Martin i have a question for you

[Money]

in this topic

 

http://www.scienceforums.net/forum/archive/index.php/index.php?t-2889.html

 

you say

"But that does not mean that the distance to a remote galaxy cannot be increasing at a FTL rate."

 

if i understand correct your basically saying the distance between us and

remote galaxies is increasing at ftl speeds is this happening to all galaxies around us not just the distant ones.

and if so woudnt galaxies

that we are able to observe not be observable because they are moving

away so fast

 

and p.s. i dont know alot about all this kind of science i have a little

understanding but im still touchin the surface but i've been getting

relativity, how the universe is expanding, light all that stuff pretty well so far

[Martin]

Hi Money, I just saw your post. Would have gotten back to you sooner, but I missed it.

 

in this topic

 

http://www.scienceforums.net/forum/archive/index.php/index.php?t-2889.html

 

you say

"But that does not mean that the distance to a remote galaxy cannot be increasing at a FTL rate."

 

if i understand correct your basically saying the distance between us and

remote galaxies is increasing at ftl speeds. Is this happening to all galaxies around us not just the distant ones?

No. Near galaxies either do not recede or only very slow. Medium range typically recede, but not faster than lightspeed. There is a formula for how fast things are receding----it is proportional to their current distance from us: farther things recede faster, in proportion to their distance.

 

... woudnt galaxies

that we are able to observe not be observable because they are moving

away so fast?

 

Much of the stuff that astronomers now observe (whose light is reaching us at this moment) is galaxies which WERE receding faster than light AT THE TIME when they emitted the light which is now reaching us.

And now presumably they would be receding even faster, because farther away.

 

Like for example anything we can see now that has a redshift z = 2 or greater would be a case of that. And astronomers have a huge catalog of stuff with redshift z > 2. The cosmic microwave background comes from stuff so far back in time that it has a redshift z = 1100. there are a lot of objects to look at between z = 2 and z = 1100, and all those objects were receding FTL at the time they emitted the light which we are now receiving from them.

 

Do you have any trouble understanding how light that is emitted from an object when it is receding from us at, say, 2c (twice the speed of light) could eventually reach us?

 

Light from a lot of what astronomers study is like that. I think it's worth trying to understand how (but a lot of people find it difficult.)

[Martin]

Here's my advice

 

first get used to using Ned Wright's cosmo calculator

 

http://www.astro.ucla.edu/~wright/CosmoCalc.html

 

this does not give the recession speeds but it does convert redshift z to comoving distance (the realistic measure of current distance)

 

It also give "angular size distance" (distance gauged by how small things look)

and "luminosity distance" (distance told by how faint thing look)

but these measures have some odd behavior.

 

the comoving distance is what you plug into the Hubble law. which says

current recession speed = H x current distance

 

If you havent done so, you should play around with the cosmo calculator and get used to putting some z in the box (leave the other parameters alone, they are set right at their default values) and learning the light travel time and the distance.

 

If you ever do this, which I think is a first step, then tell me and we can go to the next step, where you try a next step calculator which actually tells you the recession speeds, as multiples of the speed of light.

[Money]

Do you have any trouble understanding how light that is emitted from an object when it is receding from us at, say, 2c (twice the speed of light) could eventually reach us?

 

i understand that pretty well if its a celestial body that emits light.

what if its a planet moving that far away that wouldnt emit light correct?

 

but i do understand that if it emits light moving 2c away that light will eventually reach us because light is constant and will keep going in whatever direction it was emitted in. but b-cuz the universe is expanding it will just take longer

Martin how do u kno so much kuz wen i graduate idk wat college

to go to for mainly science like this and u are Smart lol wat college did or do u go to

 

i hope im understanding this rite

(we havnt gone over things like this is school im just workin off the knowledge that i've gained from reading many topics on this site and other various websites)

[Martin]

i understand that pretty well ...

 

Good. Also, have you by any chance tried out that cosmological calculator?

