Universal Expansion-My First Dumb Question.

[JohnB]

The recent pictures from the Hubble Deep Space Field got me wondering.

 

They are from a time when the Universe was only 700 million years old, some 14 billion years ago. What I don't understand is this.

 

At 700 mill years old, the maximum possible distance between the observed Galaxies and Earth is 1.4 bill LY. So if the Universe wasn't expanding, then the light would take 1.4 bill years to get here.

 

The Universe is expanding of course, so, since the light has to travel further, then it takes longer to get here. 10 times as long. Now if we were at one "edge" of the Universe and the Galaxies at the other, moving away from each other either side of the "central" point, then both "edges" would need to moving at 45% C to "create" the required distance in the time available.

 

That just doesn't seem right, so where am I going wrong?

[Martin]

Nice image

http://imgsrc.hubblesite.org/hu/db/2004/28/images/c/formats/full_jpg.jpg

Do you have a link to some text that went with it, that might for example give a figure for the redshift of something in the field?

 

...At 700 mill years old' date=' the [i']maximum possible distance [/i] between the observed Galaxies and Earth is 1.4 bill LY.

...

 

I dont understand your reasoning here. It looks like you are applying the speed of light as a kind of "speed limit" to the expansion of space. But most of space is expanding away from us faster than light. Since the recession speeds of most of the stuff (distant galaxies) in the observable universe are FTL, this speed limit doesnt apply.

 

there was a tutorial article a while back that is online. It was called

"Expanding confusion". It was by Tamara Davis and Charles Lineweaver (a prominent cosmologist) and was meant to counter the misconception that recession speed has to be less than c. I will fetch the link to their article.

[YT2095]

 

 

I dont understand your reasoning here. It looks like you are applying the speed of light as a kind of "speed limit" to the expansion of space. But most of space is expanding away from us faster than light.

is it???

 

I don`t think so )

[Douglas]

But most of space is expanding away from us faster than light.

Martin, I'd like to see the article by Tamara Davis and Charles lineweaver.

I'm not a cosmologist, but am interested in the sciences.

Seems to me, that if the galaxies were receding faster than the speed of light, that we'd never ever see the light from those galaxies, or the frequency of the red shift would drop below the bandwidth of our eyes

[Martin]

I was curious about the 700 million years

 

apparently the hubble website said that something in the picture

(some of the reddest dimmest dots that appeared, I guess) was

from when the universe was only 700 million years old.

 

they may have meant "roughly"----the usual estimate of the age is around 13.7 billion years plusorminus some uncertainty. So they may have meant that the light from those dimmest reddest dots in the picture was roughly 13 billion years old.

 

Anyway I was curious as to what that means in terms of redshift so I plugged it into Morgan's calculator

http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html

 

It turns out that seeing something from when the universe was 700 million years old corresponds to it having a redshift of 7.5.

 

You can find Morgan's calculator in Alexa's collection of links here at SFN:

http://www.scienceforums.net/forums/showthread.php?p=87859#post87859

 

I still have to get that link to the Davis/Lineweaver article Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe

http://arxiv.org/abs/astro-ph/0310808

 

Anyway, assuming that there really is a dot in that picture that is redshift 7.5 (there quite possibly is, such things have been observed!) then

that thing was 3.44 billion LY from what was becoming the milkyway (from "us") when it emitted the light we are now seeing

 

and it was, at the moment it emitted the light we are now seeing, receding from us at 3.23 times c

 

and it is now receding from us at 2.12 times c and is at a distance of 29.31 billion LY.

 

I am just reading the numbers off of Morgan's calculator---what you get if you plug in redshift 7.5 (with the usual matter density 0.27 and cosmological constant 0.73 and hubble parameter 71)

 

the numbers from the calculator would make sense to be rounded off because stuff is not known to such precision, but this is about what the consensus among cosmologists would be concerning anything that comes in with a redshift of 7.5

 

"Redshift 7.5? Oh, that means the light is about 13 billion years old and the thing that emitted it was receding around 3c, and about 3.4 billion LY away when it emitted the light, and it is now receding around 2c and is some 29 billion LY away from us."

 

A french team has reported seeing something with redshift even higher than 7.5 but that is pretty high already. So I wonder what the hubble website is really claiming. It would be nice to see the text that went with the picture.

[YT2095]

what was the redshift of 7.5 callibrated against in the vicinity?

