The cosmic microwave background radiation (CMB) is an important piece of evidence supporting the notion that our universe is flat. The CMB is a faint radiation that permeates the entire universe, and it is considered a relic of the Big Bang. It was first discovered in 1965 and has since been extensively studied and analyzed.
One of the key observations about the CMB is its uniformity or isotropy. Regardless of the direction we look in the sky, the CMB appears to have a very consistent temperature of about 2.7 Kelvin (-270.45 degrees Celsius or -454.81 degrees Fahrenheit). This isotropy suggests that, at some point in the past, the entire observable universe was in a state of thermal equilibrium.
However, the distribution of temperature variations in the CMB is not perfectly uniform. There are slight temperature fluctuations or anisotropies, which are tiny differences in temperature from one region to another. These anisotropies are incredibly important for understanding the shape of the universe.
When scientists measure the anisotropies in the CMB, they find that they are consistent with a universe that is flat on large scales. In cosmology, "flatness" refers to the geometry of space. A flat universe is one in which the geometry follows Euclidean principles, similar to a flat sheet of paper. This means that the angles of a triangle in a flat universe would add up to 180 degrees, and parallel lines would never intersect.
The precise measurement of the anisotropies in the CMB, along with other cosmological observations, such as measurements of the large-scale structure of the universe and the distribution of galaxies, support the idea that our universe is very close to flat. If the universe were significantly curved, either positively (closed) or negatively (open), the patterns observed in the CMB would be different from what we observe.
It's important to note that the term "flat" in cosmology refers to the overall large-scale geometry of the universe, not to its local properties or the presence of three-dimensional objects. The idea of a flat universe does not mean that space is literally flat like a sheet of paper, but rather that it has a specific geometry that can be described as flat according to mathematical models.
In summary, the uniformity and anisotropies observed in the cosmic microwave background radiation, along with other cosmological measurements, provide strong evidence that our universe has a flat geometry on large scales.