Douglas Phillips asked Douglas Phillips:

I have a question about your discussion of critical density in the afterward to Quantum Void. You explain that to the extent we can measure it, the amount of mass in the universe is exactly the amount needed to cancel the expansive force. So why then is the universe expanding? Wouldn’t it just be at a “standstill”? John Toto (via email)

Douglas Phillips Great question! Here’s the short answer:
The Big Bang started with a period of inflation where physical space grew faster than light speed. Ever since, the universe has been expanding and the pull of gravity hasn’t been enough to slow it down. Yet. Picture a runaway train car careening down the track. But is the track level? Does it bend downhill? Or uphill? What ultimately happens to the train car depends a lot on the track.

The long answer, is… longer. 😊
American astronomer Edwin Hubble discovered expansion in 1925 while studying variable stars at the Mount Wilson Observatory in Pasadena, CA. Since Hubble’s discovery, astronomers have wondered: will it expand forever? If so, the universe will get colder, darker, and larger over trillions of years (and infinitely more). But there’s a flip side to that thought: could gravity (which pulls all matter together) slow this expansion over time? That’s the uphill train track metaphor. Perhaps gravity could even reverse the expansion, resulting in the Big Crunch? As recently as the 1990s, no one knew, but they were clever enough to figure out how to answer the question. It boils down to accurately measuring the Hubble constant (H, a measurement of expansion) and the gravitational constant (G, which Sir Isaac Newton derived). Insert those two measurements into Alexander Friedmann’s equation for Critical Density ( ρc = 3H2 / 8πG ), and you’ve computed a theoretical density of matter where the push of expansion exactly balances the pull of gravity. If the overall density of our universe happens to be this value, the expansion that began with the Big Bang will slow and eventually stop altogether (the level track). No Big Crunch (which requires gravity to win the battle), but no infinite expansion either.

So, does our universe fit this concept? Measurements of the cosmic microwave background made by the WMAP and Planck missions say, yes, it does. Actual density = critical density = 9.47 x 10-27 kg/m3, which is about 5 atoms of hydrogen per cubic meter across the whole universe. But there’s a caveat: dark energy. No one really knows what dark energy is (or even if it really exists), but astronomers agree there is something out there that seems to be accelerating the expansion rate. The Hubble constant might not be a constant, or if it is, there are other factors involved in the expansion of the universe. Critical density tells us the universe is flat – there’s a precise balance between the expansion (positive energy) and the gravitational force (negative energy), and because of this balance we can be reasonably sure that the universe sprang from nothing. But critical density isn’t enough to tell us if the expansion will halt or continue forever. We need to better understand dark energy, and we don’t have that understanding today.

So, going back to the runaway train metaphor, we find ourselves careening down what we measure to be a flat track. We know how we got into this situation (some fool lit a hundred sticks of dynamite back in the train tunnel, and out we popped) but we don’t know for sure what’s ahead. Given a flat track, we ought to slow down and stop, but at the moment we seem to be accelerating. So what’s going on? Astronomers are working on it. Stay tuned for the exciting conclusion!

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