WMAP and Our Flat Universe 229
universes, its expansion began to accelerate due to the effects of the cosmological constant.
It continues to expand perpetually (see Figure 5-12 for a full timeline).
Astronomers now know that for our universe, W = 1. This means that our universe
is flat. But how do we know that, and why doesn’t the model for our universe look like
model b, the flat universe, which shows a slowly decreasing rate of expansion?
WMAP and Our Flat Universe
The majority of the data that we use to verify the curvature of the universe comes from
images and other information taken from the Wilkinson Microwave Anisotropy Probe, or
WMAP. WMAP measured differences in the sky’s temperature, as seen in Figure 5-13, via
cosmic microwave background radiation (CMBR) (refer to Chapter 3). For almost 400,000
years after the Big Bang, the entire universe was an opaque, hot, and dense fog of photons
and baryons. Eventually, this fog cooled enough to start forming atoms, thus making the
fog more transparent (meaning visible light could penetrate it). The CMBR that we observe
is made of those photons from the early universe, although they have redshifted from vis-
ible wavelengths to microwave wavelengths. Agreeing with the cosmological principle (which
states that no place in the universe is special and that the universe appears to be the same
from every direction), the CMBR shows the homogenous nature of the universe in that the
temperature of the universe is observed to be 2.725 Kelvin from all directions, with only a
0.003 Kelvin difference between the hottest and coldest parts of the sky. WMAP was able
to show us those almost indiscernible differences in temperature. By understanding these
differences, scientists can determine all sorts of information, including how it is that our uni-
verse has very little curvature.
Average distance between galaxies
= 1
Λ > 0
= 0
< 1
= 1
> 1
Figure 5-11: Changes in the Friedmann-Lemaitre-Robertson-Walker universe models over time
230 Chapter 5 Our Ever-Expanding Universe
Open Flat Closed
Figure 5-13: The varying shades in WMAP’s readings represent the varying temperatures across
the universe—WMAP’s actual measurements are consistent with a flat universe.
Scale factor
expansion era
Accelerating expansion
Planck length
End of inflation era
Planck time (10
Planck era
Big Bang
Inflation era
Figure 5-12: A timeline of an essentially flat universe

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