How far can we see?

[sman]

I'll try to state my problem with parsimony:

 

When we look a Barnards star (for example, ~15 light years out) we see it as it was 15 years ago. When we look 1 million ly out we are looking at a scene from 1 million years ago. So I think I understand it when astronomers say that looking very far away is equivalent to looking very far into the past.

 

I've heard of telescopes that see BILLIONS of light years away. Here's my problem: the universe is only ~15 billion years old. So if we are looking at something 15 billion ly away, we are looking 15b years into the past - at that time when whatever we are looking at should've been RIGHT HERE.

 

Maybe we just can't see that far, but still, the farther we look the closer we come to the point where all mater was in the same spot. This is the paradox: the FARTHER it is away from us, the CLOSER everything sould be to everything else.

 

I'm sure I'm missing something here that someone can fill in. I'd appreciate any comment.

[sanctus]

It is based on the common misconception that the big bang was an explosion in one point (had that too until a little while ago and still often find myself thinking like that), but the big bang was an explosion everywhere since it is assumed to have created space-time...there is more to say, but not much time now...

[modest]

I've heard of telescopes that see BILLIONS of light years away. Here's my problem: the universe is only ~15 billion years old. So if we are looking at something 15 billion ly away, we are looking 15b years into the past - at that time when whatever we are looking at should've been RIGHT HERE.

 

Close, but not quite. There are two different kinds of distance that are important here. Light travel time is the distance you're thinking of. If we see a galaxy and the light took 10 billion years to reach us then you might think it is 10 billion lightyears away. But, the universe is expanding, so while the light from that galaxy traveled for 10 billion years the galaxy from which it came moved away from us.

 

The comoving distance is how far the galaxy is from us today which can be more than 13.7 billion lightyears because of expansion. If the light travel time is 10 billion years then the comoving distance is about 16.4 billion lightyears. The furthest back we can see is to when the universe was filled with opaque plasma which is today viewed as the cosmic microwave background.

 

That was more than 13 billion years ago. The light that we see as the CMB today was only 36 million lightyears from us (or from the matter that would eventually become the Milky Way) when it was released. The universe has expanded about 1300 times over since that happened meaning the matter which emitted that light which we see today is currently about 46 billion lightyears from us.

 

So, in general, your post is correct. The further back we look the closer everything was. The only reason it seems like a paradox is because you're mixing up the two different types of distance. A comoving distance of 20 or 30 billion lightyears does not mean the light was emitted 20 or 30 billion years ago.

 

~modest

[sanctus]

Ok, modest I think what I said was not relevant at all...and then you are scared to read my article :-)

[modest]

...and then you are scared to read my article :-)

 

Well, scared to discuss it at least :phones:

 

~modest

[sman]

If the light travel time is 10 billion years then the comoving distance is about 16.4 billion lightyears.

 

So this particular galaxy has put 16.4 billion light years between us in 15 billion years? This kinda throws up a flag for me.

[sman]

My instinct here (and this maybe my fallacy) is to lay cartesian coordinates over it and just do the math. This produces kindof an "event horizon" at comoving distance=15bly. I think the cosmos is bigger than that & I don't think spacetime is expanding faster than the speed of light.

[modest]

Yes, Sman, If you're talking in terms of light travel time distance then there's a cosmic horizon somewhere between 13 and 14 billion lightyears. No light can have taken longer to reach us. But, this is not the only distance cosmologists speak in terms of. In an expanding cosmos it is sensible to speak of distances which are greater than the light travel time.

 

Ned Wright's cosmology tour under the heading Many Distances explains:

Cosmology Tutorial - Part 2

And here's one of many Hypo threads on the topic:

17549

~modest

[sman]

Thank you for the references and thank you both for the feedback - all of which is very relevent.

 

I think this is just a product of my inability to wrap my mind around the concept of spacetime. I'll keep trying - there's no deadline.

[modest]

I'll keep trying - there's no deadline.

 

Fantastic outlook! :)

 

Believe me when I say, you are not alone in your confusion. The different ways of expressing distance have generated a lot of confusion. The following link shows one astronomer who appears more than a little irritated by the way the popular press expresses cosmic distances:

Light Travel Time Distance

You must imagine that this galaxy which you're looking at is moving away from you. By the time we absorb light from that galaxy it is no longer in the spot where it emitted the light which we just received. It has expanded away from us by the time we receive the light. So, how far is the galaxy really? There's more than one answer.

