Question of the week: What is the strongest physical evidence for the big bang?
My answer: I frequently get asked this question by people who lack formal education in physics and astronomy. I tell them that the strongest physical evidences for the big bang creation model are technical and need careful explanation for laypeople to understand and appreciate. I have provided those explanations with extensive documentation in my book, The Creator and the Cosmos, 4th edition.1 I find, however, that most people who ask me this question are satisfied with me giving them one or two visual demonstrations that we must be living in a universe that has been continuously expanding from a big bang beginning.
A big bang universe is one in which the universe starts off near infinitely hot and cools down from that initial hot state as the universe continues to expand throughout its history. Astronomers possess the tools to measure the present temperature of the radiation left over from the cosmic creation event and also the past temperatures at different look back times. (The look back time is the time it takes light to travel from a galaxy—in which the temperature of the radiation remaining from the cosmic creation event is measured—to the astronomer’s telescope.) Figure 1 shows the expected cooling curve of a big bang universe that is 13.8 billion years old overlapped with actual measurements astronomers have made of the present and past temperatures of the radiation left over from the cosmic creation event. The most accurate of the past measurements is the one in the middle of the cooling curve. This measurement fits the curve predicted by the big bang creation model to better than the thickness of the plotted curve.
Figure 1: Evidence of Cooling from the Big Bang Creation Event
Credit: Hugh Ross
Hubble Space Telescope images of distant galaxies provide a different visual demonstration that we must be living in a universe that arose from a big bang creation event. The photo at left in figure 2 shows galaxies located 12 billion light years away, meaning that we are seeing them as they were 12 billion years ago. The photo at right in figure 2 shows galaxies 2 billion light years away, meaning that we are seeing them as they were 2 billion years ago. Both images have the same spatial scale. In the photo to the left galaxies are jammed so tightly together that it is common for them to tear spiral arms off from one other. In the photo at right, thanks to the expansion of the universe predicted by the big bang, galaxies have spread so far apart from one another that the ripping off of spiral arms is a relatively rare phenomenon.
Figure 2: Galaxy Separations at Two Different Distances
Image credits: NASA/ESA/ACS Science Team, N. Benitez and H. Ford (JHU), T. Broadhurst (Racah Institute of Physics, Hebrew University, M. Clampin and G Hartig (STScI), G. Illinworth (UCD/Lick Observatory)
Laypeople seem especially impressed that we can actually watch the big bang phenomenon of the universe. These visual demonstrations typically motivate them to examine the even stronger physical evidences.