 

It helps you get a practical grasp of the significance of the redshift.

When some light comes in to us with its wavelengths stretched out by a factor of 3, say, then what does that mean about how long the light has been traveling and how far away the source is?

 

Stretchout by factor of 3 means z = 2

the redshift z number is always ONE LESS than the actual stretch factor. That is just a convention which astronomers have always had.

 

So if the wavelengths are all 4 times longer than when they were emitted then that means z = 3.

 

the subtracting one thing doesnt matter it is just a convention they have---a longstanding professional habit.

 

what matters is if z = 3 what does that mean about how long the light has been traveling?

 

you need to play around with the calculator and get used to answering that question for yourself

 

(after that we can talk about recession speeds)

[Money]

im understanding the calculator pretty well

and i understand wat ur saying about tha redshift formula

but im having trouble understanding wat Redshift actually is

ive read up on it but im having a hard time comprehending it

i understand all the outcomes except

 

The comoving volume within redshift z is 11486.128 Gpc3

The comoving radial distance, which goes into Hubble's law, is 13997.6 Mpc or 45.655 Gly

i dont kno wat these are and i kind of get the angular size difference

[Martin]

...The comoving volume within redshift z is 11486.128 Gpc3

The comoving radial distance, which goes into Hubble's law, is 13997.6 Mpc or 45.655 Gly

i dont kno wat these are and i kind of get the angular size difference

 

Good. It looks like you plugged in z = 1100 which is the redshift of the cosmic microwave background.

 

as usual the astronomer's technical words get in the way a bit.

"comoving radial distance" (in this context) just means the real actual distance measured today at this moment

 

dont worry about the "comoving volume" that is just the volume of a sphere with radius 45.655 billion LY.

focus on the main thing, which is that 45.655 billion LY distance

(let's round it to 45.7 for convenience)

 

What the calculator is telling you in this case is this:

 

Roughly 13.6 billion years ago when the expansion had been going on for just about 380,000 years, the universe was full of hot crud--around 3000 degrees kelvin, and stuff that hot glows with a reddish light (redder than sunlight but similiar). So space was full of a hot dilute gas, partly ionized, and glowing with light similar from what you get from a 100 watt incandesent litebulb (not as yellow as the sun, more orange-ish).

Wavelengths roughly on the scale of a millionth of a meter (one micron)

 

As the expanding universe became 380,000 years old it got transparent enough for the the light to get loose, instead of being cooped up in opaque hot gas, and it has been flying free for about 13.6 billion years all that time getting stretched out so by the time that light gets to us it's wavelengths are stretched out 1100-fold.

(light with that long wavelength is called microwave and you can't see it, so it is doesnt look like the original reddish sunlight-type light)

 

so now instead of wavelength on a millionth of a meter scale, it is on the thousandth of a meter (one millimeter) scale----more like in a microwave oven than from a litebulb

 

what the calculator says is HOW FAR AWAY that the crud is that emitted the light which we are now getting as microwave background. It says that crud is now 45.7 billion LY away.

 

===================

so here is what you can now do. Light from nearby stars has characteristic "bumps" in it similar to the characteristic colors that different chemicals make when they burn, for example there is a distinctive yellow from sodium and characteristic colors red and bluegreen that hydrogen makes, so there is a kind of NORMAL PROFILING of light from nearby stars and galaxies. You can separate the light out with a prism and find distinctive bright lines in the the "rainbow".

 

When you plot the profile of some light on a wavelength scale and you find that the distinctive sodium peak is coming at, say, 1000 nanometers instead of 500

nanometers and the distinctive bluegreen is coming at 900 instead of 450 and the distinctive red is coming at 1200 instead of 600, then you say THIS LIGHT HAS BEEN SHIFTED! All the wavelengths in this light have been increased by a factor of 2! THIS IS Z=1 LIGHT. This light did NOT come from a nearby galaxy!

