Size of Known Universe / Age of Known Universe > c. What gives?

[CurtC]

There’s an excellent article at the Scientific American site about this: Misconceptions about the Big Bang

My favorite part:

This ubiquity of the big bang holds no matter how big the universe is or even whether it is finite or infinite in size. Cosmologists sometimes state that the universe used to be the size of a grapefruit, but what they mean is that the part of the universe we can now see–our observable universe–used to be the size of a grapefruit.

Observers living in the Andromeda galaxy and beyond have their own observable universes that are different from but overlap with ours. Andromedans can see galaxies we cannot, simply by virtue of being slightly closer to them, and vice versa. Their observable universe also used to be the size of a grapefruit. Thus, we can conceive of the early universe as a pile of overlapping grapefruits that stretches infinitely in all directions. Correspondingly, the idea that the big bang was “small” is misleading. The totality of space could be infinite. Shrink an infinite space by an arbitrary amount, and it is still infinite.

[Mathochist]

@Exapno Mapcase

Exactly. We can’t make any observations of any material receding at more than the apparent speed of light. That’s why people talk of the “observable” universe, that part of the universe that is receding at less than C. It’s the observable universe that has a radius of approximately 13 billion light years.

Hold on there, chief. There are observable (well, observed) objects with redshifts corresponding to superluminal recessions. The picture explanation is in the print version of the SciAm article CurtC just linked to, but the basic idea is that the light ray is stretched as well, though the galaxy isn’t. Basically, the galaxy was close enough when the light was emitted, but the stretching of the universe puts to too far away by now.

[Exapno_Mapcase]

That’s not exactly how I read the article, but let’s go to the source:

The idea of seeing faster-than-light galaxies may sound mystical, but it is made possible by changes in the expansion rate. Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place.

Still, there is a limit to how far away something can be and emit light that can still be seen:

An accelerating universe, then, resembles a black hole in that it has an event horizon, an edge beyond which we cannot see. The current distance to our cosmic event horizon is 16 billion light-years, well within our observable range. Light emitted from galaxies that are now beyond the event horizon will never be able to reach us; the distance that currently corresponds to 16 billion light-years will expand too quickly. We will still be able to see events that took place in those galaxies before they crossed the horizon, but subsequent events will be forever beyond our view.

[Chronos]

The units for speed/distance would be 1/s, right?

Correct, and the reciprocal of that is the Hubble time, which is a first approximation for the age of the Universe (for a better approximation, you need to know what sort of stuff the Universe is made of). It’s more commonly expressed in kilometers/second/megaparsec, though. Currently, the value is around 60 km/s/mpc, which is called the Hubble constant (a misnomer, since in most cosmological models, it’s not constant).

[Marley23]

Look at it this way: the universe is expanding in multiple ‘directions’ (if I can accurately use that word), so its size would be increasing at a rate greater than the speed of light.

[Mathochist]

@Marley23

Look at it this way: the universe is expanding in multiple ‘directions’ (if I can accurately use that word), so its size would be increasing at a rate greater than the speed of light.

Read the OP. He took care of that thought…

[Mathochist]

@Exapno Mapcase

Still, there is a limit to how far away something can be and emit light that can still be seen:

Oh, yes. I didn’t mean to say that we can see everything, but within the range of what we can see are objects with redshifts corresponding to superluminal recessions.