“So if the Universe isn’t in a steady state and it’s not heading for a Big Crunch, I guess it’s getting bigger forever, huh?”

“Careful, Jeremy, the Universe expansion could maybe reach a stopping point if it happened to hold exactly the right amount of mass‑energy. The expansion could just stop when forces balance out.”

“What forces, Mr Moire? There’s gravity pulling everything together so what’s pushing them apart?”

“That is an excellent question, one that we don’t yet have an answer for. We’re about where Newton was with gravity. There was a lot of observational evidence, he had a name for it and knew how to calculate its effects, but he didn’t know how it worked. That’s us with Einstein’s Cosmological Constant.”

“Observational evidence — we can actually see things accelerate?”

“Not any one object speeding up. Human lifetimes are too short to measure acceleration in galaxies a hundred thousand lightyears across. No, we use the same strategy that Hubble used — measure many galaxies at different distances from us and graph recession speed against distance. During the century since Hubble we’ve greatly improved our estimates of astronomical speeds and distances. Dividing the known speed of light into a galaxy’s measured distance tells us time since it emitted the photons we see. Our findings confirm Hubble’s general conclusion — on average, older photons come from galaxies that fly away faster. Hubble thought that the relation was linear but our fine‑tuned numbers show otherwise. The data says that after the first few seconds the Universe stretched at a steady rate for only the first ⅔ of its life. The stretch has been accelerating since then.”

“Why wasn’t it accelerating since the beginning? Did someone cut in the afterburner?”

“More like turned one off. The evidence and theory we have so far indicate the Universe has seen a succession of phases dominated by different processes. You’ve probably heard of inflation—”

“Have I? You should see what they want for a burger these days!”

“Not that sort of inflation, but I know how you feel. No, I’m referring to cosmic inflation, very early in the Big Bang sequence, when the Universe expanded by a factor of 10^{26} within a tiny fraction of a second. It was driven by enormously powerful radiation‑linked effects we don’t understand that finally ran out of steam and let lower‑energy processes take over.”

“How’d that happen?”

“We don’t know. The general principle is that one process so dominates what’s going on in a phase that nothing else matters, until for some reason it stops mattering and we’re in a new phase with a different dominant process. The early Universe was controlled by radiative processes until things cooled off enough for particles to form and persist. That changed the game. Gravity dominated the next 8 billion years. Particles clumped together, atoms then dust then solar systems into larger and larger structures with bigger spaces between them. About 5 billion years ago the game changed again.”

“So early on there weren’t even atoms, huh? Wow. What was the next game‑changer?”

“Thanks to Einstein and Friedmann’s work we’ve got at least a guess.”

“Friedmann?”

“Alexander Friedmann. He was a Russian physicist, used Einstein’s General Relativity results to derive three equations that together model the dynamics of the overall scale of the Universe using just a few estimates for current conditions. His equations give acceleration as the difference of two terms. The positive term is simply proportional to Einstein’s Constant. The negative term depends on both average mass density and pressure. Take a moment to think.”

“Umm… Positive is acceleration, negative is deceleration, density and pressure go down … If the negative term gets smaller than the positive one, acceleration increases, right?”

“It does, and we think the constant term has been increasingly dominant for 5 billion years. Something else to consider — the equation’s result is in terms of scale change divided by current scale. What’s it mean if that ratio’s a positive constant?”

“Change by a constant positive percentage … that’s exponential growth!”

“I thought you’d recognize it. Einstein’s Constant implies the scale of the Universe grows at an exponentially accelerating rate. We’re now in the Cosmological Constant phase.”

~~ Rich Olcott