 

Let's say the galaxy has a redshift of 1.815. This will mean light traveling from that galaxy to the milky way took 10 billion years. When the light was emitted the universe was much younger than it is today (the universe was about 3.7 billion years old). So this is not a very sensible distance. Light travel time distance is comparing how far apart a young galaxy is to an old galaxy. We're looking at two different time periods.

 

The comoving distance looks at how far that other galaxy is *today* compared to the Milky Way today. This distance is necessarily going to be greater than the light travel time because these two galaxies have been moving apart during the time the light has been traversing the distance. In this case of redshift 1.815 and light travel time of 10 billion years the comoving distance is 16.165 billion light years.

 

Just as there is more than one measure of distance, so too there is more than one measure of velocity. The velocity that goes into Hubble's law is not the same as you would figure in special relativity. The link I gave yesterday gets into that.

 

But, yeah, this can get confusing quick—your puzzlement is quite understandable.

 

~modest

[modest]

So this particular galaxy has put 16.4 billion light years between us in 15 billion years? This kinda throws up a flag for me.

 

Sorry, I didn't see this question before. Yes, you're spot on. In fact, the furthest thing we can see is the cosmic microwave background which is *currently* about 46 billion lightyears from us (that is to say, the comoving distance between us and it is 46 billion lightyears).

 

This means, as you correctly infer, that the hot condensed plasma which eventually become the milky way and the hot condensed plasma which eventually became some other galaxy put 46 billion lightyears between each other in 13.6 billion years. This also means it is currently expanding away from us at faster than the speed of light.

 

The question is: how can we see something that is expanding away from us at faster than the speed of light? In fact, we cannot; we are not seeing the thing that is expanding away from us at that speed. We are *not* seeing the galaxy as it exists today—nor can we. We see it as it existed in the very young universe. Back when the light was emitted it was not expanding away from us as fast as it is today.

 

Galaxies which have a light travel time distance less than 13.7 billion lightyears with a comoving distance greater than 13.7 billion lightyears started out inside our cosmic horizon but have subsequently moved out of our horizon. So we can't see how they exist today—nor will we ever be able to no matter how long we wait. We can only see how they used to be when they were inside our past light cone.

 

~modest

[mentor]

don`t forget that at the very start the Universe was expanding faster than light (universes are allowed to do this, its things inside em that can`t go FTL), so a HUGE chunk of the Universe is so far away that we can`t see it and never could, even with a "SuperHubble", the chunk we CAN see is comparable in size (so I read once) to about the size of a grain of sand in the Superbowl stadium.

 

I`m from England so I don`t actually know how big the Superbowl stadium is, but assuming it`s bigger than a football stadium, thats one BIG universe, you just need some advanced technology to get to it, like a wormhole generator or a quantum transporter of some sort, since it`s recession velocity is still faster than light (and still accelerating by recent accounts) , so going via normal space won`t cut it.

 

afaik the current furthest object has a recession velocity of 0.84C, so thats over 4/5ths of the way back to creation (and IF the Universes expansion IS still accelerating then that object got beyond our reachable range a LONG time ago and is now (from our viewpoint) traveling faster than light.

[Michael Mooney]

According to the theory of space expanding faster than light (as per inflation cosmology), what is it exactly that is supposed to be expanding? If space is not simply empty volume*, what then is it supposed to be? (Of course it is not empty where "stuff" exists *in space*.

This is half the "what is spacetime?" question, and it has never been answered either after over 80 original pages of that (now closed) thread.

Michael

[arkain101]

Question: How is trick of perspective explained away? Is this consideration possible or fallacy.

 

From a guess, I am going to say that this (what I am going to share below) is almost certainly put under consideration. With that said, what I AM interested in, is what explanations are available to explain it.

 

In special relativity, it is said that when an object is traveling at relativistic speeds, distance shrinks in the direction of motion. However, I propose the slight flaw in that statement. Direction "OVERALL" is arbitrary(I believe that is the right word to use). In such a way that; anything moving towards you is considered the direction you are traveling. Therefore, direction of travel can exist relatively speaking more than one direction. However, you can only specify one singular direction for a result to make sense. Therefore, velocity must be applied to "the surroundings" as opposed to the observer, because logically, an observer can't travel two directions at the same time.

