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Sagar_C
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Is the Earth moving away from the moon (however tiny amount it may be) due to expansion of the universe? Or is Hubble's law applicable to only to gravitationally-UNbound object?
CaptainEvil said:In fact, galaxies as a whole are gravitationally bound i.e. a galaxy will not rip apart due to Hubble's Law - only galaxies relative to each other will accelerate away.
Bandersnatch said:This pdf in particular might be of interest to you:
http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
It deals with the balloon analogy and some common misconceptions about the expanding universe, including the one in question.
Sagar_C said:Thanks. But this logic confuses me because galaxies are gravitationally bound in a galaxy cluster and then what?
mrspeedybob said:The rate of cosmological expansion is about 74 km/s per billion light years. The moon is about 239,000 miles away. By my math that means that each second the moon is about 3 picometers further away then it would be without expansion. About 1/10 of a millimeter per year.
azizlwl said:I've heard from tv program that finally the atoms will rip apart too.
Does this mean that the radius of the electron orbiting the center increases?
Drakkith said:The universe does expand, but only on the largest scales where we can model it as being approximately homogenous. Locally the math doesn't work the same and we get zero expansion.
The Big Rip (as it is called) is one possible scenario for the future. There are other options, and at the moment nothing indicates that the Big Rip will happen.azizlwl said:I've heard from tv program that finally the atoms will rip apart too.
No - unless you look at the last 10-whatever seconds before the Big Rip (if it happens at all).Does this mean that the radius of the electron orbiting the center increases?
Drakkith said:Nope, the Moon is gravitationally bound to the Earth, the Sun, and the rest of the Milky Way and does NOT experience any recession due to the expansion rate of the universe. The key lies in the fact that under GR, the specific math model that we use to determine the expansion of the universe says that the universe is homogenous at all levels. Obviously it is not, as we have clumps of matter in the form of Stars, Planets, etc, and also have wide tracts of nearly empty space. The universe does expand, but only on the largest scales where we can model it as being approximately homogenous. Locally the math doesn't work the same and we get zero expansion.
bcrowell said:The effect on the earth-moon system is not zero, just 20 orders of magnitude too small to measure. Homogeneity doesn't explain why there is almost no local expansion. Homogeneity is just an assumption about initial conditions, but the smallness of local expansion requires an explanation in terms of the dynamical laws (the Einstein field equations) governing the expansion.
Drakkith said:Nope, the Moon is gravitationally bound to the Earth, the Sun, and the rest of the Milky Way and does NOT experience any recession due to the expansion rate of the universe. The key lies in the fact that under GR, the specific math model that we use to determine the expansion of the universe says that the universe is homogenous at all levels. Obviously it is not, as we have clumps of matter in the form of Stars, Planets, etc, and also have wide tracts of nearly empty space. The universe does expand, but only on the largest scales where we can model it as being approximately homogenous. Locally the math doesn't work the same and we get zero expansion.
bcrowell said:FAQ: Does everything expand equally because of cosmological expansion?
No. If everything expanded by the same percentage per year, then all our rulers and other distance-measuring devices would expand, and we wouldn't be able to detect any expansion at all. Actually, general relativity predicts that cosmological expansion has very little effect on objects that are small and strongly bound. Expansion is too small an effect to detect at any scale below that of distant galaxies.
Cooperstock et al. have estimated the effect for systems of interest such as the solar system. For example, the predicted general-relativistic effect on the radius of the Earth's orbit since the time of the dinosaurs is calculated to be about as big as the diameter of an atomic nucleus; if the Earth's orbit had expanded according to the cosmological scaling function [itex]a(t)[/itex], the increase would have been millions of kilometers.
