I How can the universe expand faster than light?

[PeterDonis]

Or if it experienced gravity time dilation, right?

In principle, yes. But the sequence of events that would have had to happen in that case is far more improbable even than it getting to ultrarelativistic velocity.

 

[Elliot Svensson]

During the early moments of Big Bang cosmology, the universe was denser than it is now. Is 13.8 billion years the proper time of the gravity-dilated objects or the proper time of an arbitrary clock living in a normal gravitational state?

 

[PeterDonis]

During the early moments of Big Bang cosmology, the universe was denser than it is now. Is 13.8 billion years the proper time of the gravity-dilated objects or the proper time of an arbitrary clock living in a normal gravitational state?

The concept of gravitational time dilation does not apply to the universe as a whole, because the universe as a whole is not static, and the concept of gravitational time dilation only applies to a static system.

 

[Elliot Svensson]

Do you agree with me that it is very strange to say that one of Einstein's predictions of the theory of relativity does not apply in Cosmology's most popular measurement, the age of the universe?

 

[PeterDonis]

Do you agree with me that it is very strange to say that one of Einstein's predictions of the theory of relativity does not apply in Cosmology's most popular measurement, the age of the universe?

No, because Einstein's prediction was not what you apparently believe it is. Einstein's prediction of gravitational time dilation was for a very specific situation: as I said, for an object that is static in a static gravitational field. Both "static" terms are essential. Neither one applies to the universe as a whole: the universe is not static--it's expanding--and comoving objects in the universe, which are what provide the standard of time used to define the age of the universe, are not static, because they are not "holding position" in any physical sense. They are in free fall, going wherever the geometry of spacetime takes them. (Notice that a static object in a static gravitational field cannot be in free fall; it must be accelerated. Einstein's original argument for gravitational time dilation makes essential use of this fact.)

Your issue here is that you are not reasoning from fundamentals. "Gravitational time dilation" is not a fundamental of GR; it is a derived consequence of GR when applied in a particular scenario, which has properties that the scenario you are trying to apply it to, the expanding universe, does not have. The fundamentals of GR are spacetime geometry and proper time along worldlines. When we say the age of the universe is 13.7 billion years, we mean that the spacetime geometry is such that the proper time elapsed along a particular family of worldlines--the comoving ones, the worldlines of observers who always see the universe as homogeneous and isotropic--since the Big Bang is 13.7 billion years. That's all we mean.

 

[Elliot Svensson]

That's very helpful--- thanks!

 

[tgarcia39]

I know that space itself is expanding but can't figure out how the speed of expansion is superluminal..

I think expansion depends on its cause. E.g., if the cause is based on the force provided for the expansion, it may be then the force is mitigating the expansion. If so, it seems the fact it is still increasing could mean the force has yet to reach its apex, or that it never will. The Inflationary Period (IP), if it occurred, did abate somewhat, but now it seems a moot point in view of Hubble's revelation. I think the IP is no longer needed to explain the homogeneity of the elements since we now know the U. has been expanding all along, likely since the BB. The bits and pieces of scientific reseach will provide us with the so-called theory of everything, I believe.

 

[Drakkith]

I think the IP is no longer needed to explain the homogeneity of the elements since we now know the U. has been expanding all along, likely since the BB.

Inflation was developed well after we already knew the universe had been expanding. Decades after in fact. The idea of inflation isn't going anywhere anytime soon.

The bits and pieces of scientific reseach will provide us with the so-called theory of everything, I believe.

Which could very easily require inflation.

 

[tgarcia39]

Inflation was developed well after we already knew the universe had been expanding. Decades after in fact. The idea of inflation isn't going anywhere anytime soon.

Which could very easily require inflation.

The IP theory seems to have been created solely for the purpose of explaining the isotropic feature of the universe, thus to me, it is not the only possible explanation. It is a very weak idea offering no reason nor cause for it, although in my opinion, if it was the impetus given to the contents of the singularity that is causing the expansion of the U., then I wonder if that does not explain the homogeneous effect previously offered as the IP to explain the effect, thus rendering the IP unnecessary. If the force that caused the BB gave impetus to the contents of the singularity, that seems to me a more easily acceptable theory since such impetus should be expected from such a force, explaining therefore the cause of the isotropic U. Do you not agree?

 

[Drakkith]

I do not agree. Regular expansion doesn't work fast enough to create the level of homogeneity and isotropy we see today as far as I know.

The IP theory seems to have been created solely for the purpose of explaining the isotropic feature of the universe, thus to me, it is not the only possible explanation.

You can see at least part of the motivation behind it on wikipedia's article: https://en.wikipedia.org/wiki/Inflation_(cosmology)#Motivations

And remember that just because science has a predominant theory for something does not mean it is the only possibility. It's usually the simplest, most accurate one though, and inflation is no different. Nothing else explains isotropy, homogeneity, and various other things as well as inflation does.