Viewing a single comment thread. View all comments

mfb- t1_jai7pg8 wrote

Reply to comment by horsetuna in Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science by AutoModerator

That depends on what you call "appearance".

If you take the distance today and consider how fast it increases then it is the speed of light divided by the Hubble constant, around 14 billion light years. We can see light these galaxies emitted in the past, it's only ~1/3 the distance to the edge of the observable universe, the current location of the matter that emitted the oldest light we see today.

We can see things where the distance between us and them always increased faster than the speed of light because of the expansion history of the universe: Initially the distance between us and the emitted light increased but as the universe got older and the expansion rate decreased the light started catching up.

6

horsetuna t1_jai8ddp wrote

Okay but that doesn't really answer my question. How far away would they have to be to be moving at, from our point of view, the speed of light?

I see your line about the Hubble constant etc but it seems to be just a commentary about that distance, not how far away a galaxy needs to be to be moving at SoL.

And I use the word appears, because it would appear to be moving at the speed of light from our point of view on earth.

2

Ape_Togetha_Strong t1_jaihc39 wrote

Nothing will ever appear to be moving at the speed of light from our point of view, because if it's moving at C away from us information will never reach us.

The hubble horizon is the distance where something would be moving faster than C due to expansion right now. But that's 14+ billion lightyears away, so even if we could receive information from those galaxies to see them, it wouldn't arrive for many billions of years.

But it will never arrive if it was emitted after the object was already past the hubble horizon.

3

reticulated_python t1_jainlof wrote

I do not think this is quite right, though perhaps I misunderstood you. See this StackExchange discussion as well as the paper linked in one of the responses. From the abstract of that article:

> we can observe galaxies that have, and always have had, recession velocities greater than the speed of light.

3

Ape_Togetha_Strong t1_jaiow6m wrote

Yeah, it's definitely not right in the details. Maybe I shouldn't have emphasized "ever" so much. But my goal was more to dispel much bigger misconceptions that went into the question rather than cover all the little details of how these horizons work.

I don't think there's really a good way to answer these complex questions that were formulated based on a lot of complete misunderstandings of what these words even mean. At least not while still sticking to just words. But saying "nothing will ever appear to be moving at C from our point of view" was probably a bad choice. But it would be true if the hubble sphere never changed in size.

The fact that we can see light from galaxies with recession velocities higher than C is one of those facts that is truly awesome to point out, but is pretty much guaranteed to just confuse someone who probably has only learned about cosmology through popular science.

1

horsetuna t1_jaiipb4 wrote

So how can this Hubble Horizon exist in a distance smaller than the observable universe? Everything outside that Horizon should not be visible because it's moving faster than light right

2

mfb- t1_jak8660 wrote

Because of the way the expansion rate changed over time, as I discussed in my initial comment. The Hubble rate ("how much the universe expands in percent per year") decreased over time. A good analogy is the ant on a rubber rope problem where the rubber rope expands much faster than the speed of the ant, too - but the ant still makes progress on the rope and over time is less affected by its expansion. For the universe, this would be a perfect match in a scenario of constant expansion. That's not what we have, but it's reasonably close for the last 10 billion years.

5