 

you see, an object of large volume traveleing at speed behind objects of small volume traveling at relatively low speed, will apear to be the same size, and yet one is moving much faster, it does look odd I know, it`s like in trick photography when you have the moon in someones hand or have them really tiny standing on a football

it`s all perspective.

 

so i`m currious to find out what this "Dot"`s movement is taken relative to.

 

 

cheerz

[Martin]

Martin' date=' I'd like to see the article by Tamara Davis and Charles lineweaver.

I'm not a cosmologist, but am interested in the sciences.

Seems to me, that if the galaxies were receding faster than the speed of light, that we'd never ever see the light from those galaxies, or the frequency of the red shift would drop below the bandwidth of our eyes[/quote']

 

Hi Douglas, that is an astute objection. My personal experience was perhaps similar to yours in that it took me a fairly long time to get used to the idea of superluminal recession speeds.

 

Modern cosmology is based on FTL recession speeds and the standard model cosmology of big bang etc simply would not work without them. But nothing I know of to say can help you assimilate the consequences of this. It just takes getting used to.

 

the essential thing is that the "speed limit" is based on Einstein's 1905 Special Relativity which models flat space and local phenomena.

The space of 1905 SR is called Minkowski space. It does not expand. it is not curved.

 

Minkowski space can serve as a small coordinate patch on the larger spacetime continuum. Like a differentiable manifold can be coordinatized by small local coordinate patches----but one patch will usually not cover the whole thing.

 

Nothing can catch up to and pass a photon because that would be a local thing and locally nothing goes faster than c.

Nothing can come whizzing past the earth at greater than c, that would be local thing and in violation of 1905 SR.

 

But the expansion of space is not governed by this 1905 limit. The recession of a distant galaxy does not involve anything passing by us and it doesnt involve the transmission of information from point A to point B (the sort of thing that obeys the 1905 speed limit).

 

the FTL recession speeds are part of a different Einstein thing---his 1915 General theory of relativity----which allows for expansion. Expansion is the increase in the distance between two stationary points---something the spacetime metric does. It is not very intuitive.

 

the big hangup is visualizing how light can reach us from a galaxy that was receding at 3c when it emitted the light! we can talk about that some. My experience was that it takes time and quite a bit of thinking over before you get it. (the trick to it is that the hubble parameter H actually decreases, as Morgan's calculator shows you, from those early years, this is how the light manages to make it, but this takes some time to digest)

[Martin]

what was the redshift of 7.5 callibrated against in the vicinity?

 

you see' date=' an object of large volume traveleing at speed behind objects of small volume traveling at relatively low speed, will apear to be the same size, and yet one is moving much faster, it does look odd I know, it`s like in trick photography when you have the moon in someones hand or have them really tiny standing on a football

it`s all perspective.

 

so i`m currious to find out what this "Dot"`s movement is taken relative to.

 

 

cheerz [/quote']

 

Hi YT, I have no text about that particular image. But objects have been seen with redshift roughly that big, so let's discuss what that means.

 

(highest redshift i have information I feel sure about is 6.4, not 7.5 but order of magnitude its roughly the same so let us stick with 7.5)

 

I guess you know that redshift is not a doppler effect. this is one of the first things one learns in a basic beginning course in General Astronomy.

 

the formula governing redshift is

1+z = wavelength received/wavelength emitted

 

you ask "in reference to what" is the redshift

 

the redshift is in reference to some hydrogen line usually, or some line in the spectrum of some atom. the atom gives off a pattern of light of known wavelengths called a spectrum. In the received light there is the same pattern but the wavelengths have all be stretched out by the same factor.

 

In this case the factor is 8.5

 

If they say the redshift z is 7.5 what they mean is that some light emitted with wavelength 1 micron will be received with wavelength 8.5 micron.

 

Some light emitted with wavelength 100 nm will be received with wavelength 850 nm.

 

that is what z = 7.5 means

 

there is a standard way, typified by Morgan's calculator, or Ned Wright's calculator (they use code published in the magazine Sky and Telescope) to get from the redshift to things like age and distance and recession speed.

