
If the universe is 13.8B years old, how can we see 46B light years away? (2018) - allthebest
https://medium.com/starts-with-a-bang/if-the-universe-is-13-8-billion-years-old-how-can-we-see-46-billion-light-years-away-db45212a1cd3
======
nvrspyx
If what we’re seeing was 13.8B light years away 13.8B years ago and are now
46B light years away, wouldn’t that mean that those things traveled ~32.2B
light years in 13.8B years. Wouldn’t that thus mean they travelled faster than
the speed of light?

Maybe I’m misunderstanding though.

EDIT: I guess you also have to take into account the expansion of the fabric
of space as well and not just the movement of the galaxies. Still a bit
difficult to grasp because I still intuitively want to say that those galaxies
are traveling faster than the speed of light with the assistance of space
expansion like the old Hollywood trick of actors running on a moving rug to
fake superhuman speed. They’re still moving faster, just not on their own. I
guess the key difference is that an actor on a sliding rug is moving through
space faster even if their legs are moving at the same speed, but in the case
of the galaxies, space itself is moving thus they’re not technically FTL
because the speed of light is relative to its movement _through_ space.

~~~
lisper
[EDIT] Everyone please ignore this comment. It is totally wrong.

> If what we’re seeing was 13.8B light years away 13.8B years ago and are now
> 46B light years away, wouldn’t that mean that those things traveled ~32.2B
> light years in 13.8B years.

No. What we're seeing was 13.8BLY away (in our reference frame) _when the
light was emitted_. Those things are 46BLY away _now_ (again, in our reference
frame) but we can't see them _now_ , we can only see them as they were 138B
years ago.

~~~
nvrspyx
I'm sorry, but that's exactly what I said unless I'm misunderstanding your
point.

I said what we're seeing was 13.8B light years away 13.8B years ago, since it
took 13.8B years for the light to reach us from the point it was emitted. I
never said we could see now where they currently are. What I failed to account
for was the space expansion during the time that the light was traveling
meaning that the light had to travel further to reach us than the point of
emission actually was when it emitted.

So really, it wasn't 13.8B years ago; it was emitted later. I don't know by
how much, but it seems like maybe the other commenter does.

~~~
lisper
Yes, I misread your comment. And then on top of that I screwed up the answer.
I've edited it accordingly. Sorry about that, I guess I'm having a bad day.

------
ctdonath
It’s not that we can see that far, it’s that what we see has moved away from
where it was when it produced the light we see now.

~~~
mannykannot
Indeed; the issue is covered much more clearly in the article from which the
most relevant illustration was taken:
[https://calgary.rasc.ca/redshift.htm](https://calgary.rasc.ca/redshift.htm)

------
Gabrielfair
Simply put: We can see 46 Billion Light Years away now, b/c it wasn't that far
away when it was created.

~~~
brianberns
No, the object is now 46b light years away, but we can’t see what it looks
like now at that distance. We see an image of it when it was much closer and
younger.

------
ssijak
If you are turning in bed and can't sleep I'll give you some questions to
think about. Even though some are non-sensical in a way, they are a good
starting point to ponder about:

\- Is space really flat (infinite) or curved (finite)?

\- If it is flat and the universe had the beginning (big bang) then does it
just have "potential infinity", by ever-expanding?

\- If it is curved is it implied that it is encompassed by something that has
at lease 3+1 dimension into which it is curving?

\- What would happen if you would happen to stand at the edge of
space/universe and push a hand through the edge?

\- Space could be infinite but at the same time with a boundary. Can you
imagine that boundary without trying to imagine what it is bounded from?

\- Where does new space, created by the universe expansion, come from?

\- What is space fabric actually made from? Is it discrete or not.

\- Are there other universes outside ours?

\- If there are, do we live in an infinite Russian doll type of universes?

\- What gives the initial conditions and laws in the universe?

\- If the universe is cyclic or there is a multiverse, are there universes
with totally different laws and if yes what generated/allowed that?

\- Try to imagine nothing.

\- Try to imagine infinite.

\- You can't imagine how any of these 2 things would work (nothing or
infinite), but one or both must exist. Either all existence existed in some
way forever, or it had some genesis from nothing.

\- Is there free will?

\- Can you even imagine a proper mechanism that would allow for free will to
exist?

\- What would prevent you from "cutting" the smallest possible particle in 2
parts?

\- Are there several dimensions of time?

\- Is there a beginning of time?

\- Is math invented or discovered?

\- Can we ever, in theory, discover every law in the universe if we are
looking at it only from the inside.

\- Try to imagine 4.5 billion years of evolution (or more if life crashed to
earth from a comet). How many organisms lived, and died, how many actions were
made, how many found a partner and reproduced for you to be here and read this
text at these exact moment. How many chances for failure at any point in that
period were there?

...

Enough for tonight. Happy dreaming.