 

Now, in a distance past, a very distant past, near the beginning of the universe; we could suggest that, the beginning of the universe, things were in a very very rapid inward velocity.

 

This naturally gives us the image from an external universe view of a very very very large universe collapsing inwards. To keep track of "perspectives" we will call this: The Perspective of separation. (PS) . This perspective is the imaginary kind, how we see the world naturally.

 

However, this natural intuitive perspective "PS", is false in the following sense: According to the accuracy of Special relativity, if a universe was collapsing inwards very close to the velocity of C, such that space is shrunk from any direction of observation. Then, it would infact be that relative to an participating observer the universe would be INCREDIBLY SMALL, NOT INCREDIBLE LARGE. At this point in time where velocity is highest space is literally shrunk. We can call this perspective: The perspective of a participant" (PP). This perspective is the realistic kind, how we are actually forced to observe anything.

 

In this collapsing model, we actually have to stop thinking of the universe as "collapsing", because the beginning of the space and time, the instant of creation would be equivalent to an inward velocity to become LESS than C, so that space and time could be greater than 0.

 

In respect to PP, the center of the universe is any location of observation. As this instant of creation occurs, it must be considered that the universe is to begin decelerating.

 

Special Relativity; a working accurate theory in our point and time and "PERSPECTIVE ON THE UNIVERSE"; SR's relativistic equations would suggest that as the universe slows down, space would expand in the direction of travel. Geometrically speaking, the direction of travel is equal to all directions. This translates into saying the universe beings to expand in every direction.

 

Another way of phrasing this expansion of space-time, or slowing down of collapse; in respect to our physics is; The potential energy of the universe, or the Total energy of the universe decreased.

 

That is, if at the beginning of the universe's total energy was equal to velocity C, with distance 0 etc, then a decrease in Total energy, equates, space-time expansion.

 

(Before the expansions of space-time, was Time (t), unknown, singularity, where mass is both impossible and excluded, "i couldnt begin to guess physics of a singularity" other than, a state of velocity C)

 

As this expansion occurs, one could also calculate that the Potential energy of the universe as one entity, decreases proportionally. That is, the rate at which the universe slows down increases per unit exponentially. That is, the deceleration of the universe, produces an acceleration of space-time.

 

As such, The act of a universe decreasing from velocity C produces space-time value, and exponential expansion in all directions.

 

In respect to physics, a prediction or expectation is that the; If this were "TRUE" one could calculate the EXACT age of the universe by taking in the account of the current stage off acceleration, the initial velocity ©, and the current velocity of expansion.

 

Furthermore, one could predict, that the accelerating universe will eventually stop, as relativistic speeds reduce.

 

 

So ALL of the above is a consideration. A specific consideration that I will call "TRICK of Perspective".

 

I ask, How is this consideration explained away as invalid data and theory?

[modest]

According to the theory of space expanding faster than light (as per inflation cosmology), what is it exactly that is supposed to be expanding?

On cosmic scales everything is expanding away from everything else. The simplest way to express such a thing is to put everything on a metric (or a grid, if you like), and to scale the metric. This accomplishes something very specific which I will describe.

 

If you consider a galaxy such as the Milky Way and measure the speed at which other galaxies are moving away from it then the further the galaxies are, the faster they will be moving away. That is a physical law of astronomy known as Hubble's law.

 

It is sensible to assume that our position in the universe is not special so that any other galaxy out there also has galaxies receding from it in such a way that the recession velocity increases with distance. An easy way to show this (to model it) is to use a rubber sheet (think of a piece of paper made of rubber). You can mark a bunch of dots on the sheet representing galaxies.

 

If you have 4 people each grab a corner of the sheet and stretch it apart then you have effectively modeled the manner in which galaxies move away from one another. One of those dots on the rubber sheet has every other dot moving away from it and the further the other dots are from it, the faster they move away. This is true for any dot.