To see why the solar-system effect is so small, let's consider how it can depend on [itex]a(t)[/itex]. The Milne universe is just flat spacetime described in silly coordinates, and it has [itex]\dot{a}\ne 0[/itex], i.e., a nonvanishing value of [itex]H_o[/itex]. This shows that we should not expect any expansion of the solar system due to [itex]\dot{a}\ne 0[/itex]. The lowest-order effect requires [itex]\ddot{a}\ne 0[/itex]. Since a rescaling like [itex]a(t)\rightarrow 2a(t)[/itex] has no physical meaning, we can guess that the effect is proportional to [itex]\ddot{a}/a[/itex]. Based on units, we expect that multiplying this by the size of the solar system might give an estimate of the anomalous acceleration with which the solar system expands, and this is indeed the result of Cooperstock's rigorous calculation. The fractional rate of anomalous acceleration [itex]\ddot{r}/r[/itex] is about [itex]H_o^2\sim 10^{-35}\ s^{-2}[/itex]. The result for [itex]\ddot{r}/r[/itex] is valid for [itex]r\ll 1/H_o[/itex] and is independent of r, so it can be applied to similar systems with circular orbits at other scales, such as the earth-moon system or a pair of galaxies in circular orbits about their common center of mass. It can't be applied to systems bound by non-gravitational forces, such as atoms and nuclei.
A nice way of discussing atoms, nuclei, photons, and solar systems all on the same footing is to note that in geometrized units, the units of mass and length are the same. Therefore the existence of any fundamental massive particle sets a universal length scale, one that will be known to any intelligent species anywhere in the universe. Since photons are massless, they can't be used to set a universal scale in this way; a photon has a certain mass-energy, but that mass-energy can take on any value. Similarly, a solar system sets a length scale, but not a universal one; the radius of a planet's orbit can take on any value. A universe without massive fundamental particles would be a universe without distance measurement. It would obey the laws of conformal geometry, in which angles and light-cones were the only measures. This is the reason that atoms and nuclei, which are made of massive fundamental particles, do not expand.
Cooperstock, Faraoni, and Vollick, "The influence of the cosmological expansion on local systems," http://arxiv.org/abs/astro-ph/9803097v1
This is a good question. This is indeed what happens, for example, due to the small outward acceleration of the Earth due to the 1/r^2 force of the sun's light pressure. The result is exactly like dialing down the strength of gravity a little.mrspeedybob said:I didn't follow most of that. I was under the impression that expansion was just another factor in what is or is not a stable orbit. For example in the Earth's orbit that you mentioned, I would think that expansion along with momentum would pull the Earth away from the sun while gravity pulls it toward and the forces balance out. In other words, if expansion stopped, the Earth would, over many years, drop to a lower orbit. Is this incorrect? If so, why? My knowledge of math is limited to algebra and the most basic calculus. Is it possible to dumb it down to that level? :-)
I think this is probably misleading. It makes the gravitational and electrical cases sound analogous, but they aren't, because you get a trend in orbital radius in one case but not the other. It makes it sound like the effect would be proportional to [itex]\dot{a}[/itex], when actually it has to go like [itex]\ddot{a}[/itex].mrspeedybob said:I think I get the fact that solid objects are unaffected by expansion because the chemical bonds which hold them together have preferred length. Try to stretch them and electrostatic forces pull them together, try to shorten them and pauli exclusion trys to push them apart, there is a certain length that has the lowest energy. Did I get that about right?
Right. Our observable universe is just a small part of the whole universe. And we are in the center of our observable universe.RobinSky said:However, it's space that expands, and the the map of the galaxies is just light, of the observable universe. And it's the observable universe that is spherical, right? Not the entire universe itself! Space can be flat happily by itself while the observable universe is spherical, right?
The Big Bang theory is a scientific explanation for the origin of the universe. It proposes that the universe began as a single point of infinite density and temperature, and has been expanding and cooling ever since.
The Big Bang theory was developed by scientists using observations and mathematical equations to understand the behavior of the universe. It is based on evidence such as the cosmic microwave background radiation and the expanding universe.
The Big Bang theory is a widely accepted scientific theory, but it cannot be proven in the traditional sense. Instead, it is supported by a vast amount of evidence and has successfully predicted many observable phenomena in the universe.
The Big Bang theory does not explain the creation of the universe, but rather describes what happened after the universe began. The theory does not address the concept of a creator or the origin of the universe itself.
There are alternative theories to the Big Bang, such as the steady state theory and the oscillating universe theory. However, the Big Bang theory is currently the most widely accepted explanation for the origin and evolution of the universe.