 

You see now you are very difficult to fool. You see the light, you notice the shift, you realize it is z = 1, and you CAN EVEN TELL HOW FAR AWAY THE OBJECT IS at present THAT EMITTED IT.

 

How far? What (comoving, present) distance corresponds to z=1?

[Money]

kan we move to the next step Martin

i've been messin wit that Calc. for about 2 days now lol

[Martin]

kan we move to the next step Martin

i've been messin wit that Calc. for about 2 days now lol

 

embarrassed face

I forgot about this thread. things were happening here. sorry.

 

Money, please keep in mind that I am not a professional teacher, with the deep pockets of knowledge and the resources of a teacher. I'm a retired guy.

 

but I can coach you a little ways along.

 

The thing to take away from the Wright calculator is that they have MEASURED THE NUMBERS 71 AND 0.73 AND 0.27

BY FITTING THE MODEL TO THE DATA.

 

These numbers are what you see as the DEFAULT values of H, Omega_Lambda, and Omega_Matter, when you use Wright's calculator. he is nice to you so he puts in the correct default values.

 

You can't move on to Morgan's calculator unless you are prepared to type in those values, because Morgan's calculator makes you type in the three key parameters.

 

It is actually easy, H is "Hubble parameter" you type in 71 because

the recession speed increases by 71 km/s for every Megaparsec you go out.

It's measured.

 

And Omega_Matter (also called Omega for short) is the "matter fraction" the percentage of the energy density that is matter (either dark or ordinary).

It is 27 percent.

 

And Omega_Lambda (also called Lambda for short which is kind of sloppy notation) is the "dark energy fraction and it is estimated 73 percent.

 

You have to kind of take it on faith for now. They put a lot of effort getting these estimates as good as possible.

 

I will get the URL for Morgan's calculator. It gives the recession SPEEDS.

 

First you type in those three numbers 0.27

and 0.73

and 71

and then you type in the redshift z = 3 or 7 or 1100 or whatever

and it tells you both how fast the crud was receding when it emitted the light we are now getting from it

and how fast it is receding now at the present day.

 

Bad news! THE URL FOR MORGAN'S CALCULATOR NO LONGER WORKS. I checked it within the past two weeks and it worked, suddenly it's gone.

Here is Morgan's cosmology website:

http://www.uni.edu/morgans/cosmos/index.html

 

Morgan has reorganized the website and closed old links, I am still searching for the cosmo-calculator java applet. Here is an index of java applets at the site:

 

http://www.uni.edu/morgans/ajjar/index.html

 

Ahhhh! here it is

http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

 

The first number you have to type in is 0.27 for Omega_Matter (here called Omega for short)

the next number is 0.73 for Omega_Lambda (here called Lambda for short)

then you type 71 for H, and then you are ready

 

put in z = 2, for example, and get the recession speed when that z=2 light was emitted

It should give 1.17 c for the speed when the light was emitted.

[Money]

Martin thanks for helping me understand

an sorry if im a bother lol but im just really interested

but yea im a little confused about sum of the calculations

 

Z=2 distance now= 17.1 GLY

speed moving away now= 1.24

distance then= 3.34 GLY

speed moving then= 1.17c -----------i get that

 

Z=7 Distance now= 28.75

speed moving away now= 2.08c

Distance then= 3.59 GLY

Speed moving away then= 3.07c -------- i get this one too

 

Z=1100 Distance Now= 45.49 GLY

speed moving away now= 3.3c

Distance then= 0.04 GLY

Speed moving away- 56.95c ---------im confused on this one

 

if the object was closer to us "back then" then how was it

traveling faster away from us then now when its further away ???

 

i thought the farther away the faster it will move

i have a thought in the back of my head that it might be

have somthing to do with the time wen tha universe was opaque

and light first started moving as i've seen u describe in other posts

but idk

 

i've seen som1 describe the universe like a rubberband being stretched

and i see that as a good way to picture it the edges strech faster than

the center

[Martin]

...

