The discussion centers on the relationship between the flatness problem, as articulated by Bob Dicke, and the expansion rate of the universe. It asserts that a flat universe is inherently unstable and requires precise conditions in the early universe to maintain its flatness. The prevailing theory, cosmic inflation, is credited with driving the universe close to flatness by acting as a dark energy during its early expansion. The conversation concludes that the fine-tuning of the expansion rate and the flatness of the universe are fundamentally interconnected claims.
PREREQUISITESAstronomers, cosmologists, and physics students interested in the dynamics of the universe's expansion and the fundamental principles of cosmology.
More precisely: a matter-dominated or radiation-dominated universe will move further away from flatness as it expands (unless it happens to have exactly the critical density and be perfectly flat).windy miller said:the flatness problem of Bob Dicke, says a flat universe in unstable and so has to be set very precisely in the early universe to give us the flat universe we see today
Though even with dark energy, flatness remains a problem early-on. For much of the history of our universe, it was dominated by matter (and earlier, radiation). During this period, the effect of spatial curvature grew by many orders of magnitude. So for it to be observed flat today, it had to be extremely extremely flat early-on, such as when Big Bang Nucleosynthesis was going on.PeterDonis said:More precisely: a matter-dominated or radiation-dominated universe will move further away from flatness as it expands (unless it happens to have exactly the critical density and be perfectly flat).
Our universe is currently dark energy dominated, so it is (very slowly) moving closer to flatness as it expands (assuming it isn't already perfectly flat).
According to inflation models, our universe was also (effectively) dark energy dominated (the inflaton field acts like dark energy) during inflation, which (rapidly) drove the universe very, very close to flatness, close enough that the period of 10 billion years or so after inflation ended when the universe was radiation and then matter dominated did not move it far enough away from flatness for us to detect the difference.
Yes, that's the issue that inflation models claim to solve.kimbyd said:even with dark energy, flatness remains a problem early-on
Where have you seen the claim that the expansion rate of the universe is delicately fine tuned?windy miller said:the claim that the expansion rate of the universe is delicately fine tuned and the flatness of the geometry is delicately fine tuned is one and the same claim
Alan Guth says it about 3 minutes into this videoPeterDonis said:Where have you seen the claim that the expansion rate of the universe is delicately fine tuned?