~~~
csomar
> \- What would happen if you would happen to stand at the edge of
> space/universe and push a hand through the edge?

That doesn't make sense if you say the universe is curved or infinite. There
will be no edge.

> \- Space could be infinite but at the same time with a boundary. Can you
> imagine that boundary without trying to imagine what it is bounded from?

No. That's what the definition of infinite is. There are no bounds.

> \- What is space fabric actually made from? Is it discrete or not.

Probably numbers. But then I like Math, so...

> \- Are there other universes outside ours?

I don't think we are any special. But I'd be happy to be proven otherwise!

> \- Try to imagine nothing.

> \- Try to imagine infinite.

You are having trouble with nothingness because you are linking it to your
consciousness. Infinity is easy though. You keep pushing _infinitely_ and
there is no edge!

> \- Is there free will?

No. But don't go and do stupid stuff because of that.

> \- Can you even imagine a proper mechanism that would allow for free will to
> exist?

There isn't. You are the product of your environment and your parents.

> \- What would prevent you from "cutting" the smallest possible particle in 2
> parts?

Max Planck.

> \- Are there several dimensions of time?

> \- Is there a beginning of time?

There is no time. Try understanding the theory of general relativity.

> \- Is math invented or discovered?

Math is very basic. It starts with set theory and then builds on top of that.
It is invented since set theory is invented.

~~~
ozzmotik
>No. That's what the definition of infinite is.. There are no bounds.

just a thought: it sounds like you are more describing the concept of an
uncountable infinity in a mathematical sense, a set that is infinite and with
no discernible end, as opposed to a countable one, where the set is certainly
infinite but it can be iterated through and conceptually has some point where
it transitions into a higher meta-set that encompasses it and far more. at
least, that's the metaphor i use to interpret the concept of an infinity that
is bounded; conceptually, everything that exists must be contained within
__something __if you ask me. it could even be contained within a conceptual
nothing, but that nothing itself acts as a boundary point for the infinite.

also: i am definitely not a cosmologist or a scientist in any rigorous sense,
im just a person that likes to think a lot. my arguments or thoughts likely
wouldn't hold their weight compared to one substantiated with support and
such, so I just want to make it known that I don't exactly treat my views
internally as the canonical explanation, just the one that makes the most
sense to me given my limited scope of understanding. thanks for your time :)

------
meteor333
Sorry, can some explain the ‘universe is expanding’ concept? So far I thought
it only meant that the space universe occupies is expanding, meaning the
matter keeps going farther from its origin point, but I didn’t know the
‘fabric’ of the universe is expanding. I’m not able wrap my head around this
concept. Can some please point me a resource that dumbs it down?

~~~
fghtr
"The universe does not expand "into" anything and does not require space to
exist "outside" it. Technically, neither space nor objects in space move.
Instead it is the metric governing the size and geometry of spacetime itself
that changes in scale."

[https://en.wikipedia.org/wiki/Expansion_of_the_universe](https://en.wikipedia.org/wiki/Expansion_of_the_universe)

------
ldoughty
What does it mean: "if we left today we could only reach one third of it"?

