What are the Physical effects of the expansion of the Universe

AI Thread Summary
The discussion focuses on the physical effects of the universe's expansion, particularly how it interacts with gravity and atomic forces. It emphasizes that while the universe is expanding, local forces like gravity prevent smaller systems, such as galaxies and atoms, from experiencing this expansion. The concept of a "small force" from the expansion is debated, with some suggesting it has no significant impact on gravitationally bound systems. The potential future scenario known as "The Big Rip," where expansion could eventually overcome gravitational forces, is acknowledged as speculative. Overall, the conversation highlights the complexity of cosmic expansion and its limited effects on smaller scales.
ashwinnarayan
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Homework Statement



I trying to learn general relativity and ran into a few questions while learning about the expansion of the universe.

I found a really good intuitive explanation of the expansion of the universe here: http://curious.astro.cornell.edu/question.php?number=274

However, I am still left with a few questions, particularly about this paragraph from the article:

Finally, I should point out that not everything in the universe is "stretching" or "expanding" in the way that the spaces between faraway galaxies stretch. For example, you and I aren't expanding, the Earth isn't expanding, the sun isn't expanding, even the entire Milky Way galaxy isn't expanding. That's because on these relatively small scales, the effect of the universe's stretching is completely overwhelmed by other forces (i.e. the galaxy's gravity, the sun's gravity, the Earth's gravity, and the atomic forces which hold people's bodies together). It is only when we look across far enough distances in the universe that the effect of the universe's stretching becomes noticeable above the effects of local gravity and other forces which tend to hold things together. (That is why, in the analogy of the tape measure I discussed above, the tape measure that you keep in your pocket does not get stretched, while the one that goes between two galaxies does get stretched. I bet some people were wondering about that!)

What exactly does it mean that gravity and atomic forces are stronger than the expansion of the universe? Does this mean that the expansion exerts a small constant force on every particle that forces them away from each other? And if such a small force does indeed exist then shouldn't we be able to find objects on the universe which are at a distance where the force of the expansion of the universe exactly cancels out the force of gravity?

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The Attempt at a Solution

 
Space news on Phys.org
I'm not sure what the exact distance would be. However, galaxies, galactic clusters, and super-clusters seem to hold together.
 
An analogy that I like is that the effect of "dark energy" on gravitationally bound systems is exactly the same as the effect of an ant pushing on a tank. It's not that the force exerted has a tiny effect, it's that it has no effect at all.
 
I think I understand. So there is a small force. That's the reason one of the possible theories for the end of the universe is that the expansion of space overcomes the force of gravity and electromagnetism cause atoms to rip apart.

I think they call it "The Big Rip". They're quite fond of repeating that on all those shows about the universe on The History Channel
 
The Force increases in the future because the expansion of the universe is accelerating.
 
ashwinnarayan said:
I think I understand. So there is a small force. That's the reason one of the possible theories for the end of the universe is that the expansion of space overcomes the force of gravity and electromagnetism cause atoms to rip apart.

I think they call it "The Big Rip". They're quite fond of repeating that on all those shows about the universe on The History Channel

The force does NOT increase as far as is known. Its EFFECTS increase, outside of gravitationally bound systems, because it has a cummulative effect, but inside gravitationally bound systems, there is no evidence that it will ever have any effect. The big rip is total speculation.
 
https://en.wikipedia.org/wiki/Recombination_(cosmology) Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in context of the calculation of the observable universe radius, where the time integral of the inverse of the scale factor is multiplied by the constant speed of light ##c##.
The formal paper is here. The Rutgers University news has published a story about an image being closely examined at their New Brunswick campus. Here is an excerpt: Computer modeling of the gravitational lens by Keeton and Eid showed that the four visible foreground galaxies causing the gravitational bending couldn’t explain the details of the five-image pattern. Only with the addition of a large, invisible mass, in this case, a dark matter halo, could the model match the observations...
Hi, I’m pretty new to cosmology and I’m trying to get my head around the Big Bang and the potential infinite extent of the universe as a whole. There’s lots of misleading info out there but this forum and a few others have helped me and I just wanted to check I have the right idea. The Big Bang was the creation of space and time. At this instant t=0 space was infinite in size but the scale factor was zero. I’m picturing it (hopefully correctly) like an excel spreadsheet with infinite...
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