If the Universe is expanding, and the expansion is accelerating, what does that tells us about the cause of the expanding Universe?
Even though it’s been nearly 100 years since its initial discovery, the expanding Universe still puzzles almost everyone who thinks about it. What causes the Universe to expand? Why did it start off expanding in the first place? What determines the rate of expansion, and how does that rate translate into something that we can actually go and observe? What do we mean when we say “the expansion is accelerating,” and why is that such a profound, revolutionary statement that we only started making in the 1990s? And, behind the scenes, what is the ultimate cause of each aspect of our cosmic expansion, and how confidently can we state such things?
There’s a remarkable scientific story to be told here, and even seasoned scientists and science communicators frequently make mistakes when trying to provide answers to these questions. It makes educating the general public, especially young people, a particular challenge for father-of-a-curious-13-year-old Philip Gee, who writes in to ask:
“I read everything you write, absorb 5% and talk to my kids like I’m an expert haha… I so often read this sort of thing:
‘Well, our cosmological model predicts an expanding universe, and as a consequence, the existence of an event that we call Hot Big Bang. Yet, the current state of the expansion is not constant in time, instead is increasing; thus this growing rate in the expansion has to be driven by a different factor, something that wasn’t predominantly acting during the early stages of the Universe or at times where galaxies formed.’
The expansion of space isn’t accelerating right? Well, it is, but not because the Hubble Constant is increasing? Shouldn’t we just be saying: ‘the Hubble Constant, which is a measure of expanding space over time, has to be driven by something?’”
Let’s break down and answer the different parts of this question, because even though it’s complicated, we can come to an understanding by unpacking it one step at a time. Let’s check it out, one step at a time.