 

Ned Wright has a Cosmology FAQ. you might check it out to see if you like it. He teaches graduate level cosmology at UCLA and is one of the people in charge of the WMAP satellite. has a good site. Has a Cosmology Tutorial at the site. a calculator like Morgan's, and all that good stuff. cosmology animations etc. The link to it is also at Alexa's link library

[Martin]

I'm still wondering about that figure of 700 million years

(it is very early to have a galaxy)

 

it would correspond to z =7.5

(that is higher than all but one observation I know of)

 

I went looking for reliable info on high z galaxies and quasars and I found

this recent paper

http://arxiv.org/astro-ph/0406140

 

this has a table with high-z results from a number of observation team

and a couple of 6.6

plus three or so with 6.5

 

already in 2003 I knew of one 6.4 quasar (which made it to Ned Wright's website's "News of the Universe")

 

So although there is this one instance of a z = 10 galaxy reported by a French team this year, I still am skeptical about that, the highest I feel sure about is z= 6.6

 

We can plug that into the Ned Wright, or the Morgan, calculators and see what the age and distance are for that redshift. It will not be so extreme as the 700 million years that started this thread! (but it will still be pretty far and receding pretty fast)

 

Yeah, a z=6.6 thing emitted the light when universe was 830 million year old, and thing was then 3.7 billion LY distant and was receding 2.94 c.

And the thing is now 28 billion LY distant and receding 2.05 c.

 

So it is qualitatively about the same---except now we have reasonably solid evidence from several teams of astronomers!

 

Several things have been confirmably seen that emitted the light when receding almost 3 c and are now receding 2 c, and when they emitted the light the universe was a bit over 800 million year old. That is, redshift 6.6 things have been seen.

 

But about the French team's redshift 10 there is still some doubt (Ned Wright discusses the criticism of this)

 

for an article discussing the range 6.6-7.1 try this

http://arxiv.org/abs/astro-ph/0402319

it is by J-P Kneib, one of the French team that thought they had a z = 10

but the article is more cautious and only talks about maybe seeing something around z=7. It can give you an idea of the difficulties.

 

I think one has to say 6.6 is highest reliable todate (despite some pressreleases)

[Martin]

here is the report of a quasar found at z = 6.41

where they were actually able to measure the mass of the black hole

at its center, powering the quasar by stuff falling into it

 

http://arxiv.org/astro-ph/0303062

 

the black hole turned out to be 3 billion solar masses,

quite a large black hole.

 

a lot of the study depends on the 280 nm wavelength UV light

coming from doubly ionized Magnesium, this would be shifted

down into the infrared (multiply 280 by 7.41)

 

it is pretty remarkable they can see something at that distance

let alone measure its mass!

[JohnB]

Martin, thaks for the explanations. I was assuming C as a speed limit. I'm not a Cosmologist, just a layman trying to a handle on the concepts involved.

 

The 700 million year figure was quoted on news reports that made me look for the pic in the first place, so it could be wrong.

 

The pic is from the Hubble site and very small. The one I d/loaded, at full size, would take about 15 monitors to view. 2401 x 3001 pixels, 495k .jpg file. Hudf_hst_big.jpg. I did find another copy on the net at http://www.fearme.com/img/imgview.cgi?file=&series=astro&page=0

it's the thirteenth one down on the lefthand column.

 

I'll follow your links, I see I have a fair bit of reading ahead of me.

Your explanation leads to further questions. ( As all good explanations do.)

and it was, at the moment it emitted the light we are now seeing, receding from us at 3.23 times c

 

and it is now receding from us at 2.12 times c and is at a distance of 29.31 billion LY.

 

How do we arrive at the figure of deceleration of the expansion? Is it a function derived by comparing the redshift of nearer (newer) Galaxies to further (older) Galaxies? As there is no way to confirm the figure for deceleration, how do we know it's right?

[Martin]

How do we arrive at the figure of deceleration of the expansion? Is it a function derived by comparing the redshift of nearer (newer) Galaxies to further (older) Galaxies? As there is no way to confirm the figure for deceleration' date=' how do we know it's right?[/quote']

 

almost all cosmology is based on two fairly simple-looking differential equations called the Friedmann equations, and when you ask a question like that to a cosmologist he/she will give you a verbal interpretation of the solution of the F. eqns.

