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?
The relationship between geometry and expansion rate is a fundamental concept in physics and cosmology. It refers to the way in which the shape of the universe affects the rate at which it is expanding. This relationship is described by Einstein's theory of general relativity.
The geometry of the universe plays a crucial role in determining its expansion rate. In a flat universe, where the geometry is Euclidean, the expansion rate is constant. However, in a curved universe, such as a spherical or hyperbolic geometry, the expansion rate can vary depending on the amount of matter and energy present.
Currently, the expansion rate of the universe is accelerating. This was first discovered in the late 1990s through observations of distant supernovae. It is believed that this acceleration is due to the presence of dark energy, a mysterious force that counteracts the effects of gravity and causes the expansion of the universe to speed up.
The expansion rate of the universe is a key component of the Big Bang theory. According to this theory, the universe began as a singularity and has been expanding ever since. The expansion rate determines the age and size of the universe, as well as the distribution of matter and energy within it.
Yes, the expansion rate of the universe can change over time. As mentioned earlier, the current expansion rate is accelerating, but it is possible that in the future, it may slow down or even reverse. This is dependent on the amount and nature of dark energy, as well as the amount of matter and energy in the universe.