I couldn't find a the limiting factor... The stars would be dead before we
arrived? My best guess. Of course, the qualification would be that we continue
to be bound to the speed of light maximum.

~~~
leonroy
The universe is expanding, at a rate which I believe is heading past the speed
of light.

This limits our ability to reach galaxies further away from us as they are
speeding away faster than we could conceivably travel since our engines would
of course be bound by the speed of light.

Some more info:
[https://www.forbes.com/sites/startswithabang/2016/06/10/can-...](https://www.forbes.com/sites/startswithabang/2016/06/10/can-
the-universe-expand-faster-than-the-speed-of-light/#64e0a9473605)

~~~
hunter2_
Why would some things be bound by the speed of light and not others? I always
thought of this limitation as an all-or-nothing law.

~~~
michelpp
Everything that moves through space is bound by the speed of light. But space
_itself_ is expanding at a rate such that two distant points can move away
from each other faster than the speed of light. Think of it like two ants on
opposite sides of a balloon, one ant starts moving toward the other as fast as
it can go, but then you start to inflate the balloon. The distance between the
two ants expands faster than an ant can move and they'll never meet.

~~~
rpq
Thanks for this clarification.

------
rfhjt
So space expands itself at every point at a certain rate. There is an elephant
in the room here: how do all the parts in space coordinate to expand at about
the same steady rate? Why don't we see radically different expansion rates in
different parts of the universe, e.g. stars on the left are getting further
apart 10x faster than those on the right? We often use the air balloon
analogy, but an air ballool is inflated by the same force which equally
applies to every part of the balloon. My point is that, there is probably a
simple underlying mechanism that drives space expansion and the clever nature
of this mechanism would explain the relativity axiom used in GTR.

~~~
raattgift
> at a certain rate

No, the evidence suggests that the rate is time-dependent; that is, the
expansion is accelerating.

> how do all the parts in space coordinate to expand at about the same steady
> rate?

This is a good question!

The answer is: the coordination happens in the much earlier much denser
universe in our past, just like the coordination that happened among the
fields of the Standard Model that led to the earliest atoms and molecules.

Let's first take the case where there is no accelerated expansion (which is
unlikely with current evidence). In this case, the mechanism can simply be
inertia, with an enormous early impulse like those in the family of cosmic
inflation theories. There must be a fine balance between the _density_ of
matter in the earlier universe and the impulse, because gravitational
interactions will decelerate the expansion from the early impulse until the
matter-density drops/dilutes-with-expansion to a critical point after which
the rate of expansion will be constant.

Observations support an early deceleration and subsequent fixed-rate
expansion, but about 8 billion years ago the expansion began to accelerate, as
seen in images of galaxies with redshifts less than about z ~ 0.5.

> a simple underlying mechanism

This is what the study of dark energy is about: a simple explanation for the
accelerating expansion of the universe.

Typically one starts with a field that fills the whole of spacetime, with
enormous energy-density in the early universe, with that energy-density
decaying at later times. In extremely high-quality vacuum and far from
structures like stars and black holes, the decay of this field is faster than
near those structures (too near and the field becomes locally stable; the
solar system, for example, is not expanding). When the field decays, it gives
a local increase to the inertial expansion rate described in the no-dark-
energy case a couple paragraphs up. Because there is so much empty space
outside galaxy clusters, there is a lot of _local_ , and the amount of _local_
increases over time, so larger regions of the field decay at faster rates as
the unvierse gets older.

There are other approaches too, but the effect is the same: the amount of
space outside matter structures increases at an accelerating rate in the late
universe, and this corresponds _in a frame-dependent way_ to an energy-density
that remains constant as the energy-density of the fields of the Standard
Model decreases. That is, in such a frame of reference, matter and radiation
dilute away with the expansion _MUCH_ faster than dark energy does.

Another approach is literally the cosmological constant, which if zero gives
the inertial picture above, but if it takes on a small positive value, it
produces a late-time acceleration of the expansion. No _known_ fields behave
like this: matter-density varies by location in spacetime (clumpier in space,
tending to diffuse apart (especially light) or collapse gravitationally
(especially baryons) over time), gravitational fields vary with position in
spacetime and typically with the distribution of matter. By contrast, the
cosmological constant is literally identically everywhere-and-everywhen. More
likely we will find evidence for something which closely approximates the
cosmological constant at large scales, with differences in the density of the
"cosmological not-quite-constant field" lining up very closely with observed
differences in the density of matter.

> clever nature of this mechanism

Cleverness shouldn't be expected of the mechanism itself, although it will
probably take some cleverness to describe the mechanism in a
tractable/scalable-yet-accurate-in-detail manner.

WRT your final sentence, General Relativity works just fine in the total
absence of dark energy, and dark energy as the cosmological constant has been
captured as the factor \Lambda in the Einstein Field Equations. That's like
saying that all matter has been captured as a factor in the Einstein Field
Equations, though: it has taken a lot of work including things like studying
[https://en.wikipedia.org/wiki/Deep_inelastic_scattering](https://en.wikipedia.org/wiki/Deep_inelastic_scattering)
to work out many of the finer details of the stress-energy tensor. A
comparable program likely is needed to work out the finer details of \Lambda,
which might lead to a formal explanation along the lines of the informal one a
few paragraphs above.

------
coding123
I feel like we are all idiots repeating "space is expanding" if humans could
live 14billion years in age, and we were present when it started, but we left
good old Bob on the part of the universe that was travelling away from us, he
would have traveled FTL from us. Even if you said space expanded, the
traveling has been accomplished. So now we know that space expansion is how
you travel FTL.

In other words if humans ever invent warp, the space expansion work around we
would effectively have FTL travel.