 

The expansion of the universe is very similar. Saying "space is expanding faster than the speed of light" means that the distance between two objects on the metric is increasing faster than c. This is an inevitable conclusion given two things:

1. The further away one galaxy is from another the greater the rate at which their distance increases.
   
2. The universe is infinite in size.
   

You're probably wondering what difference it makes if we say something is moving through space or if we say space is expanding between things. And, it indeed does make a difference which can only be revealed by measuring and understanding the apparent motion of cosmic objects. To give one example, if an object is moving away from us *through space* then we expect it to exhibit a redshift which can be calculated with and is due to special relativistic Doppler shift. If, however, space is expanding SR doppler shift will not give the correct redshift results. Cosmological redshift must be used. Wiki summarizes the difference:

Hubble's law of the correlation between redshifts and distances is required by models of cosmology derived from general relativity that have a metric expansion of space. As a result, photons propagating through the expanding space are stretched, creating the cosmological redshift. This differs from the Doppler effect redshifts described above because the velocity boost (i.e. the Lorentz transformation) between the source and observer is not due to classical momentum and energy transfer, but instead the photons increase in wavelength and redshift as the space through which they are traveling expands. The observational consequences of this effect can be derived using the equations from general relativity that describe a homogeneous and isotropic universe.

 

Redshift - Wikipedia, the free encyclopedia

That quote references "the metric expansion of space" which is a good article at wikipedia. If you want to get a good understanding of just what astrophysicists mean by expanding space then that would be a good place to start.

 

I should probably also note that cosmic inflation which you mention is not any more fundamentally superluminal expansion than the expansion of space today. This is a common misconception which is addressed here:

Inflation is sometimes described as “superluminal expansion” [App. B: 22–23]. This is misleading because any expansion described by Hubble’s law has superluminal expansion for sufficiently distant objects. Even during inflation, objects within the Hubble sphere (D < c/H) recede at less than the speed of light, while objects beyond the Hubble sphere (D > c/H) recede faster than the speed of light. This is identical to the situation during non-inflationary expansion, except the Hubble constant during inflation was much larger than subsequent values.

Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe (page 6)

If space is not simply empty volume*, what then is it supposed to be?

I don't think "volume" is that bad of a word to substitute for "space" in the setting of astronomy and cosmology. We could say that the volume between galaxies increases over time rather than saying the space between them increases. We would also say that the rate at which volume expands is proportional to the volume itself much in the way we would say the expansion of space between objects is proportional to the distance between them. :eek2:

 

~modest

[Pluto]

G'day Modest

 

You said

On cosmic scales everything is expanding away from everything else. The simplest way to express such a thing is to put everything on a metric (or a grid, if you like), and to scale the metric. This accomplishes something very specific which I will describe.

 

If you consider a galaxy such as the Milky Way and measure the speed at which other galaxies are moving away from it then the further the galaxies are, the faster they will be moving away. That is a physical law of astronomy known as Hubble's law.

 

I understand what you say and yet when I look at images near and far 13,2 Gyrs I see galaxies mature and young, various forms from spiral to elliptical and so on as we see them close. In addition the images show a clustering effect proven by galaxies merging and clustering into a super cluster of clusters of galaxies.

 

I may have posted thes links before.

These are for the people who have not read them and may express their own opinion on them.

 

There are a number of papers that are of interest.

 

 

[0806.4481] Hubble's Cosmology: From a Finite Expanding Universe to a Static Endless Universe

Hubble's Cosmology: From a Finite Expanding Universe to a Static Endless Universe

[0806.4481] Hubble's Cosmology: From a Finite Expanding Universe to a Static Endless Universe

 

[0810.0153] Expanding Space: The Root of Conceptual Problems of the Cosmological Physics

Expanding Space: The Root of Conceptual Problems of the Cosmological Physics

[0810.0153] Expanding Space: The Root of Conceptual Problems of the Cosmological Physics

 

[0811.3968] The origin of redshift asymmetries: How LambdaCDM explains anomalous redshift

The origin of redshift asymmetries: How LambdaCDM explains anomalous redshift

[0811.3968] The origin of redshift asymmetries: How LambdaCDM explains anomalous redshift

 

On The Origin Of The Highest Redshift Gamma-Ray Burst GRB 080913

[0812.2470] On The Origin Of The Highest Redshift Gamma-Ray Burst GRB 080913

Authors: Krzysztof Belczynski, Dieter H. Hartmann, Chris L. Fryer, Daniel E. Holz, Brian O'Shea

(Submitted on 12 Dec 2008)

[0812.2470] On The Origin Of The Highest Redshift Gamma-Ray Burst GRB 080913

 

Could these papers be wrong or is the Big Bang theory in question.

[modest]

Pluto, I've answered your post here: Re: Origin of the Universe,,,,Bang or no Bang

 

~modest