 

the F. eqns. are about a variable a(t) called the "scale factor" or the average distance between galaxies---an indicator of the size of the universe---and a(t) increasing means universe expanding

 

the point of dark energy is that matter only recently thinned out enough for dark energy (which has a constant density and doesnt thin out) to become important. So at least up until 4 or 5 billion years ago the expansion was actually slowing down! matter was concentrated enough for the first 9 billion years or so that it's gravity was pulling things together and slowing down the expansion! but by a kind of momentum the expansion kept on going and by thinning out the matter, it won. so now there is no possibility of expansion ever stopping

 

this comes from playing with the parameters of the F. eqns. which draws pretty curves which show the evolution of the size a(t) with time.

you can put in more or less matter, more or less dark energy, start with H(t) something at a certain time, change what H(t) you start with, and so on. H(t) is the hubble parameter (dont call it hubble "constant" because it is not constant, the F.eqns show how it evolves)

 

Do you do differential equations? they dont get much simpler than F.eqn (unless it is the eqn for a pendulum or an harmonic oscillator)

 

Have you tried Morgan's calculator? It shows H(t) declining over time.

this is basically the reason light can get to us!

and also the reason why something receding at 3 c back then could now be receding at only 2 c. even tho it is farther away.

 

(usually one thinks of the more distant things as receding faster, that is hubble law, but hubble law is with all quantities measured at the present moment-----v = H D-----that means at the present time t, that

v(t) = H(t) D(t)-----they just dont write the t, sometimes, out of laziness.)

 

hope you are OK, not too confused, and getting something out of this, it aint easy conceptually

[Douglas]

Hi Douglas' date=' that is an astute objection. My personal experience was perhaps similar to yours in that it took me a fairly long time to get used to the idea of superluminal recession speeds.

the big hangup is visualizing how light can reach us from a galaxy that was receding at 3c when it emitted the light! we can talk about that some. My experience was that it takes time and quite a bit of thinking over before you get it. (the trick to it is that the hubble parameter H actually decreases, as Morgan's calculator shows you, from those early years, this is how the light manages to make it, but this takes some time to digest)[/quote']

Thanks for the explanation Martin, though I'm not sure if I can digest it.

[JohnB]

Thanks Martin.

 

I haven't done differential equations since high school. Oh well, time to hit the books again.

 

When you say "FTL recession speeds" are you referring to the galaxy actually travelling FTL or apparently FTL because of the expansion?

 

Strangely enough, I don't have any problem with the concept of something apparently travelling FTL due to expansion, I just thought that everybody else would.

 

Concerning things like Morgan's Calculator, I think I'll need to do a bit more reading to be able to phrase my questions correctly. I hope you don't mind if I return to the topic at a later date?

[Martin]

 

...When you say "FTL recession speeds" are you referring to the galaxy actually travelling FTL or apparently FTL because of the expansion?

 

Strangely enough' date=' I don't have any problem with the concept of something apparently travelling FTL due to expansion, I just thought that everybody else would.

 

...[/quote']

 

you have the right idea

cosmologists use a metric, or distance measure, called the FRW

(friedmann, robertson, walker) metric

 

and in the formula for the distance between two points there is this parameter a(t) called the scale factor

 

and the expansion of the universe is shown by the fact that the distance between two points keeps increasing---because a(t) is increasing

 

and the distance between two points will be increasing FTL, if the two points are far enough apart to start with

 

 

but objects like galaxies are gravitationally bound and they fall back together in a way that balances the very slight expansion of space (which is negligibly small at scales we are familiar with) so galaxies stay the same size

 

it is only distances between very widely separated galaxies that are affected

 

the FRW metric is, essentially, a solution of the Friedmann equations (which are simplifications of the original 1915 Einstein equation of GR)

 

that is, a(t) a simple increasing function of time is a solution of the Friedmann equations, and a(t) plugs into the metric and is what makes it expand.

 

and the expansion is proportional to distance so of course if you go far enough out you find that the distance to something is increasing faster than the speed of light---it cannot be otherwise

 

you can say that the recession speed is "apparent" because due to the expansion of space----Davis and Lineweaver say it differently, they say there are two kinds of speed, ordinary velocity and recession velocity.

It is just a verbal difference, if I understand your meaning.

 

However I personally would not like to say "apparent" because it makes it seem not real. It is a very real rate of change of distance, and since speed is a rate of change of distance I have to think of it as a real speed. Just a different kind, originating differently from ordinary local speeds of motion.

 

Douglas and JohnB, you were asking stimulating questions so thanks in turn, you might like to have a look at the Friedmann equations, just if you are curious

http://www.scienceforums.net/forums/showthread.php?p=56563#post56563

 

this is not homework but just if you are curious as to what the

basic eqns of cosmology look like. they are really not too bad and the

k can be taken equal to zero so they are even simpler than they look at first sight