irregularities that existed before. Inflation stretches out not only these previous irregularities, but also the quantum fluctuations that continue to be generated on a microscopic scale, so it renders the entire universe uniform and isotropic, and also plays the role of introducing minute fluctuations of approximately the same size at each scale. This is the origin of the temperature fluctuations on a scale of one part in 10⁵ observed in the cosmic microwave background radiation and the origin of the large-scale structures of the universe such as galaxies and clusters of galaxies.

To ascertain when inflation occurred, we can try to measure the energy density of the universe at that time, and to do this, we need to detect the long-wavelength quantum gravitational waves that were generated during inflation. These long-wavelength primordial gravitational waves leave traces in the B-mode polarization of the cosmic microwave background radiation, the existence of which can be verified if they can be measured. Similar to the situation for magnetic and electrical fields, there are two types of polarization patterns, outflow-type and rotational, or E-mode and B-mode, respectively.

The B-mode polarization pattern directly communicates information about gravitational waves. The Kusaka Laboratory in the Graduate Department of Physics conducts ground-based observations with the aim of measuring this B-mode pattern. To obtain similar measurements from a platform in space, the LiteBIRD satellite will be launched in 2027, promising to yield results within 10 years. Hence, we may be able to test the model in which the rate of cosmic expansion is hypothesized to increase gradually during inflation, which we discovered when we proposed the G-inflation theory, a generalized framework for single-field inflation theory.

Addressing the question from another angle, the Ando Group is conducting basic research to directly detect quantum gravitational waves, via the DECIGO project. If this project succeeds, we may well ascertain when the Big Bang occurred after inflation.

Figure
Left: Conceptual design of LiteBIRD (Source: JAXA Institute of Space and Astronautical Science
Right: Expected B-mode measurement sensitivity of LiteBIRD (Source: LiteBIRD team)

― This article is from the "Mysteries in Science" series in The Rigakubu News ―

Translated by the Office of Communication

― Office of Communication ―