One of the most profound questions in cosmology concerns the overall geometry of the universe. Is it flat, like a sheet of paper extending infinitely? Or is it curved, either positively like a sphere or negatively like a saddle? The answer to this question has significant implications for the universe’s ultimate fate. Fortunately, observations of the Cosmic Microwave Background (CMB) radiation provide compelling evidence for a flat universe.
The Cosmic Microwave Background is the afterglow of the Big Bang, the residual heat from the early universe when it was incredibly hot and dense. About 380,000 years after the Big Bang, the universe had cooled enough for neutral atoms to form, allowing photons (light particles) to travel freely. This “first light” is what we observe today as the CMB, a faint microwave radiation permeating the entire cosmos.
The CMB is not perfectly uniform; it exhibits tiny temperature fluctuations, or anisotropies, across the sky. These minuscule variations represent the seeds of the large-scale structures we see today, the slight density differences in the early universe that eventually grew into galaxies and galaxy clusters. The precise pattern and size of these temperature fluctuations are incredibly sensitive to the universe’s geometry.
Imagine sound waves propagating through the early universe’s hot plasma. These sound waves left their imprint on the CMB as temperature variations. The characteristic size of these “sound horizons” at the time of the CMB’s emission is well understood. By measuring the apparent angular size of these features on the CMB sky, astronomers can effectively perform a cosmic geometry experiment.
In a flat universe, the angles of a triangle add up to 180 degrees, and light travels in straight lines. In this scenario, the observed angular size of the sound horizons in the CMB matches predictions. However, in a positively curved universe, light rays converge, making distant objects appear larger. Conversely, in a negatively curved universe, light rays diverge, making distant objects appear smaller.