Expansion of the universe

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Is the universe expanding? If so, is it really expanding faster than the speed of light?

Christian views

Resources are needed. Feel free to find and add resources.

Secular views - yes

• Is the universe expanding faster than the speed of light? - Dave Rothstein, Curious About Astronomy

"The universe is not a collection of galaxies sitting in space, all moving away from a central point. Instead, a more appropriate analogy is to think of the universe as a giant blob of dough with raisins spread throughout it (the raisins represent galaxies; the dough represents space). When the dough is placed in an oven, it begins to expand, or, more accurately, to stretch, keeping the same proportions as it had before but with all the distances between galaxies getting bigger as time goes on.

The bottom line is that different pairs of galaxies are moving at different speeds with respect to each other; the further the galaxies are, the faster they move apart. So when we ask whether the universe is "expanding faster than the speed of light," I am going to interpret that to mean, "Are there any two galaxies in the universe which are moving faster than the speed of light with respect to each other?

...the universe does expand faster than the speed of light, and, perhaps more surprisingly, some of the galaxies we can see right now are currently moving away from us faster than the speed of light!"

• expansion of the universe - Nus

"The explanation is that far away points sent out their light a long time ago, and in the mean time these points have been receding from us. By assuming the universe was always 'matter-dominated' and taking into account how the scale factor of the universe changes over time, you can calculate that the present radius of the region we can see (actually, whose past we can see) is exactly 3 times the distance light could have travelled...

It's a bit confusing to think about this, because what we see is not a large region of space at one time, but a "slice out of spacetime" where each point we see is at a different time. So the 42 Gly-radius visible universe is not actually what we see through telescopes, but a "straightened out" extrapolation; and different definitions for "visible universe" are possible...

...why does the distance between different points (say, beyond the visible universe) increase faster than light?

The answer has nothing to do with quantum mechanics or varying speed of light. Also, while "they're *not* moving apart, it's *space itself* that expands" is a good way to think about it intuitively, I don't think this distinction has any absolute meaning in general relativity. All you can say mathematically is that the difference in a certain coordinate increases, and usually it's handy to think of this as space expanding (to contrast with special relativity).

The short and unenlightening answer to the question is that "relative velocity" and "frame of reference" are special-relativistic concepts only, that special relativity is general relativity applied to Minkowski space, and that Friedman-Robertson-Walker space (our universe, or a good approximation) is not Minkowski space."

• Many distances - Edward L. Wright, Cosmology Tutorial

"In particular, galaxies that are far enough away from us necessarily have velocities greater than the speed of light."

Secular views - no

• Physics of Illusion of Expanding Space - W. Jim Jastrzebski

"The necessary conclusion is that frequency of a photon..., as observed by the observers located along the photon's path, has to be diminishing along the photon's path. Since it is not possible for the energy of a photon to drop while the photon is on its way because there are no "gravitational forces" acting on the photon... the light coming from distant sources in a stationary universe has to be redshifted roughly proportionally to the distance of the source of the light to keep energy conseved.

...This effect, which might have been already observed in the universe as Hubble's redshift (a.k.a. cosmological redshift), can't be explained by the loss of energy of the photon (a.k.a. tired light effect). The geometry of the spacetime itself has to be responsible for this effect. It means that this redshift is a metric redshift and that therefore, in homogeneous space, the time rate... at the source of the photon has to be smaller than the time rate at any observer. It means that for any stationary observer in a non-empty stationary homogeneous universe the time has to run slower in any observed location of that universe roughly proportionally to the distance to that location for energy to be conserved.

For the above reason the effect might be called general time dilation...

If this metric redshift is the observed Hubble's redshift then the spacetime metric may now remain stationary (no real expansion of space needed) to be consistent with observations. It seems, based on the obtained result for the density of the universe, that it may be safely assumed that the whole observed expansion of the universe is an illusion."