~~~
simonh
Ok, but you can’t use it to go to anywhere FTL. You can only use it to get
further away from somewhere FTL.

~~~
gls2ro
I think we can use it to go back to a place. It is the same as going further
just that now we “just” need to shrink the space. Of course all is so far just
an exercise of our imagination.

You can take a look at the curvature or more concrete to the density of the
Universe. It is possible to be positive and in this case at one point in the
future the Universe will collapse back on itself. Nothing will stop it to do
this apparently FTL.

------
donpark
My two cents worth: One is a measure of time. The other is a measure of
distance. The two are unrelated.

~~~
brianberns
Um, no. Distance and time are related by the speed of light, which travels 1
light year (distance) in 1 year (time).

If space wasn’t expanding, the distance to a stationary object (in light
years) would be exactly equal to the travel time of light from it (in years).

~~~
csomar
They are related (in fact there is no "time") but I think what the OP wanted
to say is that you can have a universe that it is "that" big from the get go
and so objects will be very far from the beginning.

------
gesman
All this assumes that time is a single thing (sort of gigantic X-axis with
"t") that works for everyone in universe equally.

What if we have multiple (billions) of realities with their own t-axis and
with us being only one of these with our own timeline?

~~~
tsimionescu
What if the entire universe with all of its history was just created 1ms ago?

The answer is, if you can't even conceivably test for it, it's mostly
irrelevant to think too much about.

------
Koshkin
I find the notion of the distance between two objects increasing without the
objects moving apart highly unintuitive.

~~~
rfhjt
That's an easy one. We hold the same chain and the distance between us is 100
links. The chain has a curious property: its links subdivide like living cells
and on average 1 link takes a minute to split into 2. Thus, every minute the
distance between us doubles and half hour later we'll be 1 billion times
further from each other. We can replace the chain with a net or even with a 3d
net.

------
finchisko
how can universe be expanding faster than speed of light? general relativity
doesn't apply here?

