Gravitational waves and inflation are not considered two contradicting theories of physics. In fact, they are interconnected concepts that play important roles in our understanding of the early universe.
Let's start with inflation. Inflation is a theory that suggests the universe underwent a rapid and exponential expansion in the very early stages of its existence. This theory helps explain certain observed features of the universe, such as its overall uniformity on large scales and the absence of certain types of relics predicted by other models. Inflation also provides a mechanism for the formation of the initial density fluctuations that later gave rise to the structures we see in the universe today, such as galaxies and galaxy clusters.
Gravitational waves, on the other hand, are ripples in the fabric of spacetime itself, caused by the acceleration of massive objects. According to Einstein's theory of general relativity, accelerating masses can produce these waves that propagate outward at the speed of light. Gravitational waves were directly detected for the first time in 2015, confirming their existence and opening up a new way to observe and study the universe.
Now, the connection between gravitational waves and inflation comes from the fact that during inflation, the rapid expansion of the universe generates gravitational waves as a byproduct. Quantum fluctuations in the early universe get stretched across cosmic scales during inflation, leading to variations in the gravitational field, and thus, the production of gravitational waves.
The detection of primordial gravitational waves, which are gravitational waves generated during inflation, would provide strong evidence for the inflationary theory. These primordial gravitational waves imprint a distinct signature on the cosmic microwave background (CMB), the radiation left over from the early universe. The B-mode polarization pattern in the CMB is one of the key signatures that scientists are searching for, as it would indicate the presence of primordial gravitational waves.
In summary, gravitational waves and inflation are not contradictory theories. Instead, they are connected in the sense that inflation predicts the generation of gravitational waves during the early universe. Detecting these primordial gravitational waves would provide crucial support for the inflationary model and deepen our understanding of the early stages of the universe.