Z=1100 Distance Now= 45.49 GLY

speed moving away now= 3.3c

Distance then= 0.04 GLY

Speed moving away= 56.95c ---------im confused on this one

...

 

thanks for typing in these cases. they all make sense to me at first look. It's good you are playing around with different numerical cases, you pick up a quantitative feel for it that way.

 

we can compare Morgan's result for z = 1100 with Wright's calculator (which even tho it doesn't give the speeds still has the advantage of slightly better precision.)

 

When you look back to the origin of the CMB you are looking at when the universe was 1100 times more concentrated and everything was 1000 times closer together. And the glowing CRUD that made the CMB light was only about 40 million LY from the crud that became us.

 

and the U was only about 380,000 years old.

 

At that time the expansion speeds of even nearby stuff was surprisingly fast! That just means that the HUBBLE parameter was much larger then.

Indeed the basic differential equation that describes the standard model of cosmology (the Friedmann equation, one of two actually) tells the evolution of the Hubble parameter.

 

Nowadays the Hubble parameter is a modest 71 km/s per Mpc which means for every additional Megaparsec (Mpc) you go out the recession speed is faster by 71 km/s.

 

Back in those days a Hubble parameter was really a Hubble parameter!

Like 1.4 million km/s per Megaparsec. That is about 4.7c per Megaparsec. Twentythousand times bigger than it is today.

BTW it may seem strange astronomers use two different scales of distance----LY and parsec that are the same order of size, one parsec is 3.26 LY. It is basically just a historical accident. Hidebound traditionalism. Individually they are really nice people. But they use a mix of units which are almost impossible to reform and streamline.

 

Admittedly space is not a material like rubber and the universe is not really like the 3D analog of a rubber balloon being inflated. BUT if you ever watched balloons being inflated from a waterfaucet (as when making water bombs to drop out of the window onto obnoxious persons) what you see first is this very little balloon expanding VERY FAST and then as it gets bigger and bigger the diameter increases slower and slower. After a while it gets boring because the diameter size is barely increasing. Expansion is still going on but it has been enormously slowed down.

 

That is too material to be a good analogy for the universe but it does mimic the business about the Hubble expansion parameter being extremely much larger in early times and recession speeds, even at rather modest distances, being amazingly fast. As you noted the crud that gave us the CMB light we are seeing now was receding THEN at some 57c.

 

I should check some of these numbers by comparing with Wright's calculator. I will put the links here to make comparison easy. One of the next things we should check is the ANGULAR SIZE DISTANCE of stuff at a certain Z, which Wright's calculator gives you. this is the distance gauged by how big things look or by what angle they subtend in the sky. My intuition is that the angular size distance (which Wright gives) of an object at some redshift z should have something to do with the "distance then" which Morgan gives.

 

 

 

http://www.astro.ucla.edu/~wright/CosmoCalc.html

http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

[Martin]

I just lost a post

 

http://www.astro.ucla.edu/~wright/CosmoCalc.html

http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

 

Angular size distance and "distance then" are the same.

Wright and Morgan agree that the crud that emitted the light we now receive as CMB microwave was 40 million LY away from us when it emitted it.

that is fairly close.

 

here is a diagram of the history of the universe. Just look at the top stripe. the other two are in special coordinates like "log plots" for convenience in studying detail---they are purposefully distorted.

The top stripe is the straight picture.

 

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg

 

If you print it out you may get it bigger and more legible. See what you can make of it. NOTICE THAT OUR PAST LIGHTCONE IS NOT CONESHAPE BUT RATHER IS TEARDROP SHAPE.

 

Because if you go way back in time the stuff emitting the light we are now getting was actually rather close to us. That's why the cone comes together at the bottom and makes a teardrop shape.

 

If you want to see the diagram in context, here are some links:

The PDF version http://arxiv.org/abs/astro-ph/0305179

The Cal Tech HTML version http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/frames.html

The key figure http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg

The same figure in context with surrounding text and caption

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver2.html