~~~
goodluckchuck
Idk, but if we travel 13B light years one way, it moves 13B the other then
were at 26, and space itself has to have expanded 20 billion light years
within the time it takes light to travel 13... So yeah, sounds like something
is moving faster than light.

~~~
tialaramex
Things can't move faster than light, that isn't possible, but we can think
about abstract concepts that aren't things. Fairness for example. Infinity.
These are not things. Expanding space isn't a thing, it's just a concept. So
that can go however fast you want.

Let's try something much smaller and easier to reason about. Take a small
fairly directional lamp (in my country we'd say a "torch" but words vary) and
hold up a coin in front of it so that it makes a big obvious shadow on a far
away wall, you may need to turn down ambient lighting. Relatively small
movements of the coin result in the shadow moving quickly. Huh. How fast can
that shadow move?

While the coin is a _thing_ the shadow is not, it's just your imagination
processing the absence of light.

If you have a big enough room and a strong enough light source, you can do
this trick so that a flick of the wrist appears to move the shadow much faster
than the speed of sound. If a _thing_ did that in a room filled with
breathable air you'd hear a sonic boom. But the shadow isn't a thing, it
exists only in your imagination and so you hear nothing.

Large enough objects can occlude a star to have this effect on a cosmic scale.
The shadow of a planet can pass over the face of another planet at faster than
the speed of light. But no actual _thing_ moved, so no rules are broken by
this happening, "faster than the speed of light" was only in your head, all
the actual things (including the photons) obeyed the rules and stayed at or
below the speed limit.

------
herodotus
I like the thought that, since the Universe is expanding, so is my brain.

~~~
deckar01
> Once objects are formed and bound by gravity, they "drop out" of the
> expansion and do not subsequently expand under the influence of the
> cosmological metric...

[https://en.m.wikipedia.org/wiki/Expansion_of_the_universe#To...](https://en.m.wikipedia.org/wiki/Expansion_of_the_universe#Topology_of_expanding_space)

------
Fakira
Isn't light-years a unit of distance and not time ?

~~~
dmidnight
space and time are the same thing

~~~
saagarjha
They are not.

------
lmilcin
And... another author that doesn't get it or looks for a controversial,
clickbaity title.

The correct way to state it would be to say: "We are seing things that are
currently 46B light years away but since they are so far away we see them as
they were 13.8B years ago when they were much closer than they are now."

I see absolutely nothing difficult about that statement unless you really need
to create some piece of content and you figured out you will make something
difficult so that you can then make it a little bit easier and be a hero.

~~~
jeffdavis
It's at lesst a little confusing how something went from 14B light years away
to 46B light years away (seems to be a distance of 32B light years total
travel) in 14B years.

~~~
lmilcin
Except the galaxies do not travel. Galaxies are more or less stationary, it's
the space inbetween that expands. Think in terms of raisins in a dough or
points on expanding baloon. Neither of them could be said to travel, yet they
can be seen to become more and more far away.

~~~
wahern
Technically, galaxies _do_ travel:

> In physical cosmology, peculiar velocity refers to the components of a
> galaxy's velocity that deviate from the Hubble flow. ....

> Galaxies are not distributed evenly throughout observable space, but are
> typically found in groups or clusters, where they have a significant
> gravitational effect on each other. Velocity dispersions of galaxies arising
> from this gravitational attraction are usually in the hundreds of kilometers
> per second, but they can rise to over 1000 km/s in rich clusters.

\--
[https://en.wikipedia.org/wiki/Peculiar_velocity](https://en.wikipedia.org/wiki/Peculiar_velocity)

Granted, 1000 km/s isn't even 1% of light speed, so clearly this motion
couldn't contribute much to a distant object's recession.

~~~
lmilcin
That's why I wrote "more or less stationary", which is describing 1000km/s
compared to double the speed of light that would come from naive calculation.

~~~
wahern
Ah, fair enough. I glossed right over that qualification, which is not a smart
thing to do on HN--a credit to the quality of discourse here :)

